Apparatuses and methods for fast handover and conditional fast handover of user equipment

Abstract

Various example embodiments relate to apparatuses and methods for fast handover and conditional fast handover of user equipment. An apparatus may be configured to receive a message comprising cell-specific baseline configurations and execution conditions for a conditional handover from a source access node, the cell-specific baseline configurations comprising a default signaling radio bearer configuration, determine information on radio link quality of a source cell and one or more candidate target cells, select at least one target cell from the one or more candidate target cells based on the execution conditions and the determined information on radio link quality, initiate a random-access procedure to the selected at least one target cell based on the cell-specific baseline configuration, and transmit a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration to the at least one handover target access node during the random-access procedure.

Description

APPARATUSES AND METHODS FOR FAST HANDOVERAND CONDITIONAL FAST HANDOVER OF USER EQUIPMENTTECHNICAL FIELD

[0001] The present disclosure relates generally to communication technologies and, in particular, to user equipment (UE) handover in a communication system.BACKGROUND

[0002] A communication system can be seen as a facility that enables communication sessions between two or more entities such as user terminals, base stations and / or other nodes by providing carriers between the various entities involved in the communication path. A communication system can be provided for example by means of a communication network and one or more compatible communication devices. The communication sessions may comprise, for example, communication of data for carrying communications such as voice, video, electronic mail (email), text message, multimedia and / or content data and so on. Non-limiting examples of services provided comprise two-way or multi-way calls, data communication or multimedia services and access to a data network system, such as the Internet.

[0003] In a wireless communication system at least a part of a communication session between at least two stations occurs over a wireless link. Examples of wireless systems comprise public land mobile networks (PLMN), satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). Some wireless systems can be divided into cells and are therefore often referred to as cellular systems.

[0004] A user can access the communication system by means of an appropriate communication device or terminal. A communication device of a user may be referred to as user equipment (UE) or user device. A communication device is provided with an appropriate signal receiving and transmitting apparatus for enabling communications, for example enabling access to a communication network or communications directly with other users. The communication device may access a carrier provided by a station, for example a base station of a cell, and transmit and / or receive communications on the carrier.

[0005] The communication system and associated devices typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. Communication protocols and / or parameters which shall be used for the connection are also typically defined. One example of a communication system is the Universal Mobile Telecommunications System (UMTS). Otherexamples of communication systems are Long-Term Evolution (LTE), LTE Advanced and the so-called 5G or New Radio (NR) networks. NR is being standardized by the 3rd Generation Partnership Project (3 GPP).BRIEF DESCRIPTION

[0006] An aspect provides an apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: receive, by a user equipment from a handover source access node, a message in association with a handover comprising a cellspecific baseline configuration for a handover of the user equipment to a target cell provided by a handover target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; initiate a random-access procedure to the target cell to establish a radio resource control connection between the user equipment and the handover target access node, based on the cell-specific baseline configuration; and transmit, by the user equipment to the handover target access node, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration, during the randomaccess procedure.

[0007] Another aspect provides an apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: receive, by a handover source access node from a handover target access node, a cell-specific baseline configuration for a handover to a target cell provided by the handover target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receive, by the handover source access node from a user equipment, information on radio link quality in association with a source cell provided by the handover source access node and the target cell; determine, by the handover source access node, to carry out the handover of the user equipment to the target cell using the cell-specific baseline configuration based on at least the information on radio link quality in association with the source cell and the target cell; and transmit, by the handover source access node to the user equipment, a message in association with the handover of the user equipment to the target cell comprising at least part of the cell-specific baseline configuration.

[0008] Yet another aspect provides an apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: transmit, by a handover target access node to a handover source access node, a cell-specific baseline configuration for a handover from the handover source access node to a target cell provided by the handover targetaccess node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receive, by the handover target access node from the handover source access node, information on a user equipment and an access stratum security master key for communication between the user equipment and the handover target access node; receive, by the handover target access node from the user equipment, a random-access preamble; transmit, by the handover target access node to the user equipment, a random-access response message; receive, by the handover target access node from the user equipment, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration; generate, by the handover target access node, a supplementary configuration for the handover of the user equipment to the target cell, based on the information on the user equipment; and transmit, by the handover target access node to the user equipment, the supplementary configuration.

[0009] Yest another aspect provides an apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: receive, by a user equipment from a handover source access node, a message in association with a conditional handover comprising one or more cell-specific baseline configurations and one or more execution conditions for a conditional handover of the user equipment to one or more candidate target cells provided by one or more handover candidate target access nodes, the one or more cell-specific baseline configurations comprising a default signaling radio bearer configuration; determine, by the user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more candidate target cells; select, by the user equipment, at least one target cell from the one or more candidate target cells based on the one or more execution conditions and the determined information on radio link quality in association with the source cell and the at least one of the one or more candidate target cells; initiate a random-access procedure to the selected at least one target cell to establish a radio resource control connection between the user equipment and at least one handover target access node providing the at least one target cell, based on the cell-specific baseline configuration; and when the radio resource control connection is established, transmit, by the user equipment to the at least one handover target access node, a random-access procedure related reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration.

[0010] Yet another aspect provides an apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: receive, by a handover source access node from one or more neighboring access nodes, one or more cell-specificbaseline configurations for a conditional handover to one or more neighboring cells provided by the one or more neighboring access nodes, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receive, by the handover source access node from a user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more neighboring cells; determine, by the handover source access node, to carry out the conditional handover of the user equipment to one or more candidate target cells selected from the one or more neighboring cells, based on at least the received information on radio link quality, the one or more candidate target cells being provided by one or more handover candidate target access nodes; and transmit, by the handover source access node to the user equipment, a message in association with the conditional handover of the user equipment to the one or more candidate target cells comprising at least part of the one or more cell-specific baseline configurations in association with the one or more candidate target cells.

[0011] Yet another aspect provides an apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: transmit, by a handover candidate target access node to a handover source access node, one or more cell-specific baseline configurations for a conditional handover from the handover source access node to one or more candidate target cells provided by the handover candidate target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receive, by the handover candidate target access node from the handover source access node, information on a user equipment and an access stratum security master key for communication between the user equipment and the handover candidate target access node; receive, by the handover candidate target access node from the user equipment, a random-access preamble to a target cell selected from the one or more candidate target cells according to the cell-specific baseline configuration; transmit, to the user equipment, a random-access response message; receive, by the handover candidate target access node from the user equipment, a random-access procedure related reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration; generate, by the handover candidate target access node, a supplementary configuration for the conditional handover of the user equipment to the target cell, based on the information on the user equipment; and transmit, by the handover candidate target access node to the user equipment, the supplementary configuration.

[0012] Yet other aspects provide methods, apparatuses, computer readable media and computer programs generally corresponding to the above aspects and a repetitive description thereof is omitted here for convenience.

[0013] These and other features, aspects, and advantages of the present disclosure will be apparent from a reading of the following detailed description together with the accompanying figures, which are briefly described below. The present disclosure comprises any combination of two, three, four or more features or elements set forth in this disclosure, regardless of whether such features or elements are expressly combined or otherwise recited in a specific example implementation described herein. This disclosure is intended to be read holistically such that any separable features or elements of the disclosure, in any of its aspects and example implementations, should be viewed as combinable unless the context of the disclosure clearly dictates otherwise.

[0014] It will therefore be appreciated that this Brief Description is provided merely to provide a basic understanding of some aspects of the disclosure. Accordingly, it will be appreciated that the above-described aspects are merely examples and should not be construed to narrow the scope or spirit of the disclosure in any way. Other examples, aspects and advantages will become apparent from the following detailed description taken in conjunction with the accompanying figures.BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Having thus described example implementations of the disclosure in general terms, reference will now be made to the accompanying figures, which are not necessarily drawn to scale, and wherein:

[0016] FIG. 1 illustrates a communication system that comprises one or more public land mobile networks (PLMNs) coupled to one or more external data networks, according to some examples;

[0017] FIG. 2 illustrates a deployment of a PLMN, according to some examples;

[0018] FIG. 3 is a signaling chart of a conventional handover procedure;

[0019] FIG. 4 is a signaling chart of a conventional conditional handover procedure;

[0020] FIG. 5 is a signaling chart of a fast handover procedure, according to some examples;

[0021] FIG. 6 is a signaling chart of a fast handover procedure, according to some examples;

[0022] FIG. 7 is a signaling chart of a fast handover procedure, according to some examples;

[0023] FIG. 8 is a signaling chart of a fast handover procedure, according to some examples;

[0024] FIG. 9 is a signaling chart of a fast handover procedure, according to some examples;

[0025] FIG. 10 is a signaling chart of a conditional fast handover procedure, according to some examples;

[0026] FIG. 11 is a signaling chart of a conditional fast handover procedure, according to some examples;

[0027] FIG. 12 is a signaling chart of a conditional fast handover procedure, according to some example implementations;

[0028] FIG. 13 is a signaling chart of a conditional fast handover procedure, according to some examples;

[0029] FIG. 14 is a signaling chart of a conditional fast handover procedure, according to some examples;

[0030] FIGs. 15A-15D are flowcharts of a method, according to some examples;

[0031] FIGs. 16A-16D are flowcharts of a method, according to some examples;

[0032] FIGs. 17 A- 17F are flowcharts of a method, according to some examples;

[0033] FIGs. 18 A- 18F are flowcharts of a method, according to some examples;

[0034] FIGs. 19 A- 19F are flowcharts of a method, according to some examples;

[0035] FIGs. 20A-20B are block diagrams of an apparatus, according to some examples;

[0036] FIG. 21 is a block diagram of an apparatus, according to some examples;

[0037] FIG. 22 is a block diagram of an apparatus, according to some examples;

[0038] FIG. 23 is a block diagram of an apparatus, according to some examples;

[0039] FIG. 24 is a block diagram of an apparatus, according to some examples; and

[0040] FIG. 25 is a block diagram of an apparatus, according to some examples.DETAILED DESCRIPTION

[0041] Some examples will now be described in more detail hereinafter with reference to the accompanying figures, in which some, but not all implementations of the disclosure are shown. Indeed, various implementations of the disclosure may be embodied in many different forms and should not be construed as limited to the implementations set forth herein. Like reference numerals refer to like elements throughout.

[0042] Unless specified otherwise or clear from context, references to first, second or the like should not be construed to imply a particular order. A feature described as being above another feature (unless specified otherwise or clear from context) may instead be below, and vice versa; and similarly, features described as being to the left of another feature else may instead be to the right, and vice versa. Also, while reference may be made herein to quantitative measures, values, geometric relationships or the like, unless otherwise stated, any one or more if not all of these may be absolute or approximate to account for acceptable variations that may occur, such as those due to engineering tolerances or the like.

[0043] As used herein, unless specified otherwise or clear from context, the “or” of a set of operands is the “inclusive or” and thereby true if and only if one or more of the operands is true, as opposed to the “exclusive or” which is false when all of the operands are true. Thus, forexample, “[A] or [B]” is true if [A] is true, or if [B] is true, or if both [A] and [B] are true. Further, the articles “a” and “an” mean “one or more,” unless specified otherwise or clear from context to be directed to a singular form. Furthermore, it should be understood that unless otherwise specified, the terms “data,” “content,” “digital content,” “information,” and similar terms may be at times used interchangeably. The term “network” may refer to a group of interconnected computers comprising clients and servers; and within a network, these computers may be interconnected directly or indirectly by various means comprising via one or more switches, routers, gateways, access points or the like.

[0044] Reference may be made herein to terms specific to a particular system, architecture or the like, but it should be understood that example implementations of the present disclosure may be equally applicable to any of a number of systems, architectures and the like. For example, reference may be made to 3 GPP technologies such as Global System for Mobile Communications (GSM), UMTS, LTE, LTE Advanced, 5GNR, 5G Advanced and 6G; however, it should be understood that example implementations of the present disclosure may be equally applicable to non-3GPP technologies such as IEEE 802, Bluetooth and Bluetooth Low Energy.

[0045] Further, as used in this application, the term “circuitry” may refer to one or more or all of the following: (a) hardware-only circuit implementations (such as implementations in only analog and / or digital circuitry); (b) combinations of hardware circuits and software, such as (as applicable): (i) a combination of analog and / or digital hardware circuit(s) with software / firmware and (ii) any portions of hardware processor(s) with software (comprising digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a mobile phone or server, to perform various functions); or (c) hardware circuit(s) and / or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

[0046] The above definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and / or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device.

[0047] FIG. 1 illustrates communication system 100 according to various examples of the present disclosure. The communications system generally comprises one or morecommunication networks. As shown, for example, the system comprises one or more public land mobile networks (PLMNs) 102 coupled to one or more other external data networks 104 - notably comprising a wide area network (WAN) such as the Internet. Each of the PLMNs comprises core network (CN) 106 backbone such as the Evolved Packet Core (EPC) of LTE, the 5G core network (5GC) or the like; and each of the core networks and the Internet are coupled to one or more radio access networks (RANs) 108, air interfaces or the like that implement one or more radio access technologies (RATs). As used herein, “network device” refers to any suitable device at a network side of a communication network. Examples of suitable network devices are described in greater detail below.

[0048] In addition, the system comprises one or more radio units that may be varyingly known as user equipment (UE) 110, terminal device, terminal equipment, mobile station or the like. The UE is generally a device configured to communicate with a network device or a further UE in a communication network. The UE may be a portable computer (e.g., laptop, notebook, tablet computer), mobile phone (e.g., cell phone, smartphone), wearable computer (e.g., smartwatch), or the like. In other examples, the UE may be an Internet of things (loT) device, an industrial loT (IIoT device), a vehicle equipped with a vehicle-to-everything (V2X) communication technology, or the like. In some examples, as referenced by 3GPP, the UE may be a narrowband loT (NB-IoT) device, an enhanced machine-type communication (eMTC) device, a reduced capability (RedCap) device, an ambient loT device, or the like.

[0049] In operation, UEs 110 may be configured to connect to one or more of RANs 108 according to their particular radio access technologies to thereby access particular CN 106 of PLMN 102, or to access one or more of external data networks 104 (e.g., the Internet). The external data network may be configured to provide Internet access, operator services, 3rd party services, etc. For example, the International Telecommunication Union (ITU) has classified 5G mobile network services into three categories: enhanced mobile broadband (eMBB), ultra-reliable and low-latency communications (URLLC), and massive machine type communications (mMTC) or massive internet of things (MIoT).

[0050] Examples of radio access technologies comprise 3GPP radio access technologies such as GSM, UMTS, LTE, LTE Advanced, 5GNR, 5G Advanced, and 6G. Other examples of radio access technologies comprise IEEE 802 technologies such as IEEE 802.11 (Wi-Fi), IEEE 802.15 (e.g., 802.15.1 (WPAN / Bluetooth), 802.15.4 (Zigbee) and 802.15.6 (WBAN)), Bluetooth, Bluetooth Low Energy (BLE), ultra wideband (UWB), and the like. Generally, a radio access technology may refer to any 2G, 3G, 4G, 5G, 6G or higher generation mobile communication technology and their different versions, as well as to any other wireless radio access technology that may be arranged to interwork with such a mobile communication technology to provideaccess to CN 106 of a mobile network operator (MNO).

[0051] In various examples, RAN 108 may be configured as one or more macrocells, microcells, picocells, femtocells or the like. The RAN may generally comprise one or more radio access nodes (at times more simply referred to as “access nodes” or “nodes”) that are configured to provide cells to interact with UEs 110. In various examples, a radio access node may be referred to as a base station (BS), access point (AP), base transceiver station (BTS), Node B (NB), evolved NB (eNB), macro BS, NB (MNB) or eNB (MeNB), home BS, NB (HNB) or eNB (HeNB), next generation NB (gNB), enhanced gNB (en-gNB), next generation eNB (ng-eNB), or the like. The RAN may comprise some type of network controlling / goveming entity responsible for control of the radio access nodes. The network controlling / goveming entity and radio access node may be separate or integrated into a single apparatus. The network controlling / goveming entity may comprise processing circuity configured to carry out various management functions, etc. The processing circuity may be associated with a memory, computer- readable storage medium or database for maintaining information required in the management functions.

[0052] RAN 108 may be centralized or distributed. In various examples, components of a RAN may be interconnected by Ethernet, Gigabit Ethernet, Asynchronous Transfer Mode (ATM), optical fiber, dark fiber, passive wavelength division multiplexing (WDM), WDM passive optical network (WDM-PON), optical transport network (OTN), time sensitive networking (TSN) and / or any other data link layer network, possibly comprising radio links. The RAN may be connected to CN 106 through one or more gateways, network functions or the like.

[0053] As will be appreciated, PLMN 102 may be deployed in a number of different manners. FIG. 2 illustrates deployment 200 of a PLMN, such as a 4G LTE, 5G or 6G deployment, according to some examples. As shown, the deployment comprises CN 106, and RAN 108 with one or more radio access nodes 202 configured to interact with UEs 110. In a 4G LTE deployment, the EPC is the CN, and the evolved UMTS terrestrial radio access network (E- UTRAN) is the RAN; and the E-UTRAN comprises one or more eNBs (radio access nodes) configured to connect UEs to the E-UTRAN to thereby access the EPC. Similarly, in a 5G deployment, the 5GC is CN 106, and the next generation (NG) radio access network (NG-RAN) is RAN 108; and the NG-RAN comprises one or more gNBs (radio access nodes) configured to connect UEs 110 to the NG-RAN to thereby access the 5GC. The term “gNB” in 5G may correspond to the eNB in 4G LTE.

[0054] Some deployments of 4G LTE and 5G in particular are considered standalone (SA) deployments. Other deployments combine 4G LTE and 5G technologies, and are referred to as non- standalone (NSA) deployments. In some deployments, the E-UTRAN comprises one ormore ng-eNBs that are configured to communicate with the 5GC, and that may also be configured to communicate with one or more gNBs. Similarly, in another deployment, the NG- RAN may comprise one or more en-gNBs that are configured to communicate with the EPC, and that may also be configured to communicate with one or more eNBs. In various instances, single UE 110, a dual-mode or multimode UE, may support multiple (two or more) RANs — thereby being configured to connect to multiple RANs, such as 4G LTE and 5G.

[0055] In some deployments, operations of radio access node 202 may be distributed or functionally split into components comprising one or more remote radio head (RRHs) or radio units (RUs), and a baseband unit (BBU); and in some architectures, the BBU may be split into a distributed unit (DU) and a central / centralized unit (CU), such as a server, host or node. In some architectures, the RRH / RU and DU may be collocated. It is also possible that node operations may be distributed among a plurality of servers, hosts or nodes.

[0056] It should also be understood that the distribution of work between CN 106 operations and radio access node 202 operations may vary depending on implementation. Thus, a 5G network architecture may be based on a so-called CU-DU split. One gNB-CU (central node) may control one or more gNB-DUs. The gNB-CU may control a plurality of spatially separated gNB-DUs, acting at least as transmit / receive (Tx / Rx) nodes. In some example implementations, however, the gNB-DUs (also called DU) may comprise, for example, a radio link control (RLC), medium access control (MAC) layer and a physical (PHY) layer, whereas the gNB-CU (also called a CU) may comprise the layers above the RLC layer, such as a packet data convergence protocol (PDCP) layer, a radio resource control (RRC), and an internet protocol (IP) layer. Other functional splits are also possible. It is considered that skilled person is familiar with the Open Systems Interconnection (OSI) model and the functionalities within each layer.

[0057] In some example implementations, the server or CU may generate a virtual network through which the server communicates with the radio node. In general, virtual networking may involve a process of combining hardware and software network resources and network functionality into a single, software-based administrative entity, a virtual network. Such virtual network may provide flexible distribution of operations between the server and the radio head / node. In practice, any digital signal processing task may be performed in either the CU or the DU, and the boundary where the responsibility is shifted between the CU and the DU may be selected according to implementation.

[0058] Although only one radio access node 202 is shown in FIG. 2, the deployment may comprise multiple radio access nodes, and at least some of the radio access nodes may be connected to one another by a network interface, such as an Xn interface. Similarly, the radio access nodes may be connected to CN 106 by a network interface. In 5G NR, the networkinterface between a radio access node and the CN is referred to as the NG interface, which is a network interface between the radio access node and an access and mobility management function (AMF) of the 5GC. These and other network interfaces may support the exchange of signaling messages between network entities. The signaling messages may be formatted according to an application layer protocol, such as the NG application protocol (NGAP) for the NG interface between the radio access node and the CN.

[0059] For UE 110 in an RRC connected state, it is generally desirable to keep the UE’s traffic uninterrupted when the UE moves within a cell (also referred to as radio cell) or across different radio cells of one or more radio access nodes 202. To continuously monitor the UE’s radio link condition toward a serving radio cell provided by a serving radio access node, the UE may be configured to measure received signal level and quality from the serving radio cell as well as a list of configured neighboring radio cells and report the results to the radio access node periodically and / or whenever a configured reporting event is met. These measurements may then be evaluated at the radio access node and may result in a handover (HO) of the UE from the serving radio cell provided by the serving radio access node (a handover source access node) to a new radio cell provided by a target radio access node (a handover target access node).

[0060] FIG. 3 is a signaling chart of conventional handover procedure 300 involving UE 110, handover source access node (source node) 202 A that is currently serving (connected to) the UE, and one or more handover candidate target access nodes (target nodes) 202B. As shown, the UE may at 301 be configured, such as by a radio resource control (RRC) reconfiguration, to report measurements of one or more neighboring radio cells periodically or on an event basis and, at 302, send (transmit) a measurement report indicating relevant measurements for one or more radio cells provided by the handover candidate target access node(s). Depending on various configurations and requirements, the measurement may be performed on any of a number of suitable objects. Based on measurements performed and reported by the UE and other UEs served by the handover source access node, the handover source access node may be configured to derive information, such as a number of actively connected UEs, number of RRC connections, number of physical resource blocks (PRBs) in use, the transport network load (TNL) capacity, or the like.

[0061] Handover source access node 202 A may at 303 decide to initiate the handover procedure and initiate a handover preparation in which handover source access node 202 A at 304 sends (transmits) a handover request message with a current configuration of UE 110 towards handover candidate target access node 202B controlling a target radio cell. In some cases, the handover request message may be sent over an Xn interface between the handover source access node and the handover candidate target access node, such as according to a procedure referred to as Xnhandover. In some possible cases where there is no Xn interface between these two access nodes, the handover candidate target access node may be accessed over AMF, such as according to a procedure referred to as NG application protocol (NGAP) handover.

[0062] Handover candidate target access node 202B may at 305 carry out admission control, such as to accept or reject the handover request, and at 306 provide a handover request acknowledgement (ACK) message comprising a configuration of initial access resources for UE 110 in case of acceptance. This configuration may comprise, for example, a cell radio network temporary identifier (C-RNTI), security algorithm identifiers for selected security algorithms, a data radio bearer (DRB) configuration, quality of service (QoS) flow to DRB mapping, UE capability related features enabled by the handover candidate target access node, or the like. Handover source access node 202 A may then at 307 send (transmit) a handover command message that comprises the configuration for the target radio cell towards the UE.

[0063] Upon receipt of the handover command from handover source node 202 A, UE 110 may at 308, 309, 310 obtain downlink (DL) and uplink (UL) synchronization with handover candidate target access node 202B, and thereafter complete the handover procedure. As shown, in some examples, the handover command may be provided at 307 by a RRC reconfiguration; and in some of these examples, the handover procedure may comprise a random-access procedure for handover, comprising downlink (DL) and uplink (UL) synchronization and random-access channel (RACH) access to the handover candidate target access node, and random-access response (RAR) message from the handover candidate target access node. The UE may then send (transmit) an RRC reconfiguration complete message to the handover candidate target access node to complete the handover procedure.

[0064] It should be understood that the handover preparation may take a lot of time and multiple signaling exchanges, such as to request the resource reservation at the handover candidate target access node and to receive the handover request ACK with the target radio cell configuration, thereby likely causing delay (or even leading to a failure) during the handover procedure. The delay may vary depending on the deployment (e.g., NGAP handover or Xn handover) or the architecture (e.g., disaggregated or monolithic structure). In some possible cases, delays up to 60 milliseconds may be expected for example in a disaggregated scenario with NGAP handover. This delay may in turn increase the likelihood of possible handover failure (HOF) and / or radio link failure (RLF).

[0065] The 3 GPP has introduced a so-call conditional handover (CHO) procedure, which may be considered as a potential candidate for addressing some of the issues of the conventional handover procedure.

[0066] Broadly speaking, in the case of CHO, UE 110 may be configured with a CHOcommand containing a target radio cell configuration and one or more CHO execution conditions for carrying out the handover to one or more radio cells. The execution condition may be for radio signal quality, for example, an execution condition may be that a measured reference signal received power (RSRP) from the serving radio cell falls below a threshold RSRP, and / or a measured RSRP from a candidate target radio cell is offset higher than the measured RSRP from the serving radio cell. Each of target radio cell(s) / handover candidate target access node(s) 202B may have prepared a configuration for a handover. Such configuration may then be communicated to the UE in the CHO command. When UE evaluates the CHO execution condition and determines that the execution condition is met for a specific target radio cell, UE may be configured to initiate a random-access, which may be contention free random-access (CFRA) or contention based random-access (CBRA), to the target radio cell and apply the received configuration from handover candidate target access node 202B when the UE successfully establishes an RRC connection with the target radio cell. In some examples, the UE may be configured with multiple execution conditions for multiple target radio cells provided by one or more handover candidate target access nodes.

[0067] CHO procedure may be seen to be so designed that UE 110 may perform / execute the handover without a need of serving radio cell / handover source access node 202Ato trigger the HO execution (as shown at 307 of FIG. 3 for a conventional HO) based on the measurement report (as shown at 302 of FIG. 3).

[0068] FIG. 4 is a signaling chart of CHO procedure 400 involving UE 110, handover source access node (source node) 202 A that is currently serving (connected to) the UE, and one or more handover candidate target access nodes (target nodes) 202B (two handover candidate target access nodes shown). Similar to the conventional HO procedure, the UE may at 401 be configured to report measurements of one or more neighboring radio cells, and at 402 send (transmit) a measurement report indicating relevant measurements for one or more target radio cells provided by the handover candidate target access node(s).

[0069] Handover source access node 202 A may at 403 decide to initiate the handover procedure as a conditional handover procedure based on the received measurement report; and similar to the conventional HO procedure, initiate a handover preparation in which handover source access node 202A at 404 sends (transmits) a handover request message with a current configuration of UE 110 to one or more handover candidate target access node(s) 202B.

[0070] Each handover candidate target access node 202B may at 405 carry out an admission control, such as to accept or reject the handover request, and at 406 provide a handover request ACK message comprising a configuration of initial access resources for UE 110. This configuration may be the same as or similar to the conventional HO procedure, comprising forexample, a C-RNTI, security algorithm identifiers for selected security algorithms, a DRB configuration, QoS flow to DRB mapping, UE capability related features enabled by the handover candidate target access node, or the like.

[0071] Handover source access node 202A may then at 407 send (transmit) a CHO command message that comprises the configuration for the candidate target radio cell(s) towards the UE. The CHO command message may be sent similar to the HO command message, such as by an RRC reconfiguration. The CHO command message also comprises one or more CHO execution conditions for one or more candidate target radio cells of handover candidate target access node(s) 202B.

[0072] Upon receipt of the conditional handover command from handover source access node 202A, when the CHO command message is sent by an RRC reconfiguration, UE 110 may at 408 send (transmit) an RRC reconfiguration complete message to handover source access node 202A. The UE maintains its connection with the handover source access node, and at 409 starts evaluating the CHO execution condition(s) for the target radio cell(s). If at least one target radio cell satisfies a corresponding CHO execution condition(s), the UE may at 410, 411, 412 obtain downlink (DL) and / or uplink (UL) synchronization and random-access channel (RACH) access with handover candidate target access node 202B that provides the target radio cell, and thereafter complete the handover procedure, such as in a manner similar to 308, 309, 310 described above for the conventional HO procedure.

[0073] Compared to the conventional HO, CHO may reduce the HOF and RLF rate as target radio cell(s) of handover candidate target access node(s) 202B may be prepared earlier, and UE 110 may start HO execution as soon as a CHO execution condition is met. In CHO, however, decoupling handover preparation and execution may lead to unnecessary handover preparation with inefficient dedicated resources at the network. Additionally, as CHO configuration is prepared earlier, there may be more time from preparation to execution with higher probability of a change in the handover source access node 202A configuration, which may lead to a mismatch at execution time.

[0074] Examples of the present disclosure therefore provide enhanced support for mobility use cases, such as (inter-node) handover or the like. Some example implementations provide fast handover (FHO) and conditional FHO (CFHO) procedures that employ a baseline configuration (also referred to as a default configuration) to save time spent on preparing a target radio cell and avoid resources allocation before they are needed, and thereby initiate the HO / CHO without preparation as compared to conventional HO / CHO. Throughout the present disclosure, (conditional) fast handover may also be referred to as (conditional) unprepared handover. The baseline configuration may comprise a default signaling radio bearer configuration and / or accessstratum (AS) security information so that the HO / CHO can be carried out without preparation but still in a secure manner. In some example implementations, the baseline configuration may comprise user plane (UP) tunnel information to facilitate early data forwarding during the HO / CHO procedures.

[0075] FIG. 5 is a signaling chart of fast handover (FHO) procedure 500, according to some examples. As shown, at least one handover candidate target access node 202B may at 501 generate a cell-specific baseline configuration (a default configuration) for the FHO to a radio cell provided by the handover candidate target access node and send (transmit) the cell-specific baseline configuration to handover source access node 202A, without a previous handover request from the handover source access node (e.g., a handover request message shown in 304 of FIG. 3). In the case multiple radio cells (e.g., Cell 1 to Cell N) are provided by handover candidate target access node 202B, the handover candidate target access node may generate and share multiple cell-specific baseline configurations in association with the multiple radio cells respectively with handover source access node 202A. The cell-specific baseline configuration(s) may be sent (transmitted) to the handover source access node in a RAN node configuration update message, or any other suitable (existing or new) message. Handover source access node 202A may at 502 send (transmit) a corresponding acknowledgment message (e.g., RAN node configuration update ACK) back to the handover candidate target access node. In some examples, the cell-specific baseline configuration may be generally considered specific to a source radio cell (source cell) provided by the handover source access node and a candidate target radio cell (candidate target cell) provided by the handover candidate target access node (or in other words, handover source and target radio cell-specific).

[0076] Cell-specific baseline configuration generated by the handover candidate target access node may comprise baseline configuration information (at times referred to as default or initial configuration information) required to access a corresponding candidate target radio cell. For instance, the baseline configuration information may comprise a candidate target radio cell identifier (ID) (e.g., physical cell ID, PCI) and system information of the candidate target radio cell, including for example carrier frequency (e.g., absolute radio frequency channel number, ARFCN), security information (e.g., information on security algorithms applied at the candidate target radio cell), and beam- and / or network slice-related information if available.

[0077] Depending on various implementations and / or requirements, the baseline configuration information may comprise baseline low layer configuration information, that is, configuration information for lower layer(s), such as layer 1 / LI (e.g., PHY layer), layer 2 / L2 (e.g., MAC layer, RLC layer, or PDCP layer), or the like. In some more particular examples, the baseline low layer configuration information may comprise suitable Ll / 2 (e.g., cell-specific)configurations, and possibly also configurations related to LI measurement. In some examples, the baseline configuration information may comprise multiple lower layer configurations (possibly for different use cases / scenarios) as part of the baseline configuration information. One example of multiple low layer configurations may comprise configuration information (e.g., PDCP and / or RLC configurations) of a radio bearer that is specific to a network slice identifier (ID) and / or QoS parameters associated with the bearer. In some examples, then, the handover candidate target access node may be configured to provide a lower layer configuration usable only for DRBs associated with a URLLC packet data unit (PDU) session or which generally requires high reliability. This baseline configuration may as well comprise other suitable (low layer related) configuration.

[0078] In some examples, baseline configuration information may also comprise baseline high layer configuration information, that is, a configuration for higher layer(s), such as layer 3 / L3 or even above (e.g., RRC layer, non-access stratum (NAS) layer). The baseline higher layer configuration may also comprise measurement related configurations.

[0079] In some examples, baseline high layer configuration information may comprise some or all of the L2 sub-layers, such as the service data adaptation protocol (SDAP) layer (e.g., for signaling radio bearers - SRBs), instead of low lower configuration information. In some examples, baseline low layer configuration information and baseline high layer configuration information may be collectively referred to as baseline configuration information for a target radio cell of the handover candidate target access node.

[0080] In some examples, baseline configuration information may comprise configuration information of a default signaling radio bearer(s) (SRB). For instance, configuration information of the default SRB may comprise at least configuration information of a default SRB1, and optionally configuration information of a default SRB2. It can facilitate SRB1 / SRB2 establishment and thus RRC and NAS signaling transfer during the fast handover procedure.

[0081] In some examples, baseline configuration information may comprise user plane (UP) tunnel information indicating a default UP tunnel towards a candidate target radio cell. The default UP tunnel may be configured with a tunnel endpoint identifier (TEID) parameter and a corresponding internet protocol (IP) address. It would be appreciated that the UP tunnel information may indicate more than one default UP tunnels. The source access node may use the default UP tunnel(s) to carry out user data forwarding during the handover execution.

[0082] In addition to the baseline configuration information, the cell-specific baseline configuration may comprise at least one criterion (e.g., a set of various criteria) that generally governs the decision as to whether to use, initiate, execute or carry out the FHO procedure with the respective handover candidate target access node (cell). One example of a suitable criterionindicates that UE must not use a guaranteed bit rate (GBR) configuration that cannot be supported by the target radio cell provided by the handover candidate target access node. In other words, if UE is configured with a GBR that cannot be supported by a target radio cell provided by the handover candidate target access node, the FHO procedure should not be used or initiated (by the handover source access node) for the UE towards the target radio cell. Some other possible examples relating to radio (related) configurations for the UE that may be supported or not supported by the handover candidate target access node (cell) may comprise for example a bandwidth configuration used by the UE, service delay requirements of the UE, service reliability requirements of the UE, certain higher layer configuration (e.g., RLC, PDCP related configurations) for example regarding robust header compression (ROHC), or the like.

[0083] As another example, a criterion that may govern the decision regarding the FHO may be that the (current) load of the target radio cell is lower than a predetermined or predefined load criterion (e.g., a threshold, such as 50%, 30%, or the like). In some examples, the load criterion or threshold may be determined by the target radio cell or node itself, or by any other suitable network entity / element (e.g., operations, administration and maintenance (0AM)) of the communication system. The (current) load information of the target radio cell may be (recently) reported by the handover candidate target access node (e.g., periodically or on request of the handover source access node) or estimated by the handover source access node (e.g., based on an early report from the target radio cell). The load information (and correspondingly also the load criterion / threshold) may be indicated in terms of any suitable parameter, which may comprise (but is not limited to), the number of RRC connections maintained by the handover candidate target access node / cell, the number of active UEs served by the handover candidate target access node / cell, the number of PRBs in use in the handover candidate target access node / cell, the transport network load (TNL) capacity of the handover candidate target access node / cell, or the like. Some other possible examples of the criterion may comprise traffic amount of the UE, capabilities of the UE and / or target radio cell, features supported at the UE and / or target radio cell, and the like.

[0084] In some examples, cell-specific baseline configuration may yet further comprise a pool (list) of one or more random-access preambles (also referred to as RACH preambles or dedicated preambles) that can be used for the FHO procedure. The pool of RACH preambles may be predetermined, preconfigured or (pre-)reserved at the handover candidate target access node. Any one of the preambles selected (by the handover source access node) from this pool and assigned to the UE may be later recognizable by the handover candidate target access node during the subsequent synchronization / random-access process. In some examples, the handover candidate target access node may prepare different pools of RACH preambles for different sourcecells / nodes. Then in the subsequent synchronization / random-access process, the handover candidate target access node can identify the source cell / node of a UE according to the RACH preamble received from the UE, which will be described in detail below.

[0085] Similarly, in some examples, the cell-specific baseline configuration may comprise a pool (list) of one or more identifiers e.g. C-RNTIs that can be used / controlled / selected (to assign to UE) by the handover source node in case of handover to a target radio cell provided by the handover candidate target access node. The pool of C-RNTIs may also be predetermined, preconfigured or (pre-)reserved at the handover candidate target access node. Any one of the C- RNTIs selected (by the handover source access node) from this pool and assigned to the UE may be later readily recognizable by the handover candidate target access node during any suitable subsequent process. In some examples, the handover candidate target access node may prepare different pools of C-RNTIs for different source cells / nodes. Then in the subsequent synchronization / random-access process, the handover candidate target access node can identify the source cell / node of a UE according to the C-RNTI used by the UE, which will be described in detail below.

[0086] In some examples, a pool of RACH preambles may be paired / associated to a pool of C- RNTIs in the cell-specific baseline configuration. In other words, each RACH preamble may be paired / associated to a corresponding C-RNTI. The source access node may select a {RACH preamble, C-RNTI} pair for UE to use in the subsequent synchronization / random-access process. Then the handover candidate target access node can identify the UE based on either the RACH preamble or the C-RNTI, which will be described in detail below.

[0087] As discussed above, handover source access node 202A may control determination or selection of the RACH preamble and / or C-RNTI from the respective pools, such as to avoid a possible collision of the RACH preamble and / or C-RNTI. In some other examples, however, handover candidate target access node 202B may be configured to determine or select (e.g., reserve) one RACH preamble and / or C-RNTI for UE 110.

[0088] In some options, handover candidate target access node 202B may provide more than one candidate target radio cell. As a result, in some examples, the handover candidate target access node may be configured to generate cell-specific baseline configurations for respective candidate target radio cells provided by it. These cell-specific baseline configurations may be transmitted (sent) to handover source access node 202A via separate messages, or via a single (e.g., aggregated or the like) message. In some examples, the cell-specific baseline configurations may be divided into a common node-specific part and a cell-specific part to reduce overall size and thereby load requirements. The common node-specific part may comprise configuration information that applies across the multiple target radio cells provided by thehandover candidate target access node, and it may be provided once as a reference configuration. The cell-specific part may comprise cell-specific configuration information for each of the multiple target radio cells on top of the reference configuration. The cell-specific configuration information may comprise, for example, a number of supported multiple-input and multipleoutput (MIMO) layers, frequency, beams or the like. In some examples, both parts of the cellspecific baseline configurations may be provided to handover source access node 202A in a single message (e.g., RAN node configuration update message). In other examples, parts of the cell-specific baseline configurations may be provided in separate messages, such as one message for the common node-specific part, and one message for the node-specific part for each of the target radio cells.

[0089] It would be appreciated that handover candidate target access node 202B may transmit (send) its cell-specific baseline configurations to more than one handover source access nodes (neighboring nodes of the handover candidate target access node). On the other hand, handover source access node 202A may receive cell-specific baseline configurations from more than one handover candidate target access nodes (neighboring nodes of the handover source access node). In this sense, it may be considered that the cell-specific baseline configuration is specific to the pair of the handover source access node and the handover candidate target access node.

[0090] Returning to FIG. 5, in some examples, handover candidate target access node 202B may at 503 determine a change in the baseline configuration(s). For instance, either or both of the pool of RACH preambles or the pool of C-RNTIs may be updated, such as after some of the RACH preambles and / or C-RNTIs have been used / occupied, reconfigured, freed, or the like. In these examples, handover candidate target access node 202B may generate an updated baseline configuration according to the change, and at 503-1 share the updated baseline configuration with handover source access node 202A. In the case the baseline configuration is divided into the common node-specific part and the cell-specific part and only one of the two parts is updated, handover candidate target access node 202B may generate an updated part and share it with handover source access node 202A. It can also reduce the load requirements.

[0091] Similar to the conventional HO procedure, UE 110 may at 504 be configured to report measurements of radio link quality in association with the source / serving radio cell and one or more neighboring radio cells, and at 505 send (transmit) a measurement report indicating relevant measurements for one or more candidate target radio cells provided by handover candidate target access node(s). UE 110 may be configured to report the measurements in a periodic manner or on an event basis.

[0092] Based on the measurements report received from UE 110, handover source access node 202A may at 506 decide to perform a handover to a target radio cell(s) reported by the UE thathas suitable measurement results (similar to 303 in FIG. 3). According to some example implementations of the present disclosure, handover source access node 202A may at 507 determine whether the UE and / or the target radio cell(s) reported by the UE satisfy at least one criterion for the fast handover (FHO). As discussed above, the at least one criterion may be obtained from the handover candidate target access node at 501 (or at 503-1), or at least a part of the criterion(s) is predefined / preconfigured at the handover source access node.

[0093] Depending on the at least one criterion for FHO, in some examples, handover source access node 202 A may check whether UE 110 uses a GBR configuration (or other suitable radio configuration of the UE) that cannot be supported by the target radio cell, whether the current load of the handover candidate target access node is below the (predetermined) criterion / threshold (e.g., 50%), whether the handover candidate target access node satisfies the service delay and / or reliability requirements of the UE, whether the handover candidate target access node supports capabilities and / or features of the UE, or the like. It would be appreciated that criteria for FHO are not limited to examples set forth above.

[0094] If handover source access node 202A determines that the at least one criterion for FHO is satisfied, handover source access node 202A may at 508-1 send (transmit) an FHO command comprising FHO-related configuration to UE 110. In the FHO procedure, the message / signaling exchanges between the handover source access node and the handover candidate target access node for handover preparation in the conventional HO are skipped. In some examples, handover source access node 202A may send (transmit) the FHO command via an RRC reconfiguration message including a reconfigurationWithSync information element (IE), or via any other suitable message. The FHO-related configuration may comprise (at least part of) the configuration information of the cell-specific baseline configuration in association with the target radio cell obtained from handover candidate target access node 202B at 501 (or at 503-1). Since the configuration information of the cell-specific baseline configuration has been described above in detail, a repetitive description thereof is omitted here for convenience.

[0095] In some examples, the FHO-related configuration may comprise a RACH preamble selected by the handover source access node from the pool of RACH preambles provided from the handover candidate target access node. With the RACH preamble, UE 110 can carry out a contention free random-access (CFRA) procedure to establish an RRC connection with handover candidate target access node 202B (target radio cell). In some other examples, the FHO-related configuration may not comprise a RACH preamble, and UE 110 can carry out a contention based random-access (CBRA) procedure. Alternatively or additionally, the FHO-related configuration may comprise an identifier e.g. C-RNTI for UE 110 in the target radio cell. The C-RNTI may be selected by the handover source access node from the pool of C-RNTIs provided from thehandover candidate target access node. As discussed above, the selected RACH preamble and C-RNTI may or may not be paired / associated to each other. In some other examples, the UE identifier e.g. C-RNTI may be generated by the handover source access node. In some examples, instead of C-RNTI, any other UE identifier, e.g., a random number or sequence, may be used to identify UE 110 in the FHO procedure.

[0096] In some examples, the FHO-related configuration may comprise security-related information in association with the target radio cell. For instance, the FHO-related configuration may comprise security-related information for generating one or more access stratum (AS) security keys to ensure secure communication between UE 110 and handover candidate target access node 202B. The security-related information may comprise for example physical cell identifier (PCI), absolute radio frequency channel number (ARFCN), security algorithm identifiers of security algorithms applied at the target radio cell, or the like. The FHO-related configuration may also comprise an indication of key update, e.g., a masterKeyUpdate field, to inform UE 110 to update the AS security keys based on the security-related information. In response to the key update indication, UE 110 may generate one or more AS security keys based on the security-related information. For instance, UE 110 may generate an AS security maser key KgNB in association with the handover candidate target access node using the current AS security maser key in association with the handover source access node, the PCI and ARFCN of the target radio cell. UE 110 may further derive four AS security keys from the master key KgNB using security algorithms identified by the security algorithm identifiers. The four AS security keys may comprise a key KRRCint for integrity protection of RRC signaling, a key KRRCenc for cyphering of RRC signaling, a key Kupint for integrity protection of user data, and a key Kupenc for cyphering of user data.

[0097] The FHO-related configuration may further comprise any other suitable configuration / information for UE 110 to be successfully handed over and connected to the handover candidate target access node 202B (target radio cell). In some examples, the FHO- related configuration may comprise minimum configuration information necessary for UE 110 to be connected to the target radio cell, and a supplementary configuration may be provided later to UE 110 to complete the handover, which will be described in detail below.

[0098] Before, after or as the FHO command message is sent to UE 110, handover source access node 202A may at 508-2 send (transmit) FHO information to handover candidate target access node 202B (target radio cell) to initiate UE-specific preparation at the target radio cell. The FHO information may be prepared / generated by the handover source access node and transmitted (sent) to the target radio cell in a suitable (existing or newly introduced) message or as separate messages. In some examples, the message conveying the FHO information may bereferred to as an FHO / HO notification message. The FHO information sent to the handover candidate target access node may comprise information on the UE to be handed over and connected to the target radio cell, including for example UE context information, basic AS configuration information such as antenna information and downlink (DL) carrier frequency, UE capabilities, etc. In some examples, UE information may comprise the UE identifier e.g. C-RNTI and optionally the RACH preamble assigned to the UE by the handover source node. In some examples, UE information may comprise user plane (UP) configuration of the UE, including for example DRB configuration information, PDU session-related information, current QoS flow to DRB mapping information, etc. In some examples, if the cell-specific baseline configuration in association with the target radio cell received from the handover candidate target access node at 501 (or at 503-1) comprises the UP tunnel information indicating at least one default UP tunnel to the handover candidate target access node, UE information may further comprise information indicating mapping of the at least one UP tunnel to the DRB configuration information. In some examples, the FHO information may also comprise a part of the FHO-related configuration information sent at 508-1 to UE 110.

[0099] In some examples, the FHO information may comprise security information necessary for protecting communication between UE 110 and handover candidate target access node 202B (target radio cell). For instance, the security information may comprise an AS security master key KgNB (or K§NB*) for the target radio cell. Source access node 202 A may generate the AS security master key KgNB (or KgNB*) for the target radio cell based on the current AS security master key in association with the source radio cell, and the PCI and ARFCN of the target radio cell. The security information may also comprise a parameter for counting next hop (NH) chaining (referred to as NCC). Handover candidate target access node 202B may directly use the received key KgNB as the AS security master key and associate the received NCC parameter with the master key. Handover candidate target access node 202B may also derive AS security keys including KRRCint, KRRCenc, Kupint, Kupenc from the AS security master key KgNB. It can ensure that the same or corresponding AS security keys are used at UE 110 and handover candidate target access node 202B.

[0100] Upon receipt of the FHO command, UE 110 may be configured to, at 509, obtain DL and UL synchronization with the target radio cell and initiate a random-access procedure towards the target radio cell by transmitting (sending) a random-access preamble (also known as MSG1) to handover candidate target access node 202B. The random-access preamble may comprise the RACH preamble assigned by the source access node to the UE if available, or a preamble selected by the UE from a predefined preamble group.

[0101] Notably, as the FHO command message at 508-1 towards UE 110 and the FHOinformation message at 508-2 towards handover candidate target access node 202B (target radio cell) may be sent (transmitted) in any sequence or in parallel, it is possible that, in some examples, the random-access preamble at 509 may take place before the FHO information message at 508- 2 (or before the complete FHO information is fully updated at the handover candidate target access node side).

[0102] In response to the random-access preamble (MSG1), handover candidate target access node 202B (target radio cell) may at 510 transmit (send) a random-access response (RAR) message (also known as MSG2) to UE 110. The RAR message may comprise a temporary C- RNTI (TC-RNTI), and an uplink (UL) grant for UE 110 to transmit a subsequent UL message.

[0103] If the RACH preamble received in MSG1 is selected from a pool of (preconfigured / reserved) RACH preambles provided by handover candidate target access node 202B at 501 or 503-1 (and / or indicated in the FHO information received at 508-2), handover candidate target access node 202B can identify that UE 110 is undergoing the FHO procedure. Handover candidate target access node 202B may check whether it has obtained information on UE 110. In an example, if UE information received at 508-2 contains the same RACH preamble or an C-RNTI paired / associated to the RACH preamble, handover candidate target access node 202B can determine that the information on UE 110 is available and start to prepare UE-specific configuration for the FHO of UE 110 based on the received UE information.

[0104] Upon receipt of the RAR message at 510, UE 110 may at 511 transmit (send) an RRC reconfiguration complete message (MSG3) to handover candidate target access node 202B. If a UE ID e.g. C-RNTI (either selected from the preconfigured / reserved pool provided by the handover candidate target access node or generated by the handover source access node) is assigned to UE 110 at 508-1, UE 110 may include the UE ID e.g. C-RNTI in a message header of the RRC reconfiguration complete message. As a back-up measure to protect the message against a fake UE who happens to get the C-RNTI, UE 110 may also add a message authentication code (MAC) e.g. a short message authentication code for integrity (shortMAC-I) in the RRC reconfiguration complete message. The shortMAC-I may be generated from the PCI of the source radio cell, the PCI of the target radio cell, and the current C-RNTI of UE 110 in association with the source radio cell, and it can be used to identify and verify UE 110. In some examples, UE 110 may also apply the key KRRCint to the RRC reconfiguration complete message for integrity protection and cypher the RRC reconfiguration complete message with the key KRRCenc. Then UE 110 may transmit (send) the RRC reconfiguration complete message on the default SRB1 indicated in the default signaling radio bearer configuration received from the source access node at 508-1. Handover candidate target access node 202B may decipher the received RRC reconfiguration complete message with the key KRRCenc and verify the integrity ofthe message with the key KRRCint. Handover candidate target access node 202B can also determine based on the shortMAC-I that the RRC reconfiguration complete message is generated and transmitted by UE 110. It can ensure secure communication between UE 110 and handover candidate target access node 202B.

[0105] If the RRC reconfiguration complete message comprises a C-RNTI selected from a pool of (preconfigured / reserved) C-RNTIs provided by handover candidate target access node 202B at 501 or 503-1 (and / or indicated in the FHO information received at 508-2), handover candidate target access node 202B can also identify that UE 110 is undergoing the FHO procedure. Handover candidate target access node 202B may check whether it has obtained information on UE 110. If UE information received at 508-2 contains the same C-RNTI, handover candidate target access node 202B can determine that the information on UE 110 is available and start to prepare UE-specific configuration for the FHO of UE 110 based on the received UE information.

[0106] Handover candidate target access node 202B may at 512 generate a supplementary configuration for the FHO of UE 110 based on the UE information and send (transmit) the supplementary configuration via an RRC Reconfiguration message (MSG4) to UE 110. Upon receipt of the RRC Reconfiguration message, UE 110 may apply the supplementary configuration to complete the FHO procedure. The supplementary configuration may comprise configuration information needed at UE 110 but not provided to UE 110 in the cell-specific baseline configuration. In some examples, the supplementary configuration may comprise DRB configuration, QoS flow to DRB mapping configuration, beam-related configuration, network slice-related configuration, among others.

[0107] In FHO procedure, there is a possibility that handover candidate target access node 202B does not receive or find the information on UE 110, or source access node 202 A does not transmit (send) the information on UE 110 to handover candidate target access node 202B at all. As a result, handover candidate target access node 202B cannot generate and transmit (send) the supplementary configuration to UE 110, and the FHO procedure may end up in failure. In some examples, UE 110 may start a timer (e.g., Tconfig) when it transmits (sends) the RRC reconfiguration complete message (MSG3) to handover candidate target access node 202B. If UE 110 determines at 513 that it has not received the supplementary configuration when the timer expires, UE 110 may initiate an RRC connection re-establishment procedure towards the target radio cell. Handover candidate target access node 202B can attempt to retrieve the UE information from handover source access node 202A during the RRC connection reestablishment procedure.

[0108] UE 110 may, at 514-1, transmit (send) an RRC re-establishment request message tohandover candidate target access node 202B, including PCI and C-RNTI in association with the target radio cell. In response to the request, handover candidate target access node 202B may, at 514-2, request handover source access node 202Ato provide UE context of UE 110. The request may also comprise the C-RNTI. Handover source access node 202A may identify from the C- RNTI that UE 110 is undergoing an FHO procedure, and at 514-3 provide UE context information comprising information necessary for generating the supplementary configuration to handover candidate target access node 202B. Handover candidate target access node 202B may at 514-4 transmit (send) an RRC re-establishment message to UE 110 to continue the RRC re-establishment, and at 514-5 generate and transmit (send) the supplementary configuration to UE 110 in an RRC reconfiguration message. In some examples, the supplementary configuration may be transmitted in the RRC re-establishment message at 514-4, the RRC reconfiguration message at 514-5 may be omitted.

[0109] Although the 4-step random-access procedure is used in the FHO procedure shown in FIG. 5, a 2-step random-access procedure may be used in place of the 4-step random-access procedure, where the random-access preamble (MSG1) and the RRC reconfiguration complete message (MSG3) may be transmitted (sent) in a message MSG A, the random-access response (MSG2) and the RRC reconfiguration message (MSG4) may be received in a message MSG B. Other aspects may be similar to the 4-step random-access procedure and a repetitive description thereof is omitted here for convenience.

[0110] FIG. 6 is a signaling chart of fast handover (FHO) procedure 600, according to some examples. In FHO procedure 600, signaling and operations similar to those in FHO procedure 500 are denoted with similar numerals, and a repetitive description thereof is omitted here for convenience. Below the description will focus on the new signaling and operations.

[0111] Referring to FIG. 6, the FHO procedure may proceed with 501 to 511. If a RACH preamble received at 509 in the random-access MSG1 and / or a C-RNTI received at 511 in the RRC reconfiguration complete message (MSG3) are selected from respective pools of (preconfigured / reserved) RACH preambles and C-RNTIs provided by handover candidate target access node 202B, handover candidate target access node 202B can identify from the RACH preamble and / or C-RNTI that UE 110 is undergoing an FHO procedure. In response, handover candidate target access node 202B may check whether it has obtained information on UE 110. If handover candidate target access node 202B has not yet obtained the information on UE 110, e.g., due to transmission failure of 508-2 or a fact that source access node 202A does not transmit (send) the information on UE 110 to handover candidate target access node 202B at all, handover candidate target access node 202B may start a timer (e.g., TuEinfo) for receipt of information on UE 110.

[0112] Handover candidate target access node 202B may, at 601, determine that the timer TUEinfo expires, and it has not yet received the information on UE 110. As a result, handover candidate target access node 202B may, at 602, request handover source access node 202A to provide the information on UE 110 by transmitting (sending) a UE context request message. The request message may comprise the C-RNTI or any other information by which handover source access node 202A can identify UE 110. In response to the request, handover source access node 202A may at 603 provide the information on UE 110 in a UE context response message to handover candidate target access node 202B.

[0113] Handover candidate target access node 202B may at 604 generate a supplementary configuration for the FHO of UE 110 based on the received information on UE 110 and send (transmit) the supplementary configuration via an RRC Reconfiguration message (MSG4) to UE 110. Upon receipt of the RRC Reconfiguration message, UE 110 may apply the supplementary configuration to complete the FHO procedure. FHO procedure 600 comprises additional signaling over Xn for the UE context fetch, but it can reduce probability of FHO failure and consequent RRC re-establishment.

[0114] FIG. 7 is a signaling chart of fast handover (FHO) procedure 700, according to some examples. In FHO procedure 700, signaling and operations similar to those in FHO procedure 500 are denoted with similar numerals, and a repetitive description thereof is omitted here for convenience. Below the description will focus on the new signaling and operations.

[0115] Referring to FIG. 7, FHO procedure may proceed with 501 to 511. In some examples, handover candidate target access node 202B provides at 501 pools of (preconfigured / reserved) RACH preambles and C-RNTIs in the cell-specific baseline configuration, and the respective RACH preambles are paired / associated to the respective C-RNTIs. Handover source access node 202A assigns at 508-1 a {RACH preamble, C-RNTI} pair to UE 110 in an FHO command message. It is worth noting that the {RACH preamble, C-RNTI} pair is provided in a secured RRC message from handover source access node 202A to UE 110. Then when handover candidate target access node 202B receives at 509 a random-access preamble (MSG1) comprising the RACH preamble selected from the pool of (preconfigured / reserved) RACH preambles, handover candidate target access node 202B can identify that UE 110 is undergoing an FHO procedure and determine the C-RNTI paired / associated to the RACH preamble.

[0116] In response, handover candidate target access node 202B may check whether it has received information on UE 110. If handover candidate target access node 202B determines at 701 that it has yet to receive the information on UE 110, handover candidate target access node 202B may, immediately or within a predefined time window, initiate a UE context fetch procedure to fetch the information on UE 110 from handover source access node 202A.Handover candidate target access node 202B may at 702 transmit (send) a UE context request message to handover source access node 202A. The request message may comprise the C-RNTI paired / associated to the RACH preamble received from UE 110. In response to the request, handover source access node 202A may at 703 provide the information on UE 110 in a UE context response message to handover candidate target access node 202B.

[0117] Handover candidate target access node 202B may at 704 generate a supplementary configuration for the FHO of UE 110 based on the received information on UE 110 and send (transmit) the supplementary configuration in a random-access response (RAR) message (MSG2) to UE 110. In some examples, if the RAR message has a limited size that cannot accommodate the supplementary configuration, the RAR message may instead indicate scheduling information (e.g., a DL grant) for transmission of the supplementary configuration. Based on the scheduling information, UE 110 may at 705 receive the supplementary configuration for example in an RRC reconfiguration message. UE 110 may apply the supplementary configuration to complete the FHO procedure. Accordingly, the FHO procedure can be completed with the RAR message, and an explicit transmission of the RRC reconfiguration complete message (MSG3) can be avoided. In some examples, if handover candidate target access node 202B receives the UE context response message after a time window for transmitting the RAR message, handover candidate target access node 202B may transmit the supplementary configuration in a subsequent message e.g., the RRC reconfiguration message (MSG4) to UE 110.

[0118] FIG. 8 is a signaling chart of fast handover (FHO) procedure 800, according to some examples. The signaling chart also shows early data forwarding during the FHO procedure. Similar signaling and operations are denoted with similar numerals, and a repetitive description thereof is omitted here for convenience. Below the description will focus on the new aspects.

[0119] Referring to FIG. 8, the FHO procedure may proceed with 501 to 508-2. It is noted that the cell-specific baseline configuration provided at 501 by handover candidate target access node 202B may comprise user plane (UP) tunnel information indicating at least one default UP tunnel towards the candidate target radio cell. Each default UP tunnel may be configured with a tunnel endpoint identifier (TEID) parameter and a corresponding internet protocol (IP) address. In some examples, if the pool of (preconfigured / reserved) C-RNTIs are also provided in the cellspecific baseline configuration, the default UP tunnel may be provided for each C-RNTI in the pool. In some examples, multiple default UP tunnels may be provided for one C-RNTI.

[0120] Handover source access node 202A sends (transmits) at 508-2 FHO information to handover candidate target access node 202B, the FHO information may comprise information indicating mapping of at least one default UP tunnel to DRB information of UE 110. The DRB information may comprise for example DRB configuration information, PDU session-relatedinformation, and QoS flow information of UE 110. The mapping information may be determined at handover source access node 202A and transmitted together with the FHO information in the FHO notification message (508-2) to handover candidate target access node 202B. In some other examples, the mapping information may be transmitted via a separate message, e.g., an Xn data forwarding bearer information update message (Xn-DF -Bearer-Information-Update) to handover candidate target access node 202B.

[0121] Handover source access node 202 A may at 801 transmit (send) an early status transfer message or a sequence number (SN) status transfer message to handover candidate target access node 202B to convey PDCP SN-related information. Then handover source access node 202A may at 802 forward user data of UE 110 via the at least one default UP tunnel to handover candidate target access node 202B.

[0122] In some examples, cell-specific baseline configuration provided at 501 by handover candidate target access node 202B may not comprise a pool of (preconfigured / reserved) C- RNTIs. In this case, the user plane (UP) tunnel information may indicate one or more common UP tunnels, not associated to any particular C-RNTI. Handover source access node 202A may generate the C-RNTI for UE 110, select a default / common UP tunnel for data forwarding of UE 110, and send (transmit) the mapping information to handover candidate target access node 202B via the FHO notification message (508-2) or a separate message. In some examples, handover source access node 202 A may not send (transmit) the mapping information to handover candidate target access node 202B via the FHO notification message (508-2) or the separate message. Instead, handover source access node 202A may include the mapping information in a header (e.g., packet header) of the user data forwarded at 802 through the default UP tunnel.

[0123] FIG. 9 is a signaling chart of fast handover (FHO) procedure 900, according to some examples. As shown, the FHO procedure may proceed with 501 to 508-2. Cell-specific baseline configuration provided at 501 by handover candidate target access node 202B may not comprise the UP tunnel information. In other words, handover candidate target access node 202B does not need to reserve a default UP tunnel for the FHO procedure. It can improve the resource efficiency.

[0124] FHO information provided at 508-2 to handover candidate target access node 202B may comprise DRB information of UE 110, including for example DRB configuration information, PDU session-related information, and QoS flow(s) information. In response to the UE DRB information, handover candidate target access node 202B may at 901 provide UP tunnel information in association with the UE DRB information to handover source access node 202A. The UP tunnel information may indicate at least one UP tunnel mapped to the DRB information of UE 110. Handover candidate target access node 202B may send (transmit) the UP tunnel information in an FHO information ACK message or any other suitable message.

[0125] Upon receipt of the UP tunnel information, handover source access node 202A may at 902 transmit (send) an early / SN status transfer message to handover candidate target access node 202B, and at 903 start the user date forwarding for UE 110 using the UP tunnel indicated in the UP tunnel information.

[0126] FIG. 10 is a signaling chart of conditional FHO (CFHO) procedure 1000, according to some examples. As shown, the CFHO procedure involves one or more handover candidate target access nodes (target access nodes) 202B that provide one or more (N) target radio cells. Similar to the FHO procedure, at least one handover candidate target access node 202B may, at 1001, generate one or more cell-specific baseline configurations for the CFHO from a source radio cell provided by handover source access node 202A to one or more target radio cells, and send (transmit) the cell-specific baseline configuration to handover source access node 202A, such as in a RAN node configuration update message or any other suitable (existing or new) message. Actually, at this point of time, handover candidate target access node(s) 202B does not know whether handover source access node 202A will initiate a FHO procedure or a CFHO procedure for a UE. The cell-specific baseline configuration may be used for either an FHO procedure or a CFHO procedure, depending on the handover decision made by handover source access node 202 A later. Handover source access node 202 A may at 1002 send (transmit) a corresponding acknowledgment message (e.g., RAN node configuration update ACK) back to handover candidate target access node(s) 202B.

[0127] In the case handover candidate target access node 202B provides multiple target radio cells, the multiple cell-specific baseline configurations in association with the multiple target radio cells may be divided into a common node-specific part and a cell-specific part to reduce overall size and thereby load requirements. The common node-specific part may comprise configuration information that applies across the multiple target radio cells provided by the handover candidate target access node, and it may be provided once as a reference configuration. The cell-specific part may comprise cell-specific configuration information for each of the multiple target radio cells on top of the reference configuration. In some examples, both parts of the cell-specific baseline configurations may be provided to the handover source access node in a single message (e.g., RAN node configuration update message). In other examples, parts of the cell-specific baseline configurations may be provided in separate messages, such as one message for the common node-specific part, and one message for the node-specific part for each of the target radio cells.

[0128] Like the FHO procedure, in some examples, handover candidate target access node(s) 202B may, at 1003, determine a change in the baseline configuration(s). In response to the change, the handover candidate target access node(s) may generate an updated baseline configurationand, at 1003-1, share the updated baseline configuration with handover source access node 202 A. Also similar to the FHO procedure, UE 110 may, at 1004, be configured to report measurements of radio link quality in association with the source / serving radio cell and one or more neighboring radio cells and, at 1005, send (transmit) a measurement report indicating relevant measurements for one or more candidate target radio cells provided by handover candidate target access node(s) 202B. UE 110 may be configured to report the measurements in a periodic manner or on an event basis.

[0129] Based on the measurements report received from UE 110, handover source access node 202A may, at 1006, decide to initiate a CFHO procedure for UE 110. In this regard, the handover source access node may decide to apply a conventional / basic HO procedure or CHO procedure, such as based on radio link quality in association with the source / serving radio cell and the neighboring radio cells indicated in the measurement report. Likewise, the handover source access node may decide to apply a CHO procedure or CFHO procedure, such as based on one or more service reliability requirements (e.g., QoS on reliability), one or more service delay requirements (e.g., expected QoS on delay), or the like. Handover source access node 202A also determines one or more candidate target radio cells selected from the neighboring radio cells for the CFHO of UE HO.

[0130] In some examples, a handover decision may be based on one or more criteria comprising radio link conditions observed by UE 110 for handover source access node 202A and handover candidate target access node(s) 202B, in order that the UE is served by the cell with the best radio link condition. Additionally or alternatively, the handover decision may also be based on network load balancing. In particular, for example, a handover may be triggered due to load balancing, as long as another cell can provide an acceptable radio link condition.

[0131] In some examples, handover source access node 202A may determine to initiate a conventional HO / CHO procedure where target radio cell(s) are fully prepared or initiate an FHO / CFHO procedure without preparation and admission control at the target radio cell(s). The handover source access node may determine to initiate an FHO procedure when the current QoS flow requires low delay, such as a low packet delay budget (PDB). The handover source access node may determine to initiate an FHO procedure when UE 110 is experiencing high mobility (and the UE’s radio link condition is quickly changing) to reduce the risk of failure before a convention HO could be executed. The handover source access node may determine to initiate a conventional CHO procedure (with prior target radio cell preparation) when the current QoS flow requires high reliability. In some examples, the handover source access node may determine to initiate a CFHO procedure (without prior target radio cell preparation) based on the same criteria as for FHO but higher service reliability and / or robustness is required. In examples inwhich the determination is to initiate FHO / CFHO, the handover source access node may determine capacity of handover candidate target access node(s) 202B, and thereby the possibility of the handover candidate target access node(s) accepting the UE to handover without prior target radio cell preparation. In an example, if UE 110 has a large amount of traffic and the current QoS flow requires high reliability, the handover source access node may determine to initiate a CFHO procedure. In some examples, the handover source access node may also consider capabilities of the UE and / or the handover candidate target access nodes to determine the handover procedure. In some examples, the handover decision may be based on criteria provided from handover candidate target access node(s) 202B or predefined / preconfigured at handover source access node 202A.

[0132] Handover source access node 202 A may at 1007 determine one or more execution conditions for the one or more candidate target radio cells provided by handover candidate target access node(s) 202B. The execution condition(s) may be based on radio measurements, such as measured RSRP from the candidate target radio cells. The execution condition(s) may be the same or different for each candidate target radio cell. In some examples, the execution condition(s) may comprise a different threshold RSRP for one or more candidate target radio cells based on a different priority of the candidate target radio cell(s). In some examples, the priority may depend on the candidate target radio cell’s capacity (e.g., in GBR). In some examples, the handover source access node may determine execution condition(s) for multiple candidate target radio cells that support all or a subset of features supported at UE 110, and allow the UE to down-select the candidate target radio cell(s) based on active feature(s) needed for the UE’s current operation.

[0133] Handover source access node 202Amay at 1008-1 send (transmit) a CFHO command message (e.g., RRC reconfiguration with sync) comprising the execution condition(s) and CFHO-related configuration to UE 110. Similar to the FHO procedure, the CFHO-related configuration may comprise (at least part of) the configuration information of the cell-specific baseline configuration(s) in association with the candidate target radio cell(s) obtained from handover candidate target access node(s) 202B at 1001 (or 1003-1), including for example a default SRB configuration indicating at least a default SRB1, or additionally a default SRB2. The CFHO-related configuration may also comprise a UE identifier e.g. C-RNTI and optionally a RACH preamble selected from the pools of (preconfigured / reserved) C-RNTIs and RACH preambles for each candidate target radio cell, security-related information in association with each candidate target radio cell, among others. In some examples, the UE identifier e.g. C-RNTI may be generated by handover source access node 202A. If the candidate target radio cell is selected as the target radio cell for the handover, UE 110 may generate one or more AS securitykeys (e.g., KgNB, KRRCint, KRRCenc, Kupint, Kupenc) using the security-related information to protect communication between UE 110 and the target radio cell, as in the FHO procedure.

[0134] Before, after or as the CFHO command message is sent to UE 110, handover source access node 202 A may, at 1008-2, send (transmit) CFHO information to handover candidate target access node(s) 202B (target radio cell(s)) to initiate UE-specific preparation, similar to 508-2 in the FHO procedure. The CFHO information may be prepared / generated by the handover source access node and transmitted (sent) to the handover candidate target access node(s) in a suitable (existing or newly introduced) message or as separate messages. In some examples, the message conveying the CFHO information may be referred to as a CFHO / HO notification message. The CFHO information sent to the handover candidate target access node(s) may comprise information on UE 110, including for example UE context information, UP configuration information (e.g., DRB related configuration information), basic AS configuration information such as antenna information and downlink (DL) carrier frequency, UE capabilities, etc. The CFHO information may comprise at least part of the information sent to the UE, including for example the UE identifier e.g. C-RNTI and optionally the RACH preamble assigned to the UE in association with each candidate target radio cell, security information for each candidate target radio cell, and the like. The security information may comprise for example an AS security master key (KgNB or KgNB*), and a parameter for counting next hop (NH) chaining in association with the AS security master key. Handover candidate target access node 202B may derive AS security keys such as KRRCint, KRRCenc, Kupint, Kupenc from the AS security master key and the security algorithms applied at the candidate target radio cell.

[0135] Upon receipt of the CFHO command, if the CFHO command is transmitted by an RRC reconfiguration message, UE 110 may at 1009 send (transmit) an RRC reconfiguration complete message to handover source node 202 A. UE 110 maintains its connection with handover source access node 202 A and, at 1010, starts evaluating the execution conditions for the candidate target radio cells received in the CFHO command. For instance, UE 110 keeps monitoring radio link quality in association with the source radio cell and the candidate target radio cells and evaluating whether the candidate target radio cells satisfy corresponding execution conditions. Meanwhile, if handover source access node 202 A determines at 1008-3 a change in the CFHO information in association with the candidate target radio cell(s), handover source access node 202A may at 1008-4 send (transmit) a CFHO information update to handover candidate target access node(s) 202B.

[0136] Based on the condition evaluation, handover source access node 202A may select at least one target radio cell from the candidate target radio cells that satisfies the corresponding execution condition(s). UE 110 may be configured to perform a random-access procedure toestablish an RRC connection with handover candidate target access node 202B (handover target access node) providing the target radio cell, similar to the FHO procedure. In this regard, UE 110 may at 1011 obtain DL and UL synchronization with the target radio cell and initiate the randomaccess procedure by transmitting (sending) a random-access preamble (MSG1) to the target radio cell. The random-access preamble (MSG1) may comprise the RACH preamble selected from the pool of (preconfigured / reserved) RACH preambles by handover source access node 202A and received by UE 110 at 1008-1 if available, or a preamble selected by UE 110 from a predefined preamble group.

[0137] In response to the random-access preamble (MSG1), handover candidate target access node 202B (target radio cell) may at 1012 transmit (send) a random-access response (RAR) message (MSG2) to UE 110. The RAR message may comprise a temporary C-RNTI (TC-RNTI), and an uplink (UL) grant for UE 110 to transmit a subsequent UL message.

[0138] If the RACH preamble received in the random-access MSG1 is selected from a pool of (preconfigured / reserved) RACH preambles provided by handover candidate target access node 202B at 1001 or 1003-1 (and / or indicated in the CFHO information received at 1008-2 or 1008-4), handover candidate target access node 202B can identify that UE 110 is undergoing a CFHO procedure (or FHO procedure because the handover candidate target access node 202B may not know which of CFHO and FHO procedures is decided by handover source access node 202A if handover candidate target access node 202B has not yet received CFHO or FHO information from handover source access node 202A). Then handover candidate target access node 202B may check whether it has obtained information on UE 110. In an example, if the UE information received at 1008-2 (or 1008-4) contains the same RACH preamble or an C-RNTI paired / associated to the RACH preamble, handover candidate target access node 202B can determine that the information on UE 110 is available and start to prepare UE-specific configuration for the CFHO of UE 110 based on the received UE information.

[0139] Upon receipt of the RAR message at 1012, UE 110 may at 1013 transmit (send) an RRC reconfiguration complete message (MSG3) to handover candidate target access node 202B. If a UE ID e.g. C-RNTI (either selected from the preconfigured / reserved pool provided by the handover candidate target access node or generated by the handover source access node) is assigned to UE 110 at 1008-1, UE 110 may include the UE ID e.g. C-RNTI in a message header of the RRC reconfiguration complete message. As a back-up measure to protect the message against a fake UE who happens to get the C-RNTI, UE 110 may also add a message authentication code (MAC) e.g. a short message authentication code for integrity (shortMAC-I) in the RRC reconfiguration complete message. The shortMAC-I may be generated from the PCI of the source radio cell, the PCI of the target radio cell, and the current C-RNTI of UE 110 inassociation with the source radio cell, and it can be used to identify and verify UE 110. In some examples, UE 110 may also apply the key KRRCint to the RRC reconfiguration complete message for integrity protection and cypher the RRC reconfiguration complete message with the key KRRCenc. Then UE 110 may transmit (send) the RRC reconfiguration complete message on the default SRB1 indicated in the default signaling radio bearer configuration received at 1008-1 from the source access node. Handover candidate target access node 202B may decipher the received RRC reconfiguration complete message with the key KRRCenc and verify the integrity of the message with the key KRRCint. Handover candidate target access node 202B can also determine based on the shortMAC-I that the RRC reconfiguration complete message is generated and transmitted by UE 110. It can ensure secure communication between UE 110 and handover candidate target access node 202B.

[0140] If the RRC reconfiguration complete message comprises a C-RNTI selected from a pool of (preconfigured / reserved) C-RNTIs provided by handover candidate target access node 202B at 1001 or 1003-1 (and / or indicated in the CFHO information received at 1008-2 or 1008- 4), handover candidate target access node 202B can also identify that UE 110 is undergoing a CFHO procedure. Then handover candidate target access node 202B may check whether it has obtained information on UE 110. If the UE information received at 1008-2 (or 1008-4) contains the same C-RNTI, handover candidate target access node 202B can determine that the information on UE 110 is available and start to prepare UE-specific configuration for the CFHO of UE 110 based on the received UE information.

[0141] Handover candidate target access node 202B may at 1014 generate a supplementary configuration for the CFHO of UE 110 based on the UE information and send (transmit) the supplementary configuration via an RRC Reconfiguration message (MSG4) to UE 110. Upon receipt of the RRC Reconfiguration message, UE 110 may apply the supplementary configuration to complete the FHO procedure. The supplementary configuration may comprise configuration information needed at UE 110 but not provided to UE 110 in the cell-specific baseline configuration. In some examples, the supplementary configuration may comprise DRB configuration, QoS flow to DRB mapping configuration, beam-related configuration, network slice-related configuration, etc.

[0142] In the CFHO procedure, there is also a possibility that handover candidate target access node 202B does not receive or find the information on UE 110, or source access node 202 A does not transmit (send) the information on UE 110 (e.g., the CFHO information / notification message) to handover candidate target access node 202B at all. As a result, handover candidate target access node 202B cannot generate and transmit (send) the supplementary configuration to UE 110, and the CFHO procedure may end up in failure. In some examples, UE 110 may start atimer (e.g., Tconfig) when it transmits (sends) the RRC reconfiguration complete message (MSG3) to handover candidate target access node 202B. If UE 110 determines at 1015 that it has not received the supplementary configuration when the timer expires, UE 110 may initiate an RRC connection re-establishment procedure towards the target radio cell. Handover candidate target access node 202B can attempt to retrieve the UE information from handover source access node 202A during the RRC connection re-establishment procedure.

[0143] UE 110 may at 1016-1 transmit (send) an RRC re-establishment request message to handover candidate target access node 202B, including PCI and C-RNTI in association with the target radio cell. In response to the request, handover candidate target access node 202B may at 1016-2 request handover source access node 202Ato provide UE context of UE 110. The request may also comprise the C-RNTI. Handover source access node 202A may identify from the C- RNTI that UE 110 is undergoing a CFHO procedure, and at 1016-3 provide UE context information comprising information necessary for generating the supplementary configuration to handover candidate target access node 202B. Handover candidate target access node 202B may at 1016-4 transmit (send) an RRC re-establishment message to UE 110 to continue the RRC re-establishment, and at 1016-5 generate and transmit (send) a supplementary configuration to UE 110 in an RRC reconfiguration message. In some examples, the supplementary configuration may be transmitted in the RRC re-establishment message at 1016-4, and the RRC reconfiguration message at 1016-5 may be omitted.

[0144] Although the 4-step random-access procedure is used in the CFHO procedure shown in FIG. 10, a 2-step random-access procedure may be used in place of the 4-step random-access procedure, where the random-access preamble (MSG1) and the RRC reconfiguration complete message (MSG3) may be transmitted (sent) in a message MSG Ato the handover candidate target access node, the random-access response (MSG2) and the RRC reconfiguration message (MSG4) may be received in a message MSG B from the handover candidate target access node. Other aspects may be similar to the 4-step random-access procedure and a repetitive description thereof is omitted here for convenience.

[0145] FIG. 11 is a signaling chart of conditional fast handover (CFHO) procedure 1100, according to some examples. In CFHO procedure 1100, signaling and operations similar to those in CFHO procedure 1000 are denoted with similar numerals, and a repetitive description thereof is omitted here for convenience. Below the description will focus on the new signaling and operations.

[0146] Referring to FIG. 11, the CFHO procedure may proceed with 1001 to 1013. If a RACH preamble received at 1011 in the random-access MSG1 and / or a C-RNTI received at 1013 in the RRC reconfiguration complete message (MSG3) are selected from respective pools of(preconfigured / reserved) RACH preambles and C-RNTIs provided by handover candidate target access node 202B, handover candidate target access node 202B can identify from the RACH preamble and / or C-RNTI that UE 110 is undergoing a CFHO (or FHO) procedure. In response, handover candidate target access node 202B may check whether it has obtained information on UE 110. If handover candidate target access node 202B has not yet obtained the information on UE 110, e.g., due to a transmission failure at 1008-2 or a fact that source access node 202A does not transmit (send) the information on UE 110 to handover candidate target access node 202B at all, handover candidate target access node 202B may start a timer (e.g., TuEinfo) for receipt of information on UE 110.

[0147] Handover candidate target access node 202B may at 1101 determine that the timer TUEinfo expires, and it has not yet received the information on UE 110. As a result, handover candidate target access node 202B may at 1102 transmit (send) a UE context request message to handover source access node 202A. The request message may comprise a C-RNTI or any other information by which handover source access node 202A can identify UE 110. In response to the request, handover source access node 202A may at 1103 provide information on UE 110 in a UE context response message to handover candidate target access node 202B.

[0148] Handover candidate target access node 202B may at 1104 generate a supplementary configuration for the CFHO of UE 110 based on the received information on UE 110 and send (transmit) the supplementary configuration via an RRC Reconfiguration message (MSG4) to UE 110. Upon receipt of the RRC Reconfiguration message, UE 110 may apply the supplementary configuration to complete the CFHO procedure. The CFHO procedure 1100 comprises additional signaling over Xn interface for the UE context fetch, but it can reduce probability of CFHO failure and consequent RRC re-establishment.

[0149] FIG. 12 is a signaling chart of conditional fast handover (CFHO) procedure 1200, according to some examples. In CFHO procedure 1200, signaling and operations similar to those in CFHO procedure 1000 are denoted with similar numerals, and a repetitive description thereof is omitted here for convenience. Below the description will focus on the new signaling and operations.

[0150] Referring to FIG. 12, the CFHO procedure may proceed with 1001 to 1011. In some examples, handover candidate target access node 202B provides, at 1001, pools of (preconfigured / reserved) RACH preambles and C-RNTIs in a cell-specific baseline configuration, and respective RACH preambles are paired / associated to respective C-RNTIs. Handover source access node 202 A assigns, at 1008-1, a {RACH preamble, C-RNTI} pair to UE 110 in a CFHO command message. It is worth noting that the {RACH preamble, C-RNTI} pair is provided in a secured RRC message from handover source access node 202 A to UE 110. Whenhandover candidate target access node 202B receives, at 1011, a random-access preamble (MSG1) comprising the RACH preamble selected from the pool of (preconfigured / reserved) RACH preambles, handover candidate target access node 202B can identify that UE 110 is undergoing a CFHO (or FHO) procedure and determine the C-RNTI paired / associated to the RACH preamble.

[0151] In response, handover candidate target access node 202B may check whether it has received information on UE 110. If handover candidate target access node 202B determines at 1201 that it has yet to receive information on UE 110, handover candidate target access node 202B may, immediately or within a predefined time window, initiate a UE context fetch procedure to fetch information on UE 110 from handover source access node 202A. Handover candidate target access node 202B may at 1202 transmit (send) a UE context request message to handover source access node 202A. The request message may comprise the C-RNTI paired / associated to the RACH preamble received from UE 110. In response to the request, handover source access node 202A may at 1203 provide the information on UE 110 in a UE context response message to handover candidate target access node 202B.

[0152] Handover candidate target access node 202B may at 1204 generate a supplementary configuration for the CFHO of UE 110 based on the received information on UE 110 and send (transmit) the supplementary configuration in a random-access response (RAR) message (MSG2) to UE 110. In some examples, if the RAR message has a limited size that cannot accommodate the supplementary configuration, the RAR message may instead indicate scheduling information (e.g., a DL grant) for transmission of the supplementary configuration. Based on the scheduling information, UE 110 may at 1205 receive the supplementary configuration for example in an RRC reconfiguration message. UE 110 may apply the supplementary configuration to complete the CFHO procedure. Accordingly, the CFHO procedure can be completed with the RAR message, and an explicit transmission of the RRC reconfiguration complete message (MSG3) can be avoided. In some examples, if handover candidate target access node 202B receives the UE context response message after a time window for transmitting the RAR message, handover candidate target access node 202B may transmit the supplementary configuration in a subsequent message e.g., the RRC reconfiguration message (MSG4) to UE 110.

[0153] FIG. 13 is a signaling chart of conditional fast handover (CFHO) procedure 1300, according to some examples. The signaling chart also shows early data forwarding during a CFHO procedure. Similar signaling and operations are denoted with similar numerals, and a repetitive description thereof is omitted here for convenience. Below the description will focus on the new aspects.

[0154] Referring to FIG. 13, the CFHO procedure may proceed with 1001 to 1008-2. It isnoted that a cell-specific baseline configuration provided at 1001 by handover candidate target access node 202B may comprise user plane (UP) tunnel information indicating at least one default UP tunnel towards the candidate target radio cell. Each default UP tunnel may be configured with a tunnel endpoint identifier (TEID) parameter and a corresponding internet protocol (IP) address. In some examples, if the pool of (preconfigured / reserved) C-RNTIs are also provided in the cell-specific baseline configuration, the default UP tunnel may be provided for each C-RNTI in the pool. In some examples, multiple default UP tunnels may be provided for one C-RNTI.

[0155] Handover source access node 202 A sends (transmits) at 1008-2 CFHO information to handover candidate target access node 202B, the CFHO information may comprise information indicating mapping of at least one default UP tunnel to DRB information of UE 110. The DRB information may comprise for example DRB configuration information, PDU session-related information, and QoS flow information of UE 110. The mapping information may be determined at handover source access node 202A and transmitted together with the CFHO information in the CFHO notification message (1008-2) to handover candidate target access node 202B. In some other examples, the mapping information may be transmitted via a separate message, e.g., an Xn data forwarding bearer information update message (Xn-DF -Bearer-Information-Update) to the handover candidate target access node 202B.

[0156] Handover source access node 202 A may at 1301 transmit (send) an early status transfer message or a sequence number (SN) status transfer message to handover candidate target access node 202B to convey PDCP SN-related information. Then handover source access node 202A may at 1302 forward user data of UE 110 via the at least one default UP tunnel to handover candidate target access node 202B.

[0157] In some examples, a cell-specific baseline configuration provided at 1001 by handover candidate target access node 202B may not comprise a pool of (preconfigured / reserved) C- RNTIs. In this case, the user plane (UP) tunnel information may indicate one or more common UP tunnels, not associated to any particular C-RNTI. Handover source access node 202A may generate a C-RNTI for UE 110, select a default / common UP tunnel for data forwarding of UE 110, and send (transmit) the mapping information to handover candidate target access node 202B via the CFHO notification message (1008-2) or a separate message. In some examples, handover source access node 202 A may not send (transmit) the mapping information to handover candidate target access node 202B via the CFHO notification message (1008-2) or the separate message. Instead, handover source access node 202A may include the mapping information in a header (e.g., packet header) of the user data forwarded at 1302 through the default UP tunnel.

[0158] FIG. 14 is a signaling chart of conditional fast handover (CFHO) procedure 1400,according to some examples. As shown, the CFHO procedure may proceed with 1001 to 1008- 2. A cell-specific baseline configuration provided at 1001 by handover candidate target access node 202B may not comprise UP tunnel information. In other words, handover candidate target access node 202B does not need to reserve a default UP tunnel for a CFHO procedure. It can improve the resource efficiency.

[0159] CFHO information provided at 1008-2 to handover candidate target access node 202B may comprise DRB information of UE 110, including for example DRB configuration information, PDU session-related information, and QoS flow(s) information. In response to the UE DRB information, handover candidate target access node 202B may at 1401 provide UP tunnel information in association with the UE DRB information to handover source access node 202A. The UP tunnel information may indicate at least one UP tunnel mapped to the DRB information of UE 110. Handover candidate target access node 202B may send (transmit) the UP tunnel information in an FHO information ACK message or any other suitable message.

[0160] Upon receipt of the UP tunnel information, handover source access node 202A may at 1402 transmit (send) an early / SN status transfer message to handover candidate target access node 202B, and at 1403 start the user date forwarding for UE 110 using the UP tunnel indicated in the UP tunnel information.

[0161] FIGs. 15A-15D are flowcharts illustrating method 1500, according to some examples. Method 1500 may be implemented at a user equipment (UE), e.g., UE 110 discussed above.

[0162] As shown, the method may comprise, at 1502, receiving a message in association with a handover from handover source access node 202A. In some examples, the message in association with the handover may be a handover (HO) command, e.g., a fast handover (FHO) command. The FHO command may comprise a cell-specific baseline configuration for a handover (e.g., a fast handover) of UE 110 to a target radio cell provided by handover target access node 202B. The cell-specific baseline configuration may comprise an identifier (e.g., physical cell identifier, PCI) of the target radio cell and information necessary for connecting to the target radio cell without UE-specific handover preparation. In other words, the cell-specific baseline configuration may not comprise UE-specific configuration information. In some examples, the cell-specific baseline configuration may comprise a default signaling radio bearer configuration indicating at least a default SRB1, or additionally a default SRB2. In some examples, the cell-specific baseline configuration may comprise an indication of key update, e.g., a masterKeyUpdate field, and security information for generating one or more access stratum (AS) security keys to protect communication with the target radio cell. In some examples, the cell-specific baseline configuration may comprise a (dedicated) RACH preamble assigned to the UE for random-access to the target radio cell. In some examples, the cell-specific baselineconfiguration may further comprise an identifier e.g. C-RNTI for UE 110.

[0163] The method may further comprise, at 1504, initiating a random-access procedure to establish an RRC connection with handover target access node 202B, based on the cell-specific baseline configuration. For instance, UE 110 may obtain DL and UL synchronization with the target radio cell and send (transmit) a random-access preamble comprising the RACH preamble indicated in the cell-specific baseline configuration (if available) to handover target access node 202B. If no RACH preamble is indicated in the cell-specific baseline configuration, UE 110 may select a RACH preamble from a preconfigured RACH preamble group. Although not shown, UE 110 may receive a random-access response (RAR) message from handover target access node 202B.

[0164] The method may further comprise, at 1506, transmitting a reconfiguration complete message to handover target access node 202B during the random-access procedure. For instance, the reconfiguration complete message may be transmitted after UE 110 receives a random-access response (RAR) message from handover target access node 202B in the random-access procedure. The reconfiguration complete message may be transmitted on the default SRB1 indicated in the default signaling radio bearer configuration. In some examples, the reconfiguration complete message may comprise the UE ID (e.g., C-RNTI) indicated in the cellspecific baseline configuration if available. In some examples, the reconfiguration complete message may further comprise a message authentication code (MAC), e.g., a short message authentication code for integrity (shortMAC-I). The shortMAC-I may be used to identify and verify UE 110.

[0165] In some examples, in the case the cell-specific baseline configuration received at 1502 comprises the indication of key update and the security information for generating one or more AS security keys, the methods may further comprise, at 1508 of FIG. 15B, in response to the indication of key update, generating the one or more AS security keys based on the security information. In some examples, the one or more AS security keys may comprise a master key KgNB and one or more keys derived from the master key, e.g., a key KRRCint for integrity protection of RRC signaling, a key KRRCenc for cyphering of RRC signaling, a key Kupint for integrity protection of user data, and a key Kupenc for cyphering of user data. The methos may further comprise, at 1510, cyphering the reconfiguration complete message with the key KRRCenc for cyphering of RRC signaling, before transmitting at 1506 the reconfiguration complete message to handover target access node 202B.

[0166] In some examples, the method may further comprise, at 1512 of FIG. 15C, receiving a supplementary configuration from handover target access node 202B. The supplementary configuration may comprise UE-specific configuration information to complete the handover ofUE 110 to the target radio cell. In some examples, the supplementary configuration may be received after transmitting the reconfiguration complete message. In some other examples, in the case the cell-specific baseline configuration received from handover source access node 202A comprises a pair of RACH preamble and UE ID e.g. C-RNTI for use in the handover, the supplementary configuration may be received in a random-access response (RAR) message during the random-access procedure, or in a message scheduled based on the RAR message. In other words, the RAR message may comprise scheduling information for the supplementary configuration.

[0167] In some examples, if the supplementary configuration is not received within a predefined time window from transmitting at 1506 the reconfiguration complete message, the method may further comprise, at 1514 of FIG. 15D, initiating an RRC connection reestablishment procedure towards the target radio cell. UE 110 may receive the supplementary configuration, e.g., in an RRC re-establishment message or an RRC reconfiguration message, from handover target access node 202B during the RRC connection re-establishment procedure.

[0168] FIGs. 16A-16D are flowcharts illustrating method 1600, according to some examples. Method 1600 may be implemented at a user equipment (UE), e.g., UE 110 discussed above.

[0169] As shown, the method may comprise, at 1602, receiving a message in association with a handover from handover source access node 202A. In some examples, the message in association with the handover may be a conditional handover (CHO) command, e.g., a conditional fast handover (FHO) command. The CFHO command may comprise one or more cell-specific baseline configurations and one or more execution conditions for a CHO (e.g., a CFHO) of UE 110 to one or more candidate target radio cells provided by one or more handover candidate target access nodes 202B. The cell-specific baseline configuration may comprise an identifier (e.g., physical cell identifier, PCI) of the candidate target radio cell and information necessary for connecting to the candidate target radio cell without UE-specific handover preparation. In other words, the cell-specific baseline configuration may not comprise UE- specific configuration information. In some examples, the cell-specific baseline configuration may comprise a default signaling radio bearer configuration indicating at least a default SRB1, or additionally a default SRB2. In some examples, the cell-specific baseline configuration may comprise an indication of key update, e.g., a masterKeyUpdate field, and security information for generating one or more access stratum (AS) security keys to protect communication with the candidate target radio cell. In some examples, the cell-specific baseline configuration may comprise a (dedicated) RACH preamble assigned to the UE for random-access to the target radio cell. In some examples, the cell-specific baseline configuration may further comprise an identifier e.g. C-RNTI for UE 110.

[0170] The method may further comprise, at 1604, determining information on radio link quality in association with a source radio cell provided by handover source access node 202A and the one or more candidate target radio cells provided by one or more handover candidate target access nodes 202B. For example, UE 110 may carry out measurements on the source and candidate target radio cells to determine the radio link quality information. In some other examples, UE 110 may receive the radio link quality information from network nodes or other UEs, or UE 110 may execute artificial intelligence (Al) or machine learning (ML) models / algorithms to determine / predict the radio link quality information in association with the source and candidate target radio cells.

[0171] Then UE 110 may, at 1606, select at least one target radio cell from the one or more candidate target radio cells, based on the one or more execution conditions and the radio link quality information in association with the source and candidate target radio cells. For example, UE 110 may keep monitoring radio link quality in association with the source radio cell and the one or more candidate target radio cells and evaluating whether the candidate target radio cells satisfy corresponding execution conditions. If multiple candidate target radio cells satisfy the execution conditions, UE 110 may select at least one target radio cell from the one or more candidate target radio cells that has the best radio link quality.

[0172] The method may further comprise, at 1608, initiating a random-access procedure to the selected target radio cell to establish an RRC connection with handover target access node 202B, based on the cell-specific baseline configuration. For instance, UE 110 may obtain DL and UL synchronization with the target radio cell and send (transmit) a random-access preamble comprising the RACH preamble indicated in the cell-specific baseline configuration (if available) to handover target access node 202B. If no RACH preamble is indicated in the cell-specific baseline configuration, UE 110 may select a RACH preamble from a preconfigured RACH preamble group. Although not shown, UE 110 may receive a random-access response (RAR) message from handover target access node 202B.

[0173] During the random-access procedure, UE 110 may, at 1610, transmit a reconfiguration complete message to handover target access node 202B. For example, the reconfiguration complete message may be transmitted after UE 110 receives a random-access response (RAR) message from handover target access node 202B. The reconfiguration complete message may be transmitted on the default SRB1 indicated in the default signaling radio bearer configuration. In some examples, the reconfiguration complete message may comprise the UE ID (e.g., C-RNTI) indicated in the cell-specific baseline configuration if available. In some examples, the reconfiguration complete message may further comprise a message authentication code (MAC), e.g., a short message authentication code for integrity (shortMAC-I). The shortMAC-I may beused to identify and verify UE 110.

[0174] In some examples, in the case the cell-specific baseline configuration received at 1602 comprises the indication of key update and the security information for generating one or more AS security keys, the methos may further comprise, at 1612 of FIG. 16B, in response to the indication of key update, generating the one or more AS security keys based on the security information. The one or more AS security keys may comprise for example a master key KgNB and one or more keys derived from the master key, e.g., a key KRRCint for integrity protection of RRC signaling, a key KRRCenc for cyphering of RRC signaling, a key Kupint for integrity protection of user data, and a key Kupenc for cyphering of user data. The AS security keys may be used to protect communication between UE 110 and handover target access node 202B. For example, UE 110 may, at 1614, cypher the reconfiguration complete message with the key KRRCenc for cyphering of RRC signaling, before transmitting the reconfiguration complete message to handover target access node 202B.

[0175] In some examples, the method may further comprise, at 1616 of FIG. 16C, receiving a supplementary configuration from handover target access node 202B. The supplementary configuration may comprise UE-specific configuration information to complete the handover of UE 110 to the target radio cell. In some examples, the supplementary configuration may be received after transmitting the reconfiguration complete message. In some other examples, in the case the cell-specific baseline configuration received from handover source access node 202A comprises a pair of RACH preamble and UE ID e.g. C-RNTI for use in the handover, the supplementary configuration may be received in a random-access response (RAR) message during the random-access procedure, or in a message scheduled based on the RAR message. In other words, the RAR message may comprise scheduling information for the supplementary configuration.

[0176] In some examples, if the supplementary configuration is not received within a predefined time window from transmitting the reconfiguration complete message, the method may further comprise, at 1618 of FIG. 16D, initiating an RRC connection re-establishment procedure towards the target radio cell. UE 110 may receive the supplementary configuration during the RRC connection re-establishment procedure, e.g., in an RRC reconfiguration message or in an RRC re-establishment message.

[0177] FIGs. 17A-17F are flowcharts illustrating method 1700, according to some examples. Method 1700 may be implemented at a handover source access node, e.g., handover source access node 202A discussed above.

[0178] As shown, the method may comprise, at 1702, receiving a cell-specific baseline configuration from handover target access node 202B. The cell-specific baseline configurationmay comprise cell-specific configuration information for a handover (e.g., a fast handover) of a UE to a target radio cell without UE-specific handover preparation. For instance, the cell-specific baseline configuration may comprise an identifier (e.g., physical cell identifier, PCI) of the target radio cell and information necessary for a UE to connect to the target radio cell without reading system information of the target radio cell. In some examples, the cell-specific baseline configuration may comprise a default signaling radio bearer configuration indicating at least a default SRB1, or additionally a default SRB2. The cell-specific baseline configuration may also comprise security algorithm identifiers indicating security algorithms applied at the target radio cell, user plane (UP) tunnel information indicating at least one UP tunnel to handover target access node 202B, etc. In some examples, the cell-specific baseline configuration may comprise a pool of preconfigured / reserved RACH preambles, and / or a pool of preconfigured / reserved C- RNTIs. Handover source access node 202A may select a RACH preamble and / or a C-RNTI for the handover of a UE to the target radio cell.

[0179] The method may further comprise, at 1704, receiving from UE 110 information on radio link quality in association with a source cell provided by handover source access node 202A and the target radio cell. For instance, the information on radio link quality may be comprised in a measurement report received from UE 110.

[0180] Based on at least the information on radio link quality, handover source access node 202 A may determine, at 1706, to hand over UE 110 to the target radio cell using the cell-specific baseline configuration. In other words, handover source access node 202A may determine to carry out the fast handover procedure for UE 110. In some examples, handover source access node 202A may determine to carry out the fast handover procedure based on the information on radio link quality and one or more of the following conditions: service delay requirements of UE, service reliability requirements of UE, load information of target radio cell, traffic amount of UE, capability of UE, and capability of target radio cell.

[0181] Then handover source access node 202A may, at 1708, transmit a message in association with the determined handover to UE 110. In some examples, the message in association with the handover may be a handover command, e.g., a fast handover command, and it may comprise at least part of the cell-specific baseline configuration in association with the target radio cell received from handover target access node 202B. In some examples, the message may comprise an indication of key update and security information for generating one or more access stratum (AS) security keys. In response to the indication of key update, UE 110 will generate one or more AS security keys using the security information to protect communication with the target radio cell, e.g., a master key KgNB and AS security keys KRRCint, KRRCenc, Kupint, Kupenc derived from the master key K§NB. In some examples, the message may comprise a RACHpreamble and / or a UE ID e.g., C-RNTI assigned to UE 110. The RACH preamble may be selected from the pool of (preconfigured / reserved) RACH preambles provided by handover target access node 202B. The UE ID e.g. C-RNTI may be selected from the pool of (preconfigured / reserved) C-RNTIs provided by handover target access node 202B, or generated by handover source access node 202A.

[0182] If handover source access node 202A determines to carry out the fast handover of UE 110 to the target radio cell, the method may further comprise, at 1710 of FIG. 17B, transmitting a handover notification message comprising information related to the fast handover to handover target access node 202B. In some examples, the handover notification message may comprise an identifier of UE 110, e.g., the C-RNTI, information on UE 110, an AS security master key, and a parameter for counting next hop chaining in association with the AS security master key. The handover notification message may also comprise at least part of the information sent at 1708 to UE 110.

[0183] In some examples, the method may further comprise, at 1712 of FIG. 17C, receiving a request for information on UE 110 from handover target access node 202B and, at 1714, transmitting a response message comprising the information on UE 110 to handover target access node 202B. For example, if the fast handover of UE 110 to the target radio cell is initiated and handover target access node 202B has not yet received any information on UE 110, then handover target access node 202B may request handover source access node 202 to provide the information on UE 110. In some examples, the response message may further comprise an AS security master key and a parameter for counting next hop chaining in association with the AS security master key.

[0184] In some examples, the request for information on UE 110 may comprise the UE ID e.g. C-RNTI assigned to UE 110 by handover source access node 202A, and a cyphered message. Handover source access node 202A may verify whether the cyphered message is cyphered by UE 110, based on the security information provided to UE 110 in the message in association with the fast handover e.g. the fast handover command. If the verification is successful, then handover source access node 202A may transmit the response message to handover target access node 202B.

[0185] In the case the cell-specific baseline configuration received from handover target access node 202B comprises UP tunnel information indicating at least one UP tunnel, the method may further comprise, at 1716 of FIG. 17D, transmitting information indicating mapping of the at least one UP tunnel to DRB information of UE 110 to handover target access node 202B when handover source access node 202 A determines to carry out the fast handover of UE 110 and, at 1718, forwarding user data of UE 110 to handover target access node 202B using the at least oneUP tunnel, based on the mapping of the at least one UP tunnel to the DRB information of UE 110.

[0186] In some other examples, in the case the cell-specific baseline configuration received from handover target access node 202B comprises the UP tunnel information indicating at least one UP tunnel, the method may further comprise, at 1720 of FIG. 17E, forwarding user data of UE 110 to handover target access node 202B using the at least one UP tunnel when handover source access node 202A determines to carry out the fast handover of UE 110. The forwarded user data may comprise a header indicating mapping of the at least one UP tunnel to the DRB information of UE 110.

[0187] In some examples, the cell-specific baseline configuration received from handover target access node 202B may not comprise the UP tunnel information. When handover source access node 202 A determines to carry out the fast handover of UE 110, handover source access node 202A may, at 1722 of FIG. 17F, transmit a request for UP tunnel information to handover target access node 202B. The request may also indicate DRB information of UE 110. In response, handover source access node 202 A may, at 1724, receive the UP tunnel information from handover target access node 202B indicating at least one UP tunnel in association with the DRB information of UE 110. Then handover source access node 202Amay, at 1726, forward user data of UE 110 to handover target access node 202B using the at least one UP tunnel.

[0188] FIGs. 18A-18F are flowcharts illustrating method 1800, according to some examples. Method 1800 may be implemented at a handover source access node, e.g., handover source access node 202A discussed above.

[0189] As shown, the method may comprise, at 1802, receiving one or more cell-specific baseline configurations from one or more neighboring access nodes 202B. The one or more cellspecific baseline configurations each may comprise cell-specific configuration information for a handover, e.g., a fast handover (FHO) or a conditional fast handover (CFHO), of a UE to a neighboring radio cell without UE-specific handover preparation. The neighboring radio cell is also referred to as candidate target radio cell, and the neighboring access node providing the neighboring cell is also referred to as handover candidate target access node. The cell-specific baseline configuration may comprise an identifier (e.g., physical cell identifier, PCI) of the candidate target radio cell and information necessary for a UE to connect to the candidate target radio cell without reading system information of the target radio cell. In some examples, the cellspecific baseline configuration may comprise a default signaling radio bearer configuration indicating at least a default SRB1, or additionally a default SRB2. The cell-specific baseline configuration may also comprise security algorithm identifiers indicating security algorithms applied at the target radio cell, user plane (UP) tunnel information indicating at least one UPtunnel to handover target access node 202B, etc. In some examples, the cell-specific baseline configuration may comprise a pool of preconfigured / reserved RACH preambles, and / or a pool of preconfigured / reserved C-RNTIs. Handover source access node 202A may select a RACH preamble and / or a C-RNTI for the handover of a UE to the candidate target radio cell.

[0190] The method may further comprise, at 1804, receiving from UE 110 information on radio link quality in association with a source cell provided by handover source access node 202A and at least one of the one or more candidate target radio cells. For instance, handover source access node 202A may receive a measurement report comprising the information on radio link quality from UE 110.

[0191] Based on at least the information on radio link quality, handover source access node 202A may determine, at 1806, to hand over UE 110 to one or more candidate target radio cells using one or more corresponding cell-specific baseline configurations. In other words, handover source access node 202A may determine to carry out the conditional fast handover (CFHO) procedure for UE 110. In some examples, handover source access node 202Amay determine the CFHO procedure based on the information on radio link quality and one or more of the following conditions: service delay requirements of UE, service reliability requirements of UE, load information of candidate target radio cell, traffic amount of UE, capability of UE, and capability of candidate target radio cell.

[0192] Then handover source access node 202A may, at 1808, transmit a message in association with the determined handover to UE 110. In some examples, the message in association with the handover may be a handover command, e.g., a conditional fast handover command, and it may comprise at least part of the cell-specific baseline configuration(s) in association with the candidate target radio cell(s) received from handover candidate target access node(s) 202B. In some examples, the message may comprise an indication of key update and security information for generating one or more access stratum (AS) security keys. In response to the indication of key update, UE 110 will generate one or more AS security keys using the security information to protect communication with the candidate target radio cell, e.g., a master key KgNB and AS security keys KRRCint, KRRCenc, Kupint, Kupenc derived from the master key KgNB. In some examples, the message may comprise a RACH preamble and / or a UE ID e.g., C-RNTI assigned to UE 110 with respect to each candidate target radio cell. The RACH preamble may be selected from the pool of (preconfigured / reserved) RACH preambles provided by handover target access node 202B. The UE ID e.g. C-RNTI may be selected from a pool of (preconfigured / reserved) C-RNTIs provided by handover target access node 202B, or generated by handover source access node 202A.

[0193] If handover source access node 202A determines to carry out the conditional fasthandover of UE 110 to one or more candidate target radio cells, the method may further comprise, at 1810 of FIG. 18B, transmitting a handover notification message comprising information related to the conditional fast handover to handover candidate target access node(s) 202B. In some examples, the handover notification message may comprise an identifier of UE 110, e.g., C-RNTI, information on UE 110, an AS security master key, and a parameter for counting next hop chaining in association with the AS security master key. The handover notification message may also comprise at least part of the information sent at 1808 to UE 110.

[0194] In some examples, the method may further comprise, at 1812 of FIG. 18C, receiving a request for information on UE 110 from at least one handover target access node and, at 1814, transmitting a response message comprising the information on UE 110 to the at least one handover target access node. For example, in the case UE 110 selects a target access node from the one of the one or more candidate target access nodes and initiates the handover to the selected target access node, if handover target access node 202B providing the target radio cell does not have information on UE 110, handover target access node 202B may transmit the request for information on UE 110 to handover source access node 202A. In some examples, the response message may further comprise an AS security master key and a parameter for counting next hop chaining in association with the AS security master key for the target radio cell.

[0195] In some examples, the request for information on UE 110 may comprise a UE ID e.g. C-RNTI assigned to UE 110 by handover source access node 202 A, and a cyphered message. Handover source access node 202A may verify whether the cyphered message is cyphered by UE 110, based on the security information provided to UE 110 in the message in association with the conditional fast handover e.g. the conditional fast handover command. If the verification is successful, then handover source access node 202A may transmit the response message to handover target access node 202B.

[0196] In the case the cell-specific baseline configuration(s) received from one or more handover candidate target access nodes comprises UP tunnel information indicating at least one UP tunnel, the method may further comprise, at 1816 of FIG. 18D, transmitting information indicating mapping of the at least one UP tunnel to DRB information of UE 110 to at least one of the one or more handover candidate target access nodes when handover source access node 202 A determines to carry out the conditional fast handover of UE 110 and, at 1818, forwarding user data of UE 110 to the at least one handover candidate target access node using the at least one UP tunnel, based on the mapping of the at least one UP tunnel to the DRB information of UE 110. It would be appreciated that handover source access node 202A may not know which one of the one or more handover candidate target access nodes is selected by UE 110 as the target access node for the handover before handover source access node 202A receives a handoversuccess message from the target access node, and handover source access node 202A may transmit the mapping information to any one of the one or more handover candidate target access nodes to initiate the early data forwarding for UE 110.

[0197] In some other examples, in the case the cell-specific baseline configuration(s) received from one or more handover candidate target access nodes comprises UP tunnel information indicating a UP tunnel, the method may further comprise, at 1820 of FIG. 18E, forwarding user data of UE 110 to at least one of the one or more handover candidate target access nodes using the UP tunnel when handover source access node 202A determines to carry out the conditional fast handover of UE 110. Handover source access node 202A may forward the user data of UE 110 to any one of the one or more handover candidate target access nodes. The forwarded user data may comprise a header indicating mapping of the UP tunnel to the DRB information of UE 110.

[0198] In some examples, the cell-specific baseline configuration(s) received from one or more handover candidate target access nodes may not comprise UP tunnel information. When handover source access node 202A determines to carry out the conditional fast handover of UE 110, handover source access node 202 A may, at 1822 of FIG. 18F, transmit a request for UP tunnel information to at least one of the one or more handover candidate target access nodes. The request may also indicate DRB information of UE 110. In response, handover source access node 202 A may, at 1824, receive UP tunnel information from the at least one handover candidate target access node indicating at least one UP tunnel in association with the DRB information of UE 110. Then handover source access node 202A may, at 1826, forward user data of UE 110 to the at least one handover candidate target access node using the at least one UP tunnel.

[0199] FIGs. 19A-19F are flowcharts illustrating method 1900, according to some examples. Method 1900 may be implemented at a handover candidate target access node (handover target access node), e.g., handover candidate target access node 202B discussed above.

[0200] As shown, the method may comprise, at 1902, transmitting one or more cell-specific baseline configurations to handover source access node 202A. The one or more cell-specific baseline configurations each may comprise cell-specific configuration information for a handover, e.g., a fast handover (FHO) or a conditional fast handover (CFHO), of a UE to a candidate target radio cell provided by handover candidate target access node 202B. At the time of transmitting the one or more cell-specific baseline configurations, handover candidate target access node 202B may not know which UE will be handed over from handover source access node 202 A to handover candidate target access node 202B. Accordingly, handover candidate target access node 202B may prepare and transmit the one or more cell-specific baseline configurations without UE-specific handover preparation. The cell-specific baselineconfiguration may comprise an identifier (e.g., physical cell identifier, PCI) of the candidate target radio cell and information necessary for a UE to connect to the candidate target radio cell without reading system information of the candidate target radio cell. In some examples, the cellspecific baseline configuration may comprise a default signaling radio bearer configuration indicating at least a default SRB1, or additionally a default SRB2. The cell-specific baseline configuration may also comprise security algorithm identifiers indicating security algorithms applied at the target radio cell, user plane (UP) tunnel information indicating at least one UP tunnel to handover candidate target access node 202B, among others. In some examples, the cellspecific baseline configuration may comprise a pool of preconfigured / reserved RACH preambles, and / or a pool of preconfigured / reserved C-RNTIs. When handover source access node 202A decides to carry out fast handover or conditional fast handover of a UE to a candidate target radio cell provided by handover candidate target access node 202B, handover source access node 202A may select a RACH preamble and / or a C-RNTI for the UE from the respective pools.

[0201] The method may further comprise, at 1904, receiving information on UE 110 from handover source access node 202A. In some examples, the information on UE 110 may be received in a handover notification message from handover source access node 202A indicating that handover source access node 202 A has decided to hand over UE 110 to one or more candidate target radio cells provided by handover candidate target access node 202B. The handover notification message may also indicate a fast handover (e.g., to one target radio cell) or a conditional fast handover (e.g., to one or more candidate target radio cells) that has been decided for UE 110. The information on UE 110 may comprise for example UE context information, UP configuration information (e.g., DRB related configuration information), basic AS configuration information such as antenna information and downlink (DL) carrier frequency, UE capabilities, among others. Upon receipt of the UE information, handover candidate target access node 202B may start to prepare UE-specific configuration for the handover of UE 110. In addition to the information on UE 110, the handover notification message may comprise, with respect to each candidate target radio cell, a RACH preamble and / or a C-RNTI assigned to UE 110, security information, and the like. The security information may comprise for example an AS security master key (KgNB or KgNB*), and a parameter for counting next hop (NH) chaining in association with the AS security master key. Handover candidate target access node 202B may use the AS security master key directly and associate the AS security master key with the parameter for counting next hop (NH) chaining. Handover candidate target access node 202B may also derive AS security keys such as KRRCint, KRRCenc, Kupint, Kupenc from the AS security master key using the security algorithms applied at the candidate target radio cell.

[0202] The method may further comprise, at 1906, receiving a random-access preamblecomprising a RACH preamble from UE 110. If the RACH preamble is selected from the pool of (preconfigured / reserved) RACH preambles provided by handover candidate target access node 202B, handover candidate target access node 202B can identify that UE 110 is undergoing an FHO or CFHO procedure.

[0203] In response to the random-access preamble, handover candidate target access node 202B may, at 1908, transmit a random-access response (RAR) message to UE 110.

[0204] As a part of the random-access procedure, handover candidate target access node 202B may, at 1910, receive an RRC reconfiguration complete message from UE 110. The RRC reconfiguration complete message may be received on the default SRB 1 indicated in the default signaling radio bearer configuration comprised in the cell-specific baseline configuration, and it may comprise the UE ID e.g., the C-RNTI assigned by handover source access node 202 A. If the C-RNTI is selected from the pool of (preconfigured / reserved) C-RNTIs provided by handover candidate target access node 202B, handover candidate target access node 202B can identify that UE 110 is undergoing an FHO or CFHO procedure. Handover candidate target access node 202B can also identify the information on UE 110 received from handover source access node 202A. In some examples, the RRC reconfiguration complete message may comprise a message authentication code e.g. a short message authentication code for integrity (shoarMAC- I) for identifying UE 110. The RRC reconfiguration complete message may be cyphered (e.g., with the AS security key KRRCenc) to protect information contained therein. If handover candidate target access node 202B has received (e.g., at 1904) the AS security information from handover source access node 202A, handover candidate target access node 202B can decipher the RRC reconfiguration complete message with the AS security key(s).

[0205] The method may further comprise, at 1912, generating a supplementary configuration for the handover (fast handover or conditional fast handover) of UE 110 based on the received information on UE 110 and, at 1914, transmitting the supplementary configuration to UE 110. The supplementary configuration may comprise configuration information needed atUE 110 but not provided to UE 110 in the cell-specific baseline configuration. In some examples, the supplementary configuration may comprise DRB configuration, QoS flow to DRB mapping configuration, beam-related configuration, network slice-related configuration, among others, to complete the handover.

[0206] In some examples, if handover candidate target access node 202B does not transmit the supplementary configuration to UE 110 due to a fact that it does not receive the information on UE 110 from handover source access node 202 A, handover candidate target access node 202B may, at 1916 of FIG. 19B, carry out an RRC connection re-establishment procedure. The RRC connection re-establishment procedure may be initiated by UE 110. During the re-establishmentprocedure, handover candidate target access node 202B may receive the information on UE 110 as well as the AS security master key and the parameter for counting next hop chaining in a retrieve UE context response message from handover source access node 202A, and generate and transmit the supplementary configuration in an RRC reconfiguration message or an RRC reestablishment message to UE 110. It can reduce the handover failure and consequent radio link failure.

[0207] As mentioned above, handover candidate target access node 202B may identify from a RACH preamble received in the random-access MSG1 and / or a UE ID e.g. C-RNTI received in the reconfiguration complete message (MSG3) that UE 110 is undergoing an FHO or CFHO procedure. If handover candidate target access node 202B dose not receive the information on UE 110 within a predefined time window from receiving the random-access preamble or the reconfiguration complete message, handover candidate target access node 202B may, at 1918 of FIG. 19C, transmit a request for the information on UE 110 to handover source access node 202A. The request may comprise the UE ID e.g. C-RNTI to identify UE 110. In some examples, the request may comprise the RRC reconfiguration complete message as cyphered received from UE 110 to identify and verify UE 110. In response, handover candidate target access node 202B may, at 1920, receive the information on UE 110 as well as the AS security master key and the parameter for counting next hop chaining from handover source access node 202A. Then handover candidate target access node 202B may generate a supplementary configuration and transmit the supplementary configuration to UE 110. In some examples, the supplementary configuration may be transmitted in the random-access response (RAR) message or a message scheduled based on the RAR message to UE 110. In some other examples, the supplementary configuration may be transmitted after receiving the RRC reconfiguration complete message from UE 110, e.g., in an RRC reconfiguration message to UE 110.

[0208] As mentioned above, the cell-specific baseline configuration provided to handover source access node 202A may comprise UP tunnel information indicating at least one UP tunnel to handover candidate target access node 202B. In these examples, the method may further comprise, at 1922 of FIG. 19D, receiving information on mapping of the at least one UP tunnel to DRB information of UE 110 from handover source access node 202 A and, at 1924, receiving user data of UE 110 on the UP tunnel, based on the mapping information. It can accomplish early data forwarding for UE 110 during the handover execution.

[0209] In some other examples, in case the cell-specific baseline configuration comprises UP tunnel information indicating at least one UP tunnel to handover candidate target access node 202B, the method may further comprise, at 1926 of FIG. 19E, receiving user data of UE 110 on the at least one UP tunnel. The user data may have a header comprising information on mappingof the at least one UP tunnel to DRB information of UE 110.

[0210] In some examples, the cell-specific baseline configuration provided by handover candidate target access node 202B may not comprise UP tunnel information. The method may further comprise, at 1928 of FIG. 19F, receiving a request for UP tunnel information of a candidate target radio cell from handover source access node 202A. The request may also indicate DRB information of UE 110. In response to the request, handover candidate target access node 202B may, at 1930, transmit UP tunnel information to handover source access node 202A indicating at least one UP tunnel in association with the DRB information of UE 110. Then handover candidate target access node 202B may, at 1932, receive user data of UE 110 on the at least one UP tunnel from handover source access node 202A.

[0211] According to example implementations of the present disclosure, a communication system 100 or PLMN 102, and its components such as UE 110, CN 106, RAN 108, radio access node 202, handover source access node 202A and / or handover candidate target access node 202B, may be implemented by various means. Means for implementing the system and its components may comprise hardware, firmware, software, or combinations thereof. In some examples, one or more apparatuses may be configured to function as or otherwise implement the system and its components shown and described herein. In examples involving more than one apparatus, the respective apparatuses may be connected to or otherwise in communication with one another in a number of different manners, such as directly or indirectly via a wired or wireless network or the like.

[0212] According to some example implementations, method 1500 described with respect to FIGs. 15A-15D and method 1600 described with respect to FIGs. 16A-16D each may be carried out by an apparatus comprising means for performing functions corresponding to steps of the method. Examples of a suitable apparatus may comprise user equipment (UE), which may refer to any device or entity that can wirelessly connect to a communication network and communicate with network devices and / or other UEs. Similarly, method 1700 described with respect to FIGs. 17A-17F, method 1800 described with respect to FIGs. 18A-18F and method 1900 described with respect to FIGs. 19A-19F each may be carried out by an apparatus comprising means for performing functions corresponding to steps of the method. Examples of a suitable apparatus may comprise a base station which can provide cells or coverage through which a UE can access the network and receive services. Examples of the base station may comprise gNB (e.g., gNB- DU, gNB-CU), ng-eNB or any suitable apparatus, such as a server, host or node.

[0213] FIGs. 20A, 20B illustrate apparatuses 2000A, 2000B, according to some examples. Apparatus 2000A may comprise at least one processing circuitry 2002A, at least one computer- readable storage medium or other memory 2004A, which is configured to store instructions2006 A, and at least one communication interface 2008 A. Apparatus 2000A may be implemented as UE to carry out methods, procedures and examples discussed above with regard to UE 110. Apparatus 2000B may comprise at least one processing circuitry 2002B, at least one computer- readable storage medium or other memory 2004B, which is configured to store instructions 2006B, and at least one communication interface 2008B. Apparatus 2000B may be implemented as a network access node e.g. a base station to carry out methods, procedures and examples discussed above with regard to handover source access node 202A and / or handover (candidate) target access node 202B. It would be appreciated that apparatuses 2000 A, 2000B each may comprise additional components or means to implement the respective methods, procedures and examples than those shown in the figures.

[0214] Processing circuitry 2002A, 2002B may be composed of one or more processors alone or in combination with one or more computer-readable storage media. The processing circuitry may be generally any piece of computer hardware that is capable of processing information such as, for example, data, computer programs and / or other suitable electronic information. The processing circuitry is composed of a collection of electronic circuits some of which may be packaged as an integrated circuit or multiple interconnected integrated circuits (an integrated circuit at times more commonly referred to as a “chip”). The processing circuitry may be configured to execute computer programs, which may be stored onboard the processing circuitry or otherwise stored in memory 2004A, 2004B (of the same or another apparatus).

[0215] Processing circuitry 2002A, 2002B may be a number of processors, a multi-core processor or some other type of processor, depending on the particular implementation. Further, the processing circuitry may be implemented using a number of heterogeneous processor systems in which a main processor is present with one or more secondary processors on a single chip. As another illustrative example, the processing circuitry may be a symmetric multiprocessor system containing multiple processors of the same type. In yet another example, the processing circuitry may be embodied as or otherwise comprise one or more ASICs, FPGAs or the like. Thus, although the processing circuitry may be capable of executing a computer program to perform one or more functions, the processing circuitry of various examples may be capable of performing one or more functions without the aid of a computer program. In either instance, the processing circuitry may be appropriately programmed to perform functions or operations according to example implementations of the present disclosure.

[0216] Memory 2004A, 2004B may be generally any piece of computer hardware that is capable of storing information such as, for example, data, computer programs, instructions 2006A, 2006B (e.g., computer-readable program code) and / or other suitable information either on a temporary basis and / or a permanent basis. The memory may comprise volatile and / or non-volatile memory, and may be fixed or removable. Examples of suitable memory comprise recording media, random access memory (RAM), read-only memory (ROM), a hard drive, a flash memory, a thumb drive, a removable computer diskette, an optical disk or some combination thereof.

[0217] Memory 2004A, 2004B may be a non-transitory device capable of storing information. One example of a suitable memory is a computer-readable storage medium, which is distinguishable from a computer-readable transmission medium capable of carrying information from one location to another. Examples of suitable computer-readable transmission media comprise electronic carrier signals, communication signals, software distribution packages, or some combination thereof. As used herein, the term “non-transitory” is a limitation of the medium itself (i.e., tangible, not a signal) as opposed to a limitation on data storage persistency (e.g., RAM versus ROM). A computer-readable medium as described herein generally refers to a computer-readable storage medium or computer-readable transmission medium. A computer- readable medium is any entity or device capable in which information, such as one or more computer programs or portions thereof, may be stored and carried.

[0218] In addition to memory 2004 A, 2004B (e.g., computer-readable storage medium), processing circuitry 2002A, 2002B may also be connected to one or more interfaces for displaying, transmitting and / or receiving information. The interfaces may comprise communication interface 2008A, 2008B and / or one or more user interfaces (e.g., display, user input interface). The communications interface may be configured to transmit and / or receive information, such as to and / or from other apparatus(es), UE(s), network access node(s), network function(s) or the like. The communications interface may be configured to transmit and / or receive information by physical (wired) and / or wireless communications links. Examples of suitable communication interfaces comprise a network interface controller (NIC), wireless NIC (WNIC) or the like.

[0219] Execution of instructions 2006 A, 2006B by corresponding processing circuitry 2002 A, 2002B, or storage of the instructions in corresponding memory 2004A, 2004B, supports combinations of operations for implementing example implementations of the present disclosure. In this manner, apparatus 2000A, 2000B each may comprise at least one processing circuitry and at least one memory coupled to the at least one processing circuitry, where the at least one processing circuitry is configured to execute instructions stored in the at least one memory. It will also be understood that one or more functions, and combinations of functions, may be implemented by special purpose hardware-based computer systems and / or processing circuitry which perform the specified functions, or combinations of special purpose hardware and program code instructions.

[0220] Some example implementations of the present disclosure may also be carried out in the form of a computer process defined by one or more computer programs or portions thereof. Example implementations of the present disclosure may be carried out by executing at least one portion of a computer program comprising instructions. The computer program may be in source code form, object code form, or in some intermediate form. The computer program may be stored in a computer-readable medium that is readable by a computer, processing circuitry or other suitable apparatus. As indicated above, for example, the computer program may be stored in a memory, such as a computer-readable storage medium. Additionally or alternatively, for example, the computer program may be stored in a computer-readable transmission medium. The coding of software for carrying out example implementations of the present disclosure is well within the scope of a person of ordinary skill in the art.

[0221] As will be appreciated, any suitable instructions may be loaded onto a computer, a processing circuitry or other programmable apparatus from a memory or a computer-readable medium (e.g., computer-readable storage medium, computer-readable transmission medium) to produce a particular machine, such that the particular machine becomes a means for implementing the functions specified herein. The instructions may also be stored in a computer- readable medium that can direct a computer, a processing circuitry or other programmable apparatus to function in a particular manner to thereby generate a particular machine or particular article of manufacture. In some examples, the instructions stored in the computer-readable medium may produce an article of manufacture, where the article of manufacture becomes a means for implementing functions described herein. The instructions may be retrieved from a computer-readable medium and loaded into a computer, processing circuitry or other programmable apparatus to configure the computer, processing circuitry or other programmable apparatus to execute operations to be performed on or by the computer, processing circuitry or other programmable apparatus.

[0222] Retrieval, loading and execution of instructions comprising program code instructions may be performed sequentially such that one instruction is retrieved, loaded and executed at a time. In some example implementations, retrieval, loading and / or execution may be performed in parallel such that multiple instructions are retrieved, loaded, and / or executed together. Execution of the program code instructions may produce a computer-implemented process such that the instructions executed by the computer, processing circuitry or other programmable apparatus provide operations for implementing functions described herein.

[0223] FIG. 21 is a block diagram illustrating apparatus 2100 according to some examples. Apparatus 2100 may be implemented to comprise or to form at least part of a UE such as UE 110 discussed above. As shown, apparatus 2100 may comprise means 2102 for receiving amessage in association with a handover from handover source access node 202A. The message may comprise a cell-specific baseline configuration for a handover of UE 110 to a target radio cell provided by handover target access node 202B, and the cell-specific baseline configuration may comprise a default signaling radio bearer configuration indicating at least a default SRB1, or additionally a default SRB2.

[0224] Apparatus 2100 may further comprise means 2104 for initiating, based on the cellspecific baseline configuration, a random-access procedure to the target radio cell to establish an RRC connection with handover target access node 202B, and means 2106 for transmitting an RRC reconfiguration complete message on an SRB e.g. the default SRB1 indicated in the default signaling radio bearer configuration to handover target access node 202B during the randomaccess procedure.

[0225] Although not shown in FIG. 21, apparatus 2100 may further comprise additional means for performing functions corresponding to operations, steps or elements in procedures, methods and examples discussed above with regard to UE 110. In some examples, the cell-specific baseline configuration may further comprise an indication of key update and security information for generating one or more AS security keys. Apparatus 2100 may further comprise means for carrying out, in response to the indication of key update, the generating of the one or more AS security keys based on the security information. The one or more AS security keys may comprise a key for cyphering of RRC signaling. Apparatus 2100 may further comprise means for cyphering the RRC reconfiguration complete message with the key for cyphering of RRC signaling before transmitting the RRC reconfiguration complete message to handover target access node 202B.

[0226] In some examples, apparatus 2100 may further comprise means for receiving, from handover target access node 202B, a supplementary configuration for the handover of UE 110 to the target cell, and / or means for initiating an RRC connection re-establishment procedure to the target radio cell in the case the supplementary configuration is not received within a predefined time window from transmitting the RRC reconfiguration complete message. The supplementary configuration may be received in a message from handover target access node 202B after transmitting the RRC reconfiguration complete message, or in the case the cellspecific baseline configuration received from handover source access node 202A comprises a pair of RACH preamble and UE ID e.g. C-RNTI, the supplementary configuration may be received in a random-access response (RAR) message during the random-access procedure or in a message scheduled based on the RAR message.

[0227] FIG. 22 is a block diagram illustrating apparatus 2200 according to some examples. Apparatus 2200 may be implemented to comprise or to form at least part of a UE such as UE110 discussed above. As shown, apparatus 2200 may comprise means 2202 for receiving a message in association with a conditional handover from handover source access node 202A. The message may comprise one or more cell-specific baseline configurations and one or more execution conditions for a conditional handover of UE 110 to one or more candidate target radio cells provided by one or more handover candidate target access nodes 202B. The one or more cell-specific baseline configurations each may comprise a default signaling radio bearer configuration indicating at least a default SRB1, or additionally a default SRB2. Apparatus 2200 may further comprise means 2204 for determining information on radio link quality in association with a source radio cell provided by handover source access node 202A and at least one of the one or more candidate target radio cells, means 2206 for selecting at least one target radio cell from the one or more candidate target radio cells based on the one or more execution conditions and the determined information on radio link quality, means 2208 for initiating, based on the cell-specific baseline configuration, a random-access procedure to the selected at least one target radio cell to establish an RRC connection with at least one handover target access node 202B providing the at least one target radio cell, and means 2210 for transmitting an RRC reconfiguration complete message on an SRB indicated in the default signaling radio bearer configuration to handover target access node 202B during the random-access procedure.

[0228] Although not shown in FIG. 22, apparatus 2200 may further comprise additional means for performing functions corresponding to operations, steps or elements in procedures, methods and examples discussed above with regard to UE 110. In some examples, the cell-specific baseline configuration may further comprise an indication of key update and security information for generating one or more AS security keys. Apparatus 2200 may further comprise means for carrying out, in response to the indication of key update, the generating of the one or more AS security keys based on the security information. The one or more AS security keys may comprise a key for cyphering of RRC signaling. Apparatus 2200 may further comprise means for cyphering the RRC reconfiguration complete message with the key for cyphering of RRC signaling before transmitting the RRC reconfiguration complete message to handover candidate target access node 202B.

[0229] In some examples, apparatus 2200 may further comprise means for receiving a supplementary configuration for the handover of UE 110 to the target radio cell from handover candidate target access node 202B, and / or means for initiating an RRC connection reestablishment procedure to the target radio cell in the case the supplementary configuration is not received within a predefined time window from transmitting the RRC reconfiguration complete message. The supplementary configuration may be received in a message from handover candidate target access node 202B after transmitting the RRC reconfiguration completemessage, or in the case the cell-specific baseline configuration received from handover source access node 202A comprises a pair of RACH preamble and UE ID e.g. C-RNTI, the supplementary configuration may be received in a random-access response (RAR) message during the random-access procedure or in a message scheduled based on the RAR message.

[0230] FIG. 23 is a block diagram illustrating apparatus 2300 according to some examples. Apparatus 2300 may be implemented to comprise or to form at least part of a network access node such as handover source access node 202A discussed above. As shown, apparatus 2300 may comprise means 2302 for receiving, from handover target access node 202B, a cell-specific baseline configuration for a handover to a target radio cell provided by handover target access node 202B. The cell-specific baseline configuration may comprise a default signaling radio bearer configuration indicating at least a default SRB1, or additionally a default SRB2. Apparatus 2300 may further comprise means 2304 for receiving, from UE 110, information on radio link quality in association with a source radio cell provided by handover source access node 202 A and the target radio cell, means 2306 for determining, based on at least the information on radio link quality, to carry out the handover of UE 110 to the target radio cell using the cell-specific baseline configuration, and means 2308 for transmitting a message in association with the handover to UE 110. The message may comprise at least part of the cellspecific baseline configuration for the handover of UE 110 to the target radio cell.

[0231] Although not shown in FIG. 23, apparatus 2300 may further comprise additional means for performing functions corresponding to operations, steps or elements in procedures, methods and examples discussed above with regard to handover source access node 202A. In some examples, the message in association with the handover may further comprise an indication of key update and security information for UE 110 to generate one or more AS security keys. Apparatus 2300 may further comprise means for transmitting a handover notification message to handover target access node 202B in response to determining to carry out the handover of UE 110 to the target radio cell provided by handover target access node 202B. The handover notification message may comprise an identifier for identifying UE 110, information on UE 110, an AS security master key, and a parameter for counting next hop chaining in association with the AS security master key.

[0232] In some example, apparatus 2300 may further comprise means for receiving a request for information on UE 110 from handover target access node 202B after transmitting the message in association with the handover, and means for transmitting a response message comprising information on UE 110, an AS security master key, and a parameter for counting next hop chaining in association with the AS security master key to handover target access node 202B. In some examples, the message in association with the handover may provide an identifier assignedto UE 110, and the request for information on UE 110 may comprise the identifier assigned to UE 110 and a cyphered message. The response message may be transmitted to handover target access node 202B when handover source access node 202A verifies that the cyphered message is cyphered by UE 110.

[0233] The cell-specific baseline configuration may further comprise user plane (UP) tunnel information indicating at least one UP tunnel to handover target access node 202B. In some examples, apparatus 2300 may further comprise means for transmitting, in response to determining to carry out the handover, information indicating mapping of the at least one UP tunnel to DRB information of UE 110 to handover target access node 202B, and means for forwarding, based on the mapping information, user data of UE 110 using the at least one UP tunnel to handover target access node 202B. In some other examples, apparatus 2300 may further comprise means for forwarding, in response to determining to carry out the handover, user data of UE 110 using the at least one UP tunnel to handover target access node 202B, where the user data comprises a header indicating mapping of the at least one UP tunnel to DRB information of UE 110.

[0234] In some examples, apparatus 2300 may further comprise means for transmitting, in response to determining to carry out the handover, a request for UP tunnel information to handover target access node 202B, the request also indicating DRB information of UE 110, means for receiving UP tunnel information indicating at least one UP tunnel in association with the DRB information of UE 110 from handover target access node 202B, and means for forwarding user data of UE 110 using the at least one user plane tunnel to handover target access node 202B.

[0235] FIG. 24 is a block diagram illustrating apparatus 2400 according to some examples. Apparatus 2400 may be implemented to comprise or to form at least part of a network access node such as handover source access node 202A discussed above. As shown, apparatus 2400 may comprise means 2402 for receiving, from one or more neighboring access nodes 202B, one or more cell-specific baseline configurations for a conditional handover to one or more neighboring radio cells provided by one or more neighboring access nodes 202B, the cellspecific baseline configuration comprising a default signaling radio bearer configuration indicating at least a default SRB1, or additionally a default SRB2, means 2404 for receiving, from UE 110, information on radio link quality in association with a source radio cell provided by handover source access node 202A and at least one of the one or more neighboring radio cells, means 2406 for determining, based on at least the received information on radio link quality, to carry out the conditional handover of UE 110 to one or more candidate target radio cells selected from the one or more neighboring radio cells provided by one or more handover candidate targetaccess nodes 202B, and means 2408 for transmitting a message in association with the conditional handover to UE 110, the message comprising at least part of the one or more cellspecific baseline configurations in association with the one or more candidate target radio cells.

[0236] Although not shown in FIG. 24, apparatus 2400 may further comprise additional means for performing functions corresponding to operations, steps or elements in procedures, methods and examples discussed above with regard to handover source access node 202A. In some examples, apparatus 2400 may further comprise means for transmitting handover information in association with UE 110 to one or more handover candidate target access nodes 202B after determining to carry out the conditional handover of UE 110, the handover information comprising an identifier for identifying UE 110, information on UE 110, an AS security master key, and a parameter for counting next hop chaining in association with the AS security master key.

[0237] In some examples, apparatus 2400 may further comprise means for receiving a request for information on UE 110 from at least one handover target access node 202B after transmitting the message in association with the conditional handover, and means for transmitting a response message to at least one handover target access node 202B, the response message comprising information on UE 110, an AS security master key, and a parameter for counting next hop chaining in association with the AS security master key. In some examples, in the case the message in association with the conditional handover provides an identifier assigned to UE 110 and the request for the information on UE 110 comprises the identifier assigned to UE 110 and a cyphered message, the response message may be transmitted to at least one handover target access node 202B when handover source access node 202A verifies that the cyphered message is cyphered by UE 110.

[0238] The cell-specific baseline configuration may further comprise UP tunnel information indicating at least one UP tunnel to handover candidate target access node 202B. In some examples, apparatus 2400 may further comprise means for transmitting, in response to determining to carry out the conditional handover, information indicating mapping of the at least one UP tunnel to DRB information of UE 110 to at least one handover candidate target access node 202B, and means for forwarding, based on the mapping information, user data of UE 110 using the at least one UP tunnel to at least one handover candidate target access node 202B. In some other examples, apparatus 2400 may further comprise means for forwarding, in response to determining to carry out the conditional handover, user data of UE 110 using the at least one user plane tunnel to at least one handover candidate target access node 202B, the user data comprising a header indicating mapping of the at least one UP tunnel to DRB information of UE 110.

[0239] In some examples, apparatus 2400 may further comprise means for transmitting, in response to determining to carry out the conditional handover, a request for UP tunnel information to at least one handover candidate target access node 202B, the request also indicating DRB information of UE 110, means for receiving UP tunnel information indicating at least one UP tunnel in association with the DRB information from at least one handover candidate target access node 202B, and means for forwarding user data of UE 110 using the at least one UP tunnel.

[0240] FIG. 25 is a block diagram illustrating apparatus 2500 according to some examples. Apparatus 2500 may be implemented to comprise or to form at least part of a network access node such as handover (candidate) target access node 202B discussed above. As shown, apparatus 2500 may comprise means 2502 for transmitting, to handover source access node 202A, one or more cell-specific baseline configurations for a handover or conditional handover from handover source access node 202A to one or more (candidate) target radio cells provided by the handover (candidate) target access node 202B, the cell-specific baseline configuration comprising a default signaling radio bearer configuration indicating at least a default SRB1, or additionally a default SRB2, means 2504 for receiving information on UE 110 to be handed over and an AS security master key from handover source access node 202A, means 2506 for receiving, from UE 110, a random-access preamble to a target radio cell of the one or more (candidate) target radio cells provided by handover (candidate) target access node 202B, means 2508 for transmitting a random-access response (RAR) message to UE 110, means 2510 for receiving an RRC reconfiguration complete message on a SRB (e.g., SRB1) indicated in the default signaling radio bearer configuration from UE 110, means 2512 for generating a supplementary configuration for the handover or conditional handover of UE 110 based on the received information on UE 110, and means 2514 for transmitting the supplementary configuration to UE 110.

[0241] Although not shown in FIG. 25, apparatus 2500 may further comprise additional means for performing functions corresponding to operations, steps or elements in procedures, methods and examples discussed above with regard to handover (candidate) target access node 202B. In some examples, means 2504 for receiving the information on UE 110 may comprise means for receiving a (basic or conditional) handover notification message from handover source access node 202A comprising the information on UE 110, the AS security master key, and a parameter for counting next hop chaining in association with the AS security master key. In some other examples, means 2504 for receiving the information on UE 110 may comprise means for carrying out an RRC connection re-establishment procedure initiated by UE 110 to receive from handover source access node 202 A the information on UE 110, the AS security master key, anda parameter for counting next hop chaining in association with the AS security master key. In some yet other examples, means 2504 for receiving the information on UE 110 may comprise means for transmitting a request for the information on UE 110 to handover source access node 202A within a predefined time window from receiving the random-access preamble or the RRC reconfiguration complete message from UE 110, and means for receiving a response message from handover source access node 202A comprising the information on UE 110, the AS security master key, and a parameter for counting next hop chaining in association with the AS security master key.

[0242] The cell-specific baseline configuration may further comprise user plane (UP) tunnel information indicating at least one UP tunnel to handover candidate target access node 202B. In some examples, apparatus 2500 may further comprise means for receiving information indicating mapping of the at least one UP tunnel to DRB information of UE 110 from handover source access node 202A, and means for receiving user data of UE 110 using the at least one UP tunnel from handover source access node 202A based on the mapping information. In some other examples, apparatus 2500 may further comprise means for receiving user data of UE 110 using the at least one UP tunnel from handover source access node 202A, the user data comprising a header indicating mapping of the at least one UP tunnel to DRB information of UE 110.

[0243] In some examples, apparatus 2500 may further comprise means for receiving a request for UP tunnel information from handover source access node 202A, the request also indicating DRB information of UE 110, means for transmitting UP tunnel information indicating at least one UP tunnel in association with the DRB information of UE 110 to handover source access node 202 A, and means for receiving user data of UE 110 using the at least one UP tunnel from handover source access node 202A.

[0244] It would be understood that blocks in the drawings may be implemented in various manners, including software, hardware, firmware, or any combination thereof. In some embodiments, one or more blocks may be implemented using software and / or firmware, for example, machine-executable instructions stored in the storage medium. In addition to or instead of machine-executable instructions, parts or all of the blocks in the drawings may be implemented, at least in part, by one or more hardware logic components. For example, and without limitation, illustrative types of hardware logic components that can be used include Field-Programmable Gate Arrays (FPGAs), Application-Specific Integrated Circuits (ASICs), Application-Specific Standard Products (ASSPs), System-on-Chip systems (SOCs), Complex Programmable Logic Devices (CPLDs), etc.

[0245] As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two ormore elements are joined by “and” or “or”, mean at least any one of the elements, or at least any two or more of the elements, or at least all the elements.

[0246] As explained above and reiterated below, the present disclosure comprises, without limitation, the following example implementations.

[0247] Clause 1. An apparatus, comprising: means for receiving, by a user equipment from a handover source access node, a message in association with a handover comprising a cellspecific baseline configuration for a handover of the user equipment to a target cell provided by a handover target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; means for initiating a random-access procedure to the target cell to establish a radio resource control connection between the user equipment and the handover target access node, based on the cell-specific baseline configuration; and means for transmitting, by the user equipment to the handover target access node, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration, during the random-access procedure.

[0248] Clause 2. The apparatus of clause 1, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2. The reconfiguration complete message is transmitted on the default signaling radio bearer 1.

[0249] Clause 3. The apparatus of clause 1, wherein the cell-specific baseline configuration further comprises an indication of key update, and security information for generating one or more access stratum security keys for communication with the target cell. The apparatus further comprises: means for carrying out, by the user equipment in response to the indication of key update, the generating of the one or more access stratum security keys for communication with the target cell based on the security information, the one or more access stratum security keys comprising a key for cyphering of radio resource control signaling; and means for cyphering the reconfiguration complete message with the key for cyphering of radio resource control signaling before transmitting the reconfiguration complete message to the handover target access node.

[0250] Clause 4. The apparatus of clause 1, wherein the reconfiguration complete message comprises a short message authentication code to identify the user equipment.

[0251] Clause 5. The apparatus of clause 1, further comprising: means for receiving, by the user equipment from the handover target access node, a supplementary configuration for the handover of the user equipment to the target cell; or means for initiating a radio resource control connection re-establishment procedure to the target cell in the case the supplementary configuration is not received within a predefined time window from transmitting the reconfiguration complete message.

[0252] Clause 6. The apparatus of clause 5, wherein the supplementary configuration is comprised in a message received from the handover target access node after transmitting the reconfiguration complete message, or in the case the cell-specific baseline configuration received from the handover source access node comprises a pair of a random-access preamble and a user equipment identifier for use in the handover, the supplementary configuration is comprised in a random-access response message received from the handover target access node as a part of the random access procedure, or in a message scheduled based on the random access response message.

[0253] Clause 7. An apparatus, comprising: means for receiving, by a handover source access node from a handover target access node, a cell-specific baseline configuration for a handover to a target cell provided by the handover target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; means for receiving, by the handover source access node from a user equipment, information on radio link quality in association with a source cell provided by the handover source access node and the target cell; means for determining, by the handover source access node, to carry out the handover of the user equipment to the target cell using the cell-specific baseline configuration based on at least the information on radio link quality in association with the source cell and the target cell; and means for transmitting, by the handover source access node to the user equipment, a message in association with the handover of the user equipment to the target cell comprising at least part of the cell-specific baseline configuration.

[0254] Clause 8. The apparatus of clause 7, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2.

[0255] Clause 9. The apparatus of clause 7, wherein the handover of the user equipment to the target cell using the cell-specific baseline configuration is determined based on the information on radio link quality in association with the source cell and the target cell and at least one of: service delay requirements of the user equipment; service reliability requirements of the user equipment; load information of the target cell; traffic amount of the user equipment; capability of the user equipment; and capability of the target cell.

[0256] Clause 10. The apparatus of clause 7, wherein the message in association with the handover of the user equipment further comprises an indication of key update, and security information for the user equipment to generate one or more access stratum security keys for communication with the target cell.

[0257] Clause 11. The apparatus of clause 7, further comprising: means for transmitting, by the handover source access node to the handover target access node, in response to determiningto carry out the handover of the user equipment to the target cell, a handover notification message comprising an identifier for identifying the user equipment, information on the user equipment, an access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0258] Clause 12. The apparatus of clause 7, further comprising: means for receiving, by the handover source access node from the handover target access node, in response to the message in association with the handover, a request for information on the user equipment; and means for transmitting, by the handover source access node to the handover target access node, a response message comprising the information on the user equipment, an access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0259] Clause 13. The apparatus of clause 12, wherein the message in association with the handover provides an identifier assigned to the user equipment, the request for information on the user equipment comprises the identifier assigned to the user equipment, and a cyphered message, and the response message is transmitted to the handover target access node when the handover source access node verifies that the cyphered message is cyphered by the user equipment.

[0260] Clause 14. The apparatus of clause 7, wherein the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node. The apparatus further comprises: means for transmitting, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and means for forwarding, by the handover source access node to the handover target access node, user data for the user equipment using the at least one user plane tunnel to the handover target access node, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0261] Clause 15. The apparatus of clause 7, wherein the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node. The apparatus further comprises means for forwarding, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, user data for the user equipment using the at least one user plane tunnel to the handover target access node indicated in the userplane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0262] Clause 16. The apparatus of clause 11, further comprising: means for transmitting, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; means for receiving, by the handover source access node from the handover target access node, the user plane tunnel information indicating at least one user plane tunnel to the handover target access node in association with the information on the data radio bearer of the user equipment; and means for forwarding, by the handover source access node to the handover target access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0263] Clause 17. An apparatus, comprising: means for transmitting, by a handover target access node to a handover source access node, a cell-specific baseline configuration for a handover from the handover source access node to a target cell provided by the handover target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; means for receiving, by the handover target access node from the handover source access node, information on a user equipment and an access stratum security master key for communication between the user equipment and the handover target access node; means for receiving, by the handover target access node from the user equipment, a random-access preamble; means for transmitting, by the handover target access node to the user equipment, a random-access response message; means for receiving, by the handover target access node from the user equipment, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration; means for generating, by the handover target access node, a supplementary configuration for the handover of the user equipment to the target cell, based on the information on the user equipment; and means for transmitting, by the handover target access node to the user equipment, the supplementary configuration.

[0264] Clause 18. The apparatus of clause 17, wherein the default signaling radio bearer configuration comprises at least configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and the reconfiguration complete message is received on the default signaling radio bearer 1.

[0265] Clause 19. The apparatus of clause 17, wherein the reconfiguration complete message is cyphered with an access stratum security key for cyphering of radio resource control signaling derived from the access stratum security master key.

[0266] Clause 20. The apparatus of clause 17, wherein the reconfiguration complete messagecomprises a short message authentication code to identify the user equipment.

[0267] Clause 21. The apparatus of clause 17, wherein the information on the user equipment and the access stratum security master key are comprised in a handover notification message received from the handover source access node, and the handover notification message further comprises a parameter for counting next hop chaining in association with the access stratum security master key.

[0268] Clause 22. The apparatus of clause 17, wherein means for receiving the information on the user equipment and the access stratum security master key comprises: means for carrying out, by the handover target access node, a radio resource control connection re-establishment procedure initiated by the user equipment to receive, from the handover source access node, the information on the user equipment, the access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0269] Clause 23. The apparatus of clause 17, wherein means for receiving the user equipment information and the access stratum security master key comprises: means for transmitting, by the handover target access node to the handover source access node, a request for the information on the user equipment within a predefined time window from receiving the random-access preamble or the reconfiguration complete message from the user equipment; and means for receiving, by the handover target access node from the handover source access node, a response message comprising the information on the user equipment, the access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0270] Clause 24. The apparatus of clause 23, wherein the supplementary configuration is comprised in a message transmitted to the user equipment after receiving the reconfiguration complete message, or the supplementary configuration is comprised in the random-access response message or in a message scheduled based on the random-access response message.

[0271] Clause 25. The apparatus of clause 23, wherein the reconfiguration complete message received from the user equipment is cyphered with a key derived from the access stratum security master key, and the user equipment information request transmitted to the handover source access node comprises an identifier assigned to the user equipment and the reconfiguration complete message as cyphered.

[0272] Clause 26. The apparatus of clause 17, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node. The apparatus furthercomprises: means for receiving, by the handover target access node from the handover source access node, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and means for receiving, by the handover target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0273] Clause 27. The apparatus of clause 17, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node. The apparatus further comprises: means for receiving, by the handover target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0274] Clause 28. The apparatus of clause 17, further comprising: means for receiving, by the handover target access node from the handover source access node, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; means for transmitting, by the handover target access node to the handover source access node, the user plane tunnel information indicating at least one user plane tunnel to the handover target access node in association with the information on the data radio bearer of the user equipment; and means for receiving, by the handover target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0275] Clause 29. An apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: receive, by a user equipment from a handover source access node, a message in association with a handover comprising a cellspecific baseline configuration for a handover of the user equipment to a target cell provided by a handover target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; initiate a random-access procedure to the target cell to establish a radio resource control connection between the user equipment and the handover target access node, based on the cell-specific baseline configuration; and transmit, by the user equipment to the handover target access node, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration, during the randomaccess procedure.

[0276] Clause 30. The apparatus of clause 29, wherein the default signaling radio bearerconfiguration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and wherein the reconfiguration complete message is transmitted on the default signaling radio bearer 1.

[0277] Clause 31. The apparatus of clause 29, wherein the cell-specific baseline configuration further comprises an indication of key update, and security information for generating one or more access stratum security keys for communication with the target cell, and the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: carry out, by the user equipment in response to the indication of key update, the generating of the one or more access stratum security keys for communication with the target cell based on the security information, the one or more access stratum security keys comprising a key for cyphering of radio resource control signaling; and cypher the reconfiguration complete message with the key for cyphering of radio resource control signaling before transmitting the reconfiguration complete message to the handover target access node.

[0278] Clause 32. The apparatus of clause 29, wherein the reconfiguration complete message comprises a short message authentication code to identify the user equipment.

[0279] Clause 33. The apparatus of clause 29, wherein the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: receive, by the user equipment from the handover target access node, a supplementary configuration for the handover of the user equipment to the target cell; or initiate a radio resource control connection re-establishment procedure to the target cell in the case the supplementary configuration is not received within a predefined time window from transmitting the reconfiguration complete message.

[0280] Clause 34. The apparatus of clause 33, wherein the supplementary configuration is comprised in a message received from the handover target access node after transmitting the reconfiguration complete message, or in the case the cell-specific baseline configuration received from the handover source access node comprises a pair of a random-access preamble and a user equipment identifier for use in the handover, the supplementary configuration is comprised in a random-access response message received from the handover target access node as a part of the random access procedure, or in a message scheduled based on the random access response message.

[0281] Clause 35. An apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: receive, by a handover source access node from a handover target access node, a cell-specific baseline configuration for a handover to a target cell provided by the handover target access node, the cell-specific baselineconfiguration comprising a default signaling radio bearer configuration; receive, by the handover source access node from a user equipment, information on radio link quality in association with a source cell provided by the handover source access node and the target cell; determine, by the handover source access node, to carry out the handover of the user equipment to the target cell using the cell-specific baseline configuration based on at least the information on radio link quality in association with the source cell and the target cell; and transmit, by the handover source access node to the user equipment, a message in association with the handover of the user equipment to the target cell comprising at least part of the cell-specific baseline configuration.

[0282] Clause 36. The apparatus of clause 35, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2.

[0283] Clause 37. The apparatus of clause 35, wherein the apparatus is caused to determine to carry out the handover of the user equipment to the target cell using the cell-specific baseline configuration based on the information on radio link quality in association with the source cell and the target cell and at least one of service delay requirements of the user equipment; service reliability requirements of the user equipment; load information of the target cell; traffic amount of the user equipment; capability of the user equipment; and capability of the target cell.

[0284] Clause 38. The apparatus of clause 35, wherein the message in association with the handover of the user equipment further comprises an indication of key update, and security information for the user equipment to generate one or more access stratum security keys for communication with the target cell.

[0285] Clause 39. The apparatus of clause 35, wherein the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: transmit, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, a handover notification message comprising an identifier for identifying the user equipment, information on the user equipment, an access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0286] Clause 40. The apparatus of clause 35, wherein the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: receive, by the handover source access node from the handover target access node, in response to the message in association with the handover, a request for information on the user equipment; and transmit, by the handover source access node to the handover target access node, a response message comprising the information on the user equipment, an access stratum security master key forcommunication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0287] Clause 41. The apparatus of clause 40, wherein the message in association with the handover provides an identifier assigned to the user equipment, the request for information on the user equipment comprises the identifier assigned to the user equipment, and a cyphered message, and the response message is transmitted to the handover target access node when the handover source access node verifies that the cyphered message is cyphered by the user equipment.

[0288] Clause 42. The apparatus of clause 35, wherein the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: in case the cellspecific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node, transmit, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and forward, by the handover source access node to the handover target access node, user data for the user equipment using the at least one user plane tunnel to the handover target access node, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0289] Clause 43. The apparatus of clause 35, wherein the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: in case the cellspecific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node, forward, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, user data for the user equipment using the at least one user plane tunnel to the handover target access node indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0290] Clause 44. The apparatus of clause 39, wherein the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: transmit, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; receive, by the handover source access node from the handover target access node, the user planetunnel information indicating at least one user plane tunnel to the handover target access node in association with the information on the data radio bearer of the user equipment; and forward, by the handover source access node to the handover target access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0291] Clause 45. An apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: transmit, by a handover target access node to a handover source access node, a cell-specific baseline configuration for a handover from the handover source access node to a target cell provided by the handover target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receive, by the handover target access node from the handover source access node, information on a user equipment and an access stratum security master key for communication between the user equipment and the handover target access node; receive, by the handover target access node from the user equipment, a random-access preamble; transmit, by the handover target access node to the user equipment, a random-access response message; receive, by the handover target access node from the user equipment, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration; generate, by the handover target access node, a supplementary configuration for the handover of the user equipment to the target cell, based on the information on the user equipment; and transmit, by the handover target access node to the user equipment, the supplementary configuration.

[0292] Clause 46. The apparatus of clause 45, wherein the default signaling radio bearer configuration comprises at least configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and the reconfiguration complete message is received on the default signaling radio bearer 1.

[0293] Clause 47. The apparatus of clause 45, wherein the reconfiguration complete message is cyphered with an access stratum security key for cyphering of radio resource control signaling derived from the access stratum security master key.

[0294] Clause 48. The apparatus of clause 45, wherein the reconfiguration complete message comprises a short message authentication code to identify the user equipment.

[0295] Clause 49. The apparatus of clause 45, wherein the information on the user equipment and the access stratum security master key are comprised in a handover notification message received from the handover source access node, and the handover notification message further comprises a parameter for counting next hop chaining in association with the access stratum security master key.

[0296] Clause 50. The apparatus of clause 45, wherein the apparatus being caused to receivethe information on the user equipment and the access stratum security master key comprises the apparatus being caused to: carry out, by the handover target access node, a radio resource control connection re-establishment procedure initiated by the user equipment to receive, from the handover source access node, the information on the user equipment, the access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0297] Clause 51. The apparatus of clause 45, wherein the apparatus being caused to receive the user equipment information and the access stratum security master key comprises the apparatus being caused to: transmit, by the handover target access node to the handover source access node, a request for the information on the user equipment within a predefined time window from receiving the random-access preamble or the reconfiguration complete message from the user equipment; and receive, by the handover target access node from the handover source access node, a response message comprising the information on the user equipment, the access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0298] Clause 52. The apparatus of clause 51, wherein the supplementary configuration is comprised in a message transmitted to the user equipment after receiving the reconfiguration complete message, or the supplementary configuration is comprised in the random-access response message, or in a message scheduled based on the random-access response message.

[0299] Clause 53. The apparatus of clause 51, wherein the reconfiguration complete message received from the user equipment is cyphered with a key derived from the access stratum security master key, and the user equipment information request transmitted to the handover source access node comprises an identifier assigned to the user equipment and the reconfiguration complete message as cyphered.

[0300] Clause 54. The apparatus of clause 45, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node, and the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: receive, by the handover target access node from the handover source access node, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and receive, by the handover target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel, based on the mapping of the at least one user plane tunnel to the information on the dataradio bearer of the user equipment.

[0301] Clause 55. The apparatus of clause 45, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node, and the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: receive, by the handover target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0302] Clause 56. The apparatus of clause 45, wherein the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: receive, by the handover target access node from the handover source access node, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; transmit, by the handover target access node to the handover source access node, the user plane tunnel information indicating at least one user plane tunnel to the handover target access node in association with the information on the data radio bearer of the user equipment; and receive, by the handover target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0303] Clause 57. A method, comprising: receiving, by a user equipment from a handover source access node, a message in association with a handover comprising a cell-specific baseline configuration for a handover of the user equipment to a target cell provided by a handover target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; initiating a random-access procedure to the target cell to establish a radio resource control connection between the user equipment and the handover target access node, based on the cell-specific baseline configuration; and transmitting, by the user equipment to the handover target access node, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration, during the random-access procedure.

[0304] Clause 58. The method of clause 57, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and wherein the reconfiguration complete message is transmitted on the default signaling radio bearer 1.

[0305] Clause 59. The method of clause 57, wherein the cell-specific baseline configuration further comprises an indication of key update, and security information for generating one or more access stratum security keys for communication with the target cell, and the method furthercomprises: carrying out, by the user equipment in response to the indication of key update, the generating of the one or more access stratum security keys for communication with the target cell based on the security information, the one or more access stratum security keys comprising a key for cyphering of radio resource control signaling; and cyphering the reconfiguration complete message with the key for cyphering of radio resource control signaling before transmitting the reconfiguration complete message to the handover target access node.

[0306] Clause 60. The method of clause 57, further comprising: receiving, by the user equipment from the handover target access node, a supplementary configuration for the handover of the user equipment to the target cell; or initiating a radio resource control connection reestablishment procedure to the target cell in the case the supplementary configuration is not received within a predefined time window from transmitting the reconfiguration complete message.

[0307] Clause 61. The method of clause 60, wherein the supplementary configuration is comprised in a message received from the handover target access node after transmitting the reconfiguration complete message, or in the case the cell-specific baseline configuration received from the handover source access node comprises a pair of a random-access preamble and a user equipment identifier for use in the handover, the supplementary configuration is comprised in a random-access response message received from the handover target access node as a part of the random access procedure, or in a message scheduled based on the random access response message.

[0308] Clause 62. A method, comprising: receiving, by a handover source access node from a handover target access node, a cell-specific baseline configuration for a handover to a target cell provided by the handover target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receiving, by the handover source access node from a user equipment, information on radio link quality in association with a source cell provided by the handover source access node and the target cell; determining, by the handover source access node, to carry out the handover of the user equipment to the target cell using the cell-specific baseline configuration based on at least the information on radio link quality in association with the source cell and the target cell; and transmitting, by the handover source access node to the user equipment, a message in association with the handover of the user equipment to the target cell comprising at least part of the cell-specific baseline configuration.

[0309] Clause 63. The method of clause 62, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2.

[0310] Clause 64. The method of clause 62, wherein the message in association with thehandover of the user equipment further comprises an indication of key update, and security information for the user equipment to generate one or more access stratum security keys for communication with the target cell.

[0311] Clause 65. The method of clause 62, further comprising: transmitting, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, a handover notification message comprising an identifier for identifying the user equipment, information on the user equipment, an access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0312] Clause 66. The method of clause 62, further comprising: receiving, by the handover source access node from the handover target access node, in response to the message in association with the handover, a request for information on the user equipment; and transmitting, by the handover source access node to the handover target access node, a response message comprising the information on the user equipment, an access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0313] Clause 67. The method of clause 66, wherein the message in association with the handover provides an identifier assigned to the user equipment, the request for information on the user equipment comprises the identifier assigned to the user equipment, and a cyphered message, and the response message is transmitted to the handover target access node when the handover source access node verifies that the cyphered message is cyphered by the user equipment.

[0314] Clause 68. The method of clause 62, further comprising: in case the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node, transmitting, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and forwarding, by the handover source access node to the handover target access node, user data for the user equipment using the at least one user plane tunnel to the handover target access node, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0315] Clause 69. The method of clause 62, further comprising: in case the cell-specificbaseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node, forwarding, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, user data for the user equipment using the at least one user plane tunnel to the handover target access node indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0316] Clause 70. The method of clause 65, further comprising: transmitting, by the handover source access node to the handover target access node, in response to determining to carry out the handover of the user equipment to the target cell, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; receiving, by the handover source access node from the handover target access node, the user plane tunnel information indicating at least one user plane tunnel to the handover target access node in association with the information on the data radio bearer of the user equipment; and forwarding, by the handover source access node to the handover target access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0317] Clause 71. A method, comprising: transmitting, by a handover target access node to a handover source access node, a cell-specific baseline configuration for a handover from the handover source access node to a target cell provided by the handover target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receiving, by the handover target access node from the handover source access node, information on a user equipment and an access stratum security master key for communication between the user equipment and the handover target access node; receiving, by the handover target access node from the user equipment, a random-access preamble; transmitting, by the handover target access node to the user equipment, a random-access response message; receiving, by the handover target access node from the user equipment, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration; generating, by the handover target access node, a supplementary configuration for the handover of the user equipment to the target cell, based on the information on the user equipment; and transmitting, by the handover target access node to the user equipment, the supplementary configuration.

[0318] Clause 72. The method of clause 71, wherein the default signaling radio bearer configuration comprises at least configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and the reconfiguration complete message is received on the default signaling radio bearer 1.

[0319] Clause 73. The method of clause 71, wherein the reconfiguration complete message iscyphered with an access stratum security key for cyphering of radio resource control signaling derived from the access stratum security master key.

[0320] Clause 74. The method of clause 71, wherein the information on the user equipment and the access stratum security master key are comprised in a handover notification message received from the handover source access node, and the handover notification message further comprises a parameter for counting next hop chaining in association with the access stratum security master key.

[0321] Clause 75. The method of clause 71, wherein the receiving information on the user equipment comprises: carrying out, by the handover target access node, a radio resource control connection re-establishment procedure initiated by the user equipment to receive, from the handover source access node, the information on the user equipment, the access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0322] Clause 76. The method of clause 71, wherein the apparatus being caused to receive the user equipment information and the access stratum security master key comprises the apparatus being caused to: transmit, by the handover target access node to the handover source access node, a request for the information on the user equipment within a predefined time window from receiving the random-access preamble or the reconfiguration complete message from the user equipment; and receive, by the handover target access node from the handover source access node, a response message comprising the information on the user equipment, the access stratum security master key for communication between the user equipment and the handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0323] Clause 77. The method of clause 76, wherein the supplementary configuration is comprised in a message transmitted to the user equipment after receiving the reconfiguration complete message, or the supplementary configuration is comprised in the random-access response message, or in a message scheduled based on the random-access response message.

[0324] Clause 78. The method of clause 76, wherein the reconfiguration complete message received from the user equipment is cyphered with a key derived from the access stratum security master key, and the user equipment information request transmitted to the handover source access node comprises an identifier assigned to the user equipment and the reconfiguration complete message as cyphered.

[0325] Clause 79. The method of clause 71, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel informationindicating at least one user plane tunnel to the handover target access node, and the method further comprises: receiving, by the handover target access node from the handover source access node, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and receiving, by the handover target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0326] Clause 80. The method of clause 71, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover target access node, and the method further comprises: receiving, by the handover target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0327] Clause 81. The method of clause 71, further comprising: receiving, by the handover target access node from the handover source access node, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; transmitting, by the handover target access node to the handover source access node, the user plane tunnel information indicating at least one user plane tunnel to the handover target access node in association with the information on the data radio bearer of the user equipment; and receiving, by the handover target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0328] Clause 82. A computer-readable medium comprising instructions that, in response to execution by at least one processing circuitry, cause an apparatus to perform the method of any of clauses 57-81.

[0329] Clause 83. A computer program comprising instructions that, in response to execution by at least one processing circuitry, cause an apparatus to perform the method of any of clauses 57-81.

[0330] Clause 84. An apparatus, comprising: means for receiving, by a user equipment from a handover source access node, a message in association with a conditional handover comprising one or more cell-specific baseline configurations and one or more execution conditions for a conditional handover of the user equipment to one or more candidate target cells provided by one or more handover candidate target access nodes, the one or more cell-specific baseline configurations comprising a default signaling radio bearer configuration; means for determining,by the user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more candidate target cells; means for selecting, by the user equipment, at least one target cell from the one or more candidate target cells based on the one or more execution conditions and the determined information on radio link quality in association with the source cell and the at least one of the one or more candidate target cells; means for initiating a random-access procedure to the selected at least one target cell to establish a radio resource control connection between the user equipment and at least one handover target access node providing the at least one target cell, based on the cell-specific baseline configuration; and means for transmitting, by the user equipment to the at least one handover target access node, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration, during the random-access procedure.

[0331] Clause 85. The apparatus of clause 84, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and the reconfiguration complete message is transmitted on the default signaling radio bearer 1.

[0332] Clause 86. The apparatus of clause 84, wherein the cell-specific baseline configuration further comprises an indication of key update, and security information for generating one or more access stratum security keys for communication with the at least one candidate target cell, and the apparatus further comprises: means for carrying out, in response to the indication of the key update, the generating of the one or more access stratum security keys for the communication with the at least one target cell, the one or more access stratum security keys comprising a key for cyphering of radio resource control signaling; and means for cyphering the reconfiguration complete message with the key for cyphering of the radio resource control signaling for the transmitting of the reconfiguration complete message to the at least one handover target access node.

[0333] Clause 87. The apparatus of clause 84, wherein the reconfiguration complete message comprises a short message authentication code to identify the user equipment.

[0334] Clause 88. The apparatus of clause 84, further comprising: receiving, by the user equipment from the at least one handover target access node, a supplementary configuration for the handover of the user equipment to the at least one target cell; or initiating a radio resource control connection re-establishment procedure to the at least one target cell in the case the supplementary configuration is not received within a predefined time window from transmitting the reconfiguration complete message.

[0335] Clause 89. The apparatus of clause 88, wherein the supplementary configuration iscomprised in a message received from the at least one handover target access node after transmitting the reconfiguration complete message, or in the case the cell-specific baseline configuration received from the handover source access node comprises a pair of a random access preamble and a user equipment identifier for use in the conditional handover, the supplementary configuration is comprised in a random-access response message received from the at least one handover target access node as a part of the random-access procedure, or in a message scheduled based on the random-access response message.

[0336] Clause 90. An apparatus, comprising: means for receiving, by a handover source access node from one or more neighboring access nodes, one or more cell-specific baseline configurations for a conditional handover to one or more neighboring cells provided by the one or more neighboring access nodes, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; means for receiving, by the handover source access node from a user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more neighboring cells; means for determining, by the handover source access node, to carry out the conditional handover of the user equipment to one or more candidate target cells selected from the one or more neighboring cells, based on at least the received information on radio link quality, the one or more candidate target cells being provided by one or more handover candidate target access nodes; and means for transmitting, by the handover source access node to the user equipment, a message in association with the conditional handover of the user equipment to the one or more candidate target cells, the message comprising at least part of the one or more cell-specific baseline configurations in association with the one or more candidate target cells.

[0337] Clause 91. The apparatus of clause 90, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2.

[0338] Clause 92. The apparatus of clause 90, wherein the conditional handover of the user equipment to the one or more candidate target cells selected from the one or more neighboring cells is determined based on the information on radio link quality and at least one of: service delay requirements of the user equipment; service reliability requirements of the user equipment; load information of the one or more candidate target cells; traffic amount of the user equipment; capability of the user equipment; and capability of the one or more candidate target cells.

[0339] Clause 93. The apparatus of clause 90, wherein the message in association with the conditional handover further comprises an indication of key update, and security information for the user equipment to generate one or more access stratum security keys for communication with the candidate target cell.

[0340] Clause 94. The apparatus of clause 90, further comprising: means for transmitting, by the handover source access node to the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment to the one or more candidate target cells, handover information in association with the user equipment comprising an identifier for identifying the user equipment, information on the user equipment, an access stratum security master key for communication between the user equipment and the handover candidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0341] Clause 95. The apparatus of clause 90, further comprising: receiving, by the handover source access node from at least one handover target access node of the one or more handover candidate target access nodes, in response to the transmitting of the message in association with the conditional handover, a request for information on the user equipment; and transmitting, by the handover source access node to the at least one handover target access node, a response message comprising information on the user equipment, an access stratum security master key for communication between the user equipment and the at least one handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0342] Clause 96. The apparatus of clause 95, wherein the message in association with the conditional handover provides an identifier assigned to the user equipment, the request for the information on the user equipment comprises the identifier assigned to the user equipment, and a cyphered message, and the response message is transmitted to the at least one handover target access node when the handover source access node verifies that the cyphered message is cyphered by the user equipment.

[0343] Clause 97. The apparatus of clause 90, wherein the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node. The apparatus further comprises: means for transmitting, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and means for forwarding, by the handover source access node to the at least one handover candidate target access node, user data for the user equipment using the at least one user plane tunnel to the at least one handover candidate target access node, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0344] Clause 98. The apparatus of clause 90, wherein the cell-specific baseline configurationfurther comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node. The apparatus further comprises: means for forwarding, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, user data for the user equipment using the at least one user plane tunnel to the at least one handover candidate target access node indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0345] Clause 99. The apparatus of clause 94, further comprising: means for transmitting, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; means for receiving, by the handover source access node from the at least one handover candidate target access node, the user plane tunnel information indicating at least one user plane tunnel in association with the information on the data radio bearer of the user equipment; and means for forwarding, by the handover source access node to the at least one handover candidate target access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0346] Clause 100. An apparatus, comprising: means for transmitting, by a handover candidate target access node to a handover source access node, one or more cell-specific baseline configurations for a conditional handover from the handover source access node to one or more candidate target cells provided by the handover candidate target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; means for receiving, by the handover candidate target access node from the handover source access node, information on a user equipment and an access stratum security master key for communication between the user equipment and the handover candidate target access node; means for receiving, by the handover candidate target access node from the user equipment, a random-access preamble to a target cell selected from the one or more candidate target cells; means for transmitting, by the handover candidate target access node to the user equipment, a randomaccess response message; means for receiving, by the handover candidate target access node from the user equipment, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration; means for generating, by the handover candidate target access node, a supplementary configuration for the conditional handover of the user equipment to the target cell, based on the information on the user equipment; and means for transmitting, by the handover candidate target access node to the user equipment,the supplementary configuration.

[0347] Clause 101. The apparatus of clause 100, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and the reconfiguration complete message is received on the default signaling radio bearer 1.

[0348] Clause 102. The apparatus of clause 100, wherein the reconfiguration complete message is cyphered with an access stratum security key for cyphering of radio resource control signaling.

[0349] Clause 103. The apparatus of clause 100, wherein the reconfiguration complete message comprises a short message authentication code to identify the user equipment.

[0350] Clause 104. The apparatus of clause 100, wherein the information on the user equipment and the access stratum security master key are comprised in a conditional handover notification message received from the handover source access node, and the conditional handover notification message further comprises a parameter for counting next hop chaining in association with the access stratum security master key.

[0351] Clause 105. The apparatus of clause 100, wherein means for receiving the information on the user equipment and the access stratum security master key comprises: means for carrying out, by the handover candidate target access node, a radio resource control connection reestablishment procedure initiated by the user equipment to receive, from the handover source access node, the information on the user equipment, the access stratum security master key for communication between the user equipment and the handover candidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0352] Clause 106. The apparatus of clause 100, wherein means for receiving the information on the user equipment and the access stratum security master key comprises: means for transmitting, by the handover candidate target access node to the handover source access node, a request for the information on the user equipment within a predefined time window from receiving the random-access preamble or the reconfiguration complete message from the user equipment; and means for receiving, by the handover candidate target access node from the handover source access node, a response message comprising the information on the user equipment, the access stratum security master key for communication between the user equipment and the handover candidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0353] Clause 107. The apparatus of clause 106, wherein the supplementary configuration is comprised in a message transmitted to the user equipment after receiving the reconfigurationcomplete message, or the supplementary configuration is comprised in the random-access response message or in a message scheduled based on the random-access response message.

[0354] Clause 108. The apparatus of clause 106, wherein the reconfiguration complete message received from the user equipment is cyphered with a key derived from the access stratum security master key, and the request for the information on the user equipment transmitted to the handover source access node comprises an identifier assigned to the user equipment and the reconfiguration complete message as cyphered.

[0355] Clause 109. The apparatus of clause 100, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node. The apparatus further comprises: means for receiving, by the handover candidate target access node from the handover source access node, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and means for receiving, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0356] Clause 110. The apparatus of clause 100, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node. The apparatus further comprises: means for receiving, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0357] Clause 111. The apparatus of clause 100, further comprising: means for receiving, by the handover candidate target access node from the handover source access node, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; transmitting, by the handover candidate target access node to the handover source access node, the user plane tunnel information indicating at least one user plane tunnel in association with the information on the data radio bearer of the user equipment; and means for receiving, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0358] Clause 112. An apparatus, comprising: at least one memory configured to storeinstructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: receive, by a user equipment from a handover source access node, a message in association with a conditional handover comprising one or more cell-specific baseline configurations and one or more execution conditions for a conditional handover of the user equipment to one or more candidate target cells provided by one or more handover candidate target access nodes, the one or more cell-specific baseline configurations comprising a default signaling radio bearer configuration; determine, by the user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more candidate target cells; select, by the user equipment, at least one target cell from the one or more candidate target cells based on the one or more execution conditions and the determined information on radio link quality in association with the source cell and the at least one of the one or more candidate target cells; initiate a random-access procedure to the selected at least one target cell to establish a radio resource control connection between the user equipment and at least one handover target access node providing the at least one target cell, based on the cell-specific baseline configuration; and transmit, by the user equipment to the at least one handover target access node, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration, during the random-access procedure.

[0359] Clause 113. The apparatus of clause 112, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and the reconfiguration complete message is transmitted on the default signaling radio bearer 1.

[0360] Clause 114. The apparatus of clause 112, wherein the cell-specific baseline configuration further comprises an indication of key update, and security information for generating one or more access stratum security keys for communication with the at least one candidate target cell, and the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: carry out the, in response to the indication of the key update, generating of the one or more access stratum security keys for the communication with the at least one target cell, the one or more access stratum security keys comprising a key for cyphering of radio resource control signaling; and cypher the reconfiguration complete message with the key for cyphering of the radio resource control signaling for the transmitting of the reconfiguration complete message to the at least one handover target access node.

[0361] Clause 115. The apparatus of clause 112, wherein the reconfiguration complete message comprises a short message authentication code to identify the user equipment.

[0362] Clause 116. The apparatus of clause 112, wherein the at least one processing circuitryis further configured to execute the instructions to cause the apparatus to at least: receive, by the user equipment from the at least one handover target access node, a supplementary configuration for the handover of the user equipment to the at least one target cell; or initiate a radio resource control connection re-establishment procedure to the at least one target cell in the case the supplementary configuration is not received within a predefined time window from transmitting the reconfiguration complete message.

[0363] Clause 117. The apparatus of clause 116, wherein the supplementary configuration is comprised in a message received from the at least one handover target access node after transmitting the reconfiguration complete message, or in the case the cell-specific baseline configuration received from the handover source access node comprises a pair of a random access preamble and a user equipment identifier for use in the conditional handover, the supplementary configuration is comprised in a random-access response message received from the at least one handover target access node as a part of the random-access procedure, or in a message scheduled based on the random-access response message.

[0364] Clause 118. An apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: receive, by a handover source access node from one or more neighboring access nodes, one or more cell-specific baseline configurations for a conditional handover to one or more neighboring cells provided by the one or more neighboring access nodes, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receive, by the handover source access node from a user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more neighboring cells; determine, by the handover source access node, to carry out the conditional handover of the user equipment to one or more candidate target cells selected from the one or more neighboring cells, based on at least the received information on radio link quality, the one or more candidate target cells being provided by one or more handover candidate target access nodes; and transmit, by the handover source access node to the user equipment, a message in association with the conditional handover of the user equipment to the one or more candidate target cells, the message comprising at least part of the one or more cell-specific baseline configurations in association with the one or more candidate target cells.

[0365] Clause 119. The apparatus of clause 118, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2.

[0366] Clause 120. The apparatus of clause 118, wherein the apparatus is caused to determineto carry out the conditional handover of the user equipment to the one or more candidate target cells selected from the one or more neighboring cells based on the information on radio link quality and at least one of: service delay requirements of the user equipment; service reliability requirements of the user equipment; load information of the one or more candidate target cells; traffic amount of the user equipment; capability of the user equipment; and capability of the one or more candidate target cells.

[0367] Clause 121. The apparatus of clause 118, wherein the message in association with the conditional handover further comprises an indication of key update, and security information for the user equipment to generate one or more access stratum security keys for communication with the candidate target cell.

[0368] Clause 122. The apparatus of clause 118, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: transmit, by the handover source access node to the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment to the one or more candidate target cells, handover information in association with the user equipment comprising an identifier for identifying the user equipment, information on the user equipment, an access stratum security master key for communication between the user equipment and the handover candidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0369] Clause 123. The apparatus of clause 118, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: receive, by the handover source access node from at least one handover target access node of the one or more handover candidate target access nodes, in response to the transmitting of the message in association with the conditional handover, a request for information on the user equipment; and transmit, by the handover source access node to the at least one handover target access node, a response message comprising information on the user equipment, an access stratum security master key for communication between the user equipment and the at least one handover target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0370] Clause 124. The apparatus of clause 123, wherein the message in association with the conditional handover provides an identifier assigned to the user equipment, the request for the information on the user equipment comprises the identifier assigned to the user equipment, and a cyphered message, and the response message is transmitted to the at least one handover target access node when the handover source access node verifies that the cyphered message is cyphered by the user equipment.

[0371] Clause 125. The apparatus of clause 118, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: in case the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, transmit, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and forward, by the handover source access node to the at least one handover candidate target access node, user data for the user equipment using the at least one user plane tunnel to the at least one handover candidate target access node, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0372] Clause 126. The apparatus of clause 118, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: in case the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, forward, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, user data for the user equipment using the at least one user plane tunnel to the at least one handover candidate target access node indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0373] Clause 127. The apparatus of clause 122, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: transmit, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; receive, by the handover source access node from the at least one handover candidate target access node, the user plane tunnel information indicating at least one user plane tunnel in association with the information on the data radio bearer of the user equipment; and forward, by the handover source access node to the at least one handover candidate target access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0374] Clause 128. An apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: transmit, by a handover candidatetarget access node to a handover source access node, one or more cell-specific baseline configurations for a conditional handover from the handover source access node to one or more candidate target cells provided by the handover candidate target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receive, by the handover candidate target access node from the handover source access node, information on a user equipment and an access stratum security master key for communication between the user equipment and the handover candidate target access node; receive, by the handover candidate target access node from the user equipment, a random-access preamble to a target cell selected from the one or more candidate target cells; transmit, to the user equipment, a random-access response message; receive, by the handover candidate target access node from the user equipment, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration; generate, by the handover candidate target access node, a supplementary configuration for the conditional handover of the user equipment to the target cell, based on the information on the user equipment; and transmit, by the handover candidate target access node to the user equipment, the supplementary configuration.

[0375] Clause 129. The apparatus of clause 128, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and the reconfiguration complete message is received on the default signaling radio bearer 1.

[0376] Clause 130. The apparatus of clause 128, wherein the reconfiguration complete message is cyphered with an access stratum security key for cyphering of radio resource control signaling.

[0377] Clause 131. The apparatus of clause 128, wherein the reconfiguration complete message comprises a short message authentication code to identify the user equipment.

[0378] Clause 132. The apparatus of clause 128, wherein the information on the user equipment and the access stratum security master key are comprised in a conditional handover notification message received from the handover source access node, and the conditional handover notification message further comprises a parameter for counting next hop chaining in association with the access stratum security master key.

[0379] Clause 133. The apparatus of clause 128, wherein the apparatus being caused to receive the user equipment information and the access stratum security master key comprises the apparatus being caused to: carry out, by the handover candidate target access node, a radio resource control connection re-establishment procedure initiated by the user equipment to receive, from the handover source access node, the information on the user equipment, the access stratum security master key for communication between the user equipment and the handovercandidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0380] Clause 134. The apparatus of clause 128, wherein the apparatus being caused to receive the user equipment information and the access stratum security master key comprises the apparatus being caused to: transmit, by the handover candidate target access node to the handover source access node, a request for the information on the user equipment within a predefined time window from receiving the random-access preamble or the reconfiguration complete message from the user equipment; and receive, by the handover candidate target access node from the handover source access node, a response message comprising the information on the user equipment, the access stratum security master key for communication between the user equipment and the handover candidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0381] Clause 135. The apparatus of clause 134, wherein the supplementary configuration is comprised in a message transmitted to the user equipment after receiving the reconfiguration complete message, or the supplementary configuration is comprised in the random-access response message or in a message scheduled based on the random-access response message.

[0382] Clause 136. The apparatus of clause 134, wherein the reconfiguration complete message received from the user equipment is cyphered with a key derived from the access stratum security master key, and the request for the information on the user equipment transmitted to the handover source access node comprises an identifier assigned to the user equipment and the reconfiguration complete message as cyphered.

[0383] Clause 137. The apparatus of clause 128, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, and the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: receive, by the handover candidate target access node from the handover source access node, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and receive, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0384] Clause 138. The apparatus of clause 128, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, and the at least one processing circuitry is configured to execute the instructions to cause theapparatus to at least: receive, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0385] Clause 139. The apparatus of clause 128, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: receive, by the handover candidate target access node from the handover source access node, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; transmit, by the handover candidate target access node to the handover source access node, the user plane tunnel information indicating at least one user plane tunnel in association with the information on the data radio bearer of the user equipment; and receive, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0386] Clause 140. A method, comprising: receiving, by a user equipment from a handover source access node, a message in association with a conditional handover comprising one or more cell-specific baseline configurations and one or more execution conditions for a conditional handover of the user equipment to one or more candidate target cells provided by one or more handover candidate target access nodes, the one or more cell-specific baseline configurations comprising a default signaling radio bearer configuration; determining, by the user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more candidate target cells; selecting, by the user equipment, at least one target cell from the one or more candidate target cells based on the one or more execution conditions and the determined information on radio link quality in association with the source cell and the at least one of the one or more candidate target cells; initiating a random-access procedure to the selected at least one target cell to establish a radio resource control connection between the user equipment and at least one handover target access node providing the at least one target cell, based on the cell-specific baseline configuration; and transmitting, by the user equipment to the at least one handover target access node, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration, during the random-access procedure.

[0387] Clause 141. The method of clause 140, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and thereconfiguration complete message is transmitted on the default signaling radio bearer 1.

[0388] Clause 142. The method of clause 140, wherein the cell-specific baseline configuration further comprises an indication of key update, and security information for generating one or more access stratum security keys for communication with the at least one candidate target cell, and the method further comprises: carrying out the, in response to the indication of the key update, generating of the one or more access stratum security keys for the communication with the at least one target cell, the one or more access stratum security keys comprising a key for cyphering of radio resource control signaling; and cyphering the reconfiguration complete message with the key for cyphering of the radio resource control signaling for the transmitting of the reconfiguration complete message to the at least one handover target access node.

[0389] Clause 143. The method of clause 140, wherein the reconfiguration complete message comprises a short message authentication code to identify the user equipment.

[0390] Clause 144. The method of clause 140, further comprising: receiving, by the user equipment from the at least one handover target access node, a supplementary configuration for the handover of the user equipment to the at least one target cell; or initiating a radio resource control connection re-establishment procedure to the at least one target cell in the case the supplementary configuration is not received within a predefined time window from transmitting the reconfiguration complete message.

[0391] Clause 145. The method of clause 144, wherein the supplementary configuration is comprised in a message received from the at least one handover target access node after transmitting the reconfiguration complete message, or in the case the cell-specific baseline configuration received from the handover source access node comprises a pair of a random access preamble and a user equipment identifier for use in the conditional handover, the supplementary configuration is comprised in a random-access response message received from the at least one handover target access node as a part of the random-access procedure, or in a message scheduled based on the random-access response message.

[0392] Clause 146. A method, comprising: receiving, by a handover source access node from one or more neighboring access nodes, one or more cell-specific baseline configurations for a conditional handover to one or more neighboring cells provided by the one or more neighboring access nodes, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receiving, by the handover source access node from a user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more neighboring cells; determining, by the handover source access node, to carry out the conditional handover of the user equipment to one or more candidate target cells selected from the one or more neighboring cells, based on at least the receivedinformation on radio link quality, the one or more candidate target cells being provided by one or more handover candidate target access nodes; and transmitting, by the handover source access node to the user equipment, a message in association with the conditional handover of the user equipment to the one or more candidate target cells, the message comprising at least part of the one or more cell-specific baseline configurations in association with the one or more candidate target cells.

[0393] Clause 147. The method of clause 146, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2.

[0394] Clause 148. The method of clause 146, wherein the conditional handover of the user equipment to the one or more candidate target cells selected from the one or more neighboring cells is determined based on the information on radio link quality and at least one of: service delay requirements of the user equipment; service reliability requirements of the user equipment; load information of the one or more candidate target cells; traffic amount of the user equipment; capability of the user equipment; and capability of the one or more candidate target cells.

[0395] Clause 149. The method of clause 146, wherein the message in association with the conditional handover further comprises an indication of key update, and security information for the user equipment to generate one or more access stratum security keys for communication with the candidate target cell.

[0396] Clause 150. The method of clause 146, further comprising: transmitting, by the handover source access node to the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment to the one or more candidate target cells, handover information in association with the user equipment comprising an identifier for identifying the user equipment, information on the user equipment, an access stratum security master key for communication between the user equipment and the handover candidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0397] Clause 151. The method of clause 146, further comprising: receiving, by the handover source access node from at least one handover target access node of the one or more handover candidate target access nodes, in response to the transmitting of the message in association with the conditional handover, a request for information on the user equipment; and transmitting, by the handover source access node to the at least one handover target access node, a response message comprising information on the user equipment, an access stratum security master key for communication between the user equipment and the at least one handover target access node, and a parameter for counting next hop chaining in association with the access stratum securitymaster key.

[0398] Clause 152. The method of clause 151, wherein the message in association with the conditional handover provides an identifier assigned to the user equipment, the request for the information on the user equipment comprises the identifier assigned to the user equipment, and a cyphered message, and the response message is transmitted to the at least one handover target access node when the handover source access node verifies that the cyphered message is cyphered by the user equipment.

[0399] Clause 153. The method of clause 146, further comprising: in case the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, transmitting, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and forwarding, by the handover source access node to the at least one handover candidate target access node, user data for the user equipment using the at least one user plane tunnel to the at least one handover candidate target access node, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0400] Clause 154. The method of clause 146, further comprising: in case the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, forwarding, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, user data for the user equipment using the at least one user plane tunnel to the at least one handover candidate target access node indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0401] Clause 155. The method of clause 150, further comprising: transmitting, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; receiving, by the handover source access node from the at least one handover candidate target access node, the user plane tunnel information indicating at least one user plane tunnel in association with the information on the data radio bearer of the user equipment; and forwarding, by the handover source access node to the at least one handovercandidate target access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0402] Clause 156. A method, comprising: transmitting, by a handover candidate target access node to a handover source access node, one or more cell-specific baseline configurations for a conditional handover from the handover source access node to one or more candidate target cells provided by the handover candidate target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receiving, by the handover candidate target access node from the handover source access node, information on a user equipment and an access stratum security master key for communication between the user equipment and the handover candidate target access node; receiving, by the handover candidate target access node from the user equipment, a random-access preamble to a target cell selected from the one or more candidate target cells; transmitting, to the user equipment, a random-access response message; receiving, by the handover candidate target access node from the user equipment, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration; generating, by the handover candidate target access node, a supplementary configuration for the conditional handover of the user equipment to the target cell, based on the information on the user equipment; and transmitting, by the handover candidate target access node to the user equipment, the supplementary configuration.

[0403] Clause 157. The method of clause 156, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and the reconfiguration complete message is received on the default signaling radio bearer 1.

[0404] Clause 158. The method of clause 156, wherein the reconfiguration complete message is cyphered with an access stratum security key for cyphering of radio resource control signaling.

[0405] Clause 159. The method of clause 156, wherein the reconfiguration complete message comprises a short message authentication code to identify the user equipment.

[0406] Clause 160. The method of clause 156, wherein the information on the user equipment and the access stratum security master key are comprised in a conditional handover notification message received from the handover source access node, and the conditional handover notification message further comprises a parameter for counting next hop chaining in association with the access stratum security master key.

[0407] Clause 161. The method of clause 156, wherein the receiving the information on the user equipment and the access stratum security master key comprises: carrying out, by the handover candidate target access node, a radio resource control connection re-establishment procedure initiated by the user equipment to receive, from the handover source access node, theinformation on the user equipment, the access stratum security master key for communication between the user equipment and the handover candidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0408] Clause 162. The method of clause 156, the receiving the information on the user equipment and the access stratum security master key comprises: transmitting, by the handover candidate target access node to the handover source access node, a request for the information on the user equipment within a predefined time window from receiving the random-access preamble or the reconfiguration complete message from the user equipment; and receiving, by the handover candidate target access node from the handover source access node, a response message comprising the information on the user equipment, the access stratum security master key for communication between the user equipment and the handover candidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

[0409] Clause 163. The method of clause 162, wherein the supplementary configuration is comprised in a message transmitted to the user equipment after receiving the reconfiguration complete message, or the supplementary configuration is comprised in the random-access response message or in a message scheduled based on the random-access response message.

[0410] Clause 164. The method of clause 162, wherein the reconfiguration complete message received from the user equipment is cyphered with a key derived from the access stratum security master key, and the request for the information on the user equipment transmitted to the handover source access node comprises an identifier assigned to the user equipment and the reconfiguration complete message as cyphered.

[0411] Clause 165. The method of clause 156, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, and the method further comprises: receiving, by the handover candidate target access node from the handover source access node, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and receiving, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

[0412] Clause 166. The method of clause 156, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, and the method further comprises: receiving, by the handover candidate target access node from thehandover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

[0413] Clause 167. The method of clause 156, further comprising: receiving, by the handover candidate target access node from the handover source access node, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; transmitting, by the handover candidate target access node to the handover source access node, the user plane tunnel information indicating at least one user plane tunnel in association with the information on the data radio bearer of the user equipment; and receiving, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

[0414] Clause 168. A computer-readable medium comprising instructions that, in response to execution by at least one processing circuitry, cause an apparatus to perform the method of any of clauses 140-167.

[0415] Clause 169. A computer program comprising instructions that, in response to execution by at least one processing circuitry, cause an apparatus to perform the method of any of clauses 140-167.

[0416] Further, while operations are depicted in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and para...

Claims

CLAIMS:

1. An apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: receive, by a user equipment from a handover source access node, a message in association with a conditional handover comprising one or more cell-specific baseline configurations and one or more execution conditions for a conditional handover of the user equipment to one or more candidate target cells provided by one or more handover candidate target access nodes, the one or more cell-specific baseline configurations comprising a default signaling radio bearer configuration; determine, by the user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more candidate target cells; select, by the user equipment, at least one target cell from the one or more candidate target cells based on the one or more execution conditions and the determined information on radio link quality in association with the source cell and the at least one of the one or more candidate target cells; initiate a random-access procedure to the selected at least one target cell to establish a radio resource control connection between the user equipment and at least one handover target access node providing the at least one target cell, based on the cell-specific baseline configuration; and transmit, by the user equipment to the at least one handover target access node during the random-access procedure, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration.

2. The apparatus of claim 1, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, andwherein the reconfiguration complete message is transmitted on the default signaling radio bearer 1.

3. The apparatus of claim 1, wherein the cell-specific baseline configuration further comprises an indication of key update, and security information for generating one or more access stratum security keys for communication with the at least one candidate target cell, and the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: carry out the, in response to the indication of the key update, generating of the one or more access stratum security keys for the communication with the at least one target cell , , the one or more access stratum security keys comprising a key for cyphering of radio resource control signaling; and cypher the reconfiguration complete message with the key for cyphering of the radio resource control signaling for the transmitting of the reconfiguration complete message to the at least one handover target access node.

4. The apparatus of claim 1, wherein the reconfiguration complete message comprises a short message authentication code to identify the user equipment.

5. The apparatus of claim 1, wherein the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: receive, by the user equipment from the at least one handover target access node, a supplementary configuration for the handover of the user equipment to the at least one target cell; or initiate a radio resource control connection re-establishment procedure to the at least one target cell in the case the supplementary configuration is not received within a predefined time window from transmitting the reconfiguration complete message.

6. An apparatus, comprising: at least one memory configured to store instructions; andat least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: receive, by a handover source access node from one or more neighboring access nodes, one or more cell-specific baseline configurations for a conditional handover to one or more neighboring cells provided by the one or more neighboring access nodes, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receive, by the handover source access node from a user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more neighboring cells; determine, by the handover source access node, to carry out the conditional handover of the user equipment to one or more candidate target cells selected from the one or more neighboring cells, based on at least the received information on radio link quality, the one or more candidate target cells being provided by one or more handover candidate target access nodes; and transmit, by the handover source access node to the user equipment, a message in association with the conditional handover of the user equipment to the one or more candidate target cells comprising one or more cell-specific baseline configurations in association with the one or more candidate target cells.

7. The apparatus of claim 6, wherein the cell-specific baseline configuration further comprises an indication of key update, and security information for the user equipment to generate one or more access stratum security keys for communication with the candidate target cell.

8. The apparatus of claim 6, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: transmit, by the handover source access node to the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment to the one or more candidate target cells, handover information in association with the user equipment comprising an identifier for identifying the user equipment, information onthe user equipment, an access stratum security master key for communication between the user equipment and the handover candidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

9. The apparatus of claim 6, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: in case the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, transmit, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and forward, by the handover source access node to the at least one handover candidate target access node, user data for the user equipment using the at least one user plane tunnel to the at least one handover candidate target access node, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

10. The apparatus of claim 6, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: in case the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, forward, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, user data for the user equipment using the at least one user plane tunnel to the at least one handover candidate target access node indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

11. The apparatus of claim 6, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least:transmit, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; receive, by the handover source access node from the at least one handover candidate target access node, the user plane tunnel information indicating at least one user plane tunnel in association with the information on the data radio bearer of the user equipment; and forward, by the handover source access node to the at least one handover candidate target access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

12. An apparatus, comprising: at least one memory configured to store instructions; and at least one processing circuitry configured to access the at least one memory, and execute the instructions to cause the apparatus to at least: transmit, by a handover candidate target access node to a handover source access node, one or more cell-specific baseline configurations for a conditional handover from the handover source access node to one or more candidate target cells provided by the handover candidate target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receive, by the handover candidate target access node from the handover source access node, information on a user equipment and an access stratum security master key for communication between the user equipment and the handover candidate target access node; receive, by the handover candidate target access node from the user equipment, a random-access preamble to a target cell selected from the one or more candidate target cells; transmit, to the user equipment, a random-access response message; receive, by the handover candidate target access node from the user equipment, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration,generate, by the handover candidate target access node, a supplementary configuration for the conditional handover of the user equipment to the target cell, based on the information on the user equipment; and transmit, by the handover candidate target access node to the user equipment, the supplementary configuration.

13. The apparatus of claim 12, wherein the reconfiguration complete message is cyphered with an access stratum security key for cyphering of radio resource control signaling.

14. The apparatus of claim 12, wherein the supplementary configuration is comprised in a message transmitted to the user equipment after receiving the reconfiguration complete message, or the supplementary configuration is comprised in the random-access response message or in a message scheduled based on the random-access response message.

15. The apparatus of claim 12, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, and the at least one processing circuitry is further configured to execute the instructions to cause the apparatus to at least: receive, by the handover candidate target access node from the handover source access node, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and receive, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

16. The apparatus of claim 12, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, and the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: receive, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

17. The apparatus of claim 12, wherein the at least one processing circuitry is configured to execute the instructions to cause the apparatus to at least: receive, by the handover candidate target access node from the handover source access node, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; transmit, by the handover candidate target access node to the handover source access node, the user plane tunnel information indicating at least one user plane tunnel in association with the information on the data radio bearer of the user equipment; and receive, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

18. A method, comprising: receiving, by a user equipment from a handover source access node, a message in association with a conditional handover comprising one or more cell-specific baseline configurations and one or more execution conditions for a conditional handover of the user equipment to one or more candidate target cells provided by one or more handover candidate target access nodes, the one or more cell-specific baseline configurations comprising a default signaling radio bearer configuration;determining, by the user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more candidate target cells; selecting, by the user equipment, at least one target cell from the one or more candidate target cells based on the one or more execution conditions and the determined information on radio link quality in association with the source cell and the at least one of the one or more candidate target cells; initiating a random-access procedure to the selected at least one target cell to establish a radio resource control connection between the user equipment and at least one handover target access node providing the at least one target cell, based on the cell-specific baseline configuration; and transmitting, by the user equipment to the at least one handover target access node during the random-access procedure, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration.

19. The method of claim 18, wherein the default signaling radio bearer configuration comprises configuration information on default signaling radio bearer 1, or additionally configuration information on default signaling radio bearer 2, and wherein the reconfiguration complete message is transmitted on the default signaling radio bearer 1.

20. The method of claim 18, wherein the cell-specific baseline configuration further comprises an indication of key update, and security information for generating one or more access stratum security keys for communication with the at least one candidate target cell, and the method further comprises: carrying out, in response to the indication of key update, the generating of the one or more access stratum security keys for the communication with the at least one target cell, the one or more access stratum security keys comprising a key for cyphering of radio resource control signaling; andcyphering the reconfiguration complete message with the key for cyphering of the radio resource control signaling for the transmitting of the reconfiguration complete message to the at least one handover target access node.

21. The method of claim 18, further comprising: receiving, by the user equipment from the at least one handover target access node, a supplementary configuration for the handover of the user equipment to the at least one target cell; or initiating a radio resource control connection re-establishment procedure to the at least one target cell in the case the supplementary configuration is not received within a predefined time window from transmitting the reconfiguration complete message.

22. A method, comprising: receiving, by a handover source access node from one or more neighboring access nodes, one or more cell-specific baseline configurations for a conditional handover to one or more neighboring cells provided by the one or more neighboring access nodes, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receiving, by the handover source access node from a user equipment, information on radio link quality in association with a source cell provided by the handover source access node and at least one of the one or more neighboring cells; determining, by the handover source access node, to carry out the conditional handover of the user equipment to one or more candidate target cells selected from the one or more neighboring cells, based on at least the received information on radio link quality, the one or more candidate target cells being provided by one or more handover candidate target access nodes; and transmitting, by the handover source access node to the user equipment, a message in association with the conditional handover of the user equipment to the one or more candidate target cells comprising one or more cell-specific baseline configurations in association with the one or more candidate target cells.

23. The method of claim 22, wherein the cell-specific baseline configuration further comprises an indication of key update, and security information for the user equipment to generate one or more access stratum security keys for communication with the candidate target cell.

24. The method of claim 22, further comprising: transmitting, by the handover source access node to the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment to the one or more candidate target cells, handover information in association with the user equipment comprising an identifier for identifying the user equipment, information on the user equipment, an access stratum security master key for communication between the user equipment and the handover candidate target access node, and a parameter for counting next hop chaining in association with the access stratum security master key.

25. The method of claim 22, further comprising: in case the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, transmitting, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and forwarding, by the handover source access node to the at least one handover candidate target access node, user data for the user equipment using the at least one user plane tunnel to the at least one handover candidate target access node, based on the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

26. The method of claim 22, further comprising: in case the cell-specific baseline configuration further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node,forwarding, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, user data for the user equipment using the at least one user plane tunnel to the at least one handover candidate target access node indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

27. The method of claim 22, further comprising: transmitting, by the handover source access node to at least one of the one or more handover candidate target access nodes, in response to determining to carry out the conditional handover of the user equipment, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; receiving, by the handover source access node from the at least one handover candidate target access node, the user plane tunnel information indicating at least one user plane tunnel in association with the information on the data radio bearer of the user equipment; and forwarding, by the handover source access node to the at least one handover candidate target access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

28. A method, comprising: transmitting, by a handover candidate target access node to a handover source access node, one or more cell-specific baseline configurations for a conditional handover from the handover source access node to one or more candidate target cells provided by the handover candidate target access node, the cell-specific baseline configuration comprising a default signaling radio bearer configuration; receiving, by the handover candidate target access node from the handover source access node, information on a user equipment and an access stratum security master key for communication between the user equipment and the handover candidate target access node;receiving, by the handover candidate target access node from the user equipment, a randomaccess preamble to a target cell selected from the one or more candidate target cells; transmitting, to the user equipment, a random-access response message; receiving, by the handover candidate target access node from the user equipment, a reconfiguration complete message on a signaling radio bearer indicated in the default signaling radio bearer configuration, generating, by the handover candidate target access node, a supplementary configuration for the conditional handover of the user equipment to the target cell, based on the information on the user equipment; and transmitting, by the handover candidate target access node to the user equipment, the supplementary configuration.

29. The method of claim 28, wherein the reconfiguration complete message is cyphered with an access stratum security key for cyphering of radio resource control signaling.

30. The method of claim 28, wherein the supplementary configuration is comprised in a message transmitted to the user equipment after receiving the reconfiguration complete message, or the supplementary configuration is comprised in the random-access response message or in a message scheduled based on the random-access response message.

31. The method of claim 28, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, and the method further comprises: receiving, by the handover candidate target access node from the handover source access node, information indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment; and receiving, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel, basedon the mapping of the at least one user plane tunnel to the information on the data radio bearer of the user equipment.

32. The method of claim 28, wherein the cell-specific baseline configuration transmitted to the handover source access node further comprises user plane tunnel information indicating at least one user plane tunnel to the handover candidate target access node, and the method further comprises: receiving, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information, the user data comprising a header indicating mapping of the at least one user plane tunnel to information on a data radio bearer of the user equipment.

33. The method of claim 28, further comprising: receiving, by the handover candidate target access node from the handover source access node, a request for user plane tunnel information, the request also indicating information on a data radio bearer of the user equipment; transmitting, by the handover candidate target access node to the handover source access node, the user plane tunnel information indicating at least one user plane tunnel in association with the information on the data radio bearer of the user equipment; and receiving, by the handover candidate target access node from the handover source access node, user data for the user equipment using the at least one user plane tunnel indicated in the user plane tunnel information.

34. An apparatus, comprising means for performing the method of any of claims 18-33.

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