Apparatuses and methods for fast handover and conditional fast handover of user equipment without preallocated c-rnti

Abstract

Various example embodiments relate to apparatuses and methods for fast handover and conditional fast handover of user equipment without a pre-allocated C-RNTI An apparatus may be configured to receive a message in association with a handover from a handover source access node, the message comprising a cell-specific baseline configuration for a handover of user equipment to a target cell and a handover identifier assigned to the user equipment, to initiate a random-access procedure to the target cell according to the cell-specific baseline configuration, to transmit a reconfiguration complete message comprising the handover identifier to the handover target access node during the random-access procedure, and to receive a message comprising a supplementary configuration for the user equipment and a cell radio network temporary identifier assigned to the user equipment from the handover target access node.

Description

APPARATUSES AND METHODS FOR FAST HANDOVERAND CONDITIONAL FAST HANDOVER OF USER EQUIPMENT WITHOUT PREALLOCATED C-RNTITECHNICAL 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 to a target cell provided by a handover target access node, the message comprising a cell-specific baseline configuration for the handover of the user equipment to the target cell and a handover identifier assigned to the user equipment; initiate a random-access procedure to the target cell to establish a radio resource control connection with the handover target access node, according to the cell-specific baseline configuration; transmit, by the user equipment to the handover target access node during the random-access procedure, a reconfiguration complete message comprising the handover identifier; and receive, by the user equipment from the handover target access node, a message comprising a supplementary configuration for the handover of the user equipment to the target cell and a cell radio network temporary identifier assigned to the user equipment.

[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 user equipment from a handover source access node, a message in association with a conditional handover to one or more candidate target cells provided by one or more handover candidate target access nodes, the message in association with the conditional handover comprising one or more cell-specific baseline configurations for the conditional handover of the user equipment to the one or more candidate target cells, one or more execution conditions for the one or more candidate target cells, and one or more handover identifiers assigned to the user equipment with respect to the one or more candidate target cells; 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, a target cell from the one or more candidate target cells for the conditional handover based on the one or more execution conditions and the determined information on radio link quality; initiate a random-access procedure to the selected target cell to establish a radio resource control connection with a handover target access node providing the target cell, according to the cellspecific baseline configuration; transmit, by the user equipment to the handover target accessnode during the random-access procedure, a reconfiguration complete message comprising the handover identifier assigned to the user equipment with respect to the target cell; and receive, from the handover target access node, a message comprising a supplementary configuration for the conditional handover of the user equipment and a cell radio network temporary identifier assigned to the user equipment.

[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: 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; 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 based on the received information on radio link quality; determine, by the handover source access node, a handover identifier for the user equipment; 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, the message comprising at least part of the cell-specific baseline configuration in association with the target cell and the handover identifier for the user equipment; and transmit, by the handover source access node to the handover target access node, handover information comprising information on the user equipment and the handover identifier for the user equipment.

[0009] 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-specific baseline configurations for conditional handover to one or more neighboring cells provided by the one or more neighboring access nodes; receive, by the handover source access node from a user equipment, information on radio link quality in association with at least one source cell and at least one of the one or more neighboring cells; determine, by the handover source access node, to carry out conditional handover of the user equipment to one or more candidate target cells provided by one or more handover candidate target access nodes based on the received information on radio link quality, the one or more candidate target cells being selected from the one or more neighboring cells; determine, by the handover source access node, one or more handover identifiers for the user equipment with respect to the one or more candidate target cells; 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 and the one or more handover identifiers for the user equipment with respect to the one or more candidate target cells; and transmit, by the handover source access node to the one or more handover candidate target access nodes, handover information comprising information on the user equipment and the one or more handover identifiers for the user equipment with respect to the one or more candidate target cells.

[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: transmit, by a handover target access node to a handover source access node, one or more cell-specific baseline configurations in association with one or more candidate target cells provided by the handover target access node; receive, by the handover target access node from the handover source access node, handover information comprising information on a user equipment and one or more handover identifiers assigned to the user equipment with respect to the one or more candidate target cells; carry out, by the handover target access node, a random-access procedure initiated by the user equipment in association with a target cell selected from the one or more candidate target cells to establish a radio resource control connection with the user equipment; receive, by the handover target access node from the user equipment during the random-access procedure, a reconfiguration complete message comprising the handover identifier assigned to the user equipment with respect to the target cell; generate a supplementary configuration for the user equipment based on the information on the user equipment; and transmit, by the handover target access node to the user equipment, a message comprising the supplementary configuration and a cell radio network temporary identifier assigned to the user equipment in the target cell.

[0011] 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.

[0012] 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.

[0013] 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

[0014] 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:

[0015] FIG. 1 illustrates a telecommunications system that comprises one or more public land mobile networks (PLMNs) coupled to one or more external data networks, according to some examples;

[0016] FIG. 2 illustrates a deployment of a PLMN, according to some examples;

[0017] FIG. 3 is a signaling chart of a conventional handover procedure;

[0018] FIG. 4 is a signaling chart of a conventional conditional handover procedure;

[0019] FIG. 5 is a signaling chart of a fast handover procedure, according to some examples;

[0020] FIG. 6 is a signaling chart of a fast handover procedure, according to some examples;

[0021] FIG. 7 is a signaling chart of a conditional fast handover procedure, according to some examples;

[0022] FIG. 8 is a flowchart of a method, according to some examples;

[0023] FIG. 9 is a flowchart of a method, according to some examples;

[0024] FIG. 10 is a flowchart of a method, according to some examples;

[0025] FIG. 11 is a flowchart of a method, according to some examples;

[0026] FIG. 12 is a flowchart of a method, according to some examples;

[0027] FIG. 13 A is a block diagrams of an apparatus, according to some examples;

[0028] FIG. 13B is a block diagrams of an apparatus, according to some examples;

[0029] FIG. 14 is a block diagram of an apparatus, according to some examples;

[0030] FIG. 15 is a block diagram of an apparatus, according to some examples;

[0031] FIG. 16 is a block diagram of an apparatus, according to some examples;

[0032] FIG. 17 is a block diagram of an apparatus, according to some examples; and

[0033] FIG. 18 is a block diagram of an apparatus, according to some examples.DETAILED DESCRIPTION

[0034] Some examples will now be described in more detailed 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.

[0035] 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.

[0036] 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, for example, “[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.

[0037] 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.

[0038] 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) withsoftware / 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.

[0039] 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.

[0040] FIG. 1 illustrates communication system 100 according to various examples of the present disclosure. The communication system generally comprises one or more communication networks. As shown, for example, the system may comprise one or more public land mobile networks (PLMNs) 102 coupled to one or more external data networks (DN) 104 - notably comprising a wide area network (WAN) such as the Internet. Each of the PLMNs may comprise 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.

[0041] 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 another 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.

[0042] 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).

[0043] 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 provide access to CN 106 of a mobile network operator (MNO).

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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 or more 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.

[0048] 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.

[0049] 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 (alsocalled 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.

[0050] In some examples, 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.

[0051] 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 network interface 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.

[0052] 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 (handover source cell) provided by the serving radio access node (handover source access node) to a new radio cell (handover target cell) provided by a target radio access node (handover target access node).

[0053] 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 handover target access node (target node) 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 provided by one or more neighboring access nodes periodically or on an event basis and, at 302, send (transmit) a measurement report indicating relevant measurements for the one or more neighboring radio cells. 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.

[0054] Based on the measurement report, handover source access node 202 A may at 303 decide to initiate a handover procedure for UE 110, 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 target access node 202B controlling a target radio cell selected from the one or more neighboring radio cells. 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 Xn handover. 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.

[0055] 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.

[0056] Upon receipt of the handover command from handover source access node 202A, UE 110 may at 308, 309, 310 obtain downlink (DL) and uplink (UL) synchronization with handover target access node 202B (target radio cell) and thereafter complete the handover procedure. As shown, in some examples, the handover command may be provided at 307 by a RRC reconfiguration; and the handover procedure may comprise a random-access procedure for handover, comprising downlink (DL) and uplink (UL) synchronization and random-accesschannel (RACH) access (random-access preamble) to the handover target access node, and random-access response (RAR) message from the handover target access node. The UE may then send (transmit) an RRC reconfiguration complete message to the handover target access node to complete the handover procedure.

[0057] 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 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, a delay 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).

[0058] 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.

[0059] Broadly speaking, in the case of CHO, UE 110 may be configured with a CHO command containing a candidate 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 (source 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 candidate target radio cell(s) / handover candidate target access node(s) 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(s) and determines that the execution condition is met for a certain candidate 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 the handover candidate target access node 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 candidate target radio cells provided by one or more handover candidate target access nodes.

[0060] 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 measurementreport (as shown at 302 of FIG. 3).

[0061] 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) UE 110, and one or more handover candidate target access nodes 202B (two handover candidate target access nodes shown). Similar to the conventional HO procedure, UE 110 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 candidate target radio cells provided by the handover candidate target access node(s).

[0062] 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 may at 404 send (transmit) a handover request message with a current configuration of UE 110 to one or more handover candidate target access node(s) 202B.

[0063] 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 for example C-RNTI, security algorithm identifiers for selected security algorithms, DRB configuration, QoS flow to DRB mapping, UE capability related features enabled by the handover candidate target access node, or the like.

[0064] Handover source access node 202A may then at 407 send (transmit) a CHO command message to UE 110. The CHO command message may be sent similar to the HO command message, such as by an RRC reconfiguration. The CHO command message may comprise configuration for one or more candidate target radio cells and one or more execution conditions for the one or more candidate target radio cells.

[0065] Upon receipt of the CHO 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 202 A. UE 110 may maintain its connection with handover source access node 202A and at 409 starts evaluating the execution condition(s) for the one or more candidate target radio cells. If at least one candidate target radio cell satisfies a corresponding execution condition(s), UE 110 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 candidate target radio cell (target radio cell) and thereafter complete the CHO procedure, such as in a manner similar to 308, 309, 310 described above for the conventional HO procedure.

[0066] 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 handover execution as soon as an execution condition is met. In CHO, however, decoupling handover preparation and execution may lead to unnecessary handover preparation with inefficient dedicated resources (e.g., RACH preambles and C-RNTIs) 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.

[0067] Fast handover, including basic / normal fast handover (FHO) and conditional fast handover (CFHO), has been proposed to address at least some of the issues relating to the conventional HO and CHO procedures. FIG. 5 illustrates basic FHO procedure 500 involving UE 110, handover source access node (source node) 202 A and one or more handover candidate target access nodes 202B (only one is shown). As shown, handover candidate target access node(s) 202B may at 501 provide a cell-specific baseline configuration for a candidate target radio cell to handover source access node 202A. The cell-specific baseline configuration may comprise basic configuration information applicable for any UE served by handover source access node 202A to initiate a random-access procedure towards the corresponding target radio cell during a handover procedure. In other words, the cell-specific baseline configuration is provided without UE-specific handover preparation. Accordingly, fast handover may also be referred to as unprepared handover. It is suggested that the baseline configuration comprises a pool of preconfigured / reserved RACH preambles and a pool of preconfigured / reserved C-RNTIs for UEs to carry out random-access to the target radio cell. Upon receipt of the cell-specific baseline configuration, handover source access node 202A may send (transmit) an acknowledge message to handover candidate target access node(s) 202B.

[0068] Similar to the conventional HO procedure, UE 110 may at 503 be configured to report measurements of one or more neighboring radio cells, and at 504 send (transmit) a measurement report indicating relevant measurements for one or more candidate target radio cells provided by the handover candidate target access node(s). UE 110 may transmit the measurement report periodically or on event basis.

[0069] Based on the received measurement report, handover source access node 202A may at 505 decide to initiate a fast handover (FHO) for UE 110 to a target radio cell and at 506 send (transmit) an FHO command (e.g., by RRC reconfiguration with synchronization) to UE 110. The FHO command may comprise the baseline configuration of the target radio cell, a C-RNTI, and optionally a RACH preamble for UE 110 to access the target radio cell. The RACH preamble and C-RNTI may be selected from respective pools of preconfigured / reserved RACH preamblesand C-RNTIs provided by handover candidate target access node 202B. Handover source access node 202A may at 507 send (transmit) handover information comprising the selected C-RNTI and information on UE 110 to handover candidate target access node 202B.

[0070] Upon receipt of the FHO command from handover source access node 202 A, UE 110 may at 508, 509, 510 obtain downlink (DL) and / or uplink (UL) synchronization and randomaccess channel (RACH) access with handover candidate target access node 202B that provides the target radio cell and thereafter complete the FHO procedure, such as in a manner similar to 308, 309, 310 described above for the conventional HO procedure.

[0071] Although not shown here, a CFHO procedure may be similar to the FHO procedure except that UE 110, instead of handover source access node 202 A, decides execution of the handover by evaluating one or more execution conditions. The FHO and CFHO procedures can greatly reduce a time interval from handover decision to handover execution by skipping the handover preparation phase, thereby overcome issues caused by the time interval. Throughout the present disclosure, (conditional) fast handover may also be referred to as (conditional) unprepared handover.

[0072] In the FHO and CFHO procedures, a pool of preconfigured / reserved C-RNTIs is provided in the baseline configuration to the handover source access node. When the handover source access node decides (conditional) fast handover of a UE to a (candidate) target radio cell, the handover source access node selects a C-RNTI from the pool of preconfigured / reserved C- RNTIs and assigns the selected C-RNTI to the UE. The selected C-RNTI is used by the UE when the UE moves its RRC connection to the target radio cell. However, it may happen that such a C-RNTI pool is not provided to the handover source access node, or the pool becomes empty. In addition, when one or more C-RNTIs are selected from the pool and used for the handover of UEs to the target radio cell, the handover candidate target access node needs to update the pool, i.e., removing the used C-RNTIs from the pool and adding new C-RNTIs to the pool, which increases signaling overhead of the network.

[0073] Example implementations of the present disclosure therefore provide enhanced support for mobility use cases, such as (inter-node) handover or the like. Some examples provide fast handover (FHO) and conventional FHO (CFHO) procedures that employ a handover identifier for identifying a UE during the FHO and CFHO procedures. The handover identifier is different from C-RNTI. In other words, a handover (candidate) target access node does not need to reserve / pre-allocate a C-RNTI for the UE before the handover execution. The handover (candidate) target access node may allocate a C-RNTI to the UE when the UE successfully establishes an RRC connection with the target radio cell. It can improve resource efficiency. The handover identifier may be generated by a handover source access node when it determines theFHO or CFHO of the UE, or selected from a pool of preconfigured / reserved handover identifiers provided by the handover (candidate) target access node. Since the handover identifier is temporarily used during the FHO and CFHO procedures and it is released / reusable when the FHO and CFHO procedures are completed, the handover (candidate) target access node does not need to update the pool of preconfigured / reserved handover identifiers, which can reduce signaling overhead of the network. It would be appreciated that the handover identifier may be also used in a similar way in conventional HO and CHO procedures discussed above or in lower layer triggered mobility (LTM) use cases.

[0074] FIG. 6 is a signaling chart of fast handover (FHO) procedure 600, according to some examples. As shown, at least one handover candidate target access node (target node) 202B may at 601 generate a cell-specific baseline configuration (a default configuration) for the FHO to a candidate target radio cell provided by handover candidate target access node 202B and send (transmit) the cell-specific baseline configuration to handover source access node (source node) 202 A, 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, handover candidate target access node 202B 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) messages. Handover source access node 202A may at 602 send (transmit) an acknowledgment message (e.g., RAN node configuration update ACK) back to the handover candidate target access node to confirm receipt of the cell-specific baseline configuration.

[0075] 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 (also 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), beam- and / or network slice-related information if available, among others.

[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 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). 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.

[0078] In some examples, cell-specific baseline configuration may further comprise a pool (list) of one or more random-access channel (RACH) preambles (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 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 RACH preamble received from the UE, which will be described in detail below.

[0079] Similarly, in some examples, cell-specific baseline configuration may further comprise a pool (list) of one or more handover identifiers 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 handover identifier can be used to identify a UE across the handover source access node, the handover candidate target access node and the UE during the handover procedure, and hence it may be also referred to as UE identifier or UE handover identifier. The pool of handover identifiers may also be predetermined, preconfigured or reserved at the handover candidate target access node. Any one of the handover identifiers 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 the handover execution. In some examples, the handover candidate target access node may prepare different pools of handover identifiers for different source cells / nodes. For instance, the handover 1identifiers may be generated based on one or more of an identifier (e.g., physical cell identifier, PCI) of the source radio cell, an identifier of the handover source access node, an identifier (e.g., PCI) of the candidate target radio cell, an identifier of the handover candidate target access node, and a random number or sequence. Then during the handover execution, the handover candidate target access node can identify the source cell / node of the UE according to the handover identifier used by the UE, which will be described in detail below.

[0080] In some examples, respective RACH preambles may be paired / associated to respective handover identifiers in the cell-specific baseline configuration. The handover source access node may select and assign a {RACH preamble, handover identifier} pair to a UE when a handover is decided for the UE. Then the handover candidate target access node can identify the UE and its source cell / node based on either the RACH preamble or the handover identifier, which will be described in detail below.

[0081] As discussed above, handover source access node 202A may control determination or selection of the RACH preamble and / or handover identifier from the respective pools, such as to avoid a possible collision of the RACH preamble and / or handover identifier. 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 handover identifier for UE 110.

[0082] 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 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. For example, the cell-specific configuration information may comprise the respective pools of preconfigured / reserved RACH preambles and handover identifiers, a number of supported multiple-input and multiple-output (MIMO) layers, frequency, beams, and the like. In some examples, both parts of the cell-specific 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 cellspecific baseline configurations may be provided in separate messages, such as one message forthe common node-specific part, and one message for the node-specific part for each of the target radio cells.

[0083] 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 (i.e., specific to the source radio cell and the candidate target radio cell).

[0084] Returning to FIG. 6, similar to the conventional HO procedure, UE 110 may at 603 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 604 send (transmit) a measurement report indicating relevant measurements for one or more candidate target radio cells. UE 110 may be configured to report the measurements in a periodic manner or on event basis.

[0085] Handover source access node 202 A may at 605 decide to carry out a fast handover (FHO) of UE 110 to a candidate target radio cell(s), based on the measurements report received from UE 110 and one or more criteria for the FHO. The one or more criteria for the FHO may be obtained from the handover candidate target access node, or at least part of the criteria may be predefined / preconfigured at the handover source access node. Some examples of the criteria may comprise for example but are not limited to service delay requirements of UE, service reliability requirements of UE, load information of target radio cell, traffic amount of UE, capability of UE, capability of target radio cell, etc.

[0086] In response to determination of the FHO for UE 110, handover source access node 202 A may at 606 determine a handover identifier for the handover of UE 110. In some examples, handover source access node 202A may select a handover identifier from the pool of preconfigured / reserved handover identifiers received at 601 from handover candidate target access node 202B. In some other examples, if handover candidate target access node 202B does not provide the pool of preconfigured / reserved handover identifiers, handover source access node 202Amay generate a handover identifier for UE 110. For example, handover source access node 202A may generate the handover identifier based on one or more of a current C-RNTI of UE 110 in the source radio cell, an identifier (e.g., PCI) of the source radio cell, an identifier of a node hosting RRC of UE 110 (e.g., source gNB ID or source gNB-CU ID), an identifier of an entity in the handover source access node that hosts RRC context of UE 110 (e.g., an RRC entity ID), an identifier (e.g., PCI) of the target radio cell, and an identifier of the handover target accessnode. Handover source access node 202A may also select a RACH preamble for the handover of UE 110 if a pool of preconfigured / reserved RACH preambles are received at 601 from handover candidate target access node 202B.

[0087] Handover source access node 202A may at 607 send (transmit) a handover command (e.g., FHO command) comprising FHO-related configuration to UE 110. In some examples, the FHO command may be sent via an RRC reconfiguration message including a reconfigurationWithSync information element (IE), or via any other suitable message. The FHO- related configuration may comprise the handover identifier, and (at least part of) configuration information of the cell-specific baseline configuration in association with the target radio cell obtained from handover candidate target access node 202B at 601. 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. In some examples, the FHO- related configuration may further 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. It is worth noting that handover source access node 202A does not provide a new C-RNTI for UE 110 in the target radio cell.

[0088] Before, after or as the FHO command message is sent to UE 110, handover source access node 202A may at 608 send (transmit) handover information to handover candidate target access node 202B (target radio cell) to initiate UE-specific preparation at the target radio cell. The handover 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 handover information may be referred to as an FHO / HO notification message. The handover information sent to the handover candidate target access node may comprise information on a UE to be handed over, including UE context such as UE capabilities, AS security information, basic AS configuration information such as antenna information and downlink (DL) carrier frequency, index to RAT / frequency selection priority, UE aggregate MBR (Maximum Bit Rate), DRB configuration, PDU session to be setup list, RRC context, location reporting, mobility restriction, etc. The handover information may also comprise the handover identifier assigned to the UE. Upon receipt of the handover information, handover candidate target access node 202B knows that a UE will be handed over to one of the candidate target radio cells and it may start to prepare a UE-specific supplementary configuration for the handover based on the received UE information.

[0089] In response to the FHO command, UE 110 may be configured to, at 609, 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) tohandover candidate target access node 202B. The random-access MSG1 comprises a RACH preamble, which may use 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.

[0090] Notably, as the FHO command message at 607 towards UE 110 and the handover information message at 608 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 609 may take place before the handover information message at 608.

[0091] In response to the random-access preamble (MSG1), handover candidate target access node 202B (target radio cell) may at 610 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.

[0092] UE 110 may at 611 transmit (send) an RRC reconfiguration complete message (MSG3) comprising the handover identifier to handover candidate target access node 202B, using the UL grant received in the RAR message. In some examples, UE 110 may transmit (send) the RRC reconfiguration complete message on a default SRB1 indicated in the cell-specific baseline configuration received from handover source access node 202A.

[0093] Handover candidate target access node 202B can know from the handover identifier that UE 110 is undergoing an FHO (or CFHO) procedure, and it may check whether it has received information on UE 110 corresponding to the handover identifier. If UE information received at 608 corresponds to the same handover identifier, handover candidate target access node 202B can determine that the information on UE 110 is locally available, and it may start to prepare a UE-specific supplementary configuration for the handover of UE 110 based on the received information on UE 110. If handover candidate target access node 202B has not yet received information on UE 110, (e.g., due to transmission failure at 608 or a fact that handover source access node 202 A does not transmit (send) information on UE 110 to handover candidate target access node 202B at all), it may start a timer (e.g., TuEinfo) for receipt of information on UE 110.

[0094] Handover candidate target access node 202B may at 612 determine that the timer TuEinfo expires but it has not yet received information on UE 110. As a result, handover candidate target access node 202B may at 613 request handover source access node 202 A to provide the information on UE 110 by transmitting (sending) a UE context request message. The request message may comprise the handover identifier to identify UE 110. In response to the request, handover source access node 202A may at 614 provide information on UE 110 in a UE context response message to handover candidate target access node 202B.

[0095] Handover candidate target access node 202B may generate a supplementaryconfiguration for the handover of UE 110 based on the received information on UE 110 and assign a C-RNTI to UE 110. The supplementary configuration may comprise UE-specific configuration information needed for a full handover of UE 110 but not provided to UE 110 in the cell-specific baseline configuration. In some examples, the supplementary configuration may comprise for example DRB configuration, QoS flow to DRB mapping configuration, beam- related configuration, network slice-related configuration, and the like. Then at 615, handover candidate target access node 202B may send (transmit) the supplementary configuration and the C-RNTI 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 and use the C-RNTI for future communication with handover candidate target access node 202B. In other words, UE 110 may release the handover identifier, and handover source access node 202A may reuse the released handover identifier for FHO (or CFHO) of other UEs to the target radio cell.

[0096] Although the 4-step random-access procedure is described in the FHO procedure shown in FIG. 6, 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.

[0097] FIG. 7 is a signaling chart of conditional FHO (CFHO) procedure 700, 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 701 generate one or more cell-specific baseline configurations for the CFHO from a source radio cell provided by handover source access node 202Ato one or more candidate target radio cells, and send (transmit) the cell-specific baseline configuration(s) to handover source access node 202A, such as in a RAN node configuration update message or any other suitable (existing or new) message. At this point of time, handover candidate target access node(s) 202B may not know whether handover source access node 202 A will initiate an 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 202A later. As discussed above, the cell-specific baseline configuration may comprise a pool (list) of one or more RACH preambles and / or a pool (list) of one or more handover identifiers.

[0098] Handover source access node 202A may at 702 send (transmit) a correspondingacknowledgment message (e.g., RAN node configuration update ACK) back to handover candidate target access node(s) 202B.

[0099] Similar to the FHO procedure, UE 110 may at 703 be configured to report measurements of radio link quality in association with a source / serving radio cell and one or more neighboring radio cells, and at 704 send (transmit) a measurement report indicating relevant measurements for one or more candidate target radio cells provided by one or more handover candidate target access node(s) 202B. UE 110 may be configured to report the measurements in a periodic manner or on event basis.

[0100] Handover source access node 202A may at 705 decide to initiate a CFHO procedure for UE 110, based on the measurements report received from UE 110 and optionally one or more criteria for CFHO. The one or more criteria for CFHO may be configured in the cell-specific baseline configuration received from handover candidate target access node 202B, or at least part of the one or more criteria for CFHO may be predefined / preconfigured at handover source access node 202A. In some examples, handover source access node 202A may make a determination to apply a CFHO procedure based on radio link quality in association with the source / serving radio cell and the neighboring radio cells indicated in the measurement report, one or more service reliability requirements (e.g., QoS on reliability) of UE 110, one or more service delay requirements (e.g., expected QoS on delay) of UE 110, or the like. Handover source access node 202Amay at 705 also determine one or more execution conditions for the CFHO of UE 110 to one or more candidate target radio cells.

[0101] If CFHO is determined at 705 for UE 110, handover source access node 202A may at 706 determine a handover identifier for the CFHO of UE 110. If a pool of preconfigured / reserved handover identifiers are received in the cell-specific baseline configuration from handover candidate target access node 202B, handover source access node 202A may select a handover identifier from the pool. If handover candidate target access node 202B does not provide the pool of preconfigured / reserved handover identifiers, handover source access node 202Amay generate a handover identifier for the CFHO of UE 110. If a pool of preconfigured / reserved RACH preambles are received in the cell-specific baseline configuration from handover candidate target access node 202B, handover source access node 202A may also select a RACH preamble from the pool for UE 110. It would be appreciated that if handover source access node 202A determines multiple candidate target radio cells for the CFHO of UE 110, it will determine a handover identifier (and optionally a RACH preamble) for each of the multiple candidate target radio cells.

[0102] Handover source node 202A may at 707 send (transmit) a handover command (e.g., CFHO command) comprising CFHO-related configuration and one or more execution conditionsto UE 110. Similar to the FHO command, the CFHO command may be transmitted for example via an RRC reconfiguration with synchronization message or any other suitable message. The CFHO-related configuration may comprise (at least part of) 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. The CFHO-related configuration may also comprise the handover identifier and optionally the RACH preamble for each candidate target radio cell.

[0103] Before, after or as the CFHO command is sent to UE 110, handover source access node 202A may at 708 send (transmit) handover information to handover candidate target access node(s) 202B (target radio cell(s)). The handover 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 handover information may be referred to as a CFHO / HO notification message. The handover information sent to the handover candidate target access node(s) may comprise information on a UE to be handed over, including UE context such as UE capabilities, AS security information, basic AS configuration information such as antenna information and downlink (DL) carrier frequency, index to RAT / frequency selection priority, UE aggregate MBR, DRB configuration, PDU session to be setup list, RRC context, location reporting, mobility restriction, etc. The handover information may also comprise the handover identifier assigned to the UE. Upon receipt of the handover information, handover candidate target access node 202B knows that a UE will be handed over to one of the candidate target radio cells and it may start to prepare a UE-specific supplementary configuration for the handover based on the received UE information.

[0104] Upon receipt of the CFHO command, if the CFHO command is transmitted by an RRC reconfiguration message, UE 110 may at 709 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 710, starts evaluating the execution condition(s) for the one or more candidate target radio cells indicated 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 the corresponding execution condition(s).

[0105] Based on the condition evaluation, handover source access node 202A may select at least one target radio cell from the one or more candidate target radio cells that satisfies the corresponding execution condition(s). UE 110 may be configured to perform a random-access procedure to establish 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 711 obtain DL and UL synchronization with the target radio cell and initiate the random-access procedure by transmitting (sending) a random-access preamble (MSG1) to the target radio cell. The random-access MSG1 may comprise a RACH preamble, which may be selected by handover source access node 202A from the pool of (preconfigured / reserved) RACH preambles received from handover candidate target access node 202B (if available), or selected by UE 110 from a predefined preamble group.

[0106] In response to the random-access preamble (MSG1), handover candidate target access node 202B (target radio cell) may at 712 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.

[0107] UE 110 may at 713 transmit (send) an RRC reconfiguration complete message (MSG3) comprising the handover identifier to handover candidate target access node 202B, using the UL grant received in the RAR message. In some examples, UE 110 may transmit (send) the RRC reconfiguration complete message on a default SRB1 indicated in the cell-specific baseline configuration received from handover source access node 202A.

[0108] Handover candidate target access node 202B can identify from the handover identifier that UE 110 is undergoing a CFHO (or FHO) procedure and check whether it has received information on UE 110. If UE information received at 708 corresponds to the same handover identifier, handover candidate target access node 202B can determine that the information on UE 110 is locally available, and it may start to prepare a UE-specific supplementary configuration for the handover of UE 110 based on the received information on UE 110. If handover candidate target access node 202B has not yet received information on UE 110, (e.g., due to transmission failure at 708 or a fact that handover source access node 202A does not transmit (send) information on UE 110 to handover candidate target access node 202B at all) it may start a timer (e.g., TuEinfo) for receipt of information on UE 110.

[0109] Handover candidate target access node 202B may at 714 determine that the timer TuEinfo expires but it has not yet received information on UE 110. As a result, handover candidate target access node 202B may at 715 send (transmit) a UE context request comprising the handover identifier to handover source access node 202A. In response to the request, handover source access node 202 A may at 716 provide information on UE 110 in a UE context response message to handover candidate target access node 202B.

[0110] Handover candidate target access node 202B may generate a supplementary configuration for the handover of UE 110 based on the received information on UE 110 and assign a C-RNTI to UE 110. The supplementary configuration may comprise UE-specificconfiguration information needed for a full handover of UE 110 but not provided to UE 110 in the cell-specific baseline configuration. In some examples, the supplementary configuration may comprise for example DRB configuration, QoS flow to DRB mapping configuration, beam- related configuration, network slice-related configuration, and the like. Then at 717, handover candidate target access node 202B may send (transmit) the supplementary configuration and the C-RNTI 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 and use the C-RNTI for future communication with handover candidate target access node 202B. In other words, UE 110 may release the handover identifier, and handover source access node 202A may reuse the released handover identifier for CFHO (or FHO) of other UEs to the target radio cell.

[0111] Although the 4-step random-access procedure is described in the CFHO procedure shown in FIG. 7, a 2-step random-access procedure may be used in place of the 4-step randomaccess 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.

[0112] FIG. 8 is a flowchart illustrating method 800 according to some examples. Method 800 may be implemented at a user equipment (UE), e.g., UE 110 discussed above.

[0113] As shown, the method may comprise, at 802, 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 comprise a handover (HO) command, e.g., a fast handover (FHO) command. The FHO command may comprise a handover identifier and 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. Unlike a legacy handover command, the message does not provide a new C-RNTI for UE 110 in the target radio cell. It is worth noting that the handover identifier is different from C-RNTI. The former is used to identify a UE (or a handover of the UE) during a handover procedure, while the latter is used to identify a UE as long as the UE is connected to a radio cell. The cell-specific baseline configuration may comprise for example an identifier (e.g., 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 for the handover of UE 110. In some examples, the message in association with the handover may further comprise a RACH preamble for the UE to access the target radio cell.

[0114] The method may further comprise, at 804, initiating a random-access procedure to the target radio cell to establish an RRC connection with handover target access node 202B providing the target radio cell, according to the received 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 cellspecific 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.

[0115] During the random-access procedure, UE 110 may, at 806, transmitting a reconfiguration complete message comprising the handover identifier to handover target access node 202B. 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. In some examples, the reconfiguration complete message may be transmitted on a default SRB1 indicated in the cell-specific baseline configuration received from handover source access node 202A.

[0116] The method may further comprise, at 808, receiving a message comprising a supplementary configuration and a C-RNTI from handover target access node 202B. The supplementary configuration may comprise UE-specific configuration information for the handover of UE 110 to the target radio cell. In some examples, the supplementary configuration may comprise for example DRB configuration, QoS flow to DRB mapping configuration, beam- related configuration, network slice-related configuration, and the like. UE 110 may apply the supplementary configuration to complete the handover and use the C-RNTI for future communication with handover candidate target access node 202B.

[0117] FIG. 9 is a flowchart illustrating method 900 according to some examples. Method 900 may be implemented at a user equipment (UE), e.g., UE 110 discussed above.

[0118] As shown, the method may comprise, at 902, receiving a message in association with a conditional handover from handover source access node 202A. In some examples, the message in association with the conditional handover may comprise a conditional handover (CHO) command, e.g., a conditional fast handover (CFHO) command. The CFHO command may comprise one or more cell-specific baseline configurations and one or more execution conditions for 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 CFHO command may further comprise one or more handover identifiers in association with the one or more cell-specific baseline configurations / candidate target radio cells, respectively. Unlike a legacy handover command, theCFHO command does not provide a new C-RNTI for UE 110 in a candidate target radio cell. The cell-specific baseline configuration may comprise for example an identifier (e.g., PCI) of the target radio cell and information necessary for a UE to connect 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 for the handover of UE 110. In some examples, the CFHO command may further comprise one or more RACH preambles for the UE to access the one or more candidate target radio cells respectively.

[0119] The method may further comprise, at 904, 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 determine radio link quality information based on history measurements stored locally, receive radio link quality information from network nodes or other UEs, or execute artificial intelligence (Al) or machine learning (ML) models / algorithms to predict radio link quality information in association with the source and candidate target radio cells.

[0120] Then UE 110 may, at 906, 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 and 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.

[0121] The method may further comprise, at 908, initiating a random-access procedure to the selected target radio cell to establish an RRC connection with handover target access node 202B providing the target radio cell, 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 randomaccess response (RAR) message from handover target access node 202B.

[0122] During the random-access procedure, UE 110 may, at 910, transmit a reconfiguration complete message to handover target access node 202B. For example, the reconfigurationcomplete 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 a default SRB1 indicated in the cell-specific baseline configuration. The reconfiguration complete message may comprise the handover identifier in association with the target radio cell received in the CFHO command from handover source access node 202A.

[0123] The method may further comprise, at 912, receiving a message comprising a supplementary configuration and a C-RNTI from handover target access node 202B. The supplementary configuration may comprise UE-specific configuration information for the handover of UE 110 to the target radio cell. In some examples, the supplementary configuration for example DRB configuration, QoS flow to DRB mapping configuration, beam-related configuration, network slice-related configuration, and the like. UE 110 may apply the supplementary configuration to complete the handover and use the C-RNTI for future communication with handover candidate target access node 202B.

[0124] FIG. 10 illustrates method 1000 according to some examples. Method 1000 may be implemented at a handover source access node, e.g., handover source access node 202 A discussed above.

[0125] As shown, the method may comprise, at 1002, receiving a cell-specific baseline configuration from handover target access node 202B. The cell-specific baseline configuration may 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 pool of preconfigured / reserved RACH preambles, and / or a pool of preconfigured / reserved handover identifiers. It is worth noting that the handover identifier is different from C-RNTI. The former is used to identify a UE (or a handover of the UE) during a handover procedure, while the latter is used to identify a UE as long as the UE is connected to a radio cell. The cell-specific baseline configuration does not provide a new C-RNTI for UE 110 in the target radio cell. It would be appreciated that handover source access node 202 A may receive one or more cell-specific baseline configurations for a potential fast handover of UE 110 to one or more candidate target radio cells provided by one or more handover candidate target access nodes 202B.

[0126] The method may further comprise, at 1004, receiving from UE 110 information on radio link quality in association with a source / serving radio cell and the target radio cell. For instance, the information on radio link quality may be received in a measurement report fromUE 110.

[0127] Based on at least the information on radio link quality, handover source access node 202 A may determine, at 1006, to carry out a fast handover of UE 110 to a target radio cell using a cell-specific baseline configuration in association with the target radio cell. In some examples, handover source access node 202 A may determine to carry out the fast handover of UE 110 based on the radio link quality information received from UE 110 and one or more criteria for the fast handover. The one or more criteria for the fast handover may be indicated in the cell-specific baseline configuration received from handover candidate target access node 202B, or at least part of the criteria may be predefined / preconfigured at handover source access node 202A. Some examples of the criteria may comprise for example service delay requirements of UE, service reliability requirements of UE, load information of target radio cell, traffic amount of UE, capability of UE, capability of target radio cell, etc.

[0128] The method may further comprise, at 1008, determining a handover identifier for the determined fast handover of UE 110. In some examples, handover source access node 202 A may select a handover identifier from a pool of preconfigured / reserved handover identifiers received in the cell-specific baseline configuration from handover candidate target access node 202B. In some other examples, if handover candidate target access node 202B does not provide the pool of preconfigured / reserved handover identifiers, handover source access node 202Amay generate a handover identifier for UE 110. For example, handover source access node 202Amay generate the handover identifier based on one or more of a current C-RNTI of UE 110 in the source radio cell, an identifier (e.g., PCI) of the source radio cell, an identifier of a node hosting RRC of UE 110 (e.g., source gNB ID or source gNB-CU ID), an identifier of an entity in the handover source access node that hosts RRC context of UE 110 (e.g., an RRC entity ID), an identifier (e.g., PCI) of the target radio cell, and an identifier of the handover target access node. Handover source access node 202A may also select a RACH preamble for the handover of UE 110 if a pool of preconfigured / reserved RACH preambles are received from handover candidate target access node 202B.

[0129] Then handover source access node 202 A may, at 1010, transmit a message in association with the determined fast handover to UE 110. In some examples, the message in association with the fast handover may comprise a fast handover (FHO) command, which may comprise the handover identifier and at least part of configuration information of the cell-specific baseline configuration in association with the target radio cell. In some examples, the FHO command may further comprise a RACH preamble selected from the pool of (preconfigured / reserved) RACH preambles received from handover target access node 202B.

[0130] The method may further comprise, at 1012, transmitting handover information tohandover target access node 202B. The handover information may comprise information on the fast handover of UE 110 to the target radio cell, such as the handover identifier and information on UE 110. In some examples, the information on UE 110 may comprise UE context, such as UE capabilities, AS security information, basic AS configuration information such as antenna information and downlink (DL) carrier frequency, index to RAT / frequency selection priority, UE aggregate MBR, DRB configuration, PDU session to be setup list, RRC context, location reporting, mobility restriction, etc. In some examples, The handover information may also comprise at least part of the baseline configuration sent at 1010 to UE 110.

[0131] FIG. 11 is a flowchart illustrating method 1100, according to some examples. Method 1100 may be implemented at a handover source access node, e.g., handover source access node 202A discussed above.

[0132] As shown, the method may comprise, at 1102, 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 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., 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 cell-specific baseline configuration may comprise a pool of preconfigured / reserved RACH preambles, and / or a pool of preconfigured / reserved handover identifiers. In some examples, the handover identifiers may be generated based on one or more of an identifier (e.g., PCI) of the source radio cell, an identifier of the handover source access node, an identifier (e.g., PCI) of the candidate target radio cell, an identifier of the handover candidate target access node, and a random number or sequence. The handover candidate target access node may prepare different pools of handover identifiers for different source radio cells / nodes. Then during the handover execution, the handover candidate target access node can identify the source cell / node of the UE according to the handover identifier used by the UE. It is worth noting that the handover identifier is different from C-RNTI. The former is used to identify a UE (or a handover of the UE) during a handover procedure, while the latter is used to identify a UE as long as the UE is connected to a radio cell. The cell-specific baseline configuration does not provide a new C-RNTI for UE 110 in a candidate target radio cell.

[0133] The method may further comprise, at 1104, receiving from UE 110 information on radio link quality in association with a source cell and at least one of the neighboring radio cells.For instance, handover source access node 202A may receive a measurement report comprising the information on radio link quality from UE 110.

[0134] Based on at least the information on radio link quality, handover source access node 202 A may determine, at 1106, to carry out a conditional fast handover (CFHO) of UE 110 to one or more candidate target radio cells using one or more corresponding cell-specific baseline configurations. The one or more candidate target radio cells are selected from the neighboring radio cells based on the radio link quality of the neighboring radio cells. In some examples, handover source access node 202 A may determine the CFHO for UE 110 based on the information on radio link quality and optionally one or more criteria for CFHO. The one or more criteria for CFHO may be configured in the cell-specific baseline configuration received from handover candidate target access node 202B, or at least part of the one or more criteria for CFHO may be predefined / preconfigured at handover source access node 202A. In some examples, handover source access node 202A may make a determination to initiate a CFHO procedure based on radio link quality in association with the source / serving radio cell and the neighboring radio cells indicated in the measurement report, one or more service reliability requirements (e.g., QoS on reliability) of UE 110, one or more service delay requirements (e.g., expected QoS on delay) of UE 110, or the like. Handover source access node 202A may at 1106 also determine one or more execution conditions for the CFHO of UE 110 to the one or more candidate target radio cells.

[0135] In response to the determined CFHO of UE 110, handover source access node 202 A may, at 1108, determine one or more handover identifiers for the CFHO of UE 110 to the one or more candidate target radio cells. If a pool of preconfigured / reserved handover identifiers are provided in the cell-specific baseline configuration in association with a candidate target radio cell received from handover candidate target access node 202B, handover source access node 202A may select a handover identifier from the pool for the candidate target radio cell. If handover candidate target access node 202B does not provide the pool of preconfigured / reserved handover identifiers for a candidate target radio cell, handover source access node 202A may generate a handover identifier for the candidate target radio cell. For example, handover source access node 202A may generate the handover identifier based on one or more of a current C- RNTI of UE 110 in the source radio cell, an identifier (e.g., PCI) of the source radio cell, an identifier of a node hosting RRC of UE 110 (e.g., source gNB ID or source gNB-CU ID), an identifier of an entity in the handover source access node that hosts RRC context of UE 110 (e.g., an RRC entity ID), an identifier (e.g., PCI) of the target radio cell, and an identifier of the handover target access node. If a pool of preconfigured / reserved RACH preambles are provided in the cell-specific baseline configurations received from handover candidate target access node202B, handover source access node 202A may also select a RACH preamble from the pool for each candidate target radio cell.

[0136] Then handover source access node 202 A may, at 1110, transmit a message in association with the determined CFHO to UE 110. In some examples, the message in association with the CFHO may comprise a handover command, e.g., a CFHO command, and it may comprise (at least part of) configuration information in the one or more cell-specific baseline configurations and the one or more execution conditions for the one or more candidate target radio cells. The message may also comprise the handover identifier and optionally the RACH preamble for each candidate target radio cell.

[0137] Handover source access node 202A may also, at 1112, transmitting handover information related to the determined CFHO to one or more handover candidate target access nodes 202B. The handover information may comprise for example the handover identifier for each candidate target radio cell, and information on UE to be handed over. In some examples, the information on UE may comprise UE context, such as UE capabilities, AS security information, basic AS configuration information such as antenna information and downlink (DL) carrier frequency, index to RAT / frequency selection priority, UE aggregate MBR, DRB configuration, PDU session to be setup list, RRC context, location reporting, mobility restriction, etc. In some examples, the handover information may also comprise at least part of configuration information in the one or more cell-specific baseline configurations sent at 1110 to UE 110.

[0138] FIGs. 12 is a flowchart illustrating method 1200, according to some examples. Method 1200 may be implemented at a handover candidate target access node (handover target access node), e.g., handover candidate target access node 202B discussed above.

[0139] As shown, the method may comprise, at 1202, transmitting to handover source access node 202A one or more cell-specific baseline configurations in association with one or more candidate target radio cells provided by handover candidate target access node 202B. 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. 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 202A to which candidate target radio cell(s) provided by 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. In this regard, the (conditional) fast handover may be also referred to as (conditional) unprepared handover. The cell-specific baseline configuration may comprise an identifier (e.g., PCI) of the candidate targetradio 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 cell-specific baseline configuration may comprise a pool of preconfigured / reserved RACH preambles, and / or a pool of preconfigured / reserved handover identifiers. 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, handover source access node 202A may select a RACH preamble and / or a handover identifier for the handover of the UE from the respective pools. The handover identifiers may be generated based on one or more of an identifier (e.g., PCI) of a source radio cell, an identifier of a handover source access node (e.g., source gNB ID), an identifier (e.g., PCI) of a candidate target radio cell, an identifier of a handover candidate target access node (e.g., candidate target gNB ID), and a random number or sequence. Handover candidate target access node 202B may prepare different pools of handover identifiers for different source radio cells / nodes. Then during handover execution, handover candidate target access node 202B can identify a source cell / node of a UE according to the handover identifier used by the UE. It is worth noting that the handover identifier is different from C-RNTI. Handover candidate target access node 202B does not provide a new C-RNTI in the cell-specific baseline configurations.

[0140] The method may further comprise, at 1204, receiving handover information indicating a handover, e.g., a fast handover (FHO) or a conditional fast handover (CFHO), of UE to one or more candidate target radio cells from handover source access node 202A. The handover information may comprise information on UE to be handed over, and one or more handover identifiers assigned to the UE with respect to the one or more candidate target radio cells. In some examples, the information on UE may comprise UE context, such as UE capabilities, AS security information, basic AS configuration information such as antenna information and downlink (DL) carrier frequency, index to RAT / frequency selection priority, UE aggregate MBR, DRB configuration, PDU session to be setup list, RRC context, location reporting, mobility restriction, etc. The handover identifiers may be selected from respective pools of preconfigured / reserved handover identifiers provided by handover candidate target access node 202B for respective candidate target radio cells (if available), or generated by handover source access node 202A. In some examples, the handover information may also comprise at least part of configuration information in the one or more cell-specific baseline configurations in association with the one or more candidate target radio cells.

[0141] Handover candidate target access node 202B may, at 1206, start to carry out a randomaccess procedure initiated by UE 110 to establish an RRC connection with UE 110. For example, handover candidate target access node 202B may receive a random-access preamble towards a target radio cell from UE 110. The random-access preamble may comprise a RACH preambleselected from the pool of (preconfigured / reserved) RACH preambles provided by handover candidate target access node 202B (if available), or a RACH preamble selected from a preamble group predefined / preconfigured at UE 110. Although not shown in FIG. 12, handover candidate target access node 202B may respond to the request by transmitting a random-access response (RAR) to UE 110.

[0142] As a part of the random-access procedure, handover candidate target access node 202B may, at 1208, receive a reconfiguration complete message comprising a handover identifier from UE 110. In some examples, the reconfiguration complete message may be received on a default SRB1 indicated in the cell-specific baseline configuration in association with the target radio cell. Handover candidate target access node 202B can identify from the handover identifier that UE 110 is undergoing an FHO or CFHO procedure and check whether it has received information on UE 110. If handover candidate target access node 202B has received, at 1204, information on UE corresponding to the handover identifier, handover candidate target access node 202B can determine that the information on UE 110 is locally available, and it may start to prepare a UE- specific supplementary configuration for the handover of UE 110 based on the received information on UE 110. If handover candidate target access node 202B determines that it has not yet received information on UE 110, it may, immediately or within a predetermined time period, initiate a UE context fetch procedure to fetch information on UE 110 from handover source access node 202A.

[0143] Then handover candidate target access node 202B may, at 1210, generate a supplementary configuration for the handover (fast handover or conditional fast handover) of UE 110 based on the received information on UE 110. The supplementary configuration may comprise UE-specific 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. Handover candidate target access node 202B may also determine a C-RNTI for UE 110.

[0144] The method may further comprise, at 1212, transmit the supplementary configuration and the C-RNTI to UE 110 to complete the handover procedure. In some examples, the supplementary configuration and the C-RNTI may be transmitted via an RRC reconfiguration message to UE 110.

[0145] According to some examples, methods 800, 900 described above with respect to FIGs. 8-9 may be carried out by an apparatus comprising means for performing functions corresponding to steps of the methods. Examples of a suitable apparatus may comprise user equipment (UE), which may refer to any device or entity that can wirelessly connect to acommunication network and communicate with network devices and / or other UEs. Similarly, methods 1000, 1100, 1200 described above with respect to FIGs. 10-12 may be carried out by an apparatus comprising means for performing functions corresponding to steps of the methods. Examples of a suitable apparatus may comprise a base station which can provide cells or coverage through which 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.

[0146] FIGs. 13A, 13B illustrate apparatuses 1300A, 130013, according to some examples. Apparatus 1300A may comprise at least one processing circuitry 1302A, at least one computer- readable storage medium or other memory 1304A which is configured to store instructions 1306A, and at least one communication interface 1308A. Apparatus 1300Amay be implemented as UE to carry out methods, procedures and examples discussed above with regard to UE 110. Apparatus 1300B may comprise at least one processing circuitry 1302B, at least one computer- readable storage medium or other memory 1304B which is configured to store instructions 1306B, and at least one communication interface 1308B. Apparatus 13006 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 1300 A, 13006 each may comprise additional components or means to implement the respective methods, procedures and examples than those shown in the figures.

[0147] Processing circuitry 1302 A, 1302B 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 1304A, 1304B (of the same or another apparatus).

[0148] Processing circuitry 1302 A, 1302B 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, theprocessing 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.

[0149] Memory 1304 A, 1304B may be generally any piece of computer hardware that is capable of storing information such as, for example, data, computer programs, instructions 1306A, 1306B (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 nonvolatile 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.

[0150] Memory 1304 A, 1304B 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, telecommunications 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.

[0151] In addition to memory 1304 A, 1304B (e.g., computer-readable storage medium), processing circuitry 1302A, 1302B may also be connected to one or more interfaces for displaying, transmitting and / or receiving information. The interfaces may comprise communication interface 1308A, 1308B 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.

[0152] Execution of instructions 1306 A, 1306B by corresponding processing circuitry 1302 A, 1302B, or storage of the instructions in corresponding memory 1304 A, 1304B, supports combinations of operations for implementing example implementations of the present disclosure. In this manner, apparatus 1300 A, 13006 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] FIG. 14 is a block diagram illustrating apparatus 1400 according to some examples. Apparatus 1400 may be implemented to comprise or to form at least part of UE such as UE 110 discussed above. As shown, apparatus 1400 may comprise means 1402 for receiving, from handover source access node 202A, a message in association with a handover to a target cell provided by handover target access node 202B. The message may comprise a cell-specific baseline configuration for the handover of UE 110 to the target cell, and a handover identifier in association with the handover of UE 110.

[0157] Apparatus 1400 may further comprise means 1404 for initiating, based on the cellspecific baseline configuration, a random-access procedure towards the target cell to establish an RRC connection with handover target access node 202B providing the target cell. For example, means 1404 may transmit a random-access preamble to handover target access node 202B, according to the cell-specific baseline configuration in association with the target cell.

[0158] Apparatus 1400 may further comprise means 1406 for transmitting an RRC reconfiguration complete message comprising the handover identifier to handover target access node 202B during the random-access procedure, and means 1408 for receiving, from handover target access node 202B, a message comprising a supplementary configuration for the handover of UE 110 to the target cell and a C-RNTI for UE 110 in the target cell. The C-RNTI is different from the handover identifier used during the handover of UE 110.

[0159] Although not shown in FIG. 14, apparatus 1400 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.

[0160] FIG. 15 is a block diagram illustrating apparatus 1500 according to some examples. Apparatus 1500 may be implemented to comprise or to form at least part of UE such as UE 110 discussed above. As shown, apparatus 1500 may comprise means 1502 for receiving, from handover source access node 202A, a message in association with a conditional handover to one or more conditional target cells provided by one or more handover candidate target access nodes 202B. The message may comprise one or more cell-specific baseline configurations and one or more execution conditions for the conditional handover of UE 110 to one or more candidatetarget cells, and one or more handover identifiers assigned to UE 110 with respect to the one or more candidate target cells. The one or more cell-specific baseline configurations each may comprise basic configuration information for UE 110 to connect to a corresponding candidate target cell.

[0161] Apparatus 1500 may further comprise means 1504 for determining information on radio link quality in association with a source cell and at least one of the one or more candidate target cells, and means 1506 for selecting at least one target cell from the one or more candidate target cells based on the determined information on radio link quality and the one or more execution conditions for the one or more candidate target cells. For example, means 1504 may periodically measure radio link quality of the source cell and one or more candidate target cells, and means 1506 may evaluate whether the one or more candidate target cells satisfy corresponding execution conditions. If one or more candidate target cells satisfy the execution conditions, means 1506 may select at least one of the one or more candidate target cells having the best radio link quality as the target cell.

[0162] Apparatus 1500 may further comprise means 1508 for initiating a random-access procedure towards the selected at least one target cell based on the cell-specific baseline configuration, so as to establish an RRC connection with at least one handover target access node 202B providing the at least one target radio cell, means 1510 for transmitting a reconfiguration complete message comprising a handover identifier in association with the target cell to handover target access node 202B during the random-access procedure, and means 1502 for receiving a message comprising a supplementary configuration for the handover of UE 110 to the target cell and a C-RNTI from handover target access node 202B.

[0163] Although not shown in FIG. 15, apparatus 1500 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.

[0164] Fig 16 is a block diagram illustrating apparatus 1600 according to some examples. Apparatus 1600 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 1600 may comprise means 1602 for receiving, from handover target access node 202B, a cell-specific baseline configuration for a handover to a target cell provided by handover target access node 202B, means 1604 for receiving, from UE 110, information on radio link quality in association with a source cell and the target cell, means 1606 for determining, based on at least the information on radio link quality, to carry out the handover of UE 110 to the target cell using the cell-specific baseline configuration, and means 1608 for determining a handover identifier for the determined handover of UE 110. The handover identifier is different from a C-RNTI for UE110 in the target cell. In some examples, the cell-specific baseline configuration may comprise a pool of preconfigured / reserved handover identifiers, and means 1608 may comprise means 1608- 1 for selecting a handover identifier from the pool for the determined handover of UE 110. In some other examples, means 1608 may comprise means 1608-2 for generating a handover identifier for the determined handover of UE 110. For example, means 1608-2 can generate a handover identifier for UE 110 based on one or more of a current C-RNTI of UE 110 in association with the source cell, an identifier (e.g., PCI) of the source cell, an identifier of a node hosting RRC of UE 110, an identifier of an entity in handover source access node 202Athat hosts RRC context of UE 110, an identifier (e.g., PCI) of the target cell, and an identifier of handover target access node 202B.

[0165] Apparatus 1600 may further comprise means 1610 for transmitting a message in association with the determined handover to UE 110, the message comprising the handover identifier and at least part of configuration information in the cell-specific baseline configuration in association with the target cell, and means 1612 for transmitting handover information comprising information on UE 110 and the handover identifier to handover target access node 202B.

[0166] It would be appreciated that apparatus 1600 may further comprise additional means (not shown in FIG. 16) for performing functions corresponding to operations, steps or elements in procedures, methods and examples discussed above with regard to handover source access node 202A.

[0167] FIG. 17 is a block diagram illustrating apparatus 1700 according to some examples. Apparatus 1700 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 1700 may comprise means 1702 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 cells provided by one or more neighboring access nodes 202B, means 1704 for receiving, from UE 110, information on radio link quality in association with a source cell and at least one of the one or more neighboring cells, means 1706 for determining, based on at least the received information on radio link quality, to carry out a conditional handover of UE 110 to one or more candidate target cells selected from the one or more neighboring cells, and means 1708 for determining one or more handover identifier for UE 110 with respect to the one or more candidate target cells. The handover identifier is different from a C-RNTI for UE 110 in a candidate target cell. In some examples, the cell-specific baseline configuration may comprise a pool of preconfigured / reserved handover identifiers, and means 1708 may comprise means 1708- 1 for selecting a handover identifier from the pool for the handover of UE 110 to a correspondingcandidate radio cell. In some other examples, means 1708 may comprise means 1708-2 for generating a handover identifier for the handover of UE 110 to a corresponding candidate target cell. For example, means 1708-2 can generate a handover identifier for UE 110 based on one or more of a current C-RNTI of UE 110 in association with the source cell, an identifier (e.g., PCI) of the source cell, an identifier of a node hosting RRC of UE 110, an identifier of an entity in handover source access node 202A that hosts RRC context of UE 110, an identifier (e.g., PCI) of the target cell, and an identifier of handover target access node 202B.

[0168] Apparatus 1700 may further comprise means 1710 for transmitting a message in association with the conditional handover to UE 110, the message comprising the handover identifier and at least part of configuration information indicated in the cell-specific baseline configurations in association with the one or more candidate target cells, and means 1712 for transmitting handover information comprising information on UE 110 and the one or more handover identifiers to handover target access node 202B. It would be appreciated that apparatus 1700 may further comprise additional means (not shown in FIG. 17) for performing functions corresponding to operations, steps or elements in procedures, methods and examples discussed above with regard to handover source access node 202A.

[0169] FIG. 18 is a block diagram illustrating apparatus 1800 according to some examples. Apparatus 1800 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 1800 may comprise means 1802 for transmitting one or more cell-specific baseline configurations in association with one or more candidate target cells provided by handover candidate target access node 202B to handover source access node 202A, and means 1804 for receiving handover information from handover source access node 202A. The handover information may comprise information on UE 110 to be handed over and one or more handover identifiers assigned to UE 110 with respect to the one or more candidate target cells. The handover identifier is different from a C-RNTI for UE 110 in a candidate target cell. In some examples, the cell-specific baseline configuration may comprise a pool of preconfigured / reserved handover identifiers, and the one or more handover identifiers received in the handover information are selected from respective pools in association with the one or more candidate target cells. Although shown, apparatus 1800 may further comprise means for generating a pool of handover identifiers for a corresponding candidate target cell. For example, a handover identifier may be generated based on one or more of an identifier (e.g., PCI) of a source cell, an identifier of a handover source access node, an identifier of a candidate target cell, an identifier of a handover candidate target access node, and a random number or sequence.

[0170] Apparatus 1800 may further comprise means 1806 for carrying out a random-accessprocedure initiated by UE 110 to establish an RRC connection with UE 110. For example, means 1806 may receive a random-access preamble towards a target radio cell from UE 110. The target radio cell is selected from the one or more candidate target cells provided by handover candidate target access node 202B.

[0171] Apparatus 1800 may further comprise means 1808 for receiving a reconfiguration complete message comprising a handover identifier in association with the target radio cell from UE 110 during the random-access procedure, and means 1810 for generating a supplementary configuration for the handover (fast handover or conditional fast handover) of UE 110 to the target cell based on the information on UE 110. The supplementary configuration may comprise UE-specific configuration information needed to complete the handover 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.

[0172] Apparatus 1800 may further comprise means 1812 for transmitting the supplementary configuration and a C-RNTI to UE 110 to complete the handover procedure. It would be appreciated that apparatus 1800 may further comprise additional means (not shown in FIG. 18) 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.

[0173] 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.

[0174] As used herein, “at least one of the following: ” and “at least one of ” and similar wording, where the list of two or more 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.

[0175] 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. Incertain circumstances, multitasking and parallel processing may be advantageous. Likewise, while several specific implementation details are contained in the above discussions, these should not be construed as limitations on the scope of the present disclosure, but rather as descriptions of features that may be specific to particular embodiments. Certain features that are described in the context of separate embodiments may also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment may also be implemented in multiple embodiments separately or in any suitable sub-combination.

[0176] Although the subject matter has been described in a language that is specific to structural features and / or method actions, it is to be understood the subject matter defined in the appended claims is not limited to the specific features or actions described above. On the contrary, the above-described specific features and actions are disclosed as an example of implementing the claims.

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 handover to a target cell provided by a handover target access node, the message comprising a cell-specific baseline configuration for the handover of the user equipment to the target cell and a handover identifier assigned to the user equipment; initiate a random-access procedure to the target cell to establish a radio resource control connection with the handover target access node, according to the cell-specific baseline configuration; transmit, by the user equipment to the handover target access node during the randomaccess procedure, a reconfiguration complete message comprising the handover identifier; and receive, by the user equipment from the handover target access node, a message comprising a supplementary configuration for the handover of the user equipment to the target cell and a cell radio network temporary identifier assigned to the user equipment.

2. The apparatus of claim 1, wherein the handover identifier is different from the cell radio network temporary identifier.

3. 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 to one or more candidate target cells provided by one or more handover candidate target access nodes, the message in association with the45conditional handover comprising one or more cell-specific baseline configurations for the conditional handover of the user equipment to the one or more candidate target cells, one or more execution conditions for the one or more candidate target cells, and one or more handover identifiers assigned to the user equipment with respect to the one or more candidate target cells; 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, a target cell from the one or more candidate target cells for the conditional handover based on the one or more execution conditions and the determined information on radio link quality; initiate a random-access procedure to the selected target cell to establish a radio resource control connection with a handover target access node providing the target cell, according to the cell-specific baseline configuration; transmit, by the user equipment to the handover target access node during the randomaccess procedure, a reconfiguration complete message comprising the handover identifier assigned to the user equipment with respect to the target cell; and receive, from the handover target access node, a message comprising a supplementary configuration for the conditional handover of the user equipment and a cell radio network temporary identifier assigned to the user equipment.

4. 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 cellspecific baseline configuration for a handover to a target cell provided by the handover target access node;46receive, 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 based on the received information on radio link quality; determine, by the handover source access node, a handover identifier for the user equipment; 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, the message comprising at least part of the cell-specific baseline configuration in association with the target cell and the handover identifier for the user equipment; and transmit, by the handover source access node to the handover target access node, handover information comprising information on the user equipment and the handover identifier for the user equipment.

5. The apparatus of claim 4, wherein the handover identifier is different from a cell radio network temporary identifier of the user equipment in association with the target cell.

6. The apparatus of claim 4 or 5, wherein the apparatus being caused to determine the handover identifier for the user equipment comprises the apparatus being caused to at least: generate, by the handover source access node, the handover identifier for the user equipment with respect to the target cell, or in case the cell-specific baseline configuration in association with the target cell comprises a pool of reserved handover identifiers, select the handover identifier for the user equipment from the pool of reserved handover identifiers.

7. The apparatus of claim 6, wherein the handover identifier is generated based on one or more of: a cell radio network temporary identifier of the user equipment in association with a source cell provided by the handover source access node;47an identifier of the source cell; an identifier of a node hosting radio resource control of the user equipment; an identifier of an entity in the handover source access node that hosts radio resource control context of the user equipment; an identifier of the target cell; and an identifier of the handover target access node.

8. 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 conditional handover to one or more neighboring cells provided by the one or more neighboring access nodes; receive, by the handover source access node from a user equipment, information on radio link quality in association with at least one source cell and at least one of the one or more neighboring cells; determine, by the handover source access node, to carry out conditional handover of the user equipment to one or more candidate target cells provided by one or more handover candidate target access nodes based on the received information on radio link quality, the one or more candidate target cells being selected from the one or more neighboring cells; determine, by the handover source access node, one or more handover identifiers for the user equipment with respect to the one or more candidate target cells; 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 and the one or more handover identifiers for the user equipment with respect to the one or more candidate target cells; andtransmit, by the handover source access node to the one or more handover candidate target access nodes, handover information comprising information on the user equipment and the one or more handover identifiers for the user equipment with respect to the one or more candidate target cells.

9. 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, one or more cell-specific baseline configurations in association with one or more candidate target cells provided by the handover target access node; receive, by the handover target access node from the handover source access node, handover information comprising information on a user equipment and one or more handover identifiers assigned to the user equipment with respect to the one or more candidate target cells; carry out, by the handover target access node, a random-access procedure initiated by the user equipment in association with a target cell selected from the one or more candidate target cells to establish a radio resource control connection with the user equipment; receive, by the handover target access node from the user equipment during the random-access procedure, a reconfiguration complete message comprising the handover identifier assigned to the user equipment with respect to the target cell; generate a supplementary configuration for the user equipment based on the information on the user equipment; and transmit, by the handover target access node to the user equipment, a message comprising the supplementary configuration and a cell radio network temporary identifier assigned to the user equipment in the target cell.

10. The apparatus of claim 9, wherein the handover identifier is different from the cell radio network temporary identifier.

11. The apparatus of claim 9 or 10, wherein the cell-specific baseline configuration in association with the candidate target cell comprises a pool of reserved handover identifiers, and the handover identifier received from the user equipment is selected from the pool of reserved handover identifiers.

12. The apparatus of claim 11, wherein the reserved handover identifiers are generated based on one or more of: an identifier of a source cell; an identifier of a handover source access node; an identifier of the candidate target cell; an identifier of the handover target access node; and a random number or sequence.

13. A method, comprising: receiving, by a user equipment from a handover source access node, a message in association with a handover to a target cell provided by a handover target access node, the message comprising a cell-specific baseline configuration for the handover of the user equipment to the target cell and a handover identifier assigned to the user equipment; initiating a random-access procedure to the target cell to establish a radio resource control connection with the handover target access node, according to the cell-specific baseline configuration; transmitting, by the user equipment to the handover target access node during the randomaccess procedure, a reconfiguration complete message comprising the handover identifier; and receiving, by the user equipment from the handover target access node, a message comprising a supplementary configuration for the handover of the user equipment to the target cell and a cell radio network temporary identifier assigned to the user equipment.

14. The method of claim 13, wherein the handover identifier is different from the cell radio network temporary identifier.

15. A method, comprising: receiving, by a user equipment from a handover source access node, a message in association with a conditional handover to one or more candidate target cells provided by one or more handover candidate target access nodes, the message in association with the conditional handover comprising one or more cell-specific baseline configurations for the conditional handover of the user equipment to the one or more candidate target cells, one or more execution conditions for the one or more candidate target cells, and one or more handover identifiers assigned to the user equipment with respect to the one or more candidate target cells; 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, a target cell from the one or more candidate target cells for the conditional handover based on the one or more execution conditions and the determined information on radio link quality; initiating a random-access procedure to the selected target cell to establish a radio resource control connection with a handover target access node providing the target cell, according to the cell-specific baseline configuration; transmitting, by the user equipment to the handover target access node during the randomaccess procedure, a reconfiguration complete message comprising the handover identifier assigned to the user equipment with respect to the target cell; and receiving, from the handover target access node, a message comprising a supplementary configuration for the conditional handover of the user equipment and a cell radio network temporary identifier assigned to the user equipment.

16. A method, comprising: receiving, by a handover source access node from a handover target access node, a cellspecific baseline configuration for a handover to a target cell provided by the handover target access node;51receiving, 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 based on the received information on radio link quality; determining, by the handover source access node, a handover identifier for the user equipment; 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, the message comprising at least part of the cell-specific baseline configuration in association with the target cell and the handover identifier for the user equipment; and transmitting, by the handover source access node to the handover target access node, handover information comprising information on the user equipment and the handover identifier for the user equipment.

17. The method of claim 16, wherein the handover identifier is different from a cell radio network temporary identifier of the user equipment in association with the target cell.

18. The method of claim 16 or 17, wherein determining the handover identifier for the user equipment comprises: generating, by the handover source access node, the handover identifier for the user equipment with respect to the target cell, or in case the cell-specific baseline configuration in association with the target cell comprises a pool of reserved handover identifiers, selecting the handover identifier for the user equipment from the pool of reserved handover identifiers.

19. The method of claim 18, wherein the handover identifier is generated based on one or more of: a cell radio network temporary identifier of the user equipment in association with a source cell provided by the handover source access node;52an identifier of the source cell; an identifier of a node hosting radio resource control of the user equipment; an identifier of an entity in the handover source access node that hosts radio resource control context of the user equipment; an identifier of the target cell; and an identifier of the handover target access node.

20. 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 conditional handover to one or more neighboring cells provided by the one or more neighboring access nodes; receiving, by the handover source access node from a user equipment, information on radio link quality in association with at least one source cell and at least one of the one or more neighboring cells; determining, by the handover source access node, to carry out conditional handover of the user equipment to one or more candidate target cells provided by one or more handover candidate target access nodes based on the received information on radio link quality, the one or more candidate target cells being selected from the one or more neighboring cells; determining, by the handover source access node, one or more handover identifiers for the user equipment with respect to the one or more candidate target cells; 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 and the one or more handover identifiers for the user equipment with respect to the one or more candidate target cells; and transmitting, by the handover source access node to the one or more handover candidate target access nodes, handover information comprising information on the user equipment and the one or more handover identifiers for the user equipment with respect to the one or more candidate target cells.5321. A method, comprising: transmitting, by a handover target access node to a handover source access node, one or more cell-specific baseline configurations in association with one or more candidate target cells provided by the handover target access node; receiving, by the handover target access node from the handover source access node, handover information comprising information on a user equipment and one or more handover identifiers assigned to the user equipment with respect to the one or more candidate target cells; carrying out, by the handover target access node, a random-access procedure initiated by the user equipment in association with a target cell selected from the one or more candidate target cells to establish a radio resource control connection with the user equipment; receiving, by the handover target access node from the user equipment during the randomaccess procedure, a reconfiguration complete message comprising the handover identifier assigned to the user equipment with respect to the target cell; generating a supplementary configuration for the user equipment based on the information on the user equipment; and transmitting, by the handover target access node to the user equipment, a message comprising the supplementary configuration and a cell radio network temporary identifier assigned to the user equipment in the target cell.

22. The method of claim 21, wherein the handover identifier is different from the cell radio network temporary identifier.

23. The method of claim 21 or 22, wherein the cell-specific baseline configuration in association with the candidate target cell comprises a pool of reserved handover identifiers, and the handover identifier received from the user equipment is selected from the pool of reserved handover identifiers.

24. The apparatus of claim 23, wherein the reserved handover identifiers are generated based on one or more of54an identifier of a source cell; an identifier of a handover source access node; an identifier of the candidate target cell; an identifier of the handover target access node; and a random number or sequence.

25. An apparatus, comprising means for performing the method of any of claims 13-24.55

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