Method, first distributed unit and user equipment

Abstract

The present disclosure relates to apparatus and methods for lower layer triggered mobility. Improved methods and apparatus for inter-CU lower layer triggered mobility are disclosed. Methods of base station initiated inter-CU lower layer triggered mobility, and base station initiated uplink measurement based inter-CU lower layer triggered mobility, are disclosed. Methods for inter-CU lower layer triggered mobility with conditional handover are also disclosed.

Description

METHOD, FIRST DISTRIBUTED UNIT AND USER EQUIPMENT

[0001] The present disclosure relates to a communication system. The disclosure has particular but not exclusive relevance to wireless communication systems and devices thereof operating according to the 3rd Generation Partnership Project (3GPP) standards or equivalents or derivatives thereof (including LTE-Advanced, Next Generation or 5G networks, future generations, and beyond), and to lower layer triggered mobility (LTM).

[0002] Earlier developments of the 3GPP standards were referred to as the Long-Term Evolution (LTE) of Evolved Packet Core (EPC) network and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), also commonly referred as '4G'. More recently, the terms '5G' and 'new radio' (NR) are used to refer to an evolving communication technology that is expected to support a variety of applications and services. Various details of 5G networks are described in, for example, NPL 1. 3GPP intends to support 5G by way of the so-called 3GPP Next Generation (NextGen) radio access network (RAN) and the 3GPP NextGen core network.

[0003] Under the 3GPP standards, a NodeB (or an eNB in LTE, and gNB in 5G) is the radio access network (RAN) node (or simply 'access node', 'access network node' or 'base station') via which communication devices (user equipments or 'UEs') connect to a core network and communicate with other communication devices or remote servers. For simplicity, the present application will use the term access network node, RAN node or base station to refer to any such access nodes.

[0004] Also for simplicity, the present application will use the term mobile device, user device, or UE to refer to any communication device that is able to connect to the core network via one or more base stations. Although the present application may refer to mobile devices in the description, it will be appreciated that the technology described can be implemented on any communication devices (mobile and / or generally stationary) that can connect to a communications network for sending / receiving data, regardless of whether such communication devices are controlled by human input or software instructions stored in memory.

[0005] In the current 5G architecture, the gNB structure may be split into two or more parts. In some RAN implementations there are two parts, known as the Central Unit (CU or gNB-CU) - sometimes referred to as a 'control unit' - and the Distributed Unit (DU or gNB-DU), connected by an F1 interface. This enables the use of a 'split' architecture in which the typically 'higher' CU layers (for example, but not necessarily or exclusively, Packet Data Convergence Protocol (PDCP) and Radio Resource Control (RRC) layers) and the, 'lower' DU layers (for example, but not necessarily or exclusively, Radio Link Control (RLC), Media Access Control (MAC), and Physical (PHY) layers) are separated between a particular CU, and one or more DUs that are connected to and controlled by that CU via the F1 interface. Thus, for example, the higher layer CU functionality for a number of gNBs may be implemented centrally (for example, by a single processing unit, or in a cloud-based or virtualised system), whilst retaining the lower layer DU functionality locally separately for each gNB.

[0006] Historically, mobility between different cells in cellular communications has been based on communication at higher layers, such as layer 3 (e.g., the L3 or radio resource control (RRC) layer) signalling. More recently, with a view to providing enhanced mobility, consideration has given to developing and providing support for layer 1 (e.g., the L1 or physical (PHY) layer) and / or layer 2 (e.g., the L2 or media access control (MAC) layer) centric mobility (also referred to as L1 / L2 centric mobility) rather than at higher layers (e.g., the RRC layer). Such L1 / L2 centric mobility (also referred to as L1 / L2 triggered mobility, or 'lower layer triggered mobility' (LTM)) has prospects for improving mobility for devices operating both below 7 GHz and in mmWave bands, for example by supporting lower latency handover and improved robustness. However, LTM is currently only supported for the case of intra-CU handover (including inter-DU and intra-DU scenarios).

[0007] NPL 1: Next Generation Mobile Networks (NGMN) Alliance, 'NGMN 5G White Paper' V1.0, February 17, 2015 <https: / / www.ngmn.org / 5g-white-paper.html>

[0008] There is a need for the development of communication devices (such as base stations and / or UEs), and corresponding methods, that support efficient and robust mechanisms for inter-CU based LTM. In particular, there is a need for improved methods and apparatus for supporting reliable, secure and efficient inter-CU based LTM handover.

[0009] The disclosure aims to provide apparatus and methods that at least partially address the above needs and / or issues.

[0010] A method performed by a first distributed unit of a first base station according to the present disclosure, the method comprising: 　　transmitting, to a second central unit of a second base station via a first central unit of the first base station, first information of candidate Layer 1 (L1) / Layer 2 (L2) triggered mobility (LTM) cells; and 　　receiving, from the second central unit of the second base station via the first central unit of the first base station, information for configuration on L1 measurements on the candidate LTM cells on which the second central unit of the second base station has configured a second distributed unit of the second base station, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements.

[0011] A method performed by a user equipment (UE) according to the present disclosure, the method comprising: 　　receiving, from a second central unit of a second base station via a first central unit of a first base station and a first distributed unit of the first base station, information of configuration on layer 1 (L1) measurements on candidate L1 / layer 2 (L2) triggered mobility (LTM) cells on which the second central unit of the second base station has configured the second distributed unit of the second base station; and 　　transmitting, to the first distributed unit of the first base station, L1 measurement reports of results of the L1 measurement performed based on the information of configuration, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements.

[0012] A first distributed unit of a first base station according to the present disclosure, comprising: 　　means for transmitting, to a second central unit of a second base station via a first central unit of the first base station, first information of candidate Layer 1 (L1) / Layer 2 (L2) triggered mobility (LTM) cells; and 　　means for receiving, from the second central unit of the second base station via the first central unit of the first base station, information for configuration on L1 measurements on the candidate LTM cells on which the second central unit of the second base station has configured a second distributed unit of the second base station, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements.

[0013] A user equipment (UE) according to the present disclosure, comprising: 　　means for receiving, from a second central unit of a second base station via a first central unit of a first base station and a first distributed unit of the first base station, information of configuration on layer 1 (L1) measurements on candidate L1 / layer 2 (L2) triggered mobility (LTM) cells on which the second central unit of the second base station has configured the second distributed unit of the second base station; and 　　means for transmitting, to the first distributed unit of the first base station, L1 measurement reports of results of the L1 measurement performed based on the information of configuration, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements.

[0014] Detailed examples of the disclosure will now be described, by way of example, with reference to the accompanying drawings in which:Fig. 1 schematically illustrates a mobile ('cellular' or 'wireless') communication system 1;Fig. 2 illustrates a typical frame structure that may be used in the communication system 1 of Fig. 1;Fig. 3 illustrates a user plane protocol stack;Fig. 4 illustrates a control plane protocol stack;Fig. 5 illustrates examples of intra-CU and inter-CU handovers;Fig. 6 illustrates an example of base station initiated inter-CU LTM;Fig. 7 illustrates an example of base station initiated uplink measurement based inter-CU LTM;Fig. 8 illustrates an example of inter-CU LTM with conditional handover;Fig. 9 illustrates information elements included in a CSI report;Fig. 10 is a schematic block diagram illustrating the main components of a UE 3 for the communication system 1 of Fig. 1;Fig. 11 is a schematic block diagram illustrating the main components of a base station 5 of a distributed type for the communication system 1 of Fig. 1; andFig. 12 is a schematic block diagram illustrating the main components of a core network node or function 7 for the communication system 1 of Fig. 1.

[0015] Overview 　　An exemplary communication system will now be described in general terms, by way of example only, with reference to Figs. 1 to 5.

[0016] Fig. 1 schematically illustrates a mobile ('cellular' or 'wireless') communication system 1 to which examples of the present disclosure are applicable.

[0017] In the communication system 1, user equipment (UEs) 3-1, 3-2, 3-3 (e.g. mobile telephones and / or other mobile devices) can communicate with each other via a radio access network (RAN) node 5 that operates according to one or more compatible radio access technologies (RATs). In the illustrated example, the RAN comprises a distributed base station 5 or 'gNB' operating one or more associated cells 9. Communication via the RAN is typically routed through an associated core network 7 (e.g. a 5G / 6G or later generations core network or evolved packet core network (EPC)).

[0018] As those skilled in the art will appreciate, whilst three UEs 3 and one base station 5 are shown in Fig. 1 for illustration purposes, the system, when implemented, will typically include other base stations 5 and UEs 3.

[0019] Each base station 5 controls one or more associated cells 9 either directly, or indirectly via one or more other nodes (such as home base stations, relays, remote radio heads, distributed units, and / or the like). It will be appreciated that the base stations 5 may be configured to support 4G, 5G, 6G and / or later generations, and / or any other 3GPP or non-3GPP communication protocols.

[0020] In this example the illustrated RAN comprises a distributed base station 5 comprising at least one distributed unit (DU) 5b (e.g., a gNB-DU or the like), and a central unit (CU) 5c (e.g., a gNB-CU or the like). The CU 5c employs a separated control plane and user plane and so is, itself, split between a control plane function (CU-CP) and a user plane function (CU-UP) which respectively communicate, with the DU 5b via an appropriate interface (e.g. an F1-C interface) and an appropriate interface (e.g. an F1-U logical interface) (together forming an appropriate interface (e.g. an F1 interface (or 'reference point'))), and with one another via an appropriate interface (e.g. an E1 logical interface). It will be appreciated that while, in this example, the DU 5b includes the physical and virtual elements required to provide the functionality of the lower parts of the PHY layer and hence communicate with the UEs 3 over the air interface, the RAN may alternatively (or additionally) include one or more separate radio units (RUs) (e.g., providing this functionality of the lower parts of the PHY layer). It will, nevertheless, be appreciated that whilst distributed RAN node 5 is shown and described, the RAN node 5 may be provided in a non-distributed form, for example as an integrated base station.

[0021] The UEs 3 and their serving base station 5 are connected via an appropriate air interface (for example the so-called 'Uu' interface and / or the like). Neighbouring base stations 5 may be connected to each other via an appropriate base station to base station interface (such as the so-called 'X2' interface, 'Xn' interface and / or the like).

[0022] The core network 7 includes a number of logical nodes (or 'functions') for supporting communication in the communication system 1. In this example, the core network 7 comprises control plane functions (CPFs) 10 and one or more network node entities for the communication of user data (e.g. user plane functions (UPFs) 11). The CPFs 10 include one or more network node entities for the communication of control signalling (e.g. Access and Mobility Management Functions (AMFs) 10-1), one or more network node entities for session management (e.g. Session Management Functions (SMFs) 10-2) and a number of other functions 10-n (such as, for example, an Authentication Server Function (AUSF) which facilitates security processes, a Unified Data Management (UDM) entity for managing user specific data (e.g., for access authorization, user registration, and data network profiles), a Policy Control Function (PCF), an Application Function (AF), and / or the like). It will be appreciated that the nodes or functions may have different names in different systems.

[0023] The RAN node 5 is connected to the core network nodes via appropriate interfaces (or 'reference points') such as an N2 reference point between the CU 5c (CU-CP) of the RAN node 5 and the AMF 10-1 for the communication of control signalling, and an N3 reference point between the CU 5c (CU-UP) of the RAN node 5 and each UPF 11 for the communication of user data. The UEs 3 are each connected to the AMF 10-1 via a non-access stratum (NAS) connection over an appropriate interface (e.g. an N1 reference point (analogous to the S1 reference point in LTE)). It will be appreciated that N1 communications are routed transparently via the RAN node 5.

[0024] One or more UPFs 11 are connected to an external data network (e.g., an IP network such as the Internet) via an appropriate reference point (e.g. an N6 reference point) for communication of the user data.

[0025] The AMF 10-1 performs mobility management related functions, maintains the NAS connection with each UE 3 and manages UE registration. The AMF 10-1 is also responsible for managing paging. The SMF 10-2 provides session management functionality (that formed part of MME functionality in LTE) and additionally combines some control plane functions (provided by the serving gateway and packet data network gateway in LTE). The SMF 10-2 also allocates IP addresses to each UE 3.

[0026] The base station 5 of the communication system 1 may be configured to operate at least one cell 9 on an associated time-division duplex (TDD) carrier that operates in unpaired spectrum. It will be appreciated that the base station 5 may also operate at least one cell 9 on an associated frequency-division duplex (FDD) carrier that operates in paired spectrum.

[0027] The base station 5 is also configured for transmission of, and the UEs 3 are configured for the reception of, control information and user data via a number of downlink (DL) physical channels and for transmission of a number of physical signals. The DL physical channels correspond to resource elements (REs) carrying information originating from a higher layer, and the DL physical signals are used in the physical layer and correspond to REs which do not carry information originating from a higher layer.

[0028] The physical channels may include, for example, a physical downlink shared channel (PDSCH), a physical broadcast channel (PBCH), and a physical downlink control channel (PDCCH). The PDSCH carries data sharing the PDSCH's capacity on a time and frequency basis. The PDSCH can carry a variety of items of data including, for example, user data, UE-specific higher layer control messages mapped down from higher channels, system information blocks (SIBs), and paging. The PDCCH carries downlink control information (DCI) for supporting a number of functions including, for example, scheduling the downlink transmissions on the PDSCH and also the uplink data transmissions on a physical uplink shared channel (PUSCH). The PBCH provides UEs 3 with the Master Information Block (MIB). It also, in conjunction with the PDCCH, supports the synchronisation of time and frequency, which aids cell acquisition, selection and re-selection. The UE 3 may receive a Synchronization Signal Block (SSB), and the UE 3 may assume that reception occasions of a PBCH, primary synchronization signal (PSS) and secondary synchronization signal (SSS) are in consecutive symbols and form a SS / PBCH block. The base station 5 may transmit a number of synchronization signal (SS) blocks corresponding to different DL beams. The total number of SS blocks may be confined, for example, within a 5 ms duration as an SS burst. The periodicity of the SSB transmissions may be indicated to the UE using any suitable signalling (e.g. per serving cell using ssb-periodicityServingCell). The periodicity value for the SSB may be, for example, greater than or equal to 20 ms. For initial cell selection, the UE 3 may be configured to assume that an SS burst occurs with a periodicity of 2 frames. The UE 3 may also be provided with an indication of which SSBs within a 5 ms duration are transmitted (e.g. using ssb-PositionsInBurst).

[0029] The DL physical signals may include, for example, reference signals (RSs) and synchronization signals (SSs). A reference signal (sometimes known as a pilot signal) is a signal with a predefined special waveform known to both the UE 3 and the base station 5. The reference signals may include, for example, cell specific reference signals, UE-specific reference signal (UE-RS), downlink demodulation signals (DMRS), and channel state information reference signal (CSI-RS).

[0030] Similarly, the UEs 3 are configured for transmission of, and the base station 5 is configured for the reception of, control information and user data via a number of uplink (UL) physical channels corresponding to REs carrying information originating from a higher layer, and UL physical signals which are used in the physical layer and correspond to REs which do not carry information originating from a higher layer. The physical channels may include, for example, the PUSCH, a physical uplink control channel (PUCCH), and / or a physical random-access channel (PRACH). The UL physical signals may include, for example, demodulation reference signals (DMRS) for a UL control / data signal, and / or sounding reference signals (SRS) used for UL channel measurement.

[0031] When the UE 3 initially establishes a radio resource control (RRC) connection with a base station 5 via a cell 9 it registers with an appropriate core network node (e.g., AMF 10-1, MME). The UE 3 is in the so-called RRC connected state and an associated UE context is maintained by the network. When the UE 3 is in the so-called RRC idle state, or is in the RRC inactive state, it selects an appropriate cell for camping so that the network is aware of the approximate location of the UE 3 (although not necessarily on a cell level).

[0032] As mentioned above, the base station 5 in this example is a 'distributed' base station 5 that is split between one or more distributed units (DUs) 5b and a central unit (CU) 5c, with a CU 5c typically performing higher level functions and communication with the next generation core, and with the DU 5b performing lower level functions and communication over an air interface with UEs 3 in the vicinity (i.e. in a cell operated by the base station 5). A distributed base station 5 may, for example, include the following functional units hosting the following functions: 　　 Central Unit (CU): a logical node hosting Radio Resource Control (RRC), Service Data Adaptation Protocol (SDAP) and Packet Data Convergence Protocol (PDCP) layers of the base station 5 that controls the operation of one or more DUs. The CU terminates an appropriate interface (e.g. the so-called F1 interface) connected with the DU. 　　 Distributed Unit (DU): a logical node hosting Radio Link Control (RLC), Medium Access Control (MAC) and Physical (PHY) layers of the base station 5, and its operation is partly controlled by the CU. One DU supports one or multiple cells. One cell is supported by only one DU. The DU terminates an appropriate interface (e.g. the F1 interface) connected with the CU. 　　 CU-Control Plane (CU-CP): a logical node hosting the RRC and the control plane part of the PDCP protocol of the CU for the base station 5. The CU-CP terminates an appropriate interface (e.g. the so-called E1 interface) connected with the CU-UP and an appropriate interface (e.g. the F1-C (F1 control plane) interface) connected with the DU. 　　 CU-User Plane (CU-UP): a logical node hosting the user plane part of the PDCP protocol and the SDAP protocol of the CU for the base station 5. The CU-UP terminates an appropriate interface (e.g. the E1 interface) connected with the CU-CP and an appropriate interface (e.g. the F1-U (F1 user plane) interface) connected with the DU.

[0033] Frame Structure 　　Referring to Fig. 2, which illustrates a typical frame structure that may be used in the communication system 1, the base station 5 and UEs 3 of the communication system 1 communicate with one another using resources that are organised, in the time domain, into frames of length 10 ms. Each frame comprises ten equally sized subframes of 1 ms length. Each subframe is divided into one or more slots comprising 14 orthogonal frequency-division multiplexing (OFDM) symbols of equal length.

[0034] As seen in Fig. 2, the communication system 1 supports multiple different numerologies (subcarrier spacing (SCS), slot lengths and hence OFDM symbol lengths). Specifically, each numerology is identified by a parameter, μ, where μ=0 represents 15 kHz (corresponding to the LTE SCS). Currently, the SCS for other values of μ can, in effect, be derived from μ = 0 by scaling up in powers of 2 (i.e. SCS = 15 x 2μkHz). The relationship between the parameter, μ, and SCS (Δf) is as shown in Table 1:

[0035] Protocol Stacks 　　Fig. 3 illustrates a user plane protocol stack that can be used in the communication system illustrated in Fig. 1. The protocol stack includes a number of protocol layers that are terminated at the UE 3 and at the base station 5. As shown in Fig. 3, the protocol stack includes a physical (PHY) layer, Medium Access Control (MAC) layer, Radio Link Control (RLC) layer, Packet Data Convergence Protocol (PDCP) layer, and Service Data Adaptation Protocol (SDAP) layer. Fig. 4 illustrates a control plane protocol stack that can be used in the communication system illustrated in Fig. 1. As shown in Fig. 4, the control plane protocol stack includes the PHY, MAC, RLC and PDCP layers, as well as the radio resource control (RRC) layer.

[0036] As described above, the SDAP and PDCP layers may be hosted at a CU 5c. The RLC, MAC and PHY layers may be hosted at a DU 5b.

[0037] The MAC layer is a layer 2 (L2) sublayer. The MAC layer receives RLC PDUs (as MAC service data units (SDU)s) from the RLC sublayer and processes them to form MAC PDUs (which are packaged as transport blocks (TBs)) for transmission via the PHY sublayer. PDCP PDUs carrying data from the upper layers may also be referred to as PDCP data PDUs to distinguish them from a PDCP control PDU carrying control data. The MAC layer supports the HARQ protocol. The PHY layer is a layer 1 (L1) layer that can apply segmentation to large TBs to maintain the transmitted packet size below the configured maximum packet size, and enables transmission and reception via the air interface. Cyclic redundancy check (CRC) bits can be included in each TB at the PHY layer, to enable the receiving device to check for errors in the received TBs.

[0038] The RLC layer is a L2 sublayer. The RLC sublayer provides a radio link protocol used over the air interfaces between a UE 3 and the base station 5 (i.e., Uu). The RLC sublayer provides a number of functions depending on requirements including, for example: transfer of upper layer (PDCP) PDUs in one of three modes including acknowledged mode (AM), unacknowledged mode (UM) and transparent mode (TM); error correction through automatic repeat requests (ARQ) for AM data transfer; concatenation, segmentation and reassembly of RLC SDUs (UM and AM); re-segmentation of RLC data PDUs when a complete RLC PDU cannot be transmitted (AM); reordering of RLC data PDUs (UM and AM); duplicate detection (UM and AM); RLC SDU discard (UM and AM); RLC re-establishment; protocol error detection and recovery. One or more RLC entities may be established in the RLC sublayer for receiving data from higher layers and for processing the RLC SDUs through the RLC layer to become RLC PDUs (and vice versa). Following the processing of the RLC SDUs to form RLC PDUs, the RLC entity transfers the RLC PDUs to the MAC sublayer (where they are received as MAC SDUs). A receiving RLC entity in one device (e.g., the base station 5 or UE 3) can generate an RLC status report indicating the receive status of packets (RLC SDUs) successfully received (or not received) from a transmitting RLC entity at a corresponding peer device (e.g., the base station 5, or UE 3) and send the RLC status report as an RLC status PDU to the peer device. An RLC status report / PDU will typically include the receive status for a plurality of received RLC SDUs (and possibly RLC SDU segments). On receipt of the RLC status report / PDU, the RLC entity at the peer (transmitting) device will iterate through the positively acknowledged packets in the RLC status report / PDU and notify the PDCP layer of each RLC SDU (and hence PDCP PDU) that has been positively acknowledged. Accordingly the base station 5 and remote UE 3 may operate data radio bearers (DRBs) in an RLC acknowledge mode (AM).

[0039] The PDCP sublayer is the L2 sublayer that sits below the SDAP sublayer for user plane data, and below the RRC sublayer for control plane data. The PDCP sublayer provides a number of functions depending on requirements including, for example: transfer of user plane data (to / from the SDAP sublayer); transfer of control plane data (to / from the RRC sublayer); maintenance of PDCP sequence numbers; header compression and decompression (e.g., using the robust header compression (ROHC) protocol); ciphering and deciphering; integrity protection and integrity verification; timer based discarding of packets (PDCP SDUs); routing (e.g., for split bearers); duplication; reordering and in-order delivery; out-of-order delivery; and / or duplicate discarding.

[0040] The SDAP layer is the highest L2 sublayer of the protocol stack. It is responsible for quality of service (QoS) flow handling across the air (Uu) interface and N3 interface. The SDAP sublayer can have multiple SDAP entities (e.g., one for each protocol data unit (PDU) session between the base station 5 and remote UE 3) where SDAP entity establishment / release is initiated by RRC. The SDAP layer is responsible for the transfer of user plane data between the remote UE 3 and the core network (UPF 11). An SDAP entity maps each QoS flow from higher layers, within a particular PDU session, to a respective data radio bearer (DRB) established, via lower layers (e.g., PDCP and RLC) with the appropriate level of QoS, over the air interface between the UE 3 and the base station 5. As those skilled in the art will appreciate, the QoS flow may be in either the downlink or in the uplink and there may be more than one such QoS flow within the PDU session. It will be appreciated that the SDAP layer is not present in some architectures (e.g., in a non-stand-alone (NSA) architecture).

[0041] The RRC layer is considered to be a layer 3 (L3) sublayer and is the highest layer in the control plane of the access stratum (AS). It is also responsible for transferring messages of the non-access stratum (NAS), which is located above the RRC layer. The RRC layer is responsible for handling many RAN-related control plane procedures such as: broadcasting system Information (SI); transmission of paging messages to notify a UE about incoming connection requests; connection management; handling UE capabilities; and measurement configuration and reporting.

[0042] CU-DU Information Exchange 　　In the communication system 1, various messages may be exchanged between the CU 5c and the DU 5b, for instance during a setup procedure for configuring communication between the DU 5b and CU 5c (e.g., an 'F1 setup procedure'). The purpose of the setup procedure is to exchange application level data needed for the DU 5b and the CU 5c to interoperate correctly (e.g., on the F1 interface). This procedure is the initial procedure triggered for control plane communication (e.g., over the F1-C interface) after a transport network layer (TNL) association has become operational. Typically, this procedure will use non-UE associated signalling.

[0043] During such a procedure, the DU 5b may transmit a setup request message (e.g., an F1 Setup Request) to the CU 5c. Where the setup request message is an F1 Setup Request it may, for example, include the following information: 　　 -　　DU Served Cells List IE: Information about the cells supported by the DU 5b and associated features (e.g., NR-U); 　　 -　　DU System Information IE.

[0044] It will be appreciated that differently named information elements, having a similar purpose, may be used.

[0045] In response, the CU 5c typically transmits an F1 Setup Response message to the DU 5b. The following information may be included in the F1 Setup Response message: 　　 -　　Cells to be Activated List IE: a list of cells that the CU 5c requests the DU 5b to activate.

[0046] Further to the above messages, other messages (with associated information elements) may be used to update the configuration between the CU 5c and the DU 5b. For example, Public Land Mobile Network (PLMN) list, served cell information, DU Configuration Update or CU Configuration Update messages may be used to exchange information between the CU 5c and the DU 5b.

[0047] Additional messaging may be used between the CU 5c and the DU 5b, for instance when establishing a UE's context during a UE Context Setup procedure. The purpose of the UE Context Setup procedure is to establish the UE Context including, signalling radio bearer (SRB), and data radio bearer (DRB). This procedure uses UE associated signalling.

[0048] During such a procedure, the CU 5c may transmit a UE Context Setup Request message to the DU 5b (this message may comprise an RRC container used to carry additional information, e.g. in an RRC information IE). The following information may be included in the UE Context Setup Request message: 　　 -　　UE-CapabilityRAT-ContainerList: the DU 5b may take this information into account for UE specific configurations; 　　 -　　DRX Cycle IE: the DU 5b may use the provided value from the CU 5c; 　　 -　　RRC Information IE; in the case where the CU 5c receives a UEAssistanceInformation IE from the UE 3, the UEAssistanceInformation IE may be included in the RRC Information IE. The DU 5b may, if supported, take the UE's assistance information into account when configuring resources for the UE 3. 　　 -　　SRB To Be Setup List; 　　 -　　DRB To Be Setup List.

[0049] In response, the DU 5b transmits a UE Context Setup Response message to the CU 5c. The following information may be included in the UE Context Setup Response message: 　　 -　　A list of DRBs (and SRBs) which are successfully established and DRBs (and SRBs) which failed to establish; 　　 -　　When the DU 5b reports the unsuccessful establishment of a DRB or SRB, an associated cause value should be precise enough to enable the CU 5c to know the reason for the unsuccessful establishment. This may include the reason that a given feature is not supported by the DU 5b; 　　 -　　CellGroupConfig: As an Octet string which is included by CU 5c transparently within an RRC reconfiguration message sent to UE 3; 　　 -　　DRX Config: As an Octet string which is included by CU 5c transparently within the RRC reconfiguration message sent to UE 3.

[0050] Instead of the above messages, alternative messages (with associated information elements) may be used to update the UE's configuration between the CU 5c and the DU 5b.

[0051] For example, to further update the UE 3 configuration between CU 5c and DU 5b (e.g., DRB / SRB), UE Context Modification Request message (from CU 5c to DU 5b) or UE Context Modification Required (from DU 5b to CU 5c) messages may be used for information exchange between the CU 5c and the DU 5b.

[0052] Radio Link Monitoring (RLM) and Radio Link Failure (RLF) 　　The UE 3 may be configured to perform one or more radio link monitoring (RLM) procedures to monitor a radio link for communications. The UE 3 performs measurements for the RLM using the physical layer. The UE 3 performs the measurements to monitor the status of serving cell, and may also perform measurements for candidate cells for lower-layer triggered mobility. The measurement results may be passed to both the MAC and RRC layers at the UE 3. Radio link failure (RLF) can be detected using the measurements and the RRC layer. In other words, the RRC layer evaluates conditions for RLF based on the measurements performed by the UE 3. If it is determined that RLF has occurred, then corresponding RLF procedures are triggered, and RRC re-establishment may be triggered. Configuration information for beam failure, and beam failure recovery parameters, may also be passed to the MAC layer from the RRC layer. Configuration information for the UE 3 measurements may be passed from the RRC layer to the physical layer, for example a set of radio link monitoring reference signal resources (RLM-RS) may be provided. The RLM-RS may comprise one or more SS / PBCH Blocks (SSB), and / or one or more channel state information reference signals (CSI-RS).

[0053] RLF may occur, for example, due to congestion in a cell of a base station 5, or due to a change in radio conditions (e.g. poor weather, or an obstruction between the UE 3 and the base station 5). In response to detecting RLF, the UE 3 may cease to transmit one or more uplink transmissions to avoid generating uplink interference (e.g. within 40 ms of detecting RLF).

[0054] Beams and Reference Signals 　　A base station 5 may transmit signals over a plurality of different beam directions. Each beam may have a corresponding index for identifying the beam. For SSB transmissions, the identifying index may be an SSB index. Whilst each beam is transmitted in a generally different direction, it will be appreciated that there may be some spatial overlap between the beams. For SIB1 / SI / paging transmission, there may be one or multiple beam sweeping cycles within a SIB1 / SI / paging transmission window / transmission occasion.

[0055] SSB beams may be transmitted in the time domain as a group of SSB transmissions, which may be referred to as an 'SSB burst set'. Each SSB in the SSB burst set may be referred to as an 'SSB block'. For example, a 5 ms SSB burst set in which SSB beams are transmitted by the base station 5 sequentially within a 5 ms time period may be used, resulting in the 'beam sweeping' effect. However, the burst set need not necessarily be of 5 ms duration. Various other transmission configurations in the time domain may be used depending on the configuration of the base station 5 and the available communication resources.

[0056] The UE 3 and the base station 5 may perform an initial access procedure after the UE 3 has received one of the beamformed signals transmitted by the base station 5, and the UE 3 may be configured to transmit a corresponding measurement report to the base station 5. The UE 3 may perform measurements of, for example, synchronisation signal RSRP (SS-RSRP), reference signal received quality (RSRQ), RSRP and signal to noise interference ratio (RSRP-SINR), or physical broadcast channel demodulation reference signal (PBCH DMRS).

[0057] The UE 3 may be configured to determine an SSB index corresponding to a beam by decoding the PBCH DMRS. The UE 3 may determine a particular beam (and / or corresponding time or frequency resource) to be used for communication with the base station 5 based on corresponding signal measurements performed by the UE 3. Alternatively, the UE 3 may report the measurements to the base station 5, and the base station 5 may determine the beam (and / or corresponding time or frequency resource) to be used for communication with the UE 3.

[0058] The base station 5 is operable to transmit reference signals (RS) in the one or more cells 9 that it operates. These reference signals include channel state information RS (CSI-RS). The CSI-RS may be used by the UE 3 for a number of different purposes including, for example, CSI reporting in which the UE 3 derives channel state information (CSI) including one or more channel quality indicators (CQI), rank indicators (RI), and / or precoding matrix indicators (PMI) from CSI-RS measurements and reports them to the base station 5 in a CSI report. The CQI is an index (typically 4 bits) value representing a signal to interference and noise ratio (SINR). The CQI value also corresponds to a modulation and coding scheme (MCS) to be used for each layer. The RI indicates a number of MIMO transmission layers requested by the UE 3 (albeit the base station 5 might not necessarily use the requested number of MIMO transmission layers). The PMI is used by the UE 3 to report parameters defining a preferred precoding matrix to be applied for downlink transmissions (albeit the base station 5 may not use the requested precoding). A layer indicator (LI) may also be included in the CSI report for identifying the strongest layer from the set of layers indicated by the RI.

[0059] The CSI-RS may also be used by the UE 3 for beam management, including the refinement of initial beam selection based on SSBs. For example, the base station 5 may use a set of relatively broad beams for transmission of the SSBs and a set of narrower (more directional) beams for the CSI-RS. The UE 3 can be configured, by the base station 5, to measure each CSI-RS transmission to identify the best CSI-RS beam and to report this to the base station 5 (e.g., by means of a CSI report including a CSI-RS indicator (CRI) identifying the strongest CSI-RS and hence CSI-RS beam). The UE 3 may also be configured to report the (Layer 1) RSRP which has been measured for the strongest CSI-RS.

[0060] CSI-RS may either be either zero power (ZP-CSI-RS) or non-zero power (NZP-CSI-RS). ZP-CSI-RS are empty resource elements, used primarily for interference measurement. NZP-CSI-RS are used for most of the procedures including channel measurement, beam management, beam measurement, connected mode mobility etc. A non-zero-power CSI-RS may be configured, for example, using a NZP-CSI-RS-Resource information element (IE), or using a CSI-RS-Resource-Mobility field in an CSI-RS-ResourceConfigMobility IE. NZP CSI-RS can be used for interference measurement (IM), for example as part of determining a Signal to Interference plus Noise Ratio (SINR). For cases in which interference is likely to be primarily due to inter-cell interference, CSI IM resources may be used. These resources may be used to measure background interference originating from neighbouring cells. The UE 3 may be provided a configuration for receiving (and measuring) the CSI-RS from the base station 5 (e.g. using a CSI Report Configuration, CSI-ReportConfig, transmitted from the base station 5 to the UE 3). The CSI Report Configuration includes an indication of resources for channel measurement, NZP-CSI-RS resources for interference management, and CSI-IM resources.

[0061] There are also several other ways in which the CSI-RS may be used including, for example, for connected mode mobility, radio link failure detection, beam failure detection / recovery, and fine timing of time and / or frequency synchronisation.

[0062] CSI Reporting 　　The base station 5 can configure how the UE 3 measures CSI-RS and transmits corresponding reports to the base station 5 using appropriate measurement configuration signalling. The base station 5 can, for example, use the measurement configuration signalling (e.g., using a CSI-measconfig IE) to configure the UE 3 to measure and report specific resources used for CSI-RS.

[0063] The base station 5 can also configure the UE 3 to provide CSI reports based on different report timing configurations. For example, the UE 3 may be configured for persistent reporting, semi-persistent reporting on the PUSCH, semi-persistent reporting on the PUCCH, or aperiodic reporting. Aperiodic reporting and semi-persistent reporting on PUSCH may be triggered using a PUSCH DCI. For example, DCI (e.g., using DCI format 0_1) may trigger aperiodic reporting by providing a CSI request that points to a respective index of each of one or more corresponding aperiodic trigger states. Each of these trigger states is associated with one or more corresponding CSI report configurations. Semi-persistent reporting on PUSCH may be triggered in a similar way.

[0064] Each CSI report configuration identifies at least one CSI resource configuration for measurement (e.g., channel measurement). The identified CSI resource configuration is defined by a corresponding IE that includes a list of identifiers corresponding to one or more sets of CSI resources and associated configuration information. The associated configuration information may, for example, identifying an associated bandwidth part and a resource type. The identified resource type may, for example, identify the CSI-RS resource to be a periodic, a semi-persistent, or an aperiodic type. Each resource set comprises one or more specific CSI resource configurations represented by associated identifiers that each point to the specific configuration information for that CSI resource configuration). Accordingly, the base station 5 can configure multiple CSI report configuration instances and CSI resource configuration instances. It will be appreciated that multiple resource sets can be configured per CSI resource config for the case of aperiodic CSI RS resources.

[0065] The CSI-RS transmissions may be beamformed and each CSI-RS resource mapped to a different respective beam (and to an associated set of physical antenna elements). Since the CSI-RS is already beamformed the UE 3 measures the CSI-RS, identifies the one or more directional beams on which it can successfully receive data, and reports the one or more CSI-RS resources associated with the one or more identified beams (or with the best identified beams) to the base station 5. Thus, the base station 5 can schedule resources for the PDSCH (and associated DMRS) using the one or more identified beams and the PDSCH (and associated DMRS) can be precoded / beamformed using the same weights as were used for the CSI-RS beamforming of the identified beams.

[0066] Quasi Co-Location (QCL) and Transmission Configuration Indicator (TCI) 　　It will be appreciated that transmissions that share the same antenna port experience the same propagation channel. However, transmissions using different antenna ports can have radio channels that also share common characteristics, in which case the different antenna ports can be said to be "Quasi Co-Located" (QCL). The characteristics that are common for the radio channels may be, for example, the Doppler shift, Doppler spread, average delay, delay spread, or spatial receiver parameters (properties associated with the downlink received signal angle of arrival at the UE 3), depending on the type of QCL. These characteristics may be referred to as 'large scale' radio channel characteristics. QCL information can be used as part of channel estimation, frequency offset estimation and synchronisation procedures performed by a UE 3. For example, the UE 3 can determine a characteristic for a transmission transmitted using a particular antenna port, and assume that the characteristic is the same for a transmission using a quasi co-located antenna port. It will be appreciated that antenna ports may be geographically separated and yet still have common channel characteristics and be considered to be quasi co-located.

[0067] There are four types of QCL, each type corresponding to a set of channel characteristics that are common between the QCL antenna ports. The four types of QCL and the corresponding common channel characteristics are: 　　 -　　QCL Type A: Doppler shift; Doppler spread; average delay; delay spread 　　 -　　QCL Type B: Doppler shift; Doppler spread 　　 -　　QCL Type C: Doppler shift; average delay 　　 -　　QCL Type D: spatial receiver parameters (for spatial quasi co-location, e.g. the UE 3 can use the same Tx and / or Rx beam for DL reception of PDCCH / PDSCH as used for the quasi co-located transmission)

[0068] The base station 5 may be configured to provide an indication to the UE 3 that the antenna port used for a specific synchronisation signal or reference signal (e.g. CSI-RS) is quasi co-located with an antenna port used for PDSCH or PDCCH. Therefore, by performing a measurement of the synchronisation signal or refence signal, the UE 3 can determine a configuration to be used for transmission or reception of the PDSCH or PDCCH. The base station 5 may provide the indication of which transmissions can be considered to be quasi co-located using RRC signalling, a MAC CE, or DCI (e.g. to provide an indication of which CSI-RS can be considered to be QCL with a PDSCH or PDCCH). A number of "transmission configuration indicator" (TCI) states can be configured for the PDSCH and PDCCH (e.g. up to 128 TCI states for the PDSCH and up to 64 TCI states for the PDCCH). The TCI states are deactivated by default. The TCI includes an indication of the SS / PBCH block or CSI-RS that is QCL with the PDSCH or PDCCH, and an indication of the type of QCL. The UE 3 can receive and decode the PDSCH / PDCCH based on the QCL information indicated by the TCI state. If multiple TCI states are associated with a particular PDSCH or PDDCH, then the base station 5 may provide an indication of which TCI state to use (e.g. using an activation command included in a MAC CE), or the UE 3 may assume, for example, QCL between the PDSCH or PDCCH and an SSB selected during an initial access procedure.

[0069] Lower Layer Triggered Mobility 　　When UE 3 moves from one cell to another, a serving cell may need to be changed. Serving cell change may be triggered by layer 3 (L3) measurements and can be achieved using radio resource control (RRC) signalling. However, this process involves layer 1 (L1) and layer 2 (L2) resets, resulting in increased latency, larger overhead and longer interruption time. For inter-cell mobility the UE may need to perform reconfiguration and downlink / uplink (DL / UL) synchronisation towards the target cell. In order to enable more efficient handover, lower-layer based (L1 or L2) handover may be used. Beneficially, the UE 3 and base station 5 can be configured for implementing an LTM procedure in which the UE 3 is able to switch between pre-configured candidate lower-layer triggered mobility (LTM) cells, based on the content of lower layer (L1 and / or L2) measurement reports, relatively swiftly (e.g., potentially without requiring any RRC reconfiguration). Accordingly, as the UE 3 moves around the pre-configured candidate LTM cells, it can execute fast cell switches, potentially without RRC reconfiguration.

[0070] Fig. 5 illustrates examples of intra-CU and inter-CU handovers for which LTM may be used. An inter-DU intra-CU handover is illustrated by arrow A. In the inter-DU intra-CU handover the UE 3 is handed over from a first DU 5b-1 to a second DU 5b-2, but the CU 5c-1 remains unchanged. An inter-DU inter-CU handover is illustrated by arrow B. In the inter-DU inter-CU handover the DU 5b changes from DU 5b-2 to DU 5b-3, and the CU 5c changes from CU 5c-1 to CU 5c-2. It will be appreciated that an intra-DU intra-CU handover is also possible, in which both the DU 5b and the CU 5c are unchanged (e.g. handover of the UE 3 to another cell provided by the same DU 5b and CU 5c).

[0071] Conditional Handover (CHO) 　　Conditional Handover (CHO) is a handover that is executed by the UE 3 when one or more handover execution conditions are met. The UE 3 starts evaluating the one or more execution conditions upon receiving a CHO configuration (from the network, e.g. from the base station 5), and stops evaluating the one or more execution conditions once the handover is executed. The execution conditions may be based, for example, on measurements performed by the UE 3 of reference signal received power (RSRP), reference signal received quality (RSRQ), RSRP and signal to noise interference ratio (RSRP-SINR). For layer 1 / layer 2 (L1 / L2) mobility, handover is initiated based on L1 / L2 measurement results.

[0072] An example of CHO will now be described. A 'CHO candidate cell' is a candidate cell for CHO, and has a corresponding CHO configuration. The CHO configuration comprises the configuration of one or more CHO candidate cells generated by the candidate base stations 5 and one or more execution conditions generated by the source base station 5.

[0073] An execution condition may comprise, for example, one or two trigger conditions, which may also be referred to as CHO events.

[0074] In CHO the preparation and execution phases of the CHO procedure may be performed without involvement of the core network; i.e. preparation messages are directly exchanged between base stations 5. The release of the resources at the source base station during the conditional handover completion phase is triggered by the target base station 5.

[0075] In a CHO method, the source base station 5 may determine that CHO should be used. The source base station 5 may request CHO for one or more candidate cells belonging to one or more candidate base stations 5. A CHO request message can be sent for each candidate cell.

[0076] The candidate base stations 5 send a CHO response, including a configuration of one or more CHO candidate cells, to the source base station 5. The CHO response message may be sent for each candidate cell.

[0077] The source base station 5 may send an RRC Reconfiguration message to the UE 3, containing the configuration of one or more CHO candidate cells and one or more CHO execution conditions. The UE 3 may transmit an RRC Reconfiguration Complete message to the source base station 5. If early data forwarding is applied, the source base station 5 may transmit an early status transfer message.

[0078] The UE 3 maintains connection with the source base station after receiving CHO configuration, and starts evaluating the CHO execution conditions for the one or more candidate cells. If at least one CHO candidate cell satisfies the corresponding CHO execution condition, the UE 3 detaches from the source base station, applies the stored corresponding configuration for that selected candidate cell, synchronises to that candidate cell and completes the RRC handover procedure by sending an RRC Reconfiguration Complete message to the target base station 5. The UE 3 releases stored CHO configurations after successful completion of the handover procedure.

[0079] A target base station 5 transmits a handover success message to the source base station 5 to inform that the UE 3 has successfully accessed the target cell. In return, the source base station 5 transmits a sequence number status transfer (e.g. SN STATUS TRANSFER) message. The source base station 5 can then transmit a handover cancel message toward the other signalling connections or other candidate target base stations, if any, to cancel CHO for the UE 3.

[0080] Conditional configurations for a conditional handover may be provided as a 'delta configuration' with respect to the configuration of the serving cell. In other words, parameters and setting for the conditional configuration may be indicated by indicating the differences between the conditional configuration and the configuration of the serving cell.

[0081] A UE 3 may be configured to indicate to another entity in the network 1 that the UE 3 supports conditional handover by, for example, transmitting a signal that includes an indication in a conditional handover field or information element.

[0082] A CHO candidate cell list can be used to indicate a list of candidate target cells for a conditional handover. A candidate target cell for CHO may be referred to as a CHO candidate. For example, up to 8 candidate cells with associated conditional handover execution conditions may be configured for a UE 3. The number of execution conditions may be two (alternatively one execution condition, or three or more execution conditions, could conceivably be used). The UE 3 executes the CHO towards a selected target cell when the conditions are met by applying the corresponding conditional reconfigurations. This improves mobility robustness since the CHO configuration can be sent before the serving cell quality drops, and the UE 3 may avoid mobility failure due to a missed HO command.

[0083] Timing Advance (TA) 　　The UE 3 may be provided with timing advance (TA) information that refers to a timing adjustment indication indicating a value of a timing offset (NTA) between uplink and downlink radio frames, for the UE 3 to use for a target timing advance group (TAG) (e.g., a primary TAG (PTAG) in the case of a handover or a primary secondary TAG (PSTAG) in the case of a secondary cell group (SCG) change). A TAG is a group of cells sharing the same uplink transmission timing (e.g. a group of cells provided by the same RAN node). A time alignment timer (e.g. timeAlignmentTimer) can be configured to define the maximum time since the UE 3 has received TA information (e.g. a TA command) from the base station 5, during which the UE 3 is considered to be synchronised for uplink transmissions in the cell. In other words, the UE 3 is considered to be synchronised for UL transmissions in a particular cell whilst the corresponding time alignment timer is running. If the time alignment timer expires (because the UE 3 has not received TA information from the base station 5 whilst the timer is running), the UE 3 can determine that the UE 3 is no longer synchronised for uplink transmissions in the corresponding cell. Synchronisation can be restored, for example, using a random access procedure. It will be appreciated that some cells may not require the UE 3 to be provided with TA information in order for synchronisation to be achieved. For example, for small cells the propagation delay of transmissions between the UE 3 and the base station 5 may be negligible.

[0084] The timing advance is used to control UL transmission timing for a UE 3 (e.g. for PUSCH and PUCCH), and improves synchronisation of communication between the UE 3 and the base station 5. UEs 3 that are further from a base station 5 may be configured to use larger TA values to compensate for the propagation delay of radio signals between the UE3 and the base station 5. The TA value corresponds to the time difference between the beginning of an uplink radio frame transmitted by the UE 3, and a corresponding downlink radio frame received at the UE 3.

[0085] A UE 3 can acquire DL synchronization for a target cell with SSB and TA before a cell switch for LTM. However, there is a problem that without TRS tracking and CSI information acquisition at the UE 3 before the cell switch, the UE 3 cannot maintain a high date rate for transmissions with the target cell immediately after the cell switch. Improved methods that mitigate against this issue will now be described.

[0086] Base Station Initiated inter-CU LTM Fig. 6 illustrates an improved example of base station initiated inter-CU LTM. In step S601, UE Context Setup / Modification is performed. The purpose of the UE Context Setup procedure is to establish the UE Context including signalling radio bearer (SRB) and data radio bearer (DRB), and the procedure uses UE associated signalling.

[0087] In step S602 RRC configuration in preparation for handover is performed. Step S602 includes layer 3 (L3) measurement configuration, and Inter-CU SSB-based RRM Measurement Timing Configuration Window (SMTC) coordination. The RRC reconfiguration information may include measurement configuration information (for configuring a measurement performed by the UE 3) and / or measurement report configuration information (for configuring a measurement report transmitted to the base station 5 by the UE 3). The RRC reconfiguration information may also include a target cell list and RACH configuration, SMTC (Inter-CU) SSB-related information for a target cell and the associated DU ID. Advantageously, the RRC reconfiguration information may also include the associated CU ID, and inter-frequency measurement gap configuration information. If the measurement gap is used, then inter-frequency measurement may be performed using OFDM symbol corresponding to the overlapping time span between the SMTC window duration and the measurement gap (as defined by a higher layer for the minimum measurement time).

[0088] In steps S603 to S607, activation of reference signal transmission (SSB or CSI-RS) is performed if the candidate cell is not the current serving cell for the UE 3. For example, the candidate cell may be a deactivated secondary cell.

[0089] In step S603, LTM candidate cell subset selection is performed at the source DU 5b-2.

[0090] In step S604, the source DU 5b-2 transmits a request for L1 measurement configuration (for the LTM candidate cell subset) to the target CU 5c-2 via the source CU 5c-1.

[0091] In step S605, the target CU 5c-2 transmits an L1 measurement configuration request to the target DU 5b-3, including an indication of one or more tracking reference signals (TRS) and channel state information reference signals (CSI-RS). In other words, the target CU 5c-2 provides information for configuration of L1 measurements for candidate cells of target CU 5c-2.

[0092] In step S606, the target DU 5b-3 transmits an L1 measurement configuration response to the target CU 5c-2.

[0093] In step S607, the target CU 5c-2 transmits an L1 measurement configuration response to the source DU 5b-2 via the source CU 5c-1. The L1 measurement configuration response includes an L1 measurement configuration and / or L1 measurement report configuration for the target CU 5c-2 candidate cells (the target CU candidate cells). The source CU 5c-1 provides an L1 measurement configuration and / or L1 measurement report configuration for the candidates cells of the source CU 5c-1 and transmits the configuration to the source DU 5b-2.

[0094] In step S608, the source DU 5b-2 transmits an L1 measurement configuration, including an indication of the TRS and / or CSI-RS, to the UE 3. The L1 measurement configuration may be, for example, for triggering TRS monitoring and / or CSI acquisition for the candidate cells by the UE 3. The presence of information elements in the transmission of step S608 indicating, for the candidate cells, one or more of SSB information, TRS configuration information, CSI-RS configuration information or SRS configuration information may be configurable by the network.

[0095] In step S609, the UE 3 transmits an L1 measurement report to the source DU 5b-2 for the determined subset of the candidate cells (which may include cells of both the source CU 5c-1 and the target CU 5c-2).

[0096] In step S610 the source DU 5b-2 transmits, to the UE 3, an indication of TCI states to be activated. Based on the measurement report received from the UE 3, the network may activate some transmission configuration information (TCI) states quasi co-located (QCL-ed) with cells whose physical cell ID (PCI) is different from serving cell. DL synchronisation is then performed at the UE 3, and a determination of whether one or more handover conditions are met is performed at the source DU 5b-2. The source DU 5b-2 also determined the cell or beam for handover if the one or more handover conditions are met. For beam indication, TCI state activation via MAC CE before the cell switch command for one or more of the candidate cells may be used. Advantageously, for TCI state activation, within the subset of the selected candidate cells, one or more beams (e.g. SSB and / or CSI-RS) per candidate cell may be selected for TCI state activation, and only the TCI states for beams above a configurable (by the network) measurement threshold may be activated. The number of TCI states that can be activated simultaneously for the UE 3 may be defined using a parameter Ntci. The number of TCI states that may be activated simultaneously may be, for example, 2, 3, 4, 5, 6 or 7 (although any other suitable value for Ntci could alternatively be used.

[0097] In step S611 the source DU 5b-2 transmits, to the UE 3, a cell switch command using a MAC control element (MAC-CE). The cell switch command includes an indication of the target cell ID and the TCI state. Advantageously, the MAC-CE (or DCI) may indicate the one or more TCI states or reference signal identifiers for the target cell. The information transmitted in step S611 may also include an indication of the target cell index. The transmission may include a field (e.g. information element) indicating a joint UL and DL unified TCI state index, or a pair of UL and UL TCI state indices for PRACH and / or PDCCH of the target cell, and another field (e.g. information element) may be used for the PDSCH and / or PUCCH of the target cell.

[0098] In step S612 UL synchronisation is performed, optionally including a random access procedure.

[0099] Base Station Initiated UL-Measurement-Based inter-CU LTM 　　Fig. 7 shows a modified version of the method of Fig. 6, in which the LTM is base station initiated UL-measurement-based inter-CU LTM, and the UE 3 is configured to transmit periodic or aperiodic SRS for the candidate cells.

[0100] Steps S701 to S703 are the same as steps S601 to S603 of Fig. 6, and so will not be described again here.

[0101] In step S704, the source DU 5b-2 transmits a request for SRS configuration (for the LTM candidate cell subset) to the target CU 5c-2 via the source CU 5c-1.

[0102] In step S705, the target CU 5c-2 transmits an L1 measurement configuration request to the target DU 5b-3, including an indication of one or more SRS. In other words, the target CU 5c-2 provides information for configuration of SRS measurements for candidate cells of target CU 5c-2.

[0103] In step S706, the target DU 5b-3 transmits an L1 measurement configuration response to the target CU 5c-2.

[0104] In step S707, the target CU 5c-2 transmits an L1 measurement configuration response to the source DU 5b-2 via the source CU 5c-1. The L1 measurement configuration response includes configuration information for the SRS.

[0105] In step S708, the source DU 5b-2 transmits an L1 SRS configuration, to the UE 3. The SRS configuration may be, for example, for triggering transmission of the SRS by the UE 3. The transmission of step S708 may include information for triggering periodic or aperiodic SRS transmissions for the candidate cell by the UE 3.

[0106] In step S709, the target DU 5b-3 performs L1 SRS measurements (of the SRS transmitted by the UE 3) for the candidate cells, and in step S710 the target DU 5b-3 transmits a corresponding L1 SRS measurement report to the target CU 5c-2. The target CU 5c-2 transmits the L1 SRS measurement report to the source DU 5b-2 via the source CU 5c-1. DL synchronisation is then performed at the UE 3, and a determination of whether one or more handover conditions are met is performed at the source DU 5b-2. The source DU 5b-2 also determined the cell or beam for handover if the one or more handover conditions are met.

[0107] Steps S712 and S713 are the same as steps S611 and S612 of Fig. 6, and so will not be described again here.

[0108] Inter-CU LTM with CHO 　　Fig. 8 illustrates an example of inter-CU LTM with conditional handover (CHO).

[0109] Steps S801 to S808 are the same as steps S601 to S608 of Fig. 6, and so will not be described again here.

[0110] In step S809, The UE 3 transmits an L1 measurement report (for source and target cells) to the source DU 5b-2. The L1 measurement report may be transmitted periodically by the UE 3.

[0111] In step S810, an information element indicating TCI state activation for a subset of candidate cells is transmitted from the source DU 5b-2 to the UE 3. The event triggered measurement report may act as a cell switch indication. A target cell ID and a TCI state or RS identifier indicating the DL / UL beam of the selected target cell may be included in the measurement report. An optional handover condition configuration may also be transmitted to the UE 3 by the source DU 5b-2 in step S810. The base station 5 may configure a CHO threshold based on L3 measurement reporting or L1 measurement reporting. However, beneficially, this configuration may not be performed to enable the UE 3 to derive the LTM handover threshold (e.g. autonomously).

[0112] For L1 measurement reporting, by the UE 3, for candidate cells, a pool index corresponding to a CU-ID can be used to enable the source DU 5b-2 to distinguish the candidate cell L1 measurement reports. A pool index corresponding to the CU-ID can be used to group the source cell and target and / or candidate cell measurement reports. A CSI-RS index associated with a configured candidate cell may be included for each L1-RSRP or L1-SINR measurement report.

[0113] Fig. 9 shows an example of information that may be included in the CSI report. For beam selection for SSB based L1-RSRP measurement reports, the values of M and L define the number of beams that are reported in a single report instance. SSB Beam selection is performed across the L cells from configured cells, and there are M SSB beams for each of the L configured cells. M*L beams (M multiplied by L) are reported in a single report instance. The RRC configured candidate values of M may be 1, 2, 3 or 4, and the RRC configured candidate values of L may be 1, 2, 3 or 4. The maximum value of M*L and the combination of M and L may be based on a capability of the UE 3. As shown in Fig. 9, the CSI report includes CSI fields that are defined per CU pool. Beneficially, there can be L * Mcsi CSI measurement value field per CU. Mcsi represents the number of CSI beams for each of the L candidate cells hosted on a CU in a measurement report, and the value of Mcsi may be configurable by the network. When a CHO measurement reporting threshold is configured, Mcsi can be a variable. Likewise, these measurement values can be for just SSB candidate beams. The CSI report also includes an optional CHO measurement reporting threshold for each CU pool (e.g. RSRP threshold or SINR threshold).

[0114] Following step S810, DL synchronisation is performed at the UE 3, and a determination of whether a CHO handover condition is met is performed at the UE 3. The UE 3 is also configured to determine the cell / beam for handover if the CHO handover condition is met.

[0115] In step S811a, the UE 3 transmits a source and / or target cell L1 measurement report to the source DU 5b-2, and in optional steps S811b and 811c a cell switch indication is transmitted from the source DU 5b-2 to the target CU 5c-2 via the source CU 5c-1. The transmission of step S811a advantageously enables the UE 3 to indicate the new TCI state and / or beam or RS identifier for the PDSCH / PUSCH of the target cell to enable a higher data rate and reduced interruption time during the conditional handover. Following the cell switch indication of steps S811b and S811c, the activated TCI states for other candidates cells are deactivated (this may be determined implicitly). In step S811a the UE 3 may optionally provide an indication of a contention free random access (CFRA) resource for RACH based access by the UE 3 to the target cell, or an indication of a configured grant resource for RACH-less access by the UE 3 to the target cell.

[0116] Step S812 is the same as step S612 of Fig. 6, and so will not be described again here. In step S813, an optional cell switch success / failure indication is transmitted from the target DU 5b-3 to the target CU 5c-2, which may be forwarded to the source DU 5b-2 via the source CU 5c-1 by the target CU 5c-2.

[0117] User Equipment 　　Fig. 10 is a schematic block diagram illustrating the main components of a UE 3 as shown in Fig. 1.

[0118] As shown, the UE 3 has a transceiver circuit 310 that is operable to transmit signals to and to receive signals from a base station 5 via one or more antenna 330 (e.g., comprising one or more antenna elements). The UE 3 has a controller 370 to control the operation of the UE 3. The controller 370 is associated with a memory 390 and is coupled to the transceiver circuit 310. Although not necessarily required for its operation, the UE 3 might, of course, have all the usual functionality of a conventional UE 3 (e.g. a user interface 350, such as a touch screen / keypad / microphone / speaker and / or the like for, allowing direct control by and interaction with a user) and this may be provided by any one or any combination of hardware, software, and firmware, as appropriate. Software may be pre-installed in the memory 390 and / or may be downloaded via the telecommunications network or from a removable data storage device (RMD), for example.

[0119] The controller 370 is configured to control overall operation of the UE 3 by, in this example, program instructions or software instructions stored within memory 390. As shown, these software instructions include, among other things, an operating system 410, a communications control module 430, and an L1 / L2 mobility module 450.

[0120] The communications control module 430 is operable to control the communication between the UE 3 and its one or more serving base stations 5 (and other communication devices connected to the base station 5, such as further UEs and / or core network nodes). The communications control module 430 is configured for the overall handling uplink communications via associated uplink channels (e.g. via a physical uplink control channel (PUCCH), random access channel (RACH), and / or a physical uplink shared channel (PUSCH)) including both dynamic and semi-static signalling (e.g., SRS). The communications control module 430 is also configured for the overall handling of receipt of downlink communications via associated downlink channels (e.g. via a physical downlink control channel (PDCCH) and / or a physical downlink shared channel (PDSCH)) including both dynamic and semi-static signalling (e.g., CSI-RS). The communications control module 430 is responsible, for example: for determining where to monitor for downlink control information (e.g., the location of CSSs / USSs, CORESETs, and associated PDCCH candidates to monitor); for determining the resources to be used by the UE 3 for transmission / reception of UL / DL communications (including interleaved resources and resources subject to frequency hopping); for managing frequency hopping at the UE side; for determining how slots / symbols are configured (e.g., for UL, DL or SBFD communication, or the like); for determining which one or more bandwidth parts are configured for the UE 3; for determining how uplink transmissions should be encoded; for applying any SBFD specific communication configurations appropriately; and the like. The L1 / L2 mobility module 450 may be configured to control communications in accordance with any of the L1 / L2 methods described above (for example, to perform measurements of one or more reference signals transmitted by a base station 5, or to transmit an SRS).

[0121] RAN (Distributed Type) 　　Fig. 11 is a simplified block schematic illustrating the main components of a distributed RAN comprising a distributed type of base station 5 for implementation in the system of Fig. 1.

[0122] As shown, the RAN node 5 includes a central unit 5c and a distributed unit 5b (although it may include other DUs as described above). Each unit 5c, 5b includes respective transceiver circuitry 51c, 51b.

[0123] The transceiver circuitry 51b of the distributed unit 5b is operable to transmit signals to and to receive signals from UEs 3 via an air interface 53b and one or more antennas and is also operable to transmit signals to and to receive signals from the central unit 5c via an interface, for example the distributed unit side of an F1 interface (which may be provided over a satellite radio interface).

[0124] The transceiver circuitry 51c of the central unit 5c is operable to transmit signals to and to receive signals from functions of the core network 7 and / or other RANs via a network interface 55c. The network interface typically includes an N2 and / or N3 interfaces for communicating with the core network 7 and a base station to base station (e.g. Xn) interface for communicating with other RANs. The transceiver circuitry 51c of the central unit 5c is also operable to transmit signals to and to receive signals from one or more distributed units 5b, for example the central unit side of the F1 interface provided.

[0125] Each unit 5c, 5b includes a respective controller 57c, 57b which controls the operation of the corresponding transceiver circuitry 51c, 51b in accordance with software stored in the respective memories 59c and 59b of the distributed unit 5b and the central unit 5c. The software of each unit may be pre-installed in the memory 59c, 59b and / or may be downloaded via the communication system 1 or from a removable data storage device (RMD), for example. The software of each unit includes, among other things, a respective operating system 61c, 61b, a respective communications control module 63c, 63b and an L1 / L2 mobility module 65c, 65b.

[0126] Each communications control module 63c, 63b is operable to control the communication of its corresponding unit 5c, 5b including the communication from one unit to the other. The communications control module 63b of the distributed unit 5b controls communication between the distributed unit 5b and the UEs 3, and the communications control module 63c of the central unit 5c controls communication between the central unit 5c and other network entities that are connected to the distributed RAN node 5.

[0127] The communications control modules 63c, 63b also respectively control the part played by the distributed unit 5b and central unit 5c in the flow of uplink and downlink user traffic and control data to be transmitted to the communications devices served by the RAN node 5 including, for example, control data for managing operation of the UEs 3. Each communication control module 63c, 63b is responsible, for example, for controlling the respective part played by the distributed unit 5b and central unit 5c in the reception and decoding of uplink communications, via associated uplink channels (e.g. via a physical uplink control channel (PUCCH), a random-access channel (RACH), and / or a physical uplink shared channel (PUSCH)) including both dynamic and semi-static signalling (e.g., SRS). Each communication control module 63c, 63b is responsible for controlling the respective part played by the distributed unit 5b and central unit 5c in the transmission of downlink communications via associated downlink channels (e.g. via a physical downlink control channel (PDCCH) and / or a physical downlink shared channel (PDSCH)) including both dynamic and semi-static signalling (e.g., CSI-RS, SSBs etc.).

[0128] The L1 / L2 mobility module 65c, 65b is configured for performing control of communications in accordance with any of the L1 / L2 methods described above (for example, to perform measurements of one or more SRS transmitted by a UE 3, or to determine a candidate cell set for LTM).

[0129] It will be appreciated that the communications control modules 63b, 63c may also include a number of sub-modules (or layers) to support specific functionalities for the corresponding unit 5c, 5b. The modules included will depend on how the corresponding unit 5c, 5b is configured (e.g., the precise CU-DU split). For example, the communications control modules 63b of the distributed unit 5b may include a PHY sub-module, a MAC sub-module, and an RLC sub-module, whereas the communications control modules 63c of the central unit 5c may include a PDCP sub-module, an SDAP sub-module, an IP sub-module, an RRC sub-module, etc. The communication control modules 63b, 63c, may perform control as part of any of the methods described above (for example to provide the air interface protocols, or methods of feedback-based retransmission, described above).

[0130] Core Network Node / Function 　　Fig. 12 is a block diagram illustrating the main components of a core network node or function, such as the AMF 10-1, CPF 10, the UPF 11, the SMF 10-2 or OAM. As shown, the core network function includes a transceiver circuit 710 which is operable to transmit signals to and to receive signals from other nodes (including the UE 3, the base station 5, and other core network nodes) via a network interface 720. A controller 730 controls the operation of the core network function in accordance with software stored in a memory 740. The software may be pre-installed in the memory 740 and / or may be downloaded via the communication system 1 or from a removable data storage device (RMD), for example. The software includes, among other things, an operating system 750, and a communications control module 760.

[0131] The communications control module 760 is responsible for handling (generating / sending / receiving) signalling between the core network function and other nodes, such as the UE 3, the base station 5, and other core network nodes, and may perform control according to any of the methods described above.

[0132] Modifications and Alternatives 　　As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above examples whilst still benefiting from the disclosure therein.

[0133] It will be appreciated, for example, that whilst cellular communication generation (2G, 3G, 4G, 5G, 6G etc.) specific terminology may be used, in the interests of clarity, to refer to specific communication entities, the technical features described for a given entity are not limited to devices of that specific communication generation. The technical features may be implemented in any functionally equivalent communication entity regardless of any differences in the terminology used to refer to them.

[0134] In the above description, the UEs and the base station are described for ease of understanding as having a number of discrete functional components or modules. Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the disclosure, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating system or code and so these modules may not be discernible as discrete entities.

[0135] In the above detailed examples, a number of software modules were described. As those skilled in the art will appreciate, the software modules may be provided in compiled or un-compiled form and may be supplied as a signal over a computer network, or on a recording medium. Further, the functionality performed by part, or all of this software may be performed using one or more dedicated hardware circuits. However, the use of software modules is preferred as it facilitates the updating of the base station or the UE in order to update their functionalities.

[0136] Each controller may comprise any suitable form of processing circuitry including (but not limited to), for example: one or more hardware implemented computer processors; microprocessors; central processing units (CPUs); arithmetic logic units (ALUs); input / output (IO) circuits; internal memories / caches (program and / or data); processing registers; communication buses (e.g. control, data and / or address buses); direct memory access (DMA) functions; hardware or software implemented counters, pointers and / or timers; and / or the like. Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.

[0137] The User Equipment (or "UE", "mobile station", "mobile device" or "wireless device") in the present disclosure is an entity connected to a network via a wireless interface. 　　It should be noted that the present disclosure is not limited to a dedicated communication device and can be applied to any device having a communication function as explained in the following paragraphs.

[0138] The terms "User Equipment" or "UE" (as the term is used by 3GPP), "mobile station", "mobile device", and "wireless device" are generally intended to be synonymous with one another, and include standalone mobile stations, such as terminals, cell phones, smart phones, tablets, cellular IoT devices, IoT devices, and machinery. It will be appreciated that the terms "mobile station" and "mobile device" also encompass devices that remain stationary for a long period of time.

[0139] A UE may, for example, be an item of equipment for production or manufacture and / or an item of energy related machinery (for example equipment or machinery such as: boilers; engines; turbines; solar panels; wind turbines; hydroelectric generators; thermal power generators; nuclear electricity generators; batteries; nuclear systems and / or associated equipment; heavy electrical machinery; pumps including vacuum pumps; compressors; fans; blowers; oil hydraulic equipment; pneumatic equipment; metal working machinery; manipulators; robots and / or their application systems; tools; molds or dies; rolls; conveying equipment; elevating equipment; materials handling equipment; textile machinery; sewing machines; printing and / or related machinery; paper converting machinery; chemical machinery; mining and / or construction machinery and / or related equipment; machinery and / or implements for agriculture, forestry and / or fisheries; safety and / or environment preservation equipment; tractors; precision bearings; chains; gears; power transmission equipment; lubricating equipment; valves; pipe fittings; and / or application systems for any of the previously mentioned equipment or machinery etc.).

[0140] A UE may, for example, be an item of transport equipment (for example transport equipment such as: rolling stocks; motor vehicles; motorcycles; bicycles; trains; buses; carts; rickshaws; ships and other watercraft; aircraft; rockets; satellites; drones; balloons etc.). A UE may, for example, be an item of information and communication equipment (for example information and communication equipment such as: electronic computer and related equipment; communication and related equipment; electronic components etc.).

[0141] A UE may, for example, be a refrigerating machine, a refrigerating machine applied product, an item of trade and / or service industry equipment, a vending machine, an automatic service machine, an office machine or equipment, a consumer electronic and electronic appliance (for example a consumer electronic appliance such as: audio equipment; video equipment; a loud speaker; a radio; a television; a microwave oven; a rice cooker; a coffee machine; a dishwasher; a washing machine; a dryer; an electronic fan or related appliance; a cleaner etc.).

[0142] A UE may, for example, be an electrical application system or equipment (for example an electrical application system or equipment such as: an x-ray system; a particle accelerator; radio isotope equipment; sonic equipment; electromagnetic application equipment; electronic power application equipment etc.).

[0143] A UE may, for example, be an electronic lamp, a luminaire, a measuring instrument, an analyser, a tester, or a surveying or sensing instrument (for example a surveying or sensing instrument such as: a smoke alarm; a human alarm sensor; a motion sensor; a wireless tag etc.), a watch or clock, a laboratory instrument, optical apparatus, medical equipment and / or system, a weapon, an item of cutlery, a hand tool, or the like.

[0144] A UE may, for example, be a wireless-equipped personal digital assistant or related equipment (such as a wireless card or module designed for attachment to or for insertion into another electronic device (for example a personal computer, electrical measuring machine)).

[0145] A UE may be a device or a part of a system that provides applications, services, and solutions described below, as to "internet of things (IoT)", using a variety of wired and / or wireless communication technologies.

[0146] Internet of Things devices (or "things") may be equipped with appropriate electronics, software, sensors, network connectivity, and / or the like, which enable these devices to collect and exchange data with each other and with other communication devices. IoT devices may comprise automated equipment that follow software instructions stored in an internal memory. IoT devices may operate without requiring human supervision or interaction. IoT devices might also remain stationary and / or inactive for a long period of time. IoT devices may be implemented as a part of a (generally) stationary apparatus. IoT devices may also be embedded in non-stationary apparatus (e.g. vehicles) or attached to animals or persons to be monitored / tracked.

[0147] It will be appreciated that IoT technology can be implemented on any communication devices that can connect to a communications network for sending / receiving data, regardless of whether such communication devices are controlled by human input or software instructions stored in memory.

[0148] It will be appreciated that IoT devices are sometimes also referred to as Machine-Type Communication (MTC) devices or Machine-to-Machine (M2M) communication devices. It will be appreciated that a UE may support one or more IoT or MTC applications. Some examples of MTC applications are listed in the following table. This list is not exhaustive and is intended to be indicative of some examples of machine type communication applications.

[0149] Applications, services, and solutions may be an MVNO (Mobile Virtual Network Operator) service, an emergency radio communication system, a PBX (Private Branch eXchange) system, a PHS / Digital Cordless Telecommunications system, a POS (Point of sale) system, an advertise calling system, an MBMS (Multimedia Broadcast and Multicast Service), a V2X (Vehicle to Everything) system, a train radio system, a location related service, a Disaster / Emergency Wireless Communication Service, a community service, a video streaming service, a femto cell application service, a VoLTE (Voice over LTE) service, a charging service, a radio on demand service, a roaming service, an activity monitoring service, a telecom carrier / communication NW selection service, a functional restriction service, a PoC (Proof of Concept) service, a personal information management service, an ad-hoc network / DTN (Delay Tolerant Networking) service, etc.

[0150] Further, the above-described UE categories are merely examples of applications of the technical ideas described in the present document. Needless to say, these technical ideas are not limited to the above-described UE and various modifications can be made thereto.

[0151] Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.

[0152] While the present disclosure has been particularly shown and described with reference to example embodiments thereof, the present disclosure is not limited to these example embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present disclosure as defined by the claims. And each example embodiment can be appropriately combined with at least one of example embodiments.

[0153] Each of the drawings or figures is merely an example to illustrate one or more example embodiments. Each figure may not be associated with only one particular example embodiment, but may be associated with one or more other example embodiments. As those of ordinary skill in the art will understand, various features or steps described with reference to any one of the figures can be combined with features or steps illustrated in one or more other figures, for example, to produce example embodiments that are not explicitly illustrated or described. Not all of the features or steps illustrated in any one of the figures to describe an example embodiment are necessarily essential, and some features or steps may be omitted. The order of the steps described in any of the figures may be changed as appropriate.

[0154] The whole or part of the example embodiments disclosed above can be described as, but not limited to, the following supplementary notes. 　　(Supplementary Note 1) 　　A method performed by a first distributed unit of a first base station, the method comprising: 　　transmitting, to a second central unit of a second base station via a first central unit of the first base station, first information of candidate Layer 1 (L1) / Layer 2 (L2) triggered mobility (LTM) cells; and 　　receiving, from the second central unit of the second base station via the first central unit of the first base station, information for configuration on L1 measurements on the candidate LTM cells on which the second central unit of the second base station has configured a second distributed unit of the second base station, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements. 　　(Supplementary Note 2) 　　The method according to supplementary note 1, further comprising: 　　transmitting, to a user equipment (UE), the information of configuration. 　　(Supplementary Note 3) 　　The method according to supplementary note 2, further comprising: 　　receiving, from the UE, L1 measurement reports of results of the L1 measurement performed based on the information of configuration; and 　　transmitting, to the UE, information for activating at least one transmission configuration indicator (TCI) state on at least one beam per candidate LTM cell. 　　(Supplementary Note 4) 　　The method according to supplementary note 3, wherein 　　a maximum number of at least one TCI state activated is 2, 3, 4, 5, 6 or 7. 　　(Supplementary Note 5) 　　The method according to supplementary note 3 or 4, wherein 　　the information for activating the at least one TCI state is transmitted via a medium access control (MAC) control element (CE) or downlink control information (DCI). 　　(Supplementary Note 6) 　　The method according to any one of supplementary notes 3 to 5, wherein 　　an inter-central unit LTM procedure is performed after transmitting the information for activating the at least one TCI state. 　　(Supplementary Note 7) 　　The method according to supplementary note 6, wherein 　　at least one source TCI state activated is reused by the UE for beam failure recovery in a case where a beam failure is detected during the inter-central unit LTM procedure. 　　(Supplementary Note 8) 　　The method according to any one of supplementary notes 3 to 7, wherein 　　the L1 measurement reports from the UE is transmitted by being triggered by an event, and 　　the L1 measurement reports includes at least one of: 　　　　an identifier of the target cell, 　　　　a TCI state, or 　　　　an identifier of a reference signal, indicating at least one beam corresponding to the target cell. 　　(Supplementary Note 9) 　　The method according to any one of supplementary notes 3 to 8, wherein 　　the L1 measurement reports include information indicating at least one of: 　　　　at least one contention-free random access (CFRA) resource for access to the candidate LTM cells, or 　　　　at least one configured grant resource for access to the candidate LTM cells. 　　(Supplementary Note 10) 　　The method according to supplementary note 9, wherein 　　transmissions of at least one of: 　　　　a physical random access channel (PRACH), 　　　　a physical uplink control channel (PUCCH) / a physical uplink shared channel (PUSCH) carrying at least one of: 　　　　　　a hybrid automatic repeat request (HARQ)-acknowledgment (ACK), 　　　　　　a scheduling request (SR), 　　　　　　periodic / semi-persistent (P / SP) CSI, or 　　　　　　aperiodic CSI, 　　on at least one serving cell is kept by the UE in a case where the transmissions are overlapped to transmissions of PRACH on the candidate LTM cells. 　　(Supplementary Note 11) 　　The method according to any one of supplementary notes 3 to 10, further comprising: 　　transmitting, to the second central unit of the second base station via the first central unit of the first base station, a cell switch indication for deactivating TCI states for the candidate LTM cells. 　　(Supplementary Note 12) 　　The method according to any one of supplementary notes 3 to 11, wherein 　　the L1 measurement reports includes at least one of: 　　　　at least one CSI-RS index corresponding to a respective candidate LTM cells, or 　　　　an index for at least one resource pool corresponding to the first central unit of the first base station or the second central unit of the second base station. 　　(Supplementary Note 13) 　　The method according to supplementary note 2, further comprising: 　　receiving, from the second distributed unit of the second base station via the second central unit of the second base station and the first central unit of the first base station, L1 measurement reports of results of the L1 measurement performed based on the information of configuration; and 　　transmitting, to the UE, a cell switch command based on the L1 measurement reports. 　　(Supplementary Note 14) 　　The method according to supplementary note 13, wherein 　　a beam application time reference subcarrier spacing (SCS) is based on at least one serving cell and / or the candidate LTM cells, whichever is larger. 　　(Supplementary Note 15) 　　A method performed by a user equipment (UE), the method comprising: 　　receiving, from a second central unit of a second base station via a first central unit of a first base station and a first distributed unit of the first base station, information of configuration on layer 1 (L1) measurements on candidate L1 / layer 2 (L2) triggered mobility (LTM) cells on which the second central unit of the second base station has configured the second distributed unit of the second base station; and 　　transmitting, to the first distributed unit of the first base station, L1 measurement reports of results of the L1 measurement performed based on the information of configuration, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements. 　　(Supplementary Note 16) 　　A first distributed unit of a first base station comprising: 　　means for transmitting, to a second central unit of a second base station via a first central unit of the first base station, first information of candidate Layer 1 (L1) / Layer 2 (L2) triggered mobility (LTM) cells; and 　　means for receiving, from the second central unit of the second base station via the first central unit of the first base station, information for configuration on L1 measurements on the candidate LTM cells on which the second central unit of the second base station has configured a second distributed unit of the second base station, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements. 　　(Supplementary Note 17) 　　A user equipment (UE) comprising: 　　means for receiving, from a second central unit of a second base station via a first central unit of a first base station and a first distributed unit of the first base station, information of configuration on layer 1 (L1) measurements on candidate L1 / layer 2 (L2) triggered mobility (LTM) cells on which the second central unit of the second base station has configured the second distributed unit of the second base station; and 　　means for transmitting, to the first distributed unit of the first base station, L1 measurement reports of results of the L1 measurement performed based on the information of configuration, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements.

[0155] This application is based upon and claims the benefit of priority from Great Britain Patent Application No. 2315416.4, filed on October 6, 2023, the disclosure of which is incorporated herein in its entirety by reference.

[0156] 1 COMMUNICATION SYSTEM 3 USER EQUIPMENT 310 TRANSCEIVER CIRCUIT 330 ANTENNA 350 USER INTERFACE 370 CONTROLLER 390 MEMORY 410 OPERATING SYSTEM 430 COMMUNICATIONS CONTROL MODULE 450 L1 / L2 MOBILITY MODULE 5 RADIO ACCESS NETWORK NODE 5b DISTRIBUTED UNIT 51b TRANSCEIVER CIRCUITRY 53b AIR INTERFACE 57b CONTROLLER 59b MEMORY 61b OPERATING SYSTEM 63b COMMUNICATIONS CONTROL MODULE 65b L1 / L2 MOBILITY MODULE 5c CENTRAL UNIT 51c TRANSCEIVER CIRCUITRY 55c NETWORK INTERFACE 57c CONTROLLER 59c MEMORY 61c OPERATING SYSTEM 63c COMMUNICATIONS CONTROL MODULE 65c L1 / L2 MOBILITY MODULE 7 CORE NETWORK NODE 710 TRANSCEIVER CIRCUIT 720 NETWORK INTERFACE 730 CONTROLLER 740 MEMORY 750 OPERATING SYSTEM 760 COMMUNICATIONS CONTROL MODULE 9 CELL 10 CONTROL PLANE FUNCTION 10-1 ACCESS AND MOBILITY MANAGEMENT FUNCTION 10-2 SESSION MANAGEMENT FUNCTION 10-3 UNIFIED DATA MANAGEMENT FUNCTION 10-n OTHER FUNCTION 11 USER PLANE FUNCTION 20 EXTERNAL DATA NETWORK

Claims

1. A method performed by a first distributed unit of a first base station, the method comprising: 　　transmitting, to a second central unit of a second base station via a first central unit of the first base station, first information of candidate Layer 1 (L1) / Layer 2 (L2) triggered mobility (LTM) cells; and 　　receiving, from the second central unit of the second base station via the first central unit of the first base station, information for configuration on L1 measurements on the candidate LTM cells on which the second central unit of the second base station has configured a second distributed unit of the second base station, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements.

2. The method according to claim 1, further comprising: 　　transmitting, to a user equipment (UE), the information of configuration.

3. The method according to claim 2, further comprising: 　　receiving, from the UE, L1 measurement reports of results of the L1 measurement performed based on the information of configuration; and 　　transmitting, to the UE, information for activating at least one transmission configuration indicator (TCI) state on at least one beam per candidate LTM cell.

4. The method according to claim 3, wherein 　　a maximum number of at least one TCI state activated is 2, 3, 4, 5, 6 or 7.

5. The method according to claim 3 or 4, wherein 　　the information for activating the at least one TCI state is transmitted via a medium access control (MAC) control element (CE) or downlink control information (DCI).

6. The method according to any one of claims 3 to 5, wherein 　　an inter-central unit LTM procedure is performed after transmitting the information for activating the at least one TCI state.

7. The method according to claim 6, wherein 　　at least one source TCI state activated is reused by the UE for beam failure recovery in a case where a beam failure is detected during the inter-central unit LTM procedure.

8. The method according to any one of claims 3 to 7, wherein 　　the L1 measurement reports from the UE is transmitted by being triggered by an event, and 　　the L1 measurement reports includes at least one of: 　　　　an identifier of the target cell, 　　　　a TCI state, or 　　　　an identifier of a reference signal, indicating at least one beam corresponding to the target cell.

9. The method according to any one of claims 3 to 8, wherein 　　the L1 measurement reports include information indicating at least one of: 　　　　at least one contention-free random access (CFRA) resource for access to the candidate LTM cells, or 　　　　at least one configured grant resource for access to the candidate LTM cells.

10. The method according to claim 9, wherein 　　transmissions of at least one of: 　　　　a physical random access channel (PRACH), 　　　　a physical uplink control channel (PUCCH) / a physical uplink shared channel (PUSCH) carrying at least one of: 　　　　　　a hybrid automatic repeat request (HARQ)-acknowledgment (ACK), 　　　　　　a scheduling request (SR), 　　　　　　periodic / semi-persistent (P / SP) CSI, or 　　　　　　aperiodic CSI, 　　on at least one serving cell is kept by the UE in a case where the transmissions are overlapped to transmissions of PRACH on the candidate LTM cells.

11. The method according to any one of claims 3 to 10, further comprising: 　　transmitting, to the second central unit of the second base station via the first central unit of the first base station, a cell switch indication for deactivating TCI states for the candidate LTM cells.

12. The method according to any one of claims 3 to 11, wherein 　　the L1 measurement reports includes at least one of: 　　　　at least one CSI-RS index corresponding to a respective candidate LTM cells, or 　　　　an index for at least one resource pool corresponding to the first central unit of the first base station or the second central unit of the second base station.

13. The method according to claim 2, further comprising: 　　receiving, from the second distributed unit of the second base station via the second central unit of the second base station and the first central unit of the first base station, L1 measurement reports of results of the L1 measurement performed based on the information of configuration; and 　　transmitting, to the UE, a cell switch command based on the L1 measurement reports.

14. The method according to claim 13, wherein 　　a beam application time reference subcarrier spacing (SCS) is based on at least one serving cell and / or the candidate LTM cells, whichever is larger.

15. A method performed by a user equipment (UE), the method comprising: 　　receiving, from a second central unit of a second base station via a first central unit of a first base station and a first distributed unit of the first base station, information of configuration on layer 1 (L1) measurements on candidate L1 / layer 2 (L2) triggered mobility (LTM) cells on which the second central unit of the second base station has configured the second distributed unit of the second base station; and 　　transmitting, to the first distributed unit of the first base station, L1 measurement reports of results of the L1 measurement performed based on the information of configuration, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements.

16. A first distributed unit of a first base station comprising: 　　means for transmitting, to a second central unit of a second base station via a first central unit of the first base station, first information of candidate Layer 1 (L1) / Layer 2 (L2) triggered mobility (LTM) cells; and 　　means for receiving, from the second central unit of the second base station via the first central unit of the first base station, information for configuration on L1 measurements on the candidate LTM cells on which the second central unit of the second base station has configured a second distributed unit of the second base station, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements.

17. A user equipment (UE) comprising: 　　means for receiving, from a second central unit of a second base station via a first central unit of a first base station and a first distributed unit of the first base station, information of configuration on layer 1 (L1) measurements on candidate L1 / layer 2 (L2) triggered mobility (LTM) cells on which the second central unit of the second base station has configured the second distributed unit of the second base station; and 　　means for transmitting, to the first distributed unit of the first base station, L1 measurement reports of results of the L1 measurement performed based on the information of configuration, and 　　wherein the information for configuration indicates at least one of: 　　　　tracking reference signal (TRS) configuration for the L1 measurements, 　　　　channel state information reference signal (CSI-RS) configuration for the L1 measurements, or 　　　　sounding reference signal (SRS) configuration for the L1 measurements.

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