Method, user equipment and source secondary base station

Abstract

A wireless communication system is described that consisting of at least one master node (MN) and one secondary node (SN) that support a master cell group (MCG) of cells and a secondary cell group (SGC) of cells respectively. The wireless communication system described supports SCG L1 / L2 triggered mobility (LTM) without MN involvement, for intra-SN based SCG LTM, and / or for inter-SN based SCG LTM (e.g., in conjunction with support for inter-CU LTM).

Description

METHOD, USER EQUIPMENT AND SOURCE SECONDARY BASE STATION

[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). The disclosure has particular, although not necessarily exclusive relevance to, inter-cell mobility triggered by L1 / L2 signalling.

[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 term '5G' and 'new radio' (NR) has started to be 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 equipment or 'UE') 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] 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] Both 5G and earlier communication technologies support dual connectivity (DC) in which a UE with appropriate capabilities may be configured, by the network, to communicate using radio resources provided via different base stations (or, potentially, different cells of the same base station). DC is conceptually similar to carrier aggregation (CA) in which the UE may be configured to use the resources of different carriers / cells provided via a single serving base station to allow the UE to operate in different frequency bands at substantially the same time. Generally, DC splits user plane traffic at the PDCP layer, whereas CA splits traffic at the MAC layer although it will be appreciated that as technology is developed this may change. Moreover, with DC the UE may communicate via different cells using the same radio access technologies (RATs) or different RATs whereas in CA the same RAT is generally used for the different carriers / cells via which the UE communicates. Typically, CA is used in scenarios where the backhaul between nodes is ideal, while DC is used for non-ideal backhaul scenarios, for example where there is the potential for a relatively large delay between nodes.

[0007] The base stations via which a UE is configured to communicate using DC typically comprise a base station configured as a master node (MN) and at least one base station (but possibly more) configured as a secondary node (SN). A group of one or more cells provided by the MN and configured for use by the UE for DC is known as a master cell group (MCG), whereas a group of one or more cells provided by the SN and configured for use by the UE for DC is known as a secondary cell group (SCG). The MCG includes a cell configured as a primary cell (PCell) whereas the MCG includes a cell configured as a primary secondary cell (PSCell).

[0008] Whilst the base stations acting as the MN and the SN are typically different, current technology (e.g., NR DC technology) allows for DC configurations in which a UE is configured to communicate via a single base station that acts as both an MN and as an SN, and that provides one or more cells configured as the MCG for that UE and one or more cells configured as the SCG.

[0009] It will be appreciated that CA and DC are not mutually exclusive and can be implemented together, for the same UE, for example where there are potentially multiple carriers in the MCG and potentially multiple carriers in the SCG.

[0010] 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 'LTM') has prospects for improving mobility for devices operating both below 7 GHz and in mmWave bands, for example by supporting lower handover latency and improved robustness.

[0011] 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>

[0012] The development of efficient LTM procedures provides a number of challenges in the context of DC. For example, it is currently envisaged that LTM may be implemented PCell change (MCG) using LTM without an SCG configuration albeit that if there is an SCG configuration, the SCG may be released at LTM execution. It is also envisaged that LTM may need to be supported for SCG LTM without MN involvement, for intra-SN based SCG LTM, and / or for inter-SN based SCG LTM (e.g., in conjunction with support for inter-CU LTM).

[0013] There is, therefore, a need for the development of communication devices (such as base stations and / or UEs) that support efficient / flexible mechanisms for supporting LTM in one or more of the above scenarios.

[0014] One or more apparatus and / or one or more associated methods are disclosed that at least partially addresses the above need.

[0015] The various functional means described below that are part of the UE may be provided by a memory and one or more processors that execute instructions stored in the memory. Similarly, the various functional means described below that are part of the access network node may be provided by a memory and one or more processors that execute instructions stored in the memory.

[0016] Various example described below may be implemented by means of a computer program product comprising computer implementable instructions for causing a programmable computer to carry out the any of the methods described below. The computer implementable instructions may be provided as a signal or on a tangible computer readable medium.

[0017] A method performed by a user equipment (UE) according to the present disclosure configured to communicate using radio resources provided by a master base station and a source secondary base station, the method comprising: 　　receiving, from the source secondary base station directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　transmitting, to the source secondary base station, a Layer 1 (L1) measurement report including results of L1 measurement performed on cells configured by the information; 　　receiving, from the source secondary base station, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell; and 　　performing SCG LTM cell switch procedure towards to the target cell using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier.

[0018] A method performed by a source secondary base station according to the present disclosure configured to provide radio resources for communicating with a user equipment (UE), together with a master base station, the method comprising: 　　transmitting, to the UE directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　receiving, from the UE, a Layer 1 (L1) measurement report including results of L1 measurement performed by the UE on cells configured by the information; 　　transmitting, to the UE, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell, wherein 　　SCG LTM cell switch procedure towards to the target cell is performed by the UE using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier.

[0019] A user equipment (UE) according to the present disclosure configured to communicate using radio resources provided by a master base station and a source secondary base station, the UE comprising: 　　means for receiving, from the source secondary base station directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　means for transmitting, to the source secondary base station, a Layer 1 (L1) measurement report including results of L1 measurement performed on cells configured by the information; 　　means for receiving, from the source secondary base station, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell; and 　　means for performing SCG LTM cell switch procedure towards to the target cell using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier.

[0020] A source secondary base station according to the present disclosure configured to provide radio resources for communicating with a user equipment (UE), together with a master base station, the source secondary base station comprising: 　　means for transmitting, to the UE directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　means for receiving, from the UE, a Layer 1 (L1) measurement report including results of L1 measurement performed by the UE on cells configured by the information; 　　means for transmitting, to the UE, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell, wherein 　　SCG LTM cell switch procedure towards to the target cell is performed by the UE using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier.

[0021] Examples of apparatus and methods 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;Fig. 2 illustrates an exemplary intra-CU L1 / L2 triggered mobility (LTM) cell switch scenario that may be used in the communication system of Fig. 1;Fig. 3 illustrates a possible grouping of cells into a master cell group (MCG) and one or more secondary cell groups (SCGs) that may be employed in the communication system of Fig. 1;Fig. 4A illustrates possible connectivity between a UE and nodes in a dual connectivity (DC) mode that may be employed in the communication system of Fig. 1;Fig. 4B illustrates a user plane from a UE perspective showing the signalling radio bearers (SRBs) for the UE and nodes connected in NR-DC mode;Fig. 5 is a simplified sequence diagram illustrating, in general overview, an LTM procedure that may be implemented in the communication system 1;Fig. 6 is a simplified sequence diagram illustrating the LTM procedure of Fig. 5 in more detail;Fig. 7 is a simplified sequence diagram illustrating a typical RRC procedure for switching between a source and a target secondary node that may be employed in the communication system of Fig. 1;Fig. 8 is a simplified sequence diagram illustrating a possible part of the typical RRC procedure for switching between a source and a target secondary node shown in Fig. 7;Fig. 9 is a simplified sequence diagram illustrating an intra-SN LTM handover procedure that may be employed in the communication system of Fig. 1;Fig. 10 is a simplified sequence diagram illustrating another intra-SN LTM handover procedure that may be employed in the communication system of Fig. 1;Fig. 11 is a simplified sequence diagram illustrating a first part of an inter-SN LTM handover procedure that may be employed in the communication system of Fig. 1;Fig. 12 is a simplified sequence diagram illustrating a second part of the inter-SN LTM handover procedure of Fig. 11;Fig. 13 a simplified sequence diagram illustrating a first part of another inter-SN LTM handover procedure;Fig. 14 is a simplified sequence diagram illustrating a second part of the inter-SN LTM handover procedure in Fig. 13;Fig. 15 is a simplified block schematic illustrating the main components of a user equipment that may be used in the communication system of Fig. 1;Fig. 16 is a simplified block schematic illustrating the main components of a RAN node (base station / access network node) that may be used in the communication system of Fig. 1; andFig. 17 is a simplified block schematic illustrating the main components of a distributed-type RAN (with a central unit (CU) and a distributed unit (DU)) that may be used in the communication system of Fig. 1.

[0022] Overview 　　An exemplary communication system will now be described in general terms, by way of example only, with reference to Figs. 1 to 8.

[0023] Fig. 1 schematically illustrates a mobile ('cellular' or 'wireless') communication system (communication system 1) to which the examples described herein are applicable.

[0024] 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 radio access network (RAN) node 5A that operates according to one or more compatible radio access technologies (RATs). In the illustrated example, the RAN node 5A comprises a distributed base station 5A or 'gNB' operating one or more associated cells 9. Communication via the RAN 5 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)).

[0025] As those skilled in the art will appreciate, whilst three UEs 3, and one RAN 5 are shown in Fig. 1 for illustration purposes, the system, when implemented, will typically include other RAN node 5A and UEs 3.

[0026] The RAN 5 controls one or more associated cells 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 RAN node 5A may be configured to support both 4G and 5G, and / or later generations, and / or any other 3GPP or non-3GPP communication protocols.

[0027] In this example the illustrated RAN node 5A comprises a distributed base station 5A 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 via an appropriate interface (e.g. an F1-C interface) and an appropriate interface (e.g. an F1-U 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 5 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 base station 5A is shown and described, the base station 5A may be provided in a non-distributed form, for example as an integrated base station 5A.

[0028] The UEs 3 and their serving RAN 5 are connected via an appropriate air interface (for example the so-called 'Uu' interface and / or the like). Equipment of neighbouring RANs 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).

[0029] 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.

[0030] The RAN node 5A 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 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 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 reference point (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 5.

[0031] Each UPF 11 is 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.

[0032] 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.

[0033] The SMF 10-2 is connected to the AMF 10-1 via an appropriate reference point (e.g. an N11 reference point). 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. The SMF 10-2 uses user information provided via the AMF 10-1 to determine what session manager would be best assigned to the user. The SMF 10-2 may be considered effectively to be a gateway from the user plane to the control plane of the network. The SMF 10-2 also allocates IP addresses to each UE 3.

[0034] The RAN 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 originated from a higher layer, and the DL physical signals are used in the physical layer and correspond to REs which do not carry information originated from a higher layer.

[0035] 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.

[0036] 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 RAN 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).

[0037] Similarly, the UEs 3 are configured for transmission of, and the RAN 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 originated from a higher layer, and UL physical signals which are used in the physical layer and correspond to REs which do not carry information originated 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.

[0038] Intra-CU LTM Cell Switches 　　Referring to Fig. 2, which illustrates an exemplary intra-CU L1 / L2 triggered mobility (LTM) cell switch scenario that may take place in the communication system 1, a UE 3 may move among the cells provided via the same CU.

[0039] For example, as seen in Fig. 2, a CU 5c may control a plurality of DUs 5b-1, 5b-2, 5b-3 and each DU 5b-1, 5b-2, 5b-3 may respectively operate a plurality of cells 9-1a to 9-1c, 9-2a to 9-2c, and 9-3a to 9-3c (and / or beams). As a UE 3 moves through the cells (or beams), as illustrated by the dashed arrow A, it passes through a number of cells (in the example 9-1b --> 9-1c --> 9-2a --> 9-2b --> 9-3c --> 9-3a) involving multiple cell-to-cell transitions (denoted 'X'). The cell transitions include intra-DU cell transitions (e.g., 9-1b --> 9-1c; 9-2a --> 9-2b; and 9-3c --> 9-3a) and inter-DU cell transitions (e.g., 9-1c --> 9-2a and 9-2b --> 9-3c).

[0040] Beneficially, the UE 3 and RAN node 5A (i.e., CU 5c and DUs 5b-1, 5b-2, 5b-3) are mutually configured for implementing an intra-CU LTM procedure in which the UE 3 is able to switch between pre-configured candidate LTM cells 9-1a to 9-1c, 9-2a to 9-2c, and 9-3a to 9-3c, 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 9-1a to 9-1c, 9-2a to 9-2c, and 9-3a to 9-3c, it can execute fast cell switches, potentially without RRC reconfiguration.

[0041] It will be appreciated that while the cell switches in Fig. 2 are necessitated by movement of the UE 3 relative to the cells 9, cell switching of this type could be necessitated by movement of the cells 9 relative to the UE 3 (e.g., in a non-terrestrial network (NTN) where each cell is are provided via a moving satellite). Moreover, cell switching of this type could be necessitated by a change of cell size arising from a network energy saving (NES) mode being initiated in the RAN by / for one or more DUs. Moreover, cell switching of this type could be necessitated by a cell being turned off (or on) for energy saving (or load management) purposes.

[0042] Master / Secondary Cell Groups 　　Fig. 3 illustrates possible groupings of cells into a master cell group (MCG) and one or more secondary cell groups (SCGs) that may be employed in the communication system 1.

[0043] Fig. 3 illustrates a first group of cells 9-1a, 9-1b, 9-1c provided by a first base station 5-1 that is configured for use by a UE 3 as an MN, and two other groups of cells (9-2a, 9-2b, 9-2c; and 9-3a, 9-3b, 9-3c) each provided by another respective base station 5-2, 5-3 that may be configured for use by the UE 3 as a corresponding SN. It will be appreciated that while Fig. 3 shows two other base stations (5-2, 5-3), each providing a respective group of cells 9-2, 9-3 for the purposes of illustration, any number of such base stations may be provided.

[0044] As seen in Fig, 3, the first group of cells 9-1 provided by the base station 5-1 configured as MN 5-1 is configured as an MCG for the UE 3 and comprises a primary cell (PCell) 9-1a, and two secondary cells (SCells) 9-1b, 9-1c. It will be appreciated that while the MCG depicted comprises two SCells, the MCG may comprise any number of SCells. The MN 5-1 depicted may be any type of node that is capable of supporting DC, for example, it may be an eNB, a ng-eNB, a gNB, a distributed gNB, or the like.

[0045] In Fig. 3 a first of the other base stations 5-2 is configured as a current (source) SN 5-2 for the UE 3 and the group of cells 9-2 provided by that other base station 5-2 is configured as a first (e.g., source) SCG for the UE 3 (although it will be appreciated that the SCG may not include all cells provided by that base station 5-2). The SCG provided by the first of the other base stations 5-2 includes a primary secondary cell (PSCell) 9-2a, and two SCells 9-2b, 9-2c. As shown in Fig. 3, the cells of this SCG at least partially overlap with the cells of the MCG such that the UE 3 may, based on its positioning in Fig. 3, be able to access and use a cell provided by the MN 5-1 and / or a cell provided by the SN 5-2 for the purposes of DC. The MCG provided by the MN 5-1 may, for example, provide the UE 3 with access to one RAT (e.g., NR), while the SCG provided by the SN 5-2 may provide the UE 3 with access to a different RAT (e.g., LTE). It will nevertheless be appreciated that the MCG and SCG provided by the MN 5-1 and the SN 5-2 respectively may provide the UE 3 with access to the same RAT.

[0046] In Fig. 3 a second of the other base stations 5-3 provides a group of cells that may be configured as a second SCG for the UE 3 (e.g., if the UE 3 moves to the coverage of that base station 5-3). For example, the cells provided by the second of the other base stations 5-3 may be configured as a PSCell 9-3a, and two SCells 9-3b, 9-3c (or another similar configuration).

[0047] As with the first SCG, the potential second SCG provided by the second of the other base stations 5-3 may provide the UE 3 with access to a different RAT to the one provided by the MCG. Furthermore, the potential second SCG may provide the UE 3 with access to a different RAT to the one provided by the first SCG, although typically the RATs of multiple SCGs are the same and may provide coverage to the UE 3 in different geographical areas around the MCG.

[0048] The other base stations 5-2, 5-3 depicted may also be any type of node that is capable of supporting DC, for example, it may be an eNB, a ng-eNB, a gNB, a distributed gNB, or the like.

[0049] It will be appreciated that during use the UE 3 may move geographically and thus between areas covered by different cells or groups of cells. As such the (re)configuration of the UE and the cells may be necessary to maintain optimal communication between the UE 3 and a RAN 5.

[0050] For example, the movement of the UE 3 may necessitate a reconfiguration of the SCG provided via the current SN 5-2 to comprise a different configuration of PSCell and one or more SCells. This may be facilitated by an intra-SN cell switching procedure. Alternatively, the movement of the UE 3 may necessitate a change of SN that provides the SCG (and hence the cells of the SCG) from the first of the other base stations 5-2 to the second of the other base stations 5-3. This may be facilitated by an inter-SN cell switching procedure.

[0051] New Radio (NR)-DC 　　Fig. 4A shows an example connectivity of a MN of an MCG and a SN of an SCG in a DC scenario (in this example an NR-DC scenario using a 5G standalone core) that may be implemented in the communication system 1. In this exemplary configuration both the master and secondary RAN nodes 5A are gNBs although it will be appreciated that this need not be the case and either node may be a node of a different RAT (e.g., an LTE node or the node of a more advanced generation of wireless technology).

[0052] As shown in Fig. 4A, the UE 3 is connected, in the DC scenario, via a master node (MN) 5-1 that is connected to the core network 7 via a control plane interface (in this example an NG-C interface) between the MN 5-1 and a mobility management entity (e.g., the AMF 10-1 of the core network 7). It will be appreciated that MN 5-1 may be a split node i.e., a distributed-type node 5A. In that case the central unit (CU) 5c of the MN 5-1 would be connected to the AMF 10-1 via the NG-C interface. The MN 5-1 is also connected to the UE 3 via an air interface (Uu).

[0053] At the same time there is also provided a secondary node (SN) 5-2 that connects to the MN 5-1 via a base station-to-base station control plane interface (in this example an Xn-C interface) which allows signalling between the MN 5-1 and the SN 5-2. It will be appreciated that SN 5-2 may be a split node i.e., a distributed-type node 5A. In that case the CU 5c of the SN 5-2 would be connected to MN 5-1 via the Xn-C interface. Where both the MN 5-1 and the SN 5-2 are distributed types, then it is the respective CUs 5c of those nodes that are connected via the Xn-C interface. As with the MN 5-1, the SN 5-2 is connected to the UE 3 via an air interface (Uu).

[0054] The UE 3 has a single RRC state based on the MN 5-1's RRC state. Furthermore, both the MN 5-1 and the SN 5-2 have their own RRC entity which can generate their own RRC protocol data units (PDUs) that can be sent to the UE 3. The MN 5-1 sends its initial SN RRC configuration via the MCG signalling radio bearer (SRB1). However, subsequent reconfigurations may be transported via the MN 5-1 or the SN 5-2. Additionally, based on the connections provided between the MN 5-1, SN 5-2, and UE 3, RRC PDUs generated by the SN 5-2 can be transported to the UE 3 via the MN 5-1 (for example, over the Xn-C interface between the SN 5-2 and the MN 5-1, and then over the Uu interface between the MN 5-1 and the UE 3). Alternatively, the SN 5-2 may send RRC PDUs to the UE 3 directly over its Uu interface via a SCG bearer (SRB3).

[0055] This configuration means that the MN 5-1 and the SN 5-2 can be operated with shared spectrum channel access.

[0056] It will be appreciated that while Fig. 4A shows two separate base stations 5-1, 5-2, (one MN 5-1 and one SN 5-2), there may instead be provided a single base station 5A to which the UE 3 is connected, and which acts as both a MN 5-1 and a SN 5-2. In that case the single base station 5A may configure both the MCG and any SCGs.

[0057] Fig. 4B shows a user plane from a UE perspective and shows the bearers for the UE 3 and nodes connected in the DC (e.g., NR-DC) mode described above with reference to Fig. 4A. It will be appreciated that the bearers in Fig. 4B can refer to SRBs or data radio bearers (DRBs).

[0058] As seen in Fig. 4B there user plane from the UE's perspective has several different layers including the service data adaption protocol (SDAP) layer, a packet data convergence protocol (PDCP) layer, a radio link connection (RLC) layer, and a medium access control (MAC) layer.

[0059] The SDAP layer is responsible for quality of service (QoS) flow handling across the air interface between the UE 3 and the RAN nodes 5A. The SDAP will map a specific QoS flow within a protocol data unit (PDU) session to a corresponding DRB which has been established with the appropriate level of QoS. In addition, SDAP will mark the transmitted packets with the correct QoS flow ID (QFI), ensuring that the packet receives the correct forwarding treatment as it traverses the communication system. Typically, for each PDU session, a single protocol entity of SDAP will be configured.

[0060] Nevertheless, for DC, an MCG and a SCG may each have a separate SDAP configuration on the device i.e., the MCG and SCG may have their own SDAP entity (entities not shown), and a separate SRB, referred to in Fig. 4B as 'MCG Bearer' and 'SCG Bearer'. The MCG and SCG bearers can utilise resources while following the radio protocols located in only MN 5-1 and SN 5-2, respectively. A 'Split Bearer' may also be configured that allows the UE 3 (e.g., when in non-standalone (NSA) multi-RAT (MR) DC) to utilise resources from two distinct schedulers (i) one in the MN 5-1 and (ii) the other in the SN 5-2.

[0061] Beneath the SDAP layer is provided a split NR PDCP layer which facilitates transfer of user plane data, transfer of control plane data, header compression, ciphering, and integrity protection functions. As shown in Fig. 4B, the PDCP layer is where the splitting of signals and data occurs for the MCG, SCG, and split bearers. From the PDCP layer there follows a RLC layer which performs functions such as the transfer of upper layer PDUs in one of three modes: Acknowledged Mode (AM), Unacknowledged Mode (UM) and Transparent Mode (TM), error correction, concatenation, segmentation, and reassembly of RLC service data units (SDUs), re-segmentation of RLC data PDUs, reordering of RLC data PDUs, duplicate detection, and the like. From the RLC layer follows a MAC layer.

[0062] In the case of the SRBs (which are used for RRC signalling, NAS messages, and the like) the provision of the separate MCG and SCG bearers along with the split bearer allows the SN 5-2 to send RRC PDUs directly between the SN 5-2 and the UE 3 via the SRB3 as described above where the RRC PDUs do not require any coordination from the MN 5-1. Similarly, measurement reporting for mobility within an SN 5-2 can be done directly from the UE 3 to the SN 5-2 if the SRB3 is configured. Alternatively, or additionally (whether coordination from the MN 5-1 is required or not), the SN 5-2 may send RRC PDUs to the UE 3 that are routed via the MN 5-1 i.e., the RRC PDUs are sent to the UE 3 from the MN 5-1 via SRB1 as described above.

[0063] In the case of the DRBs, the provision of the separate MCG and SCG bearers along with the split bearer allows the splitting of data transmissions to the UE 3 via the MN 5-1 and the SN 5-2. For example, some data may be transmitted to the UE 3 via the MN 5-1, and some may be transmitted via the SN 5-2, with each of those nodes providing a different type of network access. For instance, the MN 5-1 may provide NR access and the SN 5-2 may provide LTE access.

[0064] General LTM procedure 　　An exemplary intra-CU LTM procedure will now be described, by way of example only, with reference to Figs. 5 and 6. Possible beneficial variations to these procedures are described later.

[0065] Fig. 5 is a simplified sequence diagram illustrating, in general overview, air-interface signalling for an LTM procedure that may be implemented in the communication system 1.

[0066] Referring to Fig. 5, the LTM procedure in this example may be for a change in a cell of an MCG for example involving an intra-CU inter-DU cell switch in the RAN 5 between a cell as previously described with reference to Fig. 3.

[0067] As seen in Fig. 5, the UE 3 (which is in an RRC CONNECTED state) and the RAN 5 engage in measurement reporting procedure at S504. This procedure typically involves the UE 3 sending, to the RAN 5, an RRC measurement report (e.g., in a 'MeasurementReport' message or the like) containing the results of measurements for one or more cells (e.g., of measurements of reference signals in a serving cell and / or one or more neighbouring cells). The measurement results may include, for example, L3 filtered measurement results for a beam and / or cell (but this need not be the case).

[0068] The RAN 5 can then make a decision, at S506, to initiate an L1 / L2 inter-cell triggered mobility (LTM) (pre)configuration, and initiate preparation of one or more candidate cells. For example, preparation of candidate cells may include the preparation of a respective LTM candidate cell configuration for each candidate cell.

[0069] The RAN 5 transmits an RRC (re)configuration message (e.g., an 'RRCReconfiguration' message or the like), at S508, to the UE 3 including one or more LTM candidate cell configurations of one or multiple candidate cells, which are subsequently stored at the UE 5 for future use. Having stored one or more LTM candidate cell configurations, the UE 3 transmits, at S510, an appropriate message to confirm completion of the (re)configuration (e.g., an 'RRCReconfigurationComplete' message or the like) to the RAN 5. The sending of this message completes an LTM preparation part of the LTM procedure (also known as an LTM pre-configuration procedure).

[0070] Following transmission of the RRC (re)configuration completion message by the UE 3, the UE 3 and RAN 5 may perform an early synchronisation procedure. For example, the UE 3 may perform, at S512a DL synchronisation with one or more candidate cells before receiving a cell switch command, DL synchronisation for one or more candidate cells before cell switch command being supported at least based on SSB.

[0071] Moreover, the UE 3 may also perform early synchronisation including early timing advance (TA) acquisition with one or more candidate cells requested by the network before receiving the cell switch command i.e., the UE 3 may perform, at S512b, UL synchronisation with one or more candidate cells before receiving a cell switch command. This may be performed via contention-free random access (CFRA) triggered by a physical downlink channel (PDCCH) order from the source cell, following which the UE 3 sends a preamble towards the indicated candidate cell. In order to minimize the data interruption of the source cell due to CFRA towards each candidate cell, the UE 3 does not receive random access response (RAR) for the purpose of TA value acquisition, and the TA value of the candidate cell is indicated in the cell switch command. The UE 3 does not maintain the TA timer for the candidate cell and relies on network implementation to guarantee the TA validity.

[0072] Following transmission of the RRC (re)configuration completion message by the UE 3 (and optional early synchronisation) the UE 3 enters an LTM execution phase involving the performance of L1 measurements on one or more configured candidate cells and transmits, at S514, one or more associated lower-layer measurement reports to the RAN 5. L1 measurements are performed for as long as the RRC reconfiguration provided at S508 applies.

[0073] Considering those L1 measurements transmitted by the UE 3 to the RAN 5, the RAN 5 makes a decision as to whether to execute cell switch to a target cell at S516. If the RAN 5 decides to execute cell to a target cell, the RAN 5 transmits, at S518, a cell switch command to trigger cell switch (e.g., in an appropriate MAC control element (CE)). That cell switch command includes a candidate configuration index for the target cell.

[0074] The UE 3 then switches, at S520, to the target cell and applies the configuration indicated by the candidate configuration index. For example, the UE 3 may detach from its source cell of one DU of the RAN 5 and applies the configuration of a target cell of another DUI of the RAN 5. Finally, the UE 3 completes the LTM cell switch procedure, at S524, for example by sending an RRC (re)configuration message (e.g., an 'RRCReconfiguration' message or the like) to target cell.

[0075] Alternatively, if the UE 3, having received the cell switch command to trigger cell switch, does not have a valid TA of the target cell (for example, if the UE 3 did not perform early synchronisation) then the UE 3 may perform a RACH procedure toward the target cell to obtain a valid TA of that target cell at S522. In that case, the UE 3 considers that LTM execution is successfully completed when the random-access procedure is successfully completed. For RACH-less LTM, the UE considers that LTM execution is successfully completed when the UE determines that the network has successfully received its first UL data.

[0076] It will be appreciated that S512 through S524 of the LTM procedure shown in Fig. 5 can be performed multiple times for any subsequent LTM cell switch using one or more LTM candidate cell configurations provided at S506.

[0077] Fig. 6 is a simplified sequence diagram illustrating the LTM procedure of Fig. 5 in more detail for a specific scenario. Specifically, Fig. 6 illustrates an intra-CU inter-DU cell switch scenario and includes, for this example, typical signalling that may take place between the CU 5c, the source DU 5b-1 and a target DU 5b-2.

[0078] As seen in Fig. 6, the UE 3, source DU 5b-1, and associated CU 5c engage in a measurement control and reporting procedure at S604. This procedure typically involves the UE 3 sending, to the source DU 5b-1, an RRC measurement report (e.g., in a 'MeasurementReport' message or the like) containing the results of measurements for one or more cells (e.g., of measurements of reference signals in a serving cell and / or one or more neighbouring cells). The measurement results may include, for example, L3 filtered measurement results for a beam and / or cell (but this need not be the case). The source DU 5b-1 may then send an appropriate message (e.g., UL RRC Message Transfer message or the like) for conveying the received measurement report to the CU 5c.

[0079] The CU 5c can then make a decision, at S606, to initiate an L1 / L2 inter-cell triggered mobility (pre) configuration and initiate preparation of one or more candidate cells. For example, preparation of the candidate cells may include the preparation of one or more candidate DUs 5b-3 of respective candidate cells (one of which will be the target DU 5b-2 of a target candidate cell). Preparation of each candidate DU 5b-3 includes preparation of a LTM candidate cell configuration for each candidate cell.

[0080] The CU 5c sends one or more messages for requesting the setting up a context for the UE 3 to one or more candidate DUs 5b-3, 5b-2 containing (candidate) target cells (shown only for DU 5b-3 for exemplary purposes). This message, sent at S608a is, in effect, a request for LTM configuration at the recipient DU. If a candidate DU 5b-2 accepts the request for LTM configuration in one or more (candidate) target cells, it responds to the CU 5c with an appropriate response including a generated lower layer configuration for one or more accepted target candidate cells. It will be appreciated that, potentially, either a single UE context setup procedure (i.e., one procedure per candidate DU 5b-2 to prepare one or multiple candidate cells of the DU 5b-2) or multiple UE context setup procedures (i.e., one procedure per candidate cell) may be used at this stage. For example, at least the target DU 5b-2 responds with such a response message including the generated lower layer configuration for each accepted target candidate cell of the target DU 5b-2 at S608b.

[0081] The CU 5c sends, at S610, an appropriate RRC message (e.g., an RRC (re)configuration message) to the UE 3, via the source DU 5b-1, including the L1 / L2 triggered mobility configuration for each candidate cell. This message may, for example, be sent to the source DU 5b-1 as part of a UE Context Modification Request or another message such as, for example, a DL RRC Message Transfer message or the like, from where it is sent to the UE 3. The RRC message may, for example, include a new cell-radio network temporary identifier (C-RNTI) of the UE 3 to identify that UE 3.

[0082] The UE 3 responds, at S612, with a RRC reconfiguration complete message sent to the CU 5c, via the source DU 5b-1. The source DU 5b-1 may transfer the message to the CU 5c using an appropriate message, corresponding to the message sent at S610. This message may, for example, be a UE Context Modification Response or another message such as, for example, an UL RRC Message Transfer message or the like. The sending of this message completes an LTM preparation part of the LTM procedure (e.g., as described with reference to Fig. 5).

[0083] Following the LTM preparation, the UE 3 and RAN 5 may engage in an LTM execution phase, corresponding to that described with reference to Fig. 5, involving a cell switch procedure to switch to a cell of the target DU 5b-2.

[0084] Specifically, following the LTM pre-configuration the UE 3 and RAN 5 may send a lower layer measurement report (e.g., including one or more L1 / L2 measurement results) to the source DU 5b-1 (e.g., for the serving and / or one or more target / candidate cells) as seen at S614.

[0085] The source DU 5b-1 may then decide to execute LTM to switch to a candidate target cell at S616. It will be appreciated that the source DU 5b-1 may also notify the LTM cell switch decision to the other nodes as well. The source DU 5b-1 sends, at S618, an LTM command, to initiate the cell switch, to the UE 3 (e.g., using a MAC control element (CE) or the like). It will be appreciated that any notification of the LTM cell switch decision to other nodes may occur after the source DU 5b-1 sends the LTM command to the UE 3.

[0086] The UE 3 is thus able to detach from the current cell of the source DU 5b-1 (and synchronise to the target cell of the target DU 5b-2 as necessary) as described with reference to Fig. 5. The UE 3 may then engage in a random access channel (RACH) based initial access procedure (or alternatively a RACH-less initial access procedure) to connect to the target DU 5b-2 at S620.

[0087] The source DU 5b-1 may also notify the CU 5c about the initiation of LTM / the sending of the LTM command to the UE 3 (e.g., over the F1 interface using an F1 application protocol (F1AP) or the like). The target DU 5b-2 detects the UE access and may notify the CU 5c of the access success at S622.

[0088] It will be appreciated that, as a skilled person would understand, for inter-DU LTM, any release of resources of the source cell (and any prepared cell) in the source DU 5b-1, may be achieved in any appropriate manner (if at all).

[0089] It will be appreciated that the detailed steps of the procedure described with reference to Fig. 6 is exemplary is for illustrative purposes, as those skilled in the art will be aware, there are a number of possible variations to the procedure and, in particular, to the LTM preparation part and / or the LTM execution part of the procedures.

[0090] Secondary node switch procedure 　　Whilst the LTM procedure described previously may be used in the communication system 1, for example where cells are grouped together into a MCG and SCGs, the communication system 1 is also configured to support layer 3 (L3) RRC based mobility procedures (e.g., for switching between a source and target secondary node of a SCG).

[0091] Figs. 7 and 8 illustrate a simplified sequence diagram illustrating a typical RRC procedure for switching between a source and a target secondary node of a SCG that may be used in the communication system 1.

[0092] Referring to Fig. 7, the RRC procedure in this case concerns a secondary node (SN) switch in a SCG between a source secondary node (S-SN) 5-2 and a target secondary node (T-SN) 5-3, which is facilitated via a master node (MN) 5-1 of an MCG.

[0093] As seen in Fig. 7, the S-SN 5-2 initiates the SN change procedure by sending a message to indicate that a change of SN is required (e.g., an 'SN Change Required' message or the like), at S702, which contains a candidate target SN ID. The message may also include an SCG configuration (e.g., to support delta configuration) and measurement results relating to the candidate target SN (T-SN) 5-3.

[0094] The MN 5-1 requests, at S704, the candidate target SN (T-SN) 5-3 to allocate resources for the UE 3 by means of an SN addition procedure. For example, the MN 5-1 may send a secondary node addition request to the candidate target secondary node (T-SN) 5-3 which may include E-UTRAN radio access bearer (E-RAB) characteristics (e.g., E-RAB parameters, transport network layer (TNL) address information), SCG configuration information including the UE capabilities and UE capability coordination results, the latest measurement results for the candidate target SN (T-SN) 5-3 received from the source SN (S-SN) 5-2, security Information to enable SRB3 (e.g., a signal radio bearer for the transmission of RRC messages when a UE is in E-UTRA to NR-DC), X2-U TNS address information (when necessary e.g., when the radio bearer requires X2-U between MN and SN), and the maximum supportable QoS level when SN terminated split bearers are implemented. If data forwarding is needed, the target SN (T-SN) 5-3 provides data forwarding addresses to the MN 5-1. The target SN (T-SN) 5-3 includes the indication of the full or delta RRC configuration.

[0095] If a candidate target SN (T-SN) 5-3 decides to accept the request, the candidate target SN (T-SN) 5-3 sends, at S706, an addition request acknowledgement to the MN 5-1, and allocates the necessary radio resources and transport network resources. The candidate target SN (T-SN) 5-3 (now the target SN (T-SN) 5-3) also decides which cells should form its SCG and designates a PSCell and other SCG SCells which are indicated to the MN 5-1 in a new SCG radio resource configuration sent by the target SN (T-SN) 5-3.

[0096] In addition, if the source SN (S-SN) 5-2 is communicating via SN terminated bearers, then the MN 5-1 may provide base station-to-base station user plane interface (Xn-U) downlink (DL) TNL address information to the target SN (T-SN) 5-3 (e.g., in an Xn-U Address Indication message) at S706b.

[0097] The MN 5-1 then triggers, at S708, a RRC reconfiguration with the UE 3 to apply the new configuration received from the target SN (T-SN) 5-3. The MN 5-1 indicates the new configuration to the UE 3 in an MN RRC reconfiguration message including the SN RRC reconfiguration message generated by the target SN (T-SN) 5-3. The UE 3 then applies the new configuration and sends an MN RRC reconfiguration complete message, at S710, including an SN RRC response message for sending to the target SN (T-SN) 5-3, if needed. In case the UE 3 is unable to comply with (part of) the configuration included in the MN RRC reconfiguration message, it may perform a reconfiguration failure procedure (not shown).

[0098] If the allocation of the target SN (T-SN) 5-3 resources is successful, the MN 5-1 confirms the change of the source SN (S-SN) 5-2 by sending a message to confirm SN change (e.g., an 'SN Change Confirm' message or the like) to source SN (S-SN) 5-2 at S712. Reception of the message to confirm SN change triggers the source SN (S-SN) 5-2 to stop providing user data to the UE 3 and, if applicable, to start data forwarding as described in more detail with reference to Fig. 8.

[0099] If the RRC connection reconfiguration procedure is successful, the MN 5-1 informs the target SN (T-SN) 5-3, at S714, via an appropriate message (e.g., an 'SN Reconfiguration Complete' message or the like), which may also include an SN RRC response message for the target SN (T-SN) 5-3, if received from the UE 3. The UE 3 then may begin synchronisation with and connection to, at S716, the target SN (T-SN) 5-3 via a typical RACH procedure.

[0100] Following completion of the RACH procedure, the MN 5-1 may send, at S720 a message to the source SN (S-SN) 5-2 to trigger release of radio and C-plane related resources associated with the UE context (e.g., a UE Context Release message or the like). It will be appreciated that, at the same time, any ongoing data forwarding may continue.

[0101] In between (or during) the RACH procedure at S716, and the transmission of the message for triggering release of resources at S720, several optional procedures at S718 may be performed. Examples of such optional procedures are described with reference to Fig. 8.

[0102] Fig. 8 is a simplified sequence diagram illustrating a possible part of the typical RRC procedure for switching between a source and a target secondary node shown in Fig. 7 including a number of optional steps in more detail.

[0103] For example following, at S802, if the packet data convergence protocol PDCP entity which is responsible for processing both UL and DL data is changed such that the PDCP termination point is changed for bearers using radio link control (RLC) acknowledgement mode (AM), then the S-SN 5-2 sends will send a SN Status Transfer message to the MN 5-1, which the MN 5-1 then sends to the T-SN 5-3, if needed at S804.

[0104] At S806, if applicable, data forwarding from the S-SN 5-2 may take place. It may be initiated as early as when the S-SN 5-2 receives the SN change confirmation message from the MN 5-1 (sent at S712 in Fig. 7). If data forwarding is needed the MN 5-1 provides data forwarding addresses to the source SN (S-SN) 5-2. If direct data forwarding is used for SN terminated bearers, the MN 5-1 provides data forwarding addresses as received from the target SN (T-SN) 5-3 to source SN (S-SN) 5-2.

[0105] At S810, if applicable, a PDU session path update procedure may be triggered by the MN 5-1 by sending a PDU session resource modify indication message or the like to the AMF 10-1 of the core network 7 including at least one PDU session ID to modify the PDU session. This may include, for example bearer modification and the establishment of a new PDU session path. The AMF 10-1 may then coordinate with a UPF 11 to modify the bearer and once completed, the AMF 10-1, at S812, may send a PDU session resource modify confirm message to the MN 5-1.

[0106] As can be seen in Figs. 7 and 8, the use of typical RRC procedures for the switching of secondary nodes in a communication system supporting an MCG and a SCG can involve a relatively lengthy procedure. Beneficially, therefore, the communication system 1 is configured to support one or more LTM procedures to support a number of different mobility scenarios involving the SCG.

[0107] As can be seen in Figs. 7 and 8, the use of typical RRC procedures for the switching of secondary nodes in a communication system supporting an MCG and an SCG can involve a relatively lengthy procedure. Beneficially, therefore, the communication system 1 is configured to support one or more LTM procedures for a number of different mobility scenarios involving, in particular, a change of a secondary cell (e.g., a PSCell) of an SCG both when there is no change of SN (intra-SN) and when there is a change of SN (inter-SN).

[0108] As described in more detail below, the LTM procedures supported by the communication system 1 beneficially include, for example, intra-SN LTM based procedures both for scenarios in which the SN can communicate directly with a UE 3 using RRC signalling (i.e., SRB3 based RRC signalling) and when the SN sends RRC signalling indirectly to a UE 3 via the MN (i.e., SRB1 based RRC signalling). As described in more detail below, the LTM procedures supported by the communication system 1 also beneficially include, for example, inter-SN LTM based procedures both for scenarios in which the source SN (S-SN) 5-2 can communicate directly with a target SN (T-SN) 5-3 (e.g., over a base station-to-base station interface such as the Xn interface) and for scenarios in which the source SN (S-SN) 5-2 cannot communicate directly with a target SN (T-SN) 5-3 (e.g., over a base station-to-base station interface such as the Xn interface).

[0109] It will be appreciated that the various procedures described below are neither mutually exclusive nor reliant on one another. More specifically, all or a subset of the procedures may be incorporated into a communication system together (or individually) to provide a commensurate benefit.

[0110] Secondary Cell Group LTM 　　Intra-Secondary Node (SN) SCG based LTM Handover 　　As mentioned above, the LTM procedures supported by the communication system 1 beneficially include intra-SN LTM based procedures both for scenarios in which the SN can communicate directly with a UE 3 using RRC signalling (i.e., SRB3 based RRC signalling) and when the SN sends RRC signalling indirectly to a UE 3 via the MN (i.e., SRB1 based RRC signalling). Two such procedures will now be described by way of example only with reference to Fig. 9 and Fig.10

[0111] Intra-SN SRB3 based SCG based LTM Handover 　　Fig. 9 is a simplified sequence diagram illustrating an intra-SN LTM handover procedure involving SRB3 based signalling.

[0112] As shown in Fig. 9, there is a UE 3 in communication with a source node 5-1 (i.e., a source MN 5-1) of an MCG, and a source SN (S-SN) 5-2 of an SCG. It will be appreciated that the source SN (S-SN) 5-2 is effectively also the 'target' SN (or 'T-SN') for the intra-SN LTM procedure described. For clarity, the term source SN (or 'S-SN') will be used throughout the description of Fig. 9.

[0113] The source node (MN) 5-1 and the S-SN 5-2 may exchange, at S902, SCG LTM support information. This information may include, for example, an indicator provided by the MN 5-1 to the S-SN 5-2 to indicate that SCG LTM is allowed / supported (e.g., an "Allow SCG LTM indicator" or the like). This indicator may be sent, for example, to the S-SN 5-2 via an appropriate base station-to-base station interface application protocol message (e.g., an Xn message such as an Xn Setup Request, Xn Configuration Update Request message or the like). Following transmission of the message including the indicator the S-SN 5-2 may acknowledge the message via an appropriate base station-to-base station interface application protocol response message (e.g., an Xn message such as an XN Setup Response or an Xn Configuration Update Response message). Alternatively (or possibly additionally), the S-SN 5-2 may declare its capability to support SCG LTM to the MN 5-1 via an appropriate base station-to-base station interface application protocol message (e.g., an Xn message such as an XN Setup Request or an Xn Configuration Update Request message). It will be appreciated that this exchange of information is optional procedure for SCG based LTM.

[0114] At S904, the UE 3, source node (MN) 5-1 and / or the S-SN 5-2 may engage in a measurement configuration and reporting procedure. This procedure typically involves the UE 3 being configured to perform SCG LTM related measurements and to report these when triggered do so (e.g., in response to an appropriate measurement reporting event). The measurement configuration may be sent, for example, via an appropriate RRC message including the measurement configuration sent from the S-SN 5-2 using SRB3 (although this does not preclude the configuration alternatively or additionally being provided directly from the MN 5-1). When the event is triggered the UE 3 may send the associated measurement report to the S-SN 5-2 using SRB3 (although this does not preclude the report alternatively or additionally being sent to the MN 5-1). The measurement results may be provided, for example, via an appropriate RRC message (e.g., in a 'MeasurementReport' message or the like). The measurement report may, for example, contain the results of measurements for one or more cells (e.g., of measurements of reference signals in a serving cell and / or one or more neighbouring cells). The measurement results may include, for example, L3 filtered measurement results for a beam and / or cell (but this need not be the case). The measurement reporting event may, for example, be triggered when a reference signal received power (RSRP) measurement and / or reference signal received quality (RSRQ) measurement for a target cell becomes a (pre)configured offset better than that of the serving PSCell (e.g., a so-called 'A3' event)).

[0115] The S-SN 5-2 then makes an SCG LTM decision, at S906, based on the measurement report received from the UE 3. Based on the SCG LTM decision, internal candidate LTM preparation is performed within the S-SN 5-2 (e.g., following a similar intra-CU LTM configuration procedure described above with reference to Fig. 2). In this scenario, the S-SN 5-2 and the T-SN are the same, however the cells forming the SCG and supported by the S-SN / T-SN 5-2 are changed.

[0116] The S-SN 5-2 sends, at S908, an RRC (re)configuration message (e.g., an 'RRCReconfiguration' message or the like) to the UE 3. This RRC message carries the SCG LTM configuration. The configuration can be sent over SRB3 between S-SN 5-2 and the UE 3. The configuration may include an indicator sent to the UE 3 to indicate it is for SCG LTM. Furthermore, there may be multiple SCG LTM configurations carried by the RRC (re)configuration message. Each SCG LTM configuration may be identified by a respective ID (e.g., an 'SCG LTM configuration ID' or the like). Each SCG LTM configuration may include a combination of new SCG cell sets, each new SCG cell set including a respective PSCell and other cells. Each SCG LTM configuration may also include a timing advance (TA) value and / or a difference in the TA value between the PSCell and the other cells of each new SCG cell set. Among each SCG LTM configuration, transmission configuration indication (TCI) state information and reference signal (RS) configuration information (e.g., CSI-RS, and / or SSB) may be included for each cell.

[0117] By way of example only, assuming there are six cells hosted by the S-SN 5-2 (Cell-1, Cell-2, Cell-3, Cell4, Cell-5, and Cell-6), the SCG before SCG LTM configured within the S-SN 5-2 may be summarised as {PSCell = Cell-1; Other cells within SCG = Cell-2 and Cell-3}. During the procedure described above with reference to Fig. 9, the RRC (re)configuration message (carrying the SCG LTM configuration) sent to the UE 3 by the S-SN 5-2 may include, for example, one or more (in this example eight) different SCG LTM configurations as follows: 　　SCG LTM configuration-1 　　　　{PSCell= Cell-4; Other cells within SCG = Cell-5 and Cell-6}; 　　SCG LTM configuration-2 　　　　{PSCell= Cell-4; Other cells within SCG = Cell-2 and Cell-6}; 　　SCG LTM configuration-3 　　　　{PSCell= Cell-4; Other cells within SCG = Cell-2 and Cell-3}; 　　SCG LTM configuration-4 　　　　{PSCell= Cell-2; Other cells within SCG = Cell-5 and Cell-6}; 　　SCG LTM configuration-5 　　　　{PSCell= Cell-2; Other cells within SCG = Cell-3 and Cell-6}; 　　SCG LTM configuration-6 　　　　{PSCell= Cell-2; Other cells within SCG = Cell-1 and Cell-6}; 　　SCG LTM configuration-7 　　　　{PSCell= Cell-1; Other cells within SCG = Cell-5 and Cell-6}; 　　SCG LTM configuration-8 　　　　{PSCell= Cell-1; Other cells within SCG = Cell-2 and Cell-6}.

[0118] It will be appreciated that the SCG LTM configurations shown above are by way of example only and that other SCG LTM configurations are possible. For example, for each new SCG LTM configuration, the cell set may include a new PSCell and / or other cell(s), that were not part of any previous SCG LTM configuration.

[0119] Following transmission RRC (re)configuration message to the UE 3, the UE 3, at S910, sends an appropriate message to confirm completion of the (re)configuration (e.g., an 'RRCReconfigurationComplete' message or the like) to the S-SN 5-2 to acknowledge the RRC (re)configuration message received for the purpose of SCG LTM.

[0120] At S912, the UE 3 reports the (L1) measurements to the S-SN 5-2 for SCG LTM based on the configuration received at S908. The S-SN 5-2 makes an SCG LTM decision at S914, based on the (L1) measurement report. During the SCG LTM decision, the PSCell of the previous SCG is changed to one of the cells hosted by the same S-SN 5-2 and one or more other cells within the previous SCG may also be changed to other cells hosted by the same S-SN 5-2. Alternatively, the PSCell of the previous SCG is not changed, but one or more other cells within the previous SCG may be changed to other cells hosted by the same S-SN 5-2. This decision can result in a single SCG LTM configuration identity, which means the network will configure a new SCG for the UE 3 within the same S-SN 5-2 (because the S-SN 5-2 is the new T-SN).

[0121] The S-SN 5-2 sends an LTM cell switch MAC CE to the UE 3 at S916 which carries a SCG LTM configuration ID (corresponding to the cell set configured during the SCG LTM preparation stage; for example, the SCG LTM configuration ID may correspond to one of the eight possible example configurations listed above). The LTM cell switch MAC CE may also carry beam information (for the UE to access the target cell for LTM), the TA value for the target PSCell (which may indicate the same TA as the source PSCell, or the exact value of the target PSCell) and an optional CFRA index indicating the CFRA resources if a RACH-less access procedure is to be used during LTM handover. The MAC CE may indicate that the LTM is an SCG LTM via a specific LTM indicator.

[0122] The UE 3 processes the SCG LTM command and then accesses the target PSCell via RACH-based or RACH-less access at S918. The UE 3 may also need access to other cells within the target SCG. However, if the same TA was assumed for both PSCell and the other cells, one single UE access will be sufficient. If the UE needs access to other cells within the target SCG but the same TA was not assumed for both the PSCell and the other cells, then the UE may need to perform RACH-based or RACH-less access for the different cells it wishes to access. An appropriate message (e.g., an 'RRCReconfigurationComplete' message, or the like) may be included in a first uplink MAC PDU / transmission from the UE 3 to the S-SN 5-2, to indicate the UE's arrival to the network node during SCG LTM.

[0123] Following access, the S-SN 5-2 notifies the occurrence of the SCG LTM to the MN 5-1 at S920. This notification may carry, for example, the change in the UE context including the new SCG information, e.g., the PSCell identity and the other cell identities within the new SCG. The notification to the MN 5-1 may include a UE ID, an indicator of the occurrence of SCG LTM, and the SCG LTM configuration including the cell set combination.

[0124] Alternatively, the S-SN 5-2 may send the SCG LTM configuration to the MN 5-1 before the occurrence of the SCG LTM, and then when the SCG LTM happens, the notification sent from the S-SN 5-2 to the MN 5-1 may include an indication that describes which SCG LTM configuration is being used for the UE 3 (e.g., a UE ID, an indicator of the occurrence of SCG LTM, and a SCG LTM configuration ID). When receiving the notification, the MN 5-1 may update the UE context accordingly.

[0125] In the case of an SCG LTM cell switch, a specific supervision timer (e.g., a timer like legacy T304) may be used to monitor the procedure. It will be appreciated that, in the event of an SCG LTM cell switch failure (i.e., arising from a supervision timer expiry), or radio link failure (RLF), a fast recovery procedure similar to conditional handover (CHO) may be applied. For example, in the event of such a failure, the UE 3 may perform cell selection using known techniques. If the selected cell is an SCG LTM candidate cell, the UE 3 may perform RACH-less or RACH-based SCG LTM cell switch on the selected cell. If the selected cell is not an SCG LTM candidate cell, the UE 3 may transmit an RRC re-establishment request to the selected cell. Alternatively, the SCG LTM recovery is not executed by the UE in the case of SCG LTM cell switch failure. In this scenario the UE may simply drop the SCG portion of the DC communication and only maintains communication with the network via the MCG.

[0126] In the case of an SCG LTM switch failure, the UE 3 may first report the SCG LTM cell switch failure to the MN 5-1 via the MN SRB (e.g., SRB1). Within this SCG LTM cell switch failure report, there may be an SCG LTM cell switch failure indicator as well as other appropriate information (e.g., an SCG LTM configuration, an indication of the cell set within the new SCG, the latest (L1) measurement on the cells, the (L1) measurements previously reported to S-SN 5-2 for SCG LTM decision, and the like).

[0127] While in Fig. 9 the MN 5-1 and the S-SN 5-2 are depicted as if they are separate base stations 5A, it will be appreciated that the UE 3 may instead be connected to a single base station 5A that acts as both a MN and as a SN, and which is configured for both an MCG and a SCG. In such a scenario a similar SCG LTM procedure described above may be applied.

[0128] Intra-Secondary Node (SN) SRB1 Handover 　　Fig. 10 is a simplified sequence diagram illustrating an intra-SN LTM handover procedure involving SRB1 based signalling.

[0129] As shown in Fig. 10, there is a UE 3 in communication with a source node 5-1 (i.e., a source MN 5-1) of an MCG, and a source SN 5-2 of an SCG. It will be appreciated that the source SN (S-SN) 5-2 is effectively also the 'target' SN (or 'T-SN') for the intra-SN LTM procedure described. For clarity, the term source SN (or 'S-SN') will be used throughout the description of Fig. 10.

[0130] As with the intra-SN handover procedure described with reference to Fig. 9, the source node (MN) 5-1 and the S-SN 5-2 may exchange, at S1002, SCG LTM support information. This information may include, for example, an indicator provided by the MN 5-1 to the S-SN 5-2 to indicate that SCG LTM is allowed / supported (e.g., an "Allow SCG LTM indicator" or the like). This indicator may be sent, for example, to the S-SN 5-2 via an appropriate base station-to-base station interface application protocol message (e.g., an Xn message such as an Xn Setup Request, Xn Configuration Update Request message or the like). Following transmission of the message including the indicator the S-SN 5-2 may acknowledge the message via an appropriate base station-to-base station interface application protocol response message (e.g., an Xn message such as an XN Setup Response or an Xn Configuration Update Response message). Alternatively (or possibly additionally), the S-SN 5-2 may declare its capability to support SCG LTM to the MN 5-1 via an appropriate base station-to-base station interface application protocol message (e.g., an Xn message such as an XN Setup Request or an Xn Configuration Update Request message). It will be appreciated that this exchange of information is optional procedure for SCG based LTM.

[0131] At S1004a-d, the source MN 5-1 and the S-SN 5-2 may engage in a measurement configuration / reporting and acknowledgement procedure. This procedure typically involves the source SN (S-SN) 5-2 preparing the measurement configuration and sending it to the source MN 5-1 (e.g., as an RRC measurement configuration) at S1004a, which is then forwarded by the MN 5-1 to the UE 3 over the MN SRB i.e., SRB1 at S1004b. The transmission of the measurement configuration from the S-SN 5-2 to the MN 5-1 may occur over the Xn interface between source node (MN) 5-1 and source SN (S-SN) 5-2 using a new inter-node message (e.g., an 'RRC Forward Message') to transfer the (RRC) measurement configuration (e.g., by including the measurement configuration within a transparent container in the inter-node message).

[0132] Following receipt of the measurement configuration, the UE 3 is configured to perform SCG LTM related measurements and to report these to the S-SN 5-2, via the MN 5-1. For example, the measurement report may be sent using SRB1 to the MN 5-1 at S1004c and from the MN 5-1 to the S-SN 5-2 at S1004d. The measurement results may be provided, for example, via an appropriate RRC message (e.g., in a 'MeasurementReport' message or the like). The measurement report may, for example, contain the results of measurements for one or more cells (e.g., of measurements of reference signals in a serving cell and / or one or more neighbouring cells). The measurement results may include, for example, L3 filtered measurement results for a beam and / or cell (but this need not be the case). The measurement reporting event may, for example, be triggered when a reference signal received power (RSRP) measurement and / or reference signal received quality (RSRQ) measurement for a target cell becomes a (pre)configured offset better than that of the serving PSCell (e.g., a so-called 'A3' event)). The transmission of the measurement configuration from the S-SN 5-2 to the MN 5-1 may occur over the Xn interface between source node (MN) 5-1 and source SN (S-SN) 5-2 using a new inter-node message (e.g., an 'RRC Forward Message') to transfer the (RRC) measurement configuration (e.g., by including the measurement configuration within a transparent container in the inter-node message).

[0133] As the measurement configurations and reports are sent between the MN 5-1 and the UE 3 over SRB1, the S-SN 5-2 must first prepare the measurement configurations for SCG LTM to send to the MN 5-1, which, as indicated above, is subsequently sent to the UE 3 over SRB1.

[0134] The S-SN 5-2 then makes an SCG LTM decision at S1006, based on the measurement report received from the UE 3. Based on the SCG LTM decision, internal candidate LTM preparation is performed within the S-SN 5-2 (following a similar intra-CU LTM configuration procedure described above with reference to Fig. 2). In this scenario, the S-SN 5-2 and the T-SN are the same, however the cells forming the SCG and supported by the S-SN / T-SN 5-2 are changed.

[0135] The S-SN 5-2 then sends, at S1008a an RRC (re)configuration message (e.g., an 'RRCReconfiguration' message or the like) to the MN 5-1. This RRC message carries the SCG LTM configuration. Each SCG LTM configuration may be as previously described above with reference to Fig. 9. For example, the configuration may include an indicator sent to the UE 3 to indicate it is for SCG LTM. Furthermore, there may be multiple SCG LTM configurations carried by the RRC (re)configuration message. Each SCG LTM configuration may be identified by a respective ID (e.g., an 'SCG LTM configuration ID' or the like). Each SCG LTM configuration may include a combination of new SCG cell sets, each new SCG cell set including a respective PSCell, and other cells as previously described. Each SCG LTM configuration may also include a timing advance (TA) value and / or a difference in the TA value between the PSCell and the other cells of each new SCG cell set. Among each SCG LTM configuration, TCI state information and RS configuration information (e.g., CSI-RS, and / or SSB) may be included for each cell.

[0136] The SCG LTM configuration is then subsequently transferred to the UE 3 via the MN SRB (SRB1) at S1008b. As the SCG LTM configuration is sent to the UE 3 in this step-wise manner, the MN 5-1 can first store the SCG LTM configuration within the UE context, prior to sending the SCG LTM configuration on to the UE 3. Alternatively, the MN 5-1 may not resolve the message but instead just transparently forward the SCG LTM configuration to the UE 3 via SRB1 over the air interface.

[0137] The UE 3 then sends a RRC (re)configuration complete message (e.g., an 'RRCReconfigurationComplete' message or the like) at S1010a to the MN 5-1 to acknowledge the RRC (re)configuration message for SCG LTM, which is then in turn forwarded by the MN 5-1 to S-SN 5-2 at S1010b.

[0138] At S1012, the UE 3 reports the (L1) measurements to the S-SN 5-2 for SCG LTM. The S-SN makes an SCG LTM decision at S1014, based on the (L1) measurement report. During the SCG LTM decision, the PSCell of the previous SCG is changed to one of the cells hosted by the same S-SN 5-2 and one or more other cells within the previous SCG may also be changed to one or more other cells hosted by the same S-SN 5-2. This decision can result in a single SCG LTM configuration identity, which means the network will configure a new SCG for the UE 3 within the same S-SN 5-2 (because the S-SN 5-2 is the new T-SN).

[0139] The S-SN 5-2 sends an LTM cell switch MAC CE to the UE 3 at S1016 which carries a SCG LTM configuration ID (corresponding to the cell set configured during the SCG LTM preparation stage; for example, the SCG LTM configuration ID may correspond to one of the eight possible example configurations listed above). The LTM cell switch MAC CE may also carry beam information (for the UE to access the target cell for LTM), the TA value for the target PSCell (which may indicate the same TA as the source PSCell, or the exact value of the target PSCell) and an optional CFRA index indicating the CFRA resources if a RACH-less access procedure is to be used during LTM handover. The MAC CE may indicate that the LTM is an SCG LTM via a specific LTM indicator.

[0140] The UE 3 processes the SCG LTM command and then accesses the target PSCell via RACH-based or RACH-less access at S1018. The UE 3 may also need access to other cells within the target SCG. However, if the same TA was assumed for both PSCell and the other cells, one single UE access will be sufficient. If the UE needs access to other cells within the target SCG but the same TA was not assumed for both the PSCell and the other cells, then the UE may need to perform RACH-based or RACH-less access for the different cells it wishes to access. An appropriate new RRC message (e.g., RRCReconfigurationComplete), or a specific MAC CE, may be included in the first uplink MAC PDU / transmission from the UE 3 to the MN 5-1, to indicate the UE 3's arrival to the network node during SCG LTM.

[0141] The MN 5-1 may resolve the message and then send an indication to S-SN 5-2 to notify the S-SN 5-2 of the UE 3's arrival. Alternatively, the MN 5-1 may forward the received message (e.g., RRC message) or specific MAC CE to the S-SN 5-2. In this scenario, the S-SN 5-2 resolves the message and thus becomes aware of the UE 3's arrival via SCG LTM.

[0142] Following access, the S-SN 5-2 notifies the occurrence of the SCG LTM to the MN 5-1 at S1020. This notification may carry, for example, the change in the UE context including the new SCG information, e.g., the PSCell identity and the other cell identities within the new SCG. The notification the to the MN 5-1 may include a UE ID, an indicator of the occurrence of SCG LTM, and the SCG LTM configuration including the cell set combination.

[0143] Alternatively, the S-SN 5-2 may send the SCG LTM configuration to the MN 5-1 before the occurrence of the SCG LTM, and then when the SCG LTM happens, the notification sent from the S-SN 5-2 to the MN 5-1 may include an indication that describes which SCG LTM configuration is being used for the UE 3 (e.g., a UE ID, an indicator of the occurrence of SCG LTM, and a SCG LTM configuration ID). When receiving the notification, the MN 5-1 may update the UE context accordingly.

[0144] In the case of an SCG LTM cell switch, a specific supervision timer (e.g., a timer like legacy T304) may be used to monitor the procedure. It will be appreciated that in the event of an SCG LTM cell switch failure (i.e., arising from a supervision timer expiry), or radio link failure (RLF), a fast recovery procedure similar to conditional handover (CHO) may be applied. For example, in the event of such a failure, the UE 3 may perform cell selection using known techniques. If the selected cell is an SCG LTM candidate cell, the UE 3 may perform RACH-based SCG LTM cell switch on the selected cell. If the selected cell is not an SCG LTM candidate cell, the UE 3 may transmit an RRC re-establishment request to the selected cell. Alternatively, the SCG LTM recovery is not executed by the UE 3 in the case of SCG LTM cell switch failure. In this scenario the UE 3 may simply drop the SCG portion of the DC communication and only maintains communication with the network via the MCG.

[0145] In the case of an SCG LTM switch failure, the UE 3 may first report the SCG LTM cell switch failure to the MN 5-1 via the MN SRB (e.g., SRB1). Within this SCG LTM cell switch failure report, there may be an SCG LTM cell switch failure indicator as well as other appropriate information (e.g., an SCG LTM configuration, an indication of the cell set within the new SCG, the latest (L1) measurement on the cells, the (L1) measurements previously reported to S-SN 5-2 for SCG LTM decision, and the like).

[0146] While in Fig. 10 the MN 5-1 and the S-SN 5-2 are depicted as if they are separate base stations 5A, it will be appreciated that the UE 3 may instead be connected to a single base station 5A that acts as both a MN and as a SN, and which is configured for both an MCG and a SCG. In such a scenario a similar SCG LTM procedure described above may be applied.

[0147] Inter-Secondary Node (SN) Handover 　　As mentioned above, the LTM procedures supported by the communication system 1 beneficially include inter-SN LTM based procedures both for scenarios in which the source SN can communicate directly with a target SN (e.g., over a base station-to-base station interface such as the Xn interface) and for scenarios in which the source SN cannot communicate directly with a target SN (e.g., over a base station-to-base station interface such as the Xn interface). Two such procedures will now be described by way of example only with reference to Figs. 11 to Fig. 14.

[0148] Inter-SN Handover - Direct SN-to-SN Communication 　　Figs. 11 and 12 together provide a simplified sequence diagram illustrating an inter-SN LTM handover procedure. As shown, there is a UE 3 in communication with a source node 5-1 (i.e., an MN 5-1) of a MCG, a source SN (S-SN) 5-2 of a secondary cell group, and a target SN (T-SN) 5-3.

[0149] The source node (MN) 5-1 and the S-SN 5-2 may exchange, at S1102a, SCG LTM support information. This information may include, for example, an indicator provided by the MN 5-1 to the S-SN 5-2 to indicate that SCG LTM is allowed / supported (e.g., an "Allow SCG LTM indicator" or the like). This indicator may be sent, for example, to the S-SN 5-2 via an appropriate base station-to-base station interface application protocol message (e.g., an Xn message such as an Xn Setup Request, Xn Configuration Update Request message or the like). Following transmission of the message including the indicator the S-SN 5-2 may acknowledge the message via an appropriate base station-to-base station interface application protocol response message (e.g., an Xn message such as an XN Setup Response or an Xn Configuration Update Response message). Alternatively (or possibly additionally), the S-SN 5-2 may declare its capability to support SCG LTM to the MN 5-1 via an appropriate base station-to-base station interface application protocol message (e.g., an Xn message such as an XN Setup Request or an Xn Configuration Update Request message). It will be appreciated that this exchange of information is optional procedure for SCG based LTM.

[0150] Similarly, the S-SN 5-2 and T-SN 5-3 may exchange, at S1102b, SCG LTM support information. This information may include, for example, an indicator to indicate that SCG LTM is allowed / supported (e.g., an "Allow SCG LTM indicator" or the like). This indicator may be sent, for example, from the S-SN 5-2 (or from the T-SN 5-3) to the T-SN 5-3 (or to the S-SN 5-2) via an appropriate base station-to-base station interface application protocol message (e.g., an Xn message such as an Xn Setup Request, Xn Configuration Update Request message or the like). Following transmission of the message including the indicator the S-SN 5-2 (or T-SN 5-3) may acknowledge the message via an appropriate base station-to-base station interface application protocol response message (e.g., an Xn message such as an XN Setup Response or an Xn Configuration Update Response message). Alternatively (or possibly additionally), the S-SN 5-2 (or T-SN 5-3) may declare its capability to support SCG LTM to the T-SN 5-3 (or S-SN 5-2) via an appropriate base station-to-base station interface application protocol message (e.g., an Xn message such as an XN Setup Request or an Xn Configuration Update Request message). It will be appreciated that this exchange of information is optional procedure for SCG based LTM.

[0151] At S1104, the UE 3, source node (MN) 5-1 and / or the S-SN 5-2 may engage in a measurement configuration and reporting procedure. This procedure typically involves the UE 3 being configured to perform SCG LTM related measurements and to report these when triggered do so (e.g., in response to an appropriate measurement reporting event). The measurement configuration may be sent, for example, via an appropriate RRC message including the measurement configuration sent from the S-SN 5-2 using SRB3 (although this does not preclude the configuration alternatively or additionally being provided directly from the MN 5-1). When the event is triggered the UE 3 may send the associated measurement report to the S-SN 5-2 using SRB3 (although this does not preclude the report alternatively or additionally being sent to the MN 5-1). The measurement results may be provided, for example, via an appropriate RRC message (e.g., in a 'MeasurementReport' message or the like). The measurement report may, for example, contain the results of measurements for one or more cells (e.g., of measurements of reference signals in a serving cell and / or one or more neighbouring cells). The measurement results may include, for example, L3 filtered measurement results for a beam and / or cell (but this need not be the case). The measurement reporting event may, for example, be triggered when a reference signal received power (RSRP) measurement and / or reference signal received quality (RSRQ) measurement for a target cell becomes a (pre)configured offset better than that of the serving PSCell (e.g., a so-called 'A3' event)).

[0152] The S-SN 5-2 then makes an SCG LTM decision, at S1106, based on the measurement report received from the UE 3 (e.g., as described with reference to Fig. 9).

[0153] Following the SCG LTM Decision, the S-SN 5-2 sends, at S1108a, an appropriate base station-to-base station interface application protocol message to the T-SN 5-3 (e.g., a new Xn message such as a 'SN Change Request', an existing Xn Handover (HO) request message, or any other suitable Xn message). That message includes an LTM handover preparation request information element (IE), with an indication to initiate SCG based LTM handover. When the T-SN 5-3 receives such a message, if the LTM HO preparation request IE is contained in the message, then the T-SN 5-3 considers that the request concerns a SCG based LTM handover.

[0154] The T-SN 5-3 then sends, at S1108b, an appropriate base station-to-base station interface application protocol message to the S-SN 5-2 (e.g., a new Xn message such as a 'SN Change Response', or the HO request acknowledgement message, or any other suitable Xn message) as a response message to the message received at S1108a. As part of the acknowledgement procedure, the T-SN 5-3 performs an LTM preparation procedure at S1108c. For example, the T-SN 5-3 prepares candidate cells by preparing one or more SCG LTM configurations as previously described. The T-SN 5-3 then sends one or more prepared SCG LTM configurations in a new IE (e.g., a "LTM Handover preparation Response IE"), within the response message sent at S1108b, over the base station-to-base station interface. The response message may also include an SCG based LTM handover acceptance indication sub-IE to indicate the acceptance of inter-SN SCG LTM HO.

[0155] The prepared SCG LTM configuration or configurations may include, for example, an indicator to indicate it is for SCG LTM. Furthermore, there may be multiple SCG LTM configurations carried by the response message. Each SCG LTM configuration may be identified by a respective ID (e.g., an 'SCG LTM configuration ID' or the like). Each SCG LTM configuration may include a combination of new SCG cell sets, each new SCG cell set including a respective PSCell and other cells. Each SCG LTM configuration may also include a timing advance (TA) value and / or a difference in the TA value between the PSCell and the other cells of each new SCG cell set.

[0156] The S-SN 5-2 then sends, at S1112 an RRC (re)configuration message (e.g., an 'RRCReconfiguration' message or the like) to the UE 3. This RRC message carries the SCG LTM configuration or configurations. The configuration can be sent over, in this example, SRB3 between S-SN 5-2 and the UE 3 (although it will be appreciated that the procedure could be adapted to provide the configuration via the MN and SRB1).

[0157] Following transmission RRC (re)configuration message to the UE 3, then the UE 3, at S1114, sends an appropriate message to confirm completion of the (re)configuration (e.g., an 'RRCReconfigurationComplete' message or the like) to the S-SN 5-2 to acknowledge the RRC (re)configuration message received for the purpose of SCG LTM.

[0158] At S1116, the UE 3 reports the (L1) measurements to the S-SN 5-2 for SCG LTM. The S-SN 5-2 makes an SCG LTM decision at S1118, based on the (L1) measurement report. During the SCG LTM decision, the PSCell of the previous SCG is changed to one of the cells hosted by the T-SN 5-3 and one or more other cells within the previous SCG may also be changed to other cells hosted by the T-SN 5-3.

[0159] The remaining operations are shown in Fig. 12. Following the S-SN 5-2 performing a SCG LTM decision at S1118, the S-SN 5-2 sends, at S1120, the S-SN 5-2 sends an LTM cell switch MAC CE to the UE 3 which carries a SCG LTM configuration ID (corresponding to the cell set configured during the SCG LTM preparation stage). The LTM cell switch MAC CE may also carry beam information (for the UE 3 to access the target cell for LTM), the TA value for the target PSCell (which may indicate the same TA as the source PSCell, or the exact value of the target PSCell) and an optional CFRA index indicating the CFRA resources if a RACH-less access procedure is to be used during LTM handover. The MAC CE may indicate that the LTM is an SCG LTM via a specific LTM indicator.

[0160] The UE 3 processes the SCG LTM command and then accesses the target PSCell via RACH-based or RACH-less access at S1122. The UE 3 may also need access to other cells within the target SCG. However, if the same TA was assumed for both PSCell and the other cells, one single UE access will be sufficient. If the UE 3 needs access to other cells within the target SCG but the same TA was not assumed for both the PSCell and the other cells, then the UE 3 may need to perform RACH-based or RACH-less access for the different cells it wishes to access.

[0161] Following access, the T-SN 5-3 notifies the occurrence of the SCG LTM to the MN 5-1 at S1124 over the base station-to-base station interface. This notification may carry, for example, the change in the UE context including the new SCG information, e.g., the PSCell identity and the other cell identities within the new SCG. The notification to the MN 5-1 may include a UE ID, an indicator of the occurrence of SCG LTM, and the SCG LTM configuration including the cell set combination.

[0162] When the MN 5-1 receives the SCG LTM handover notification from the T-SN 5-3, the MN 5-1 sends an appropriate message to the S-SN 5-2 to request the release of resources at S-SN 5-2 allocated for the UE 3 (e.g., an 'S-NODE Release Request' which is an Xn message). A special cause value (e.g., SCG LTM handover to a new SN) may be included in this message to describe the reason for such release.

[0163] In the event of an SCG LTM cell switch failure (i.e., supervision timer expiry) or radio link failure (RLF), a fast recovery procedure similar to conditional handover (CHO) may be applied. For example, in the event of such a switch failure, the UE 3 may first report the SCG LTM cell switch failure to the MN 5-1 via MN service radio bearer (SRB). The UE 3 may then perform cell selection using known techniques. If the selected cell is an SCG LTM candidate cell, the UE 3 will perform RACH-based SCG LTM cell switch on the selected cell. Alternatively, if the selected cell is not an SCG LTM candidate cell, UE 3 transmits an RRC re-establishment request to the selected cell.

[0164] Whilst the procedure shown in Figs. 11 and 12 is such that the measurement configuration and measurement reporting procedure at S1104 occurs via SRB3, it will however be appreciated the procedure may alternatively be configured such that the measurement configuration and measurement reporting procedure at S1104 occurs via SRB1.

[0165] It can be seen that the general principle behind the procedure shown in Figs. 11 and 12 is similar to the procedure of Fig. 9. However, the signalling flow is based on Inter-SN signalling. The signalling flow in this example is based on inter-CU LTM signalling in which inter-CU signalling between the MN 5-1 and the SN 5-2 / 5-3 is supported for inter-SN SCG LTM.

[0166] Inter-SN Handover - Indirect SN-to-SN Communication via MN 　　Figs. 13 and 14 together illustrate a simplified sequence diagram for another inter-SN LTM handover procedure. As shown, there is a UE 3 in communication with a source node 5-1 (i.e., an MN 5-1) of a MCG, a source SN (S-SN) 5-2 of a secondary cell group, and a target SN (T-SN) 5-3.

[0167] In Fig. 13 the procedure is shown starting at S1302, in which the UE 3, source node (MN) 5-1 and / or the S-SN 5-2 engage in a measurement configuration and reporting procedure. It will, nevertheless, be appreciated that the source node (MN) 5-1 and the S-SN 5-2 may have previously exchanged SCG LTM support information as described above.

[0168] This measurement configuration and reporting procedure typically involves the UE 3 being configured to perform SCG LTM related measurements and to report these when triggered do so (e.g., in response to an appropriate measurement reporting event). The measurement configuration may be sent, for example, via an appropriate RRC message including the measurement configuration sent from the S-SN 5-2, e.g., using SRB3 (although this does not preclude the configuration alternatively or additionally being provided directly from the MN 5-1 using SRB1). When the event is triggered the UE 3 may send the associated measurement report to the S-SN 5-2 using SRB3 (although this does not preclude the report alternatively or additionally being sent to the MN 5-1 using SRB1). The measurement results may be provided, for example, via an appropriate RRC message (e.g., in a 'MeasurementReport' message or the like). The measurement report may, for example, contain the results of measurements for one or more cells (e.g., of measurements of reference signals in a serving cell and / or one or more neighbouring cells). The measurement results may include, for example, L3 filtered measurement results for a beam and / or cell (but this need not be the case). The measurement reporting event may, for example, be triggered when a reference signal received power (RSRP) measurement and / or reference signal received quality (RSRQ) measurement for a target cell becomes a (pre)configured offset better than that of the serving PSCell (e.g., a so-called 'A3' event).

[0169] The S-SN 5-2 then makes an SCG LTM Decision at S1304 based on the measurement report received from the UE 3.

[0170] Following the SCG LTM Decision, the S-SN 5-2, at S1306, initiates an SN change procedure by sending a message to the MN 5-1 to indicate that SN change is required (e.g., an 'SN Change Required' message) with an indication of an SCG LTM preparation request for inter-SN SCG LTM. This indication may or may not be transparent to the MN 5-1. If it is transparent, the indication may be included in an RRC Inter-node message (e.g., CG-Config). If it is not transparent, the indication may be included as a base station-to-base station interface (e.g., Xn) application protocol (AP) IE.

[0171] The MN 5-1 then sends a request to add an SN at S1308 (e.g., an 'SN Addition Request') to the T-SN 5-3 that includes an indication that the request is for the purposes of an SCG LTM request. The T-SN 5-3 may respond with an appropriate acknowledgement (e.g., an 'SN Addition Request Acknowledgement' message) to the MN 5-1 at S1310, to acknowledge the SCG LTM and an appropriate SCG configuration.

[0172] At S1312 the MN 5-1 sends to the S-SN 5-2 a message confirming SN change (e.g., an 'SN Change Confirm' message or the like) that includes an indication of an SCG LTM preparation response for inter-SN SCG LTM (e.g., an acknowledgement of SCG LTM). This indication may or may not be transparent to the MN 5-1. As part of the acknowledgement procedure, the S-SN 5-2 performs an LTM preparation procedure at S1314. For example, the S-SN 5-2 prepares candidate cells by preparing one or more SCG LTM configurations as previously described.

[0173] The S-SN 5-2 then sends, at S1316 the S-SN 5-2 sends an RRC (re)configuration message (e.g., an 'RRCReconfiguration' message or the like) to the UE 3. This RRC message carries the SCG LTM configuration. The configuration can be sent over SRB3 between S-SN 5-2 and the UE 3 (or alternatively indirectly via the MN 5-1 and SRB1). The configuration may include an indicator to indicate it is for SCG LTM. Furthermore, there may be multiple SCG LTM configurations carried by the RRC (re)configuration message. Each SCG LTM configuration may be identified by a respective ID (e.g., an 'SCG LTM configuration ID' or the like). Each SCG LTM configuration may include a combination of new SCG cell sets, each new SCG cell set including a respective PSCell and other cells. Each SCG LTM configuration may also include a timing advance (TA) value and / or a difference in the TA value between the PSCell and the other cells of each new SCG cell set.

[0174] Following transmission RRC (re)configuration message to the UE 3, then the UE 3, at S1318, sends an appropriate message to confirm completion of the (re)configuration (e.g., an 'RRCReconfigurationComplete' message or the like) to the S-SN 5-2 to acknowledge the RRC (re)configuration message received for the purpose of SCG LTM.

[0175] The remaining operations are shown in Fig. 14. At S1320, the UE 3 reports the (L1) measurements to the S-SN 5-2 for SCG LTM. The S-SN 5-2 makes an SCG LTM decision at S1322, based on the (L1) measurement report. During the SCG LTM decision, the PSCell of the previous SCG is changed to one of the cells hosted by the T-SN 5-3 and one or more other cells within the previous SCG may also be changed to other cells hosted by the T-SN 5-3.

[0176] The S-SN 5-2 sends an LTM cell switch MAC CE to the UE 3 at S1324 which carries a SCG LTM configuration ID (corresponding to the cell set configured during the SCG LTM preparation stage). The LTM cell switch MAC CE may also carry beam information (for the UE to access the target cell for LTM), the TA value for the target PSCell (which may indicate the same TA as the source PSCell, or the exact value of the target PSCell) and an optional CFRA index indicating the CFRA resources if a RACH-less access procedure is to be used during LTM handover. The MAC CE may indicate that the LTM is an SCG LTM via a specific LTM indicator.

[0177] The UE 3 processes the SCG LTM command and then accesses the target PSCell via RACH-based or RACH-less access at S1326. The UE 3 may also need access to other cells within the target SCG. However, if the same TA was assumed for both PSCell and the other cells, one single UE access will be sufficient. If the UE 3 needs access to other cells within the target SCG but the same TA was not assumed for both the PSCell and the other cells, then the UE 3 may need to perform RACH-based or RACH-less access for the different cells it wishes to access.

[0178] Finally, at S1328 the T-SN 5-3 notifies the occurrence of the SCG LTM to the MN 5-1 at S1328 over the base station-to-base station interface. This notification may carry, for example, the change in the UE context including the new SCG information, e.g., the PSCell identity and the other cell identities within the new SCG. The notification to the MN 5-1 may include a UE ID, an indicator of the occurrence of SCG LTM, and the SCG LTM configuration including the cell set combination.

[0179] It can be seen that this is another example that provides for inter-SN based SCG LTM based on an inter-CU procedure. Unlike the previous example, however, this beneficially addresses the situation where there is no direct interface between the different SNs during DC operation. Specifically, this example makes use of an SN-initiated SN change procedure starting from an SN change required message from the SN 5-2 to the MN 5-1. In particular in this example the S-SN 5-2 beneficially sends an SN change required message, with an appropriate indication of an SCG LTM preparation request for inter-SN SCG LTM included in the message.

[0180] User Equipment 　　Fig. 15 is a simplified block schematic illustrating the main components of a UE 3 for implementation in the communication system 1 of Fig. 1.

[0181] As shown, the UE 3 comprises transceiver circuitry 31 that is operable to transmit signals to and to receive signals from a RAN 5 via an air interface 33 and one or more antennas.

[0182] The UE 3 has a controller 37 to control the operation of the UE 3. The controller 37 is associated with a memory 39 and is coupled to the transceiver circuit 31. 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 35, 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.

[0183] The controller 37 is configured to control overall operation of the UE 3 by, in this example, program instructions or software instructions stored within the memory 39. The software may be pre-installed in the memory 39 and / or may be downloaded via the communication system 1 or from a removable data storage device (RMD), for example. As shown, these software instructions include, among other things, an operating system 41 and a communications control module 43.

[0184] The communications control module 43 is operable to control the communication between the UE 3 and one or more serving RANs 5 (and other communication devices connected to the RAN 5, such as further UEs and / or core network nodes). The communications control module 43 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 43 is also configured for the overall handling 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, SSBs etc.).

[0185] It will be appreciated that the communications control module 43 may include a number of sub-modules (or 'layers') to support specific functionalities. For example, the communications control module 63 may include a PHY sub-module, a MAC sub-module, an RLC sub-module, a PDCP sub-module, an SDAP sub-module, an IP sub-module, an RRC sub-module, etc.

[0186] The communications control module 43 is configured in particular, to control the UE's communications, in accordance with any of the methods described herein.

[0187] RAN 　　Fig. 16 is a simplified block schematic illustrating the main components of a (non-distributed) base station 5A for implementation in the system of Fig. 1.

[0188] As shown, the base station 5A has a transceiver circuit 51 for transmitting signals to and for receiving signals from the communication devices (such as UEs 3) via one or more antenna 53 (e.g., a single or multi-panel antenna array / massive antenna), and a core network interface 55 for transmitting signals to and for receiving signals from network nodes in the core network 7. Although not shown, the base station 5A may also be coupled to other base stations 5A via an appropriate interface (e.g., the so-called 'X2' interface in LTE or the 'Xn' interface in NR). The base station 5A has a controller 57 to control the operation of the base station 5A. The controller 57 is associated with a memory 59. Software may be pre-installed in the memory 59 and / or may be downloaded via the communication system 1 or from a removable data storage device (RMD), for example. The controller 57 is configured to control the overall operation of the base station 5A by, in this example, program instructions or software instructions stored within memory 59.

[0189] As shown, these software instructions include, among other things, an operating system 61, and a communications control module 63.

[0190] The communications control module 63 is operable to control the communication between the base station 5A and UEs 3 and other network entities (e.g., core network nodes) that communicate with the base station 5A. The communications control module 63 is configured for the overall control of 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). The communications control module 63 is also configured for the overall control of 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.).

[0191] It will be appreciated that the communications control module 63 may include a number of sub-modules ('layers' or 'entities') to support specific functionalities. For example, the communications control module 63 may include, for communicating with a UE 3, a PHY sub-module, a MAC sub-module, an RLC sub-module, a PDCP sub-module, an RRC sub-module, etc. Moreover, the communications control module 63 may include, for communicating with a core network entity such as an MME (or similar node such as an AMF 10-1), an S1 application protocol (S1-AP) sub-module, a stream control transmission protocol (SCTP) sub-module, an IP sub-module, a layer 1 (L1) sub-module, a layer 2 (L2) sub-module, etc (or corresponding sub-modules for communicating with an AMF 10-1).

[0192] The communications control module 63 is configured in particular, to control the base station's communications, in accordance with any of the methods described herein.

[0193] RAN (Distributed Type) 　　Fig. 17 is a simplified block schematic illustrating the main components of a distributed RAN 5 comprising a distributed type of base station 5A for implementation in the system of Fig. 1.

[0194] As shown, the RAN 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.

[0195] 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).

[0196] 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 5 via a network interface 55c. The network interface typically includes an N2 and / or N3 interfaces for communicating with the core network and a base station to base station (e.g., Xn) interface for communicating with other RANs 5. The central unit 5c transceiver circuitry 51c 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.

[0197] 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 and a respective communications control module 63c, 63b.

[0198] 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 5.

[0199] 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 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.).

[0200] It will be appreciated that the communications control modules 63c, 63b 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 63c 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.

[0201] Modifications and Alternatives 　　A detailed embodiment has been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein.

[0202] 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.

[0203] 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 invention, 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.

[0204] In the above embodiments, 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 to the base station or to the UE 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.

[0205] 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.

[0206] The base station may comprise a 'distributed' base station having a central unit 'CU' and one or more separate distributed units (DUs).

[0207] 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.

[0208] 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.

[0209] 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.

[0210] 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.).

[0211] 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.).

[0212] 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.).

[0213] 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.).

[0214] 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.).

[0215] A UE may, for example, be an electronic lamp, a luminaire, a measuring instrument, an analyzer, 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.

[0216] 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)).

[0217] 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.

[0218] 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.

[0219] 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.

[0220] 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. table01

[0221] 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.

[0222] Further, the above-described UE categories are merely examples of applications of the technical ideas and exemplary embodiments described in the present document. Needless to say, these technical ideas and embodiments are not limited to the above-described UE and various modifications can be made thereto.

[0223] Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.

[0224] 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.

[0225] 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.

[0226] 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 user equipment (UE) configured to communicate using radio resources provided by a master base station and a source secondary base station, the method comprising: 　　receiving, from the source secondary base station directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　transmitting, to the source secondary base station, a Layer 1 (L1) measurement report including results of L1 measurement performed on cells configured by the information; 　　receiving, from the source secondary base station, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell; and 　　performing SCG LTM cell switch procedure towards to the target cell using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier. 　　(Supplementary Note 2) 　　The method according to supplementary note 1, wherein 　　the target cell is operated by the source secondary base station. 　　(Supplementary Note 3) 　　The method according to supplementary note 1, wherein 　　the target cell is operated by a target secondary base station different from the source secondary base station. 　　(Supplementary Note 4) 　　The method according to any one of supplementary notes 1 to 3, further comprising: 　　detecting SCG LTM cell switch failure; and 　　reporting the SCG cell switch failure to the master base station. 　　(Supplementary Note 5) 　　A method performed by a source secondary base station configured to provide radio resources for communicating with a user equipment (UE), together with a master base station, the method comprising: 　　transmitting, to the UE directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　receiving, from the UE, a Layer 1 (L1) measurement report including results of L1 measurement performed by the UE on cells configured by the information; 　　transmitting, to the UE, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell, wherein 　　SCG LTM cell switch procedure towards to the target cell is performed by the UE using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier. 　　(Supplementary Note 6) 　　A user equipment (UE) configured to communicate using radio resources provided by a master base station and a source secondary base station, the UE comprising: 　　means for receiving, from the source secondary base station directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　means for transmitting, to the source secondary base station, a Layer 1 (L1) measurement report including results of L1 measurement performed on cells configured by the information; 　　means for receiving, from the source secondary base station, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell; and 　　means for performing SCG LTM cell switch procedure towards to the target cell using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier. 　　(Supplementary Note 7) 　　A source secondary base station configured to provide radio resources for communicating with a user equipment (UE), together with a master base station, the source secondary base station comprising: 　　means for transmitting, to the UE directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　means for receiving, from the UE, a Layer 1 (L1) measurement report including results of L1 measurement performed by the UE on cells configured by the information; 　　means for transmitting, to the UE, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell, wherein 　　SCG LTM cell switch procedure towards to the target cell is performed by the UE using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier.

[0227] This application is based upon and claims the benefit of priority from Great Britain Patent Application No. 2315413.1, filed on October 6, 2023, the disclosure of which is incorporated herein in its entirety by reference.

[0228] 1 COMMUNICATION SYSTEM 3 USER EQUIPMENT 31 TRANSCEIVER CIRCUIT 33 ANTENNA 35 USER INTERFACE 37 CONTROLLER 39 MEMORY 41 OPERATING SYSTEM 43 COMMUNICATIONS CONTROL MODULE 5 RADIO ACCESS NETWORK 5A RAN NODE 51 TRANSCEIVER CIRCUIT 53 ANTENNA 55 CORE NETWORK INTERFACE 57 CONTROLLER 59 MEMORY 61 OPERATING SYSTEM 63 COMMUNICATIONS CONTROL MODULE 5b DISTRIBUTED UNIT 51b TRANSCEIVER CIRCUITRY 53b AIR INTERFACE 57b CONTROLLER 59b MEMORY 61b OPERATING SYSTEM 63b COMMUNICATIONS CONTROL MODULE 5c CENTRAL UNIT 51c TRANSCEIVER CIRCUITRY 55c NETWORK INTERFACE 57c CONTROLLER 59c MEMORY 61c OPERATING SYSTEM 63c COMMUNICATIONS CONTROL MODULE 7 CORE NETWORK 9 CELL 10 CONTROL PLANE FUNCTION 10-1 ACCESS AND MOBILITY MANAGEMENT FUNCTION 10-2 SESSION MANAGEMENT FUNCTION 10-n OTHER FUNCTION 11 USER PLANE FUNCTION 20 EXTERNAL DATA NETWORK

Claims

1. A method performed by a user equipment (UE) configured to communicate using radio resources provided by a master base station and a source secondary base station, the method comprising: 　　receiving, from the source secondary base station directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　transmitting, to the source secondary base station, a Layer 1 (L1) measurement report including results of L1 measurement performed on cells configured by the information; 　　receiving, from the source secondary base station, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell; and 　　performing SCG LTM cell switch procedure towards to the target cell using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier.

2. The method according to claim 1, wherein 　　the target cell is operated by the source secondary base station.

3. The method according to claim 1, wherein 　　the target cell is operated by a target secondary base station different from the source secondary base station.

4. The method according to any one of claims 1 to 3, further comprising: 　　detecting SCG LTM cell switch failure; and 　　reporting the SCG cell switch failure to the master base station.

5. A method performed by a source secondary base station configured to provide radio resources for communicating with a user equipment (UE), together with a master base station, the method comprising: 　　transmitting, to the UE directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　receiving, from the UE, a Layer 1 (L1) measurement report including results of L1 measurement performed by the UE on cells configured by the information; 　　transmitting, to the UE, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell, wherein 　　SCG LTM cell switch procedure towards to the target cell is performed by the UE using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier.

6. A user equipment (UE) configured to communicate using radio resources provided by a master base station and a source secondary base station, the UE comprising: 　　means for receiving, from the source secondary base station directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　means for transmitting, to the source secondary base station, a Layer 1 (L1) measurement report including results of L1 measurement performed on cells configured by the information; 　　means for receiving, from the source secondary base station, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell; and 　　means for performing SCG LTM cell switch procedure towards to the target cell using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier.

7. A source secondary base station configured to provide radio resources for communicating with a user equipment (UE), together with a master base station, the source secondary base station comprising: 　　means for transmitting, to the UE directly via a signaling radio bearer 3 (SRB3) or via the master base station, information for secondary cell group (SCG) Layer 1 / Layer 2 triggered mobility (LTM) configuration using a Radio Resource Control (RRC) reconfiguration message, wherein the information includes an identifier indicating a respective SCG LTM configuration per cell; 　　means for receiving, from the UE, a Layer 1 (L1) measurement report including results of L1 measurement performed by the UE on cells configured by the information; 　　means for transmitting, to the UE, an LTM cell switch command Medium Access Control (MAC) Control Element (CE) triggering cell switch by including a configuration identifier for a target configuration corresponding to a target cell, wherein 　　SCG LTM cell switch procedure towards to the target cell is performed by the UE using SCG LTM configuration included in the information including the identifier corresponding to the configuration identifier.

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