WO2025164347A1

WO2025164347A1 - Method performed by mobile terminal, method performed by satellite, mobile terminal, and satellite

Info

Publication number: WO2025164347A1
Application number: PCT/JP2025/001251
Authority: WO
Inventors: Yuhua Chen; Sadafuku Hayashi; Xuelong Wang; Hisashi Futaki
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 2024-02-01
Filing date: 2025-01-17
Publication date: 2025-08-07
Anticipated expiration: 2026-08-01
Also published as: GB202401345D0

Abstract

A method performed by a mobile terminal configured to communicate with a satellite having at least a part of functions of a base station is disclosed. The method includes receiving, from the satellite, information for switching a serving satellite from the satellite to another satellite; performing the switching the serving satellite based on the information, and wherein the performing the switching includes: synchronizing with a target cell which is different from a serving cell served by the satellite; resetting configuration related to a Medium Access Control (MAC) layer; and re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).

Description

METHOD PERFORMED BY MOBILE TERMINAL, METHOD PERFORMED BY SATELLITE, MOBILE TERMINAL, AND SATELLITE

　　The present disclosure relates to a communication system and to parts thereof. 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 Long Term Evolution (LTE)-Advanced, Next Generation or 5G networks, future generations, and beyond). The disclosure has particular but not exclusive relevance to improvements relating to mobility scenarios such as satellite switching for non-terrestrial networks (NTNs) in regenerative mode deployment scenarios.

　　Earlier developments of the 3GPP standards were referred to as the Long-Term Evolution (LTE) of Evolved Packet Core (EPC) network and Evolved Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (E-UTRAN), also commonly referred as '4G'. More recently, the term '5G' and 'new radio' (NR) is used to refer to an evolving communication technology that supports a variety of applications and services. Various details of 5G networks are described in, for example, the 'NGMN 5G White Paper' V1.0 (NPL 2) by the Next Generation Mobile Networks (NGMN) Alliance, which document is available from https://www.ngmn.org/5g-white-paper.html. 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.

　　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 one or more devices (user equipments or 'UEs') connect to a core network and communicate with one or more other communication devices or one or more remote servers. For simplicity, the present application will use the term access network node, RAN node (or simply RAN) or base station to refer to any such access nodes.

　　For simplicity, the present application will use the term mobile device, user device, or UE to refer to any communication device that is able to connect to the core network via one or more base stations. Although the present application may refer to one or more 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 communication network for sending/receiving data, regardless of whether such communication devices are controlled by human input or software instructions stored in memory.

　　In the current 5G architecture, the base station 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 (sometimes referred to as 'Medium') 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 base stations 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 base station.

　　In 5G, core network entities comprise logical nodes (or 'functions') including one or more control plane functions (CPFs) and one or more user plane functions (UPFs). The CPFs include, amongst other things, one or more Access and Mobility Management Functions (AMFs), a session management function (SMF), and one or more location management functions (LMFs). The AMF generally corresponds to the MME in 4G and performs many of the functions performed by the MME. Each UPF combines functionality of both the S-GW and P-GW - specifically user plane functionality of the S-GW (SGW-U) and user plane functionality of the P-GW (PGW-U). The SMF provides session management functionality (that formed part of MME functionality in 4G). The SMF also combines the some of the functionality provided by the S-GW and P-GW - specifically control plane functionality of the S-GW (SGW-C) and control plane functionality of the P-GW (PGW-C). The SMF also allocates IP addresses to each UE.

　　3GPP is also working with the satellite communication industry to specify an integrated satellite and terrestrial network infrastructure in the context of 5G. This is referred to as non-terrestrial networks (NTN) which term refers to networks, or segments of networks, using an airborne or spaceborne vehicle for transmission of data and control signalling. Satellites refer to spaceborne vehicles in Low Earth Orbits (LEO), Medium Earth Orbits (MEO), Geostationary Earth Orbit (GEO) or in Highly Elliptical Orbits (HEO). Airborne vehicles refer to High Altitude Platforms (HAPs) encompassing Unmanned Aircraft Systems (UAS) - including tethered UAS, Lighter than Air UAS and Heavier than Air UAS - all operating quasi-stationary at an altitude typically between 8 and 50 km.

　　3GPP Technical Report (TR) 38.811 (NPL 1) is a study on New Radio to support such non-terrestrial networks. The study includes, amongst other things, NTN deployment scenarios and related system parameters (such as architecture, altitude, orbit etc.) and a description of adaptation of the 3GPP channel models for non-terrestrial networks (propagation conditions, mobility, etc.). Non-terrestrial networks are expected to:
-　　help foster the 5G service roll out in un-served or underserved areas to upgrade the performance of terrestrial networks;
-　　reinforce service reliability by providing service continuity for user equipment or for moving platforms (e.g. passenger vehicles - aircraft, ships, high speed trains, buses);
-　　increase service availability everywhere; especially for critical communications, future railway/maritime/aeronautical communications; and
-　　enable 5G network scalability through the provision of efficient multicast/broadcast resources for data delivery towards the network edges or even directly to the user equipment.

　　Non-Terrestrial Network access typically features the following elements (amongst others):
-　　NTN Terminal: This may refer to the 3GPP UE or to a UE specific to the satellite system in the case that the satellite does not serve directly 3GPP UEs;
-　　A service link which refers to the radio link between the user equipment and the space/airborne platform (which may be in addition to a radio link with a terrestrial based RAN);
-　　A space or an airborne platform (e.g., a satellite or the like);
-　　Gateways that connect the satellite or aerial access network to the core network. It will be appreciated that gateways will mostly likely be collocated with a base station (e.g. a gNB); and
-　　Feeder links which refer to the radio links between the Gateways and the space/airborne platform.

　　Satellite or aerial vehicles typically generate several satellite beams over a given area. The beams have a typically elliptic footprint on the surface of the earth. The beam footprint may be moving over the earth with the satellite or the aerial vehicle motion on its orbit. Alternatively, the beam footprint may be earth fixed (albeit temporarily), in such case some beam pointing mechanisms (mechanical or electronic steering feature) may be used to compensate for the satellite or the aerial vehicle motion. There are different options for beam identification purposes. In one option multiple (nearby/neighbouring) satellite beams may have the same associated physical cell identifier (PCI) and hence the PCI can remain unchanged as a UE moves from beam-to-beam of the set of beams sharing a PCI. Alternatively, there may be a one-to-one relationship between the PCIs and the satellite beams (at least within a particular satellite's coverage area comprising multiple beams).

　　The coverage in 5G is primarily beam-based rather than cell based. There is no cell-level reference channel from where the coverage of the cell could be measured. Instead, each cell has one or more so-called synchronization signal / physical broadcast channel (PBCH) block (SSB) beams (which are different to satellite or NTN beams). SSB beams form a matrix of beams covering an entire cell area. Each SSB beam carries an SSB comprising a primary synchronization signal (PSS), secondary synchronization signal (SSS), and physical broadcast channel (PBCH).

　　The UE searches for and performs measurements on the SSB beams (e.g. of the synchronization signal reference signal received power, 'SS-RSRP', synchronization signal reference signal received quality, 'SS-RSRQ', and/or the synchronization signal to noise or interference ratio, 'SS-SINR'). The UE maintains a set of candidate beams which may contain beams from multiple cells. A PCI and beam identifier (ID) (or SSB index) thus distinguish the SSB beams from each other. Effectively, therefore, the SSB beams are like mini cells which may be within a larger cell. Once a UE has detected and selected a cell (and/or an SSB beam in the case of 5G) it may attempt to access that cell and/or SSB beam using an initial RRC connection setup procedure comprising a random-access procedure.

　　For example, once a UE has detected and selected a cell (and/or a beam in the case of 5G) it may attempt to access that cell and/or beam using an initial radio resource control (RRC) connection setup procedure comprising a random access (RACH) procedure that typically involves four distinct steps. Alternatively, the UE may attempt to access that cell and/or beam using a so-called two-step RACH procedure. Both the four step and two step RACH procedures are well known to those skilled in the art.

　　As those skilled in the art will appreciate, while a contention based PRACH procedure is described, a non-contention based (or 'contention free') procedure may also be used in which a dedicated preamble is assigned by the base station to the UE.

　　Random access procedures such as those described may also be used in other contexts including, for example, handover, connection reestablishment, requesting UL scheduling where no dedicated resource for a scheduling-request has been configured for the UE, etc.

　　Nevertheless, whilst a RACH procedure may be used to access a target cell of a target RAN node during handover, the UE may attempt to access that cell and/or beam using a so called 'RACH-less' based handover which provides reductions in the data connectivity interruption time at each handover as it removes the need for performing random access when first accessing the target cell, and hence reduces overall handover execution time.

　　In conventional handover procedures (including RACH-less handover procedures) a source base station may initially decide to initiate a handover based on measurement reporting by the UE (e.g., a measurement report triggered by a particular measurement reporting event, or periodically, or the like). In response, the source RAN node initiates preparation of a target RAN node for handover by sending a handover request message to the target RAN node.

　　Assuming the target RAN node decides to allow the handover request (e.g., based on appropriate admission control), the target RAN node then prepares handover and sends a handover request acknowledgement message to the source RAN node. This handover request acknowledgement message includes an RRC message generated by the target RAN node for instructing modification/reconfiguration of the UE's RRC connection for the purposes of handover.

　　The source RAN node then initiates a handover execution phase by sending the RRC reconfiguration message (including the mobility control information) to the UE. The UE receives the RRC reconfiguration message and is thus commanded by the source RAN node to perform the handover. The UE derives target RAN node specific keys and configures the selected security algorithms to be used in the target cell. After receiving the RRC Reconfiguration message, the UE will attempt to access a primary cell (PCell) of the target RAN node at the first available physical uplink shared channel (PUSCH) occasion.

　　To confirm the handover the UE may send an RRC reconfiguration complete message to the target RAN node. The RRC reconfiguration complete message includes a cell radio network temporary identifier (C-RNTI), e.g., along with an uplink buffer status report, and/or uplink data, whenever possible. The target RAN node verifies the C-RNTI sent in the RRC reconfiguration complete message. The target RAN node can then begin sending data to the UE after scheduling appropriate downlink resources using the PDCCH.

　　The handover procedure is completed for the UE when the UE receives a UE contention resolution identity MAC control element (MAC CE) from the target RAN node or the UE receives a PDCCH addressed to its C-RNTI from the target RAN node after sending the initial uplink transmission.

NPL 1: 3GPP Technical Report (TR) 38.811
NPL 2: NGMN 5G White Paper' V1.0

　　The disclosure aims to provide one or more apparatus and/or one or more associated methods that at least partially addresses or contributes to one or more of the above issues.

　　In the following disclosure 'satellite' based NTN will generally be referred to but it will be appreciated that the principles and methods described are more widely applicable to other space or airborne platforms used for implementing NTNs.

　　In one aspect there is provided a method performed by a mobile terminal configured to communicate with a satellite having at least a part of functions of a base station, the method comprising:
　　　　　　receiving, from the satellite, information for switching a serving satellite from the satellite to another satellite;
　　　　　　performing the switching the serving satellite based on the information, and
　　　　　　wherein the performing the switching includes:
　　　　　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).

　　In one aspect there is provided a method performed by a satellite having at least a part of functions of a base station, the method comprising:
　　transmitting, to a mobile terminal configured to communicate with the satellite, information for switching a serving satellite from the satellite to another satellite to cause the mobile terminal to perform the switching the serving satellite, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).

　　In one aspect there is provided a mobile terminal configured to communicate with a satellite having at least a part of functions of a base station, the mobile terminal comprising:
　　means for receiving, from the satellite, information for switching a serving satellite from the satellite to another satellite;
　　means for performing the switching the serving satellite based on the information, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).

　　In one aspect there is provided a satellite having at least a part of functions of a base station, the satellite comprising:
　　means for transmitting, to a mobile terminal configured to communicate with the satellite, information for switching a serving satellite from the satellite to another satellite to cause the mobile terminal to perform the switching the serving satellite, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).

　　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.

　　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.

　　According to the present disclosure, it is possible to provide a method performed by a mobile terminal, a method performed by a satellite, a mobile terminal, and a satellite.

　　Examples of the disclosure will now be described, by way of example, with reference to the accompanying drawings in which:

Fig. 1 illustrates schematically a mobile (cellular or wireless) communication system; Fig. 2A illustrates a possible architecture of an NTN RAN; Fig. 2B illustrates a possible architecture of an NTN RAN; Fig. 2C illustrates a possible architecture of an NTN RAN; Fig. 3 illustrates schematically a non-terrestrial network (NTN) radio access network that may be used in the communication system of Fig. 1; Fig. 4A illustrates a satellite switching mechanism of from an old space (or air) borne platform to a new space (or air) borne platform that are in transparent mode; Fig. 4B illustrates another satellite switching mechanism from an old space (or air) borne platform to a new space (or air) borne platform that are in transparent mode; Fig. 5 is a simplified sequence diagram illustrating a RACH-less satellite switching procedure for NTNs in transparent mode; Fig. 6 illustrates communication links (direct and indirect) that may be formed between two satellites in regenerative mode; Fig. 7 illustrates a simplified sequence diagram illustrating a first regenerative mode satellite switching procedure; Fig. 8 illustrates a simplified sequence diagram illustrating a second regenerative mode satellite switching procedure; Fig. 9 illustrates a simplified sequence diagram illustrating a third regenerative mode satellite switching procedure; Fig. 10 illustrates a simplified sequence diagram illustrating a fourth regenerative mode satellite switching procedure; Fig. 11 illustrates a simplified sequence diagram illustrating a fifth regenerative mode satellite switching procedure; Fig. 12A illustrates another simplified sequence diagram illustrating a sixth regenerative mode satellite switching procedure; Fig. 12B illustrates another simplified sequence diagram illustrating a sixth regenerative mode satellite switching procedure; Fig. 13 is a simplified block schematic illustrating the main components of a UE 3 for implementation in the system of Fig. 1; and Fig. 14 is a simplified block schematic illustrating the main components of a base station 5A for implementation in the system of Fig. 1.

　　< Overview >
　　An exemplary communication system 1 will now be described in general terms, by way of example only, with reference to Figs. 1 to 3.

　　Fig. 1 schematically illustrates a mobile ('cellular' or 'wireless') telecommunication system (e.g. communication system 1) to which the examples described herein are applicable.

　　In the communication system 1, user equipments (UEs) 3 (3-1, 3-2, 3-3) (e.g. mobile telephones and/or other mobile devices) can communicate with each other via a corresponding (radio) access network ((R)AN) 5-1, 5-2 that operates according to one or more compatible radio access technologies (RATs). In the illustrated example, each RAN 5-1, 5-2 includes a base station 5A-1, 5A-2 (e.g., a base station such as a gNB) that respectively operates one or more associated cells 9 (9-1, 9-2).

　　As those skilled in the art will appreciate, whilst three UEs 3, and two RANs 5-1, 5-2 are shown in Fig. 1 for illustration purposes, the system, when implemented, will typically include one or more other RANs 5 and UEs 3.

　　In the illustrated example, both RANs 5-1, 5-2 are non-terrestrial network (NTN) RANs, nevertheless it will be appreciated that at least one of the RANs may be a terrestrial network (TN) RAN. In the exemplary communication system 1, either of the RANs 5 may also be configured to support operation in one or more network energy saving (NES) modes.

　　Each 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 each RAN 5 may be configured to support 4G, 5G, and/or later generations and/or any other 3GPP or non-3GPP communication protocols.

　　The base station 5A of each RAN 5 may be a distributed base station comprising at least one distributed unit (DU) (e.g., a gNB-DU or the like), and a central unit (CU) (e.g., a gNB-CU or the like). In such a distributed base station the CU 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 F1-U interface) (together forming an F1 interface (or 'reference point')), and with one another via an appropriate interface (e.g. an E1 interface). It will be appreciated that while the DU may include 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 base station 5A 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 the base station may be in a non-distributed form, for example as an integrated base station 5A or function node with equivalent functionality under a different name.

　　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). Base stations 5A of one or more 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 - not shown in Fig. 1).

　　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) (or one or more function nodes with equivalent functionality under a different name), one or more network node entities for session management (e.g. Session Management Functions (SMFs) 10-2) (or one or more function nodes with equivalent functionality under a different name) 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.

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

　　One or more UPFs 11 are connected to an external data network 20 (e.g. an IP network such as the internet) via an appropriate interface (e.g. an N6 reference point) for communication of the user data.

　　The AMF 10-1 performs mobility management related functions, maintains the NAS connection with each UE 3 and manages UE registration. The AMF 10-1 is also responsible for managing paging.

　　The SMF 10-2 is connected to the AMF 10-1 via an appropriate interface (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.

　　Each base station 5A is also configured for transmission of, and the one or more 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 one or more 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.

　　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 one or more UEs 3 with the Master Information Block (MIB). It also, in conjunction with the PDCCH, supports the synchronisation of time and frequency, which aids cell acquisition, selection and re-selection.

　　The DL physical signals may include, for example, one or more reference signals (RSs) and one or more synchronization signals (SSs). A reference signal (sometimes known as a pilot signal) is a signal with a predefined special waveform known to both the UE 3 and the base station of 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).

　　Similarly, the UEs 3 are configured for transmission of, and the base station of 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.

　　The UEs 3 and base station 5A of the RAN 5 of the communication system 1 are mutually configured for performing a random-access channel (RACH) procedure for the UE 3 to access the network. Specifically, on detection and selection of a cell (and/or a beam in the case of 5G) the UE 3 is able to attempt access to that cell and/or beam using an initial radio resource control (RRC) connection setup procedure comprising a random-access procedure. Prior to attempting initial access, the UE 3 chooses random access resources (including, for example, a preamble) to use to initiate the RACH procedure. The UE 3 sends the selected preamble (e.g., in 'Msg1') to the base station of the RAN 5 over a physical random-access channel (PRACH) for initiating the process to obtain synchronization in the uplink (UL). In response, the base station of the RAN 5 responds with a random-access response (RAR) (or 'Msg2'). The RAR indicates reception of the preamble and includes: a timing-alignment (TA) command for adjusting the transmission timing of the UE 3 based on the timing of the received preamble; an uplink grant field indicating the resources to be used in the uplink for a physical uplink shared channel (PUSCH); a frequency hopping flag to indicate whether the UE 3 is to transmit on the PUSCH with or without frequency; a modulation and coding scheme (MCS) field from which the UE 3 can determine the MCS for the PUSCH transmission; and a transmit power control (TPC) command value for setting the power of the PUSCH transmission. The UE 3 then sends a third message ('Msg3') to the network over a physical uplink shared channel (PUSCH) based on the information in the RAR. The specific message sent by the UE 3 in this step, and the content of the message, depends on the context in which the random-access procedure is being used. In the example of initial radio RRC connection setup, however, Msg3 typically comprises an RRC Setup request or similar message carrying a temporary randomly generated UE identifier. The network responds with a fourth message ('Msg4') which carries the randomly generated UE identifier received in Msg3 for contention purposes to resolve any collisions between different UEs 3 using the same preamble sequence. When successful, Msg4 also transfers the UE 3 to a connected state.

　　While a four-step contention-based RACH procedure is described it will be appreciated that the UE 3 and the base station 5A of either RAN 5 of the communication system 1 may also perform a non-contention based (or 'contention free') procedure in which a dedicated preamble is assigned by the base station of the RAN 5 to the UE 3. Moreover, the UE 3 and the base station 5A of the RAN 5 of the communication system 1 may perform a two-step RACH procedure (e.g., as described in the introduction).

　　It will be appreciated that while the UE 3 can trigger initiation of the RACH procedure itself (e.g., when the UE 3 needs to connect to the network), initiation of the RACH procedure may be by the network. For example, a RACH procedure may be initiated via a message sent via downlink control information (DCI) with an appropriate DCI format (e.g. 1_0) in a physical downlink control channel (PDCCH) - such a message is commonly known as a PDCCH order. A RACH procedure may be also initiated by the base station 5A of the RAN 5 when handover is required (e.g., using a handover command message).

　　< NTN RAN Architecture >
　　Fig. 2A illustrates a possible architecture of an NTN RAN 5 that may be used.
　　Fig. 2B illustrates a possible architecture of an NTN RAN 5 that may be used.
　　Fig. 2C illustrates a possible architecture of an NTN RAN 5 that may be used.

　　The architecture of Fig. 2A may be referred to as a 'transparent satellite' based RAN architecture. In this architecture, the base station 5A-1 is a terrestrially located base station that sends and receives communications respectively destined for and originating from the UEs 3 via a terrestrially located gateway 5B-1 and via a non-terrestrial space (or air) borne platform 5C-1 that has no base station functionality. The non-terrestrial space (or air) borne platform 5C-1 relays these communications to and from the UEs 3 in one or more cells operated by the base station 5A-1, and from and to the gateway 5B-1 as required. The non-terrestrial space (or air) borne platform 5C-1 relays these communications transparently without on-board processing them in effect acting as a so-called 'bent-pipe'. In this implementation, the feeder link between the gateway 5B-1 and the non-terrestrial space (or air) borne platform 5C-1 effectively acts as part of the NR-Uu interface (or reference point) between the base station 5A-1 and one or more UEs 3. Similarly, the service link between the non-terrestrial space (or air) borne platform 5C-1 and one or more UEs 3 effectively acts as another part of the NR-Uu interface (or reference point) between the base station 5A-1 and one or more UEs 3. The base station's communication link with the core network 7 (e.g. for signalling over the N2, N3 interface/reference point etc.) is provided solely terrestrially.

　　The architecture of Fig. 2B may be referred to as a 'regenerative satellite' based RAN architecture (i.e., in which the satellite performs on board processing of the payload being communicated between the UE 3 and the core network 7). In this architecture, the base station 5A-1 is a base station 5A-1 of a distributed type having a terrestrially located central unit (CU) 5A_CU-1 and a distributed unit (DU) 5A_DU-1 provided on-board the non-terrestrial space (or air) borne platform 5C-1. The terrestrially located CU 5A_CU-1 performs some of the (typically higher layer) functionality of the base station 5A-1 whereas the non-terrestrially located DU 5A_DU-1 performs other (typically lower layer) functionality of the base station 5A-1. The terrestrially located CU 5A_CU-1 communicates with the non-terrestrially located DU 5A_DU-1 via the gateway 5B-1 and an F1 interface implemented via a satellite radio interface between the gateway 5B-1 and the non-terrestrial space (or air) borne platform 5C-1 in which the DU 5A_DU-1 is provided.

　　The non-terrestrial space (or air) borne platform 5C-1 transmits communications destined for and originating from the UEs 3 in one or more cells operated by the base station 5A-1, and from and to the gateway 5B-1 as required. However, in this implementation lower layer processing of communication respectively destined for and originating from the UEs 3 is performed on-board the non-terrestrial space (or air) borne platform 5C-1 by the DU 5A_DU-1 and higher layer processing of that communication respectively destined for and originating from the UEs 3 is performed by the terrestrially located CU 5A_CU-1.

　　Accordingly, in this implementation, the feeder link between the gateway 5B-1 and the non-terrestrial space (or air) borne platform 5C-1 effectively acts as the F1 interface (or reference point) between the CU 5A_CU-1 and DU 5A_DU-1 of the base station 5A-1. The service link between the non-terrestrial space (or air) borne platform 5C-1 and one or more UEs 3, on the other hand, effectively acts as the NR-Uu interface (or reference point) between the base station 5A-1 and one or more UEs 3. The base station's communication link with the core network 7 (e.g. for signalling over the N2, N3 interface/reference point etc.) is provided solely terrestrially.

　　The architecture of Fig. 2C may also be referred to as a 'regenerative satellite' based RAN architecture (i.e., in which the satellite performs on board processing of the payload being communicated between the UE 3 and the core network 7). In this architecture, the base station 5A-1 is provided on-board the non-terrestrial space (or air) borne platform 5C-1. The base station 5A-1 on board the non-terrestrial space (or air) borne platform 5C-1 transmits communications destined for and originating from the UEs 3 in one or more cells operated by the base station 5A-1, and from and to the core network 7 via the gateway 5B-1 as required. However, in this implementation, processing of communication respectively destined for and originating from the UEs 3 is performed on-board the non-terrestrial space (or air) borne platform 5C-1 by the base station 5A-1.

　　Accordingly, in this implementation, the feeder link between the gateway 5B-1 and the non-terrestrial space (or air) borne platform 5C-1 effectively acts as part of the N2/N3 interfaces (or reference points) between the base station 5A-1 and the core network 7. The base station's communication link with the core network 7 (e.g. for signalling over the N2, N3 interface/reference point etc.) is thus provided partly via the feeder link and partly terrestrially. The service link between the non-terrestrial space (or air) borne platform 5C-1 and one or more UEs 3, on the other hand, effectively acts as the NR-Uu interface (or reference point) between the base station 5A-1 and one or more UEs 3.

　　The base station 5A-1 thus controls one or more associated cells via the non-terrestrial space (or air) borne platform 5C-1. It will be appreciated that the base station 5A-1 may be configured to support 4G, 5G and/or later generations, and/or any other 3GPP or non-3GPP communication protocols.

　　< NTN RAN >
　　Fig. 3 illustrates schematically one NTN RAN 5-1 architecture that may be used in the communication system 1 of Fig. 1.

　　Fig. 3 depicts an NTN RAN 5-1 whose satellite is in regenerative mode as shown in Fig. 2C. It will nevertheless be appreciated that NTN RAN 5-1 depicted is by way of example only, and that the NTN RAN 5-1 may comprise a satellite in regenerative mode as shown in Fig. 2B, or alternative transparent (bent pipe) mode as shown in Fig. 2A.

　　As seen in Fig. 3, the NTN RAN 5-1 comprises a base station 5A-1 operating one or more associated cells 9, a gateway 5B-1, and a non-terrestrial space (or air) borne platform 5C-1 (e.g. comprising one or more satellites and/or airborne vehicles), which may be referred to generally as a 'satellite' 5C-1 for simplicity. Communication via the NTN RAN 5-1 is routed through the core network 7 and external data network 20 (e.g. via the N6 interface / reference point).

　　The NTN RAN 5-1 controls a number of directional satellite beams via which associated NTN cells 9 may be provided. Specifically, each satellite beam has an associated footprint on the surface of the Earth which forms an NTN cell, or part of an NTN cell. Each NTN cell has an associated Physical Cell Identity (PCI). The satellite beam footprints may be moving as the space (or air) borne platform 5C-1 is travelling along its orbit (e.g. as illustrated by the arrows A in Fig. 3). Alternatively, the satellite beam footprint may be earth fixed, in which case an appropriate satellite beam pointing mechanism (mechanical or electronic steering) may be used to compensate for the movement of the non-terrestrial space (or air) borne platform 5C-1. Satellite beams and satellites are not considered visible from a UE 3 perspective in NTN. This does not, however, preclude differentiating at the public land mobile network (PLMN) level the type of network (e.g. NTN vs. terrestrial).

　　The base station 5A-1 of the NTN RAN 5-1 is configured to provide ephemeris data for the non-terrestrial space (or air) borne platform 5C-1, to the UEs 3, to help UEs 3 perform measurement and cell selection/reselection and for supporting initial access. This ephemeris data may comprise information on orbital information such as information on orbital plane level or on satellite level and/or information (e.g. a pointer or index) from which more detailed ephemeris data stored in the UE 3 (e.g. in a universal subscriber identity module, 'USIM') may be obtained. At least some of this ephemeris information may, for example, be provided in system information and/or may be provided using UE specific (dedicated) signalling such as RRC signalling.

　　Specifically, the base station 5A-1 is able to provide satellite assistance information for the satellite as part of a dedicated system information block (SIB) that is broadcast to one or more UEs 3 in a corresponding cell 9 of the NTN RAN 5-1 (for 5G NTN this may, for example, be SIB19 but for future generations it may be provided in another SIB or in a different way). The satellite assistance information may include, for example, information identifying at least one associated NTN configuration (e.g., as part of an NTN-Config IE or the like). The NTN configuration includes parameters for assisting the UE 3 to access the network using NTN access (e.g., ephemeris data, common timing alignment parameters, a scheduling (e.g., k_offset), validity duration for uplink synchronisation information, and an epoch time (a reference time for which assistance information is valid)).

　　The satellite assistance information may include, for example, an indication of a time information on when a cell provided via NTN quasi-Earth fixed system is going to stop serving the area it is currently covering (e.g., in a t-Service IE). This may be indicated, for example, as a time in multiples of 10 ms after 00:00:00 on a Gregorian calendar date of 1 January 1900 (midnight between Sunday, December 31, 1899, and Monday, January 1, 1900). The exact stop time may be between the time indicated by the value of this field minus 1 and the time indicated by the value of this field.

　　With the help of this ephemeris data, a UE 3 may search for the first NTN cell it can connect to. After detecting a synchronization signal block (SSB) of a cell 9 broadcasted via a non-terrestrial space (or air) borne platform 5C-1, the UE 3 may be able to read initial system information of that cell which may contain further ephemeris information relating to the exact location of the cell 9 (and/or to the satellite broadcasting the cell 9). This ephemeris information may be given relative to information relating, for example, to the orbital plane that the UE 3 may already have obtained.

　　The accuracy of the prediction of a satellite orbit or the satellite position can decrease with time and so, to help ensure accuracy, the ephemeris data provided to the UE 3 is updated (a)periodically.

　　The same PCI may be used for several satellite beams, or there may be one PCI per satellite beam. A satellite beam can consist of one or more SSB beams with one cell (PCI) having a maximum of L SSB beams, where L can typically be 4, 8 or 64 depending on the band. During initial access, the UEs 3 perform cell search based on SSBs where each SSB is transmitted in a different respective beam. Each SSB comprises a primary synchronization signal (PSS), secondary synchronization signal (SSS), and physical broadcast channel (PBCH). As the SSB carries synchronization signals (SSs) / PBCH (SS/PBCH) transmissions, it is sometimes referred to as an SS/PBCH block.

　　As those skilled in the art will understand, while the disclosure is described in the context of an NTN based RAN 5 / base station 5A, many of the technical features described are generally applicable to, and can be implemented in, any RAN 5 / base station 5A of a more conventional (non-NTN) based communication system 1.

　　< Satellite Switching: Hard & Soft Switching >
　　It will be appreciated that to support mobility, switching mechanisms and procedures may be implemented to allow switching between non-terrestrial space (or air) borne platform with which a UE 3 is in communication.

　　Fig. 4A illustrates a satellite switching mechanism of a UE 3 from an old non-terrestrial space (or air) borne platform 5C-1_Old to a new non-terrestrial space (or air) borne platform 5C-1_New that are in transparent mode. It will be appreciated that the term 'satellite' as used herein may also include a non-terrestrial space (or air) borne platform.

　　Fig. 4A depicts an example of a hard satellite switching mechanism/procedure wherein the serving time of the old (e.g., source) space (or air) borne platform 5C-1_Old and the serving time of the new space (or air) borne platform 5C-1_New have no overlap such that the UE 3 begins communicating with base station 5A-1 via the new non-terrestrial space (or air) borne platform 5C-1_New only after communication via the old space (or air) borne platform 5C-1_Old ceases or is disconnected.

　　As shown in Fig. 4A (top), the UE 3 may initially be in communication with base station 5A-1 of the NTN RAN 5-1 over cell 9-1 via an old (e.g., source) non-terrestrial space (or air) borne platform 5C-1_Old. The cell 9-1 is provided by the base station 5A-1. At some future time, in response to a change in situation (e.g., the UE 3 moves out of range of non-terrestrial space (or air) borne platform 5C-1_Old, or the non-terrestrial space (or air) borne platform 5C-1_Old moves and can no longer provide service to the UE 3), it may be difficult (or impossible) to maintain communication between UE 3 and the base station 5A-1 via the old non-terrestrial space (or air) borne platform 5C-1_Old. Accordingly, a satellite switching procedure may be used to switch the old non-terrestrial space (or air) borne platform 5C-1_Old to another (new) non-terrestrial space (or air) borne platform 5C-1_New to maintain the UE's 3 communication link with the base station 5A-1.

　　Fig. 4A (bottom) shows the UE 3 in communication with base station 5A-1 of the NTN RAN 5-1 over cell 9-2 via a new non-terrestrial space (or air) borne platform 5C-1_New after some switching time T-switch. The cell 9-2 is provided by the base station 5A-1. It will be appreciated that the cell provided by the base station 5A-1 following the satellite switch may (as shown here) be a different cell 9-2. Alternatively, the base station 5A-1 may provide the same cell 9-1 via the new non-terrestrial space (or air) borne platform 5C-1_New.

　　Fig. 4B illustrates another satellite switching mechanism of a UE 3 from an old non-terrestrial space (or air) borne platform 5C-1_Old to a new non-terrestrial space (or air) borne platform 5C-1_New that are in transparent mode.

　　Fig. 4B depicts an example of a soft satellite switching mechanism/procedure wherein the serving time of the old non-terrestrial space (or air) borne platform 5C-1_Old and the serving time of the new non-terrestrial space (or air) borne platform 5C-1_New have overlap such that the UE 3 begins communicating with base station 5A-1 via the new non-terrestrial space (or air) borne platform 5C-1_New while the communication via the old non-terrestrial space (or air) borne platform 5C-1_Old is still available. After the switching time has ended (T-service), communication with the base station 5A-1 via the old non-terrestrial space (or air) borne platform 5C-1_Old is terminated.

　　As shown in Fig. 4B (top), the UE 3 may initially be in communication with base station 5A-1 of the NTN RAN 5-1 over cell 9-1 via the old non-terrestrial space (or air) borne platform 5C-1_Old (e.g., a source satellite). The cell 9-1 is provided by the base station 5A-1. At some future time, in response to a change in situation (e.g., the non-terrestrial space (or air) borne platform 5C-1_Old moves and approaches a point where it will no longer connect to the gateway 5B-1/base station 5A-1 on the ground and/or provide service to the area where the UE 3 is located), a satellite switching procedure may be used to switch the old non-terrestrial space (or air) borne platform 5C-1_Old to a new non-terrestrial space (or air) borne platform 5C-1_New that is better able to communicate with the base station 5A-1 to maintain the UE's 3 communication link with the base station 5A-1.

　　Fig. 4B (middle) shows the UE 3 in communication with base station 5A-1 of the NTN RAN 5-1 over either or both of cells 9-1 and 9-2 via the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New at some switching time (T-switch). The cells 9-1 and 9-2 are both provided by the base station 5A-1 and are both in service.

　　Fig. 4B (bottom) shows the UE 3 in communication with base station 5A-1 of the NTN RAN 5-1 over cell 9-2 via the new non-terrestrial space (or air) borne platform 5C-1_New after a time at which service of over cell 9-2 via the old non-terrestrial space (or air) borne platform 5C-1_Old is terminated (T-service). The cell 9-2 is provided by the same base station 5A-1. It will be appreciated that the cell provided by the base station 5A-1 following the satellite switch may (as shown here) be a different cell 9-2. Alternatively, the base station 5A-1 may provide the same cell 9-1 via the new non-terrestrial space (or air) borne platform 5C-1_New.

　　< Satellite Switching: Transparent Mode >
　　Fig. 5 is a simplified sequence diagram illustrating a RACH-less satellite switching procedure for one or more NTNs in transparent mode deployment scenarios that may be implemented in the communication system 1.

　　As is shown in Fig. 5, there may be a UE 3 that communicates over a serving cell 9-1 provided by an old (e.g. source) non-terrestrial space (or air) borne platform 5C-1_Old, that communicates with the core network (CN) 7 via a base station 5A-1, which is located on the ground. The base station 5A-1 on the ground communicates with the old non-terrestrial space (or air) borne platform 5C-1_Old via a gateway (GW) 5B-1 also located on the ground as depicted in e.g., Figs. 4A and 4B.

　　In transparent mode, the base station 5A-1 and the gateway 5B-1 used may remain unchanged while the non-terrestrial space (or air) borne platform 5C-1 in communication with the UE 3 may be switched when necessary (e.g., satellite moves out of range). In such a scenario, a satellite switch between the old non-terrestrial space (or air) borne platform 5C-1_Old that the UE 3 was initially in communication with, and a new non-terrestrial space (or air) borne platform 5C-1_New can be performed by re-synchronising the UE 3 from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New.

　　Beneficially, in transparent mode, as the same base station 5A-1 is used irrespective of the satellite with which the UE 3 communicates, there may be no need for the UE 3 to undergo initial access and related procedures such as random access signalling, RRC signalling, user plane stack reset, and/or physical cell identifier (PCI) changes. Thus, the level of signalling and processing required to switch from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New is minimal.

　　At step S502, the UE 3 receives a system information message (e.g., a SIB19) from the old non-terrestrial space (or air) borne platform 5C-1_Old that may include NTN-specific parameters for the current serving cell 9-1 provided by the base station 5A-1 and/or neighbouring cells 9-2 provided by the base station 5A-1. In particular, the system information message may contain information required for RACH-less satellite switch to a new non-terrestrial space (or air) borne platform 5C-1_New such as satellite assistance information, ephemeris data, common timing advance parameters, k_offset, validity duration for UL synchronization epoch time, cell reference location, cell stop time, and the like. Additionally, the system information block message may optionally include an SSB index and SSB time offset.

　　At step S504, having received the system information message (e.g., SIB19), the UE 3 (which is in RRC_CONNECTED mode with the base station 5A-1) performs any necessary procedures to facilitate satellite switch from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New.

　　For example, upon receipt of a SIB19 message, the UE 3 may decide a time at which to switch (T-Switch) from its current non-terrestrial space (or air) borne platform 5C-1_Oldto a new non-terrestrial space (or air) borne platform 5C-1_New based on whether the UE 3 supports hard or soft satellite switching. Where the UE 3 supports hard switching only, then T-Switch is equal to the time at which the new non-terrestrial space (or air) borne platform 5C-1_New can provide a communication link to the UE 3 e.g., T-Switch = T-Service. However, if the UE 3 supports soft switching, then T-Switch equals some time T between the start of a switching procedure and a time when the switching procedure ends and service can be resumed (e.g., between 'T-Start' and 'T-Service').

　　Additionally, or alternatively, upon receipt of a system information message (e.g., SIB19), the UE 3 may start (or restart) a timer (e.g., a 'T430' timer) for the serving cell 9-1 with the timer value set to a validity duration (e.g., 'ntn-UlSyncValidityDuration') from the subframe indicated by an epoch time (e.g., 'epochTime') for the serving cell 9-1 indicated in the system information message. Once the timer for the serving cell 9-1 expires, UL synchronisation is deemed lost, and the UE 3 attempts to receive the system information message from the base station 5-1 again. This process is repeated until the system information message is successfully deemed to have been received.

　　Additionally, or alternatively, if the system information message contains information elements (IEs) indicating satellite switch with resynchronisation (e.g., 'SatSwitchWithReSync') and the T-service time (e.g., 't-Service'), and the UE 3 supports hard satellite switching with re-synchronisation, and if the system information message also includes a T-start time IE (e.g., 't-ServiceStart') and the UE 3 also supports soft satellite switch with resynchronisation, then the UE 3 performs satellite switch with resynchronization using a soft switching procedure (e.g., as described with reference to Fig. 4B) between the time indicated by t-ServiceStart and the time indicated by t-Service for the serving cell.

　　If, however, the system information message does not include the IE t-ServiceStart and/or the UE 3 does not support soft satellite switch with resynchronization, then the UE 3 performs satellite switch with resynchronization using a hard switching procedure (e.g., as described with reference to Fig. 4A) at the time indicated by t-Service for the serving cell 9-1. Additionally, or alternatively, upon receipt of the system information message the UE 3 may suspend its UL transmissions and flush its HARQ buffer.

　　At step S506, the UE 3 acquires/detects, from a new non-terrestrial space (or air) borne platform 5C-1_New, one or more synchronisation signals (e.g., synchronisation signal blocks (SSBs)) for synchronisation of the UE 3 with the new non-terrestrial space (or air) borne platform 5C-1_New. To detect such synchronisation signals, the UE 3 may first perform SSB-based RRM Measurement Timing Configuration (SMTC) adjustment using a propagation delay difference (PDD). A PDD indicates the service link propagation delay difference between a serving cell 9-1 and a neighbour cell 9-2 and may be used in SMTC adjustment to determine the SMTC window indicating the measurement periodicity and timings of SSBs that UE 3 can use for measurements. It will be appreciated that where an SSB time offset is configured in the system information message it too may be used for SMTC adjustment to determine the SMTC window indicating the measurement periodicity and timings of SSBs that UE 3 can use for measurements. The UE 3 detects the target non-terrestrial space (or air) borne platform 5C-1_New SSBs within the adjusted SMTC window, and based on those received SSBs, the UE 3 determines and establishes DL synchronisation with the new non-terrestrial space (or air) borne platform 5C-1_New.

　　At step S508, the UE 3 acquires synchronisation with the target non-terrestrial space (or air) borne platform 5C-1_New and starts a timer (e.g., a T430 timer) and indicates to lower layers (e.g., MAC layer), that synchronisation with a satellite is restored. Additionally, or alternatively, the UE 3 may set its timing advance (N_TA) value to zero and clear any UE-specific K_offset values it is using. Additionally, UL transmissions may be resumed and one or more timing advance reports (TARs) and/or one or more TAR-scheduling requests (TAR-SRs) for the UE 3 (if it supports TARs) may be triggered.

　　At step S510, the UE 3 initiates timing advance (TA) reporting transmissions in the first UL transmission. For example, the UE 3 may trigger TA reporting transmissions in the first UL transmission via a TAR MAC control element (CE). Once triggered, the UE 3 may transmit the TA report via PUCCH scheduling request (PUCCH-SR) or PUSCH scheduled by a dynamic grant (DG) or a configured grant (CG) when the TA timer is running (S510a). Alternatively, when the TA timer is not running and there is no PUCCH-SR, the UE 3 may initiate UL transmissions via RACH (S510b).

　　< Satellite Switching: Regenerative Mode >
　　Satellite switching procedure for one or more NTNs in regenerative mode deployment scenarios however are not as simple as that described above with reference to Fig. 5. In transparent mode the same base station 5A-1 is used irrespective of the satellite with which the UE 3 communicates. However, in the regenerative mode at least part of the base station 5A-1 (e.g., a base station distributed unit (DU) 5A_DU-1_Old, 5A_DU-1_New) may be located on the satellite with which the UE 3 communicates (e.g., as is the case in the regenerative mode shown in Fig. 2B). Alternatively, in the regenerative mode the whole of the base station 5A-1_Old, 5A-1_New may be located on the satellite with which the UE 3 communicates (e.g., as is the case in the regenerative mode shown in Fig. 2C).

　　Accordingly, the satellite with which the UE 3 communicates cannot be simply switched without initial access procedures such as RRC signalling, user plane stack reset, and/or physical cell identifier (PCI) changes; rather the UE 3 must undergo an inter-base station handover procedure that allows the UE 3 to take account of changes in protocol variables and status (e.g., DRX timers in MAC, transmission/reception window in RLC, and the like), between the different base stations (or different DUs of a base station) hosted on different satellites.

　　It will therefore be appreciated that during satellite switching procedures for one or more NTNs in regenerative mode deployment scenarios, the UE 3 must undergo a handover/switching procedure from its old non-terrestrial space (or air) borne platform 5C-1_Old to a new non-terrestrial space (or air) borne platform 5C-1_New that takes account of the change in base station (or change in base station DU). It will also be appreciated that to facilitate such a handover/switching procedure, the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New must be able to communicate with each other (directly or indirectly) to exchange information about the UE 3 (e.g., to exchange UE context information, UE configuration information, and the like). Additionally, the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New must be able to communicate with each other to indicate to one another if, and when, a handover/switch is to occur, as well as if, and when, a handover/switch has been successful (or unsuccessful as the case may be).

　　Fig. 6 illustrates communication links (direct and indirect) that may be formed between the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New when in regenerative mode.

　　As is shown in Fig. 6, the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New may communicate with each other (i.e., the old base station 5A-1_Old and the new base station 5A-1_New communicate with each other) directly using an Xn interface over an inter-satellite link (ISL), i.e. option 1 in Fig. 6. However, such ISLs can be unreliable. Typically, handover/switching procedures between one or more satellites 5C, and thus base stations 5A hosted on those satellites 5C, relies on a stable Xn interface between the base stations 5A. In the case of regenerative mode satellites however, it cannot be guaranteed that a stable Xn interface over an ISL between the satellites 5C can be established or maintained.

　　Alternatively, as is shown in Fig. 6, rather than relying on the establishment of ISLs, handover/switching procedures between the base stations 5A hosted on the satellites 5C can be facilitated through a common gateway 5B-1 and/or core network (CN) 7 i.e. option 2 in Fig. 6. For example, the satellites 5C may be able to communicate with one another indirectly, via their respective feeder links and a common gateway (GW) 5B-1 on the ground. However, even in this arrangement, there is no guarantee that feeder links between the GW 5B-1 and both the old and new satellite 5C will be established and operational at the same time.

　　There is thus a risk that, irrespective of the manner in which the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New communicate with each other, the communication link between them may not be established or operational at the time that handover/switching is required, causing a disruption in service.

　　There is therefore a need to provide improved switching procedures between satellites of NTN RANs 5-1 operating in regenerative mode.

　　In a first example in the disclosure below there is provided an improved switching procedure whereby a UE 3, and the satellites 5C /base stations 5A of the NTN RAN 5 are configured so that the UE 3 can re-synchronise itself from an old satellite 5C hosting an old base station 5A (currently connected base station 5A) to a new satellite 5C hosting a new base station 5A. That re-synchronisation may be triggered by the old base station/satellite that the UE 3 is currently in communication with. That re-synchronisation may further involve resetting the MAC, and re-establishing radio link control (RLC) and the packet data convergence protocol (PDCP) layer for all radio bearers (RB). That re-synchronisation may also involve the UE 3 keeping all or most of previous configuration. That re-synchronisation may also involve relocating UE context information at the new base station, from the old base station 5A, via a gateway 5B and/or the CN 7. Beneficially, by allowing the UE 3 to keep all or most of its previous configuration and/or by allowing the relocation of the UE context information at the new base station 5A, the UE 3 may re-synchronise itself to the new base station 5A on the new satellite 5C without having to exchange initial access information about UE contexts and thus RRC-type signalling required to facilitate the switching procedure between the old and new satellite 5C is reduced, and the switching procedure is overall simplified.

　　In another example improved switching procedure a UE 3, and the satellites 5C /base stations 5A of the NTN RAN 5 are configured so that the UE 3 can re-synchronise from an old satellite 5C hosting an old DU 5A_DU of a base station 5A (currently connected DU 5A_DU of the base station 5A) to a new satellite 5C hosting a new DU 5A_DU of the base station 5A. That re-synchronisation may be triggered by the old DU 5A_DU of the base station 5A hosted on the old satellite 5C that the UE 3 is currently in communication with. That re-synchronisation may further involve resetting the MAC, and re-establishing radio link control (RLC) and executing data recovery in the packet data convergence protocol (PDCP) layer for all radio bearers (RB). That re-synchronisation may also involve the UE 3 keeping all or most of previous configuration. That re-synchronisation may also involve relocating UE context information at the new DU 5A_DU of the base station 5A via a gateway 5B and/or the CN 7.

　　Beneficially, by allowing the relocation of the UE context information at the new base station DU 5A_DU, the UE 3 may re-synchronise itself to the new base station DU 5A_DU on the new satellite 5C without having to exchange initial access information about UE contexts and thus RRC-type signalling required to facilitate the switching procedure between the old and new satellite 5C is reduced, and the switching procedure is overall simplified.

　　In another example improved switching procedure, the UE 3 and the base stations 5A of the NTN RAN 5 are configured so that the UE 3 can perform an RRC reestablishment procedure to switch between the old and new satellite 5C based on satellite switching information provided or broadcast to the UE 3 by the old base station 5A hosted on the old satellite 5C. For example, based on that satellite switching information, the UE 3 may be configured to perform a cell selection procedure to select a cell 9 to switch to provided by a new base station 5A hosted on a new satellite 5C. Having selected a cell 9, the UE 3 may perform an RRC reestablishment procedure, which may include the new base station 5A hosted on the new satellite 5C finding/retrieving any necessary UE context information to facilitate the satellite switch.

　　Beneficially, in each of the improved switching procedures described above enhanced/simplified mobility procedures are provided for one or more UEs 3 in RRC connected mode. In those improved switching procedures, the RRC connection of the UEs 3 is switched from an old satellite 5C to a new satellite 5C in regenerative mode with less L3 signalling involvement and/or shorter interruption.

　　In another example improved switching procedure, the UE 3 and the base stations 5A of the NTN RAN 5 are configured to perform a satellite switching procedure for the UE 3 using a proxy base station 5A between the old base station 5A and the new base station 5A hosted on the old and new satellites 5C respectively.

　　In one such example using a proxy base station 5A, the proxy base station 5A may be connected to the old base station 5A on the old satellite 5C and may be configured to receive, from the old base station 5A, handover/switching requests for the UE 3 on behalf of the new satellite 5C. Additionally, the proxy base station 5A may be configured to respond to the handover/switching request, and to provide necessary UE context information/configurations used in the new base station 5A to facilitate the satellite switch.

　　In another example using a proxy base station 5A, the proxy base station 5A may initially be connected to the old base station 5A and may be configured to receive UE context/configuration information from the old base station 5A, such that once the old base station 5A is disconnected from the UE 3, necessary UE context information/configurations can be provided to the new base station 5A on the new satellite 5C by the proxy base station 5A when the UE 3 requests handover/switching to the new base station 5A.

　　In yet another example using a proxy base station 5A, proxy base stations 5A may be provided for both the old base station 5A hosted on the old satellite 5C and the new base station 5A hosted on the new satellite 5C. In such a scenario, the proxy for the new base station 5A may be located in the old base station 5A and may be configured to perform admission control procedures in response to receiving a switching/handover request from the old base station 5A hosted on the satellite 5C. For example, the using a proxy base station 5A may prepare and trigger the handover/switching procedure. At the same time the proxy for the old base station 5A may be located in the old base station 5A and may be configured to receive all UE context information of the UEs 3 connected to the old base station 5A, and thus provide access to the UE context information to the new base station 5A hosted on the new satellite 5C during the handover/switching procedure.

　　Beneficially in each of the improved switching procedures described above, even when end to end Xn interface between an old and a new base station 5A hosted on an old and new satellite 5C respectively is not available, handover preparation and UE context information retrieval for RRC Re-establishment is supported as the proxy base station 5A may prepare the necessary handover messages/triggers and assist in the relocation of necessary UE context information.

　　Each of the improved handover procedures outlined above will now be discussed in more detail with reference to Figs. 7 to 12.

　　< Detailed Description >
　　< Non-Distributed Regenerative Mode >
　　< Satellite Switching with Re-synchronisation >
　　Fig. 7 illustrates a simplified sequence diagram illustrating a first regenerative mode satellite switching procedure that may be used in the communication system 1 of Fig. 1.

　　As shown in Fig. 7, the first regenerative mode satellite handover procedure relates to satellites that host the whole base station 5A-1 onboard such as in Fig. 2C.

　　As shown in Fig. 7, initially data transmissions S702a may occur between the UE 3 and an old (e.g., source) base station 5A-1_Old hosted by the old non-terrestrial space (or air) borne platform (e.g. satellite) 5C-1_Old. At the same time data transmissions S702b may occur between the old base station 5A-1_Old and GW 5B-1 of the NTN RAN 5-1 to allow communication between the old base station 5A-1_Old and the CN 7.

　　At step S704, the old base station 5A-1_Old hosted by the old non-terrestrial space (or air) borne platform 5C-1_Old transmits appropriate system information message with respect to the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New (e.g., a SIB19 message) to the UE 3. The system information message may contain information about the new non-terrestrial space (or air) borne platform 5C-1_New and the new base station 5A-1_New to which the UE 3 is to switch.

　　For example, the system information message may contain a switch service time (e.g., T-service), the stop service timing of the old satellite 5C-1_Old, and optionally the start service timing of the new satellite 5C-1_New (e.g., T-start). It will be appreciated that where the switch to be performed is a hard switch, the start service timing of the new satellite 5C-1_New (e.g., T-start) may not be provided in the system information message as T-start = T-service. Furthermore, it will be appreciated the absence of a start service timing of the new satellite 5C-1_New (e.g., T-start) in the system information message may also be an implicit indication that a hard switch is to be performed.

　　The system information message may also include appropriate cell information for the new cell 9 that is to be provided for the UE 3 by the new base station 5A-1_New (e.g., cell identifiers, and the like).

　　Additionally, or alternatively the system information message may include an indication that a satellite switch is to be performed to switch the satellite 5C-1 in communication with the UE 3 from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New. For example, the old non-terrestrial space (or air) borne platform 5C-1_Old may transmit, to the UE 3, an indication that a satellite switch procedure using re-establishment relevant information is to be performed. Re-establishment relevant information may, by way of example only, include appropriate information to identify the new non-terrestrial space (or air) borne platform 5C-1_New, new base station 5A-1_New configuration information, satellite switching time information (e.g., a time window and/or a time point at which the switch is to occur), and the like. Additionally, the old base station 5A-1_Old may transmit other appropriate information to the UE 3 at step S704 to assist with the satellite switching procedure. For example, the old base station 5A-1_Old may transmit, to the UE 3, information relating to the type of satellite switch to be performed which may depend on the NTN payload type being implemented; for example, the information may indicate whether the switch is between two base stations 5A-1 hosted on two different satellites 5C-1, or two distributed units of a single base station 5A-1 hosted on two different satellites 5C-1.

　　Additionally, or alternatively, the old base station 5A-1_Old may transmit, to the UE 3, information relating to the frequency of a new cell to be provided by the new base station 5A-1_New via the new non-terrestrial space (or air) borne platform 5C-1_New. The old base station 5A-1_Old may also indicate the physical cell identifier (PCI) and other appropriate cell ID information to the UE 3 for the new cell to be provided by the new base station 5A-1_New.

　　Additionally, or alternatively, the old base station 5A-1_Old may transmit, to the UE 3, information relating to a common configuration of a target cell to be provided by the new base station 5A-1_New via the new non-terrestrial space (or air) borne platform 5C-1_New (e.g., ServingCellConfigCommon). The old base station 5A-1_Old may also transmit, to the UE 3, appropriate key update associated information, and the like (e.g., keySetChangeIndicator and/or nextHopChainingCount).

　　Furthermore, it will be appreciated that for Rel-18 type UEs 3 or UEs 3 that only support satellite switching with re-synchronisation (i.e., UEs 3 that only support communications with satellites in transparent mode) backward compatibility should ideally be supported so that if such UEs 3 are present in NTN RANs 5-1 in regenerative mode they do not erroneously trigger satellite switching with re-synchronisation which would not be appropriate. Accordingly, the old base station 5A-1_Old may beneficially transmit appropriate information (e.g., new information elementsSatSwitchwithgNBonboard/SatSwitchwithgNB-Duonboard/SatswitchwithReestablishement2, or the like) to the UE 3 at step S704 to indicate that the NTN RAN 5-1 is in regenerative mode, and that switching with re-synchronization should not be attempted by the UE 3.

　　Additionally, or alternatively, the old base station 5A-1_Old may transmit an appropriate trigger to the UE 3 to trigger a specific behaviour of the UE 3 necessary to facilitate the switch from the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New. For example, the old base station 5A-1_Old may transmit different triggers to the UE 3 depending on whether a) the NTN RAN 5-1 is in transparent or regenerative mode, and b) whether the UE 3 supports satellite switching where the satellites 5C-1 are in regenerative mode.

　　All information transmitted to the UE 3 at step S704 described above may be delivered to the UE 3 via any appropriate type of broadcast message or dedicate message (e.g., an RRC message, or the like). Alternatively, the information transmitted to the UE 3 at step S704 may be transmitted to the UE 3 via a mix of both broadcast messages and dedicated signalling. For example, security-based information may be transmitted to the UE 3 via dedicated signalling, while other types of information are transmitted to the UE 3 via broadcast signalling.

　　Apart from the information transmitted to the UE 3 at step S704 described above, all other aspects of the configuration of the UE 3 may remain unchanged such that they are the same before and after satellite switch. For example, some UE 3 configuration parameters (e.g., C-RNTI, radioBearerConfig, measConfig, and the like) may remain unchanged after satellite switch.

　　At step S706a, the old base station 5A-1_Old transmits context information of UE 3 to the GW 5B-1 for transfer to the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New. Specifically, in Fig. 7, the transfer of the context information for the UE 3 to the new base station 5A-1_New on the new non-terrestrial space (or air) borne platform 5C-1_New is performed indirectly via the GW 5B-1/CN 7. For example, the old base station 5A-1_Old transmits that context information of UE 3 to the CN 7 via the GW 5B-1. Having received the context information of UE 3, the CN 7 then forwards the context information to the new base station 5A-1_New on the new non-terrestrial space (or air) borne platform 5C_New via the GW 5B-1 to relocate the context information of UE 3 at the new non-terrestrial space (or air) borne platform 5C-1_New.

　　Nevertheless, it will be appreciated that where there is an Xn interface between the old base station 5A-1_Old and the new base station 5A-1_New, the context information of UE 3 may be transmitted by the old base station 5A-1_Old directly to the new base station 5A-1_New without having to go via the CN 7.

　　The context information may comprise context information for all UEs 3 that are to be switched from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New. For example, the context information transmissions may comprise individual context information messages transmitted individually for each UE 3. Alternatively, a single appropriate context relocation message may be transmitted from the old base station 5A-1_Old to the new base station 5A-1_New that contains appropriate context information for all UEs 3 collectively that are being switched from old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New.

　　In an example, the UE context relocation procedure at steps S706a and S706b may be carried out using an existing Next-Generation Application Protocol (NGAP) handover procedure. For example, existing NGAP handover required messages may be used to transmit the UE context information from the old base station 5A-1_Old hosted on the old non-terrestrial space (or air) borne platform 5C-1_Old to the CN 7. In response, the CN 7 may use existing handover request messages to transmit the UE context information from the CN 7 to the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New.

　　The handover request message and/or the handover required message (e.g., RRC messages) may include the context information of the UEs 3 itself, or alternatively the context information of the UEs 3 may be transmitted subsequent to the handover request message in another appropriate message. The handover required message and/or the handover request message may also include an indication that the context information of the UEs 3 is to be transmitted to, and stored in, the CN 7 initially until a feeder link switch time when the context information of the UEs 3 can be forwarded to the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New (e.g., if the feeder link between the GW 5B-1 and the new non-terrestrial space (or air) borne platform 5C-1_New is not yet available).

　　Alternatively, in another example the UE context relocation procedure at steps S706a and S706b may be triggered using new NGAP procedures. For example, rather than using pre-existing messages, new messages may be implemented such as a UE context relocation required message and a UE context relocation request message. The UE context relocation required message may be transmitted by the old base station 5A-1_Old hosted on the old non-terrestrial space (or air) borne platform 5C-1_Old to the CN 7, and the UE context relocation request message may be transmitted by the CN 7 to the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New.

　　The UE context relocation request message and/or the UE context relocation required message may include the context information of the UEs 3 itself, or alternatively the context information of the UEs 3 may be transmitted subsequent to the handover request message in another appropriate message. The UE context relocation request message and/or the UE context relocation required message may also include an indication that the context information of the UEs 3 is to be transmitted to, and stored in, the CN 7 initially until a feeder link switch time when the context information of UEs 3 is forwarded to the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New (e.g., if the feeder link between the GW 5B-1 and the new non-terrestrial space (or air) borne platform 5C-1_New is not yet available).

　　The UE context information included in the UE context relocation request message and/or the UE context relocation required message may be identical (or at least similar) to the pre-existing 'source NG-RAN node-to-target NG-RAN node transparent container' IE in existing NGAP procedures.

　　Alternatively, as already alluded to above, if an inter-satellite link (ISL) exists between the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New, the UE context relocation procedure at steps S706a and S706b may be performed over that ISL. For example, the UE context information may be transmitted directly from the old base station 5A-1_Old hosted on the old non-terrestrial space (or air) borne platform 5C-1_Old to the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New, i.e. over an Xn interface between the old base station 5A-1_Old and the new base station 5A-1_New. In this scenario the UE context relocation may use existing Xn Application Protocol (XnAP) handover procedures. For example, the UE context may be transmitted between the respective base stations 5A-1 on the old and new satellite 5C-1 via an appropriate message using the existing XnAP handover procedures.

　　Alternatively, in another example, if an inter-satellite link (ISL) exists between the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New, the UE context relocation procedure at steps S706a and S706b may be performed over that ISL using new XnAP procedures. For example, rather than using pre-existing messages, new messages may be implemented such as a UE context relocation request message. The UE context information may be included in the UE context relocation request message. The UE context information included in the UE context relocation request message may be identical (or at least similar) to the pre-existing UE context information IEs in the handover request messages used in existing XnAP handover procedures.

　　At step S708a, optionally, the old base station 5A-1_Old may transmit sequence number (SN) status information and buffered data held at the old base station 5A-1_Old hosted on the old non-terrestrial space (or air) borne platform 5C-1_Old to CN 7 (via the GW 5B-1) in an appropriate message. For example, the SN status information and buffered data may be transmitted to the CN 7 in a SN Status Transfer Message to convey the uplink packet data convergence protocol (PDCP) SN receiver status and the downlink PDCP SN transmitter status of data radio bearers (DRBs) for which PDCP status preservation applies i.e. for radio link control acknowledgement mode (RLC-AM).

　　The SN status information and buffered data transmitted to CN 7 may comprise the SN status information and buffered data of all UEs 3 that are to be switched from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New. That SN status information and buffered data may be transmitted individually for each UE 3 i.e., a separate appropriate SN Status Transfer Message is transmitted from the old base station 5A-1_Old to the CN 7 for each individual UE 3 that is being switched from old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New. Alternatively, a single appropriate SN Status Transfer Message may be transmitted from the old base station 5A-1_Old to the CN 7 that contains appropriate SN status information and buffered data for all UEs 3.

　　At step S708b, having received the SN status information and buffered data, the CN 7 forwards the SN status information and buffered data to the new base station 5A-1_New to relocate the SN status information and buffered data at the new non-terrestrial space (or air) borne platform 5C-1_New.

　　Although the relocation of the UE context information at steps S706a-S706b and the relocation of the SN status information and buffered data at steps S708a-S708b are described above as occurring at a time when the satellite switch is occurring, it will be appreciated that both relocations may alternatively occur in advance of any switch. For example, the relocations may occur in anticipation of the switch occurring between the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New. Alternatively, or additionally, the UE context information, the SN status information, and the buffered data may be provided to the new base station 5A-1_New hosted by the new non-terrestrial space (or air) borne platform 5C-1_New at some advanced time (even if it is not clear that a switch will be performed) and/or may be updated periodically to reduce the amount of signalling required during the actual switch procedur.

　　Although Fig. 7 suggests that the relocation of the UE context information at steps S706a-S706b and the relocation of the SN status information and buffered data at steps S708a-S708b occurs sequentially in that order, it will nevertheless be appreciated that there is no fixed order for the transmission of the UE context relocation and SN status transfer, and that both the UE context relocation and SN status transfer may be performed simultaneously either within one message, or different messages, or where they occur separately, the SN status transfer may occur prior to the UE context relocation.

　　Although Fig. 7 suggests that the relocation of the UE context information at steps S706a-S706b occurs once the system information is sent at step S704 to the UE 3 from the old base station 5A-1_Old, it will nevertheless be appreciated that the UE context information may be transmitted to the GW 5B-1 and/or the CN 7 in advance of the switch time. For example, the UE context information may be transmitted to the CN 7 in advance to avoid high amounts of data transfer and data load over the feeder links during the switching procedure.

　　At step S710, which may occur as the same time as steps S706 and S708, or alternatively, may occur at a separate time, the UE 3 performs a re-synchronisation procedure to the new base station 5A-1_New hosted by the new non-terrestrial space (or air) borne platform 5C-1_New. For example, having received, at step S704, appropriate system information from the old base station 5A-1_Old hosted by the old non-terrestrial space (or air) borne platform 5C-1_Old, the UE 3 may initiate a re-synchronisation procedure, using the system information, to re-synchronise to the new base station 5A-1_New hosted by the new non-terrestrial space (or air) borne platform 5C-1_New, which may, by way of example include re-synchronising to the downlink (DL) of the new cell 9-2 indicated as being provided by the new base station 5A-1_New in the system information.

　　In one example, at step S710, the UE 3 may be triggered, upon reaching an indicated time indicated in the appropriate system information (e.g., T-service and/or T-switch) at step S704, to consider that its UL synchronisation with the old base station 5A-1_Old on the old non-terrestrial space (or air) borne platform 5C-1_Old is lost, and all UL transmissions to the old base station 5A-1_Old on the old non-terrestrial space (or air) borne platform 5C-1_Old may be suspended. Subsequently, the UE 3 may trigger re-synchronisation to the DL of a new serving cell 9-2 that is served by the new non-terrestrial space (or air) borne platform 5C-1_New as indicated in the appropriate system information about the new non-terrestrial space (or air) borne platform 5C-1_New / new base station 5A-1_New that it received at step S704. Having re-synchronised to the DL of the new serving cell 9-2, the UE 3 may subsequently assume that UL synchronisation is established with the new base station 5A-1_New hosted by the new non-terrestrial space (or air) borne platform 5C-1_New.

　　Once synchronisation with both the DL and UL of the new serving cell 9-2 is established, the UE 3 may reset its MAC, re-establish radio link control (RLC) entities, and re-establish packet data convergent protocol (PDCP) entities for radio bearers (RBs) for transmissions to and from the UE 3 to the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New. Once those MAC/RLC/PDCP entities are established, UL transmissions from the UE 3 are resumed, and data transmissions S712a may occur between the UE 3 and the new base station 5A-1_New hosted by the new non-terrestrial space (or air) borne platform 5C-1_New. At the same time data transmissions S712b may occur between the new base station 5A-1_New hosted by the new non-terrestrial space (or air) borne platform 5C-1_New and GW 5B-1 of the NTN RAN 5-1.

　　It will be appreciated that to ensure compatibility between an old cell and a new cell for use by the UE 3 before and after the above satellite switching procedure, the old base station 5A-1_Old may be made aware of the capabilities and/or configuration of the new base station 5A-1_New, along with any capability and/or configuration restrictions of the new base station 5A-1_New, via appropriate signalling between the old and new base station 5A-1_Old, 5A-1_New (e.g., signalling over an Xn/F1 interface, or indirectly via GW 5B-1 / CN 7). To that end, the old base station 5A-1_Old may be responsible for ensuring that configurations of UEs 3 before satellite switch are compatible with the capabilities and/or configuration of the new base station 5A-1_New, along with any capability and/or configuration restrictions of the new base station 5A-1_New.

　　It will be appreciated that the capability and application layer configuration of the old and new base station 5A-1_Old, 5A-1_New hosted on the old and new non-terrestrial space (or air) borne platform 5C-1_Old, 5C-1_New respectively may be highly aligned such that the old and new base station 5A-1_Old, 5A-1_New can continue the same service to all the UEs 3 with the same configuration after completion of the switching procedure. In this scenario, beneficially, admission control procedures can be skipped and no, or little, configuration changes are required to switch the UEs 3 from the old base station 5A-1_Old to the new base station 5A-1_New. If, however the capability and application layer configuration of the old and new base station 5A-1_Old, 5A-1_New are not highly aligned, then the handover of the UE 3 from the old non-terrestrial space (or air) borne platform 5C-1_Old, to the new non-terrestrial space (or air) borne platform 5C-1_New may be carried out using a typical legacy handover procedures e.g., normal/enhanced handover, conditional handover (CHO), and/or, RRC re-establishment procedures.

　　Fig. 8 illustrates a simplified sequence diagram illustrating a second regenerative mode satellite switching procedure that may be used in the communication system 1 of Fig. 1.

　　As shown in Fig. 8, the second regenerative mode satellite switch procedure relates to satellites 5C-1 that host the distributed unit of base station 5A-1 onboard such as in Fig. 2B. As shown in Fig. 8, initially data transmissions S802a may occur between the UE 3 and an old base station DU 5A_DU-1_Old hosted by hosted by the old non-terrestrial space (or air) borne platform 5C-1_Old. At the same time data transmissions S802b may occur between the old base station DU 5A_DU-_Old and with an associated base station CU 5A_CU-1 via GW 5B-1 of the NTN RAN 5-1 to allow communication between the old base station DU 5A_DU-1_Old and the CN 7.

　　At step S804, the old base station DU 5A_DU-1 hosted by the old non-terrestrial space (or air) borne platform 5C-1_Old, which hosts, transmits appropriate system information (e.g., SIB19) to the UE 3 about the new non-terrestrial space (or air) borne platform 5C-1_New. The system information message may contain information about the new non-terrestrial space (or air) borne platform 5C-1_New and a new base station DU 5A_DU-1_New to which the UE 3 is to switch.

　　The system information message may also include appropriate cell information for the new cell 9-2 that is to be provided for the UE 3 by the new base station DU 5A_DU-1_New (e.g., cell identifiers, and the like).

　　Additionally, or alternatively the system information message may include an indication that a particular satellite switch procedure is to be performed by UE 3 upon switching the satellite 5C-1 from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New. For example, the old base station DU 5A_DU-1_Old may transmit, to the UE 3, an indication that a satellite switch procedure using re-establishment relevant information is to be performed. Re-establishment relevant information may, by way of example only, include appropriate information to identify the new non-terrestrial space (or air) borne platform 5C-1_New, new base station configuration information, satellite switching time information (e.g., a time window and/or a time point at which the switch is to occur), and the like. Additionally, the old base station DU 5A_DU-1_Old may transmit other appropriate information to the UE 3 at step S804 to assist with the satellite switching procedure. For example, the old base station DU 5A_DU-1_Old may transmit, to the UE 3, information relating to the type of satellite switch to be performed which may depend on the NTN payload type being implemented; for example, the information may indicate whether the switch is between two base stations 5A-1 hosted on two different satellites 5C-1, or two distributed units 5A_DU-1 of a single base station 5A-1 hosted on two different satellites 5C-1.

　　Additionally, or alternatively, the old base station DU 5A_DU-1_Old may transmit, to the UE 3, information relating to the frequency of a target cell 9-2 to be provided by the new base station DU 5A_DU-1_New via the new non-terrestrial space (or air) borne platform 5C-1_New. The old base station DU 5A_DU-1_Old may also indicate the physical cell identifier (PCI) and other appropriate cell ID information to the UE 3 for the target cell 9-2 to be provided by the new base station DU 5A_DU-1_New.

　　Additionally, or alternatively, the old base station DU 5A_DU-1_Old may transmit, to the UE 3, information relating to a common configuration of a target cell 9-2 to be provided by the new base station DU 5A_DU-1_New via the new non-terrestrial space (or air) borne platform 5C-1_New (e.g., ServingCellConfigCommon). The old base station DU 5A_DU-1_Old may also transmit, to the UE 3, appropriate key update associated information, and the like (e.g., keySetChangeIndicator and/or nextHopChainingCount).

　　Furthermore, it will be appreciated that for Rel-18 type UEs 3 or UEs 3 that only support satellite switching with re-synchronisation (i.e., UEs 3 that only perform re-synchronisation to the cell 9-2 by the new satellite 5C-1, which is defined for transparent mode), backward compatibility should ideally be supported so that if such UEs 3 are present in NTN RANs 5-1 in regenerative mode they do not erroneously trigger satellite switching with re-synchronisation which would not be appropriate. Accordingly, the old base station DU 5A_DU-1_Old may transmit appropriate information differently (e.g., using new information elements name SatSwitchwithgNBonboard/SatSwitchwithgNB-Duonboard/SatswitchwithReestablishement2, or the like) to the UE 3 at step S804 to differentiate satellite switch in regenerative mode from satellite switch in transparent mode, and that switching with re-synchronization should not be attempted by the UE 3.

　　Additionally, or alternatively, the old base station DU 5A_DU-1_Old may transmit an appropriate trigger to the UE 3 to trigger a specific behaviour of the UE 3 necessary to facilitate the switch from the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New. For example, the old base station DU 5A_DU-1_Old may transmit different triggers to the UE 3 depending on: a) whether the NTN RAN 5-1 is in transparent or regenerative mode, and b) whether the UE 3 supports satellite switching where the satellites 5C-1 are in regenerative mode.

　　All information transmitted to the UE 3 at step S804 described above may be delivered to the UE 3 via any appropriate type of broadcast message or dedicate message (e.g., an RRC message, or the like). Alternatively, the information transmitted to the UE 3 at step S804 may be transmitted to the UE 3 via a mix of both broadcast messages and dedicated signalling. For example, security-based information may be transmitted to the UE 3 via dedicated signalling, while other types of information are transmitted to the UE 3 via broadcast signalling.

　　Apart from the information transmitted to the UE 3 at step S804 described above, all other aspects of the configuration of the UE 3 may remain unchanged such that they are the same before and after satellite switch. For example, some UE configuration parameters (e.g., C-RNTI, radioBearerConfig, measConfig, and the like) may remain unchanged after satellite switch.

　　At step S806a the old base station DU 5A_DU-1_Old transmits context information of UE 3 to the base station CU 5A_CU-1, via the GW 5B-1, for transfer to the new base station DU 5A_DU-1_New. Specifically, in Fig. 8, the transfer of the context information of UE 3 to the new base station DU 5A_DU-1_New is performed indirectly via the GW 5B-1 / base station CU 5A_CU-1. For example, the old base station DU 5A_DU-1_Old transmits that context information of UE 3 to the base station CU 5A_CU-1 via the GW 5B-1. Having received the context information of UE 3, the base station CU 5A_DU-1 then forwards the context information to the new base station DU 5A_DU-1_New via the GW 5B-1 to relocate the context information of UE 3 at the new base station DU 5A_DU-1_New.

　　The context information may comprise context information for all UEs 3 that are to be switched from the old base station DU 5A_DU-1_Old to the new base station DU 5A_DU-1_New. For example, the context information transmissions may comprise individual context information messages transmitted individually for each UE 3. Alternatively, a single appropriate context relocation message may be transmitted from the old base station DU 5A_DU-1_Old to the new base station DU 5A_DU-1_New that contains appropriate context information for all UEs 3 collectively that are being switched from old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New.

　　In an example, the UE 3 context relocation procedure at steps S806a and S806b may be carried out using an existing Next-Generation Application Protocol (NGAP) handover procedure. For example, existing NGAP handover required messages may be used to transmit the UE context information from the old base station DU 5A_DU-1_Old hosted on the old non-terrestrial space (or air) borne platform 5C-1_Old to the base station CU 5A_CU-1. In response, the base station CU 5A_CU-1 may use existing handover request messages to transmit the UE context information from the base station CU 5A_CU-1 to the new base station DU 5A_DU-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New.

　　The handover request message and/or the handover required message (e.g., RRC messages) may include the context information of the UEs 3 itself, or alternatively the context information of the UEs 3 may be transmitted subsequent to the handover request message in another appropriate message. The handover required message and/or the handover request message may also include an indication that the context information of the UEs 3 is to be transmitted to, and stored in, the base station CU 5A_CU-1 initially until a feeder link switch time when the context information of the UEs 3 can be forwarded to the new base station DU 5A_DU-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New (e.g., if the feeder link between the GW 5B-1 and the new non-terrestrial space (or air) borne platform 5C-1_New is not yet available).

　　Alternatively, in another example the UE context relocation procedure at steps S806a and S806b may be triggered using new NGAP procedures. For example, rather than using pre-existing messages, new messages may be implemented such as a UE context relocation required message and a UE context relocation request message. The UE context relocation required message may be transmitted by the old base station DU 5A_DU-1_Old hosted on the old non-terrestrial space (or air) borne platform 5C-1_Old to the base station CU 5A_CU-1, and the UE context relocation request message may be transmitted by the base station CU 5A_CU-1 to the new base station DU 5A_DU-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New.

　　The UE context relocation request message and/or the UE context relocation required message may include the context information of the UEs 3 itself, or alternatively the context information of the UEs 3 may be transmitted subsequent to the handover request message in another appropriate message. The UE context relocation request message and/or the UE context relocation required message may also include an indication that the context information of the UEs 3 is to be transmitted to, and stored in, the base station CU 5A_CU-1 initially until a feeder link switch time when the context information of the UEs 3 is forwarded to the new base station DU 5A_DU-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New (e.g., if the feeder link between the GW 5B-1 and the new non-terrestrial space (or air) borne platform 5C-1_New is not yet available).

　　At step S808, optionally, the old base station DU 5A_DU-1_Old may transmit downlink data delivery status frames to the base station CU 5A_CU-1 via the GW 5B-1 in an appropriate message to inform the base station CU 5A_CU-1 of any downlink data from the old base station DU 5A_DU-1_Old that has not been successfully transmitted to the UE 3.

　　The downlink data delivery status frames sent via the GW 5B-1 to the base station CU 5A_CU-1 may comprise downlink data delivery status frames of all UEs 3 that are to be switched from the old base station DU 5A_DU-1_Old to the new base station DU 5A_DU-1_New. Those downlink data delivery status frames may be transmitted individually for each UE 3 i.e., separate appropriate downlink data delivery status frames are transmitted from the old base station DU 5A_DU-1_Old via the GW 5B-1 to the base station CU 5A_CU-1 for each individual UE 3 that is being switched from old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New. Alternatively, downlink data delivery status frames may be transmitted from the old base station DU 5A_DU-1_Old via the GW 5B-1 to the base station CU 5A_CU-1 that contains appropriate downlink data delivery status frames for all UEs 3.

　　At step S810, which may occur as the same time as steps S806 and S808, or alternatively, may occur at a separate time, the UE 3 performs a re-synchronisation procedure to the new base station DU 5A_DU-1_New. For example, having received, at step S804, appropriate system information from the old base station DU 5A_DU-1_Old, the UE 3 may initiate a re-synchronisation procedure, using the system information, to re-synchronise to the new base station DU 5A_DU-1_New, which may, by way of example include re-synchronising to the downlink (DL) of the new cell 9-2 indicated as being provided by the new base station DU 5A_DU-1_New in the system information.

　　In one example, at step S810, the UE 3 may be triggered, upon reaching an indicated time indicated in the appropriate system information (e.g., T-service and/or T-switch) at step S804, to consider that its UL synchronisation with the old base station DU 5A_DU-1_Old is lost, and all UL transmissions to the old base station DU 5A_DU-1_Old may be suspended. Subsequently, the UE 3 may trigger re-synchronisation to the DL of a new serving cell that is served by the new base station DU 5A_DU-1_New as indicated in the appropriate system information about the new non-terrestrial space (or air) borne platform 5C-1_New / new base station DU 5A_DU-1_New that it received from the old base station DU 5A_DU-1_Old at step S804. Having re-synchronised to the DL of the new serving cell 9-2, the UE 3 may subsequently assume that UL synchronisation is established with the new base station DU 5A_DU-1_New at some future pre-set time T after the re-synchronisation to the DL of the new serving cell 9-2.

　　Once synchronisation with both the DL and UL of the new serving cell 9-2 is established, the UE 3 may reset its MAC, re-establish radio link control (RLC), and re-establish packet data convergent protocol (PDCP) entities for radio bearers (RBs) for transmissions to and from the UE 3 to the new base station DU 5A_DU-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New. Once those PDCP entities are established, UL transmissions from the UE 3 are resumed, and data transmissions S812a may occur between the UE 3 and the new base station DU 5A_DU-1_New. At the same time data transmissions S812b may occur between the new base station DU 5A_DU-1_New and the base station CU 5A_CU-1 via the GW 5B-1 of the NTN RAN 5-1 to allow communication between the new base station DU 5A_DU-1_New and the CN 7.

　　It will be appreciated that to ensure compatibility between an old cell and a new cell 9-2 for use by the UE 3 before and after the above satellite switching procedure, the old base station DU 5A_DU-1_Old may be made aware of the capabilities and/or configuration of the new base station DU 5A_DU-1_New, along with any capability and/or configuration restrictions of the new base station DU 5A_DU-1_New, via appropriate signalling between the old and new base station DUs 5A_DU-1_Old, 5A_DU-1_New (e.g., signalling over an Xn/F1 interface, or indirectly via GW 5B-1 / the base station CU 5A_CU-1). To that end, the old base station DU 5A_DU-1_Old may be responsible for ensuring that configurations of UEs 3 before satellite switch are compatible with the capabilities and/or configuration of the new base station DU 5A_DU-1_New, along with any capability and/or configuration restrictions of the new base station DU 5A_DU-1_New.

　　It will be appreciated that the capability and application layer configuration of the old and new base station DUs 5A_DU-1_Old, 5A_DU-1_New hosted on the old and new non-terrestrial space (or air) borne platform 5C-1_Old, 5C-1_New respectively may be highly aligned such that the old and new base station DUs 5A_DU-1_Old, 5A_DU-1_New can continue the same service to all the UEs 3 with the same configuration after completion of the switching procedure. In this scenario, beneficially, admission control procedures can be skipped and no, or little, configuration changes are required to switch the UEs 3 from the old base station DU 5A_DU-1_Old to the new base station DU 5A_DU-1_New. If, however the capability and application layer configuration of the new and old base station DUs 5A_DU-1_Old, 5A_DU-1_New are not highly aligned, then the handover of the UE 3 from the old base station DU 5A_DU-1_Old, to the new base station DU 5A_DU-1_New may be carried out using a typical legacy handover procedures e.g., normal/enhanced handover, conditional handover (CHO), and/or, RRC re-establishment procedures.

　　< Satellite Switching Enhanced RRC Re-establishment Procedures >
　　Fig. 9 illustrates a simplified sequence diagram illustrating a third regenerative mode satellite switching procedure that may be used in the communication system 1 of Fig. 1.

　　As shown in Fig. 9, the third regenerative mode satellite switching procedure is described with reference to satellites 5C-1 that host base station DUs 5A_DU-1 onboard such as in Fig. 2B. Nevertheless, it will be appreciated that the third regenerative mode satellite switching procedure that will now be described may be implemented in a non-distributed base station architecture.

　　As shown in Fig. 9, at step S902 the old base station DU 5A_DU-1_Old may provide information to the UE 3 (e.g., satellite switching information) necessary for switching the satellite 5C-1 in communication with the UE 3. It will be appreciated that the satellite switching information may be provided to the UE 3 in the form of broadcast transmissions that occur periodically to broadcast information from the old base station DU 5A_DU-1_Old to the UE 3. Alternatively, the satellite switching information may be provided in appropriate RRC messages (e.g., an RRC Connection (Re)configuration message).

　　The satellite switching information may include appropriate timing information such as a satellite start switch time (T-start), and/or a satellite service time (T-service). The time T-start may indicate to the UE 3 a time that a satellite switch procedure is to be initiated, while the time T-service may indicate to the UE 3 a time at which service from a new base station DU 5A_DU-1_New may begin.

　　Additionally, or alternatively, the satellite switching information may include appropriate information about the new non-terrestrial space (or air) borne platform 5C-1_New and/or the new base station DU 5A_DU-1_New to which the UE 3 is to switch. The satellite switching information may also include new cell identification information that may be used to indicate to the UE 3 one or more target cells 9-2 provided by the new base station DU 5A_DU-1_New to which the UE 3 is to switch (e.g., a physical cell identifier of the one or more target cells 9-2 and/or a frequency of the one or more target cells 9-2). The satellite switching information may also include a common cell configuration, or the like, that contains common configuration information for the one or more target cells 9-2 provided by the new base station DU 5A_DU-1_New to which the UE 3 is to switch.

　　Additionally, or alternatively, the satellite switching information may also include appropriate backoff information to enable the UE 3 to perform a backoff procedure as it switches from the old base station DU 5A_DU-1_Old to the new base station DU 5A_DU-1_New.
Additionally, or alternatively, the satellite switching information may also include a (shared) configured grant to be used for initial access to the new base station DU 5A_DU-1_New to which the UE 3 is to switch. For example, the satellite switching information may include a (shared) configured grant to be used by the UE 3 to allow the UE 3 to skip a full RACH procedure when attempting initial access to the new base station DU 5A_DU-1_New to which the UE 3 is to switch, and to allow initial access to be achieved through the use of appropriate RRC messages (e.g., RRC re-establishment request messages).

　　It will be appreciated that the satellite switching information may be sent by the old base station DU 5A_DU-1_Old to the UE 3 in any appropriate broadcast type message. For example, the satellite switching information may be sent in any appropriate pre-exiting broadcast message, or alternatively new broadcast message with appropriate information elements (IE) for the different pieces of satellite switching information. Alternatively, the satellite switching information may be sent in another pre-existing message (e.g., an RRC (Re)configuration message).

　　Whether a new message or a pre-existing message is used, all (or most) of the satellite switching information may be grouped together and included within one IE (e.g., SatSwitchWithReestablish) whose inclusion in the message may implicitly indicate that satellite swich is to be performed with an enhanced RRC Re-establishment procedure as described in further detail below.

　　As shown in Fig. 9, data transmissions S904a may occur between the UE 3 and the old base station DU 5A_DU-1_Old. At the same time data transmissions S904b may occur between the old base station DU 5A_DU-1_Old and GW 5B-1 of the NTN RAN 5-1 to allow communication between the old base station DU 5A_DU-1_Old and the base station CU 5A_CU-1.

　　At step S906, RRC connection reestablishment is triggered to perform RRC connection reestablishment between the UE 3 and the new base station DU 5A_DU-1_New indicated to the UE 3 at step S902 in the satellite switching information sent by the old base station DU 5A_DU-1_Old to the UE 3.

　　By way of example only, RRC connection reestablishment may be triggered if:
a) all (or most) of the satellite switching information sent by the old base station DU 5A_DU-1_Old to the UE 3 at step S902 is grouped together and included within one IE (e.g., SatSwitchWithReestablish) whose inclusion in the information message implicitly indicates that satellite switch is to be performed with an enhanced RRC Re-establishment procedure; and
b) the time reaches the time T-start or T-service indicated in the satellite switching information sent by the base station DU 5A_DU-1_Old to the UE 3 at step S902.

　　At step S908, the UE 3 uses the satellite switching information sent by the old base station DU 5A_DU-1_Old to the UE 3 at step S902, to select a new cell 9-2 provide by a new base station DU 5A_DU-1_New to which the UE 3 can switch. For example, the satellite switching information may include appropriate target (i.e., new) cell identification information that may be used to indicate to the UE 3 one or more target cells 9 provided by a new base station DU 5A_DU-1_New to which the UE 3 is to switch (e.g., a physical cell identifier of the one or more new cells 9 and/or a frequency of the one or more new cells 9). Using that new cell identification information, the UE 3 may select one of the new cells 9 to which it will switch.

　　Furthermore, at step S908, the UE 3 may execute a cell selection procedure by leveraging the cell information indicated in the IE SatSwitchWithReestablish for cell selection, i.e., the UE 3 scan the one or more cells 9 indicated in the IE SatSwitchWithReestablish first, If, having detected the one or more cells 9 indicated in the IE SatSwitchWithReestablish, the UE 3 selects a cell 9-2 to switch to.

　　If the UE 3 is not able to detect the one or more cells 9 indicated in the IE SatSwitchWithReestablish, then UE 3 will further scan any other cells 9 in the vicinity of the UE 3 to attempt to identify a cell 9 to which the UE 3 should switch. UE 3, having selected a cell 9-2, initiates the RRC connection reestablishment procedure at step S910.

　　At step S910, the UE 3 transmits an RRC connection reestablishment request message to the new base station DU 5A_DU-1_New that provides the selected cell 9-2. It will be appreciated that the UE 3 may transmit the RRC connection reestablishment request message as soon as possible (e.g., as soon as the cell selection procedure at step S908 is completed). Alternatively, the UE 3 may apply a random backoff timer and the UE 3 may only send the RRC connection reestablishment request message at the expiry of the random backoff timer.

　　Beneficially, the provision of the random backoff timer may help to minimise signalling overhead at any given time, especially where there are multiple UEs 3 that need to switch from the old base station DU 5A_DU-1_Old to the new base station DU 5A_DU-1_New.

　　At step S912a the new base station DU 5A_DU-1_New performs any appropriate procedure and/or signalling necessary to obtain/retrieve context information of the UE 3. For example, the new base station DU 5A_DU-1_New may retrieve UE context information from the RRC connection reestablishment request message sent by the UE 3 to the new base station DU 5A_DU-1_New if the context information is contained within the RRC connection reestablishment request message.

　　In another example, the new base station DU 5A_DU-1_New may retrieve UE context information by signalling to the UE 3 to send its context information to the new base station DU 5A_DU-1_New if the RRC connection reestablishment request message does not contain the UE context information but indicates the UE 3.

　　In yet another example, the new base station DU 5A_DU-1_New may retrieve UE context information by signalling to base station CU 5A_CU-1 to send the UE context information to the new base station DU 5A_DU-1_New. This may occur, for example, if the RRC connection reestablishment request message does not contain the UE context information, and the base station CU 5A_CU-1 stores the appropriate UE context information.

　　In yet another example, where there is an ISL between the base station DU 5A_DU-1_Old and the new base station DU 5A_DU-1_New, the new base station DU 5A_DU-1_New may retrieve UE context information by signalling to the old base station DU 5A_DU-1_Old directly to request the old base station DU 5A_DU-1_Old to send the UE context information to the new base station DU 5A_DU-1_New.

　　Having found/retrieved the UE context information, the new base station DU 5A_DU-1_New sends, at S912b, an RRC connection reestablishment message to perform RRC connection reestablishment between the UE 3 and the new base station DU 5A_DU-1_New.

　　At step S914, the UE 3 sends an RRC connection reestablishment complete message to the new base station DU 5A_DU-1_New to confirm that RRC connection reestablishment with the new base station DU 5A_DU-1_New has been successfully achieved, and that data transmissions between the UE 3 and the new base station DU 5A_DU-1_Newmay begin. UL transmissions from the UE 3 are then resumed, and data transmissions S916a may occur between the UE 3 and the new base station DU 5A_DU-1_New. At the same time data transmissions S916b may occur between the new base station DU 5A_DU-1_New and GW 5B-1 / base station CU 5A_CU-1 of the NTN RAN 5-1 to allow communication with the CN 7.

　　< RRC (Re)configuration & Release Messages >
　　In the scenarios described above, the satellite switching information is described as being provided in system information (e.g., SIB19), and/or broadcast messages. It will nevertheless be appreciated that the satellite switching information may be provided to the UE 3 via any appropriate RRC type message (e.g., an RRC (re)configuration message). By way of example only, rather than the satellite switching information being transmitted to the UE 3 via system information and/or broadcast messages, the satellite switching information may be transmitted, by the old base station DU 5A_DU-1_Old,to the UE 3, in an RRC (re)configuration message as part of a RRC procedure to re-configure the connection of the UE 3 from the old base station DU 5A_DU-1_Old to the new base station DU 5A_DU-1_New. In another example, rather than the satellite switching information being transmitted to the UE 3 via system information and/or broadcast messages, the satellite switching information may be transmitted, by the old base station DU 5A_DU-1_Old,to the UE 3, in an RRC release message which is used to initiate release of the connection of the UE 3 from the old base station DU 5A_DU-1_Old during an RRC (re)configuration procedure.

　　< Satellite Switching using Proxy Base Stations >
　　The following satellite switching procedures make use of at least one proxy base station 5A to support handover preparation and UE context retrieval for RRC Re-establishment e.g., when end-to-end Xn interfaces between different base station 5A on board different satellites 5C does not exist. It will therefore be appreciated that the following satellite switching procedures may be used in conjunction with the first, second and third regenerative mode satellite switching procedure described above, which are directed toward enhancing/simplifying the mobility procedures to switch RRC connections of UEs 3 to the new satellites 5C with less L3 signaling involvement and/or shorter interruption.

　　Fig. 10 illustrates a simplified sequence diagram illustrating a fourth regenerative mode satellite switching procedure using a ground-based proxy base station 5A-1_Proxy for the new base station 5A-1_New that may be used in the communication system 1 of Fig. 1.

　　The fourth regenerative mode satellite switching procedure shown relates to satellites 5C-1 that host the whole base station 5A-1 onboard such as in Fig. 2C. However, it will nevertheless be appreciated that the regenerative mode satellite switching procedure is equally applicable to satellites that host base station DUs 5A_DU-1 onboard such as in Fig. 2B.

　　As shown in Fig. 10, data transmissions S1002a may occur between the UE 3 and the old base station 5A-1_Old hosted by the old non-terrestrial space (or air) borne platform 5C-1_Old. At the same time data transmissions S1002b may occur between the old base station 5A-1_Old hosted by the old non-terrestrial space (or air) borne platform 5C-1_Old and GW 5B-1 of the NTN RAN 5-1 to allow communication between the old base station 5A-1_Old and the CN 7.

　　Furthermore, as shown in Fig. 10, the old base station 5A-1_Old hosted by the old non-terrestrial space (or air) borne platform 5C-1_Old is connected to, and in communication with, a proxy base station 5A-1_Proxy based on the ground. The proxy base station 5A-1_Proxymay obtain appropriate capability information, configuration information, and limitation information about the new base station 5A-1_New hosted by the new non-terrestrial space (or air) borne platform 5C-1_New (e.g., where there is no ISL between the old non-terrestrial space (or air) borne platform 5C-1_Old and the new non-terrestrial space (or air) borne platform 5C-1_New), for example, via operations and management (OAM).

　　The proxy base station 5A-1_Proxy may respond to a handover request from the old base station 5A-1_Old through a handover request acknowledgement message. In addition, the proxy base station 5A-1_Proxy may inform the new base station 5A-1_New hosted by the new non-terrestrial space (or air) borne platform 5C-1_New of UE context information/configurations for UEs 3 to be switched from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New.

　　At step S1004, when the old base station 5A-1_Old decides to prepare the handover of a UE 3 to new base station 5A-1_New, the old base station 5A-1_Old sends a handover request message (e.g., an XnAP/NGAP message, or the like) to the proxy base station 5A-1_Proxy that is acting as a proxy for a new base station 5A-1_New hosted on a new non-terrestrial space (or air) borne platform 5C-1_New to which the UE 3 is to switch.

　　In response to the handover request message, at step S1004, the proxy base station 5A-1_Proxy may transmit to the old base station 5A-1_Olda handover request acknowledgement message (e.g., another XnAP/NGAP message, or the like)that may include an RRC configuration used in a new base station 5A-1_New after handover.

　　Additionally, the handover request acknowledgement message may include other appropriate information that is necessary for the handover procedure to be performed. For example, the handover request acknowledgement message may include capability information, configuration information, and limitation information about the new base station 5A-1_New. Alternatively, the capability information, configuration information, and limitation information about the new base station 5A-1_New may be provided to the old base station 5A-1_Old by the proxy base station 5A-1_Proxy in one or more separate messages subsequent to the handover request acknowledgement message.

　　Having received the handover request message at step S1004, the proxy base station 5A-1_Proxy may be triggered to perform appropriate admission control procedures at step S1006 before responding. For example, the proxy base station 5A-1_Proxy may perform any necessary validation processes before confirming to establishing a connection with the new base station 5A-1_New / new non-terrestrial space (or air) borne platform 5C-1_New to determine if current resources are sufficient for the proposed UE connection. The admission control procedures may also include determining which UEs 3 currently in communication via the old base station 5A-1_Old on the old non-terrestrial space (or air) borne platform 5C-1_Old are to be admitted for switching to the new base station 5A-1_New on the new non-terrestrial space (or air) borne platform 5C-1_New.

　　It will be appreciated that appropriate admission control procedures may also include preparing any necessary (conditional) handover messages that need to be used to facilitate handover (e.g., preparing the handover request acknowledgement message, and other necessary RRC messages i.e., RRCReconfiguration messages, and the like). Having prepared any necessary (conditional) handover messages that need to be used to facilitate handover (e.g., preparing the handover request acknowledgement message), the proxy base station 5A-1_Proxy sends such handover request acknowledgement messages to the old base station 5A-1_Old at step S1008.

　　Having received the handover request acknowledgement message at step S1008, the old base station 5A-1_Old, may in turn be triggered to transmit RRC Reconfiguration messages to the UE 3 at step S1010 to initiate RRC reconfiguration of the UE 3 to the new non-terrestrial space (or air) borne platform 5C-1_New. Additionally, the RRC Reconfiguration messages may include a handover trigger condition to initiate handover between the old base station 5A-1_Old and the new base station 5A-1_New.

　　At the same time, or shortly after the old base station 5A-1_Old is triggered to transmit the RRC Reconfiguration messages to the UE 3 at step S1010, the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New may connect to the CN 7 (via the GW 5B-1) at step S1012.

　　Furthermore, at the same time, or shortly after the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New connects to the CN 7 (via the GW 5B-1) at step S1012, the proxy base station 5A-1_Proxy may forward any appropriate information about all of the admitted UEs 3 that are to be switched from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New, which may include configurations and appropriate UE context information of those admitted UEs 3 at step S1014. Once the new base station 5A-1_New on the new non-terrestrial space (or air) borne platform 5C-1_New has received the appropriate information about all of the admitted UEs 3 that are to be switched from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New, the new base station 5A-1_New may be considered to be prepared for (conditional) handover (S1016).

　　At step S1018, the UE 3 performs a (conditional) handover procedure to switch its connection from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New. For example, the UE 3 may perform any appropriate (conditional) handover procedure between the old base station 5A-1_Old on the old non-terrestrial space (or air) borne platform 5C-1_Old and the new base station 5A-1_New on the new non-terrestrial space (or air) borne platform 5C-1_New to establish a connection between the UE 3 and the new base station 5A-1_New (optionally while maintaining the pre-existing connection between the UE 3 and the old base station 5A-1_Old on the old non-terrestrial space (or air) borne platform 5C-1_Old). Once the connection between the UE 3 and the new base station 5A-1_New on the new non-terrestrial space (or air) borne platform 5C-1_New has been established the UE 3 may disconnect/terminate its connection with the old base station 5A-1_Old and may send, at step S1020, an RRC reconfiguration complete message to the new base station 5A-1_New to confirm that the satellite switch has been successfully completed.

　　Once the RRC reconfiguration complete message is transmitted to the new base station 5A-1_New to confirm that the satellite switch has been successfully completed data transmission may resume. For example, having successfully performed the switch procedure, data transmissions between the UE 3 and the new base station 5A-1_New may be initiated at step S1022a, and similarly data transmissions between the new base station 5A-1_New and the CN 7 may be initiated at step S1022b.

　　Fig. 11 illustrates a simplified sequence diagram illustrating a fifth regenerative mode satellite switching procedure using a ground-based proxy base station 5A-1_Proxy for the old base station 5A-1_Old that may be used in the communication system of Fig. 1.

　　The fifth regenerative mode satellite handover procedure shown relates to satellites 5C-1 that host the whole base station 5A-1 onboard such as in Fig. 2C. However, it will nevertheless be appreciated that the regenerative mode satellite handover procedure is equally applicable to satellites 5C-1 that host the distributed unit 5A_DU-1 of a base station 5A-1 onboard such as in Fig. 2B.

　　As shown in Fig. 11, data transmissions S1102a may occur between the UE 3 and the old base station 5A-1_Old. At the same time data transmissions S1102b may occur between the old base station 5A-1_Old and GW 5B-1 of the NTN RAN 5-1 to allow communication between the old base station 5A-1_Old and the CN 7.

　　Furthermore, as shown in Fig. 11, the old base station 5A-1_Old is connected to, and in communication with (initially), a proxy base station 5A-1_Proxy. That proxy base station 5A-1_Proxy is configured to act as a proxy on behalf of the old base station 5A-1_Old hosted on the old non-terrestrial space (or air) borne platform 5C-1_Old from which the UE 3 switches. The proxy base station 5A-1_Proxyallows the old base station 5A-1_Oldto relocate UE context information, from the old base station 5A-1_Old to the new base station 5A-1_New.

　　At step S1103, old base station 5A-1_Old downloads, to the proxy base station 5A-1_Proxy, UE context information for all UEs 3 connected to the old base station 5A-1_Old that are to be switched to the new base station 5A-1_New. Once the UE context information for all UEs 3 connected to the old base station 5A-1_Old has been downloaded to the proxy base station 5A-1_Proxy, the connection between the old base station 5A-1_Old and the proxy base station 5A-1_Proxy may be terminated. For example, following the download of the UE context information for all UEs 3 connected to the old base station 5A-1_Old to the proxy base station 5A-1_Proxy, the connection between the UE 3 and old base station 5A-1_Old may be disconnected S1104 via an appropriate message and/or trigger (e.g., an RRC release message, or the like).

　　At the same time, or shortly after the disconnection between the old base station 5A-1_Old and the proxy base station 5A-1_Proxy, the proxy base station 5A-1_Proxy may establish a new connection with a new base station 5A-1_New at step S1106, which is the new base station 5A-1_New to which the UEs 3 are to be switched. For example, the proxy base station 5A-1_Proxy may perform any necessary RRC reestablishment/reconfiguration procedure necessary to connect to the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New.

　　Having (re)established a connection between the proxy base station 5A-1_Proxy and the new base station 5A-1_New, the proxy base station 5A-1_Proxy may relocate the UE context information for all the UEs 3 that it received from the old base station 5A-1_Old to the new base station 5A-1_New. In one example, the proxy base station 5A-1_Proxy may automatically upload/transmit the UE context information for all the UEs 3, that it received from the old base station 5A-1_Old to the new base station 5A-1_New upon the establishment of the connection between the proxy base station 5A-1_Proxy and the new base station 5A-1_New, (Alt 1) at step S1110a.

　　Shortly after the relocation of the UE context information, the UE 3 may request RRC reestablishment with the new base station 5A-1_New by transmitting an RRC reestablishment request message, at step S1108, to the new base station 5A-1_New. As the UE context information has been relocated at the new base station 5A-1_New, in response to the RRC reestablishment request message from the UE 3, the new base station 5A-1_Newmay retrieve that UE context information locally (e.g., from its memory), and use that information to establish a connection between the UE 3 and the new base station 5A-1_New in an RRC reestablishment procedure. For example, the new base station 5A-1_New may retrieve that UE context information locally and may use that information when sending an RRC reestablishment message at step S1112 to the UE 3 (which may include a re-establishment trigger) to establish a connection between the UE 3 and the new base station 5A-1_New.

　　In another example, the new base station 5A-1_New may send a request message to the proxy base station 5A-1_Proxy to request and retrieve UE context information from the proxy base station 5A-1_Proxy that was downloaded from the old base station 5A-1_Old and stored in its memory at step S1110c (Alt 2). Having requested the UE context information, the proxy base station 5A-1_Proxy forwards the UE context information for all the UEs 3, that it received from the old base station 5A-1_Old to the new base station 5A-1_New at step S1110d. Shortly after the relocation of the UE context information, the UE 3 may request RRC reestablishment with the new base station 5A-1_New by transmitting an RRC reestablishment request message, at step S1108, to the new base station 5A-1_New. As the UE context information has been relocated at the new base station 5A-1_New, in response to the RRC reestablishment request message from the UE 3, the new base station 5A-1_New may retrieve that UE context information locally (e.g., from its memory), and use that information to establish a connection between the UE 3 and the new base station 5A-1_New. For example, the new base station 5A-1_New may retrieve that UE context information locally and may use that information when sending RRC reestablishment messages to the UE 3 at step S1112 (which may include a reestablishment trigger) to establish a connection between the UE 3 and the new base station 5A-1_New.

　　Once the connection between the UE 3 and the new base station 5A-1_New has been established data transmission can resume at S1114a, S1114b.

　　Figs. 12A and 12B illustrate another simplified sequence diagram illustrating a sixth regenerative mode satellite switching procedure using a proxy base station 5A-1_Proxy for both the old base station 5A-1_Old and the new base station 5A-1_New that may be used in the communication system 1 of Fig. 1.

　　The sixth regenerative mode satellite handover procedure shown relates to satellites 5C-1 that host the whole base station 5A-1 onboard such as in Fig. 2C. However, it will nevertheless be appreciated that the regenerative mode satellite handover procedure is equally applicable to satellites 5C-1 that host the distributed unit of a base station 5A-1 onboard such as in Fig. 2B.

　　As shown in Fig. 12A, data transmissions S1202a may occur between the UE 3 and the old base station 5A-1_Old hosted on the old non-terrestrial space (or air) borne platform 5C-1_Old. At the same time data transmissions S1202b may occur between the old base station 5A-1_Old and GW 5B-1 of the NTN RAN 5-1 to allow communication between the old base station 5A-1_Old and the CN 7. Furthermore, as shown in Fig. 12A, the old base station 5A-1_Oldon the old non-terrestrial space (or air) borne platform 5C-1_Old is connected to, and in communication with, the CN 7, via the GW 5B-1 of the NTN RAN 5-1.

　　At step S1204 the old base station 5A-1_Old transmits a handover request message to a proxy base station 5A-1_New-Proxy for a new base station 5A-1_New. Having received the handover request message, the proxy base station 5A-1_New-Proxy for the new base station 5A-1_Newmay be triggered to perform appropriate admission control procedures at step S1206. For example, the proxy base station 5A-1_New-Proxy for a new base station 5A-1_Newmay perform any necessary validation processes, including determining which UEs 3 currently in old base station 5A-1_Old hosted on the old non-terrestrial space (or air) borne platform 5C-1_Old are to be admitted for communication via the new base station 5A-1_New on the new non-terrestrial space (or air) borne platform 5C-1_New.

　　It will be appreciated that appropriate admission control procedures may also include preparing any necessary (conditional) handover messages that need to be used to facilitate handover (e.g., preparing the handover request acknowledgement message, and other necessary RRC messages i.e., RRCReconfiguration messages, and the like). Having prepared any necessary (conditional) handover messages that need to be used to facilitate handover (e.g., preparing the handover request acknowledgement message), the proxy base station 5A-1_New-Proxy for the new base station 5A-1_Newsends such handover request acknowledgement messages to the old base station 5A-1_Old at step S1208.

　　Having received the handover request acknowledgement message at step S1208, the old base station 5A-1_Old may in turn be triggered to transmit RRC Reconfiguration messages to the UE 3 at step S1210 to initiate RRC reconfiguration of the UE 3 to the new base station 5A-1_New. For example, the handover request acknowledgement message may include a handover trigger to initiate handover between the old base station 5A-1_Old and the new base station 5A-1_New.

　　Furthermore, at the same time, or shortly after the old base station 5A-1_Old is triggered to transmit the RRC Reconfiguration messages to the UE 3 at step S1210, the new base station 5A-1_Newmay establish a connection with the CN 7, via the GW 5B-1 of the NTN RAN 5-1.

　　At step S1212, having received the handover request acknowledgement message at step S1208, the old base station 5A-1_Old, uploads/transmits any appropriate information about all of the UEs 3 connected to the old base station 5A-1_Oldto the CN 7. For example, the old base station 5A-1_Old, uploads/transmits appropriate UE context information of all of the UEs 3 connected to the old base station 5A-1_Oldto the CN 7, which may subsequently be relocated, by the CN 7, to the proxy base station 5A-1_Old-Proxy for the old base station 5A-1_Old.

　　At the same time, or shortly thereafter, the proxy base station 5A-1_New-Proxy for the new base station 5A-1_New uploads/transmits any appropriate information about all of the admitted UEs 3, that are to be switched from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New,to the CN 7 at step S1214. For example, the old non-terrestrial space (or air) borne platform 5C-1_Old, uploads/transmits appropriate UE configuration information of all of the admitted UEs 3 connected to the old non-terrestrial space (or air) borne platform 5C-1_Oldto the CN 7, which may subsequently be relocated, by the CN 7, to the new base station 5C-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New. Once the new base station 5A-1_New has received the appropriate information about all of the admitted UEs 3 that are to be switched from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New, the new base station 5A-1_New may be considered to be prepared for (conditional) handover (S1216).

　　Fig. 12B illustrates the remaining steps of the simplified sequence of the sixth regenerative mode satellite switching procedure shown in Fig. 12A.

　　As shown in Fig. 12B, once the new base station 5A-1_New is prepared for (conditional) handover at step S1216, the new base station 5A-1_New, at S1218, may transmit system information to the UE 3 to provide the UE 3 information about the new base station 5A-1_New to which it is to switch. For example, the new base station 5A-1_New may send a system information a (e.g., SIB19 message) to the UE 3 at step S1218, which may contain, by way of example only, satellite assistance information (e.g., Ephemeris data, common timing advance parameters, k_offset, validity duration for UL synchronization epoch time, cell reference location, cell stop time, and the like).

　　Upon receipt of the system information message (e.g., a SIB19 message) by the UE 3, the UE 3 may execute a (conditional) handover procedure at step S1220. For example, where the UE 3 is in an RRC_CONNECTED mode, the UE 3 may trigger an (conditional) handover procedure at step S1220 to handover the UE 3 from the old base station 5A-1_Old, to the new base station 5A-1_New.Having performed the handover procedure, the UE 3 sends, at step S1222, to the new base station 5A-1_New, a handover complete message to indicate that the handover had been successfully completed (e.g., a RRCReconfigurationComplete message for (conditional) handover).

　　Alternatively, upon receipt of the system information message (e.g., a SIB19 message) by the UE 3, the UE 3 may execute a RRC reestablishment procedure at step S1220. For example, where the UE 3 is in an RRC_IDLE mode, the UE 3 may trigger an RRC reestablishment procedure at step S1220 to (re)establish a connection between the UE 3 and the new base station 5A-1_New.Having triggered the RRC reestablishment procedure at step S1220, the UE 3 sends, at step S1222, a RRC (re)establishment request message to the new base station 5A-1_New.

　　Beneficially, as the proxy base station 5A-1_New-Proxy for the new base station 5A-1_New previously located the UE context information for all of the admitted UEs 3 to be switched from the old non-terrestrial space (or air) borne platform 5C-1_Old to the new non-terrestrial space (or air) borne platform 5C-1_New, the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_Newhas local access to all of the necessary UE context information to assist with the (conditional) handover or RRC reestablishment procedure between UE 3 and the new base station 5A-1_New hosted on the new non-terrestrial space (or air) borne platform 5C-1_New.

　　Once the UE 3 has been switched from the old base station 5A-1_Old to the new base station 5A-1_New, data transmission may resume. For example, having successfully performed the switch procedure, data transmissions between the UE 3 and the new base station 5A-1_New may be initiated at step S1224a, and similarly data transmissions between the new base station 5A-1_New and the CN 7 may be initiated at step S1224b.

　　< Components of the NTN RAN >
　　< User Equipment >
　　Fig. 13 is a simplified block schematic illustrating the main components of the UE 3 for implementation in the system of Fig. 1.

　　As shown, the UE 3 has a transceiver circuit 31 that is operable to transmit signals to and to receive signals from a base station 5A via one or more antenna 33 (e.g., comprising one or more antenna elements). 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 (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. 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.

　　The controller 37 is configured to control overall operation of the UE 3 by, in this example, program instructions or software instructions stored within memory 39. As shown, these software instructions include, among other things, an operating system 41, and a communication control module 43.

　　The communication control module 43 is operable to control the communication between the UE 3 and its serving base station or base stations 5A (and other communication devices connected to the base station 5A, such as further UEs and/or core network nodes). The communication control module 43 is configured for the overall handling of 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 communication control module 43 is also configured for the overall handling of receipt of downlink communications via associated downlink channels (e.g., of DCI via a physical downlink control channel (PDCCH) and/or a physical downlink shared channel (PDSCH)) including both dynamic and semi-persistent scheduling (e.g., SPS). The communication control module 43 is responsible, for example: for determining where to monitor for downlink control information; for determining the resources to be used by the UE 3 for transmission/reception of UL/DL communications (including interleaved resources and resources subject to frequency hopping); for managing frequency hopping at the UE side; for determining how slots/symbols are configured (e.g., for UL, DL or full duplex communication, or the like); for determining which bandwidth parts are configured for the UE 3; for determining how uplink transmissions should be encoded and the like.

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

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

　　< Base Station >
　　Fig. 14 is a simplified block schematic illustrating the main components of a base station 5A for implementation in the system of Fig. 1 (e.g. in an NTN access network or other such RAN 5).

　　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 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.

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

　　The communication 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 communication 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 communication 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-persistent scheduling (e.g., SPS). The communication control module 63 is responsible, for example: for determining where to configure the UE 3 to monitor for downlink control information (e.g., the location of search spaces, CORESETs, and associated PDCCH candidates to monitor); for determining the resources to be scheduled for UE transmission/reception of UL/DL communications (including interleaved resources and resources subject to frequency hopping); for managing frequency hopping at the base station side; for configuring slots/symbols appropriately (e.g., for UL, DL or full duplex communication, or the like); for configuring bandwidth parts for the UE 3; for providing related configuration signalling to the UE 3; and the like.

　　It will be appreciated that the communication control module 63 may include a number of sub-modules ('layers' or 'entities') to support specific functionalities. For example, the communication 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 communication 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).

　　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.

　　< Modifications and Alternatives >
　　Detailed examples been described above. As those skilled in the art will appreciate, a number of modifications and alternatives can be made to the above examples whilst still benefiting from the enhancements embodied therein.

　　It will be appreciated that description of features of and actions performed by a base station (or eNB or gNB), NTN nodes, and UEs may be applied equally to base stations and UEs that communicate in the terrestrial plane only (i.e. as part of a terrestrial RAN without features of an NTN RAN such as a gateway and space or airborne platform) as to base stations that communicate via a non-terrestrial plane.

　　Moreover, description of features of and actions performed by a base station (or eNB or gNB), apply equally to distributed type base stations as to non-distributed type base stations.

　　It will also be appreciated that whilst information elements having specific names have been described differently named information elements but having a similar purpose may be used.

　　In the above description the UE 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 disclosed enhancements, 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.

　　In the above examples, a number of software modules were described. As those skilled in the art will appreciate, the software modules may be provided in compiled or un-compiled form and may be supplied to the UE or base station 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 UE or the base station in order to update their functionalities.

　　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.

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

　　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 an extended period of time.

　　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; moulds 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.).

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

　　A UE may, for example, be an item of information and communication equipment (for example information and communication equipment such as: electronic computer and related equipment; communication and related equipment; electronic components etc.).

　　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.).

　　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.).

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

　　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)). 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.

　　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 an extended 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.

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

　　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.　　　

　　Table 1

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

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

　　Although the present disclosure has been described with reference to the example embodiments, the present disclosure is not limited to the above. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the disclosure.

　　This application is based upon and claims the benefit of priority from UK patent application No. 2401345.0, filed on February 1, 2024, the disclosure of which is incorporated herein in its entirety by reference.

　　The program can be stored and provided to the computer device using any type of non-transitory computer readable media. Non-transitory computer readable media include any type of tangible storage media. Examples of non-transitory computer readable media include magnetic storage media (such as floppy disks, magnetic tapes, hard disk drives, etc.), optical magnetic storage media (e.g. magneto-optical disks), CD-ROM (Read Only Memory), CD-R, CD-R/W, and semiconductor memories (such as mask ROM, PROM (Programmable ROM), EPROM (Erasable PROM), flash ROM, RAM (Random Access Memory), etc.). The program may be provided to the computer device using any type of transitory computer readable media. Examples of transitory computer readable media include electric signals, optical signals, and electromagnetic waves. Transitory computer readable media can provide the program to the computer device via a wired communication line, such as electric wires and optical fibers, or a wireless communication line.

　　For example, 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 mobile terminal configured to communicate with a satellite having at least a part of functions of a base station, the method comprising:
　　receiving, from the satellite, information for switching a serving satellite from the satellite to another satellite;
　　performing the switching the serving satellite based on the information, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).
(Supplementary note 2)
　　The method according to Supplementary note 1, wherein
　　the information includes at least one of:
　　　　information for indicating the target cell,
　　　　information for indicating the another satellite,
　　　　information for indicating a frequency corresponding to the target cell,
　　　　configuration information common to at least one cell including the target cell,
　　　　information for indicating the at least the part of the functions of the base station which the satellite has,
　　　　information for updating at least one security key for a connection with the mobile terminal and the another satellite,
　　　　information for indicating a backoff time for the switching, or
　　　　information for indicating a configured grant.
(Supplementary note 3)
　　The method according to Supplementary note 1 or 2, further comprising:
　　maintaining configuration related to at least one of:
　　　　at least one radio network temporary identifier (RNTI),
　　　　at least one radio bearer, or
　　　　at least one measurement.
(Supplementary note 4)
　　The method according to any one of Supplementary notes 1 to 3, wherein
　　the receiving the information is performed via at least one of:
　　　　system information, or
　　　　a dedicated signalling.
(Supplementary note 5)
　　The method according to any one of Supplementary notes 1 to 4, wherein
　　the performing the switching is performed by at least one of:
　　　　a non-Radio Resource Control (RRC) layer procedure,
　　　　a RRC re-establishment procedure, or
　　　　a RRC reconfiguration procedure based on a RRC release procedure.
(Supplementary note 6)
　　The method according to any one of Supplementary notes 1 to 5, wherein
　　context information on the mobile terminal is relocated from the satellite to the another satellite directly or via at least one network node in a terrestrial network.
(Supplementary note 7)
　　The method according to Supplementary note 6, wherein
　　information for a Packet Data Convergence Protocol (PDCP) sequence number (SN) status is transferred from the satellite to the another satellite directly or via the at least one network node, in a case where the satellite has all functions of the base station.
(Supplementary note 8)
　　The method according to Supplementary note 6, wherein
　　information for downlink data delivery status is transferred from the satellite to the another satellite directly or via the at least one network node, in a case where the satellite has the part or the functions of the base station.
(Supplementary note 9)
　　The method according to any one of Supplementary notes 6 to 8, wherein
　　the at least one network node includes a first proxy base station,
　　the context information is transmitted from the satellite to the first proxy base station directly or via the at least one network node other than the first proxy base station before the another satellite is connected with any of the at least one network node, and
　　the context information is transmitted from the first proxy base station to the another satellite after the satellite has been disconnected with the at least one network node.
(Supplementary note 10)
　　The method according to any one of Supplementary notes 6 to 9, wherein
　　the at least one network node includes a second proxy base station,
　　information for handover on the switching is transmitted from the satellite to the second proxy base station before the another satellite is connected with any of the at least one network node, and
　　information on admitted mobile terminals determined based on admission control at the second proxy base station and corresponding configurations is transmitted from the second proxy base station to the another satellite directly or via the at least one network node other than the second proxy base station after the satellite has been disconnected with the at least one network node.
(Supplementary note 11)
　　The method according to any one of Supplementary notes 6 to 10, wherein
　　the context information is transmitted from the satellite to the another satellite directly in a case where an inter satellite link between the satellite and the another satellite exists.
(Supplementary note 12)
　　The method according to any one of Supplementary notes 1 to 11, wherein
　　the performing the switching includes performing a Packet Data Convergence Protocol (PDCP) data recovery procedure for at least one corresponding data radio bearer.
(Supplementary note 13)
　　The method according to any one of Supplementary notes 1 to 12, wherein
　　the information includes:
　　　　information indicating when the another satellite is going to start serving an area currently covered by the satellite, and
　　　　information indicating when a cell served by the satellite is going to stop serving an area which the satellite is currently covering, and
　　the performing the switching is triggered in a case where the mobile station receives the information and information indicating satellite switch with connection reestablishment, from the satellite.
(Supplementary note 14)
　　A method performed by a satellite having at least a part of functions of a base station, the method comprising:
　　transmitting, to a mobile terminal configured to communicate with the satellite, information for switching a serving satellite from the satellite to another satellite to cause the mobile terminal to perform the switching the serving satellite, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).
(Supplementary note 15)
　　A mobile terminal configured to communicate with a satellite having at least a part of functions of a base station, the mobile terminal comprising:
　　means for receiving, from the satellite, information for switching a serving satellite from the satellite to another satellite;
　　means for performing the switching the serving satellite based on the information, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).
(Supplementary note 16)
　　A satellite having at least a part of functions of a base station, the satellite comprising:
　　means for transmitting, to a mobile terminal configured to communicate with the satellite, information for switching a serving satellite from the satellite to another satellite to cause the mobile terminal to perform the switching the serving satellite, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).

1　　communication system
3, 3-1, 3-2, 3-3　　UE
5, 5-1, 5-2　　radio access network (RAN), NTN (R)AN
5A, 5A-1, 5A-2　　base station
5B, 5B-1　　gateway, (GW)
5C, 5C-1　　space (or air) borne platform, satellite
7　　core network (CN)
9, 9-1, 9-2　　cell
10　　control plane functions (CPFs)
10-1　　Access and Mobility Management Functions (AMFs)
10-2　　Session Management Function (SMF)
11　　user plane functions (UPFs)
20　　external data network
31, 51　　transceiver circuit
33, 53　　antenna
35　　user interface
55　　core network interface
37, 57　　controller
39, 59　　memory
41, 61　　operating system
43, 63　　communications control module

Claims

　　A method performed by a mobile terminal configured to communicate with a satellite having at least a part of functions of a base station, the method comprising:
　　receiving, from the satellite, information for switching a serving satellite from the satellite to another satellite;
　　performing the switching the serving satellite based on the information, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).
　　The method according to claim 1, wherein
　　the information includes at least one of:
　　　　information for indicating the target cell,
　　　　information for indicating the another satellite,
　　　　information for indicating a frequency corresponding to the target cell,
　　　　configuration information common to at least one cell including the target cell,
　　　　information for indicating the at least the part of the functions of the base station which the satellite has,
　　　　information for updating at least one security key for a connection with the mobile terminal and the another satellite,
　　　　information for indicating a backoff time for the switching, or
　　　　information for indicating a configured grant.
　　The method according to claim 1 or 2, further comprising:
　　maintaining configuration related to at least one of:
　　　　at least one radio network temporary identifier (RNTI),
　　　　at least one radio bearer, or
　　　　at least one measurement.
　　The method according to any one of claims 1 to 3, wherein
　　the receiving the information is performed via at least one of:
　　　　system information, or
　　　　a dedicated signalling.
　　The method according to any one of claims 1 to 4, wherein
　　the performing the switching is performed by at least one of:
　　　　a non-Radio Resource Control (RRC) layer procedure,
　　　　a RRC re-establishment procedure, or
　　　　a RRC reconfiguration procedure based on a RRC release procedure.
　　The method according to any one of claims 1 to 5, wherein
　　context information on the mobile terminal is relocated from the satellite to the another satellite directly or via at least one network node in a terrestrial network.
　　The method according to claim 6, wherein
　　information for a Packet Data Convergence Protocol (PDCP) sequence number (SN) status is transferred from the satellite to the another satellite directly or via the at least one network node, in a case where the satellite has all functions of the base station.
　　The method according to claim 6, wherein
　　information for downlink data delivery status is transferred from the satellite to the another satellite directly or via the at least one network node, in a case where the satellite has the part or the functions of the base station.
　　The method according to any one of claims 6 to 8, wherein
　　the at least one network node includes a first proxy base station,
　　the context information is transmitted from the satellite to the first proxy base station directly or via the at least one network node other than the first proxy base station before the another satellite is connected with any of the at least one network node, and
　　the context information is transmitted from the first proxy base station to the another satellite after the satellite has been disconnected with the at least one network node.
　　The method according to any one of claims 6 to 9, wherein
　　the at least one network node includes a second proxy base station,
　　information for handover on the switching is transmitted from the satellite to the second proxy base station before the another satellite is connected with any of the at least one network node, and
　　information on admitted mobile terminals determined based on admission control at the second proxy base station and corresponding configurations is transmitted from the second proxy base station to the another satellite directly or via the at least one network node other than the second proxy base station after the satellite has been disconnected with the at least one network node.
　　The method according to any one of claims 6 to 10, wherein
　　the context information is transmitted from the satellite to the another satellite directly in a case where an inter satellite link between the satellite and the another satellite exists.
　　The method according to any one of claims 1 to 11, wherein
　　the performing the switching includes performing a Packet Data Convergence Protocol (PDCP) data recovery procedure for at least one corresponding data radio bearer.
　　The method according to any one of claims 1 to 12, wherein
　　the information includes:
　　　　information indicating when the another satellite is going to start serving an area currently covered by the satellite, and
　　　　information indicating when a cell served by the satellite is going to stop serving an area which the satellite is currently covering, and
　　the performing the switching is triggered in a case where the mobile station receives the information and information indicating satellite switch with connection reestablishment, from the satellite.
　　A method performed by a satellite having at least a part of functions of a base station, the method comprising:
　　transmitting, to a mobile terminal configured to communicate with the satellite, information for switching a serving satellite from the satellite to another satellite to cause the mobile terminal to perform the switching the serving satellite, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).
　　A mobile terminal configured to communicate with a satellite having at least a part of functions of a base station, the mobile terminal comprising:
　　means for receiving, from the satellite, information for switching a serving satellite from the satellite to another satellite;
　　means for performing the switching the serving satellite based on the information, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).
　　A satellite having at least a part of functions of a base station, the satellite comprising:
　　means for transmitting, to a mobile terminal configured to communicate with the satellite, information for switching a serving satellite from the satellite to another satellite to cause the mobile terminal to perform the switching the serving satellite, and
　　wherein the performing the switching includes:
　　　　synchronizing with a target cell which is different from a serving cell served by the satellite;
　　　　resetting configuration related to a Medium Access Control (MAC) layer; and
　　　　re-establishing configuration related to a Radio Link Control (RLC) layer and a Packet Data Convergence Protocol (PDCP) entity for at least one Radio Bearer (RB).