Method performed by access network node, method performed by user equipment, method performed by core network node, access network node, user equipment, and core network node

Abstract

A method performed by an access network node in a non-terrestrial network is disclosed. The method includes receiving, via an available link, user plane data, in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for another link than the available link; storing the user plane data until the another link becomes available; and forwarding, via the another link, the user plane data when the another link becomes available.

Description

METHOD PERFORMED BY ACCESS NETWORK NODE, METHOD PERFORMED BY USER EQUIPMENT, METHOD PERFORMED BY CORE NETWORK NODE, ACCESS NETWORK NODE, USER EQUIPMENT, AND CORE NETWORK NODE

[0001] 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 LTE-Advanced, Next Generation or 5G networks, future generations, and beyond). The disclosure has particular but not exclusive relevance to improvements relating to the use of store and forward techniques for the communication of user data in the context of Non-Terrestrial Networks (NTN).

[0002] Earlier developments of the 3GPP standards were referred to as the Long-Term Evolution (LTE) of Evolved Packet Core (EPC) network and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), also commonly referred as '4G'. More recently, the term '5G' and 'new radio' (NR) has started to be used to refer to an evolving communication technology that is expected to support a variety of applications and services. Various details of 5G networks are described in, for example, the 'NGMN 5G White Paper' V1.0 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.

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

[0004] For simplicity, the present application will use the term mobile device, user device, or UE to refer to any communication device that is able to connect to the core network via one or more base stations. Although the present application may refer to mobile devices in the description, it will be appreciated that the technology described can be implemented on any communication devices (mobile and / or generally stationary) that can connect to a communications network for sending / receiving data, regardless of whether such communication devices are controlled by human input or software instructions stored in memory. One specific type of UE supported in modern communication systems are so called Internet of Things (IoT) devices that are non-standard hardware devices (including everyday physical objects such as sensor devices, gadgets, appliances and the like) that are able to connect wirelessly to a network to transmit and receive data. Technologies for supporting such IoT UEs in cellular communication systems are often referred to as cellular IoT (CIoT) enhancements or optimisations. CIoT enhancements include, for example, narrowband IoT (NB-IoT) enhancements, which is a radio technology developed, to support cellular network IoT devices and services, in which the bandwidth is limited to a single narrowband (e.g., where transmissions are restricted to occupying a single 180KHz physical resource block (PRB) / 12 subcarriers of 15KHz each). CIoT enhancements also include features supporting so-called "LTE - Machines" (LTE Cat-M1 or simply LTE-M) technologies involving bandwidth limited UEs (BL UEs), which is faster than NB-IoT but operates over a wider narrowband (e.g., limited to 6 PRBs / 1.4MHz).

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

[0006] The core network includes a number of communication entities for providing different functions for supporting communication.

[0007] For example, in 4G the core network entities include, amongst other things, a mobility management entity (MME), a serving gateway (SGW or S-GW), a packet data network (PDN) gateway (PGW or P-GW), etc. The MME manages general mobility aspects of the UE and ensures that connectivity is maintained with the UE as it is moving within the geographical area covered by the communication system. The MME also handles control-plane signalling for the UE and manages the various bearers associated with the UE (e.g. such as an Evolved Packet System (EPS) bearer and / or a radio bearer), for example by controlling the S-GW and the P-GW (and / or possibly other network nodes) via which such bearers are provided. The S-GW provides a connection between the UE and the core network (via the base station) for sending and receiving user plane data over an associated communication bearer (e.g. an EPS bearer). The communication bearer normally terminates at the P-GW, although it is often complemented by an external bearer as well (for example, another EPS bearer and / or the like) between the P-GW and a communication end-point outside the core network (e.g. in an external network). It will be appreciated that the functionalities of the S-GW and the P-GW could be implemented in a single gateway element.

[0008] In 5G, the core network entities comprise logical nodes (or 'functions') including 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). 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.

[0009] In 4G a number of EPS optimisations for supporting CIoT (e.g., NB-IoT) were introduced (which are generally applicable to later generations of technology), these enhancements allowed for communication new user data paths for IoT. In contrast to the original data paths via the S-GW and P-GW, these new data paths allow communication of user data via the MME (in 4G) (and other CN nodes such as S-GW, P-GW and or a Service Capability Exposure Function (SCEF)) although, in later generations, this may be via a different equivalent node (e.g., the AMF in 5G). CIoT EPS optimisation using these newer path is referred to as control plane (CP) mode or 'CP-Mode' CIoT EPS optimisation whereas CIoT EPS optimisation using the original data paths is referred to as user plane (UP) mode or 'UP-Mode' CIoT EPS optimisation.

[0010] CP-Mode CIoT EPS optimisation reduces the total number of control plane messages when handling a short data transaction (as typically occurs in IoT communication), user data or SMS messages that is conveyed using a service request procedure, via the MME, by encapsulating them in non-access stratum (NAS) messages. In the case of IP data packets, UL data may be transferred from the base station, via the MME, the S-GW, and the P-GW, to the CIoT services. In the case of non-IP data packets, UL data may be transferred from the base station, via the MME, and the SCEF, to the CIoT services.

[0011] The UP-mode CIoT EPS optimisation, on the other hand, conveys user plane data without using the service request procedure to establish access stratum (AS) context in the serving base station and the UE. This UP-mode method is based on UP transport of user data in which data is transferred over the conventional user plane through the network from the base station to the S-GW and vice-versa. For UP-mode CIoT, two distinct RRC connection scenarios are possible. In a first scenario an RRC connection is released with a possible resume operation indicated, and resumption of the connection may then be requested as part of a resume procedure. If this resume procedure is successful, security is established with updated keys and the radio bearers are set up as in the original connection. In a second scenario, where there is no previous release of an RRC connection with a resume indication, or if a resume request is not accepted by the base station, security and radio bearer have to be re-established.

[0012] 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.

[0013] 3GPP Technical Report (TR) 38.811 is a study on New Radio to support such on-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.

[0014] 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); -　　Feeder links which refer to the radio links between the Gateways and the space / airborne platform.

[0015] There are a number of different architectures that may be used for providing NTN access. One such architecture is a 'regenerative' access network architecture (sometimes referred to as 'regenerative satellite', 'regenerative payload', or 'regenerative mode') in which the non-terrestrial platform (e.g., satellite) performs some on board processing of the payload being communicated between the UE and the core network. Specifically, in a regenerative architecture, at least some of the base station functionality (e.g., at least the functionality of the DU of a distributed base station, or possibly all the base station functionality) is provided on the non-terrestrial platform. Other regenerative mode architectures are also possible, for example architectures in which at least some of the core network functionality is implemented on the non-terrestrial platform.

[0016] Another possible architecture is a 'transparent' access network architecture (sometimes referred to as 'transparent satellite', 'transparent mode', or 'transparent payload') in which the base station is terrestrially located and sends and receives communications respectively destined for, and originating from, UEs via a terrestrially located gateway and via a non-terrestrial platform that has no base station functionality. The non-terrestrial platform relays these communications to and from the UEs transparently without on-board processing them in effect acting as a so-called 'bent-pipe'. In this architecture, both the service link and the feeder link effectively act as part of the air interface between the base station and the UEs.

[0017] 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 ID (PCI) and hence the PCI can remain unchanged as a UE 3 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).

[0018] 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 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).

[0019] 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 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.

[0020] Specifically, the UE may attempt to access that cell and / or beam using a random access procedure that typically involves four distinct steps. Prior to attempting initial access the UE may perform transmission of a preamble to the network (e.g. a base station such as a gNB) over a physical random access channel (PRACH / RACH) for initiating a random access procedure (also referred to as a RACH procedure or simply RACH) for obtaining synchronization in the uplink (UL). This step is often referred to as PRACH transmission or simply transmission of message 1 (Msg1). In response, the network responds with a random access response (RAR). The RAR indicates reception of the preamble and includes: a timing-alignment (TA) command for adjusting the transmission timing of the UE 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 is to transmit on the PUSCH with or without frequency; a modulation and coding scheme (MCS) field from which the UE 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 RAR transmission step is often referred to as message 2 (Msg2) transmission. The UE then sends a third message (message 3 or 'Msg3') to the network over the physical uplink shared channel (PUSCH) based on the information in the RAR. The specific message sent by the UE 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 (message 4 or 'Msg4') which carries the randomly generated UE identifier received in Msg3 for contention purposes to resolve any collisions between different UEs using the same preamble sequence. When successful, Msg4 also transfers the UE to a connected state.

[0021] A similar random access procedure 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.

[0022] A so-called two-step random access procedure has also been developed (in addition to the above described four-step random access procedure). The two-step random access is mainly intended for supporting (Ultra) Low Latency Communications, 10ms control plane latency, fast handover, efficient channel access in unlicensed spectrum, and transmission of small data packets, amongst others. However, it may also apply to large cells such as non-terrestrial cells. The main difference is that whilst the four-step random access procedure requires two round-trip cycles between the UE and the base station, the two-step random access procedure aims to reduce latency and control-signalling overhead by using a single round trip cycle between the UE and the base station. Effectively, this is achieved by combining the UE's PRACH preamble (Msg1) transmission and the scheduled PUSCH transmission (Msg3) into a single message (referred to as 'MsgA'). Similarly, the random access response (RAR / Msg2) from the base station to UE and the contention resolution message (Msg4) are combined in the two-step random access procedure (and referred to as 'MsgB').

[0023] 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.

[0024] In addition to the RACH-based initial access procedures described above, a so called RACH-less access procedure was introduced, in the context of handover procedures during the development of later releases of the LTE standards, also with a view to providing reduced latency. RACH-less based handover 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.

[0025] As a non-terrestrial platform serving a UE moves, discontinuous coverage for that UE can occur, even if the UE remains stationary, as a result of the service link dropping, e.g., due to the satellite movement. In addition to discontinuous coverage of this type, there could also be intermittent feeder link connectivity (for example with a gateway at an associated ground station) - e.g., in areas where it is not feasible to deploy a gateway or where deployment of the gateway is not cost effective.

[0026] Moreover, at different times different NTN platforms (and hence on-board base stations where present) may provide the feeder link and service link respectively. Specifically, for a UE at a given location, it is possible that one or multiple satellites may be circulating around and providing communication services for that UE at different times. Thus the UE will, effectively, see different base stations during different time windows. Similarly, one or multiple satellites may be circulating around the ground location of a gateway via which one or more feeder link connections are being provided meaning that the gateway feeder connectivity may be via different satellites (and, potentially, base stations in the case of a regenerative mode architecture).

[0027] One such scenario is illustrated, for a regenerative architecture, in Fig. 1 which illustrates a change in the satellite, and hence base station, respectively providing a service link and a feeder link in an NTN system. As seen in Fig. 1, two NTN platforms (in this example satellites), which each provide a respective base station, are shown circulating around and providing a service link to a UE and a feeder link to a gateway (GW) for accessing the core network (CN) at different times (T1) and (T2). Specifically, at T1 a first satellite / base station (base station #1) provides the feeder link, and a second satellite / base station (base station #2) provides the service link. At T2 the situation is reversed, and the first satellite / base station (base station #1) provides the service link, and the second satellite / base station (base station #2) provides the feeder link. Hence, data communicated to base station #2 via the service link, and to base station #1 via the feeder link at time T1, cannot be sent respectively on to the core network and on to the UE until time T2.

[0028] It can be seen, therefore, that at a given time a UE may not have a full (end-to-end) connection all the way to the core network because the feeder link and associated service link connections are not necessarily available at the same time. In such scenarios, to avoid loss of data, the communication over the service link needs to be stored on the non-terrestrial platform for forwarding over the feeder link to the core network, or vice versa. Such techniques are known as "store and forward" techniques. These techniques are particularly applicable for delay-tolerant communications (i.e., non-real-time communications) such as those typically used in CIoT based communications.

[0029] By way of example only, one possible store and forward technique is illustrated in Fig. 2 which is a simplified sequence diagram illustrating a generalised procedure for forming a connection in an NTN system that involves storing and forwarding of user and control data. The illustrated procedure is in the context of a CIoT CP mode procedure.

[0030] As seen in Fig. 2 the procedure begins in a scenario in which a UE 3 is in coverage of a first base station 5A-1 on a first NTN-platform but the feeder link is disconnected (at S210). The UE 3 and first base station 5A-1 of a first NTN RAN 5-1 coordinate with each other to establish an RRC connection (at S212). This procedure will typically involve, for example, a random access procedure, as seen at S214 (e.g., as described above). The random access procedure culminates with the UE 3 sending the first base station 5A-1 a message indicating that an RRC has been completed that includes as a non-access stratum (NAS) payload, a UL NAS protocol data unit (PDU) including a control plane service request (CPSR) and / or control plane data (at S216). As the feeder link between the first base station 5A-1 and the core network 7 is disconnected, the first base station 5A-1 stores the NAS PDU and / or any data at S216. The first base station 5A-1 sends, at S218, a message for releasing the RRC connection to the UE 3 that includes an indication that no feeder link is available and an indication of a scheduled time when the UE 3 might expect a response from core network 7 and hence when it can perform the next transmission. The UE 3 may then enter an idle mode - effectively while awaiting a response. When the feeder link subsequently connects, at S220, the first base station 5A-1 can send, at S222 an initial UE message to the core network 7 including the NAS PDU / data. The core network 7 can determine, at S224, a second base station 5A-2 of a second NTN RAN 5-2 (via which a feeder link is / will be connected to the core network 7) will likely provide coverage to the UE 3 at a future juncture. When a feeder link to this second base station 5A-2 is available the core network 7 can send an appropriate DL NAS response PDU with any DL data at S226. The DL NAS PDU / data are stored at the second base station 5A-2 at S228. When the UE 3 is in the coverage of the second base station 5A-2 at S230, the second base station 5A-2 can page the UE 3 at S232. The UE 3 and the second base station 5A-2 can thus coordinate with one another, at S234, to establish a connection via which the DL NAS PDU and data may be delivered.

[0031] Nevertheless, the procedure in Fig. 2 does not take into account all the impacts of / issues associated with discontinuous coverage and intermittent feeder links.

[0032] In this context, as long as a full base station is on board the NTN platform, there is likely to be a greater impact on NAS procedures then AS procedures. This is because NAS procedures require connectivity from the UE all the way to the core network in both directions. For example, for conventional registration / attach procedures, a "store and forward" scheme would typically require, for every request message, a respective stored and forward cycle in one direction and, for every corresponding response message, another store and forward cycle in the opposite direction.

[0033] Whilst there are likely to be less impacts on AS procedures (like initial access), however, data transmission normally requests an end-to-end connection and UE context establishment between the UE and the core network / packet data network.

[0034] There is, therefore, a need for further improvements to better support implementation of store and forward techniques effectively, especially in the context of (but not limited to) discontinuous coverage / intermittent feeder links arising in an NTN system.

[0035] NPL 1: 3GPP Technical Report (TR) 38.811 NPL 2: NGMN 5G White Paper' V1.0

[0036] The disclosure aims to provide one or more apparatus and / or one or more associated methods that contributes to meeting the above need.

[0037] 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.

[0038] In one aspect there is provided a method performed by an access network node in a non-terrestrial network, the method comprising: 　　receiving, via an available link, user plane data, in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for another link than the available link; 　　storing the user plane data until the another link becomes available; and 　　forwarding, via the another link, the user plane data when the another link becomes available.

[0039] In one aspect there is provided a method performed by a user equipment, UE, the method comprising: 　　transmitting, to an access network node in a non-terrestrial network via a service link between the access network node and the UE, user plane data, in a case where a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for the feeder link, and wherein 　　the user plane data is stored by the access network node until the feeder link becomes available, and 　　the user plane data is forwarded via the feeder link when the feeder link becomes available.

[0040] In one aspect there is provided a method performed by a core network node, the method comprising: 　　receiving, from an access network node in a non-terrestrial network via a feeder link between the access network node and the core network node, user plane data, in a case where a service link between the access network node and a user equipment, UE, is unavailable, without triggering of resuming a connection for the service link, and wherein 　　the user plane data is stored by the access network node until the service link becomes available, and 　　the user plane data is forwarded via the service link when the service link becomes available.

[0041] In one aspect there is provided an access network node in a non-terrestrial network, the access network node comprising: 　　means for receiving, via an available link, user plane data, in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for another link than the available link; 　　means for storing the user plane data until the another link becomes available; and 　　means for forwarding, via the another link, the user plane data when the another link becomes available.

[0042] In one aspect there is provided a user equipment, UE, comprising: 　　means for transmitting, to an access network node in a non-terrestrial network via a service link between the access network node and the UE, user plane data, in a case where a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for the feeder link, and wherein 　　the user plane data is stored by the access network node until the feeder link becomes available, and 　　the user plane data is forwarded via the feeder link when the feeder link becomes available.

[0043] In one aspect there is provided a core network node comprising: 　　means for receiving, from an access network node in a non-terrestrial network via a feeder link between the access network node and the core network node, user plane data, in a case where a service link between the access network node and a user equipment, UE, is unavailable, without triggering of resuming a connection for the service link, and wherein 　　the user plane data is stored by the access network node until the service link becomes available, and 　　the user plane data is forwarded via the service link when the service link becomes available.

[0044] 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.

[0045] 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.

[0046] According to the present disclosure, it is possible to provide a method performed by an access network node, a method performed by a user equipment, a method performed by a core network node, an access network node, a user equipment, and a core network node.

[0047] Example embodiments of the disclosure will now be described, by way of example, with reference to the accompanying drawings in which:

[0048] Fig. 1 illustrates a scenario in which there is a change in the satellite, and hence base station, respectively providing a service link and a feeder link in an NTN system;Fig. 2 is a simplified sequence diagram illustrating a generalised procedure for forming a connection in an NTN system;Fig. 3 illustrates schematically an exemplary mobile (cellular or wireless) communication system;Fig. 4 is a simplified sequence diagram illustrating an attach procedure that may be used in the communication system of Fig. 3;Fig. 5 is a simplified sequence diagram illustrating an RRC connection suspend procedure that may be used in the communication system of Fig. 3;Fig. 6 is a simplified sequence diagram illustrating an RRC connection resume procedure that may be used in the communication system of Fig. 3;Fig. 7 is a simplified sequence diagram illustrating another RRC connection resume procedure that may be used in the communication system of Fig. 3;Fig. 8 illustrates schematically a non-terrestrial network (NTN) radio access network that may be used in the communication system of Fig. 3;Fig. 9A illustrates a possible architecture of an NTN RAN;Fig. 9B illustrates a possible architecture of an NTN RAN;Fig. 9C illustrates a possible architecture of an NTN RAN;Fig. 10 is a simplified sequence diagram illustrating a CIoT optimisation based data transmission flow between a base station of an NTN RAN, that may be used in the communication system of Fig. 3, when a service link is available, but a feeder link is unavailable;Fig. 11 is a simplified sequence diagram illustrating a CIoT optimisation based data transmission flow between an NTN RAN and a core network, that may be used in the communication system of Fig. 3, when a feeder link is available, but a service link is unavailable;Fig. 12 is a simplified sequence diagram illustrating an S1 setup procedure that may be used in the communication system of Fig. 3;Fig. 13 is a simplified sequence diagram illustrating a first procedure for supporting UE context retrieval between different base stations that may be used in the communication system of Fig. 3;Fig. 14 is a simplified sequence diagram illustrating a second procedure for supporting UE context retrieval between different base stations in the communication system that may be used in the communication system of Fig. 3;Fig. 15 is a simplified sequence diagram illustrating a third procedure for supporting UE context retrieval between different base stations that may be used in the communication system of Fig. 3;Fig. 16 is a simplified sequence diagram illustrating an overall attach-resume-suspend flow that may be used in the communication system of Fig. 3;Fig. 17 is a simplified block schematic illustrating the main components of a user equipment that may be used in the communication system of Fig. 3;Fig. 18 is a simplified block schematic illustrating the main components of a base station / access network node that may be used in the communication system of Fig. 3; andFig. 19 is a simplified block schematic illustrating the main components of a core network node that may be used in the communication system of Fig. 3.

[0049] < Overview > 　　An exemplary communication system will now be described in general terms, by way of example only, with reference to Figs. 3 to 9.

[0050] Fig. 3 schematically illustrates a mobile ('cellular' or 'wireless') communication system 1 to which the examples described herein are applicable.

[0051] In the communication system 1, user equipment (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 (RAN) 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 (which may be an NTN based RAN) includes a base station 5A-1, 5A-2 (e.g., an LTE / 4G base station such as an eNB) that respectively operates one or more associated cells 9 (9-1, 9-2).

[0052] As those skilled in the art will appreciate, whilst three UEs 3, and two RANs 5-1, 5-2 are shown in Fig. 3 for illustration purposes, the system, when implemented, will typically include other RAN 5 and UEs 3.

[0053] In the exemplary system the UEs 3 include one or more so-called 'internet-of-things' ('IoT') devices such as narrowband IoT (NB-IoT) devices or the like.

[0054] Each RAN 5-1, 5-2 controls one or more associated cells either directly, or indirectly via one or more other nodes (such as home base stations, relays, remote radio heads, distributed units, and / or the like). It will be appreciated that the RAN 5 may be configured to support 4G, 5G, 6G and / or later generations, and / or any other 3GPP or non-3GPP communication protocols.

[0055] 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 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 for 4G, 'Xn' interface for 5G, and / or the like).

[0056] The core network 7 includes a number of communication nodes / logical nodes (or 'functions') for supporting communication in the communication system 1. In this example, the core network 7 comprises control one or more network node entities for the communication of control signalling (e.g. Mobility Management Entities (MMEs) 11 or mobility management nodes 11), one or more network node entities for routing incoming and outgoing packets (e.g. Serving Gateways (S-GWs) 13), one or more network node entities for connecting the core network 7 and external networks 20 (e.g. Packet Data Network Gateways (P-GWs) 15) together with a number of other functional nodes (not shown). It will be appreciated that the nodes or functions may have different names in different systems. It will be appreciated that while the core network is described in the context of 4G entities and interface / reference points, the core network 7 could be any suitable core network (e.g., a 5G / 6G and / or later generations core network) with corresponding communication entities (e.g., control functions (CPFs) such as AMF, SMF, etc. - and one or more user plane functions (UPFs)).

[0057] The RAN 5 is connected to the core network nodes via appropriate interfaces (or 'reference points') such as an S1-MME reference point between the base station 5A of the RAN 5 and the MME 11 and an S1-U reference point between the base station 5A of the RAN 5 and the S-GW 13. The UEs 3 each connect to the MME 11, when applicable, via a non-access stratum (NAS) connection over an appropriate interface (e.g. an S1 reference point (analogous to the N1 reference point in 5G)). It will be appreciated, that S1 communications are routed transparently via the RAN 5.

[0058] The core network 7 (e.g., the P-GW 15) is connected to an external network 20 (e.g. an IP network such as the internet) via another reference point (e.g., 'SGi') for communication of the user data.

[0059] The MME 11 manages general mobility aspects of the UEs 3 and ensures that connectivity is maintained with the UE 3 as it is moving within the geographical area covered by the communication system 1 (and / or as the UE 3 is handed over between base stations 5A of the communication system 1). The MME 11 also handles control-plane signalling for the UE 3 and manages the various bearers associated with the UE 3 (e.g. such as an Evolved Packet System (EPS) bearer and / or a radio bearer), for example by controlling the S-GW 13 and the P-GW 15 (and / or possibly other network nodes) via which such bearers are provided.

[0060] The S-GW 13 provides a connection between the UE 3 and the core network 7 (via a base station 5A) for sending and receiving user plane data over an associated communication bearer (e.g. an EPS bearer). The communication bearer normally terminates at the P-GW 15, although it is often complemented by an external bearer as well (for example, another EPS bearer and / or the like) between the P-GW 15 and a communication endpoint outside the core network 7 (e.g. in the external network 20). It will be appreciated that, whilst shown as separate entities, the functionalities of the S-GW 13 and the P-GW 15 could be implemented in a single gateway element.

[0061] The RAN 5 is also configured for transmission of, and the UEs 3 are configured for the reception of, control information and user data via a number of downlink (DL) physical channels and for transmission of a number of physical signals. The DL physical channels correspond to resource elements (REs) carrying information originated from a higher layer, and the DL physical signals are used in the physical layer and correspond to REs which do not carry information originated from a higher layer.

[0062] The physical channels may include, for example, a physical downlink shared channel (PDSCH), a physical broadcast channel (PBCH), and a physical downlink control channel (PDCCH). The PDSCH carries data sharing the PDSCH's capacity on a time and frequency basis. The PDSCH can carry a variety of items of data including, for example, user data, UE-specific higher layer control messages mapped down from higher channels, system information blocks (SIBs), and paging. The PDCCH carries downlink control information (DCI) for supporting a number of functions including, for example, scheduling the downlink transmissions on the PDSCH and also the uplink data transmissions on a physical uplink shared channel (PUSCH). The PBCH provides UEs 3 with a 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.

[0063] The DL physical signals may include, for example, reference signals (RSs) and synchronization signals (SSs). A reference signal (sometimes known as a pilot signal) is a signal with a predefined special waveform known to both the UE 3 and the base station 5A 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).

[0064] Similarly, the UEs 3 are configured for transmission of, and the base station 5A 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.

[0065] < Attach Procedure & Initial Access > 　　The UEs 3, base station 5A of the RAN 5, and core network entities of the communication system 1 are mutually configured for performing an attach procedure for connecting the UE 3 to the network for communication of user data.

[0066] One possible such procedure that may be performed will now be described, by way of example only, with reference to Fig. 4 which is a simplified sequence diagram illustrating an attach procedure that may be used in the communication system 1.

[0067] As seen in Fig. 4, after initial synchronisation to the network (e.g., based on receipt of the PSS and SSS) the UE 3 receives a MIB, at S410, and one or more SIBs, at S412 (in this example at least system information block type 1 (SIB1)). The MIB typically provides, for example, information identifying a system bandwidth, an antenna configuration, and a system frame number. Receipt of the MIB and other system information allows the UE 3 to further (downlink) synchronise with the base station 5A of the RAN 5.

[0068] When the UE 3 needs to connect to the network it can then perform a random access channel (RACH) procedure for the UE 3 to access the network. Specifically, the UE 3 is able to attempt access to access a cell 9 (and / or beam) using an initial 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, at S414, the selected preamble (e.g., in 'Msg1') to the base station 5A 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 5A of the RAN 5 responds, at S416, with a random access response (RAR) (or 'Msg2'). The RAR indicates reception of the preamble and may include (for example): 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. At this point an initial signalling radio bearer (SRB), 'SRB0', is established for communicating certain types of RRC messages on a common control channel (CCCH). The UE 3 then sends, at S418, a third message ('Msg3') to the network over a physical uplink shared channel (PUSCH) based on the information in the RAR (e.g., using SRB0). 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 connection request or similar message carrying a temporary randomly generated UE identifier (e.g., a serving temporary mobile subscriber identity (S-TMSI)). The network responds, at S420, with a fourth message ('Msg4') carrying the randomly generated UE identifier received in Msg3 (e.g., 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 in which and another SRB, 'SRB1', is established for communicating certain RRC and NAS messages on a dedicated control channel (DCCH).

[0069] The UE 3 then attempts to achieve packet data network (PDN) connectivity by sending to the base station 5A of the RAN 5, at S422, a message indicating that RRC has been completed. This message includes as a NAS payload, an attach request to initiate the attach procedure and a PDN connectivity request. The base station 5A of the RAN 5 then sends, at S424, its first message to the core network 7 - an initial UE message containing the attach request and the PDN Connectivity Request. This message is sent to the core network node for providing mobility management functionality (in this example an MME 11 but for 5G may be an AMF). This message is sent via the S1-MME interface / reference point and includes information such as a tracking area identify (TAI) and E-UTRAN cell global identifier (ECGI) in this 4G example (similar but differently named messages / information elements may be used for 5G and other generations).

[0070] The mobility management node 11 coordinates, at S426, with another core network node (e.g., a home subscriber server HSS and / or authentication centre (AuC) in this example) to obtain authentication information, for example security information such as: KASME (an intermediate key that is derived in the HSS, and in the UE 3 from a cipher key, an integrity key, and a serving network identity (SN id)); AUTN (a so called authentication token generated at the AuC); XRES (a so-called 'expected response' generated at the AuC); and / or RAND (a random number for use in key generation and authentication).

[0071] The mobility management node 11 sends, at S428, an authentication request to the UE 3 (including RAND and AUTN) and the UE 3 responds with an authentication response, at S430, including an authentication response parameter calculated based on the RAND and AUTN and a key (K) stored at the UE 3 (e.g., in a subscriber identity module).

[0072] The mobility management node 11 then initiates NAS signalling security between the mobility management node 11 and the UE 3, at S432, by sending an NAS security mode command message informing the UE 3 of the respective algorithms to use for integrity protection and (de)ciphering. The UE 3 responds, at S434, after deriving appropriate security information, by sending a response message informing the mobility management node 11 that NAS signalling security initialisation is complete (at S436).

[0073] The mobility management node 11 coordinates, at S438, with one or more other core network nodes (e.g. an HSS) to obtain location update related information such as PDN subscription contexts (including e.g., EPS subscribed quality of service (QoS) profile and the subscribed access point name - aggregate maximum bit rate (APN-AMBR)).

[0074] The mobility management node 11 coordinates, at S440, with one or more other core network nodes (e.g. a S-GW 13 and / or P-GW 15 or combination thereof) to initiate establishment a communication GPRS tunnelling protocol (GTP) tunnel by sending an appropriate create session request (e.g. to the S-GW 13) and receiving an appropriate response once the tunnel is established. For example, after the S-GW 13 has sent a corresponding default bearer request to the P-GW 15, to create a new entry in its EPS bearer context table and a default bearer response is sent from the P-GW 15 to the S-GW 13 containing a P-GW 15 user plane address, P-GW 15 tunnel endpoint identifiers (TEIDs) for user and control plane, EPS bearer identity and QoS information. The P-GW 15 also sends downlink data that will be buffered in the S-GW 13 until connection is complete. An acknowledgment message is typically from the S-GW 13 to the mobility management node 11 that indicates a GTP for control (GTP-C) tunnel has been established.

[0075] The mobility management node 11 sends, at S442, initial context setup request (e.g., containing an S1 interface context setup request, NAS attachment accept, and activate default bearer request).

[0076] A UE capability exchange may follow in which the base station 5A sends (typically using RRC signalling), to the UE 3 (at S444), a UE capability inquiry to request information about the UE's capabilities. The UE 3 responds, at S446, with the requested UE capability information and the base station 5A provides an indication of this UE capability information to the mobility management node 11 at S448.

[0077] Access stratum (AS) security is then established. Specifically the base station 5A sends, at S450, an RRC security mode command with the AS integrity protection and encryption algorithms and 'START' parameters to the UE 3. The UE 3 calculates appropriate security keys using the received information and sends a message to the base station 5A, at S452, to indicate that RRC security mode completion. During this stage a further signalling radio bearer (SRB2) is established. SRB2 is used for RRC messages which include logged measurement information as well as for NAS messages, all using DCCH logical channel. SRB2 has a lower priority than SRB1 and is configured by the base station 5A after security activation.

[0078] RRC reconfiguration follows in which the base station 5A sends an RRC reconfiguration to the UE 3, at S454, to activate a default radio bearer. The UE 3 configures itself based on the information in the RRC reconfiguration and sends an RRC reconfiguration complete message at S456. The base station 5A then sends, at S458, a message to the mobility management node 11 to indicate that initial context setup is complete. The mobility management node 11 then coordinates with one or more other core network nodes (e.g. the S-GW 13) to modify the bearer appropriately and establish a data radio bearer (DRB) for the UE's communication.

[0079] While a four-step contention-based RACH procedure is described it will be appreciated that a UE 3 and the base station 5A of the 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 5A of the RAN 5 to the UE 3. Moreover, a 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).

[0080] 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).

[0081] < UP CIoT EPS Optimisation > The UEs 3, base station 5A of the RAN 5, and core network entities of the communication system 1 are mutually configured for implementing a number of procedures that are particularly relevant for, but not limited to, UP CIoT EPS Optimisation. These procedures include, for example, procedures for suspending and resuming AS (e.g., RRC) connections.

[0082] < Connection Suspend Procedure > 　　As mentioned above, the UEs 3, base station 5A of the RAN 5, and core network entities of the communication system 1 are mutually configured for performing connection suspend procedures (e.g., for suspending an established RRC connection).

[0083] One such procedure will now be described, by way of example only with reference to Fig. 5 which is a simplified sequence diagram illustrating an RRC connection suspend procedure that may be used in the communication system 1. In this procedure, suspension of an established RRC connection for UP CIoT EPS optimisation is described in the context of the 4G entities illustrated in Fig. 3. It will, nevertheless, be appreciated that a similar procedure may be followed by corresponding 5G entities for UP CIoT 5GS optimisation (or by corresponding devices of future generations). It will be appreciated that the description here is only intended as an overview and all parameters are therefore not listed in the message flows.

[0084] As seen in Fig. 5, before the suspension procedure commences UL data and / or DL data may be communicated via the established RRC connection.

[0085] At S501, due to one or more triggers (e.g., the expiry of a UE inactivity timer) the base station 5A of the RAN 5 decides to suspend the RRC connection.

[0086] At S502, the base station 5A initiates an S1-AP UE context suspend procedure, e.g., by sending an S1-AP UE context suspend request, to inform the MME 11 that the RRC connection is being suspended (for 5GS, the base station 5A may initiate an NG-AP UE context suspend procedure by sending an NG-AP UE context suspend request to inform an AMF that the RRC connection is being suspended).

[0087] At S503, the MME 11 requests the S-GW 13 to release all S1-U bearers for the UE 3 (for 5GS, the AMF may request an SMF to suspend a PDU session and the SMF may request a UPF to release tunnel information for the UE 3).

[0088] At S504, the MME 11 (AMF for 5GS) acknowledges the request sent at S502, e.g., by sending an S1-AP UE context suspend response (or a by sending an NG-AP UE context suspend response for 5GS).

[0089] At S505, the base station 5A suspends the RRC connection, e.g., by sending an RRC connection release message with a release cause set to 'rrc-Suspend'. The message includes a resume identifier, which is stored by the UE 3, and optionally, for early data transmission (EDT) and transmission using a Preconfigured Uplink Resource (PUR), the message may also include at least one security parameter (e.g., 'NextHopChainingCount' (NCC)) which is stored by the UE 3 and used to determine AS security keys (for 5GS, the message may include a radio network temporary identifier (RNTI), such as the so-called inactive RNTI (I-RNTI) together with the security parameter).

[0090] At S506, the UE 3 stores the AS context, suspends all SRBs and DRBs, and enters an RRC idle mode / state (RRC_IDLE).

[0091] < Connection Resume Procedure (same base station) > 　　As mentioned above, the UEs 3, base station 5A of the RAN 5, and core network entities of the communication system 1 are mutually configured for performing connection resumption procedures (e.g., for resuming a suspended RRC connection).

[0092] One such procedure will now be described, by way of example only with reference to Fig. 6 which is a simplified sequence diagram illustrating an RRC connection resume procedure that may be used in the communication system 1 in which a suspended connection (e.g., suspended using a procedure similar to that described with reference to Fig. 5) is resumed at the same base station as the base station 5A at which the original connection was suspended. In this procedure, resumption of an RRC connection for UP CIoT EPS optimisation is described in the context of the 4G entities illustrated in Fig. 3. It will, nevertheless, be appreciated that a similar procedure may be followed by corresponding 5G entities for UP CIoT 5GS optimisation (or by corresponding devices of future generations). It will be appreciated that the description here is only intended as an overview and all parameters are therefore not listed in the message flows.

[0093] At some point after the connection is suspended, (e.g. when the UE 3 is paged or when new data arrives in an uplink buffer of the UE 3) the UE 3 requests, at S601, resumption of the connection, e.g., by sending an RRC Connection Resume Request to the base station 5A. It will be appreciated that this may be after the UE 3 has performed a random access procedure by sending a random access preamble and receiving a corresponding random access response to obtain uplink resources for the request. The UE 3 includes its resume identifier (e.g., a 'resumeID' (or I-RNTI for 5GS)), the establishment / resume cause (e.g. a 'resumeCause'), and an authentication token (e.g., a 'shortResumeMAC-I'). The authentication token is calculated in the same way as the short message authentication code - integrity (MAC-I) that is used in RRC connection re-establishment, and allows the base station 5A to verify the UE identity (for 5GS, the UE 3 may resume SRB1, derive new security keys, e.g., using the NCC provided in the RRC connection release message of the previous RRC connection, and re-establish the AS security).

[0094] At S602, provided that the resume identifier (or I-RNTI for 5GS) exists and the authentication token is successfully validated, the base station 5A responds with an RRC connection resume message. The RRC connection resume message includes (for EPS) the NCC value which is required in order to re-establish the AS security.

[0095] At S603, the UE 3 resumes all SRBs and DRBs and re-establishes the AS security (for 5GS, the UE 3 resumes all other SRBs - i.e., in addition to SRB1 resumed at S601 - and all DRBs). The UE 3 is, therefore, now in a connected mode / state (RRC_CONNECTED).

[0096] At S604, the UE 3 indicates that the RRC connection resumption is complete, e.g., by sending an RRC connection resume complete message confirming that the RRC connection was resumed successfully, along with an uplink buffer status report, and / or UL data, whenever possible, to the base station 5A.

[0097] At S605, the base station 5A initiates the S1-AP UE context resume procedure to notify the MME 11 about the UE state change, e.g., by sending an S1-AP UE context resume request message (for 5GS, the base station initiates the NG-AP context resume procedure to notify the AMF about the UE state change, e.g., by sending an NG-AP resume request message).

[0098] At S606, the MME 11 coordinates with the S-GW 13 to activate the S1-U bearers for the UE 3 (for 5GS, the AMF coordinates with the SMF to resume the PDU session and the SMF coordinates with the UPF to establish the tunnel information for the UE 3).

[0099] At S607, the MME 11 (AMF for 5GS) acknowledges the request sent at S605, e.g., by sending an S1-AP UE context resume response (or a by sending an NG-AP resume response for 5GS).

[0100] As seen in Fig. 6, after the resumption procedure is complete UL data and / or DL data may be communicated via the resumed RRC connection.

[0101] It can be seen, therefore, that the UE context is stored in the UE 3, the base station 5A of the RAN 5, and the MME 11 (or AMF for 5GS) at suspension of the connection. Mobile originated (MO) and / or mobile terminated (MT) data may trigger resumption both of an RRC and S1 connection (or NG connection for 5GS) at the same base station 5A.

[0102] < Connection Resume Procedure (different base station) > 　　In addition to resumption at the same base station 5A as described with reference to Fig. 6, the UEs 3, base station 5A of the RAN 5, and core network entities of the communication system 1 are mutually configured for performing connection resumption procedures for resuming a suspended RRC connection at a different base station 5A-2 than the base station 5A-1 at which the original connection was suspended.

[0103] One such procedure will now be described, by way of example only with reference to Fig. 7 which is a simplified sequence diagram illustrating another RRC connection resume procedure that may be used in the communication system 1. In this procedure the connection is resumed at a different base station 5A-2 than the original base station 5A-1 at which the original connection was suspended.

[0104] At some point after the connection is suspended at an original base station 5A-1 of an original RAN 5-1, (e.g. when the UE 3 is paged or when new data arrives in an uplink buffer of the UE 3) the UE 3 requests, at S701, resumption of the connection at a new base station 5A-2 of a new RAN 5-2, e.g., by sending an RRC Connection Resume Request to the new base station 5A-2. It will be appreciated that this may be after the UE 3 has performed a random access procedure by sending a random access preamble and receiving a corresponding random access response to obtain uplink resources for the request. The UE 3 includes its resume identifier (or I-RNTI for 5GS), the establishment / resume cause, and an authentication token (e.g., a 'shortResumeMAC-I'). The authentication token is calculated in the same way as the short message authentication code - integrity (MAC-I) that is used in RRC connection re-establishment, and allows the base station 5A-2 to verify the UE identity (for 5GS, the UE 3 may resume SRB1, derive new security keys, e.g., using the NCC provided in the RRC connection release message of the previous RRC connection, and re-establish the AS security).

[0105] At S702, the new base station 5A-2 locates the original base station 5A-1 using the resume identifier (or I-RNTI for 5GS) and initiates an X2-AP (or Xn-AP for 5GS) retrieve UE context procedure by sending an X2-AP (or Xn-AP for 5GS) retrieve UE context request message to the original base station 5A-1.

[0106] At S703, the original base station 5A-1 responds to the new base station 5A-2 with the UE context associated with the resume identifier (or I-RNTI for 5GS) in an X2-AP (or Xn-AP for 5GS) retrieve UE context response message.

[0107] At S704, provided that the resume identifier (or I-RNTI for 5GS) exists and the authentication token is successfully validated, the new base station 5A-2 responds with an RRC connection resume message. The RRC connection resume message includes (for EPS) the NCC value which is required in order to re-establish the AS security.

[0108] At S705, the UE 3 resumes all SRBs and DRBs and re-establishes the AS security (for 5GS, the UE 3 resumes all other SRBs - i.e., in addition to SRB1 resumed at S701 - and all DRBs). The UE 3 is, therefore, now in a connected mode / state (RRC_CONNECTED).

[0109] At S706, the UE 3 indicates that the RRC connection resumption is complete, e.g., by sending an RRC connection resume complete message confirming that the RRC connection was resumed successfully, along with an uplink buffer status report, and / or UL data, whenever possible, to the new base station 5A-2.

[0110] At S707, the new base station 5A-2 initiates an S1-AP path switch procedure (e.g., by sending an S1-AP path switch request) to establish an S1 UE associated signalling connection to the serving MME 11 and to request the MME 11 to resume the UE context (the serving MME 11 may be the same or different to the MME 11 serving the original base station 5A-1). It will be appreciated that, for 5GS, the new base station 5A-2 may initiates an NG-AP path switch procedure (e.g., by sending an NG-AP path switch request) to establish an NG UE associated signalling connection to the serving AMF and to request the AMF to resume the UE context.

[0111] At S708, the MME 11 coordinates with the S-GW 13 to activate the S1-U bearers for the UE 3 and update the downlink path (for 5GS, the AMF coordinates with the SMF to resume the PDU session and the SMF coordinates with the UPF to create the tunnel information for the UE 3 and update the downlink path).

[0112] At S709, the MME 11 (AMF for 5GS) acknowledges the request sent at S707, e.g., by sending an S1-AP path switch request acknowledgement (or a by sending an NG-AP path switch request acknowledgement for 5GS).

[0113] At S710, after the S1-AP path switch procedure the new base station 5A-2 triggers release of the UE context at the old base station 5A-1, e.g., by sending a X2-AP UE context release message to initiate the X2-AP UE context release procedure (for 5GS, after the NG-AP path switch procedure the new base station 5A-2 triggers release of the UE context at the old base station 5A-1, e.g., by sending a Xn-AP UE context release message to initiate the Xn-AP UE context release procedure).

[0114] As seen in Fig. 7, after the resumption procedure is complete UL data and / or DL data may be communicated via the resumed RRC connection.

[0115] It can be seen, therefore, that the UE context is stored in the UE 3, the base station 5A-1 of the RAN 5-1, and the MME 11 (or AMF for 5GS) at suspension of the connection. MO / MT data transmission may trigger resumption both of an RRC and S1 connection (or NG connection for 5GS) at the new base station 5A-2.

[0116] < NTN RAN > 　　In the exemplary communication system 1, each RAN 5 may be implemented as a non-terrestrial network (NTN) RAN 5.

[0117] Fig. 8 illustrates schematically one such NTN RAN 5 that may be used in the communication system 1 of Fig. 3.

[0118] As seen in Fig. 8, the NTN RAN 5 comprises a base station or 5A operating one or more associated cells 9, a gateway 5B, and a non-terrestrial (space or air borne) platform 5C (e.g. comprising one or more satellites and / or airborne vehicles), which may be referred to generally as a 'satellite' for simplicity. Communication via the NTN RAN 5 is routed through the core network 7 and external network 20 (e.g. via an appropriate interface (e.g. the N6 interface / reference point)).

[0119] The NTN RAN 5 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 9, or part of an NTN cell 9. Each NTN cell 9 has an associated Physical Cell Identity (PCI). The satellite beam footprints may be moving as the non-terrestrial (space or air borne) platform 5C is travelling along its orbit (e.g. as illustrated by the arrows A in Fig. 8). 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. Satellite beams and satellites are not considered visible from a UE 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).

[0120] The base station 5A of the NTN RAN 5 is configured to provide ephemeris data for the non-terrestrial (space or air borne) platform 5C, 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 subscriber identity module, 'SIM') 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.

[0121] Specifically, the base station 5A is able to provide satellite assistance information for the satellite as part of a dedicated system information block (SIB) that is broadcast to UEs 3 in a corresponding cell 9 of the NTN RAN 5 (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)).

[0122] < NTN RAN Architecture > 　　Figs. 9A, Fig. 9B, and Fig. 9C each respectively illustrate a possible architecture of an NTN RAN 5 that may be used.

[0123] The architecture of Fig. 9A may be referred to as a 'transparent satellite' based RAN architecture. In this architecture, the base station 5A 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 and via a non-terrestrial (space or air borne) platform 5C that has no base station functionality. The non-terrestrial (space or air borne) platform 5C relays these communications to and from the UEs 3 in each cell 9 operated by the base station 5A, and from and to the gateway 5B as required. The non-terrestrial (space or air borne) platform 5C 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 and the non-terrestrial (space or air borne) platform 5C effectively acts as part of the respective Uu interface (or reference point) between the base station 5A and each UE 3. Similarly, the respective service link between the non-terrestrial (space or air borne) platform 5C and each UE 3 effectively acts as another part of the respective Uu interface (or reference point) between the base station 5A and each UE 3. The base station's communication link with the core network 7 (e.g. for signalling over the N1, N2, N3 interface / reference point etc.) is provided solely terrestrially.

[0124] The architecture of Fig. 9B 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 is a base station 5A of a distributed type having a terrestrially located central unit (CU) 5ACUand a distributed unit (DU) 5ADUprovided on-board the non-terrestrial (space or air borne) platform 5C. The terrestrially located CU 5ACUperforms some of the (typically higher layer) functionality of the base station 5A whereas the non-terrestrially located DU 5ADUperforms other (typically lower layer) functionality of the base station 5A. The terrestrially located CU 5ACUcommunicates with the non-terrestrially located DU 5ADUvia the gateway 5B and an appropriate interface (e.g. an F1 interface) implemented via a satellite radio interface between the gateway 5B and the non-terrestrial (space or air borne) platform 5C in which the DU 5ADUis provided.

[0125] The non-terrestrial (space or air borne) platform 5C transmits communications destined for and originating from the UEs 3 in each cell 9 operated by the base station 5A, and from and to the gateway 5B 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 by the DU 5ADUand higher layer processing of that communication respectively destined for and originating from the UEs 3 is performed by the terrestrially located CU 5ACU.

[0126] Accordingly, in this implementation, the feeder link between the gateway 5B and the non-terrestrial (space or air borne) platform 5C effectively acts as the F1 interface (or reference point) between the CU 5ACUand DU 5ADUof the base station 5A. The respective service link between the non-terrestrial (space or air borne) platform 5C and each UE 3, on the other hand, effectively acts as the respective Uu interface (or reference point) between the base station 5A and each UE 3. The base station's communication link with the core network 7 (e.g. for signalling over the N1, N2, N3 interface / reference point etc.) is provided solely terrestrially.

[0127] The architecture of Fig. 9C 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 is provided on-board the non-terrestrial (space or air borne) platform 5C. The base station 5A on board the non-terrestrial (space or air borne) platform 5C transmits communications destined for and originating from the UEs 3 in each cell 9 operated by the base station 5A, and from and to the core network 7 via the gateway 5B 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 by the base station 5A.

[0128] Accordingly, in this implementation, the feeder link between the gateway 5B and the non-terrestrial (space or air borne) platform 5C effectively acts as part of the N1 / N2 / N3 interfaces (or reference points) between the base station 5A and the core network 7. The base station's communication link with the core network 7 (e.g. for signalling over the N1, N2, N3 interface / reference point etc.) is thus provided partly via the feeder link and partly terrestrially. The respective service link between the non-terrestrial (space or air borne) platform 5C and each UE 3, on the other hand, effectively acts as the respective Uu interface (or reference point) between the base station 5A and each UE 3.

[0129] The base station 5A thus controls one or more associated cells via the non-terrestrial (space or air borne) platform 5C. It will be appreciated that the base station 5A may be configured to support 4G, 5G, 6G and / or later generations, and / or any other 3GPP or non-3GPP communication protocols.

[0130] For the purposes of description, when implemented in the communication system 1, the NTN RAN 5 will be described in terms of the regenerative architecture illustrated in Fig. 9C. It will be appreciated, however, that the NTN RAN 5 could potentially use a different one of the architectures and the entities of the communication system 1 could be adapted accordingly.

[0131] < Enhanced Store and Forward Techniques based on UP CIoT EPS Optimisation > 　　Beneficially, for MO / MT data transmission without a full / end to end UE to CN / PDN connection (e.g., as result of discontinuous coverage / intermediate feeder link connection in the context of an NTN deployed RAN 5), the UE 3, base station 5A of the NTN RAN 5, and the communication entities of the core network 7 in the communication system 1 are mutually configured to implement one or more enhanced UP CIoT optimisation based features (i.e., where a suspended UE context with configured DRB can be resumed for data transmission) to support improved store and forward techniques.

[0132] Specifically, effective "store and forward" data transmission is enabled by means of one or more enhancements to the resume procedures (e.g., as described with reference to Figs. 6 and 7) in a "store and forward" mode cell 9.

[0133] These enhancements are described in more detail later with reference to user plane CIoT EPS optimisations but may be adapted appropriately based on CIoT 5GS optimisations (or similar).

[0134] In summary, the possible enhancements include: 　　Allowing data transmission, over the Uu interface between the UE 3 and base station 5A, to proceed without waiting for the S1 interface to be resumed; 　　Allowing data transmission, over the S1 interface between the base station 5A and the MME 11, to proceed without waiting for an RRC connection to be resumed; 　　Enhancement to the S1 setup procedure including introduction of a new indicator in the S1 setup procedure to support interoperability between the base station 5A and core network 7; 　　Paging enhancements including the base station 5A maintaining UE paging information at least when there is stored downlink data and introduction of a new paging trigger at the MME 11 ; 　　Introducing support for UE context retrieval from a previous 'anchor' base station 5A-1 to a new anchor base station 5A-2, including: 　　Supporting storage of a backup of the anchor base station side UE context in the core network 7; 　　Introduction of a UE triggered anchor base station reallocation procedure; 　　Introduction of a network triggered anchor base station reallocation procedure; and 　　It will be appreciated that use of the UP CIoT based data "store and forward" enhancements described herein beneficially have the potential to: reduce security risk; provide increased network control (e.g., of the maximum amount data can be stored and forwarded for a given UE 3, of other QoS capabilities); support a larger amount of data in general - this may be beneficial considering that a UE 3 may have more data waiting for transmission after losing coverage for a prolonged period.

[0135] < Store and Forward Data Transmission - Overview > 　　A possible UP CIoT optimisation based store and forward data transmission flow will now be described, by way of example only, with reference to Figs. 10 and 11.

[0136] In Figs. 10 and 11, it will be appreciated that EDT could potentially be used (in a similar manner to that described with reference to Fig. 5) with the UP CIoT based method for only one data packet in one direction for transmission. Nevertheless, in the interests of simplicity, EDT has not been included in the message flows.

[0137] < Service link available / feeder link unavailable > 　　As mentioned above, in one enhancement, data transmission is allowed to proceed over the Uu interface between the UE 3 and base station 5A without waiting for the S1 interface to be resumed. A possible implementation of this will now be described, by way of example only, with reference to Fig. 10.

[0138] Fig. 10 is a simplified sequence diagram illustrating a UP CIoT optimisation based data transmission flow between a base station 5A of an NTN RAN 5, that may be used in the communication system 1, when a service link is available, but a feeder link is unavailable.

[0139] In Fig. 10, the procedure generally follows the relevant parts of the procedures illustrated in Figs. 5 and 6 (for resume at the same base station 5A) between the base station 5A and the UE 3 and the general description related to corresponding steps of those procedures also applies here.

[0140] In Fig. 10, the base station 5A stores / maintains, for each UE 3 that the base station 5A is serving (and possibly that has an associated UE capability, is in allowed list, and / or meets some other criterion) a respective UE specific buffer / data area 1030. Each UE specific buffer / data area 1030 includes a set of data type specific (sub)buffers / data areas 1032, 1034, 1036, and 1038. In this example the data type specific (sub)buffers / data areas 1032, 1034, 1036, and 1038, shown include: an uplink data buffer 1032 for storing uplink data; a paging storage area 1034 for storing paging information; a downlink data buffer 1036 for storing downlink; and a UE context storage area 1038 for storing the UE context. It will be appreciated, nevertheless, that the UE specific buffer / data area 1030 may be configured to store a different set of information depending on requirements. It will also be appreciated that while the UE specific buffer 1030 is described as being 'logically' divided into the different (sub)buffers for clarity, in an implemented system such divisions into (sub)buffers may not be immediately apparent whilst still storing the same types of information.

[0141] In Fig. 10, at the start of the procedure it is assumed that the UE 3 is registered / attached to the core network 7 (e.g., having followed a procedure similar to that shown in Fig. 4) as indicated at S1020 and has a suspended connection (e.g., having followed a procedure similar to that shown in Fig. 5). Moreover, a UE context for the UE 3 has been stored in the UE 3 (as indicated at S1040a), the base station 5A (as indicated at S1040b), and the MME 11 (as indicated at S1040c) during or after the UE registration / UE attach procedure. It will be appreciated that the UE context for the UE 3 may have been stored at multiple base stations 5A.

[0142] In Fig. 10, when the service link becomes available, uplink data at the UE 3 (i.e., MO data) and / or downlink data stored in the downlink data buffer 1036 of the base station 5A (i.e., MT data) may trigger resumption of the RRC connection. Any uplink data transmitted over the service link is stored in the uplink data buffer 1032 of the base station 5A, and any downlink data stored in the downlink data buffer 1036 is forwarded over the service link. The storing of uplink data and / or forwarding of downlink data takes place substantially immediately after the RRC connection is resumed between the UE 3 and base station 5A (i.e., data is stored and / or forwarded without having to wait for the corresponding S1 connection to be resumed).

[0143] In more detail, when the service link becomes available, at some point (e.g. when the UE 3 is paged for the MT case, or if there is pending data in the uplink buffer of the UE 3, or if new data arrives in the uplink buffer of the UE 3 for the MO case) the UE 3 requests, at S1001, resumption of the connection, e.g., by sending an RRC Connection Resume Request to the base station 5A. This may be after the UE 3 has performed a random access procedure by sending a random access preamble (as indicated at S1000a) and receiving a corresponding random access response (as indicated at S1000b) to obtain uplink resources for the request. The UE 3 includes its resume identifier, the establishment / resume cause, and an authentication token (e.g., the 'shortResumeMAC-I'), e.g., as described in respect of step S601 of Fig. 6.

[0144] It will be appreciated that, for MT data, in order to support paging (as indicated at S1050b), paging relevant information may be stored in the paging storage area 1034. The base station 5A may be configured to initiate paging (as indicated at S1050a) when the base station 5A predicts that the UE 3 enters coverage and there is downlink data stored in the downlink data buffer 1036.

[0145] It will also be appreciated that, for MO data, the UE 3 may potentially wait until the UE 3 is in coverage of the same base station 5A that UE's connection was originally suspended at before initiating an RRC connection resume. This base station 5A will have the UE context of the UE 3 and so waiting has the potential benefit of avoiding / reducing the need for UE context reallocation.

[0146] At S1002, assuming that the resume identifier exists, and the authentication token is successfully validated, the base station 5A responds with an RRC connection resume message. The RRC connection resume message includes (for EPS) the NCC value for the purposes of re-establishing the AS security (e.g., as described in respect of step S602 of Fig. 6).

[0147] At S1003, the UE 3 resumes all SRBs and DRBs and re-establishes the AS security (e.g., as described in respect of step S603 of Fig. 6). The UE 3 is, therefore, now in a connected mode / state (RRC_CONNECTED).

[0148] At S1004, the UE 3 indicates that the RRC connection resumption is complete, e.g., by sending an RRC connection resume complete message confirming that the RRC connection was resumed successfully, along with an uplink buffer status report, and / or UL data, whenever possible, to the base station 5A (e.g., as described in respect of step S604 of Fig. 6).

[0149] Unlike the procedure of Fig. 6, however, at this stage, any pending / newly arriving uplink data is transmitted to the base station 5A (S1010a) and stored in the uplink data buffer 1032 (S1010b), and any downlink data stored in the downlink data buffer 1036 is forwarded to the UE 3 without requiring initiation of the S1-AP context UE resume procedure.

[0150] Following the uplink / downlink data exchange (e.g., if there is no further uplink / downlink data and / or the base station 5A determines that the service link will no longer be available shortly), rather than initiate the S1-AP context resume procedure, the base station 5A decides, at S1005, that the RRC connection should be suspended (e.g., as described in respect of step S501 of Fig. 5). At this stage a suspend procedure, similar to that described with reference to Fig. 5, is followed except that there is no need to perform the S1-AP UE context suspend procedure (from S502 to S504 in Fig. 5). Instead, the base station 5A suspends the RRC connection by sending, at S1006, an RRC connection release message with a release cause set to 'rrc-Suspend'. The message includes a resume identifier, which is stored by the UE 3, and optionally, the message may also include the NCC parameter) which is stored by the UE 3 and used to determine AS security keys (e.g., as described in respect of step S505 of Fig. 5).

[0151] At S1007, the UE 3 stores the AS context, suspends all SRBs and DRBs, and enters an RRC idle mode / state (RRC_IDLE) (e.g., as described in respect of step S506 of Fig. 5).

[0152] < Feeder link available / Service link unavailable > 　　As mentioned above, in one enhancement, data transmission is allowed to proceed over the S1 interface between the base station 5A and the MME 11 without waiting for an RRC connection to be resumed. A possible implementation of this will now be described, by way of example only, with reference to Fig. 11.

[0153] Fig. 11 is a simplified sequence diagram illustrating a CIoT optimisation based data transmission flow between an NTN RAN 5 and the core network 7, that may be used in the communication system 1, when a feeder link is available, but a service link is unavailable.

[0154] In Fig. 11, the procedure is generally similar the relevant parts of the procedures illustrated in Figs. 5 and 6 between the NTN RAN 5 and the core network entities such as the MME 11, and the S-GW 13 and the general description related to corresponding steps of that procedure also applies here. It will be appreciated that the procedures of Fig. 10 and Fig. 11 are not mutually exclusive and may be used in conjunction with one another in the communication system 1 depending on the service link / feeder link status.

[0155] In Fig. 11, the base station 5A stores / maintains, for each UE 3 that the base station 5A is serving (and possibly that has an associated UE capability, is in allowed list, and / or meets some other criterion) a respective UE specific buffer / data area 1030 described with reference to Fig .10. Specifically, each UE specific buffer / data area 1030 includes: an uplink data buffer 1032 for storing uplink data; a paging storage area 1034 for storing paging information; a downlink data buffer 1036 for storing downlink; and a UE context storage area 1038 for storing the UE context. It will be appreciated, nevertheless, that the UE specific buffer / data area 1030 may be configured to store a different set of information depending on requirements. It will also be appreciated that while the UE specific buffer 1030 is described as being 'logically' divided into the different (sub)buffers for clarity, in an implemented system such divisions into (sub)buffers may not be immediately apparent whilst still storing the same types of information.

[0156] In Fig. 11, at the start of the procedure it is assumed that the UE 3 is registered / attached to the core network 7 (e.g., having followed a procedure similar to that shown in Fig. 4) as indicated at S1120 and has a suspended connection (e.g., having followed a procedure similar to that shown in Fig. 5). Moreover, a UE context for the UE 3 has been stored in the UE 3 (as indicated at S1140a), the base station 5A (as indicated at S1140b), and the MME 11 (as indicated at S1140c) during or after the UE registration / UE attach procedure.

[0157] In Fig. 11, when the feeder link becomes available, uplink data stored in the uplink data buffer 1032 of the base station 5A (i.e., MO data) and / or pending / newly arriving downlink data in the core network 7 (i.e., MT data) may trigger resumption of the S1 UE context between the base station 5A and the core network 7.

[0158] Any downlink data transmitted over the feeder link is stored in the downlink data buffer 1036 of the base station 5A, and any uplink data stored in the uplink data buffer 1032 is forwarded over the feeder link. The storing of downlink data and / or forwarding of uplink data takes place substantially immediately after the UE context is resumed between the base station 5A and the core network 7.

[0159] Although not shown, it will be appreciated that, for MT data, the S-GW 13 having received, from a P-GW 15, downlink data packet for the UE 3, may send a downlink data notification message to the MME 11 for which it has control plane connectivity for the given UE 3. Similarly, the MME 11, may respond to the S-GW 13 with a downlink data notification acknowledgement message. The MME 11 may then determine which base station 5A, the UE 3 will (next) be in the coverage of, and that has the UE context for the UE 3 stored.

[0160] Moreover, for MT data, when there is data waiting for transmission to a UE 3, the MME 11 has decided the base station 5A in which the UE 3 will be in coverage (and that has the UE context for the UE 3 stored), and the feeder link between this MME 11 and the base station 5A has become available there may be different ways in which the UE context may be resumed over the S1 interface.

[0161] In one option (illustrated in box (A) in Fig. 11), the MME 11 may send a paging message to the base station 5A (as indicated at S1100). The base station 5A may store the paging information, and initiate an S1-AP UE context resume procedure, e.g., by sending an S1-AP UE context resume request message (as indicated at S1101a). The MME 11 may coordinate with the S-GW 13 to activate the S1-U bearers for the UE 3 (as indicated at S1102a). The MME 11 may then acknowledge the request sent at S1101a, e.g., by sending an S1-AP UE context resume response (as indicated at S1103a).

[0162] In another option (illustrated in box (B) in Fig. 11), MME paging of the base station 5A may be skipped and, instead, the MME 11 may initiate a UE context resume procedure, e.g., by sending a S1-AP UE context resume message to the base station 5A (as indicated at S1101b). In this case, the MME 11 may still coordinate with the S-GW 13 to activate the S1-U bearers for the UE 3 (as indicated at S1102b). The base station 5A may respond to the resume message sent at S1101b, e.g., by sending an S1-AP UE context resume complete message (as indicated at S1103b).

[0163] For MO data, base station 5A may simply initiate the UE context resume procedure shown in box (A) (without paging), e.g., by sending an S1-AP UE context resume request message (as indicated at S1101a). The MME 11 may coordinate with the S-GW 13 to activate the S1-U bearers for the UE 3 (as indicated at S1102a). The MME 11 may then acknowledge the request sent at S1101a, e.g., by sending an S1-AP UE context resume response (as indicated at S1103a).

[0164] When the UE context has been resumed between, any uplink data stored in the uplink data buffer 1032 of the base station 5A may be forwarded to the core network 7 (as indicated at S1110, and any pending / newly arriving downlink data may be transmitted to the base station 5A (as indicated at S1012) and stored in the downlink data buffer 1036.

[0165] Following the uplink / downlink data exchange (e.g., if there is no further uplink / downlink data and / or the base station 5A determines that the service link will no longer be available shortly), the base station 5A decides, at S1104, that the S1 connection should be suspended. At this stage a suspend procedure, similar to that described with reference to Fig. 5, is followed. Specifically, at S1105, the base station 5A initiates an S1-AP UE context suspend procedure, e.g., by sending an S1-AP UE context suspend request (e.g., as described in respect of step S502 of Fig. 5). At S1106, the MME 11 requests the S-GW 13 to release all S1-U bearers for the UE 3 (e.g., as described in respect of step S503 of Fig. 5). At S1107, the MME 11 acknowledges the request sent at S1105, e.g., by sending an S1-AP UE context suspend response e.g., as described in respect of step S504 of Fig. 5).

[0166] < S1 Setup Procedure > 　　As mentioned above, the communication system 1 may implement enhancements to the S1 setup procedure. The purpose of the S1 setup procedure is to exchange application level data needed for the base station 5A and the MME 11 to correctly interoperate on the S1 interface. The S1 procedure is the first S1-AP procedure triggered after a transport network layer (TNL) association has become operational. The procedure uses non-UE associated signalling.

[0167] In more detail, in this exemplary enhancement, the base station 5A and the MME 11 are mutually configured for performing an enhanced S1 Setup procedure, e.g., for use after a feeder link has become available but before any UE associated signalling has happened. Specifically, to ensure support for a multi-vendor network (RAN 5 and core network 7), the enhanced S1 Setup procedure allows for the inclusion of an indication of network capability / support for store and forward data transmission in at least one of the messages used during the S1 setup procedure.

[0168] This will now be described in more detail, by way of example only, with reference to Fig. 12, which is a simplified sequence diagram illustrating an S1 setup procedure.

[0169] As seen at S1200, in one option, an S1 setup request includes an information element to indicate capability / support for store and forward data transmission. For example, the information element may indicate support for "Store-forward mode", "Discontinuous feeder link", "Intermittent NTN connection", "UE Context suspend-resume for NTN", and / or the like. The information element may be an enumerated type indicating, for example, 'true' or 'false' for the capability / support. If the feature is supported at the MME 11, then the MME 11 may respond with a normal S1 setup response message (as seen at S1202a). If the feature is not supported at the MME 11, then the MME 11 may respond with an S1 setup failure message (as seen at S1202b).

[0170] As seen at S1210, in another option, a normal S1 setup request is sent, and the MME 11 may respond with an S1 setup response message (as seen at S1212) includes an information element to indicate a capability / support for store and forward data transmission. For example, the information element may indicate support for "Store-forward mode", "Discontinuous feeder link", "Intermittent NTN connection", "Non-contiguous S1 connection for NTN", and / or the like. The information element may be an enumerated type indicating, for example, 'true' or 'false' for the capability / support.

[0171] < Paging > 　　As mentioned above, the communication system 1 may implement one or more paging enhancements, A number of possible paging enhancements will now be described in more detail by way of example only.

[0172] As described above, in order to support paging of the UE 3 when there is no feeder link / S1 connection, paging relevant information may be stored at the base station 5A (e.g., in the paging storage area 1034) so that the base station 5A may initiate paging when the base station 5A predicts that the UE 3 enters coverage and there is downlink data in the downlink data buffer 1036 for forwarding.

[0173] It will be appreciated that there are a number of different ways in which paging relevant information may be obtained by the base station 5A from the MME 11.

[0174] For example, the paging relevant information may be included as part of the stored UE context, with paging relevant information being sent to / updated at the base station 5A during UE context management procedures. In this case there may be no separate distinct storage of the paging information and the UE context in the UE buffer 1038.

[0175] Alternatively or additionally, the paging relevant information may be included as part of a message sent by the MME 11 as part of a UE context resume / suspend procedure when the MME 11 is aware that there will be (or is likely to be) downlink data stored at the base station 5A (e.g., in the download data buffer 1036) for "store and forward" data transfer. For example, the paging relevant information may be included in one or more of the UE context resume / suspend response messages from MME 11 shown in Fig. 11 (e.g., at S1103a, S1101a, and / or S1105). Again, in this case there may be no separate distinct storage of the paging information and the UE context in the UE buffer 1038.

[0176] Alternatively or additionally, the paging relevant information may be included as part of a "paging message" sent from the MME 11 to the base station 5A before and / or after a UE context resume procedure (e.g., as shown at S1100 in Fig. 11). For example, the "paging message" may be sent when a downlink data notification is received at the MME 11, and the feeder link has become available to the base station 5A, to which the MME 11 intends to send the downlink data to for "store and forward" data transfer. This can be followed by UE context resume procedure (e.g., as described with reference to box (A), Fig. 11). Nevertheless, alternatively or additionally, a "paging message" may be sent from the MME 11 to the base station 5A after the base station 5A initiates the context resume procedure, if there is downlink data being sent to the base station 5A for "store and forward" data transmission. This may be beneficial because, in this case the paging message / information may not be available in the base station 5A yet.

[0177] In respect of the behaviour of the base station 5A, instead of trigging paging over the air interface immediately after receiving a "paging message" and / or associated paging relevant information, the base station 5A may simply store the paging relevant information for later usage (e.g., when the service link has become available).

[0178] < Anchor eNB re-allocation and UE context fetching > 　　As mentioned above, in one enhancement, the communication system 1 may provide support for UE context retrieval from a previous 'anchor' base station 5A to a new anchor base station 5A.

[0179] In this context the term "anchor" base station 5A is used to refer to a base station 5A at which the RRC connection was last suspended, and at which the UE AS context is stored. Hence, the RRC connection resumption procedure can generally be performed to such an anchor base station 5A without a need to perform UE context fetching.

[0180] In this regard, it will be appreciated that there may be no X2 interface over an inter-satellite link and so, for NTN RANs 5, normal UE context retrieval between base stations 5A may not be possible, and these enhancements are aimed at addressing this issue.

[0181] To support UE context retrieval between different base stations 5A in "store and forward" NTN RANs 5, a backup of the anchor base station side UE context may be stored in the core network 7 (and maintained to keep it up to date). When an anchor base station 5A reallocation is required, the new anchor base station 5A may retrieve the UE context directly from the MME 11 (i.e., retrieve the stored backup version). The UE context in the original anchor base station 5A may then be released when a feeder link between the original anchor base station 5A and the core network 7 becomes available.

[0182] A number of procedures for supporting UE context retrieval between different base stations 5A of "store and forward" NTN RANs 5, will now be described in more detail with reference to Figs. 13 to 15.

[0183] < Anchor base station side UE context backup stored and maintained in Core Network > 　　Fig. 13 is a simplified sequence diagram illustrating a first procedure, for supporting UE context retrieval between different base stations 5A in the communication system 1, in which a backup of an anchor base station side UE context may be stored in the core network 7.

[0184] As seen in Fig. 13, at the start of the procedure it is assumed that the UE 3 is registered / attached to the core network 7 (e.g., having followed a procedure similar to that shown in Fig. 4) as indicated at S1320. The UE 3 has previously connected to a first base station 5A-1, when the service link was available, and suspended that connection (e.g., when the service link was about to become unavailable). The first base station 5A-1 is, therefore, an anchor base station 5A-1 for a future resume procedure and the corresponding MME 11 may be considered to be an anchor MME 11. The associated UE context (e.g., including a resumeID, NextHopChainingcount, and / or the like) for the UE 3 has been stored in the UE 3 (as indicated at S1340a). Similarly, an anchor base station side UE context (e.g., including a resumeID, NextHopChainingcount, and / or the like) is stored at the anchor base station 5A-1 (as indicated at S1340b), and an anchor MME side UE context is stored at the MME 11 (as indicated at S1340c).

[0185] When the suspended connection is resumed, and then suspended again (e.g., having followed a procedure similar to that shown in Fig. 10), as indicated at S1301, an updated UE context may be stored at the anchor base station 5A-1 (e.g., in the UE context storage are 1038 of the UE specific buffer 1030).

[0186] When the feeder link between the first base station 5A-1 and the MME 11 becomes available and the S1-AP UE context resumed (e.g., as described with reference to box (A) in Fig. 11), the UE context resume request includes a container carrying an updated version of the base station stored UE context (for example, including a new resumeID, NextHopChainingcount, and / or the like).

[0187] The MME 11 stores a copy of the updated version of the base station stored UE context as indicated at S1360, which acts as a backup version for retrieval should the UE 3 wish to resume the connection at a different base station than the anchor base station 5A-1 (e.g., base station #2 5A-2). It will be appreciated that a previous (e.g., original, or previously updated) version of the base station stored UE context may have been stored at the MME 11 using a similar procedure, following a previous suspension, when the feeder link between the anchor base station 5A-1 and the MME 11 was available, and this previous version may be replaced by the updated version. Specifically, the original / previously updated version of the base station side UE context may have been provided to the MME 11 using a UE context resume request during a previous S1-AP UE context resume procedure. Moreover, it will be appreciated that updated / original UE context information may, alternatively / additionally be provided in a different message from the anchor base station 5A-1 (e.g., in the UE context resume complete message of the CN initiated context resume procedure illustrated at box (B) in Fig. 11, or in the UE context suspend request message).

[0188] As described with reference to Fig. 11, the MME 11 may then coordinate with the S-GW 13 to activate the S1-U bearers for the UE 3 (as indicated at S1302). The MME 11 may then acknowledge the request sent at S1301, e.g., by sending an S1-AP UE context resume response (as indicated at S1303).

[0189] Following an uplink / downlink data exchange (at S1310 and / or S1312), the anchor base station 5A-1 station decides, at S1304, that the S1 connection should be suspended. At this stage a suspend procedure, similar to that described with reference to Fig. 5, is followed. Specifically, at S1305, the anchor base station 5A-1 initiates an S1-AP UE context suspend procedure, e.g., by sending an S1-AP UE context suspend request (e.g., as described in respect of step S502 of Fig. 5). At S1306, the MME 11 requests the S-GW 13 to release all S1-U bearers for the UE 3 (e.g., as described in respect of step S503 of Fig. 5). At S1307, the MME 11 acknowledges the request sent at S1305, e.g., by sending an S1-AP UE context suspend response e.g., as described in respect of step S504 of Fig. 5).

[0190] In summary, therefore, it can be seen that in this example the MME 11 maintains a backup of an anchor base station side UE context. The anchor base station 5A-1 updates the MME 11 with an updated UE context if there is any change (e.g., NCC, resumeID) after the UE 3 reconnects and suspends the connection to the anchor base station 5A-1.

[0191] < UE triggered anchor base station reallocation > 　　Fig. 14 is a simplified sequence diagram illustrating a second procedure, for supporting UE context retrieval between different base stations 5A in the communication system 1, in which anchor base station reallocation is triggered, by the UE 3, from a first base station 5A-1 to a second base station 5A-2.

[0192] In this example, the UE 3 is beneficially able to trigger a procedure to reallocate the anchor base station from a first base station 5A-1 to a second base station 5A-2 during which the base station side UE context it retrieved from the MME 11 and stored at the second base station 5A-2. Hence, the UE 3 is able to resume the RRC connection to the second base station 5A-2. Nevertheless, it will be appreciated that the UE 3 may be configured to restrict attempts to connect to a different base station 5A unless needed, e.g., with the UE 3 being configured to try to reconnect to the original anchor base station 5A-1 unless the UE 3 will not be in the original anchor base station's coverage for very long time (e.g., a network configured / preconfigured period), and / or a different base station 5A-2 becomes available more regularly than the original anchor base station 5A-1.

[0193] As seen in Fig. 14, at the start of the procedure it is assumed that the UE 3 is registered / attached to the core network 7 (e.g., having followed a procedure similar to that shown in Fig. 4) as indicated at S1420. The UE 3 has previously connected to a first base station 5A-1, when the associated service link was available, and suspended that connection (e.g., when the service link was about to become unavailable). The first base station 5A-1 is, therefore, an anchor base station 5A-1. The associated UE context for the UE 3 has been stored in the UE 3 (as indicated at S1440a) and the anchor base station side UE context is stored at the anchor base station 5A-1 (as indicated at S1440b). The MME-UE context is stored at the MME 11 (as indicated at S1440c). In this example, the UE context information stored at the MME 11 includes the backup anchor base station side UE context (as indicated at S1460), e.g., following a similar procedure to that shown in Fig. 13 to store / update the anchor base station side UE context.

[0194] When the service link becomes available to the second base station 5A-2, the UE 3 sends, at S1401, a message indicating that an anchor base station change is requested to the second base station 5A-2. The message may be a conventional request for resumption of the connection (e.g., an RRC connection resume request, possibly including a dedicated resume cause indicating the connection is for requesting an anchor change). The message may include the same, or at least a relevant subset of the, information provided in a RRC connection resume request (e.g., 'resumeID', 'resumeCause', and / or 'shortResumeMAC-I'). The message may, nevertheless, trigger the anchor base station change, e.g., based on the resume cause or based on a resumeID which points to a base station (e.g., the first base station 5A-1) that is different from the base station 5A-2 to which the connection is being made.

[0195] It will be appreciated that the message indicating that an anchor base station change is requested may be sent after the UE 3 has performed a random access procedure by sending a random access preamble (as indicated at S1400a) and receiving a corresponding random access response (as indicated at S1400b) to obtain uplink resources for the request.

[0196] The second base station 5A-2 determines, at S1430, that the context is not available and / or that context fetching is needed and sends, to the UE 3 at S1402, a message acknowledging / responding to the message indicating that an anchor base station change is requested sent at S1401. This message may be an RRC connection resume reject message, may be an RRC connection reject message, or may be a dedicated anchor base station change response message. As shown, the message may include a cause value, or other parameter, to indicate that the base station 5A-2 is waiting for a serving / anchor base station reallocation, or for UE context fetching. Thus, the UE 3 is released into idle mode while waiting for the feeder link between the second base station 5A-2 and the MME 11 to become available to allow a context fetch procedure to be performed.

[0197] The second base station 5A-2 waits for the feeder link between the second base station 5A-2 and the MME 11 to become available and, when it does, the second base station 5A-2 initiates, at S1403, an S1-AP procedure for performing a path switch and retrieve the (backup) UE base station side context (e.g., by sending an S1-AP retrieve UE context and path switch request or the like). This establishes an S1 UE associated signalling connection to the serving MME 11 and requests the MME 11 to resume the UE context. At S1404, the MME 11 coordinates with the S-GW 13 to activate the S1-U bearers for the UE 3 and update the downlink path.

[0198] At S1405, the MME 11 responds to the request sent at S1405 (e.g., by sending an S1-AP retrieve UE context and path switch response or the like) with an information container carrying the stored base station side UE context for the previous anchor base station 5A-1. Hence, the second base station 5A-2 can store this UE context in the UE context storage area 1038 of the UE specific buffer 1030 for the UE 3. Accordingly, the next time the UE 3 attempts to resume the RRC connection with the second base station 5A-2 when the corresponding service link is available, the UE 3 can successfully follow a procedure similar to that shown in Fig. 10 to do so.

[0199] The MME 11 may, at this stage, forward any downlink data for the UE 3, at S1412, to the second base station 5A-2 for storage in the downlink data buffer 1036 of the UE specific buffer 1030 for that UE 3. Following this, the second (new anchor) base station 5A-2 station can decide, at S1406, that the S1 connection should be suspended. At this stage a suspend procedure, similar to that described with reference to Fig. 5, is followed. Specifically, at S1407, the new anchor base station 5A-2 initiates an S1-AP UE context suspend procedure, e.g., by sending an S1-AP UE context suspend request (e.g., as described in respect of step S502 of Fig. 5). At S1408, the MME 11 requests the S-GW 13 to release all S1-U bearers for the UE 3 (e.g., as described in respect of step S503 of Fig. 5). At S1409, the MME 11 acknowledges the request sent at S1407, e.g., by sending an S1-AP UE context suspend response e.g., as described in respect of step S504 of Fig. 5).

[0200] Subsequently, when the feeder link between the original base station 5A-1 and the MME 11 is available, the UE context may be released from that base station 5A-1 as indicated at S1470.

[0201] In summary, therefore, it can be seen that in this example the UE 3 triggers a procedure in which the second base station 5A-2 waits till the feeder link between the second base station 5A-2 and the MME 11 is available to retrieve the backup of UE context stored at the anchor base station 5A-1 from the MME 11. After retrieval, the MME 11 releases the UE context at the previous anchor base station 5A-1 when the feeder link between the first base station 5A-1 and the MME 11 becomes available. The second base station 5A-2 becomes the new anchor base station 5A-2, and the UE 3 should be able to successfully resume its RRC connection to the second base station 5A-2 for data transmission once the corresponding service link becomes available.

[0202] < Core Network CN triggered anchor base station reallocation > 　　Fig. 15 is a simplified sequence diagram illustrating a third procedure, for supporting UE context retrieval between different base stations 5A in the communication system 1, in which anchor base station reallocation is triggered by the communication system 1, for the UE 3, from a first base station 5A-1 to a second base station 5A-2.

[0203] In this example, the MME 11 is beneficially able to trigger a procedure to reallocate the anchor base station 5A-1 for a given UE 3 from a first base station 5A-1 to a second base station 5A-2 during which the base station side UE context it transferred from the MME 11 to the second base station 5A-2. Hence, that UE 3 is able to resume an RRC connection to the second base station 5A-2 even if it has not previously connected to it.

[0204] As seen in Fig. 15, at the start of the procedure it is assumed that the UE 3 is registered / attached to the core network 7 (e.g., having followed a procedure similar to that shown in Fig. 4) as indicated at S1520. The UE 3 has previously connected to a first base station 5A-1, when the associated service link was available, and suspended that connection (e.g., when the service link was about to become unavailable). The first base station 5A-1 is, therefore, the anchor base station 5A-1 for that UE 3. The associated UE context for the UE 3 has been stored in the UE 3 (as indicated at S1540a) and the anchor base station side UE context is stored at the anchor base station 5A-1 (as indicated at S1540b). The MME-UE context is stored at the MME 11 (as indicated at S1540c). In this example, the UE context information stored at the MME 11 includes the backup anchor base station side UE context (as indicated at S1560), e.g., following a similar procedure to that shown in Fig. 13 to store / update the anchor base station side UE context.

[0205] The MME 11 decides to reallocate the anchor base station 5A for the UE 3 from the first base station 5A-1 to the second base station 5A-2 as indicated at S1550. When the feeder link between the first base station 5A-1 and the MME 11 is available the MME 11 releases the UE context at the first base station 5A-1 as indicated at S1555. It will be appreciated that, while the context release is shown and described as occurring before the reallocation of the context to the second (new anchor) base station 5A-2, the MME 11 may wait to release the UE context from the first base station 5A-1 until reallocation of the context to the second base station 5A-2 is completed.

[0206] The MME 11 waits for the feeder link between the second base station 5A-2 and the MME 11 to become available and, when it does, the MME 11 sends, at S1501, an S1-AP message including an information container carrying the stored base station side UE context for the previous anchor base station 5A-1. Hence, the second base station 5A-2 can store this UE context in the UE context storage area 1038 of the UE specific buffer 1030 for the UE 3. Accordingly, when the UE 3 attempts to resume the RRC connection with the second base station 5A-2 when the corresponding service link is available, the UE 3 can successfully follow a procedure similar to that shown in Fig. 10 to do so.

[0207] The second base station 5A-2 may acknowledge the message received at S1501 using a suitable S1-AP acknowledgement message as indicated at S1502 (although such an acknowledgement may not be needed).

[0208] The MME 11 may, at this stage, forward any downlink data for the UE 3, at S1512, to the second base station 5A-2 for storage in the downlink data buffer 1036 of the UE specific buffer 1030 for that UE 3. Following this, the S1-AP context is suspended using an appropriate suspend procedure (e.g., as described with reference to steps S1406 to S1409 in Fig. 14).

[0209] When a service link becomes available between the UE 3 and the second (new anchor) base station 5A-2, the UE 3 can therefore resume a connection with the second base station 5A-2.

[0210] In the exemplary procedure, the UE 3 resumes the connection in response to the base station 5A-2 paging the UE 3, as indicated at S1506, for example based on paging relevant information provided during the UE context transfer procedure and stored at the second base station 5A-2 (e.g., in the paging storage area 1036 and / or as part of the UE context). In response to the paging, the UE 3 performs a random access procedure by sending a random access preamble (as indicated at S1507a) and receiving a corresponding random access response (as indicated at S1507b) to obtain uplink resources for a connection request. The UE 3 then requests, at S1508, resumption of the connection, e.g., by sending an RRC connection resume request to the base station 5A-2. The UE 3 includes, in the request, its resume identifier, the establishment / resume cause, and an authentication token (e.g., the 'shortResumeMAC-I'). The resumption procedure may then proceed, as described with reference to Fig. 10, between the UE 3 and the second base station 5A-2.

[0211] < Overall Attach-Resume-Suspend Flow > 　　It can be seen, therefore, that the communication system 1 may include one or more enhancements based to UP CIoT optimisation based resume and / or suspend procedures for supporting improved store and forward techniques.

[0212] By way of summary, a typical sequence of events that may occur in the communication system 1 will now be described by way of example only, with reference to Fig. 16, which is a simplified sequence diagram illustrating an overall attach-resume-suspend flow that may occur in the communication system 1.

[0213] It will be appreciated that the overall flow is generalised for illustrative purposes and does not show all the messages and associated information, that will be exchanged between the various communication entities in a fully implemented system, in detail.

[0214] In Fig. 16, it is assumed that the procedure commences with the UE 3 attached / registered (at S1600) following a UE attach / registration procedure (e.g., as described with reference to Fig. 4). The base station 5A has released an earlier RRC connection for the UE 3 and has stored the UE context (including the S-TMSI) as seen at S1610.

[0215] When the feeder link is unavailable, as indicated at S1650, but the UE 3 has some uplink data (MO data) to send it coordinates with the base station 5A to establish a connection at S1612 (e.g., as described with reference to Fig. 10). The base station 5A then checks that the UE context (e.g., the S-TMSI) matches the stored context at S1614 to verify the UE 3, and checks if "store-forward" data transfer based on UP CIoT EPS optimisation may be used (e.g., based on UE capability or the like). The base station 5A. The base station 5A activates AS security at S1616 (e.g., via the AS SMC procedure using a stored security context as described with reference to Fig. 4).

[0216] The UE 3 transmits UL data to the base station 5A and the base station 5A stores the (e.g., as in an uplink data buffer 1032 as described with reference to Fig. 10).

[0217] The base station 5A makes a decision to suspend the RRC connection at S1620 (e.g., because the service link is about to drop) and releases the RRC connection with suspend by sending, to the UE 3 at S1622, an RRC connection release (with a cause value indicating the release is for suspending the connection (e.g., 'rrc-Suspend')), e.g., as described with reference to Fig. 10.

[0218] When the service link is not available, as indicated at S1660, but the feeder link is available (i.e., following a switch from service link to the feeder link), the base station 5A sends an initial UE message to the MME 11 at S1624 (e.g., as would occur in a normal call flow following RRC connection as described with reference to Fig. 4 - but in this case there is no (active) RRC connection). The MME 11 replies appropriately and the MME 11 and base station 5A coordinate with one another to perform an initial context setup as seen at S1626.

[0219] The base station 5A can then forward the stored uplink data to the S-GW 13 (via the MME 11) as seen at S1628 (e.g., as described with reference to Fig. 11).

[0220] When there is no more uplink data to send, the base station 5A initiates a UE context suspend procedure, by sending a UE context suspend request at S1630, to inform the MME 11 that the connection is being suspended (e.g., as described in respect of step S502 of Fig. 5). Whilst not shown, the MME 11 may then proceed to coordinate with the S-GW 13 to release all S1-U bearers for the UE 3 and acknowledge the request sent at S1630 as described earlier.

[0221] < User Equipment > 　　Fig. 17 is a simplified block schematic illustrating the main components of a UE 3 for implementation in the system of Fig. 3.

[0222] 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 3 (e.g., a user interface 35, such as a touch screen / keypad / microphone / speaker and / or the like for, allowing direct control by and interaction with a user) and this may be provided by any one or any combination of hardware, software, and firmware, as appropriate. 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.

[0223] 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 communications control module 43.

[0224] The communications 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 3 and / or core network nodes). The communications 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 communications 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 communications 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 3 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.

[0225] It will be appreciated that the communications control module 43 may include a number of sub-modules ('layers' or 'entities') to support specific functionalities. For example, the communications 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.

[0226] The communications control module 43 is configured, in particular, to control the UE's communications, in accordance with any of the methods described herein.

[0227] < Base Station > 　　Fig. 18 is a simplified block schematic illustrating the main components of a base station 5A for implementation in the system of Fig. 3 (e.g. in an NTN access network or other such RAN 5).

[0228] 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 communications 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.

[0229] As shown, these software instructions include, among other things, an operating system 61 and a communications control module 63.

[0230] The communications control module 63 is operable to control the communication between the base station 5A and UEs 3 and other network entities (e.g., core network nodes) that communicate with the base station 5A. The communications control module 63 is configured for the overall control of the reception and decoding of uplink communications, via associated uplink channels (e.g., via a physical uplink control channel (PUCCH), a random-access channel (RACH), and / or a physical uplink shared channel (PUSCH)) including both dynamic and semi-static signalling (e.g., SRS). The communications control module 63 is also configured for the overall control of the transmission of downlink communications via associated downlink channels (e.g., via a physical downlink control channel (PDCCH) and / or a physical downlink shared channel (PDSCH)) including both dynamic and semi-persistent scheduling (e.g., SPS). The communications 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 5A 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.

[0231] It will be appreciated that the communications control module 63 may include a number of sub-modules ('layers' or 'entities') to support specific functionalities. For example, the communications control module 63 may include, for communicating with a UE 3, a PHY sub-module, a MAC sub-module, an RLC sub-module, a PDCP sub-module, an RRC sub-module, etc. Moreover, the communications control module 63 may include, for communicating with a core network entity such as an MME 11 (or similar node such as an AMF), 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).

[0232] 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.

[0233] < Core Network Node / Function > 　　Fig. 19 is a block diagram illustrating the main components of a core network node or function, such as the MME 11, S-GW 13, or P-GW 15 (or functionally similar node / function of 5G or other cellular technology such as an AMF, CPF, UPF, SMF etc.).

[0234] As shown, the core network function includes a transceiver circuit 71 which is operable to transmit signals to and to receive signals from other nodes (including the UE 3, the base station 5A, and other core network nodes) via a network interface 72. A controller 73 controls the operation of the core network function in accordance with software stored in a memory 74. The software may be pre-installed in the memory 74 and / or may be downloaded via the communication system 1 or from a removable data storage device (RMD), for example. The software includes, among other things, an operating system 75, and a communications control module 76.

[0235] The communications control module 76 is responsible for handling (generating / sending / receiving) signalling between the core network function and other nodes, such as the UE 3, the base station 5A, and other core network nodes.

[0236] It will be appreciated that the communications control module 63 may include a number of sub-modules ('layers' or 'entities') to support specific functionalities. For example, where the core network node is implemented as an MME 11 (or AMF for 5G), the communications control module 76 may include, for communicating with the base station 5A, an S1-AP sub-module, an SCTP sub-module, an IP sub-module, an L1 sub-module, an L2 sub-module, etc (or corresponding sub-modules for an AMF).

[0237] The communications control module 76 is configured in particular, to control the core network node's communications, in accordance with any of the methods described herein.

[0238] < 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 disclosures embodied therein.

[0239] It will be appreciated that description of features of and actions performed by a base station 5A (or eNB or gNB), NTN nodes, and UEs 3 may be applied equally to base stations 5A and UEs 3 that communicate in the terrestrial plane only (i.e. as part of a terrestrial RAN 5 without features of an NTN RAN 5 such as a gateway 5B and space or airborne platform) as to base stations 5A that communicate via a non-terrestrial plane.

[0240] Moreover, description of features of and actions performed by a base station 5A (or eNB or gNB), apply equally to distributed type base stations 5A as to non-distributed type base stations 5A.

[0241] 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.

[0242] In the above description the UE 3 and the base station 5A are described for ease of understanding as having a number of discrete functional components or modules. Whilst these modules may be provided in this way for certain applications, for example where an existing system has been modified to implement the disclosure, in other applications, for example in systems designed with the inventive features in mind from the outset, these modules may be built into the overall operating system or code and so these modules may not be discernible as discrete entities.

[0243] 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 3 or base station 5A 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 3 or the base station 5A in order to update their functionalities.

[0244] 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.

[0245] 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.

[0246] 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.

[0247] 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.

[0248] 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.).

[0249] 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.).

[0250] 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.).

[0251] 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.).

[0252] 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.).

[0253] 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.

[0254] 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)).

[0255] 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.

[0256] 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.

[0257] It will be appreciated that IoT technology can be implemented on any communication devices that can connect to a communications network for sending / receiving data, regardless of whether such communication devices are controlled by human input or software instructions stored in memory.

[0258] 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 1. This list is not exhaustive and is intended to be indicative of some examples of machine type communication applications.

[0259] Table 1

[0260] 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.

[0261] Each feature disclosed in this specification (which term includes the claims) and / or shown in the drawings may be incorporated in the disclosure independently of (or in combination with) any other disclosed and / or illustrated features where it is technically feasible to do so. In particular but without limitation the features of any of the claims dependent from a particular independent claim may be introduced into that independent claim in any combination or individually wherever doing so does not cause a technically incompatibility or result in something that does not make technical sense.

[0262] Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.

[0263] 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.

[0264] This application is based upon and claims the benefit of priority from UK patent application No. 2311351.7, filed on July 24, 2023, the disclosure of which is incorporated herein in its entirety by reference.

[0265] 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.

[0266] 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 an access network node in a non-terrestrial network, the method comprising: 　　receiving, via an available link, user plane data, in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for another link than the available link; 　　storing the user plane data until the another link becomes available; and 　　forwarding, via the another link, the user plane data when the another link becomes available. (Supplementary note 2) 　　The method according to Supplementary note 1, wherein 　　the receiving is performed using Control Plane CIoT optimization feature or Early Data Transmission, EDT. (Supplementary note 3) 　　The method according to Supplementary note 1, wherein 　　the forwarding is performed using Control Plane CIoT optimization feature or Early Data Transmission, EDT. (Supplementary note 4) 　　The method according to any one of Supplementary notes 1 to 3, further comprising: 　　receiving, via the feeder link, paging information used for the forwarding user plane data to the UE via the service link; and 　　paging the UE using the paging information in a case where the service link becomes available. (Supplementary note 5) 　　The method according to Supplementary note 4, wherein 　　the paging information is included in at least one of: 　　　　a paging message from a core network, 　　　　a message for resuming or suspending UE context, from the core network, or 　　　　UE context information from the core network. (Supplementary note 6) 　　The method according to any one of Supplementary notes 1 to 5, wherein 　　the forwarding, via the another link, the user plane data is performed in a case where a UE context for the UE has been retrieved from a core network and the another link has been resumed at the access network node. (Supplementary note 7) 　　The method according to Supplementary note 6, wherein 　　the core network receives UE context from at least one access network node via a respective feeder link. (Supplementary note 8) 　　The method according to Supplementary note 7, wherein 　　the UE context from the at least one access network node is transmitted in a case where there is a change in the UE context when the respective feeder link becomes available. (UE triggered anchor eNB reallocation from eNB1 to eNB2). (Supplementary note 9) 　　The method according to Supplementary note 7 or 8, further comprising: 　　receiving, from the UE, a request for resuming a connection for the service link to the access network node; 　　transmitting, to the core network via the feeder link, request for UE context for the UE; 　　receiving, from the core network via the feeder link, the UE context for the UE in response to the request from the access network node, and 　　wherein the UE context for the UE has been transmitted to the core network from a further access network node at which the connection for the service link has been suspended. (Supplementary note 10) 　　The method according to Supplementary note 7 or 8, further comprising: 　　receiving, from the core network via the feeder link, the UE context for the UE in a case where the core network determines that an anchor access network node which serves a connection for the service link to the access network node, and 　　wherein the UE context for the UE has been transmitted to the core network from a further access network node at which the connection for the service link has been suspended. (Supplementary note 11) 　　The method according to Supplementary note 9 or 10, wherein 　　the UE context at the further access network node is released in a case where a corresponding feeder link between the further access network node and a gateway coupled to the core network becomes available. (Supplementary note 12) 　　The method according to any one of Supplementary notes 1 to 11, further comprising: 　　transmitting, to a core network, information indicating a capability of storing and forwarding data in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable. (Supplementary note 13) 　　The method performed according to any one of Supplementary notes 1 to 11, further comprising: 　　receiving, from a core network, information indicating a capability of storing and forwarding data in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable. (Supplementary note 14) 　　A method performed by a user equipment, UE, the method comprising: 　　transmitting, to an access network node in a non-terrestrial network via a service link between the access network node and the UE, user plane data, in a case where a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for the feeder link, and wherein 　　the user plane data is stored by the access network node until the feeder link becomes available, and 　　the user plane data is forwarded via the feeder link when the feeder link becomes available. (Supplementary note 15) 　　The method according to Supplementary note 14, wherein 　　initiating resuming of a connection for the service link in a case where the UE is in coverage of an access network node at which the connection for the service link was suspended. (Supplementary note 16) 　　A method performed by a core network node, the method comprising: 　　receiving, from an access network node in a non-terrestrial network via a feeder link between the access network node and the core network node, user plane data, in a case where a service link between the access network node and a user equipment, UE, is unavailable, without triggering of resuming a connection for the service link, and wherein 　　the user plane data is stored by the access network node until the service link becomes available, and 　　the user plane data is forwarded via the service link when the service link becomes available. (Supplementary note 17) 　　An access network node in a non-terrestrial network, the access network node comprising: 　　means for receiving, via an available link, user plane data, in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for another link than the available link; 　　means for storing the user plane data until the another link becomes available; and 　　means for forwarding, via the another link, the user plane data when the another link becomes available. (Supplementary note 18) 　　A user equipment, UE, comprising: 　　means for transmitting, to an access network node in a non-terrestrial network via a service link between the access network node and the UE, user plane data, in a case where a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for the feeder link, and wherein 　　the user plane data is stored by the access network node until the feeder link becomes available, and 　　the user plane data is forwarded via the feeder link when the feeder link becomes available. (Supplementary note 19) 　　A core network node comprising: 　　means for receiving, from an access network node in a non-terrestrial network via a feeder link between the access network node and the core network node, user plane data, in a case where a service link between the access network node and a user equipment, UE, is unavailable, without triggering of resuming a connection for the service link, and wherein 　　the user plane data is stored by the access network node until the service link becomes available, and 　　the user plane data is forwarded via the service link when the service link becomes available.

[0267] 1　　communication system 3　　UEs 5　　radio access network (RAN) node, NTN RAN 5A　　base station 5A-1　　first base station, anchor base station, original base station 5A-2　　second base station, new anchor base station 5B　　gateway 5C　　platform 7　　core network 9　　cells 11　　Mobility Management Entities (MMEs) 13　　Serving Gateways (S-GWs) 15　　Packet Data Network Gateways (P-GWs) 20　　external network 31, 51, 71　　transceiver circuit 33, 53　　antenna 35　　user interface 55　　core network interface 37, 57, 73　　controller 39, 59, 74　　memory 72　　network interface 41, 61, 75　　operating system 43, 63, 76　　communications control module

Claims

1. A method performed by an access network node in a non-terrestrial network, the method comprising: 　　receiving, via an available link, user plane data, in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for another link than the available link; 　　storing the user plane data until the another link becomes available; and 　　forwarding, via the another link, the user plane data when the another link becomes available.

2. The method according to claim 1, wherein 　　the receiving is performed using User Plane CIoT optimization feature or Early Data Transmission, EDT.

3. The method according to claim 1, wherein 　　the forwarding is performed using Control Plane CIoT optimization feature or Early Data Transmission, EDT.

4. The method according to any one of claims 1 to 3, further comprising: 　　receiving, via the feeder link, paging information used for the forwarding user plane data to the UE via the service link; and 　　paging the UE using the paging information in a case where the service link becomes available.

5. The method according to claim 4, wherein 　　the paging information is included in at least one of: 　　　　a paging message from a core network, 　　　　a message for resuming or suspending UE context, from the core network, or 　　　　UE context information from the core network.

6. The method according to any one of claims 1 to 5, wherein 　　the forwarding, via the another link, the user plane data is performed in a case where a UE context for the UE has been retrieved from a core network and the another link has been resumed at the access network node.

7. The method according to claim 6, wherein 　　the core network receives UE context from at least one access network node via a respective feeder link.

8. The method according to claim 7, wherein 　　the UE context from the at least one access network node is transmitted in a case where there is a change in the UE context when the respective feeder link becomes available. (UE triggered anchor eNB reallocation from eNB1 to eNB2).

9. The method according to claim 7 or 8, further comprising: 　　receiving, from the UE, a request for resuming a connection for the service link to the access network node; 　　transmitting, to the core network via the feeder link, request for UE context for the UE; 　　receiving, from the core network via the feeder link, the UE context for the UE in response to the request from the access network node, and 　　wherein the UE context for the UE has been transmitted to the core network from a further access network node at which the connection for the service link has been suspended.

10. The method according to claim 7 or 8, further comprising: 　　receiving, from the core network via the feeder link, the UE context for the UE in a case where the core network determines that an anchor access network node which serves a connection for the service link to the access network node, and 　　wherein the UE context for the UE has been transmitted to the core network from a further access network node at which the connection for the service link has been suspended.

11. The method according to claim 9 or 10, wherein 　　the UE context at the further access network node is released in a case where a corresponding feeder link between the further access network node and a gateway coupled to the core network becomes available.

12. The method according to any one of claims 1 to 11, further comprising: 　　transmitting, to a core network, information indicating a capability of storing and forwarding data in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable.

13. The method performed according to any one of claims 1 to 11, further comprising: 　　receiving, from a core network, information indicating a capability of storing and forwarding data in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable.

14. A method performed by a user equipment, UE, the method comprising: 　　transmitting, to an access network node in a non-terrestrial network via a service link between the access network node and the UE, user plane data, in a case where a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for the feeder link, and wherein 　　the user plane data is stored by the access network node until the feeder link becomes available, and 　　the user plane data is forwarded via the feeder link when the feeder link becomes available.

15. The method according to claim 14, wherein 　　initiating resuming of a connection for the service link in a case where the UE is in coverage of an access network node at which the connection for the service link was suspended.

16. A method performed by a core network node, the method comprising: 　　receiving, from an access network node in a non-terrestrial network via a feeder link between the access network node and the core network node, user plane data, in a case where a service link between the access network node and a user equipment, UE, is unavailable, without triggering of resuming a connection for the service link, and wherein 　　the user plane data is stored by the access network node until the service link becomes available, and 　　the user plane data is forwarded via the service link when the service link becomes available.

17. An access network node in a non-terrestrial network, the access network node comprising: 　　means for receiving, via an available link, user plane data, in a case where either a service link between the access network node and a user equipment, UE, or a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for another link than the available link; 　　means for storing the user plane data until the another link becomes available; and 　　means for forwarding, via the another link, the user plane data when the another link becomes available.

18. A user equipment, UE, comprising: 　　means for transmitting, to an access network node in a non-terrestrial network via a service link between the access network node and the UE, user plane data, in a case where a feeder link between the access network node and a gateway in a terrestrial network is unavailable, without triggering of resuming a connection for the feeder link, and wherein 　　the user plane data is stored by the access network node until the feeder link becomes available, and 　　the user plane data is forwarded via the feeder link when the feeder link becomes available.

19. A core network node comprising: 　　means for receiving, from an access network node in a non-terrestrial network via a feeder link between the access network node and the core network node, user plane data, in a case where a service link between the access network node and a user equipment, UE, is unavailable, without triggering of resuming a connection for the service link, and wherein 　　the user plane data is stored by the access network node until the service link becomes available, and 　　the user plane data is forwarded via the service link when the service link becomes available.

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