Routing in Sidelink Relay Networks

Abstract

A routing method in a wireless communication system comprising a network, a destination Remote User Equipment (UE), and at least one intermediate UE, the method comprising: selecting at least one path

Description

BACKGROUND Field Certain examples of the present disclosure provide various techniques relating to routing in sidelink relay networks, in particular in a network incorporating Sidelink Relay Adaptation Protocol (SRAP), for example within 3rd Generation Partnership Project (3GPP) 5th Generation (5G) New Radio (NR) and NR-based relay networks. Description of the Related Art In 3rd Generation Partnership Project (3GPP) 5th Generation (5G) New Radio (NR), Sidelink Relay Adaptation Protocol (SRAP) is intended to allow the functioning of UE-to-Network (U2N) and UE-to-UE (U2U) Sidelink Relaying networks. For example, in the U2N case, a signal may be transmitted from a base station to a destination UE via a Relay UE which has a sidelink (PC5) connection to the destination Remote UE, and the SRAP layer is used on both the Uu link (between the base station and the Relay UE), and the PC5 link (between the Relay UE and the Remote UE). The main functionality of SRAP is mapping of UL PC5 bearers onto Uu bearers, and performing the inverse process on the DL. It is also used to identify nodes in the relaying network. Work on Release 17 of 3GPP 5G is currently underway. An aim of Release 17 is to develop and improve features relating to SRAP. The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention. SUMMARY It is an aim of certain examples of the present disclosure to address, solve and / or mitigate, at least partly, at least one of the problems and / or disadvantages associated with the related art, for example at least one of the problems and / or disadvantages described herein. It is an aim of certain examples of the present disclosure to provide at least one advantage over the related art, for example at least one of the advantages described herein. The present invention is defined in the independent claims. Advantageous features are defined in the dependent claims. Embodiments or examples disclosed in the description and / or figures falling outside the scope of the claims are to be understood as examples useful for understanding the present invention. Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 illustrates an example user plane protocol stack for L2 UE-to-NW Relay (source 3GPPTS 38.800v17.1.0); Figure 2 illustrates an example control plane protocol stack for L2 UE-to-NW Relay (source 3GPPTS 38.800v17.1.0); Figure 3 illustrates steps in routing method in a network comprising a base station (gNB), a Remote UE, and a Relay UE; Figure 4 illustrates steps in routing method in a system comprising source and target base stations (gNBs), a Remote UE, and a Relay UE; Figure 5 illustrates steps in routing method in a system comprising source and target base stations (gNBs), a Remote UE, and a Relay UE; Figure 6 is a block diagram of an exemplary network entity that may be used in examples of the present disclosure. DETAILED DESCRIPTION The following description of examples of the present disclosure, with reference to the accompanying drawings, is provided to assist in a comprehensive understanding of the present invention, as defined by the claims. The description includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the examples described herein can be made. The same or similar components may be designated by the same or similar reference numerals, although they may be illustrated in different drawings. Detailed descriptions of techniques, structures, constructions, functions or processes known in the art may be omitted for clarity and conciseness, and to avoid obscuring the subject matter of the present disclosure. The terms and words used herein are not limited to the bibliographical or standard meanings, but, are merely used to enable a clear and consistent understanding of the examples disclosed herein. Throughout the description and claims, the words “comprise”, “contain” and “include”, and variations thereof, for example “comprising”, “containing” and “including”, means “including but not limited to”, and is not intended to (and does not) exclude other features, elements, components, integers, steps, processes, functions, characteristics, and the like. Throughout the description and claims, the singular form, for example “a”, “an” and “the”, encompasses the plural unless the context otherwise requires. For example, reference to “an object” includes reference to one or more of such objects. Throughout the description and claims, language in the general form of “X for Y” (where Y is some action, process, function, activity or step and X is some means for carrying out that action, process, function, activity or step) encompasses means X adapted, configured or arranged specifically, but not necessarily exclusively, to do Y. Features, elements, components, integers, steps, processes, functions, characteristics, and the like, described in conjunction with a particular aspect, embodiment, example or claim are to be understood to be applicable to any other aspect, embodiment, example or claim disclosed herein unless incompatible therewith. The following examples are applicable to, and use terminology associated with, 3GPP 5G. However, the skilled person will appreciate that the techniques disclosed herein are not limited to these examples or to 3GPP 5G, and may be applied in any suitable system or standard, for example one or more existing and / or future generation wireless communication systems or standards. The skilled person will appreciate that the techniques disclosed herein may be applied in any existing or future releases of 3GPP 5G NR or any other relevant standard. For example, the functionality of the various network entities and other features disclosed herein may be applied to corresponding or equivalent entities or features in other communication systems or standards. Corresponding or equivalent entities or features may be regarded as entities or features that perform the same or similar role, function, operation or purpose within the network. For example, the functionality of an entity in the examples below may be applied to any other suitable type of entity performing functions of a network node. The skilled person will appreciate that certain examples of the present disclosure may not be directly related to standardization but rather proprietary implementation of some of the SRAP functions or non-SRAP related functions of NR Rel-17 and beyond networks. The skilled person will appreciate that the present invention is not limited to the specific examples disclosed herein. For example: • The techniques disclosed herein are not limited to 3GPP 5G. • The techniques disclosed herein are not limited to SRAP or particular relay networks. • One or more entities in the examples disclosed herein may be replaced with one or more alternative entities performing equivalent or corresponding functions, processes or operations. • One or more of the messages in the examples disclosed herein may be replaced with one or more alternative messages, signals or other type of information carriers that communicate equivalent or corresponding information. • One or more further elements, entities and / or messages may be added to the examples disclosed herein. • One or more non-essential elements, entities and / or messages may be omitted in certain examples. • The functions, processes or operations of a particular entity in one example may be divided between two or more separate entities in an alternative example. • The functions, processes or operations of two or more separate entities in one example may be performed by a single entity in an alternative example. • Information carried by a particular message in one example may be carried by two or more separate messages in an alternative example. • Information carried by two or more separate messages in one example may be carried by a single message in an alternative example. • The order in which operations are performed may be modified, if possible, in alternative examples. • The transmission of information between network entities is not limited to the specific form, type and / or order of messages described in relation to the examples disclosed herein. Certain examples of the present disclosure may be provided in the form of an apparatus / device / network entity configured to perform one or more defined network functions and / or a method therefor. Such an apparatus / device / network entity may comprise one or more elements, for example one or more of receivers, transmitters, transceivers, processors, controllers, modules, units, and the like, each element configured to perform one or more corresponding processes, operations and / or method steps for implementing the techniques described herein. For example, an operation / function of X may be performed by a module configured to perform X (or an X-module). Certain examples of the present disclosure may be provided in the form of a system (e.g. a network) comprising one or more such apparatuses / devices / network entities, and / or a method therefor. For example, in the following examples, a network may include one or more nodes. It will be appreciated that examples of the present disclosure may be realized in the form of hardware, software or a combination of hardware and software. Certain examples of the present disclosure may provide a computer program comprising instructions or code which, when executed, implement a method, system and / or apparatus in accordance with any aspect, claim, example and / or embodiment disclosed herein. Certain embodiments of the present disclosure provide a machine-readable storage storing such a program. Figures 1 and 2 respectively show User and Control plane protocol stacks for L2 UE-to-NW Relay, as captured in the 3GPP TS 38.800v17.1.0. 3GPP agreed to introduce the Adapt layer on the Uu link (link between Relay UE and the gNB), as shown in Figures 1 and 2 in shaded boxes. Presence of a separate Adapt layer on the Sidelink (SL), i.e. PC5 link (between Remote UE and Relay UE), has also been confirmed by 3GPP RAN2. The main agreed functionality of Adapt is mapping of UL PC5 bearers onto Uu bearers, and performing the inverse process on the DL. The Adapt layer was renamed Sidelink Relay Adaptation Protocol, or SRAP for short. As captured in 3GPP TS 38.351 specification (capturing SRAP), the following is the basic model and operation of SRAP as agreed by 3GPP: On the U2N Relay UE, the SRAP sublayer contains one SRAP entity at Uu interface and a separate collocated SRAP entity at the PC5 interface. On the U2N Remote UE, the SRAP sublayer contains only one SRAP entity at the PC5 interface. Each SRAP entity has a transmitting part and a receiving part. Across the PC5 interface, the transmitting part of the SRAP entity at the U2N Remote UE has a corresponding receiving part of an SRAP entity at the U2N Relay UE, and vice-versa. Across the Uu interface, the transmitting part of the SRAP entity at the U2N Relay UE has a corresponding receiving part of an SRAP entity at the gNB, and vice-versa. At the Remote UE, in the uplink (UL) direction the SRAP will determine SRAP UE ID and BEARER ID and add the SRAP header. At the Remote UE, on the downlink (DL), the SRAP will remove the SRAP header and deliver the packet to higher layers. At the Relay UE, on the UL the SRAP will map the packet from a PC5 channel to a Uu channel using the SRAP UE ID and BEARER ID contained in the packet itself, and the mapping configuration provided by the network. At the Relay UE, on the DL the SRAP will map the packet from a Uu channel to a PC5 channel using SRAP UE ID and BEARER ID contained in the packet itself, and the mapping configuration provided by the network. In future / upcoming Releases, multiple paths could be available for concurrent and / or alternate use between a network node (e.g. base station / gNB) and the destination Remote UE. In the present disclosure, there is provided a routing method in a wireless communication system comprising a network, a destination Remote UE, and at least one intermediate UE, the method comprising: selecting at least one path of a plurality of paths for communicating data between the network and destination Remote UE; wherein at least one of the plurality of paths is an indirect path that includes the intermediate UE; and wherein the at least one intermediate UE comprises at least one of a further Remote UE and a Relay UE. The network may comprise a core network and at least one network node such as a base station. A Relay UE may be a UE that provides functionality to support connectivity to the network for Remote UE(s). A Remote UE may be a UE that communicates with the network via a path including a Relay UE and / or which communicates with the network via a path including another Remote UE. A Remote UE may aggregate traffic from multiple Remote UEs. The plurality of paths may be used simultaneously (e.g. to transmit the same packet on each of the paths, to transmit control plane data, e.g. RRC signalling, on one path and user plane data on another path, to transmit higher priority user plane data on one path and lower priority data on another path) or may be switched between (e.g. based on priority levels). A path may be a direct path between a base station and the destination Remote UE. A path may also be an indirect path involving a Relay UE and / or Remote UE between a base station and the destination Remote UE. The Relay UE or Remote UE may be referred to as an intermediate node. An indirect path may involve additional intermediate nodes. For example, the additional intermediate nodes may be one or more additional Relay UEs, one or more additional Remote UEs, or a combination of these. Intermediate nodes may also be referred to as intermediate UEs, local nodes, or local UEs. For example, multiple paths may exist in any of the following example scenarios: A destination Remote UE can be reached via a direct path from a first base station, and an indirect path via a first Relay UE. A destination Remote UE can be reached via a direct path from a first base station, an indirect path via a first Relay UE, and an indirect path via the first Relay UE and another Remote UE. A destination Remote UE can be reached via a direct path from a first base station, and an indirect path via another Remote UE. A destination Remote UE can be reached via a direct path from a first base station, and an indirect path from a second base station via a Relay UE. A destination Remote UE can be reached via an indirect path from a first base station via a first Relay UE, and an indirect path from a second base station via a second Relay UE. A destination Remote UE can be reached via an indirect path from a first base station via a first Relay UE, and an indirect path from a second base station via the first Relay UE. It will be appreciated that these are merely examples, and that combinations of direct and indirect paths not listed above may be determined. In an example, the path selection may be performed by the network, for example by a base station / central unit (CU). The network may perform path selection and then configure base stations and / or intermediate nodes appropriately. Path selection may be performed by a base 29 07 24 station, and the base station may configure intermediate nodes accordingly. Path selection may also be performed or altered by an intermediate node (for example, by a Remote UE). Path selection may be based on factors such as network conditions, congestion indication, RLF indication, a path status including status of one or more of radio links comprising the path; 5 or any other appropriate factors. Intermediate nodes (e.g. a Remote UE) may provide feedback to the base station regarding path selection and / or path selection factors. Path selection at an intermediate node may comprise selecting a complete path between the destination the source, may comprise selecting a partial path between the intermediate node and the destination, and may comprise selecting only the next hop node. 10 In an example, the method may comprise determining the plurality of paths. Determining the plurality of paths may comprise identifying paths or defining paths. Determining the plurality of paths may comprise defining routing tables or identifying paths based on existing routing tables. Paths may be identified by one or more of: a path ID; destination node address; next 15 hop node address; the combination of destination node address and next hop node address. Paths’ identities may be determined by the network (e.g. CU / base station), and may be identified locally by intermediate nodes (e.g. a Remote UE) or the destination Remote UE. Local identification may improve adaptability, for example in the case of appearance of a new node which is only visible to local nodes. 20 Path identifications (IDs) may be used to allow identification of paths and differentiation between the paths. For example, the method may further comprise assigning Path IDs to the determined paths or selecting the at least one path based on a Path ID of the at least one path. In an example, the Path ID may be part of the SRAP packet (e.g. part of the header), 25 allowing data packets from a single end-to-end (E2E) bearer to go via different routes. In another example, the Path ID is part of a different layer (e.g. PDCP), enforcing the same path for all data packets from a single E2E bearer. Priority levels may be used to assist in selection of paths. For example, selecting the at least 30 one path may comprise selecting the at least one path based on priority levels. In an example, priority levels could always be used. In another example, priority levels could be used only in cases where a main intended path is unavailable (e.g. due to RLF / congestion), to decide which path to use. In an example, priority levels may be assigned to paths by the network (e.g. CU / base station). If a priority level is not assigned to a path by the network a certain default value may be assumed by the node performing or implementing path selection. In another example, priority levels could be decided by intermediate nodes (e.g. Relay UEs and / or Remote UEs). In this case feedback may be provided to the network on priorities assigned and / or paths used. In an example, the priority level could be the same as MAC / LCP logical channel priority (or logical channel group (LOG) that a logical channel is mapped to) for logical channel to which the SRAP packet is mapped. In this case, the priority level is set as the priority of logical channel to communicate through the path (E2E). If multiple logical channels with different priorities use the path, priority level can be assigned independently. In another example, priority level could be unrelated to the logical channel priority. Hop counts may be used to limit the number of hops per path. For example, selecting the at least one path may comprise selecting the at least one path based on a hop count. In an example, the hop count may be a limit to the number of hops per SRAP packet. The hop count can be decided on the fly by the base station or intermediate nodes (e.g. a Remote UE) depending on current link situation and / or certain requirements for packet transmission, for example latency. The hop count can be decided in advance, based on e.g. E2E QoS / delay requirements for the bearer being carried in the packets in question. The hop count may be part of the packet header and / or part of routing configuration table. This may be particularly useful for the multi-hop case (i.e. a case where there are at least two layers of U2N Relay nodes), but is also useful in the single-hop case, in case a Remote UE can be reached via a chain of other Remote UEs (thereby effectively creating multi-hop through a combination of U2N and U2U, or only U2U in a star formation but where the ‘direct’ link between source and destination UEs is unavailable so data goes around the chain). In another example, the hop count may be an E2E parameter for end-to-end bearers (e.g. traffic flow / QoS flow / radio bearer / logical channel). Path selection may be performed at least in part based on information in the packet header (e.g. SRAP header) such as destination address, maximum hop count, time stamp, expiry time, QoS requirements. Path selection may be based on routing tables. Routing tables may include information on links between nodes in the network, for example links between a base station, intermediate nodes (e.g. Remote UEs), and the destination Remote UE. Nodes may store routing tables comprising information about links between network nodes, and may select a path or next hop node based on the information in the routing table. Routing tables may be configured at nodes (e.g. Relay UEs and Remote UEs) by the network (e.g. base station / CU). In existing systems, traffic received by a Remote UE terminates at the Remote UE. In an example of the present disclosure, routing tables at Relay UEs may include links (either direct, or via a Relay UE) between Remote UEs to enable paths to a destination Remote UE via other Remote UEs. A Relay UE may store a routing table comprising information about links between Remote UEs, and may select a path or next hop node at least in part based on the information in the routing table. Routing tables may also be implemented at Remote UEs to enable a Remote UE to forward traffic to another Remote UE. That is, a Remote UE may store a routing table comprising information about links between Remote UEs, and may select a path or next hop node based on the information in the routing table. A cost metric may be determined by the network for each path, based on, for example, delay, fairness, or reliability (e.g. probability of correct packet reception). The cost metric may be the cost to a packet / bearer / traffic flow of using a particular path. The delay may be based on the delay per hop, or may be based on the E2E delay, i.e. having different delay per hop depending on overall number of hops, to ensure the same level of service regardless of the number of hops. Fairness may be defined as offering the same service to packets / bearers / traffic flows with same QoS requirement, regardless of the individual paths through the network. The cost metric may be configured at nodes (e.g. Relay UEs and Remote UEs) by the network (e.g. base station / CU). For example, the cost metric may be configured by the same signaling used to configure routing tables. The cost metric may be packet / bearer / flow specific. The cost metric may be used to assist local node (e.g. Remote UE) decision making. For example, selecting the at least one path may comprise selecting the at least one path based on a cost metric of the path. Based on the cost metric, the local node may determine to discard packets. The local node may determine a delay or reliability based on, for example, content of the packet (e.g. packet header); content of the routing table linked to a specific SRAP ID / bearer ID / flow ID; and / or direct signalling from the network. For example, the cost metric may define a maximum tolerable delay, and packets determined to exceed the delay may be discarded by the local node. This supports discarding at intermediate nodes (e.g. a Remote UE) of packets no longer ‘useful’, or packets less likely to meet their QoS requirement in case where there is possibility of buffer overflow. A user packet may be transmitted using multiple paths, for example Uu (direct) and U2N relay (indirect). In this case, for example, either the base station or the destination Remote UE may inform the Relay UE to discard a buffered adapt / SRAP layer packet at relay when the user packet is received via Uu direct. That is, when a packet is received at the first gNB or the destination Remote UE via a direct path (or a faster indirect path), one or more intermediate nodes (e.g. a Remote UE) in the (slower) indirect path may be informed by the base station or the destination Remote UE to discard the packet. This may reduce unnecessary signalling and resource usage when using multiple paths. Packets may be discarded by the intermediate nodes (e.g. a Remote UE) when it is clear they cannot be delivered while keeping the required QoS. In this case, the packets are not delivered and feedback to the originating node may be provided for purposes of retransmission / stopping or starting a delivery timer / modifying a counter of dropped packets or correctly received ones, etc. In existing systems, a Remote UE either originates or terminates traffic. In examples of the present disclosure, Remote UEs are able to act as intermediate nodes by forwarding traffic to other Remote UEs (for example the destination Remote UE) or Relay UEs. That is, a Remote UE in a wireless communication system may receive data from a first UE (e.g. Relay UE or Remote UE) and forward the data to a second UE (e.g. Relay UE or Remote UE). Some examples (where appropriate) may function without gNB involvement e.g. Remote UE and a further Remote UE and / or a Relay UE communicating between each other and transparently to gNB. This means a Remote UE may assign the path IDs etc. and then select paths (e.g. based on pre-configured conditions, path ID, a priority level, hop count, path status, routing table, cost metric, etc.). The Remote UE may also assign SRAP IDs to itself and / or other Remote UEs. Signals may be exchanged between the network nodes to support coordination. For example, the following signal exchanges may take place: Serving gNB to target gNB: L2 Relay UE configuration / L2 Remote UE configuration (e.g. SL communication parameters, SRAP configuration which may include routing table, Path IDs, priority levels, maximum hop counts, cost metrics, and SRAP IDs); Serving gNB to Remote UE / Relay UE: L2 Remote UE configurations Relay UE configuration (SRAP configuration); Target gNB to Serving gNB: refusal of path switching and / or providing level of availability (e.g. existing / expected load) and / or suggested L2 Relay UE configuration / L2 Remote UE configuration; Target gNB to Remote UE / Relay UE: L2 / SRAP reconfiguration. Figure 3 illustrates steps in routing method in a network comprising a base station (gNB), a Remote UE, and a Relay UE. In an example, the method may be based on the method described in TS 38.300 v17.0.0, 16.12.6.2. At step 0, UL and DL data is exchanged between the gNB and the Remote UE based on a current path. For example, the data may be transmitted and received via a direct path between the gNB and the Remote UE. It will be appreciated that this step may not be present, and that the method may begin at step 1. At step 1, measurement configuration and reporting takes place between the gNB and the Remote UE. For example, the L2 U2N Remote UE may report one or multiple candidate L2 U2N Relay UE(s) and Uu measurements, after it measures / discovers the candidate L2 U2N Relay UE(s): - The L2 U2N Remote UE may filter the appropriate L2 U2N Relay UE(s) according to Relay selection criteria before reporting. The L2 U2N Remote UE may report only the L2 U2N Relay UE candidate(s) that fulfil the higher layer criteria; - The reporting may include at least L2 U2N Relay UE ID, L2 U2N Relay UE's serving cell ID, and sidelink measurement quantity information. The sidelink measurement quantity can be sidelink reference signal received power (SL-RSRP) of the candidate L2 U2N Relay UE, and if SL-RSRP is not available, sidelink discovery reference signal received power (SD-RSRP) may be used. At step 2, the gNB determines to switch to an indirect path via a target Relay UE. That is, the gNB may perform path selection as described above. The determination to switch, and the selection of the indirect path or target Relay UE, may be based on the measurement configuration and reporting performed in step 1 (e.g. network conditions, congestion indication, RLF indication, path status including status of one or more of radio links comprising the path) and may alternatively or additionally be based on one or more of a Path ID, a priority level, a hop count, a routing table, and a cost metric. Then RRC Reconfiguration for the Remote UE is performed. For example, the gNB may decide to switch the L2 U2N Remote UE to a target L2 U2N Relay UE. Then the gNB may send an RRCReconfiguration message to the target L2 U2N Relay UE, which may include at least L2 U2N Remote UE's local ID and L2 ID, Uu and PC5 Relay RLC channel configuration for relaying, and bearer mapping configuration. At step 3, the gNB transmits an RRC Reconfiguration message to the Remote UE. For example, the gNB may send the RRCReconfiguration message to the L2 U2N Remote UE. The RRCReconfiguration message may include at least L2 U2N Relay UE ID, Remote UE's local ID, PC5 Relay RLC channel configuration for relay traffic and the associated end-to-end radio bearer(s). The L2 U2N Remote UE may stop UP and CP transmission over Uu after reception of RRCReconfiguration message from the gNB. At step 4, PC5 (i.e. sidelink) connection is established between the Remote UE and the Relay UE. For example, the L2 U2N Remote UE may establish PC5 RRC connection with target L2 U2N Relay UE. At step 5, the Remote UE transmits an RRC Reconfiguration Complete message to the gNB via the Relay UE. For example, the L2 U2N Remote UE may complete the path switch procedure by sending the RRCReconfigurationComplete message to the gNB via the L2 U2N Relay UE. At step 6, UL and DL data is exchanged between the gNB and Remote UE via the Relay UE. For example, the data may be transmitted and received via an indirect path between the gNB and the Remote UE, the indirect path including the Relay UE. For example, the data path may be switched from direct path to indirect path between the L2 U2N Remote UE and the gNB. For a Remote UE’s configuration at target gNB, the configuration may be transferred through RRCReconfiguration (i.e., step 3 in Figure 3) with path switch command from serving gNB to Remote UE, as in TS 38.300 v17.0.0, 16.12.6.2, for example. The parameters may be the same as in the existing intra gNB path switch command and may additionally or alternatively include one or more of the configuration parameters set out above, such as routing tables, Path IDs, priority levels, maximum hop counts, cost metrics, etc. For the Relay UE’s configuration at target gNB, the configuration may be transferred through RRCReconfiguration (i.e., step 2 in Figure 3) by target gNB to Relay UE. The parameters may be the same as in the existing intra gNB path switch and may additionally or alternatively include one or more of the configuration parameters set out above, such as routing tables, Path IDs, priority levels, maximum hop counts, cost metrics, etc. It will be appreciated that a routing method similar to the above steps for switching from a direct path to an indirect path may also be applied to the case of switching a first indirect path via a first Relay UE to a second indirect path via a second Relay UE. In this case, the relay configuration of the second Relay UE (e.g. the RRC(Re)configuration message) may include additional signalling information such as relay configuration. For example, the steps may be as follows: At step 0, UL and DL data is exchanged between the gNB and the Remote UE based on a first indirect path via a first Relay UE. At step 1, measurement configuration and reporting takes place between the gNB and the first Relay UE. In an example, measurement configuration and reporting may also take place between the gNB and a second Relay UE. At step 2, the gNB determines to switch to a second indirect path via a (target) second Relay UE. That is, the gNB may perform path selection as described above. The determination to switch, and the selection of the indirect path or second Relay UE, may be based on the measurement configuration and reporting performed in step 1 (e.g. network conditions, congestion indication, RLF indication, path status including status of one or more of radio links comprising the path) and may alternatively or additionally be based on one or more of a Path ID, a priority level, a hop count, a routing table, and a cost metric. Then RRC Reconfiguration for the second Relay UE is performed between the second Relay UE and the gNB. In an example, the gNB may communicate with the first Relay UE to inform the first Relay UE of the path switch, update routing tables (for example to remove UEs no longer accessing the network via the first Relay UE), configure operation parameters, etc. The parameters for the second Relay UE configuration may be transferred through RRCReconfiguration and may be the same as in the existing intra gNB path switch and may additionally or alternatively include one or more of the configuration parameters set out above, such as routing tables, Path IDs, priority levels, maximum hop counts, cost metrics, etc. The parameters for the Remote UE configuration may be transferred through RRC Reconfiguration and may be the same as in the existing intra gNB path switch command and may additionally or alternatively include one or more of the configuration parameters set out above, such as routing tables, Path IDs, priority levels, maximum hop counts, cost metrics, etc. At step 3, the gNB transmits an RRC Reconfiguration message to the Remote UE. At step 4, PC5 (i.e. sidelink) connection is established between the Remote UE and the second Relay UE. At step 5, the Remote UE transmits an RRC Reconfiguration Complete message to the gNB via the second Relay UE. At step 6, UL and DL data is exchanged between the gNB and Remote UE via the second Relay UE. For example, the data may be transmitted and received via the second indirect path between the gNB and the Remote UE, the indirect path including the second Relay UE. Figures 4 and 5 illustrate steps in routing methods in systems comprising source and target base stations (gNBs), a Remote UE, and a Relay UE. When multi-path involves non-intra gNB cases, then simultaneous multiple paths can be configured similar to dual connectivity, which is different from path switch (handover). In non-intra gNB case, a different message or different IE is used for relay multi-path in standards than in the case of path switch command. As an example, Remote UE’s configuration at target gNB or Relay UE’s configuration at target gNB can be transferred through RRCReconfiguration message used to configure SCG (secondary cell group configuration), where the parameters are same as in intra gNB path switch command (i.e., step 3 in Figure 3) and the parameters are same as in intra gNB path switch (i.e., step 2 in Figure 3), respectively. Figure 4 illustrates establishment of an indirect path in addition to an existing indirect path in a non-intra gNB case. Although Figure 4 illustrates addition of an indirect path via the same (first) Relay UE as between the target gNB and the Remote UE, it will be appreciated that the additional indirect path may pass via a different (second) Relay UE: At step 0, UL and DL data is exchanged between the source gNB and the Remote UE based on an indirect path via the Relay UE. At step 1, measurement configuration and reporting takes place between the source gNB and the Remote UE via the Relay UE, and optionally between the target gNB and the Remote UE via the Relay UE. The measurement configuration and reporting in each case may be the same as for step 1 of Figure 3. Although Figure 4 illustrates measurement configuration and reporting taking place between the source gNB and the Remote UE via the same (first) Relay UE as between the target gNB and the Remote UE, it will be appreciated that the measurement configuration and reporting taking place between the target gNB and the Remote UE may take place via a different (second) Relay UE. At step 2, a decision is made by the network or the source gNB to add an indirect path from the target gNB to the Remote UE via the Relay UE. The additional indirect path may use the first (i.e. same) Relay UE or may use a second (i.e. different) Relay UE. The source gNB may perform path selection as described above. The determination to add the indirect path may be based on the measurement configuration and reporting performed in step 1 (e.g. network conditions, congestion indication, RLF indication, path status including status of one or more of radio links comprising the path) and may alternatively or additionally be based on one or more of a Path ID, a priority level, a hop count, a routing table, and a cost metric. At step 3, the Remote UE configuration and / or Relay UE configuration may be transferred from the source gNB to the target gNB. The parameters for the Relay UE configuration may be transferred through an SCG-configuration-like procedure. The parameters may be the same as in the existing intra gNB path switch and may additionally or alternatively include one or more of the configuration parameters set out above, such as routing tables, Path IDs, priority levels, maximum hop counts, cost metrics, etc. The parameters for the Remote UE configuration may be transferred through an SCG-configuration-like procedure. The parameters may be the same as in the existing intra gNB path switch command and may additionally or alternatively include one or more of the configuration parameters set out above, such as routing tables, Path IDs, priority levels, maximum hop counts, cost metrics, etc. At step 4, the target gNB performs RRC Reconfiguration. Step 4 may comprise transmitting an RRC Reconfiguration message including the Relay UE configuration to the (first or second) Relay UE. At step 5, the target gNB transmits an RRC Reconfiguration message including the Remote UE configuration to the Remote UE via the (first or second) Relay UE. At step 6, the Remote UE transmits an RRC Reconfiguration Complete message to the target gNB via the (first or second) Relay UE. In some examples, steps 5 and 6 may alternatively take place between the source gNB and the Remote UE instead of the target gNB and Remote UE. At step 7, RRC Reconfiguration optionally takes place between the first Relay UE and source gNB. The RRC Reconfiguration may include updated Relay UE configuration due to the additional indirect path. The Relay UE configuration may be updated at least in part according to the configuration in step 3. At step 8, UL and DL data is exchanged between the target gNB and Remote UE via the (first or second) Relay UE. At step 9, UL and DL data continues to be exchanged between the source gNB and Remote UE via the (first) Relay UE. An additional indirect path via the (same or different) Relay UE has been established between the target gNB and Remote UE in addition to the existing indirect path via the Relay UE between the source gNB and Remote UE. Figure 5 illustrates establishment of an indirect path in addition to an existing direct path in a non-intra gNB case: At step 0, UL and DL data is exchanged between the source gNB and the Remote UE based on a direct path. At step 1, measurement configuration and reporting takes place between the source gNB and the Remote UE. The measurement configuration and reporting in each case may be the same as for step 1 of Figure 3. At step 2, the source gNB decides to add an indirect path from the target gNB to the Remote UE via the Relay UE. The source gNB may perform path selection as described above. The determination to add the indirect path may be based on the measurement configuration and reporting performed in step 1 (e.g. network conditions, congestion indication, RLF indication, path status including status of one or more of radio links comprising the path) and may alternatively or additionally be based on one or more of a Path ID, a priority level, a hop count, a routing table, and a cost metric. At step 3, the Remote UE configuration and / or Relay UE configuration may be transferred from the source gNB to the target gNB. The parameters for the Relay UE configuration may be transferred through an SCG-configuration-like procedure. The parameters may be the same as in the existing intra gNB path switch and may additionally or alternatively include one or more of the configuration parameters set out above, such as routing tables, Path IDs, priority levels, maximum hop counts, cost metrics, etc. The parameters for the Remote UE configuration may be transferred through an SCG-configuration-like procedure and may be the same as in the existing intra gNB path switch command and may additionally or alternatively include one or more of the configuration parameters set out above, such as routing tables, Path IDs, priority levels, maximum hop counts, cost metrics, etc. At step 4, the target gNB performs RRC Reconfiguration. Step 4 may comprise transmitting an RRC Reconfiguration message including the Relay UE configuration to the Relay UE. At step 5, the source gNB transmits an RRC Reconfiguration message including the Remote UE configuration to the Remote UE. At step 6, the Remote UE transmits an RRC Reconfiguration Complete message to the source gNB. At step 7, UL and DL data is exchanged between the target gNB and Remote UE via the Relay UE. At step 8, UL and DL data continues to be exchanged between the source gNB and Remote UE. An indirect path via the Relay UE has been established between the target gNB and Remote UE in addition to the existing direct path between the source gNB and Remote UE. Certain examples of the present disclosure provide a first entity (e.g. a core network, a base station, a Relay UE, a Remote UE) configured to operate according to a method according to any example, embodiment, aspect and / or claim disclosed herein. Certain examples of the present disclosure provide a second entity (e.g. a core network, a base station, a Relay UE, a Remote UE) configured to cooperate with a first network entity of the preceding example according to any example, embodiment, aspect and / or claim disclosed herein. Certain examples of the present disclosure provide a network or wireless communication system comprising a first entity and a second entity according to any example, embodiment, aspect and / or claim disclosed herein. Certain examples of the present disclosure provide a computer program comprising instructions which, when the program is executed by a computer or processor, cause the computer or processor to carry out a method according to any example, embodiment, aspect and / or claim disclosed herein. Certain examples of the present disclosure provide a computer or processor-readable data carrier having stored thereon a computer program according to the preceding examples. Figure 6 is a block diagram of an exemplary entity (e.g. a core network entity, a base station, a Relay UE, a Remote UE) that may be used in examples of the present disclosure. The skilled person will appreciate that the entity illustrated in Figure 6 may be implemented, for example, as a network element on a dedicated hardware, as a software instance running on a dedicated hardware, or as a virtualised function instantiated on an appropriate platform, e.g. on a cloud infrastructure. The entity 600 comprises a processor (or controller) 601, a transmitter 603, and a receiver 605. The receiver 605 is configured for receiving one or more messages from one or more other network entities. The transmitter 603 is configured for transmitting one or more messages to one or more other network entities. The processor 601 is configured for performing operations as described above. In a first example, a routing method is provided in a wireless communication system comprising a network, a destination Remote UE, and at least one intermediate UE, the method comprising: selecting at least one path of a plurality of paths for communicating data between the network and destination Remote UE; wherein at least one of the plurality of paths is an indirect path that includes the at least one intermediate UE; and wherein the at least one intermediate UE comprises at least one of a further Remote UE and a Relay UE. In a second example, a routing method of a base station is provided, the method comprising: selecting at least one path of a plurality of paths for communicating data between a network and a destination Remote UE via the base station; wherein at least one of the plurality of paths is an indirect path that includes at least one intermediate UE; and wherein the at least one intermediate UE comprises at least one of a further Remote UE and a Relay UE. In a third example, a routing method of a Relay UE is provided, the method comprising: selecting at least one path of a plurality of paths for communicating data between a network and destination Remote UE via the Relay UE. In a fourth example, a routing method of a Remote UE is provided, the method comprising: selecting at least one path of a plurality of paths for communicating data between a network and the Remote UE; wherein at least one of the plurality of paths is an indirect path that includes at least one intermediate UE; and wherein the at least one intermediate UE comprises at least one of a further Remote UE and a Relay UE. In a fifth example, the method of any of the first to fourth examples is provided, wherein at least one of the plurality of paths is an indirect path that includes the further Remote UE or Relay UE and further includes an additional intermediate UE. In a sixth example, the method of the fifth example is provided, wherein the additional intermediate UE is an additional Remote UE. In a seventh example, a routing method of a first Remote UE is provided, the method comprising: selecting at least one path of a plurality of paths for communicating data between a network and a destination Remote UE via at least one of a further Remote UE and a Relay UE. In an eighth example, a wireless communication system comprising a network, a destination Remote UE, and at least one intermediate UE is provided, wherein the system is configured to: select at least one path of a plurality of paths for communicating data between the network and destination Remote UE; wherein at least one of the plurality of paths is an indirect path that includes the at least one intermediate UE; and wherein the at least one intermediate UE comprises at least one of a further Remote UE and a Relay UE. In a ninth example, a base station is provided, wherein the base station is configured to: select at least one path of a plurality of paths for communicating data between a network and a destination Remote UE via the base station; wherein at least one of the plurality of paths is an indirect path that includes at least one intermediate UE; and wherein the at least one intermediate UE comprises at least one of a further Remote UE and a Relay UE. In a tenth example, a Relay UE is provided, the Relay UE configured to: select at least one path of a plurality of paths for communicating data between a network and destination Remote UE via the Relay UE. In an eleventh example, a Remote UE is provided, the Remote UE configured to: select at least one path of a plurality of paths for communicating data between a network and the Remote UE; wherein at least one of the plurality of paths is an indirect path that includes at least one intermediate UE; and wherein the at least one intermediate UE comprises at least one of a further Remote UE and a Relay UE. In a twelfth example, the apparatus of any of the eighth to eleventh examples is provided, wherein at least one of the plurality of paths is an indirect path that includes the further Remote UE or Relay UE and further includes an additional intermediate UE. In a thirteenth example, the apparatus of the twelfth example is provided, wherein the additional intermediate UE is an additional Remote UE. In a fourteenth example, a first Remote UE is provided, the first Remote UE configured to: select at least one path of a plurality of paths for communicating data between a network and a destination Remote UE via at least one of a further Remote UE and a Relay UE. In a fifteenth example, the method, apparatus, or system of any of the first to fourteenth examples is provided, wherein selecting at least one path of a plurality of paths comprises selecting the at least one path based on one or more of: a path ID; a priority level; a hop count; a path status including status of one or more of radio links comprising the path; selecting individual hops that comprise the path, the selection being done by one or more nodes; a routing table; and a cost metric. In a sixteenth example, the method, apparatus, or system of any of the first to fifteenth examples is provided, wherein selecting at least one path of a plurality of paths comprises selecting a plurality of paths. In a seventeenth example, a method of a Remote UE in a wireless communication system is provided, the method comprising: receiving data from a first UE; and forwarding the data to a second UE; wherein each of the first UE and second UE comprises a Remote UE or a Relay UE. In an eighteenth example, the method of the seventeenth example is provided, the method further comprising assigning a path ID to the path comprising the Remote UE and the second UE. In a nineteenth example, the method of the seventeenth or eighteenth example is provided, the method further comprising assigning a SRAP ID to the Remote UE. In a twentieth example, the method of the nineteenth example is provided, the method further comprising assigning a SRAP ID to at least one of the first UE and second UE. In a twenty-first example, the method of any of the seventeenth to twentieth examples is provided, wherein forwarding the data to the second UE comprises selecting a path comprising the second UE based on one or more of one or more of: a path ID; a priority level; a hop count; a path status including status of one or more of radio links comprising the path; a routing table; and a cost metric. In a twenty-second example, a Remote UE is provided, the Remote UE configured to perform the method of any of the seventeenth to twenty-first examples. While the invention has been shown and described with reference to certain examples, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention, as defined by the appended claims. Certain examples of the present disclosure provide one or more techniques as disclosed in the appended annex to the description. The skilled person will appreciate that any of these techniques may be applied in combination with any of the techniques described above and illustrated in the Figures. Abbreviations / Definitions In the present disclosure, the following abbreviations and definitions may be used. 3GPP 5G 3rd Generation Partnership Project 5th Generation 5 CP Control Plane cu Central Unit DL DownLink E2E End-to-End gNB 5G base station 10 ID Identity / ldentifi cation IE Information Element L2 Layer 2 LCG Logical Channel Group LCP Logical Channel Priority 15 MAC Medium Access Control NR New Radio PC5 Sidelink interface PDCP Packet Data Conversion Protocol QoS Quality of Service 20 RAN Radio Access Network RAN2 Radio layer 2 and Radio layer 3 Working Group Rei Release RLC Radio Link Control RLF Radio Link Failure 25 RRC Radio Resource Control SD-RSRP Sidelink Discovery Reference Signal Received Power SL-RSRP Sidelink Reference Signal Received Power SL Sidelink SRAP Sidelink Relay Adaptation Protocol 30 TS Technical Specification U2N UE-to-Network U2U UE-to-UE UE User Equipment UL UpLink 35 UP User Plane Uu Air interface between terminal and base station / access point

Claims

1. A routing method in a wireless communication system comprising a network, a destination Remote User Equipment (UE), and at least one intermediate UE, the method comprising: selecting at least one path of a plurality of paths for communicating data between5 the network and destination Remote UE;wherein at least one of the plurality of paths is an indirect path that includes the at least one intermediate UE; andwherein the at least one intermediate UE comprises at least one of a Relay UE and a further Remote UE.

102. The method according to claim 1, wherein selecting at least one path of the plurality of paths comprises selecting the at least one path based on at least one of:a path identification (ID);a priority level;15 a hop count;a path status including status of one or more of radio links comprising the path;selecting individual hops that comprise the path, the selection being done by one or more nodes;a routing table; and20 a cost metric.

3. The method according to claim 1 or 2, further comprising: determining, at the network, the plurality of paths; and configuring, at the network, for each of the plurality of determined paths, at least one25 of a path identification (ID), a priority level, and a cost metric.

4. The method according to any preceding claim, further comprising determining, at the at least one intermediate node, to discard, based on a cost metric, a data packet received through the indirect path.

305. The method according to any preceding claim, further comprising configuring, by the network, a routing table comprising information on links between the network and the destination Remote UE via the at least one intermediate node.35 6. The method according to any preceding claim, wherein the at least one intermediatenode further comprises a further Remote UE, andwherein the method further comprises storing, at the further Remote UE, a routing table.

7. The method according to claim 6, wherein selecting at least one path comprises selecting, at the further Remote UE, a path based on the stored routing table.

8. The method according to any preceding claim, wherein selecting at least one path comprises selecting, at the network, the indirect path.

9. The method according to claim 8, wherein the at least one intermediate node comprises a Relay UE, the method further comprising transmitting, from the network to the Relay UE in the indirect path, a Relay UE configuration, the Relay UE configuration comprising at least one of:a routing table;a Path identification (ID);a priority level;a maximum hop count; and a cost metric.

10. The method according to claim any preceding claim, further comprising transmitting, from the network to the destination Remote UE or to the further Remote UE, a Remote UE configuration, the Remote UE configuration comprising at least one of:a routing table;a Path identification (ID);a priority level;a maximum hop count; and a cost metric.

11. A wireless communication system configured to operate according to the method of any preceding claim.

12. A base station configured to operate according to the method of any of claims 1, 2, 3, 5, 8, 9, or 10.

13. An intermediate User Equipment (UE) configured to operate according to the method of any of claims 1,2,4, 6, 7, or 8.

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