Method and apparatus for relay reselection and fallback operations in UE-to-UE relay scenarios

Abstract

The embodiment of the invention relates to a method and equipment for relay reselection and fallback operation in a user equipment (UE)-to-UE relay scene. In accordance with an embodiment of the present application, a user equipment (UE) includes a transceiver and a processor coupled to the transceiver; and the processor configured to: select a relay UE based on a relay selection operation; communicating with a further UE via the relay UE, wherein the UE and the further UE act as a UE-to-UE (U2U) remote UE; and performing at least one of: receiving a message from the relay UE, and performing a relay reselection operation after receiving the message; or measuring the quality of the direct PC5 link between the UE and the further UE.

Description

Technical Field

[0001] Embodiments of the present application generally relate to wireless communication technology, and more particularly, to methods and apparatus for relay reselection and fallback operations in a user equipment (UE) to UE relay scenario. Background Art

[0002] Vehicle-to-everything (V2X) has been introduced into 5G wireless communication technology. Given the channel structure of V2X communication, the direct link between two user equipments (UEs) is called a sidelink. Sidelink is a Long Term Evolution (LTE) feature introduced in 3GPP Release 12, and enables direct communication between UEs in close proximity without data passing through a base station (BS) or core network.

[0003] In the 3rd Generation Partnership Project (3GPP), it is proposed to deploy relay nodes (RN) in wireless communication systems. One purpose of deploying RN is to enhance the coverage of BS by improving the throughput of UEs that are within the coverage area or far away from the BS (which may result in relatively low signal quality). In some cases, RN may also be referred to as a relay UE. A 3GPP 5G sidelink system including a relay UE may be referred to as a sidelink relay system.

[0004] Currently, in 3GPP 5G wireless systems, etc., there is no clear discussion on how to design the details of relay reselection and fallback operations in UE-to-UE (U2U) relay scenarios. Summary of the invention

[0005] Some embodiments of the present disclosure provide a first user equipment (UE). The first UE may include: a transceiver; and a processor coupled to the transceiver. The processor may be configured to: select a first relay UE based on a relay selection operation; communicate with a second UE via the first relay UE, wherein the first UE and the second UE act as UE-to-UE (U2U) remote UEs; and perform at least one of the following: receive a first message from the relay UE and perform a relay reselection operation after receiving the first message; or measure the quality of a direct PC5 link between the first UE and the second UE.

[0006] In some embodiments of the present disclosure, the processor of the first UE is configured to receive at least one of the following from a serving cell: a first indication indicating whether a Layer 2 (L2) U2U relay UE function is supported by the serving cell; or a second indication indicating whether a discovery operation for the L2 U2U relay UE is supported by the serving cell.

[0007] In some embodiments of the present disclosure, at least one of the first indication or the second indication is broadcast by the serving cell via system information.

[0008] In some embodiments of the present disclosure, the processor of the first UE is configured to: determine whether the quality of the direct PC5 link between the first UE and the second UE is less than or equal to a first threshold associated with the relay selection operation; if the quality of the direct PC5 link between the first UE and the second UE is less than or equal to the first threshold, then select the first relay UE, wherein the first UE communicates with the second UE via the direct PC5 link before performing the relay selection operation; and transmit a second message to the second UE.

[0009] In some embodiments of the present disclosure, the processor of the first UE is configured to receive the first threshold associated with the relay selection operation from a serving cell.

[0010] In some embodiments of the present disclosure, the second message includes a third indication for instructing the second UE to perform a relay selection operation.

[0011] In some embodiments of the present disclosure, if the quality of the direct PC5 link is less than or equal to the first threshold, then the processor of the first UE is configured to transmit a fourth indication of releasing the PC5 link from the access stratum (AS) layer of the first UE to the upper layer of the first UE.

[0012] In some embodiments of the present disclosure, the fourth indication is used to trigger a PC5 unicast link release operation, and the second message is a PC5 unicast link release message transmitted by the upper layer of the first UE to the second UE.

[0013] In some embodiments of the present disclosure, the second message is at least one of the following: a PC5 unicast link release message; a radio resource control (RRC) reconfiguration side link message; or a notification message of a side link.

[0014] In some embodiments of the present disclosure, the second message includes one or more UE identities of one or more candidate relay UEs.

[0015] In some embodiments of the present disclosure, the processor of the first UE is configured to receive a response message from the second UE, and the response message includes a UE identification of a second relay UE selected by the second UE.

[0016] In some embodiments of the present disclosure, the second relay UE is the same as or different from the first relay UE.

[0017] In some embodiments of the present disclosure, if the quality of the direct PC5 link is less than or equal to the first threshold, then the processor of the first UE is configured to transmit a third message to the serving cell, and the third message includes at least one of the following: the first UE will act as a U2U remote UE; a request for resources for a discovery process for the first relay UE; or a request for resources for communicating with the second UE via the first relay UE.

[0018] In some embodiments of the present disclosure, the third message is a sidelink UE information message.

[0019] In some embodiments of the present disclosure, if the quality of the direct PC5 link is less than or equal to the first threshold, then the processor of the first UE is configured to perform one of the following: triggering a discovery process of the first relay UE; and releasing the direct PC5 link between the first UE and the second UE.

[0020] In some embodiments of the present disclosure, the quality of the direct PC5 link between the first UE and the second UE is measured after communicating with the second UE via the first relay UE, and the processor of the first UE is configured to receive at least one of the following from a serving cell: a second threshold associated with a fallback operation of the first UE; or a third threshold associated with the fallback operation of the first UE.

[0021] In some embodiments of the present disclosure, after measuring the quality of the direct PC5 link, the processor of the first UE is configured to: determine whether the quality of the direct PC5 link is greater than or equal to the second threshold; and if the quality of the direct PC5 link is greater than or equal to the second threshold, then perform the fallback operation to communicate with the second UE via the direct PC5 link.

[0022] In some embodiments of the present disclosure, after measuring the quality of the direct PC5 link, the processor of the first UE is configured to: determine whether the quality of the direct PC5 link is greater than or equal to the second threshold; if the quality of the direct PC5 link is greater than or equal to the second threshold, then determine whether the quality of the PC5 link between the first UE and the first relay UE is less than or equal to the third threshold; and if the quality of the PC5 link between the first UE and the first relay UE is less than or equal to the third threshold, then perform the fallback operation to communicate with the second UE via the direct PC5 link.

[0023] In some embodiments of the present disclosure, the processor of the first UE is configured to: determine whether the quality of the PC5 link between the first UE and the first relay UE is less than or equal to a fourth threshold; and if the quality of the PC5 link between the first UE and the first relay UE is less than or equal to the fourth threshold, then start measuring the quality of the direct PC5 link.

[0024] In some embodiments of the present disclosure, in order to perform the fallback operation, the processor of the first UE is configured to: establish the direct PC5 link between the first UE and the second UE; and release the PC5 link between the first UE and the first relay UE.

[0025] In some embodiments of the present disclosure, the first message is at least one of the following: a PC5 unicast link release message; or a PC5 RRC message.

[0026] In some embodiments of the present disclosure, the first message includes a fifth indication for instructing the first UE to perform at least one of the following: the relay reselection operation; or stop the service between the relay UE and the first UE.

[0027] In some embodiments of the present disclosure, the first message includes one or more UE identities of one or more candidate relay UEs.

[0028] In some embodiments of the present disclosure, the one or more UE identifications of the one or more candidate relay UEs are received by the relay UE from the second UE.

[0029] Some embodiments of the present application provide a relay UE. The relay UE may include: a transceiver; and a processor coupled to the transceiver. The processor may be configured to: establish a first PC5 radio resource control (RRC) link between the relay UE and a first UE; establish a second PC5 RRC link between the relay UE and a second UE, wherein the first UE and the second UE act as UE-to-UE (U2U) remote UEs; receive data from the first UE; and transmit the received data to the second UE.

[0030] In some embodiments of the present disclosure, the processor of the relay UE is configured to transmit a first message to a serving cell, and the first message includes at least one of the following: the relay UE will act as a U2U relay UE; a request for resources for a discovery process for at least one of the first UE or the second UE; or a request for resources for communicating with at least one of the first UE or the second UE.

[0031] In some embodiments of the present disclosure, the processor of the relay UE is configured to transmit a second message to the first UE, and wherein the second message is at least one of the following: a PC5 unicast link release message; or a PC5 RRC message.

[0032] In some embodiments of the present disclosure, the second message includes a first instruction for instructing the first UE to perform at least one of the following: a relay reselection operation; or stopping the service between the relay UE and the first UE.

[0033] In some embodiments of the present disclosure, the second message is transmitted in response to at least one of the following: the access stratum (AS) layer of the relay UE instructs the upper layer of the relay UE to release the PC5 link between the relay UE and the second UE; the relay UE receives a PC5 unicast link release message from the second UE; the relay UE receives configuration information about not acting as a U2U relay UE from a serving cell; the relay UE detects a radio link failure (RLF) of the PC5 link between the relay UE and the second UE; the relay UE determines that the quality of the PC5 link between the relay UE and the second UE is less than or equal to a second threshold; or the relay UE receives a second indication from the second UE, wherein the second indication is associated with the relay reselection operation of the first UE.

[0034] In some embodiments of the present disclosure, the AS layer of the relay UE instructs the upper layer of the relay UE after: the AS layer of the relay UE detects the RLF of the PC5 link between the relay UE and the second UE, or the AS layer of the relay UE receives a third indication from the AS layer of the second UE, wherein the third indication indicates the release of the PC5 link between the relay UE and the second UE.

[0035] In some embodiments of the present disclosure, the second message includes one or more UE identities of one or more candidate relay UEs.

[0036] In some embodiments of the present disclosure, the processor of the relay UE is configured to receive a third message from the second UE, and the third message includes the one or more UE identities of the one or more candidate relay UEs.

[0037] Some embodiments of the present application provide a network node (e.g., BS). The network node may include: a transceiver; and a processor coupled to the transceiver. The processor may be configured to: transmit to a UE a first indication indicating whether a layer 2 (L2) UE to UE (U2U) relay user equipment (UE) function is supported by the network node; and receive a second indication from the UE, wherein the second indication indicates at least one of: the UE will act as a U2U remote UE; or the UE will act as a U2U relay UE.

[0038] In some embodiments of the present disclosure, if the second indication indicates that the UE will act as the U2U remote UE, then the processor of the network node is configured to transmit at least one of the following to the UE: a first threshold associated with the relay selection operation of the UE; or a third indication for indicating whether the discovery operation of the L2 U2U relay UE is supported by the network node.

[0039] In some embodiments of the present disclosure, at least one of the first indication or the third indication is broadcast by the network node via system information.

[0040] In some embodiments of the present disclosure, if the second indication indicates that the UE will act as the U2U remote UE, then the processor of the network node is configured to receive a first message from the UE, and the first message includes at least one of the following: a request for resources for a discovery process for a relay UE; or a request for resources for communicating with a second UE via the relay UE, wherein the second UE acts as the U2U remote UE.

[0041] In some embodiments of the present disclosure, the first message is a sidelink UE information message.

[0042] In some embodiments of the present disclosure, if the second indication indicates that the UE will act as the U2U remote UE, then the processor of the network node is configured to transmit at least one of the following to the UE: a second threshold associated with the fallback operation of the UE; or a third threshold associated with the fallback operation of the UE.

[0043] In some embodiments of the present disclosure, if the second indication indicates that the UE will act as the U2U relay UE, then the processor of the network node is configured to receive a second message from the UE, and the second message includes at least one of the following: a request for resources for a discovery process for at least one remote UE; or a request for resources for communicating with the at least one remote UE.

[0044] Some embodiments of the present disclosure provide a method performed by a first user equipment (UE). The method may include: selecting a first relay UE based on a relay selection operation; communicating with a second UE via the first relay UE, wherein the first UE and the second UE act as UE-to-UE (U2U) remote UEs; and performing at least one of the following: receiving a first message from the relay UE and performing a relay reselection operation after receiving the first message; or measuring the quality of a direct PC5 link between the first UE and the second UE.

[0045] Some embodiments of the present disclosure provide a method performed by a relay UE. The method may include: establishing a first PC5 radio resource control (RRC) link between the relay UE and a first UE; establishing a second PC5 RRC link between the relay UE and a second UE, wherein the first UE and the second UE act as UE-to-UE (U2U) remote UEs; receiving data from the first UE; and transmitting the received data to the second UE.

[0046] Some embodiments of the present disclosure provide a method performed by a network node (e.g., a BS). The method may include: transmitting a first indication to a UE indicating whether a layer 2 (L2) UE-to-UE (U2U) relay user equipment (UE) function is supported by the network node; and receiving a second indication from the UE, wherein the second indication indicates at least one of: the UE will act as a U2U remote UE; or the UE will act as a U2U relay UE.

[0047] Some embodiments of the present application also provide an apparatus for wireless communication. The apparatus includes: a non-transitory computer-readable medium having computer-executable instructions stored thereon; a receiving circuit system; a transmitting circuit system; and a processor coupled to the non-transitory computer-readable medium, the receiving circuit system, and the transmitting circuit system, wherein the computer-executable instructions cause the processor to implement any one of the above methods performed by a UE, a relay UE, or a network node (e.g., a BS).

[0048] The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] To describe the manner in which the advantages and features of the present application can be obtained, the description of the present application is presented by reference to specific embodiments of the application illustrated in the accompanying drawings. These drawings depict only example embodiments of the present application and therefore should not be considered limiting of its scope.

[0050] Figure 1A schematic diagram illustrating a wireless communication system according to some embodiments of the present application.

[0051] Figure 2 An exemplary V2X communication system according to some embodiments of the present application is described.

[0052] Figure 3 An exemplary flow chart illustrating a sidelink RRC reconfiguration process according to some embodiments of the present application.

[0053] Figure 4 An exemplary flow chart illustrating a sidelink UE information process according to some embodiments of the present application.

[0054] Figure 5 An exemplary flow chart illustrating a layer 2 link release process according to some embodiments of the present application.

[0055] Figure 6 A flow chart illustrating a method performed by a UE according to some embodiments of the present application.

[0056] Figure 7 A flow chart illustrating a method performed by a relay UE according to some embodiments of the present application.

[0057] Figure 8 A flow chart illustrating a method performed by a network node according to some embodiments of the present application.

[0058] Fig. 9 A flow chart illustrating a method for relay selection operation according to some embodiments of the present application.

[0059] Fig.10 A flow chart illustrating a method for relay reselection operation according to some embodiments of the present application.

[0060] Fig.11 A flow chart illustrating a method for fallback operation of a remote UE according to some embodiments of the present application.

[0061] Fig.12 A block diagram illustrating an exemplary apparatus according to some embodiments of the present application. DETAILED DESCRIPTION

[0062] The detailed description of the accompanying drawings is intended as a description of the preferred embodiments of the present application, and is not intended to represent the only form in which the present application can be practiced. It should be understood that the same or equivalent functions can be implemented by different embodiments intended to be included in the spirit and scope of the present application.

[0063] Reference will now be made in detail to some embodiments of the present application, examples of which are illustrated in the accompanying drawings. For ease of understanding, embodiments are provided in specific network architectures and new service scenarios, such as 3GPP 5G, 3GPP LTE Release 8, etc. It is considered that, with the development of network architectures and new service scenarios, all embodiments in the present application are also applicable to similar technical problems; and in addition, the terms stated in the present application may be changed, which should not affect the principles of the present application.

[0064] Figure 1 A schematic diagram illustrating a wireless communication system according to some embodiments of the present application.

[0065] like Figure 1 As shown in FIG. 1 , for illustrative purposes, a wireless communication system 100 includes two UEs (ie, UE 101a and UE 101b), a BS 102, and a relay UE 103. Figure 1 A specific number of UEs, relay UEs, and BSs are depicted in FIG. 1 , but it is contemplated that any number of UEs, relay UEs, and BSs may be included in the wireless communication system 100 .

[0066] Since the distance between UE 101a and UE 101b is far, the two UEs communicate with each other via relay UE 103. UE 101a and UE 101b may be connected to relay UE 103 via a network interface (e.g., a PC5 interface as specified in a 3GPP standard document). UE 101a may be connected to BS 102 via a network interface (e.g., a Uu interface as specified in a 3GPP standard document). Figure 1 , UE 101a is connected to relay UE 103 via PC5 link 1, UE 101b is connected to relay UE 103 via PC5 link 2, and UE 101a is connected to BS 102 via Uu link. Figure 1 In some embodiments, the relay UE 103 or UE 101b may be connected to a BS, such as BS 102 or Figure 1 That is, UE 101a, UE 101b or relay UE 103 may be connected to the same BS or different BSs.

[0067] In some embodiments of the present application, UE 101a, UE 101b or relay UE 103 may include a computing device, such as a desktop computer, a laptop computer, a personal digital assistant (PDA), a tablet computer, a smart TV (e.g., a TV connected to the Internet), a set-top box, a game console, a security system (including a security camera), a vehicle-mounted computer, a network device (e.g., a router, a switch, and a modem), etc.

[0068] In some further embodiments of the present application, UE 101a, UE 101b or relay UE 103 may include a portable wireless communication device, a smart phone, a cellular phone, a flip phone, a device with a subscriber identity module, a personal computer, a selective call reception circuit system, or any other device capable of sending and receiving communication signals on a wireless network.

[0069] In some other embodiments of the present application, UE 101a, UE 101b or relay UE 103 may include a wearable device, such as a smart watch, a fitness band, an optical head-mounted display, etc. In addition, UE 101a, UE 101b or relay UE 103 may be referred to as a subscriber unit, a mobile device, a mobile station, a user, a terminal, a mobile terminal, a wireless terminal, a fixed terminal, a subscriber station, a user terminal or a device, or described using other terms used in the art.

[0070] BS102 may be distributed over a geographic area. In certain embodiments of the present application, each of BS102 may also be referred to as an access point, access terminal, base station, base station unit, macro cell, Node B, evolved Node B (eNB), gNB, home Node B, relay node or device, or described using other terms used in the art. BS102 is typically part of a radio access network, which may include one or more controllers communicatively coupled to one or more corresponding BS102.

[0071] The wireless communication system 100 may be compatible with any type of network capable of sending and receiving wireless communication signals. For example, the wireless communication system 100 is compatible with a wireless communication network, a cellular telephone network, a network based on time division multiple access (TDMA), a network based on code division multiple access (CDMA), a network based on orthogonal frequency division multiple access (OFDMA), an LTE network, a network based on 3GPP, a 3GPP5G network, a satellite communication network, a high altitude platform network, and / or other communication networks.

[0072] In some embodiments of the present application, the wireless communication system 100 is compatible with 5G NR of the 3GPP protocol, wherein the BS 102 transmits data using an OFDM modulation scheme on a downlink (DL), and the UE 101 (e.g., UE 101a, UE 101b, or other similar UEs) transmits data using a discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-S-OFDM) or cyclic prefix OFDM (CP-OFDM) scheme on an uplink (UL). However, more generally, the wireless communication system 100 may implement some other open or proprietary communication protocols, such as WiMAX and other protocols.

[0073] In some embodiments of the present application, BS 102 may communicate using other communication protocols (e.g., IEEE 802.11 wireless communication protocol family). In addition, in some embodiments of the present application, BS 102 may communicate on a licensed spectrum, while in other embodiments, BS 102 may communicate on an unlicensed spectrum. The present application is not intended to be limited to implementations of any particular wireless communication system architecture or protocol. In still other embodiments of the present application, BS 102 may communicate with UE 101 using 3GPP 5G protocols.

[0074] UE 101 can access BS 102 to receive data packets from BS 102 via a downlink channel and / or transmit data packets to BS 102 via an uplink channel. In normal operation, since UE 101 does not know when BS 102 will transmit data packets to it, UE 101 must always be in an awake state to monitor downlink channels (e.g., physical downlink control channel (PDCCH)) to prepare for receiving data packets from BS 102. However, if UE 101 always monitors the downlink channel even when there is no traffic between BS 102 and UE 101, this will result in serious power waste, which is problematic for power-limited UEs or power-sensitive UEs.

[0075] Generally, sidelink communication supports UE-to-UE (U2U) direct communication using two transmission modes. Two sidelink resource allocation modes are supported, namely, Mode 1 and Mode 2. In Mode 1, the sidelink resources are scheduled by the BS. In Mode 2, the UE determines the sidelink transmission resources and timing in the resource pool based on the measurement results and sensing results. Sidelink communication includes NR sidelink communication and V2X sidelink communication. Figure 2 Demonstrate NR sidelink communication. V2X sidelink communication is specified in 3GPP TS 36.300.

[0076] Figure 2 An exemplary V2X communication system according to some embodiments of the present application is described.

[0077] like Figure 2 As shown in FIG. 1 , the V2X communication system includes a gNB 202, an ng-eNB 203, and some V2X UEs, namely, UE 201-A, UE 201-B, and UE 201-C. Each of these UEs may refer to Figure 1 UE 101a, UE 101b or relay UE 103 shown and described therein.

[0078] Specifically, UE 201-A is within the coverage of gNB 202, UE 201-B is within the coverage of ng-eNB 203, and UE 201-C is outside the coverage of gNB 202 and ng-eNB 203. Support for V2X services via the PC5 interface may be provided by NR sidelink communication and / or V2X sidelink communication. NR sidelink communication may support one of three types of transmission modes for a pair of source layer 2 (L2) identification (ID) and destination L2 ID: unicast transmission; groupcast transmission; and broadcast transmission. Sidelink transmission and reception over the PC5 interface is supported when the UE is within the NG-RAN coverage or outside the NG-RAN coverage.

[0079] UE 201-A within the coverage of gNB 202 can perform sidelink unicast transmission, sidelink multicast transmission or sidelink broadcast transmission through the PC5 interface. UE 201-C outside the coverage can also perform sidelink transmission and reception through the PC5 interface. It is considered that according to some other embodiments of the present application, the V2X communication system may include more or fewer BSs and more or fewer V2X UEs. In addition, it is considered that Figure 2 The names of the V2X UEs (which represent Tx UE, Rx UE, etc.) described and shown in the figure may be different, for example, UE 201c, UE 204f, UE 208g, etc.

[0080] In addition, despite Figure 2 Each V2X UE shown in the figure is illustrated in the shape of a mobile phone, but it is contemplated that according to some other embodiments of the present application, the V2X communication system may include any type of UE (e.g., a roadmap device, a mobile phone, a computer, a laptop, an IoT (Internet of Things) device, or other type of device).

[0081] according to Figure 2 In some embodiments, UE 201-A is used as a Tx UE, and UE 201-B and UE 201-C are used as Rx UEs. UE 201-A may exchange V2X messages with UE 201-B or UE 201-C via a side link (e.g., a PC5 interface as defined in 3GPP TS23.303). UE 201-A may transmit information or data to other UEs within the V2X communication system via side link unicast, side link multicast, or side link broadcast. Side link communications include NR side link communications and V2X side link communications. For example, UE 201-A may transmit data to UE 201-C in an NR side link unicast session, and UE 201-B may transmit data to UE 201-C in a V2X side link unicast session. UE 201-A may transmit data to UE 201-B and UE 201-C in a multicast group via a side link multicast transmission session.

[0082] Sidelink communication includes NR sidelink communication and V2X sidelink communication. Figure 2 Demonstration of NR sidelink communications as specified in 3GPP TS 38.311. V2X sidelink communications as specified in 3GPP TS 36.311.

[0083] Figure 3 An exemplary flow chart illustrating a sidelink RRC reconfiguration process according to some embodiments of the present application.

[0084] like Figure 3 As shown in FIG. 3 , in operation 301 , a UE 310 (e.g., Figure 1 101a) sends an RRCReconfigurationSidelink message to UE 320 (e.g., Figure 1 In some embodiments, when UE 310 transmits the RRCReconfigurationSidelink message, UE 310 may start timer T400.

[0085] If the sidelink RRC reconfiguration process is successfully completed, then in operation 302, UE 320 may transmit an "RRC reconfiguration complete sidelink message", for example, an RRCReconfigurationCompleteSidelink message as specified in the 3GPP standard document, to UE 310. Alternatively, if the sidelink RRC reconfiguration process is not successfully completed, then in operation 302, UE 320 may transmit an "RRC reconfiguration failure sidelink message", for example, an RRCReconfigurationFailureSidelink message as specified in the 3GPP standard document, to UE 310.

[0086] The purpose of the sidelink RRC reconfiguration procedure is to modify the PC5 RRC connection, for example, to establish, modify or release sidelink data radio bearers (DRBs), configure NR sidelink measurements and reporting, and configure sidelink channel state information (CSI) reference signal resources.

[0087] UE (e.g., Figure 3 The UE 310 described and shown in FIG. 3 may initiate a sidelink RRC reconfiguration process and perform operations on a corresponding PC5 RRC connection in the following situations:

[0088] - With a peer UE (e.g. Figure 3 320) associated with the sidelink DRB;

[0089] - Establishment of a sidelink DRB associated with a peer UE;

[0090] - modification of parameters contained in the Sidelink Radio Bearer (SLRB)-Config of a sidelink DRB associated with a peer UE;

[0091] - Configuration information for peer UE to perform NR sidelink measurements and reporting; and

[0092] - Configuration information of sidelink CSI reference signal resources.

[0093] A UE with NR sidelink communication capability may initiate a process of NR sidelink UE information to report to the network or BS that a sidelink radio link failure (RLF) has been declared (e.g., timer T400 expires) or a sidelink RRC reconfiguration failure has occurred.

[0094] The following table shows the introduction of timer T400 as specified in the 3GPP standard document, including start conditions, stop conditions, actions upon expiration, and possible generic names for the timer.

[0095]

[0096] Figure 4 An exemplary flow chart illustrating a sidelink UE information process according to some embodiments of the present application.

[0097] like Figure 4 As shown in FIG. 4 , in operation 401, a network 420 (e.g., Figure 1 ) may provide a UE 410 (e.g., Figure 1 UE 101a as described and shown in Figure 3 4 ) transmits a message (e.g., SIB12 acquisition) to the UE 310 illustrated and shown in FIG. 4 . In operation 402, the UE 410 may transmit a "sidelink UE information message" (e.g., a SidelinkUEinformationNR message as specified in a 3GPP standard document) to the network 420. Specifically, the SidelinkUEinformationNR message may include sidelink failure information. The sidelink failure information may include a sidelink destination ID and a sidelink failure cause.

[0098] According to the 3GPP standard document, during the layer 2 link release process on the PC5 link, the PC5-S protocol should support the layer 2 link release function, which is used to detect whether a specific PC5 unicast link is still valid.

[0099] Figure 5An exemplary flow chart illustrating a layer 2 link release process according to some embodiments of the present application.

[0100] like Figure 5 As shown in FIG. 1 , in step 0, UE-1 (e.g., Figures 1 to 4 101a, UE 201-A, UE 310, or UE 410) and UE-2 (e.g., Figures 1 to 3 103, UE201-C or UE 320) as described and shown in FIG. 103, UE201-C or UE 320) establishes a unicast link. In step 1, UE-1 (e.g., Figure 1 101a) to UE-2 (eg, Figure 1 103) sends a disconnect request message to release the layer 2 link and delete all context data associated with the layer 2 link. The disconnect request message includes security information. In step 2, upon receiving the disconnect request message, UE-2 may respond with a disconnect response message and delete all context data associated with the layer 2 link. The disconnect response message includes security information. The V2X layer of each UE notifies the AS layer that the unicast link has been released. The V2X layer uses the PC5 link identifier to indicate the released unicast link. This enables the AS layer to delete the context associated with the released unicast link.

[0101] Currently, the details of how to design relay reselection and fallback operations in U2U relay scenarios have not been clearly discussed. The embodiments of the present application are intended to solve such problems. In some embodiments, considering the situation where a direct PC5 link between a UE and another UE has been established, the mechanism of U2U relay selection is studied. In some embodiments, considering the situation where a remote UE communicates with another remote UE via a U2U relay UE, a condition is introduced that triggers the relay UE to transmit a PC5 unicast link release message from the relay UE to the remote UE. In some embodiments, it is studied how the remote UE can distinguish between relay reselection and stopping service when another remote UE receives a PC5 unicast link release message from the relay UE. In some embodiments, considering the situation where a remote UE communicates with another remote UE via a U2U relay UE, a mechanism is introduced for the remote UE to fall back to a direct PC5 link between the remote UE and another remote UE. More details will be described in the following text in conjunction with the accompanying drawings.

[0102] Figure 6 A flow chart illustrating a method performed by a UE according to some embodiments of the present application.

[0103] The method may be performed by a UE (eg, Figure 1 UE 101a or UE 101b described and shown in Figure 2UE 201-A or UE 201-B as described and shown in Figure 3 UE 310 or UE 320 as described and shown in Figure 4 UE410 as described and shown in Figure 5 Although described with respect to a UE, it should be understood that other devices may be configured to perform similar Figure 6 method of method of method.

[0104] In such Figure 6 In the exemplary method 600 shown in FIG. 6 , in operation 601, a UE (e.g., Figure 1 The UE 101a described and shown in FIG. 101a) may select a relay UE (e.g., Figure 1 103 as described and shown in FIG.

[0105] In operation 602, the UE may communicate with a further UE (e.g., Figure 1 For simplicity, the UE may be represented as UE#1, the further UE may be represented as UE#2, and the relay UE may be represented as relay UE#1. UE#1 and UE#2 may act as U2U remote UEs.

[0106] In the embodiments of the present application, the link between UE#1 and relay UE#1 may also be referred to as "the first hop link between UE and relay UE", "the first hop link", etc. The link RRC relay connection between UE#1 and UE#2 may also be referred to as "end-to-end RRC connection of relay link", "end-to-end RRC connection", "end-to-end relay connection", "relay RRC connection", etc.

[0107] In operation 603, UE#1 may perform at least one of the following:

[0108] (1) receiving a message (denoted as message #1) from a relay UE, and performing a relay reselection operation after receiving message #1;

[0109] or

[0110] (2) Measure the quality of the direct PC5 link between UE#1 and UE#2.

[0111] In some embodiments, UE#1 may be connected to a serving cell (e.g., Figure 1 BS 102 as described and shown in FIG. 1 ) receives at least one of the following:

[0112] (1) an indication for indicating whether the L2 U2U relay UE function is supported by the serving cell (denoted as indication #1); or

[0113] (2) Another indication (denoted as indication #2) for indicating whether the discovery operation of the L2 U2U relay UE is supported by the serving cell.

[0114] In some embodiments, at least one of indication #1 or indication #2 is broadcast by the serving cell via system information (eg, system information block 12 (SIB12)).

[0115] In some embodiments, UE#1 may determine whether the quality of the direct PC5 link between UE#1 and UE#2 is less than or equal to a threshold value (denoted as Threshold #1) associated with a relay selection operation. If the quality of the direct PC5 link between UE#1 and UE#2 is less than or equal to Threshold #1, UE#1 may select relay UE#1 by performing a relay selection operation. UE#1 may then transmit another message (denoted as Message #2) to UE#2. Prior to performing the relay selection operation, UE#1 may communicate with UE#2 via the direct PC5 link between UE#1 and UE#2.

[0116] In an embodiment, for example, if UE#1 is within the coverage of the network, then threshold #1 may be configured by the network. For example, UE#1 may receive a signal from a serving cell of the network (e.g., Figure 1 BS 102 shown in FIG. 1 receives configuration information about threshold # 1. In another embodiment, for example, if UE # 1 is out of coverage (OOC) of the network, threshold # 1 may be pre-configured.

[0117] In some embodiments, message #2 includes an indication (denoted as indication #3) for instructing UE #2 to perform a relay selection operation.

[0118] In some embodiments, if the quality of the direct PC5 link is less than or equal to threshold #1, UE#1 may transmit an indication to release the PC5 link from the AS layer of UE#1 to the upper layer of UE#1 (e.g., the PC5-S layer of UE#1). In an embodiment, this indication is used to trigger a PC5 unicast link release operation. In these embodiments, message #2 transmitted from UE#1 to UE#2 is a PC5 unicast link release message transmitted by the upper layer of UE#1 (e.g., the PC5-S layer) to UE#2. Specific examples are described in Fig. 9 Example 1a is described below.

[0119] In some embodiments, the message #2 transmitted from UE #1 to UE #2 is at least one of the following: a PC5 unicast link release message; an RRC reconfiguration sidelink message; or a sidelink notification message. Fig. 9 Example 1b is described below.

[0120] In some embodiments, message #2 transmitted from UE #1 to UE #2 includes one or more UE identities of one or more candidate relay UEs.

[0121] In some embodiments, UE#1 may receive a response message from UE#2, and the response message may include a UE identification of another relay UE (denoted as relay UE#2) selected by UE#2. For example, relay UE#2 may be the same as or different from relay UE#1.

[0122] In some embodiments, if the quality of the direct PC5 link between UE#1 and UE#2 is less than or equal to threshold #1, UE#1 may transmit a message (denoted as message #3) to the serving cell. For example, message #3 may include at least one of the following:

[0123] (1) UE#1 will act as the U2U remote UE;

[0124] (2) a request for resources to relay the discovery process of UE#1; or

[0125] (3) Request for resources for communicating with UE#2 via relay UE#1.

[0126] In some embodiments, message #3 is a sidelink UE information message.

[0127] In some embodiments, if the quality of the direct PC5 link between UE#1 and UE#2 is less than or equal to threshold #1, UE#1 may trigger the discovery process of relay UE#1 or release the direct PC5 link between UE#1 and UE#2. Fig. 9 The embodiments are described as follows.

[0128] In some embodiments, in the case where UE#1 communicates with UE#2 via a relay UE#1, the quality of the direct PC5 link between UE#1 and UE#2 is measured by UE#1, and UE#1 may obtain the quality of the direct PC5 link from the serving cell (e.g., Figure 1 BS 102 as described and shown in FIG. 1 ) receives at least one of the following:

[0129] (1) a threshold value associated with the fallback operation of UE#1 (denoted as threshold value #2);

[0130] (2) another threshold value associated with the fallback operation of UE#1 (denoted as threshold value #3); or

[0131] (3) Duration associated with threshold #2. Fig.11 The embodiments are described as follows.

[0132] In an embodiment of the present application, when a remote UE (e.g., UE#1) communicating with a peer remote UE (e.g., UE#2) via a U2U relay UE (e.g., relay UE#1) performs a fallback operation, the remote UE may release the PC5 link between the remote UE and the U2U relay UE, and communicate with the peer remote UE via a direct PC5 link between the remote UE and the peer remote UE. That is, by performing the fallback operation, the remote UE may switch from the U2U relay operation back to the direct PC5 link between the remote UE and the peer remote UE.

[0133] In some embodiments, after measuring the quality of the direct PC5 link between UE#1 and UE#2, UE#1 may determine whether the quality of the direct PC5 link between UE#1 and UE#2 is greater than or equal to threshold #2. If the quality of the direct PC5 link is greater than or equal to threshold #2, UE#1 may perform a fallback operation to communicate with UE#2 via the direct PC5 link between UE#1 and UE#2.

[0134] In an embodiment, if UE#1 receives configuration information about a duration related to threshold #2 from a serving cell, and if UE#1 determines that the quality of the direct PC5 link during the duration is greater than or equal to threshold #2, then UE#1 may perform a fallback operation to communicate with UE#2 via the direct PC5 link between UE#1 and UE#2.

[0135] In some other embodiments, after measuring the quality of the direct PC5 link between UE#1 and UE#2, UE#1 may determine whether the quality of the direct PC5 link between UE#1 and UE#2 is greater than or equal to threshold #2. If the quality of the direct PC5 link is greater than or equal to threshold #2, UE#1 may further determine whether the quality of the PC5 link between UE#1 and relay UE#1 is less than or equal to threshold #3. If the quality of the PC5 link between UE#1 and relay UE#1 is less than or equal to threshold #3, UE#1 may perform a fallback operation to communicate with UE#2 via the direct PC5 link.

[0136] In an embodiment, if UE#1 receives configuration information about a duration related to threshold #2 from the serving cell, if UE#1 determines that the quality of the direct PC5 link is greater than or equal to threshold #2 for the duration, and determines that the quality of the PC5 link between UE#1 and relay UE#1 is less than or equal to threshold #3, then UE#1 may perform a fallback operation to communicate with UE#2 via the direct PC5 link between UE#1 and UE#2.

[0137] In some embodiments, UE#1 may determine whether the quality of the PC5 link between UE#1 and relay UE#1 is less than or equal to a further threshold (denoted as threshold #4), and if the quality of the PC5 link between UE#1 and relay UE#1 is less than or equal to threshold #4, then start measuring the quality of the direct PC5 link between UE#1 and UE#2.

[0138] In some embodiments, in order to perform the fallback operation, UE#1 may establish a direct PC5 link between UE#1 and UE#2, and release the PC5 link between UE#1 and the relay UE#1. Fig.11 The embodiments are described as follows.

[0139] In some embodiments, the message #1 received in operation 603 may be at least one of the following: a PC5 unicast link release message; or a PC5 RRC message. In some embodiments, the message #1 includes an instruction for instructing UE #1 to perform at least one of the following: a relay reselection operation; or stop relaying the service between UE #1 and UE #1.

[0140] In some embodiments, message #1 includes one or more UE identifiers of one or more candidate relay UEs. In an embodiment, one or more UE identifiers of one or more candidate relay UEs are received by relay UE #1 from UE #2. Fig.10 The embodiments are described as follows.

[0141] The details described in all other embodiments of this application (e.g., details about relay reselection and fallback operations in U2U relay scenarios) apply to Figure 6 In addition, in Figure 6 The details described in the embodiments apply to Figures 1 to 5 and all embodiments 7 to 12.

[0142] Figure 7 A flow chart illustrating a method performed by a relay UE according to some embodiments of the present application.

[0143] The method may be performed by a relay UE (eg, Figure 1 The relay UE 103 described and shown in Figure 2 UE 201-C as described and shown in Figure 3 UE 310 or UE 320 as described and shown in Figure 4 UE 410 as described and shown in Figure 5 Although described with respect to a relay UE, it should be understood that other devices may be configured to perform similar Figure 7 method of method of method.

[0144] In such Figure 7 In the exemplary method 700 shown in FIG. 7 , in operation 701, a relay UE (eg, Figure 1 The relay UE 103 described and shown in FIG. 103 may be used between the relay UE and the UE (e.g., Figure 1 UE 101a or Figure 6 In operation 702, the relay UE may establish a PC5 RRC link between the relay UE and the further UE (e.g., Figure 1 UE 101b or Figure 6 Another PC5 RRC link is established between the UE#2 in the embodiment of FIG. 1 and the further UE. The UE and the further UE act as U2U remote UEs. In operation 703, the relay UE may receive data from the UE. In operation 704, the relay UE may transmit the received data to the further UE.

[0145] In some embodiments, the relay UE may send a signal to a serving cell (e.g., Figure 1 BS102 or Figure 1 A different BS (not shown in the figure) transmits a message (denoted as message #4). For example, message #4 may include at least one of the following:

[0146] (1) The relay UE will act as a U2U relay UE;

[0147] (2) a request for resources for a discovery procedure for at least one of the UE or the further UE; or

[0148] (3) A request for resources for communicating with at least one of the UE or further UEs. Fig. 9 The embodiments are described as follows.

[0149] In some embodiments, the relay UE may transmit a further message to the UE (eg, Figure 6 In some embodiments, the further message includes an instruction for instructing the UE to perform at least one of the following: a relay reselection operation; or stop relaying services between the UE and the UE.

[0150] In some embodiments, a further message (e.g., Figure 6 Message #1) in the embodiment of the present invention is transmitted by the relay UE to the UE in response to at least one of the following:

[0151] (1) The AS layer of the relay UE instructs the upper layer of the relay UE (e.g., the PC5-S layer of the relay UE) to release the PC5 link between the relay UE and the further UE. In an embodiment, the AS layer of the relay UE instructs the upper layer of the relay UE after: the AS layer of the relay UE detects the RLF of the PC5 link between the relay UE and the further UE; or the AS layer of the relay UE receives another indication from the AS layer of the further UE. This indication may indicate the release of the PC5 link between the relay UE and the further UE.

[0152] (2) The relay UE receives a PC5 unicast link release message from a further UE.

[0153] (3) Relay UE from the serving cell (e.g. Figure 1 BS102 or Figure 1 A different BS not shown in the figure) receives "configuration information about not acting as a U2U relay UE".

[0154] (4) The relay UE detects the RLF of the PC5 link between the relay UE and the further UE.

[0155] (5) The relay UE determines that the quality of the PC5 link between the relay UE and the further UE is less than or equal to a further threshold.

[0156] (6) The relay UE receives a further indication from the further UE, and the further indication is associated with the relay reselection operation of the UE.

[0157] In some embodiments, a further message (e.g., Figure 6 Message #1) in the embodiment includes one or more UE identifiers of one or more candidate relay UEs.

[0158] In some embodiments, the relay UE may receive another message from a further UE, and this message includes one or more UE identities of one or more candidate relay UEs. Fig.10 The embodiments are described as follows.

[0159] The details described in all other embodiments of this application (e.g., details about relay reselection and fallback operations in U2U relay scenarios) apply to Figure 7 In addition, Figure 7 The details described in the embodiments apply to Figures 1 to 6 and all embodiments 8 to 12.

[0160] Figure 8 A flow chart illustrating a method performed by a network node according to some embodiments of the present application.

[0161] The method may be performed by a network node (e.g., Figure 1BS102 as described and shown in Figure 2 The gNB 202 or ng-eNB 203 described and shown in Figure 4 Although described with respect to a network node (e.g., a BS), it should be understood that other devices may be configured to perform similar Figure 8 method of method of method.

[0162] In such Figure 8 In the exemplary method 800 shown in FIG. 8 , in operation 801, a network node (eg, Figure 1 ) may transmit an indication (eg, Figure 6 Instruction #1 in the embodiment of ). The UE may act as a remote UE (e.g., Figure 1 UE 101a or UE 101b or Figure 6 UE#1 or UE#2 in the embodiment of the present invention) or a relay UE (e.g., Figure 1 The relay UE 103 or Figure 6 In operation 802, the network node may receive a further indication from the UE. The further indication may indicate at least one of: the UE will act as a U2U remote UE; or the UE will act as a U2U relay UE. For example, the further indication may include: Figure 6 In the example of message #3 or Figure 7 In message #4 of the embodiment.

[0163] In some embodiments, if the further indication indicates that the UE is to act as a U2U remote UE, the network node may transmit to the UE at least one of:

[0164] (1) Thresholds associated with the UE's relay selection operation (e.g., Figure 6 Threshold #1 in the embodiment of ); or

[0165] (2) Another indication for indicating whether the discovery operation of the L2 U2U relay UE is supported by the network node (e.g. Figure 6 Instruction #2 in the embodiment of ).

[0166] In some embodiments, at least one of indication #1 or indication #2 is broadcast by the network node via system information (eg, SIB12).

[0167] In some embodiments, if the further indication indicates that the UE is to act as a U2U remote UE (e.g., Figure 6UE#1 in the embodiment of ), then the network node may receive a message from the UE (e.g., Figure 6 Message #3 in the embodiment).

[0168] In an embodiment, the message is a sidelink UE information message. The message may include at least one of the following:

[0169] (1) For relay UE (e.g. Figure 6 A request for resources of a discovery process of a relay UE#1) in an embodiment;

[0170] or

[0171] (2) For a UE that uses a relay (e.g., Figure 6 1) and a further UE (eg,

[0172] Figure 6 UE#2 in an embodiment) requests for resources for communication, where the further UE acts as a U2U remote UE.

[0173] In some embodiments, if the further indication indicates that the UE is to act as a U2U remote UE (e.g., Figure 6 If the network node is UE#1 in the embodiment of FIG. 1 , the network node may transmit at least one of the following to the UE:

[0174] (1) Thresholds associated with the UE's fallback operation (e.g., Figure 6 Threshold #2 in the embodiment of );

[0175] (2) Another threshold associated with the UE's fallback operation (e.g., Figure 6 Threshold #3 in the embodiment of ); or

[0176] (3) Duration associated with threshold #2, for example Fig.11 The embodiments are described as follows.

[0177] In some embodiments, if the further indication indicates that the UE is to act as a U2U relay UE (e.g., Figure 6 1), then the network node may receive further messages from the UE (e.g., Figure 7 Message #4 in the embodiment of the present invention). The further message includes at least one of the following:

[0178] (1) for at least one remote UE (e.g., Figure 6 A request for resources for a discovery process of UE#1 and / or UE#2 in an embodiment of the present invention; or

[0179] (2) A request for resources for communicating with at least one remote UE.

[0180] The details described in all other embodiments of this application (e.g., details about relay reselection and fallback operations in U2U relay scenarios) apply to Figure 8 In addition, Figure 8 The details described in the embodiments apply to Figures 1 to 7 and all embodiments 9 to 12.

[0181] The following text describes Figures 6 to 8 Specific embodiments of the methods shown and described in any of the.

[0182] Fig. 9 A flowchart illustrating a method for relay selection operation according to some embodiments of the present application. Fig. 9 In the flowchart 900 of the embodiment of FIG. 1 , UE 901 (e.g., Figure 1 101a), UE 902 (e.g., Figure 1 UE 101b as described and shown in FIG. 1 ), network node 903 (such as Figure 1 BS 102 as described and shown in FIG. 1 ) and relay UE 904 (e.g., as Figure 1 The relay UE 103 described and shown in Fig. 9 The following operations are performed in specific embodiments 1a and 1b.

[0183] Example 1a

[0184] In operation 911, a PC5 RRC connection has been established between UE 901 and UE 902. UE 901 and UE 902 may communicate with each other via a direct PC5 link.

[0185] In some embodiments, for example, an indication is broadcast via SIB12 indicating whether L2 U2U relay UE functionality is supported by this serving cell, for example:

[0186] sl-l2u2u-relay-enumeration {enable}

[0187] In some embodiments, for example, an indication is broadcast via SIB12 indicating whether the discovery operation of the L2 U2U relay UE is supported by this serving cell, for example:

[0188] sl-l2u2u-relay-discovery-enumeration {enabled}

[0189] In some embodiments, the threshold for relay selection (e.g., Figure 6 Threshold #1 in the embodiment of is configured by the network node 904 (eg, within coverage) or pre-configured (eg, out of courage (OOC)).

[0190] In operation 912, UE 901 performs measurement of a direct PC5 link between UE 901 and UE 902. UE1 may determine whether the quality of the direct PC5 link between UE1 and UE2 is less than a (pre)configured threshold (eg, threshold #1).

[0191] In some embodiments, once UE 901 determines that the quality of the PC5 link is less than a (pre)configured threshold, UE 901 performs a relay selection operation in operation 912. In an embodiment, the AS layer of UE 901 may instruct the upper layer of UE 901 (e.g., the PC5-S layer of UE 901) to trigger the release of the PC5 unicast link. In operation 913A, the upper layer of UE 901 (e.g., the PC5-S layer) transmits a message (e.g., Figure 6 The message may be a PC5 unicast link release message. An indication of performing a relay selection operation may be added to the message (eg, PC5 unicast link release message). Figure 6 Instruction #3 in the embodiment of ).

[0192] In step 913B, after the remote UE 902 receives the message, the UE 902 performs a relay selection operation. The traffic between the UE 901 and the UE 902 will not be stopped. In step 913C, the UE 902 may transmit a response message to the UE 901.

[0193] In some embodiments, the message transmitted from UE 901 in operation 913A may include one or more UE identities of one or more candidate relay UEs, for example, included in a list of one or more candidate relay UEs. If one or more UE identities of one or more candidate relay UEs are included, UE 902 may select one candidate relay UE. The UE identity of the selected relay UE (e.g., relay UE 904) may be included in the response message transmitted from UE 902 to UE 901 in operation 913C.

[0194] In an embodiment, in operation 913B, UE 902 may reject all candidate relay UEs in the list of one or more candidate relay UEs received from UE 901, and recommend other candidate relay UEs, for example, if relay UE 904 is not included in the list, recommend relay UE 904. UE identities of other candidate relay UEs may be included in a response message transmitted from UE 902 to UE 901 in operation 913C.

[0195] In some other embodiments, once the UE 901 determines that the quality of the direct PC5 link is less than a (pre-)configured threshold, the UE 901 is triggered to perform a discovery procedure.

[0196] In some embodiments, once UE 901 determines that the quality of the direct PC5 link is less than a (pre-)configured threshold, UE 901 is triggered to release the current direct PC5 link between UE 901 and UE 902 .

[0197] In some embodiments, once UE 901 determines that the quality of the PC5 link is less than a (pre) configured threshold, it can trigger UE 901 (i.e., remote UE) to report information to the serving cell (e.g., network node 903) via a side link UE information message, provided that UE 901 is within the coverage of the serving cell.

[0198] In an embodiment, the information reported by UE 901 may be "UE 901 will act as a U2U remote UE" and / or "UE 901 requests discovery resources or communication resources for U2U relay operation".

[0199] In some embodiments, RLF detection on a direct PC5 link may be used to trigger a relay selection operation.

[0200] In some embodiments, in operation 914 (optional), if a U2U remote UE (e.g., UE 901 and / or UE 902) is within the coverage of a serving cell, the remote UE may report information to the serving cell (e.g., network node 903) via a sidelink UE information message. In an embodiment, the information may be 'UE will act as a U2U remote UE' or 'UE requests discovery resources or communication resources for U2U relay operation'.

[0201] In operation 915, after the relay selection operation, a relay UE 904 may be selected, which may be referred to as a U2U relay UE 904. UE 901 and UE 902 will act as remote UEs. A link between remote UE 901 and remote UE 902 via U2U relay UE 904 will be established.

[0202] In operation 916 (optional), if the relay UE 904 is within the coverage of the network node, the relay UE 904 may send a sidelink UE information message to the network node (eg, the network node 903 or Fig. 9 In an embodiment, the information may be "UE 904 will act as a UE-to-UE relay UE" or "UE 904 requests discovery resources or communication resources for U2U relay operation".

[0203] In operation 917 (optional), U2U relay UE 904 may allocate a local UE ID for a U2U remote UE (e.g., UE 901 and / or UE 902). The local UE ID may be added in the header of the adaptation layer of the U2U remote UE. The adaptation layer is a new layer above the radio link control (RLC) layer of the U2U remote UE.

[0204] In some embodiments, if the U2U relay UE 904 is at a network node (e.g., network node 903 or Fig. 9 If the U2U remote UE (eg, UE 901 and / or UE 902) is within the coverage of a different network node (not shown in the figure), the network node may assign a local UE ID to the U2U remote UE (eg, UE 901 and / or UE 902).

[0205] Example 1b

[0206] In operation 911, a PC5 RRC connection has been established between UE 901 and UE 902. UE 901 and UE 902 may communicate with each other via a direct PC5 link.

[0207] In some embodiments, for example, an indication is broadcast via SIB12 indicating whether L2 U2U relay UE functionality is supported by this serving cell, for example:

[0208] sl-l2u2u-relay-enumeration {enable}

[0209] In some embodiments, for example, an indication is broadcast via SIB12 indicating whether the discovery operation of the L2 U2U relay UE is supported by this serving cell, for example:

[0210] sl-l2u2u-relay-discovery-enumeration {enabled}

[0211] In some embodiments, the threshold for relay selection (e.g., Figure 6 The threshold #1 in the embodiment of the present invention is configured (e.g., within coverage) or pre-configured (e.g., out of coverage (OOC)) by the network node 904 to the UE 901 and / or the UE 902.

[0212] In operation 912, UE 901 performs measurement of a direct PC5 link between UE 901 and UE 902. UE 901 may determine whether the quality of the direct PC5 link between UE 901 and UE 902 is less than a (pre)configured threshold (eg, threshold #1).

[0213] In some embodiments, once UE 901 determines that the quality of the PC5 link is less than a (pre)configured threshold, UE 901 performs a relay selection operation in operation 912. In operation 913A, UE 901 may transmit a message (e.g., Figure 6 The message #2 in the embodiment of the present invention is used to instruct the UE 902 to perform the relay selection operation. The message may be a PC5 RRC message. The instruction for instructing the UE 902 to perform the relay selection operation may be added to the PC5 RRC message. For example, the AS layer of the UE 901 transmits an RRCReconfigurationSidelink message or a notification message of the side link to instruct the UE 902 to perform the relay selection operation.

[0214] In operation 913B, after UE 902 receives the message (eg, PC5 RRC message), UE 902 may perform a relay selection operation. Traffic between UE 901 and UE 902 will not be stopped. In operation 913C, UE 902 may transmit a response message to UE 901.

[0215] In some embodiments, the message (e.g., PC5 RRC message) transmitted from UE 901 in operation 913A may include one or more UE identities of one or more candidate relay UEs, for example, which are included in a list of one or more candidate relay UEs. If the one or more UE identities of one or more candidate relay UEs are included in the message, UE 902 may select a candidate relay UE from the one or more candidate relay UEs. The UE identity of the selected relay UE (e.g., relay UE 904) may be included in a response message transmitted from UE 902 to UE 901 in operation 913C.

[0216] In an embodiment, in operation 913B, UE 902 may reject all candidate relay UEs in the list of one or more candidate relay UEs in the message received from UE 901, and recommend other candidate relay UEs. For example, in the case where relay UE 904 is not included in the list, UE 902 may recommend relay UE 904 as a candidate relay UE. UE identities of other candidate relay UEs may be included in a response message transmitted from UE 902 to UE 901 in operation 913C.

[0217] In some other embodiments, once UE 901 determines that the quality of the direct PC5 link between UE 901 and UE 902 is less than a (pre)configured threshold, UE 901 is triggered to perform a discovery process to discover a U2U relay UE (eg, relay UE 904).

[0218] In some embodiments, once UE 901 determines that the quality of the direct PC5 link is less than a (pre-)configured threshold, UE 901 is triggered to release the current direct PC5 link between UE 901 and UE 902 .

[0219] In some embodiments, once UE 901 determines that the quality of the PC5 link is less than a (pre) configured threshold, UE 901 (i.e., remote UE) may be triggered to report information to the serving cell (e.g., network node 903) via a sidelink UE information message, provided that UE 901 is within the coverage of the serving cell. In an embodiment, the information reported by UE 901 may be "UE 901 will act as a U2U remote UE" and / or "UE 901 requests discovery resources or communication resources for U2U relay operation".

[0220] In some embodiments, RLF detection on a direct PC5 link between UE 901 and UE 902 may be used to trigger a relay selection operation of UE 901 .

[0221] In some embodiments, in operation 914 (optional), if a U2U remote UE (e.g., UE 901 and / or UE 902) is within the coverage of a serving cell, the remote UE may report information to the serving cell (e.g., network node 903) via a sidelink UE information message. In an embodiment, the information may be 'UE will act as a U2U remote UE' or 'UE requests discovery resources or communication resources for U2U relay operation'.

[0222] In operation 915, after the relay selection operation of UE 901, relay UE 904 may be selected, which may be referred to as U2U relay UE 904. UE 901 and UE 902 will act as remote UEs. A link between remote UE 901 and remote UE 902 via U2U relay UE 904 will be established.

[0223] In operation 916 (optional), if the relay UE 904 is within the coverage of the network node, the relay UE 904 may send a sidelink UE information message to the network node (eg, the network node 903 or Fig. 9 In an embodiment, the information may be "UE 904 will act as a UE-to-UE relay UE" or "UE 904 requests discovery resources or communication resources for U2U relay operation".

[0224] In operation 917 (optional), U2U relay UE 904 may allocate a local UE ID for a U2U remote UE (e.g., UE 901 and / or UE 902). The local UE ID may be added in the header of the adaptation layer of the U2U remote UE. The adaptation layer is a new layer above the radio link control (RLC) layer of the U2U remote UE.

[0225] In some embodiments, if the U2U relay UE 904 is at a network node (e.g., network node 903 or Fig. 9If the U2U remote UE (e.g., UE 901 and / or UE 902) is within the coverage range of a different network node not shown in the figure), then the local UEID of the U2U remote UE (e.g., UE 901 and / or UE 902) can be assigned by the network node.

[0226] Fig.10 A flowchart illustrating a method for relay reselection operation according to some embodiments of the present application. Fig.10 In the flowchart 1000 of the embodiment of FIG. 1 , a remote UE 1001 (e.g., Figure 1 101a), remote UE 1002 (e.g., Figure 1 101b), the network node 1003 (e.g., Figure 1 BS 102 as described and shown in FIG. 1 ) and relay UE 1004 (e.g., Figure 1 The relay UE 103) described and shown in the figure performs the following operations.

[0227] In operation 1011, a PC5 RRC connection between the remote UE 1001 and the relay UE 1004 has been established. A PC5 RRC connection between the remote UE 1002 and the same relay UE (ie, the relay UE 1004) has been established. The remote UE 1001 may transmit data to the remote UE 1002 via the U2U relay UE 1004.

[0228] In some embodiments, U2U remote UE (e.g., remote UE 1001 and / or remote UE 1002) may report information to a serving cell (e.g., network node 1003). In an embodiment, the information may be "U2U remote UE will act as U2U remote UE" or "U2U remote UE requests discovery resources or communication resources for U2U relay operation". For example, in operation 1012 (optional), if remote UE 1001 is within the coverage of network node 1003, remote UE 1001 may report information to network node 1003 via a sidelink UE information message.

[0229] In some embodiments, in operation 1013 (optional), if the U2U relay UE 1004 is within the coverage of the serving cell, the U2U relay UE 1004 may send a sidelink UE information message to the serving cell (e.g., the network node 1003 or Fig.10 In an embodiment, the information may be “the relay UE 1004 will act as a U2U relay UE” or “the relay UE 1004 requests discovery resources or communication resources for U2U relay operation”.

[0230] In operation 1014, the relay UE 1004 transmits a message to the remote UE 1001 (eg, Figure 6 Message #1 in the embodiment of the present invention), for example, it may be a PC5 link release message or a PC5 RRC message. The message may include, for example, an indication for indicating a relay reselection operation and / or stopping relaying traffic between UE 1004 and remote UE 1001. For example, a message (e.g., a PC5 link release message or a PC5 RRC message) may be transmitted based on at least one of the following conditions:

[0231] (1) After the AS layer of the relay UE detects the PC5RLF of the PC5 link between the relay UE 1004 and the remote UE 1002 or receives an indication from the AS layer of UE 1002 to release the PC5 link between the relay UE 1004 and the remote UE 1002, the AS layer of the relay UE 1004 instructs the upper layer (e.g., PC5-S layer) of the relay UE 1004 to release the PC5 link between the relay UE 1004 and the remote UE 1002.

[0232] (2) The relay UE 1004 receives the PC5 unicast link release message from the remote UE 1002.

[0233] (3) Relay UE 1004 from the serving cell (e.g., network node 1003 or Fig.10 Different network nodes not shown in the figure) receive "configuration information about not acting as a U2U relay".

[0234] (4) The relay UE 1004 detects the PC5 RLF of the PC5 link between the relay UE 1004 and the remote UE 1002. For example, the relay UE 1004 may detect the PC5 RLF of the PC5 link in the following cases:

[0235] a) upon receiving an indication from a sidelink RLC entity that a maximum number of retransmissions for a particular destination has been reached;

[0236] or

[0237] b) upon expiry of timer T400 for a specific destination; or

[0238] c) upon receiving an indication from a Medium Access Control (MAC) entity that a maximum number of consecutive Hybrid Automatic Repeat Request (HARQ) Discontinuous Transmissions (DTX) to a particular destination has been reached; or

[0239] d) upon receiving an integrity check failure indication of a Sidelink Signaling Radio Bearer 2 (SL-SRB2) or SL-SRB3 for a specific destination from a Sidelink Packet Data Convergence Protocol (PDCP) entity.

[0240] (5) In some embodiments, the "PC5 signal strength condition" may be used as a trigger condition for causing a remote UE (e.g., remote UE 1001 and / or remote UE 1002) to perform a relay reselection operation. In these embodiments, a threshold for triggering a relay reselection operation is configured to the remote UE, e.g., remote UE 1001 and / or remote UE

[0241] 1002. In addition, the threshold value may also be configured to the U2U relay UE (e.g., relay UE 1004). For example, if the relay UE determines that the quality of the link between the relay UE 1004 and the remote UE 1002 is less than the configured threshold value, the relay UE 1004 is triggered to transmit an indication to the remote UE 1001 (e.g., in a PC5 unicast link release message) to trigger the remote UE 1001 to perform a relay reselection operation.

[0242] (6) In some other embodiments, after the remote UE 1002 detects that the quality of the link between the relay UE 1004 and the remote UE 1002 is less than a configured threshold, the remote UE 1002 transmits an indication associated with a relay reselection operation to the relay UE 1004. The relay UE 1004 may then instruct the remote UE 1001 to perform a relay reselection operation.

[0243] a) Before releasing the direct PC5 link, a candidate relay UE list may be exchanged between remote UE 1001 and remote UE 1002. For example, when remote UE 1002 determines that the quality of the link between relay UE 1004 and remote UE 1002 is less than a configured threshold, remote UE 1002 transmits a message to remote UE 1001 via relay UE 1004. The message (e.g., Figure 6 Message #1) in the embodiment of the embodiment may include a candidate relay UE list including UE identifications of one or more candidate relay UEs. Remote UE 1001 may select a candidate relay UE in the candidate relay list and then transmit a response message to remote UE 1002. For example, the response message includes the UE ID of the selected candidate relay UE.

[0244] b) In some embodiments, if there is no suitable candidate relay in the candidate relay list, the remote UE 1001 may recommend or select one or more other candidate relay UEs and transmit a response message to the remote UE 1002. In these embodiments, multi-hop may also be used. For example, the response message includes the UE identifiers of the one or more other candidate relay UEs.

[0245] In operation 1015, after the remote UE 1001 receives a message (eg, Figure 6After receiving message #1 in the embodiment of the present invention, the remote UE 1001 may perform a relay reselection operation. Alternatively, after the remote UE 1001 receives a message including an indication for indicating service stop from the relay UE 1004, the remote UE 1001 may release the direct PC5 link between the relay UE 1004 and the remote UE 1001, and does not perform a relay reselection operation.

[0246] Fig.11 A flowchart illustrating a method for fallback operation of a remote UE according to some embodiments of the present application. Fig.11 In the flowchart 1100 of the embodiment of FIG. 1 , UE 1101 (e.g., Figure 1 101a), UE 1102 (e.g., Figure 1 101b), the network node 1103 (e.g., Figure 1 BS 102 as described and shown in FIG. 1 ) and relay UE 1104 (e.g., Figure 1 The relay UE 103) described and shown in the figure performs the following operations.

[0247] exist Fig.11 In an embodiment of the present invention, when the remote UE 1101 communicating with the remote UE 1102 via the U2U relay UE 1104 performs a fallback operation, the remote UE 1101 may release the PC5 link between the remote UE 1101 and the U2U relay UE 1104, and then communicate with the remote UE 1102 via the direct PC5 link between the remote UE 1101 and the remote UE 1102. That is, by performing the fallback operation, the remote UE 1101 may switch back from the U2U relay operation to the direct PC5 link between the remote UE 1101 and the remote UE 1102.

[0248] In operation 1111, a PC5 RRC connection between a remote UE 1101 and a relay UE 1104 has been established. A PC5 RRC connection between a remote UE 1102 and the same relay UE (i.e., relay UE 1104) has been established. The remote UE 1101 may transmit data to the remote UE 1102 via the relay UE 1104. The relay UE 1104 may be referred to as a U2U relay UE 1104.

[0249] In some embodiments, in operation 1112 (optional), if the U2U remote UE (e.g., remote UE 1101 and / or remote UE 1102) is within the coverage of the serving cell, the U2U remote UE will send a sidelink UE information message to the serving cell (e.g., network node 1103 or Fig.11In an embodiment, the information may be “the remote UE will act as a U2U remote UE” or “the remote UE requests discovery resources or communication resources for U2U relay operation”.

[0250] In some embodiments, in operation 1113 (optional), if the relay UE 1104 is within the coverage of the serving cell, the relay UE 1104 sends a sidelink UE information message to the serving cell (e.g., the network node 1103 or Fig.11 In an embodiment, the information may be “the relay UE 1104 will act as a U2U relay UE” or “the U2U relay UE 1104 requests discovery resources or communication resources for U2U relay operation”.

[0251] In operation 1114 , the remote UE 1101 receives “one or more thresholds associated with a backoff operation of the remote UE 1101 ” from the network node 1103 . In operation 1115 , the remote UE 1101 measures the quality of the direct PC5 link between the remote UE 1101 and the remote UE 1102 .

[0252] In some embodiments, a threshold value (eg, Figure 6 In the embodiment of the present invention, the threshold #2) is configured to the remote UE 1101. Once the quality of the direct PC5 link between the remote UE 1101 and the remote UE 1102 is better than the configured threshold, the remote UE 1101 falls back (i.e., performs a fallback operation) to the direct PC5 link between the remote UE 1101 and the remote UE 1102. That is, the remote UE 1101 can establish a direct PC5 link to the remote UE 1102, and release the link between the remote UE 1101 and the relay UE 1104. Then, the UE 1101 and the UE 1102 do not act as remote UEs, and the UE 1101 and the UE 1102 communicate with each other via the direct PC5 link.

[0253] In an embodiment, the threshold value (e.g., Figure 6 The duration associated with the threshold value #2) in the embodiment of the present invention may be configured to the remote UE 1101. For example, a condition associated with the threshold value is required within the duration. In operation 1115, once the quality of the direct PC5 link is greater than or equal to the configured threshold value for the duration, the remote UE 1101 falls back to the direct PC5 link between UE 1101 and UE 1102. Then, UE 1101 and UE 1102 do not act as remote UEs, and they will communicate with each other via the direct PC5 link.

[0254] In some embodiments, two thresholds (e.g., Figure 6 1101 and 1102) are configured to the remote UE 1101 for link switching. One threshold (e.g., threshold #2) is used for the direct PC5 link between UE 1101 and UE 1102. Another threshold (e.g., threshold #3) is used for the PC5 link between the remote UE 1101 and the relay UE 1104. For example, in operation 1115, once the quality of the direct PC5 link is greater than or equal to the configured threshold #2, and the quality of the PC5 link between the remote UE 1101 and the relay UE 1104 is less than or equal to the configured threshold #3, the remote UE 1101 falls back to the direct PC5 link between UE 1101 and UE 1102. Then, UE 1101 and UE 1102 do not act as remote UEs, and they will communicate with each other via the direct PC5 link.

[0255] In some embodiments, in operation 1115, the remote UE 1101 determines whether the quality of the PC5 link between the remote UE 1101 and the relay UE 1104 is less than or equal to another threshold value (eg, Figure 6 and if the quality of the PC5 link between the remote UE 1101 and the relay UE 1104 is less than or equal to the above-mentioned another threshold, then the remote UE 1101 starts measuring the quality of the direct PC5 link between UE 1101 and UE 1102. That is, when to monitor the direct PC5 link between UE 1101 and UE 1102 may be based on a trigger condition, for example, which is associated with a threshold (e.g., threshold #4).

[0256] In operation 1116, the remote UE 1101 communicates with the remote UE 1102 via a direct PC5 link without a relay UE (eg, relay UE 1104).

[0257] The details described in all other embodiments of this application (e.g., details on how to design relay reselection and fallback operations in U2U relay scenarios) apply to Figures 8 to 11 In any embodiment of the present invention. In addition, Figures 8 to 11 The details described in any of the embodiments apply to Figures 1 to 7 Those skilled in the art will appreciate that the present invention may be modified without departing from the spirit and scope of the present disclosure. Figures 8 to 11 The operation sequence in the flowchart of any of the embodiments of the present invention may be eliminated or modified. Figures 8 to 11 Some of the operations in the flowcharts of any of the embodiments.

[0258] Fig.12A block diagram illustrating an exemplary apparatus 1200 according to some embodiments of the present application.

[0259] like Fig.12 , the apparatus 1200 may include at least one processor 1206 and at least one transceiver 1202 coupled to the processor 1206. The apparatus 1200 may be a UE, a relay UE, or a network node (eg, a BS).

[0260] Although in this figure, elements such as at least one transceiver 1202 and processor 1206 are described in the singular, plural forms are contemplated unless limitation to the singular is explicitly stated. In some embodiments of the present application, the transceiver 1202 may be divided into two devices, such as a receive circuit system and a transmit circuit system. In some embodiments of the present application, the apparatus 1200 may further include an input device, a memory, and / or other components.

[0261] In some embodiments of the present application, the device 1200 may be a UE. The transceiver 1202 and the processor 1206 may interact with each other to perform Figures 1 to 11 In some embodiments of the present application, the device 1200 may be a relay UE. The transceiver 1202 and the processor 1206 may interact with each other to perform Figures 1 to 11 In some embodiments of the present application, the device 1200 may be a network node (eg, a BS). The transceiver 1202 and the processor 1206 may interact with each other to perform Figures 1 to 11 The operations of network nodes are described in .

[0262] In some embodiments of the present application, the device 1200 may further include at least one non-transitory computer-readable medium. For example, in some embodiments of the present disclosure, the non-transitory computer-readable medium may have computer-executable instructions stored thereon to enable the processor 1206 to implement the method for the UE as described above. For example, when executed, the computer-executable instructions enable the processor 1206 to interact with the transceiver 1202 to perform Figures 1 to 11 The operations of the UE are described in .

[0263] In some embodiments of the present disclosure, the non-transitory computer-readable medium may have computer-executable instructions stored thereon to cause the processor 1206 to implement the method for relaying UE as described above. For example, when executed, the computer-executable instructions cause the processor 1206 to interact with the transceiver 1202 to perform Figures 1 to 11 The operations of the relay UE are described in .

[0264] In some embodiments of the present disclosure, the non-transitory computer-readable medium may have computer-executable instructions stored thereon to cause the processor 1206 to implement the method described above with respect to the network node (e.g., BS). For example, when executed, the computer-executable instructions cause the processor 1206 to interact with the transceiver 1202 to perform Figures 1 to 11 The operations of network nodes are described in .

[0265] Those of ordinary skill in the art will appreciate that the operations or steps of the methods described in conjunction with the aspects disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. The software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. In addition, in some aspects, the operations or steps of the methods may reside as one or any combination or set of codes and / or instructions on a non-transitory computer-readable medium that may be incorporated into a computer program product.

[0266] Although the present disclosure has been described with its specific embodiments, it is apparent that many alternatives, modifications and variations may be apparent to those skilled in the art. For example, the various components of the embodiments may be interchanged, added or replaced in other embodiments. In addition, all elements of each figure are not necessarily required for the operation of the disclosed embodiments. For example, a person of ordinary skill in the art of the disclosed embodiments will be able to make and use the teachings of the present disclosure by simply adopting the elements of the independent claims. Therefore, the embodiments of the present disclosure as described herein are intended to be illustrative, not restrictive. Various changes may be made without departing from the spirit and scope of the present disclosure.

[0267] In this document, the term "switching" and "path switching" are used interchangeably. The term "includes / including" or any other variant thereof is intended to cover non-exclusive inclusion, so that the process, method, article or equipment containing a series of elements not only includes the elements, but also includes other elements that are not explicitly listed or inherent to such processes, methods, articles or equipment. In the absence of more constraints, the elements starting with "a / an" or the like do not exclude the presence of additional identical elements in the process, method, article or equipment containing the elements. In addition, the term "another" is defined as at least the second or more. The term "having" and the like as used herein are defined as "including". For example, the expression of "A and / or B" or "at least one of A and B" may include any and all combinations of words listed together with the expression. For example, the expression "A and / or B" or "at least one of A and B" may include A, B or both A and B. The wording "first", "second" or the like is only used to clearly illustrate the embodiments of the present application, and is not used to limit the substantive content of the present application.

Claims

1. A first user equipment UE, comprising: Transceiver; and a processor coupled to the transceiver, wherein the processor is configured to: Selecting a first relay UE based on the relay selection operation; communicating with a second UE via the first relay UE, wherein the first UE and the second UE act as UE-to-UE U2U remote UEs; and Do at least one of the following: receiving a first message from the relay UE and performing a relay reselection operation after receiving the first message; or The quality of a direct PC5 link between the first UE and the second UE is measured.

2. The first UE according to claim 1, wherein the processor of the first UE is configured to receive at least one of the following from a serving cell: a first indication for indicating whether a layer 2 L2 U2U relay UE function is supported by the serving cell; or A second indication for indicating whether a discovery operation for the L2 U2U relay UE is supported by the serving cell.

3. The first UE according to claim 1, wherein the processor of the first UE is configured to: determining whether a quality of the direct PC5 link between the first UE and the second UE is less than or equal to a first threshold associated with the relay selection operation; If the quality of the direct PC5 link between the first UE and the second UE is less than or equal to the first threshold, selecting the first relay UE, wherein the first UE communicates with the second UE via the direct PC5 link before performing the relay selection operation; and A second message is transmitted to the second UE. 4 . The first UE according to claim 3 , wherein the second message includes a third indication for instructing the second UE to perform a relay selection operation.

5. The first UE according to claim 3, wherein if the quality of the direct PC5 link is less than or equal to the first threshold, then the processor of the first UE is configured to transmit a fourth indication of releasing the PC5 link from the access stratum (AS) layer of the first UE to the upper layer of the first UE.

6. The first UE according to claim 5, wherein the fourth indication is used to trigger a PC5 unicast link release operation, and wherein the second message is a PC5 unicast link release message transmitted by the upper layer of the first UE to the second UE.

7. The first UE according to claim 1, wherein the quality of the direct PC5 link between the first UE and the second UE is measured after communicating with the second UE via the first relay UE, and wherein the processor of the first UE is configured to receive at least one of the following from a serving cell: a second threshold associated with a fallback operation of the first UE; or A third threshold associated with the fallback operation of the first UE.

8. The first UE of claim 7, after measuring the quality of the direct PC5 link, the processor of the first UE is configured to: determining whether the quality of the direct PC5 link is greater than or equal to the second threshold; and If the quality of the direct PC5 link is greater than or equal to the second threshold, performing the fallback operation to communicate with the second UE via the direct PC5 link.

9. The first UE of claim 7, after measuring the quality of the direct PC5 link, the processor of the first UE is configured to: determining whether the quality of the direct PC5 link is greater than or equal to the second threshold; If the quality of the direct PC5 link is greater than or equal to the second threshold, determining whether the quality of the PC5 link between the first UE and the first relay UE is less than or equal to the third threshold; and If the quality of the PC5 link between the first UE and the first relay UE is less than or equal to the third threshold, performing the fallback operation to communicate with the second UE via the direct PC5 link.

10. A relay user equipment UE, comprising: Transceiver; and a processor coupled to the transceiver, wherein the processor is configured to: Establishing a first PC5 radio resource control (RRC) link between the relay UE and the first UE; establishing a second PC5 RRC link between the relay UE and a second UE, wherein the first UE and the second UE act as UE-to-UE U2U remote UEs; receiving data from the first UE; and The received data is transmitted to the second UE.

11. The relay UE of claim 10, wherein the processor of the relay UE is configured to transmit a first message to a serving cell, and wherein the first message includes at least one of: The relay UE will act as a U2U relay UE; a request for resources for a discovery procedure for at least one of the first UE or the second UE; or A request for resources for communicating with the at least one of the first UE or the second UE.

12. The relay UE of claim 10, wherein the processor of the relay UE is configured to transmit a second message to the first UE, and wherein the second message is at least one of: PC5 unicast link release message; or PC5 RRC message.

13. The relay UE according to claim 12, wherein the second message includes a first instruction for instructing the first UE to perform at least one of the following: Relay reselection operation; or Stop the service between the relay UE and the first UE.

14. The relay UE of claim 12, wherein the second message is transmitted in response to at least one of: The access layer AS layer of the relay UE instructs the upper layer of the relay UE to release the PC5 link between the relay UE and the second UE; The relay UE receives a PC5 unicast link release message from the second UE; The relay UE receives configuration information about not acting as a U2U relay UE from a serving cell; The relay UE detects a radio link failure RLF of the PC5 link between the relay UE and the second UE; The relay UE determines that the quality of the PC5 link between the relay UE and the second UE is less than or equal to a second threshold; or The relay UE receives a second indication from the second UE, wherein the second indication is associated with the relay reselection operation of the first UE.

15. A network node, comprising: Transceiver; and a processor coupled to the transceiver, wherein the processor is configured to: transmitting a first indication to the UE indicating whether a layer 2 L2 UE to UE U2U relay user equipment UE function is supported by the network node; and A second indication is received from the UE, wherein the second indication indicates at least one of: The UE will act as a U2U remote UE; or The UE will act as a U2U relay UE.

Images