Method and apparatus for relay reselection and connection procedures in UE-to-UE relay scenarios

Abstract

[0003] Embodiments of the present application relate to a method and apparatus for a relay reselection and connection procedure in a user equipment (UE)-to-UE relay scenario under Third Generation Partnership Project (3GPP) 5G New Radio (NR). According to one embodiment of the present application, the method may include: establishing a PC5 Radio Resource Control (RRC) connection for a link between a UE and a relay UE, where an RRC connection for a link between the relay UE and another UE has been established; and performing a relay reselection procedure based on a trigger condition, where the trigger condition is at least one of detecting a sidelink failure, detecting a failure in an RRC relay connection for a link between the UE and the other UE, receiving a failure notification from the relay UE, and receiving a failure indication from an upper layer of the UE. Additionally, upon detecting the sidelink failure or receiving the failure notification, the UE may report failure information to a BS.

Description

[Technical Field]

[0001] TECHNICAL FIELD Embodiments of the present application generally relate to wireless communication technologies, and more particularly to a method and apparatus for relay reselection and connection procedures in a user equipment (UE)-to-UE relay scenario. [Background technology]

[0002] Vehicle-to-Everything (V2X) has been introduced into 5G wireless communication technology. From the perspective of the channel structure of V2X communication, the direct link between two user equipments (UEs) is called sidelink. Sidelink is a Long Term Evolution (LTE) feature introduced in 3GPP Release 12 that enables direct communication between nearby UEs, and data does not need to go through a base station (BS) or core network.

[0003] The Third Generation Partnership Project (3GPP) promotes the deployment of relay nodes (RNs) in wireless communication systems. One purpose of deploying RNs is to enhance the coverage area of ​​a BS by improving the throughput of UEs located within the coverage area or far from the BS, which may experience relatively low signal quality. RNs may also be referred to as relay UEs in some cases. A 3GPP 5G sidelink system including relay UEs may be referred to as a sidelink relay system.

[0004] Currently, in 3GPP 5G New Radio (NR) systems and the like, the details of how to design relay reselection and connection processing procedures in UE-UE relay scenarios have not yet been specifically discussed. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] 3GPP TS 36.300 [Non-patent document 2] 3GPP TS 23.303 [Non-patent document 3] 3GPP TS 38.311 [Non-patent document 4] 3GPP TS 36.311 [Non-patent document 5] TS 38.331 Summary of the Invention [Means for solving the problem]

[0006] Some embodiments of the present application provide a method for wireless communication, which may be performed by a UE, and includes the steps of: establishing a PC5 Radio Resource Control (RRC) connection for a link between the UE and a relay UE, where the RRC connection for a link between the relay UE and another UE is established; and performing a relay reselection procedure based on a trigger condition, where the trigger condition is at least one of detecting a sidelink failure, detecting a failure in an RRC relay connection for a link between the UE and the other UE, receiving a failure notification from the relay UE, and receiving a failure indication from an upper layer of the UE.

[0007] In some embodiments, in the method performed by the UE, the failure notification received from the relay UE is one of: a sidelink radio link failure (RLF) notification associated with a link between the relay UE and the other UE; a notification of a failure to restore a sidelink RLF on the link between the relay UE and the other UE; and a notification of a PC5 signaling (PC5-S) link failure on the link between the relay UE and the other UE.

[0008] In some embodiments, the method is performed by the UE, and the notification of the PC5-S link failure is received after an access stratum (AS) layer of the relay UE receives an indication of the PC5-S link failure. In some other embodiments, the notification of the PC5-S link failure is received after expiration of a "keep-alive procedure timer."

[0009] In some embodiments, in the method performed by the UE, the sidelink RLF notification includes a cause, and the cause is at least one of reaching a maximum number of radio link control (RLC) retransmissions, expiry of a "timer for transmission of RRC reconfiguration for sidelink", reaching a maximum number of consecutive hybrid automatic repeat request (HARQ) discontinuous transmissions (DTX), receiving an integrity check failure indication, and occurrence of a PC5-S link failure.

[0010] In some embodiments, a PC5-S link failure is detected in response to at least one of the expiration of a "timer for keep-alive procedure" associated with the link between the UE and a relay UE and the expiration of a "further timer for keep-alive procedure" associated with the link between the UE and another UE as described above.

[0011] In some embodiments, in the method performed by the UE, the failure indication is received from a PC5-S layer of the UE, and the failure indication is an indication of a PC5-S link failure of a link between the UE and a relay UE, the indication being received by the AS layer of the UE from the PC5-S layer of the UE.

[0012] In some embodiments, the method is performed by the UE, and the sidelink failure occurs in a link between the UE and the relay UE, and the sidelink failure is at least one of an RLF in the link between the UE and the relay UE and a failure related to configuration information associated with the link between the UE and the relay UE.

[0013] In some embodiments, the method, performed by the UE, wherein a failure in the RRC relay connection of the link between the UE and said other UE is detected in response to at least one of expiry of a "timer for RRC reconfiguration procedure" associated with the RRC relay connection of the link between the UE and said other UE and expiry of a "timer for keep-alive procedure" associated with the link between the UE and said other UE.

[0014] In some embodiments, the method is performed by the UE, wherein in response to expiry of a "timer for keep-alive procedure" associated with a link between the relay UE and the other UE, a failure notification is indicated from an upper layer of the relay UE to the AS layer of the relay UE, and the failure notification is sent by the relay UE to the UE.

[0015] In some embodiments, the method performed by the UE further comprises, in response to receiving a failure notification associated with a sidelink failure in a link between the relay UE and said other UE, suspending transmission of data destined for said other UE.

[0016] In some embodiments, the method performed by the UE further comprises, in response to receiving a failure notification associated with a sidelink failure in a link between the relay UE and said another UE, continuing to transmit data destined for the relay UE and continuing to receive data from the relay UE.

[0017] In some embodiments, the method performed by the UE further includes ceasing to receive data from the relay UE in response to receiving an end mark indication from the relay UE or receiving an RRC message including an indication that a data transfer intended for the UE has been completed.

[0018] In some embodiments, the method performed by the UE further comprises releasing a PC5 RRC connection between the UE and the relay UE.

[0019] In some embodiments, the method performed by the UE further includes a step of sending, by the AS layer of the UE, an indication to the PC5-S layer of the UE, the indication indicating that the UE has stopped receiving data from the relay UE.

[0020] In some embodiments, the method performed by the UE further includes reporting failure information to a base station (BS) in response to the UE being within the coverage of the BS and detecting a sidelink failure in a link between the UE and a relay UE or detecting a failure in an RRC relay connection of a link between the UE and said other UE.

[0021] In some embodiments, the failure information includes a failure cause, and the failure cause is at least one of a failure related to configuration information, where the configuration information is associated with a link between the UE and a relay UE, a sidelink failure in the link between the UE and the relay UE, a sidelink failure in the link between the relay UE and said other UE, and a failure in the RRC relay connection of the link between the UE and said other UE.

[0022] In one embodiment, the failure information includes a set of identification information for the two end UEs of the link associated with the failure cause.

[0023] In some embodiments, the method performed by the UE further includes reporting a failure notification to the BS in response to the UE being within coverage of the BS and receiving the failure notification from the relay UE.

[0024] In some embodiments, the failure notification includes a failure cause, and the failure cause is at least one of a failure related to configuration information associated with a link between the UE and the relay UE, a sidelink failure in the link between the UE and the relay UE, a sidelink failure in the link between the relay UE and said other UE, and a failure in the RRC relay connection of the link between the UE and said other UE. In one embodiment, the failure notification includes a set of identification information for two end UEs of the link associated with the failure cause.

[0025] Some embodiments of the present application also provide an apparatus for wireless communications that includes a non-transitory computer-readable medium storing computer-executable instructions, receiving circuitry, transmitting circuitry, and a processor coupled to the non-transitory computer-readable medium, the receiving circuitry, and the transmitting circuitry, the computer-executable instructions causing the processor to implement any of the above-described methods performed by a UE.

[0026] Some embodiments of the present application provide a further method for wireless communication, which may be performed by a relay UE, and includes the steps of establishing a PC5 Radio Resource Control (RRC) connection for a link between the UE and the relay UE, establishing an RRC connection for a link between the relay UE and another UE, and sending a failure notification to the UE.

[0027] In some embodiments, in the method performed by the relay UE, the failure notification sent from the relay UE is one of: a sidelink RLF notification associated with the link between the relay UE and the other UE; a notification of a failure to restore the sidelink RLF on the link between the relay UE and the other UE; and a notification of a PC5-S link failure on the link between the relay UE and the other UE.

[0028] In one embodiment, the sidelink RLF notification includes a cause, and the cause is at least one of: reaching the maximum number of RLC retransmissions; expiry of the "timer for transmission of RRC reconfiguration for sidelink"; reaching the maximum number of consecutive HARQ DTXs; receiving an integrity check failure indication; and occurrence of a PC5-S link failure.

[0029] In some embodiments, in the method performed by the relay UE, the step of sending a failure notification to the UE further includes receiving, by the AS layer of the relay UE, an indication of a PC5-S link failure on a link between the relay UE and the other UE, and sending a notification of the PC5-S link failure to the UE.

[0030] In some other embodiments, in the method performed by the relay UE, the step of sending a failure notification to the UE further includes the steps of detecting the expiry of a "keep alive procedure timer" associated with the link between the relay UE and the other UE, and sending a notification of the PC5-S link failure to the UE.

[0031] Some embodiments of the present application also provide an apparatus for wireless communications that includes a non-transitory computer-readable medium having computer-executable instructions stored thereon, receiving circuitry, transmitting circuitry, and a processor coupled to the non-transitory computer-readable medium, the receiving circuitry, and the transmitting circuitry, the computer-executable instructions causing the processor to implement any of the above-described methods performed by a relay UE.

[0032] 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.

[0033] To describe the manner in which the advantages and features of the present application can be obtained, the present application will be described by reference to specific embodiments thereof that are illustrated in the accompanying drawings. These drawings depict only example embodiments of the present application and are therefore not to be considered limiting of its scope. [Brief explanation of the drawings]

[0034] [Figure 1] 1 is a schematic diagram of a wireless communication system in accordance with some embodiments of the present application. [Figure 2] 1 is an exemplary V2X communication system in accordance with some embodiments of the present application. [Figure 3] 10 is an example flowchart of a sidelink RRC reconfiguration procedure in accordance with some embodiments of the present application. [Figure 4] 1 is an example flowchart of a sidelink UE information procedure in accordance with some embodiments of the present application. [Figure 5] 1 is an exemplary flowchart of a Layer 2 link maintenance procedure according to some embodiments of the present application. [Figure 6] 1 is an exemplary Layer 2 UE-to-UE relay protocol stack in accordance with some embodiments of the present application. [Figure 7] 1 is a flowchart of a method for performing a relay reselection procedure according to some embodiments of the present application. [Figure 8] 1 is a flowchart of a method for sending a fault notification according to some embodiments of the present application. [Figure 9] 1 is a flowchart of a method for reporting fault information according to some embodiments of the present application. [Figure 10] FIG. 1 is a simplified block diagram of an apparatus for a fault handling procedure according to some embodiments of the present application. DETAILED DESCRIPTION OF THE INVENTION

[0035] The detailed description of the accompanying drawings is intended as an illustration of a preferred embodiment of the present application and is not intended to represent the only form in which the present application may be practiced. It should be understood that the same or equivalent functions may be accomplished by various embodiments that are intended to be encompassed within the spirit and scope of the present application.

[0036] 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, the embodiments are provided under specific network architectures and new service scenarios, such as 3GPP 5G, 3GPP LTE Release 8, etc. With the development of network architectures and new service scenarios, it is contemplated that all embodiments in the present application can also be applied to similar technical problems, and furthermore, although the technical terms listed in the present application may change, this shall not affect the principle of the present application.

[0037] FIG. 1 illustrates a schematic diagram of a wireless communication system in accordance with some embodiments of the present application.

[0038] 1, the wireless communication system 100 includes, for illustrative purposes, two UEs (i.e., UE 101a and UE 101b), a BS 102, and a relay UE 103. Although a particular number of UEs, relay UEs, and BSs are depicted in FIG. 1, it is contemplated that any number of UEs, relay UEs, and BSs may be included in the wireless communication system 100.

[0039] Due to the long distance between UE 101a and UE 101b, these 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, for example, a PC5 interface as defined in 3GPP standard documents. UE 101a may be connected to BS 102 via a network interface, for example, a Uu interface as defined in 3GPP standard documents. Referring to FIG. 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 a Uu link.

[0040] In some embodiments of the present application, the 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 television (e.g., a television connected to the Internet), a set-top box, a game console, a security system (including security cameras), a vehicle-mounted computer, a network device (e.g., a router, a switch, and a modem), etc.

[0041] In some further embodiments of the present application, the UE 101a, UE 101b or relay UE 103 may include a portable wireless communication device, a smartphone, a cellular phone, a flip phone, a device with a subscriber identity module, a personal computer, a selective call receiving network, or any other device capable of transmitting and receiving communication signals over a wireless network.

[0042] In some other embodiments of the present application, the 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. Additionally, the UE 101a, UE 101b, or relay UE 103 may be referred to as a subscriber unit, a mobile, 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 may be described using other technical terms used in the art.

[0043] The BSs 102 may be dispersed throughout a geographic region. In some embodiments of the present application, each of the BSs 102 may be referred to as an access point, access terminal, base, base unit, macro cell, Node B, evolved Node B (eNB), gNB, Home Node B, relay node or device, or may be described using other technical terms used in the art. The BSs 102 are generally part of a wireless access network, which may include one or more controllers communicatively coupled to one or more corresponding BSs 102.

[0044] The wireless communication system 100 may be compatible with any type of network capable of transmitting and receiving wireless communication signals. For example, the wireless communication system 100 may be compatible with a wireless communication network, a cellular telephone network, a time division multiple access (TDMA)-based network, a code division multiple access (CDMA)-based network, an orthogonal frequency division multiple access (OFDMA)-based network, an LTE network, a 3GPP-based network, a 3GPP 5G network, a satellite communication network, a high altitude platform network, and / or other communication networks.

[0045] In some embodiments of the present application, the wireless communication system 100 is compatible with the 3GPP protocol 5G NR, where the BS 102 transmits data on the downlink (DL) using an OFDM modulation scheme and the UE 101 (e.g., UE 101a, UE 101b, or other similar UE) transmits data on the uplink (UL) using a Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing (DFT-S-OFDM) or Cyclic Prefix OFDM (CP-OFDM) scheme. However, more generally, the wireless communication system 100 may implement some other open or proprietary communication protocol, such as WiMAX, among other protocols.

[0046] In some embodiments of the present application, the BS 102 may communicate using other communication protocols, such as the IEEE 802.11 family of wireless communication protocols. Furthermore, in some embodiments of the present application, the BS 102 may communicate over licensed spectrum, while in other embodiments, the BS 102 may communicate over unlicensed spectrum. This application is not intended to be limited to any particular wireless communication system architecture or protocol implementation. In further embodiments of the present application, the BS 102 may communicate with the UE 101 using 3GPP 5G protocols.

[0047] The UE 101 may access the BS 102 to receive data packets from the BS 102 via a downlink channel and / or transmit data packets to the BS 102 via an uplink channel. In normal operation, the UE 101 does not know when the BS 102 will transmit data packets to the UE 101, so the UE 101 must constantly be awake and monitor the downlink channel (e.g., the Physical Downlink Control Channel (PDCCH)) to prepare to receive data packets from the BS 102. However, if the UE 101 were to constantly monitor the downlink channel even when there is no traffic between the BS 102 and the UE 101, it would result in significant power waste, which is problematic for power-limited or power-sensitive UEs.

[0048] Generally, sidelink communication supports UE-to-UE direct communication using two transmission modes. Two sidelink resource allocation modes are supported: Mode 1 and Mode 2. In Mode 1, sidelink resources are scheduled by the BS. In Mode 2, the UE determines the sidelink transmission resources and timing in a resource pool based on measurement and sensing results. Sidelink communication includes NR sidelink communication and V2X sidelink communication. Figure 2 below illustrates NR sidelink communication. V2X sidelink communication is specified in 3GPP TS 36.300.

[0049] FIG. 2 illustrates an exemplary V2X communication system in accordance with some embodiments of the present application.

[0050] 2, the V2X communication system includes one gNB 202, one ng-eNB 203, and several V2X UEs, namely, UE 201-A, UE 201-B, and UE 201-C, each of which may be referred to as UE 101a, UE 101b, or relay UE 103 as shown and illustrated in FIG.

[0051] In particular, UE201-A is within the coverage of gNB202, UE201-B is within the coverage of ng-eNB203, and UE201-C is outside the coverage of gNB202 and ng-eNB203. Support for V2X services over the PC5 interface can be provided by NR sidelink communication and / or V2X sidelink communication. NR sidelink communication can support one of three transmission modes for a pair of source Layer 2 identity (ID) and destination Layer 2 ID: unicast transmission, groupcast transmission, and broadcast transmission. Sidelink transmission and reception over the PC5 interface is supported when the UE is inside or outside NG-RAN coverage.

[0052] UE 201-A is within the coverage of gNB 202 and may perform sidelink unicast, groupcast, or broadcast transmissions over the PC5 interface. UE 201-C is out of coverage but may also perform sidelink transmissions and receptions over the PC5 interface. In accordance with some other embodiments of the present application, it is contemplated that a V2X communication system may include more or fewer BSs and more or fewer V2X UEs. Furthermore, it is contemplated that the names of the V2X UEs (representing Tx UE, Rx UE, etc.) as illustrated and depicted in FIG. 2 may be different, e.g., UE 201c, UE 204f, and UE 208g.

[0053] Additionally, although each V2X UE as depicted in FIG. 2 is illustrated in the form of a mobile phone, it is contemplated that a V2X communication system may include any type of UE (e.g., a road map device, a mobile phone, a computer, a laptop, an IoT (Internet of Things) device, or other type of device) in accordance with some other embodiments of the present application.

[0054] According to some embodiments of FIG. 2 , UE 201-A functions as a Tx UE, and UE 201-B and UE 201-C function as Rx UEs. UE 201-A may exchange V2X messages with UE 201-B or UE 201-C over a sidelink, e.g., a PC5 interface as defined in 3GPP TS 23.303. UE 201-A may transmit information or data to other UEs in the V2X communication system over a sidelink unicast, sidelink groupcast, or sidelink broadcast. Sidelink communications include NR sidelink communications and V2X sidelink communications. For example, UE 201-A may transmit data to UE 201-C over an NR sidelink unicast session, and UE 201-B may transmit data to UE 201-C over a V2X sidelink unicast session. UE 201-A may transmit data to UE 201-B and UE 201-C in the groupcast group via a sidelink groupcast transmission session.

[0055] Sidelink communication includes NR sidelink communication and V2X sidelink communication. Figure 2 shows NR sidelink communication as specified in 3GPP TS 38.311. V2X sidelink communication is specified in 3GPP TS 36.311.

[0056] FIG. 3 illustrates an exemplary flowchart of a sidelink RRC reconfiguration procedure according to some embodiments of the present application.

[0057] As shown in FIG. 3, in operation 301, a UE 310 (e.g., a UE 101a as illustrated and shown in FIG. 1) initiates a sidelink RRC reconfiguration procedure for a UE 320 (e.g., a relay UE 103 as illustrated and shown in FIG. 1) by sending an RRCReconfigurationSidelink message to the UE 320.

[0058] If the sidelink RRC reconfiguration procedure is completed successfully, then in operation 302 the UE 320 may send an "RRC reconfiguration complete sidelink message", e.g., an RRCReconfigurationCompleteSidelink message as defined in the 3GPP standard documents, to the UE 310. Alternatively, if the sidelink RRC reconfiguration procedure is not completed successfully, then in operation 302 the UE 320 may send an "RRC reconfiguration failure sidelink message", e.g., an RRCReconfigurationFailureSidelink message as defined in the 3GPP standard documents, to the UE 310.

[0059] The purpose of the sidelink RRC reconfiguration procedure is to modify the PC5 RRC connection, e.g., to establish, modify, or release sidelink data radio bearers (DRBs), to configure NR sidelink measurements and reporting, and to configure sidelink channel state information (CSI) reference signal resources.

[0060] A UE (e.g., UE 310 as illustrated and shown in FIG. 3) may initiate a sidelink RRC reconfiguration procedure and take action on the corresponding PC5 RRC connection if: - release of a sidelink DRB associated with a peer UE (e.g., UE 320 as illustrated and depicted in FIG. 3 ); - Establishment of a sidelink DRB associated with the peer UE; - changes to the parameters contained in the Sidelink Radio Bearer (SLRB)-Config of the Sidelink DRB associated with the peer UE; - configuration information of the peer UE for NR sidelink measurements and reporting; and - Configuration information for sidelink CSI reference signal resources.

[0061] A UE capable of NR sidelink communication may initiate a Sidelink UE Information procedure for NR to report to the network or BS that a sidelink Radio Link Failure (RLF) (e.g., timer T400 expiration) or a sidelink RRC reconfiguration failure has been declared.

[0062] The table below shows the implementation of timer T400 as specified in the 3GPP standard documents, including the start conditions, stop conditions, action on expiry and possible common names for the timer.

[0063] [Table 1]

[0064] FIG. 4 illustrates an example flowchart of a sidelink UE information procedure according to some embodiments of the present application.

[0065] As illustrated in FIG. 4, in operation 401, a UE 410 (e.g., a UE 101a as illustrated and shown in FIG. 1 or a UE 310 as illustrated and shown in FIG. 3) sends a "sidelink UE information message," e.g., a SidelinkUEinformationNR message as defined in the 3GPP standard documents, to a BS 420 (e.g., a BS 102 as illustrated and shown in FIG. 1). Specifically, the SidelinkUEinformationNR message may include sidelink failure information. The sidelink failure information may include a sidelink destination information ID and a sidelink failure cause.

[0066] According to the 3GPP standard document, in the keep-alive procedure in the PC5-S layer, the PC5-S protocol shall support a keep-alive functionality used to detect whether a specific PC5 unicast link is still valid. The UE shall minimize keep-alive signaling, for example, cancel the procedure if data is successfully received over the PC5 unicast link.

[0067] FIG. 5 illustrates an exemplary flowchart of a Layer 2 link maintenance procedure according to some embodiments of the present application.

[0068] As illustrated in FIG. 5, in step 0, UE-1 (e.g., UE 101a, UE 201-C, UE 310, or UE 410 as illustrated and shown in FIGS. 1-4) and UE-2 (e.g., relay UE 103, UE 201-A, or UE 320 as illustrated and shown in FIGS. 1-3) establish a unicast link. In step 1, UE-1 (e.g., UE 101a as illustrated and shown in FIG. 1) sends a keep-alive message to UE-2 (e.g., relay UE 103 as illustrated and shown in FIG. 1) to determine the status of the PC5 unicast link based on a trigger condition. In step 2, in response to receiving the keep-alive message, UE-2 responds with a keep-alive Ack message to UE-1. When UE-1 receives a response from UE-2, UE-1 stops the "timer for the keep-alive procedure." Otherwise, the "timer for the keep-alive procedure" expires.

[0069] FIG. 6 illustrates an exemplary Layer 2 UE-to-UE relay protocol stack in accordance with some embodiments of the present application.

[0070] The embodiment of Figure 6 illustrates protocol stacks at each side of UE1 (e.g., UE 101a, UE 201-C, UE 310, or UE 410 as illustrated and shown in Figures 1-4), relay UE (e.g., relay UE 103, UE 201-A, or UE 320 as illustrated and shown in Figures 1-3), and UE2 (e.g., UE 101b as illustrated and shown in Figure 1 or UE 320 as illustrated and shown in Figure 3). Each of UE1 and UE2 is connected to the relay UE via a PC-5 interface, which may also be referred to as a PC5 interface.

[0071] For Layer 2 (i.e., L2) UE-to-UE relay, an adaptation layer is supported over a separate PC5 link (i.e., the PC5 link between the relay UE and the destination UE). For L2 UE-to-UE relay, the adaptation layer is placed above the RLC sublayer for both the control plane (CP) and the user plane (UP) over the separate PC5 link. The Sidelink Service Data Adaptation Protocol (SDAP) or Sidelink Packet Data Convergence Protocol (PDCP) and RRC are terminated between the two remote UEs, while RLC, MAC, and PHY are terminated in each PC5 link.

[0072] In particular, as shown in Figure 6, the UE1 side includes protocol layers of PHY, MAC, RLC, adaptation layer, PDCP, and SDAP, the relay UE side includes protocol layers of PHY, MAC, RLC, and adaptation layer, and the UE2 side includes protocol layers of PHY, MAC, RLC, adaptation layer, PDCP, and SDAP.

[0073] With respect to the Layer 3 (i.e., L3) UE-to-UE relay protocol stack, the relay UE has a complete protocol stack: the user plane (UP) protocol stack of the L3 relay UE includes PHY, MAC, RLC, PDCP, and SDAP layers, and the control plane (CP) protocol stack of the L3 relay UE includes PHY, MAC, RLC, PDCP, and RRC layers.

[0074] Currently, the following problems need to be solved in a sidelink relay system under 3GPP 5G NR: what are the trigger conditions for relay reselection, what are the conditions for triggering a relay UE to send a notification of a failure for a Layer 2 link, what is the behavior of the UE after the UE receives a notification of a failure of the link between the relay UE and another UE, and whether the UE reports failure information of the end-to-end relay connection to the BS. Details on how to design relay reselection and connection processing procedures in a UE-UE relay scenario have not yet been specifically discussed. Embodiments of the present application provide relay reselection and connection processing procedures in a UE-UE relay scenario in a 3GPP 5G NR system or the like to solve the above problems. Further details will be illustrated in the following text in combination with the accompanying drawings.

[0075] 7 illustrates a flowchart of a method for performing a relay reselection procedure according to some embodiments of the present application. The method may be performed by a UE (e.g., a UE 101a as illustrated and shown in FIG. 1, a UE 201-C as illustrated and shown in FIG. 2, a UE 310 as illustrated and shown in FIG. 3, or a UE 410 as illustrated and shown in FIG. 4). Although described with respect to a UE, it should be understood that other devices may be configured to perform a method similar to that of FIG. 7.

[0076] In the exemplary method 700 as illustrated in Figure 7, a UE (e.g., UE 101a as illustrated and shown in Figure 1) establishes a PC5 RRC connection for a link between the UE and a relay UE (e.g., relay UE 103 as illustrated and shown in Figure 1) in operation 701. The embodiment of Figure 7 assumes that a PC5 RRC connection for a link between the relay UE and another UE (e.g., UE 101b as illustrated and shown in Figure 1) has been established.

[0077] In operation 702, the UE performs a relay reselection procedure based on a trigger condition. For example, the trigger condition may be: (1) Detecting a sidelink failure. For example, the sidelink failure occurs in a link between the UE and a relay UE. The sidelink failure may be a failure in the RLF in the link between the UE and the relay UE and / or a failure in configuration information associated with the link between the UE and the relay UE. (2) Detecting a failure in the RRC relay connection of the link between the UE and the other UE. For example, the failure in the RRC relay connection of the link between the UE and the other UE is detected in response to at least one of expiry of a “further timer for RRC reconfiguration procedure” associated with the RRC relay connection of the link between the UE and the other UE and expiry of an “additional timer for keep-alive procedure” associated with the link between the UE and the other UE. (3) Receiving a fault notification from the relay UE. (4) Receiving a fault indication from an upper layer of the UE.

[0078] In some embodiments, the failure notification received from the relay UE comprises: (1) A sidelink RLF notification associated with a link between the relay UE and the other UE. In one example, the sidelink RLF notification includes a cause. The cause may be at least one of reaching a maximum number of RLC retransmissions, expiry of a "timer for RRC reconfiguration transmissions for sidelink," reaching a maximum number of consecutive HARQ DTXs, receiving an integrity check failure indication, and occurrence of a PC5-S link failure. The PC5-S link failure may also be referred to as a PC5-S unicast link failure, etc. (2) Notification of failure to restore sidelink RLF on the link between the relay UE and the other UE mentioned above. (3) Notification of a PC5-S link failure on a link between the relay UE and the other UE. In one example, the notification of the PC5-S link failure is received after the AS layer of the relay UE receives an indication of the PC5-S link failure. In a further example, the notification of the PC5-S link failure is received after the expiration of a "keep-alive procedure timer."

[0079] In another example, the PC5-S link failure is detected in response to at least one of the expiration of a "timer for keep-alive procedure" associated with the link between the UE and the relay UE and the expiration of a "different timer for keep-alive procedure" associated with the link between the UE and another UE as mentioned above.

[0080] In some embodiments, the upper layer of the UE is a PC5-S layer, and the failure notification is received from the PC5-S layer of the UE. The failure notification may be an indication of a PC5-S link failure of a link between the UE and the relay UE, the indication being received by the AS layer of the UE from the PC5-S layer of the UE.

[0081] In some embodiments, upon expiration of a "timer for keep-alive procedure" associated with the link between the relay UE and the other UE described above, the upper layer of the relay UE indicates a failure notification to the AS layer of the relay UE, which then sends the failure notification to the UE.

[0082] In some embodiments, after the UE receives a failure notification associated with a sidelink failure in a link between the relay UE and the other UE, the UE suspends transmission of data destined for the other UE.

[0083] In some embodiments, after the UE receives a failure notification associated with a sidelink failure in the link between the relay UE and the other UE, the UE continues to transmit data destined for the relay UE and to receive data from the relay UE.

[0084] In one embodiment, the UE stops receiving data from the relay UE when it receives an end mark indication from the relay UE. In a further embodiment, the UE stops receiving data from the relay UE when it receives an RRC message containing an indication that the data transfer addressed to the UE has been completed.

[0085] After stopping receiving data from the relay UE, the UE may release the PC5 RRC connection between the UE and the relay UE. Alternatively, the AS layer of the UE may send an indication to the PC5-S layer of the UE to indicate that the UE has stopped receiving data from the relay UE.

[0086] In some embodiments, if the UE is within the coverage of the BS and the UE detects a sidelink failure in the link between the UE and a relay UE, or a failure in the RRC relay connection of the link between the UE and another UE as described above, the UE reports failure information to the BS.

[0087] In some embodiments, if the UE is within the coverage of the BS and the UE receives a failure notification from the relay UE, the UE reports the received failure information to the BS. In some embodiments, the failure information or failure notification includes a failure cause. The failure cause may be: (1) A fault related to configuration information, where the configuration information is associated with the link between the UE and the relay UE; (2) Side link failure in the link between the UE and the relay UE; (3) a side link failure in the link between the relay UE and the other UE; and (4) A failure in the RRC relay connection of the link between the UE and another UE as described above.

[0088] In some other embodiments, the failure information or failure notification includes a set of identification information for the two end UEs of the link associated with the failure cause. Specifically, if the UE is within the coverage of a BS (e.g., BS 102 illustrated and shown in FIG. 1), the UE may report failure information of the first hop link (i.e., the link between the UE and the relay UE) and the end-to-end RRC relay connection (i.e., the RRC relay connection between the UE and another UE) in a UE-to-UE scenario. The UE may also report receipt of failure information from the relay UE to the BS. For a configuration failure case, a corresponding cause (e.g., configuration failure) will be added to the failure information. For a PC5-S link failure, a corresponding cause (e.g., PC5-S link failure or timer expiration) will be added to the failure information.

[0089] Details described in all other embodiments of this application (e.g., details regarding specific trigger conditions for performing a relay reselection procedure) are applicable to the embodiment of Figure 7. Moreover, details described in the embodiment of Figure 7 are applicable to all embodiments of Figures 1-6 and 8-10.

[0090] 8 illustrates a flowchart of a method for transmitting a failure notification according to some embodiments of the present application. The method may be performed by a relay UE (e.g., the relay UE 103 illustrated and shown in FIG. 1, the UE 201-A or UE 201-B as illustrated and shown in FIG. 2, or the UE 320 as illustrated and shown in FIG. 3). Although described with respect to a relay UE, it should be understood that other devices may be configured to perform a method similar to that of FIG. 8.

[0091] In the exemplary method 800 as illustrated in Figure 8, a PC5 RRC connection is established for a link between a UE (e.g., UE 101a illustrated and shown in Figure 1) and a relay UE (e.g., relay UE 103 illustrated and shown in Figure 1) at operation 801. An RRC connection is established for a link between the relay UE and another UE (e.g., UE 101b illustrated and shown in Figure 1).

[0092] In operation 803, the relay UE sends a failure notification to the UE. For example, the failure notification sent from the relay UE may include: (1) A sidelink RLF notification associated with a link between the relay UE and the other UE. In one example, the sidelink RLF notification includes a cause, which may be at least one of: reaching a maximum number of RLC retransmissions, expiry of a "timer for RRC reconfiguration transmissions for sidelink," reaching a maximum number of consecutive HARQ DTXs, receiving an integrity check failure indication, and occurrence of a PC5-S link failure. (2) Notification of failure to restore sidelink RLF on the link between the relay UE and the other UE mentioned above. (3) Notification of a PC5-S link failure on the link between the relay UE and the other UE mentioned above.

[0093] In one embodiment, the AS layer of the relay UE receives an indication of a PC5-S link failure on the link between the relay UE and the other UE mentioned above, and the relay UE then sends a notification of the PC5-S link failure to the UE.

[0094] In a further embodiment, the relay UE detects the expiration of a "keep alive procedure timer" associated with the link between the relay UE and the other UE mentioned above, and then the relay UE sends a notification of a PC5-S link failure to the UE.

[0095] Details described in all other embodiments of the present application (e.g., details regarding fault notification) are applicable to the embodiment of Fig. 8. Moreover, details described in the embodiment of Fig. 8 are applicable to all embodiments of Figs. 1 to 7, 9, and 10.

[0096] 9 illustrates a flowchart of a method for reporting fault information according to some embodiments of the present application. The method may be performed by a UE (e.g., a UE 101a as illustrated and shown in FIG. 1, a UE 201-C as illustrated and shown in FIG. 2, a UE 310 as illustrated and shown in FIG. 3, or a UE 410 as illustrated and shown in FIG. 4). Although described with respect to a UE, it should be understood that other devices may be configured to perform a method similar to that of FIG. 9.

[0097] 9, in operation 901, if a UE (e.g., UE 101a illustrated and shown in FIG. 1 or UE 410 illustrated and shown in FIG. 4) is within the coverage of a BS (e.g., BS 102 illustrated and shown in FIG. 1 or BS 420 illustrated and shown in FIG. 4), the UE reports failure information to the BS. The failure information may be regarding at least one of a failure of a link between the UE and a relay UE (e.g., relay UE 103 illustrated and shown in FIG. 1) and a failure of an RRC relay connection of a link between the UE and another UE (e.g., UE 101b illustrated and shown in FIG. 1).

[0098] A link between a UE and a relay UE may also be termed a "first hop link between the UE and the relay UE," "first hop link," etc. An RRC relay connection of a link between a UE and another UE may also be termed an "end-to-end RRC connection of a relay link," "end-to-end RRC connection," "end-to-end relay connection," "relay RRC connection," etc.

[0099] In some embodiments, the UE may receive a failure notification or information from the relay UE and then report the received failure notification or information to the BS.

[0100] The fault information reported by the UE to the BS may include a fault cause. In one example, for a configuration fault case, a corresponding cause (e.g., configuration failure) may be added to the fault information. In a further example, for a PC5-S link fault case, a corresponding cause (e.g., PC5-S link failure or timer expiration) may be added to the fault information.

[0101] In some embodiments, the two terminating UEs of the link may be added to the failure information reported to the BS. In one example, if the failure occurs in the link between the UE and the relay UE (i.e., a first-hop link failure), the failure information includes identification information (e.g., a destination ID) of the UE and the relay UE. In a further example, if the failure occurs in the RRC relay connection of the link between the UE and another UE (i.e., a failure of the end-to-end RRC connection), the failure information includes identification information (e.g., a destination ID) of the UE and the other UE. When the BS receives the failure information, based on the identification information of the terminating UE, the BS can distinguish whether the failure occurs in the first-hop link or the end-to-end RRC connection.

[0102] Details described in all other embodiments of the present application (e.g., details regarding information of failures in the first hop link or end-to-end RRC connection) are applicable to the embodiment of Figure 9. Moreover, details described in the embodiment of Figure 9 are applicable to all embodiments of Figures 1 to 8 and 10.

[0103] The following text describes specific embodiments 1-3 of the method as shown and exemplified in any of FIGS.

[0104] Embodiment 1 According to embodiment 1, a UE(a) (e.g., a UE 101a as shown and illustrated in FIG. 1 ), a relay UE (e.g., a relay UE 103 as shown and illustrated in FIG. 1 ), another UE(b) (e.g., a UE 101b as shown and illustrated in FIG. 1 ), and a BS (e.g., a BS 102 as shown and illustrated in FIG. 1 ) perform the following steps: (1) Step 1: A PC5 RRC connection is established between UE(a) and a relay UE. Another PC5 RRC connection is established between the relay UE and UE(b). (2) Step 2 (only for L2 relay scenario): An end-to-end RRC connection between UE(a) and UE(b) is established. UE(a) sends an RRCReconfiguration message for the sidelink relay connection, which is then relayed to UE(b) by the relay UE. UE(a) starts a timer to control the procedure. UE(b) sends RRC reconfiguration for sidelink relay connection to UE(a), which is relayed by the relay UE. (3) Step 3: The relay UE declares a sidelink RLF for the link between the relay UE and UE(b) when at least one of the following conditions occurs: upon an indication from the sidelink RLC entity that the maximum number of retransmissions for a particular destination has been reached, or upon expiration of timer T400, or Depending on an indication from the sidelink MAC entity of the maximum number of consecutive HARQ DTXs, or In response to an integrity check failure indication from a sidelink PDCP entity, or ●Failed keep-alive procedure: After the relay UE sends a keep-alive message to UE(b), the relay UE starts a timer. If the timer expires, the upper layer will indicate this to the AS layer. The relay UE may then send a notification of side link failure or PC5 link failure to UE(a). (4) Step 4: When the relay UE declares sidelink RLF, receives a configuration failure, or receives a PC5-S layer failure indication from the upper layer of the relay UE, the relay UE sends a failure notification to UE(a). (5) Step 5: UE(a) receives a failure notification from the relay UE. The failure information may indicate a sidelink RLF, a configuration failure, or a Layer 2 link failure at higher layers. In case 1, where the failure occurs in the second hop (i.e., the link between the relay UE and UE(b)) in both the L2 and L3 relay scenarios, the trigger condition for the relay reselection procedure is: - UE(a) receives a sidelink RLF notification from the relay UE when an RLF on the sidelink between the relay UE and UE(b) occurs; ● The sidelink RLF notification may include at least one of the following sidelink RLF causes: maximum number of RLC retransmissions, T400 expiry, maximum number of consecutive HARQ DTX, receipt of integrity check failure indication, and PC5-S link failure. - UE(a) receives a sidelink RLF recovery failure notification from the relay UE when the relay UE fails to restore RLF on the sidelink between the relay UE and UE(b); - UE(a) receives notification of a PC5-S failure from the relay UE when the AS layer of the relay UE receives an indication of a PC5 layer link failure or when the "keep-alive procedure timer" expires. (6) Step 6: UE(a) is triggered to perform relay reselection. When UE(a) receives a failure notification from the relay UE, UE(a) may be triggered to keep the first hop link (i.e., the link between UE(a) and the relay UE) and send a SidelinkUEinformation message to the serving BS (e.g., BS102 illustrated and shown in FIG. 1). After UE(a) receives notification of a failure of the link between the relay UE and UE(b), the behavior of UE(a) may be as follows: - When UE(a) receives notification of the failure from the relay UE, UE(a) may continue to maintain the first hop link. UE(a) suspends transmission of data addressed to UE(b), i.e. data intended to be sent to UE(b). However, UE(a) continues to send data addressed to the relay UE. ●UE(a) continues to receive data from the relay UE until it receives an end mark indication. UE(a) releases the PC5 RRC connection between UE(a) and the relay UE. When UE(a) receives an end mark indication from the relay UE, the AS layer of UE(a) may instruct the PC5-S layer of UE(a). The UE(a) may report failure information of the first hop link and / or the end-to-end relay connection to a BS (e.g., BS 102 illustrated and shown in FIG. 1). For example: If UE(a) is within the coverage of the BS, UE(a) may report to the BS failure information regarding the first hop link and the end-to-end RRC relay connection. UE(a) also reports to the BS failure information received from the relay UE. For configuration failure cases, the corresponding cause (e.g., configuration failure) is added to the failure information. ●For PC5-S link failure cases, the corresponding cause (e.g., PC5-S link failure or timer expiration) is added to the failure information. >>ID information about the two end UEs of the link is added to the failure information reported by UE(a), so that when the BS receives the failure information, it can distinguish between the failed first hop link and the end-to-end RRC relay connection.

[0105] Embodiment 2 According to embodiment 2, a UE(a) (e.g., a UE 101a as shown and illustrated in FIG. 1 ), a relay UE (e.g., a relay UE 103 as shown and illustrated in FIG. 1 ), another UE(b) (e.g., a UE 101b as shown and illustrated in FIG. 1 ), and a BS (e.g., a BS 102 as shown and illustrated in FIG. 1 ) perform the following steps: (1) Step 1: A PC5 RRC connection is established between UE(a) and a relay UE. Another PC5 RRC connection is established between the relay UE and UE(b). (2) Step 2 (only for L2 relay scenario): An end-to-end RRC connection between UE(a) and UE(b) is established. UE(a) sends an RRCReconfiguration message for the sidelink relay connection, which is then relayed to UE(b) by the relay UE. UE(a) starts a timer to control the procedure. UE(b) sends RRC reconfiguration for sidelink relay connection to UE(a), which is relayed by the relay UE. (3) Step 3: UE(a) declares a failure of the end-to-end RRC connection between UE(a) and UE(b) based on the following conditions: In case 2, where a failure occurs in the end-to-end connection for the L2 relay: - Timer expiration for L2 relay UE(a) sends an RRC reconfiguration for the relay side link to UE(b), which is relayed to UE(b) by the relay UE. One timer is used to control the reconfiguration procedure. When UE(a) sends an RRC reconfiguration for the relay side link, UE(a) starts the timer. When UE(a) receives a reconfiguration complete for the relay side link, it stops the timer. - The AS layer of UE(b) receives an indication of a PC5 unicast link failure from a higher layer (e.g., PC5-S layer). - The "keep alive procedure timer" associated with the link between UE(a) and UE(b) expires. (4) Step 4: UE(a) is triggered to perform relay reselection. UE(a) may be triggered to keep the first hop link (i.e., the link between UE(a) and the relay UE) if the link between UE(a) and the relay UE is still available, and to send a SidelinkUEinformation message to the serving BS (e.g., BS 102 illustrated and shown in FIG. 1). The UE(a) may report failure information of the first hop link and / or the end-to-end relay connection to a BS (e.g., BS 102 illustrated and shown in FIG. 1). For example: If UE(a) is within the coverage of the BS, UE(a) may report to the BS failure information regarding the first hop link and the end-to-end RRC relay connection. UE(a) also reports to the BS failure information received from the relay UE. For configuration failure cases, the corresponding cause (e.g., configuration failure) is added to the failure information. ●For PC5-S link failure cases, the corresponding cause (e.g., PC5-S link failure or timer expiration) is added to the failure information. >>ID information about the two end UEs of the link is added to the failure information reported by UE(a), so that when the BS receives the failure information, it can distinguish between the failed first hop link and the end-to-end RRC relay connection.

[0106] Embodiment 3 According to embodiment 3, a UE(a) (e.g., a UE 101a as shown and illustrated in FIG. 1 ), a relay UE (e.g., a relay UE 103 as shown and illustrated in FIG. 1 ), another UE(b) (e.g., a UE 101b as shown and illustrated in FIG. 1 ), and a BS (e.g., a BS 102 as shown and illustrated in FIG. 1 ) perform the following steps: (1) Step 1: A PC5 RRC connection is established between UE(a) and a relay UE. Another PC5 RRC connection is established between the relay UE and UE(b). (2) Step 2 (only for L2 relay scenario): An end-to-end RRC connection between UE(a) and UE(b) is established. UE(a) sends an RRCReconfiguration message for the sidelink relay connection, which is then relayed to UE(b) by the relay UE. UE(a) starts a timer to control the procedure. UE(b) sends an RRC reconfiguration for sidelink relay connection to UE(a), which is relayed by the relay UE. (3) Step 3: UE(a) declares sidelink RLF for the link between UE(a) and the relay UE when the following conditions occur: upon an indication from the sidelink RLC entity that the maximum number of retransmissions for a particular destination has been reached, or Upon expiration of T400, or Depending on an indication from the sidelink MAC entity of the maximum number of consecutive HARQ DTXs, or ● In response to an integrity check failure indication from a sidelink PDCP entity. In case 3, where a failure occurs in the first hop link (i.e., the link between UE(a) and the relay UE) in both the L2 and L3 relay scenarios, the trigger condition for the relay reselection procedure is: - UE(a) receives a configuration failure from the relay UE. ● This configuration is associated with the link between UE(a) and the relay UE. - The AS layer of UE(a) receives an indication of a PC5 unicast link failure from an upper layer (e.g., PC5-S layer) of UE(a). - A "keep alive procedure timer" associated with the link between UE(a) and the relay UE expires. (4) Step 4: UE(a) is triggered to perform relay reselection. Meanwhile, UE(a) may send a SidelinkUEinformation message to the serving BS. The UE(a) may report failure information of the first hop link and / or the end-to-end relay connection to a BS (e.g., BS 102 illustrated and shown in FIG. 1). For example: If UE(a) is within the coverage of the BS, UE(a) may report to the BS failure information regarding the first hop link and the end-to-end RRC relay connection. UE(a) also reports to the BS failure information received from the relay UE. For configuration failure cases, the corresponding cause (e.g., configuration failure) is added to the failure information. ●For PC5-S link failure cases, the corresponding cause (e.g., PC5-S link failure or timer expiration) is added to the failure information. >>ID information about the two end UEs of the link is added to the failure information reported by UE(a), so that when the BS receives the failure information, it can distinguish between the failed first hop link and the end-to-end RRC relay connection.

[0107] FIG. 10 illustrates a simplified block diagram of an apparatus for fault handling procedures according to some embodiments of the present application.

[0108] In some embodiments of the present application, the apparatus 1000 may be a UE (e.g., UE 101a as illustrated and shown in FIG. 1, UE 201-C as illustrated and shown in FIG. 2, UE 310 as illustrated and shown in FIG. 3, or UE 410 as illustrated and shown in FIG. 4) and may at least perform the method illustrated in FIG. 7 or 9. In some other embodiments of the present application, the apparatus 1000 may be a relay UE (e.g., relay UE 103 as illustrated and shown in FIG. 1, UE 201-A or UE 201-B as illustrated and shown in FIG. 2, or UE 320 as illustrated and shown in FIG. 3) and may at least perform the method illustrated in FIG. 8. In some additional embodiments of the present application, the apparatus 1000 may be a BS (e.g., BS 102 as illustrated and shown in FIG. 1 or BS 420 as illustrated and shown in FIG. 4).

[0109] As shown in FIG. 10, the apparatus 1000 may include at least one receiver 1002, at least one transmitter 1004, at least one non-transitory computer-readable medium 1006, and at least one processor 1008 coupled to the at least one receiver 1002, the at least one transmitter 1004, and the at least one non-transitory computer-readable medium 1006.

[0110] 10, elements such as at least one receiver 1002, at least one transmitter 1004, at least one non-transitory computer-readable medium 1006, and at least one processor 1008 are described in the singular, but the plural is contemplated unless limitation to the singular is expressly stated. In some embodiments of the present application, the at least one receiver 1002 and the at least one transmitter 1004 are combined into a single device, such as a transceiver. In some embodiments of the present application, the apparatus 1000 may further include an input device, a memory, and / or other components.

[0111] In some embodiments of the present application, at least one non-transitory computer-readable medium 1006 may store computer-executable instructions programmed to implement operations of a method, such as those described with respect to any one of Figures 7-9, by at least one receiver 1002, at least one transmitter 1004, and at least one processor 1008.

[0112] Those skilled in the art will understand that the operations of the methods described in connection with the embodiments disclosed herein may be embodied directly in hardware, in software modules executed by a processor, or in a combination of the two. The software modules 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 known storage medium. Additionally, in some embodiments, the operations of the methods may reside as one or any combination or set of codes and / or instructions on a non-transitory computer-readable medium and may be incorporated into a computer program product.

[0113] While the present disclosure has been described with reference to specific embodiments thereof, it is apparent that many alternatives, modifications, and variations may be apparent to those skilled in the art. For example, various components of the embodiments may be exchanged, added, or substituted in other embodiments. Also, not all elements in the figures are necessary for the operation of the disclosed embodiments. For example, one skilled in the art would be enabled to make and use the teachings of the present disclosure by simply utilizing the elements of the independent claims. Accordingly, the embodiments of the present disclosure described herein are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the present disclosure.

[0114] As used in this document, the terms "comprise," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus comprising a set of elements does not include only those elements, but may include other elements not expressly stated or inherent in such process, method, article, or apparatus. An element preceded by "a," "an," etc., does not, without further constraints, preclude the presence of additional identical elements in the process, method, article, or apparatus that comprises the element. Also, the term "another" is defined as at least a second or more. The term "having," etc., as used herein, is defined as "comprising." [Explanation of symbols]

[0115] 100 Wireless Communication System 101a UE 101b UE 102 BS 103 Relay UE 201-A UE 201-B UE 201-C UE 202 gNB 203 ng-eNB 310 UE 320 UE 410 UE 420 BS 1000 devices 1002 Receiver 1004 Transmitter 1006 Non-transitory computer-readable medium 1008 processor T400 Timer

Claims

1. 1. A method performed by a first user equipment (UE), comprising: establishing a PC5 Radio Resource Control (RRC) connection for a link between the first UE and a relay UE, wherein an RRC connection for a link between the relay UE and a second UE has been established; performing a relay reselection procedure based on a trigger condition, the trigger condition being reception of a failure notification from the relay UE, the failure notification received from the relay UE being a sidelink Radio Link Failure (RLF) notification associated with the link between the relay UE and the second UE; A method comprising:

2. The sidelink RLF notification includes a cause, the cause being: The maximum number of Radio Link Control (RLC) retransmissions has been reached; and expiry of a timer for RRC reconfiguration transmission for the sidelink; and The maximum number of consecutive Hybrid Automatic Repeat Request (HARQ) discontinuous transmissions (DTX) has been reached, and receiving an integrity check failure indication; The method of claim 1, wherein the cause is at least one of the occurrence of PC5 signaling (PC5-S) link failure.

3. A method performed by a first user equipment (UE), comprising: establishing a PC5 Radio Resource Control (RRC) connection for a link between the first UE and a relay UE, wherein an RRC connection for a link between the relay UE and a second UE has been established; performing a relay reselection procedure based on a trigger condition, the trigger condition being detecting a side link failure occurring on a link between the first UE and the relay UE; The side link failure occurs in the link between the first UE and the relay UE, and the side link failure is an RLF in the link between the first UE and the relay UE; a failure related to configuration information associated with the link between the first UE and the relay UE; A method in which at least one of the above is performed.

4. the failure notification is indicated from an upper layer of the relay UE to an AS layer of the relay UE in response to expiration of a timer for a keep-alive procedure, the timer for a keep-alive procedure being associated with the link between the relay UE and the second UE; the failure notification is sent by the relay UE to the first UE; The method of claim 1.

5. at least one non-transitory computer-readable medium storing computer-executable instructions; at least one receiving circuitry; at least one transmission network; at least one processor coupled to the at least one non-transitory computer-readable medium, the at least one receiving circuitry, and the at least one transmitting circuitry; The computer-executable instructions cause the at least one processor to implement the method of any one of claims 1 to 4. Device.

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