Method and apparatus for link recovery in sidelink relay communications

Abstract

A method and apparatus for link recovery in sidelink relay communication are disclosed. An operation method of a transmitting terminal includes the steps of: performing, by a first relay terminal, a first SL relay communication with a receiving terminal; declaring an RLF of a sidelink in which the first SL relay communication is performed when a preconfigured condition is satisfied; configuring a connection with a second relay terminal; and performing, by the second relay terminal, a second SL relay communication with the receiving terminal.

Description

TECHNICAL FIELD

[0001] The disclosure relates to a sidelink (SL) communication technology, and more particularly, to a technology for recovering a radio link failure (RLF) in SL relay communication. BACKGROUND

[0002] To handle wireless data that has been rapidly increasing after commercialization of a fourth generation (4G) communication system (e.g., a Long Term Evolution (LTE) communication system, an LTE-Advanced (LTE-A) communication system), a fifth generation (5G) communication system (e.g., a New Radio (NR) communication system) that uses a frequency band of the 4G communication system (e.g., a frequency band below 6 GHz) as well as a frequency band higher than the frequency band of the 4G communication system (e.g., a frequency band above 6 GHz) is considered. The 5G communication system is capable of supporting enhanced mobile broadband (eMBB) communication, ultra-reliable and low-latency communication (URLLC), massive machine type communication (mMTC), etc.

[0003] The 4G communication system and the 5G communication system are capable of supporting vehicle-to-everything (V2X) communication (e.g., sidelink communication). V2X communication supported in a cellular communication system such as the 4G communication system, the 5G communication system, etc. can be referred to as "cellular-V2X (C-V2X) communication". V2X communication (e.g., C-V2X communication) can include vehicle-to-vehicle (V2V) communication, vehicle-to-infrastructure (V2I) communication, vehicle-to-pedestrian (V2P) communication, vehicle-to-network (V2N) communication, etc.

[0004] In a cellular communication system, V2X communication (e.g., C-V2X communication) can be performed based on a sidelink communication technology (e.g., a Proximity-based Service (ProSe) communication technology, a Device-to-Device (D2D) communication technology, or the like). For example, a sidelink channel can be established for vehicles participating in V2V communication (e.g., sidelink communication), and the sidelink channel can be utilized for communication between vehicles. Sidelink communication can be performed using a configured grant (CG) resource. The CG resource can be periodically configured, and periodic data (e.g., periodic sidelink data) can be transmitted using the CG resource.

[0005] On the other hand, SL communication between a transmitting terminal and a receiving terminal can be performed in a relay manner. Relay communication performed through a sidelink can be referred to as SL relay communication. For SL relay communication, a connection between the transmitting terminal and a relay terminal can be configured, and a connection between the relay terminal and the receiving terminal can be configured. The relay terminal can relay SL communication between the transmitting terminal and the receiving terminal. A radio link failure (RLF) can occur in a link between the transmitting terminal and the relay terminal and / or a link between the receiving terminal and the relay terminal. In this case, a method of recovering from the RLF is required. SUMMARY

[0006] TECHNICAL PROBLEM

[0007] To solve the above problem, the disclosure aims to provide a method and apparatus for recovering from an RLF in SL relay communication.

[0008] TECHNICAL SOLUTION

[0009] According to a first embodiment of the disclosure for achieving the above object, an operation method of a transmitting terminal can include: performing, by a first relay terminal, first sidelink (SL) relay communication with a receiving terminal; declaring a radio link failure (RLF) of a sidelink on which the first SL relay communication is performed, when a preconfigured condition is satisfied; configuring a connection with a second relay terminal; and performing, by the second relay terminal, second SL relay communication with the receiving terminal.

[0010] The preconfigured condition can be a case where a number of reception failures of a hybrid automatic repeat request (HARQ)-acknowledgement (ACK) for data transmitted through the first relay terminal is greater than or equal to a threshold value.

[0011] The preconfigured condition can be a case where the first relay terminal does not receive data transmitted by the transmitting terminal within a preset time.

[0012] The operation method of the transmitting terminal can further include receiving, from the base station, a first message including information indicating a preconfigured condition for declaring an RLF.

[0013] The first message can further include information for determining whether the preconfigured condition is satisfied.

[0014] The preconfigured condition can be independently configured for each terminal type, and the terminal type can indicate a transmitting terminal, a relay terminal, or a receiving terminal.

[0015] The configuring of the connection with the second relay terminal can include transmitting a discovery message in a broadcast scheme, and configuring the connection with the second relay terminal based on the discovery message.

[0016] The operation method of the transmitting terminal can further include transmitting, to the receiving terminal through the second relay terminal, information of the second relay terminal connected to the transmitting terminal.

[0017] According to a second embodiment of the disclosure for achieving the above object, an operation method of a relay terminal can include performing sidelink (SL) relay communication between a transmitting terminal and a receiving terminal, determining that a radio link failure (RLF) of a sidelink in which the first SL relay communication is performed has occurred when a preconfigured condition is satisfied, and transmitting a second message indicating that the RLF has occurred.

[0018] The operation method of the relay terminal can further include receiving, from the base station, a first message including information indicating a preconfigured condition for declaring an RLF, wherein the first message further includes information for determining whether the preconfigured condition is satisfied.

[0019] The preconfigured condition can be a case where a number of reception failures of a hybrid automatic repeat request (HARQ)-acknowledgement (ACK) of data of the transmitting terminal transmitted from the relay terminal to the receiving terminal is greater than or equal to a threshold value, and when the preconfigured condition is satisfied, the second message can be transmitted to the transmitting terminal.

[0020] The preconfigured condition can be a case where data transmitted by the transmitting terminal is not received for a preset time, and when the preconfigured condition is satisfied, the second message is transmitted to the receiving terminal.

[0021] The preconfigured condition can be independently configured for each terminal type, and the terminal type can indicate a transmitting terminal, a relay terminal, or a receiving terminal.

[0022] The second message can further include information of a sidelink in which the RLF has occurred.

[0023] According to a third embodiment of the disclosure for achieving the above object, an operation method of a reception terminal can include receiving radio link failure (RLF) configuration information from a base station, performing first sidelink (SL) relay communication with a transmission terminal through a first relay terminal, declaring RLF of a sidelink in which the first SL relay communication is performed when a condition indicated by the RLF configuration information is satisfied, configuring connection with a second relay terminal, and performing second SL relay communication with the transmission terminal through the second relay terminal.

[0024] The condition can be a case where data of the transmission terminal is not received from the first relay terminal within a preset time, and information of the preset time can be included in the RLF configuration information.

[0025] The condition can be a case where the first relay terminal does not receive data from the transmission terminal within a preset time, and information of the preset time can be included in the RLF configuration information.

[0026] Configuring connection with the second relay terminal can include transmitting a discovery message in a broadcast scheme, and configuring connection with the second relay terminal determined based on the discovery message.

[0027] The operation method of the reception terminal can further include transmitting, to the transmission terminal through the second relay terminal, information of the second relay terminal connected to the reception terminal.

[0028] The RLF configuration information can be common RLF configuration information of the transmission terminal, the first relay terminal, the second relay terminal, and the reception terminal or separate RLF configuration information of the reception terminal.

[0029] Advantageous Effects

[0030] According to the disclosure, when data is not received in a SL or a hybrid automatic request repeat (HARQ)-acknowledgement (ACK) for data is not received in a SL, a transmission terminal and / or a reception terminal can declare radio link failure (RLF) and can discover a new relay terminal. The transmission terminal and / or the reception terminal can configure connection with the new relay terminal and perform SL relay communication through the new relay terminal. Accordingly, SL communication can be effectively performed. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 is a conceptual diagram illustrating a V2X communication scenario.

[0032] Figure 2 is a conceptual diagram illustrating a first embodiment of a cellular communication system.

[0033] Figure 3 is a block diagram illustrating a first embodiment of a communication node constituting a cellular communication system.

[0034] Figure 4 is a block diagram illustrating a first embodiment of a user plane protocol stack of a UE performing sidelink communication.

[0035] Figure 5 is a block diagram illustrating a first embodiment of a control plane protocol stack of a UE performing sidelink communication.

[0036] Figure 6 is a block diagram illustrating a second embodiment of a control plane protocol stack of a UE performing sidelink communication.

[0037] Figure 7 is a flowchart illustrating a first implementation of an RLF declaration and recovery method when using condition #1.

[0038] Figure 8a and Figure 8b is a flowchart illustrating a first embodiment of an RLF declaration and recovery method when using condition #2.

[0039] Figure 9a and Figure 9b is a flowchart illustrating a first embodiment of an RLF declaration and recovery method when using condition #3.

[0040] Figure 10 is a flowchart illustrating a first embodiment of an RLF declaration and recovery method when using condition #4.

[0041] Figure 11 is a flowchart illustrating a first embodiment of an RLF declaration and recovery method when using condition #5. DETAILED DESCRIPTION

[0042] The present disclosure can be variously modified and have multiple embodiments, and specific embodiments will be illustrated in the accompanying drawings and described in detail. However, it should be understood that this is not intended to limit the present disclosure to specific embodiments, and the present disclosure will encompass all modifications, equivalents, and alternatives falling within the scope of the idea and technical scope of the present disclosure.

[0043] Relationship terms such as first, second, and the like can be used to describe various elements, but the elements are not limited by these terms. These terms are used only to distinguish one element from another. For example, without departing from the scope of the present disclosure, a first component can be named a second component, and a second component can be similarly named a first component. The term "and / or" means any one or a combination of a plurality of related and described items.

[0044] In the embodiments of the disclosure, "at least one of A and B" can refer to "at least one of A or B" or "at least one of a combination of one or more of A and B". In addition, "one or more of A and B" can refer to "one or more of A or B" or "one or more of a combination of one or more of A and B".

[0045] In the embodiments of the disclosure, (re)sending can refer to "sending", "re-sending", or "sending and re-sending", (re)configuring can refer to "configuring", "reconfiguring", or "configuring and reconfiguring", (re)connecting can refer to "connecting", "reconnecting", or "connecting and reconnecting", and (re)accessing can refer to "accessing", "re-accessing", or "accessing and re-accessing".

[0046] When it is mentioned that a certain component is "coupled" or "connected" with another component, it should be understood that the component is directly "coupled" or "connected" with another component, or a further component can be interposed therebetween. In contrast, when it is mentioned that a certain component is "directly coupled" or "directly connected" with another component, it should be understood that no further component is interposed therebetween.

[0047] The terms used in the disclosure are used only to describe specific embodiments, and are not intended to limit the disclosure. Unless the context clearly dictates otherwise, a singular expression includes a plural expression. In the disclosure, terms such as "include" or "have" are intended to indicate that there is a feature described in the specification, a number, a step, an operation, a component, a part, or a combination thereof, but it should be understood that these terms do not exclude the presence or addition of one or more features, numbers, steps, operations, components, parts, or combinations thereof.

[0048] Unless otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which the disclosure belongs. Terms generally used and already in the dictionary should be interpreted to have meanings matching the context of the art. In the present specification, unless clearly defined, the terms are not necessarily interpreted as having overly formal meanings.

[0049] Hereinafter, preferred embodiments of the disclosure will be described in greater detail with reference to the accompanying drawings. In describing the disclosure, the same elements in the drawings are designated with the same reference numerals and repeated descriptions of the same components will be omitted for the sake of overall understanding of the disclosure.

[0050] Figure 1 is a conceptual diagram showing a V2X communication scenario.

[0051] As Figure 1As illustrated, the V2X communication can include vehicle-to-vehicle (V2V) communication, vehicle-to-infrastructure (V2I) communication, vehicle-to-pedestrian (V2P) communication, vehicle-to-network (V2N) communication, and / or the like. The V2X communication can be supported by the cellular communication system (e.g., the cellular communication network) 140, and the V2X communication supported by the cellular communication system 140 can be referred to as "cellular-V2X (C-V2X) communication." Here, the cellular communication system 140 can include a 4G communication system (e.g., an LTE communication system or an LTE-A communication system), a 5G communication system (e.g., an NR communication system), and / or the like.

[0052] The V2V communication can denote communication between the vehicle #1 100 (e.g., a communication node located in the vehicle #1 100) and the vehicle #2 110 (e.g., a communication node located in the vehicle #2 110). Various driving information such as speed, heading, time, location, and / or the like can be exchanged between the vehicles 100, 110 through the V2V communication. Autonomous driving (e.g., platooning) can be supported based on the driving information exchanged through the V2V communication. The V2V communication supported in the cellular communication system 140 can be performed based on a "sidelink" communication technology (e.g., a ProSe communication technology and a D2D communication technology). In this case, the communication between the vehicles 100 and 110 can be performed using a sidelink channel.

[0053] The V2I communication can denote communication between the vehicle #1 100 and an infrastructure (e.g., a road side unit (RSU)) 120 located at a roadside. The infrastructure 120 can include a traffic light or a street light located at a roadside. For example, when the V2I communication is performed, communication can be performed between a communication node located in the vehicle #1 100 and a communication node located in the traffic light. Traffic information, driving information, and / or the like can be exchanged between the vehicle #1 100 and the infrastructure 120 through the V2I communication. The V2I communication supported in the cellular communication system 140 can also be performed based on a sidelink communication technology (e.g., a ProSe communication technology and a D2D communication technology). In this case, the communication between the vehicle #1 100 and the infrastructure 120 can be performed using a sidelink channel.

[0054] V2P communication can represent communication between vehicle #1 100 (e.g., a communication node located in vehicle #1 100) and person 130 (e.g., a communication node carried by person 130). V2P communication can be used to exchange driving information of vehicle #1 100 and motion information of person 130, such as speed, direction, time, and position. The communication node located in vehicle #1 100 or the communication node carried by person 130 can determine dangerous situations based on the obtained driving and motion information, thereby generating a warning indicating danger. V2P communication supported in the cellular communication system 140 can be performed based on sidelink communication technologies (e.g., ProSe communication technology and D2D communication technology). In this case, a sidelink channel can be used to perform communication between the communication node located in vehicle #1 100 or the communication node carried by person 130.

[0055] V2N communication can refer to communication between vehicle #1 100 (e.g., a communication node located in vehicle #1 100) and cellular communication system (e.g., cellular communication network) 140. V2N communication can be performed based on 4G communication technologies (e.g., LTE or LTE-A communication technologies as defined in 3GPP standards) or 5G communication technologies (e.g., NR communication technologies as defined in 3GPP standards). Furthermore, V2N communication can be performed based on communication technologies defined in IEEE 702.11 (e.g., Wireless Access in Vehicular Environments (WAVE) communication technology, Wireless Local Area Network (WLAN) communication technology, etc.) and communication technologies defined in IEEE 702.15 (e.g., Wireless Personal Area Network (WPAN) communication technology, etc.).

[0056] On the other hand, the cellular communication system 140 that supports V2X communication can be configured as follows.

[0057] Figure 2 This is a conceptual diagram illustrating a first embodiment of a cellular communication system.

[0058] Reference Figure 2 A cellular communication system may include an access network, a core network, etc. The access network may include base stations 210, repeaters 220, user equipment (UEs) 231 to 236, etc. UEs 231 to 236 may include those located in... Figure 1 The communication nodes in vehicles 100 and 110, located Figure 1 Communication nodes in infrastructure 120Figure 1 The communication nodes and the like carried by the personnel 130. When the cellular communication system supports the 4G communication technology, the core network can include a Serving Gateway (S-GW) 250, a Packet Data Network (PDN) Gateway (P-GW) 260, a Mobility Management Entity (MME) 270, and the like.

[0059] When the cellular communication system supports the 5G communication technology, the core network can include a User Plane Function (UPF) 250, a Session Management Function (SMF) 260, an Access and Mobility Management Function (AMF) 270, and the like. Alternatively, when the cellular communication system supports Non-Stand Alone (NSA), the core network composed of the S-GW 250, the P-GW 260, and the MME 270 and the like can support both the 4G communication technology and the 5G communication technology, and the core network composed of the UPF 250, the SMF 260, and the AMF 270 and the like can support both the 5G communication technology and the 4G communication technology.

[0060] In addition, when the cellular communication system supports a network slicing technology, the core network can be divided into a plurality of logical network slices. For example, a network slice supporting V2X communication (for example, a V2V network slice, a V2I network slice, a V2P network slice, a V2N network slice, and the like) can be configured, and V2X communication can be supported through the V2X network slice configured in the core network.

[0061] The communication node constituting the cellular communication system (e.g., base station, repeater, UE, S-GW, P-GW, MME, UPF, SMF, AMF, etc.) can perform communication by utilizing at least one of the following communication technologies: Code Division Multiple Access (CDMA) technology, wideband CDMA (WCDMA) technology, Time Division Multiple Access (TDMA) technology, Frequency Division Multiple Access (FDMA) technology, Orthogonal Frequency Division Multiplexing (OFDM) technology, filtered OFDM technology, Orthogonal Frequency Division Multiple Access (OFDMA) technology, Single Carrier FDMA (SC-FDMA) technology, Non-Orthogonal Multiple Access (NOMA) technology, Generalized Frequency Division Multiplexing (GFDM) technology, Filter Bank Multi-Carrier (FBMC) technology, Universal Filtered Multi-Carrier (UFMC) technology, and Space Division Multiple Access (SDMA) technology.

[0062] The communication node constituting the cellular communication system (e.g., base station, repeater, UE, S-GW, P-GW, MME, UPF, SMF, AMF, etc.) can be configured as follows.

[0063] Figure 3 is a block diagram illustrating a first embodiment of a communication node constituting a cellular communication system.

[0064] Referring to Figure 3 , the communication node 300 can include at least one processor 310, a memory 320, and a transceiver 330 connected to a network to perform communication. In addition, the communication node 300 can further include an input interface device 340, an output interface device 350, a storage device 360, etc. Each component included in the communication node 300 can be connected and communicate with each other through a bus 370.

[0065] However, the components included in the communication node 300 can be connected to the processor 310 through separate interfaces or separate buses instead of the common bus 370. For example, the processor 310 can be connected to at least one of the memory 320, the transceiver 330, the input interface device 340, the output interface device 350, and the storage 360 through a dedicated interface.

[0066] The processor 310 can execute program instructions stored in at least one of the memory 320 and the storage 360. The processor 310 can refer to a central processing unit (CPU), a graphics processing unit (GPU), or a dedicated processor performing a method according to an embodiment of the disclosure. Each of the memory 320 and the storage 360 can include at least one of a volatile storage medium and a non-volatile storage medium. For example, the memory 320 can include at least one of read-only memory (ROM) and random access memory (RAM).

[0067] Referring again to FIG. 3, Figure 2 In the communication system, the base station 210 can form a macro cell or a small cell, and can be connected to a core network through an ideal backhaul or a non-ideal backhaul. The base station 210 can transmit a signal received from the core network to the UEs 231 to 236 and the repeater 220, and can transmit a signal received from the UEs 231 to 236 and the repeater 220 to the core network. The UE#1 231, the UE#2 232, the UE#4 234, the UE#5 235, and the UE#6 236 can belong to a cell coverage of the base station 210. The UE#1 231, the UE#2 232, the UE#4 234, the UE#5 235, and the UE#6 236 can be connected to the base station 210 by performing a connection setup procedure with the base station. The UE#1 231, the UE#2 232, the UE#4 234, the UE#5 235, and the UE#6 236 can communicate with the base station 210 after being connected to the base station 210.

[0068] The repeater 220 can be connected to the base station 210 and can relay communication between the base station 210 and the UE#3 233 and the UE#4 234. That is, the repeater 220 can transmit a signal received from the base station 210 to the UE#3 233 and the UE#4 234, and can transmit a signal received from the UE#3 233 and the UE#4 234 to the base station 210. The UE#4 234 can belong to a cell coverage of the base station 210 and a cell coverage of the repeater 220, and the UE#3 233 can belong to the cell coverage of the repeater 220. That is, the UE#3 233 can be located outside the cell coverage of the base station 210. The UE#3 233 and the UE#4 234 can be connected to the repeater 220 by performing a connection setup procedure with the repeater 220. The UE#3 233 and the UE#4 234 can communicate with the repeater 220 after being connected to the repeater 220.

[0069] The base station 210 and the repeater 220 can support a Multiple Input Multiple Output (MIMO) (e.g., Single User (SU)-MIMO, Multi User (MU)-MIMO, Massive MIMO, etc.) communication technology, a Coordinated Multipoint (CoMP) communication technology, a Carrier Aggregation (CA) communication technology, an unlicensed band communication technology (e.g., Licensed Assisted Access (LAA), Enhanced LAA (eLAA), etc.), a sidelink communication technology (e.g., ProSe communication technology, D2D communication technology), etc. The UE#1 231, the UE#2 232, the UE#5 235, and the UE#6 236 can perform operations corresponding to the base station 210 and operations supported by the base station 210, etc. The UE#3 233 and the UE#4 234 can perform operations corresponding to the repeater 220 and operations supported by the repeater 220, etc.

[0070] Here, the base station 210 can be referred to as a NodeB (NB), an evolved NodeB (eNB), a base transceiver station (BTS), a radio remote head (RRH), a transmission reception point (TRP), a radio unit (RU), a road side unit (RSU), a radio transceiver, an access point, an access node, or the like. The repeater 220 can be referred to as a small base station, a relay node, or the like. Each of the UE#1 231 to the UE#6 236 can be referred to as a terminal, an access terminal, a mobile terminal, a station, a subscriber station, a mobile station, a portable subscriber station, a node, a device, an on-board unit (OBU), or the like.

[0071] On the other hand, the communication between the UE#5 235 and the UE#6 236 can be performed based on a sidelink communication technology (e.g., a ProSe communication technology, a D2D communication technology). The sidelink communication can be performed based on a one-to-one scheme or a one-to-many scheme. When the V2V communication is performed with the sidelink communication technology, the UE#5 235 can be a communication node located in the vehicle#1 100, and the UE#6 236 can be a communication node located in the vehicle#2 110. When the V2I communication is performed with the sidelink communication technology, the UE#5 235 can be a communication node located in the vehicle#1 100, and the UE#6 236 can be a communication node located in the infrastructure 120. When the V2P communication is performed with the sidelink communication technology, the UE#5 235 can be a communication node located in the vehicle#1 100, and the UE#6 236 can be a communication node carried by the pedestrian 130. Figure 1 Figure 1 Figure 1 Figure 1 Figure 1 Figure 1

[0072] According to the locations of the UEs (e.g., the UE#5 235 and the UE#6 236) participating in the sidelink communication, the scenarios in which the sidelink communication is applied can be classified as shown in Table 1 below. For example, the scenario of the sidelink communication between the UE#5 235 and the UE#6 236 as shown in FIG. 1 can be a sidelink communication scenario #C. Figure 2

[0073] [Table 1]

[0074] Sidelink communication scenario Position of UE#5 235 Position of UE#6 236 #A Outside the coverage of base station 210 Outside the coverage of base station 210 #B In the coverage of base station 210 Outside the coverage of base station 210 #C In the coverage of base station 210 In the coverage of base station 210 #D In the coverage of base station 210 In the coverage of base station 210

[0075] ​​​​​​​On the other hand, the user plane protocol stack of the UE performing sidelink communication (e.g., UE#5 235 and UE#6 236) can be configured as follows.

[0076] Figure 4 This is a block diagram illustrating a first embodiment of the user plane protocol stack of a UE performing sidelink communication.

[0077] Reference Figure 4 UE#5 235 can be Figure 2 The UE#5 235 and UE#6 236 shown can be Figure 2 The example shown is UE#6 236. The sidelink communication scenario between UE#5 235 and UE#6 236 can be one of the sidelink communication scenarios #A to #D in Table 1. The user plane protocol stack of each of UE#5 235 and UE#6 236 may include a Physical (PHY) layer, a Medium Access Control (MAC) layer, a Radio Link Control (RLC) layer, and a Packet Data Convergence Protocol (PDCP) layer.

[0078] Sidelink communication between UE#5 235 and UE#6 236 can be performed using the PC5 interface (e.g., the PC5-U interface). Layer 2 identifiers (IDs) (e.g., source Layer 2 ID, destination Layer 2 ID) can be used for sidelink communication, and the Layer 2 ID can be an ID configured for V2X communication. Furthermore, hybrid Automatic Repeat Request (HARQ) feedback operations can be supported in sidelink communication, and RLC Acknowledged Mode (RLC AM) or RLC Unacknowledged Mode (RLC UM) can be supported.

[0079] On the other hand, the control plane protocol stack of the UE performing sidelink communication (e.g., UE#5 235 and UE#6 236) can be configured as follows.

[0080] Figure 5 This is a block diagram illustrating a first embodiment of the control plane protocol stack of a UE performing sidelink communication. Figure 6 This is a block diagram illustrating a second embodiment of the control plane protocol stack of a UE performing sidelink communication.

[0081] Reference Figure 5 and Figure 6 UE#5 235 can be Figure 2 The UE#5 235 and UE#6 shown can beFigure 2 The control plane protocol stack illustrated in FIG. 23 can be a control plane protocol stack for transmitting and receiving broadcast information (e.g., a Physical Sidelink Broadcast Channel (PSBCH)). Figure 5 The control plane protocol stack illustrated in FIG. 23 can be a control plane protocol stack for transmitting and receiving broadcast information (e.g., a Physical Sidelink Broadcast Channel (PSBCH)).

[0082] Figure 5 The control plane protocol stack illustrated in FIG. 23 can include a PHY layer, a MAC layer, an RLC layer, and a Radio Resource Control (RRC) layer. The sidelink communication between the UE#5 235 and the UE#6 236 can be performed with a PC5 interface (e.g., a PC5-C interface). Figure 6 The control plane protocol stack illustrated in FIG. 23 can be a control plane protocol stack for one-to-one sidelink communication. Figure 6 The control plane protocol stack illustrated in FIG. 23 can include a PHY layer, a MAC layer, an RLC layer, a PDCP layer, and a PC5 signaling protocol layer.

[0083] On the other hand, channels used in the sidelink communication between the UE#5 235 and the UE#6 236 can include a Physical Sidelink Shared Channel (PSSCH), a Physical Sidelink Control Channel (PSCCH), a Physical Sidelink Discovery Channel (PSDCH), a Physical Sidelink Broadcast Channel (PSBCH), and the like. The PSSCH can be used for transmitting and receiving sidelink data and can be configured in a UE (e.g., the UE#5 235 or the UE#6 236) through higher layer signaling. The PSCCH can be used for transmitting and receiving Sidelink Control Information (SCI), and can also be configured in a UE (e.g., the UE#5 235 or the UE#6 236) through higher layer signaling.

[0084] The PSDCH can be used for a discovery procedure. For example, a discovery signal can be transmitted through the PSDCH. The PSBCH can be used for transmitting and receiving broadcast information (e.g., system information). Also, a demodulation reference signal (DMRS), a synchronization signal, and the like can be used in sidelink communication between the UE#5 235 and the UE#6 236. The synchronization signal can include a primary sidelink synchronization signal (PSSS) and a secondary sidelink synchronization signal (SSSS).

[0085] On the other hand, a sidelink transmission mode (TM) can be classified into sidelink TMs #1 to #4 as shown in Table 2 below.

[0086] [Table 2]

[0087] Sidelink TM Description #1 Transmitting with resources scheduled by the base station #2 UE autonomous transmission without scheduling by the base station #3 Transmitting with resources scheduled by the base station in V2X communication #4 UE autonomous transmission without scheduling by the base station in V2X communication

[0088] When the sidelink TM #3 or the TM #4 is supported, each of the UE#5 235 and the UE#6 236 can perform sidelink communication with a resource pool configured by the base station 210. The resource pool can be configured for each of sidelink control information and sidelink data.

[0089] The resource pool for the sidelink control information can be configured based on an RRC signaling procedure (e.g., a dedicated RRC signaling procedure, a broadcast RRC signaling procedure). The resource pool for receiving the sidelink control information can be configured through the broadcast RRC signaling procedure. When the sidelink TM #3 is supported, the resource pool for transmitting the sidelink control information can be configured through the dedicated RRC signaling procedure. In this case, the sidelink control information can be transmitted through a resource scheduled by the base station 210 within the resource pool configured by the dedicated RRC signaling procedure. When the sidelink TM #4 is supported, the resource pool for transmitting the sidelink control information can be configured through the dedicated RRC signaling procedure or the broadcast RRC signaling procedure. In this case, the sidelink control information can be transmitted through a resource autonomously selected by a UE (e.g., the UE#5 235 or the UE#6 236) within the resource pool configured by the dedicated RRC signaling procedure or the broadcast RRC signaling procedure.

[0090] When the sidelink TM#3 is supported, a resource pool for transmitting and receiving sidelink data can not be configured. In this case, the sidelink data can be transmitted and received through a resource scheduled by the base station 210. When the sidelink TM#4 is supported, a resource pool for transmitting and receiving sidelink data can be configured through a dedicated RRC signaling procedure or a broadcast RRC signaling procedure. In this case, the sidelink data can be transmitted and received through a resource autonomously selected by a UE (e.g., UE#5 235 or UE#6 236) in the resource pool configured by the RRC signaling procedure or the broadcast RRC signaling procedure.

[0091] Hereinafter, a sidelink communication method will be described. Even when a method (e.g., transmission or reception of a signal) to be performed at a first communication node among communication nodes is described, a corresponding second communication node can perform a method (e.g., reception or transmission of a signal) corresponding to the method performed at the first communication node. That is, when the operation of UE#1 (e.g., vehicle#1) is described, a corresponding UE#2 (e.g., vehicle#2) can perform an operation corresponding to the operation of UE#1. Conversely, when the operation of UE#2 is described, a corresponding UE#1 can perform an operation corresponding to the operation of UE#2. In the embodiments described below, the operation of a vehicle can be the operation of a communication node located in the vehicle.

[0092] In an embodiment, the signaling can be one or a combination of two or more of high layer signaling, MAC signaling, and physical (PHY) signaling. A message for high layer signaling can be referred to as a "high layer message" or a "high layer signaling message." A message for MAC signaling can be referred to as a "MAC message" or a "MAC signaling message." A message for PHY signaling can be referred to as a "PHY message" or a "PHY signaling message." The high layer signaling can refer to an operation of transmitting and receiving system information (e.g., Master Information Block (MIB), System Information Block (SIB)) and / or an RRC message. The MAC signaling can refer to an operation of transmitting and receiving a MAC control element (CE). The PHY signaling can refer to an operation of transmitting and receiving control information (e.g., Downlink Control Information (DCI), Uplink Control Information (UCI), or SCI).

[0093] Sidelink signals can be synchronization signals and reference signals used for sidelink communication. For example, synchronization signals can be Synchronization Signal / Physical Broadcast Channel (SS / PBCH) blocks, Sidelink Synchronization Signals (SLSS), Primary Sidelink Synchronization Signals (PSSS), Secondary Sidelink Synchronization Signals (SSSS), etc. Reference signals can be Channel State Information-Reference Signals (CSI-RS), DMRS, Phase Tracking-Reference Signals (PT-RS), Cell Specific Reference Signals (CRS), Sounding Reference Signals (SRS), Discovery Reference Signals (DRS), etc.

[0094] Sidelink channels can be PSSCH, PSCCH, PSDCH, PSBCH, Physical Sidelink Feedback Channel (PSFCH), etc. Additionally, a sidelink channel can refer to a sidelink channel that includes sidelink signals mapped to specific resources within the corresponding sidelink channel. Sidelink communication can support broadcast, multicast, and unicast services.

[0095] Sidelink communication can be performed using either a single SCI scheme or a multi-SCI scheme. When using a single SCI scheme, it can be based on one SCI (e.g., Phase 1 SCI (1...)). st -stage SCI) is used to perform data transmission (e.g., sidelink data transmission, sidelink-shared channel (SL-SCH) transmission). When using a multi-SCI scheme, two SCIs can be used (e.g., stage 1 SCI and stage 2 SCI). nd Data transmission is performed using a stage SCI. SCIs can be transmitted via PSCCH and / or PSSCH. When using a single SCI scheme, SCIs can be transmitted on the PSCCH (e.g., stage 1 SCI). When using a multi-SCI scheme, stage 1 SCIs can be transmitted on the PSCCH, and stage 2 SCIs can be transmitted on either the PSCCH or PSSCH. A stage 1 SCI can be referred to as "stage 1 SCI," and a stage 2 SCI can be referred to as "stage 2 SCI." The format of a stage 1 SCI can include SCI format 1-A, and the format of a stage 2 SCI can include SCI format 2-A and SCI format 2-B.

[0096] The first-stage SCI can include one or more information elements among the following: priority information, frequency resource allocation information, time resource allocation information, resource reservation period information, demodulation reference signal (DMRS) pattern information, second-stage SCI format information, a beta_offset indicator, the number of DMRS ports, and modulation and coding scheme (MCS) information. The second-stage SCI can include one or more information elements among the following: a HARQ processor identifier (ID), a redundancy version (RV), a source ID, a destination ID, CSI request information, a zone ID, and a communication range requirement.

[0097] On the other hand, a transmitting terminal can perform SL communication with a receiving terminal. The SL communication between the transmitting terminal and the receiving terminal can be performed in a unicast scheme. The transmitting terminal can refer to a terminal that transmits data (e.g., SL data) through a sidelink. That is, the transmitting terminal can refer to a source terminal. The receiving terminal can refer to a terminal that receives SL data through a sidelink. That is, the receiving terminal can refer to a destination terminal. When relay communication (e.g., SL relay communication) is required during the SL communication between the transmitting terminal and the receiving terminal, a connection between the transmitting terminal and a relay terminal can be configured, and a connection between the receiving terminal and the relay terminal can be configured. When the above connection configuration is completed, the SL relay communication can be performed.

[0098] During the SL relay communication, the link quality between the transmitting terminal and the relay terminal and / or the link quality between the receiving terminal and the relay terminal can deteriorate, in which case the SL relay communication can not be successfully performed. As the distance between the communication nodes increases, the link quality between the communication nodes can deteriorate. The above situation can result in a radio link failure (RLF). The RLF in a sidelink can be referred to as an "SL RLF". In the SL relay communication, an RLF declaration method (e.g., a detection method) and an RLF recovery method are required.

[0099] In the SL relay communication, when one or more conditions defined in Table 3 below are satisfied, a communication node (e.g., a transmitting terminal, a relay terminal, and / or a receiving terminal) can declare (or detect) an RLF.

[0100] [table 3]

[0101]

[0102]

[0103] Among the conditions defined in Table 3, the RLF declaration subject, the link in which the RLF occurs, and the parameters required to declare the RLF can be as shown in Table 4 below.

[0104] [table 4]

[0105]

[0106]

[0107] The SL RLF configuration information (e.g., RLF configuration information) can include one or more conditions defined in Table 3 and / or one or more information elements (e.g., RLF parameters) defined in Table 4. The base station can transmit the SL RLF configuration information to the communication nodes (e.g., transmitting terminal, relay terminal, and / or receiving terminal) with at least one of system information, an RRC message, a MAC control element (CE), or control information (e.g., downlink control information (DCI)). The communication nodes can receive the SL RLF configuration information from the base station. The SL RLF configuration information can be transmitted through an RRC message when the communication nodes are located within the coverage of the base station.

[0108] The SL RLF configuration information can be signaled in various ways. For example, the base station can generate the SL RLF configuration information including all the conditions defined in Table 3 and the RLF parameters for all the conditions, and transmit the SL RLF configuration information to the communication nodes with system information and / or an RRC message. The SL RLF configuration information can be common SL RLF configuration information for all the transmitting terminals, relay terminals, and receiving terminals. After transmitting the above-described SL RLF configuration information, the following operations can be additionally performed. The base station can transmit a MAC CE and / or control information including information indicating available conditions among all the conditions to the communication nodes.

[0109] As other methods, the base station can independently generate the SL RLF configuration information for each type of communication node (e.g., transmitting terminal, relay terminal, or receiving terminal) and transmit the SL RLF configuration information to each communication node. Here, the SL RLF configuration information can be individual SL RLF configuration information. For example, the SL RLF configuration information for the transmitting terminal can include at least one of information indicating that condition #2 will be used, RLF parameters required for condition #2, information indicating that condition #3 will be used, RLF parameters required for condition #3, information indicating that condition #4 will be used, or RLF parameters required for condition #4.

[0110] The SL RLF configuration information for the relay terminal can include at least one of information indicating that condition #2 will be used, RLF parameters required for condition #2, information indicating that condition #5 will be used, or RLF parameters for condition #5. The SL RLF configuration information for the receiving terminal can include at least one of information indicating that condition #1 will be used, RLF parameters required for condition #1, information indicating that condition #5 will be used, or RLF parameters for condition #5.

[0111] To support the above operations, the communication node can notify the base station of the type of the communication node (e.g., a transmitting terminal, a relaying terminal, or a receiving terminal) in advance. The base station can generate SL RLF configuration information (e.g., separate SL RLF configuration information) according to the type indicated by the communication node, and can transmit the SL RLF configuration information to the communication node. The RLF parameters (e.g., a preset time, a preset number, and a threshold) defined in Table 4 can be independently configured for each communication node.

[0112] The communication node can declare (or detect) an RLF based on the SL RLF configuration information, and can perform an RLF recovery procedure. The RLF declaration method and / or the RLF recovery method can be performed as follows.

[0113] 1. RLF declaration method and / or RLF recovery method when using condition #1 (i.e., first condition)

[0114] Figure 7 is a flowchart illustrating a first embodiment of an RLF declaration and recovery method when using condition #1.

[0115] Referring to Figure 7 , the communication system can include a transmitting terminal, a receiving terminal, a relaying terminal, and a neighboring relaying terminal. Each of the transmitting terminal, the receiving terminal, the relaying terminal, and the neighboring relaying terminal can be configured identically or similarly to the communication node 300 illustrated in Figure 3 . The transmitting terminal, the receiving terminal, the relaying terminal, and the neighboring relaying terminal can support the protocol stack illustrated in Figures 4 to 6 . The transmitting terminal can be a terminal that transmits data (e.g., sidelink data), and the receiving terminal can be a terminal that receives data (e.g., sidelink data). The relaying terminal can be a terminal that relays sidelink communication between the transmitting terminal and the receiving terminal.

[0116] A connection (e.g., a unicast link) between the transmitting terminal and the relaying terminal can be configured, and sidelink communication between the transmitting terminal and the relaying terminal can be performed in a unicast scheme. A connection (e.g., a unicast link) between the receiving terminal and the relaying terminal can be configured, and sidelink communication between the receiving terminal and the relaying terminal can be performed in a unicast scheme. The transmitting terminal, the relaying terminal, and the receiving terminal can be in a state of receiving the above-described SL RLF configuration information from the base station.

[0117] The transmitting terminal can transmit data to the receiving terminal through the relay terminal (S701). The data can be periodic data. For example, the transmitting terminal can periodically transmit data to the relay terminal using resources configured by a configured grant (CG). The relay terminal can receive the data from the transmitting terminal. The relay terminal can transmit the data of the transmitting terminal to the receiving terminal (S702). In step S702, the data can be periodically transmitted in resources configured by the CG. Alternatively, when the link quality between the transmitting terminal and the relay terminal is poor, the relay terminal can not be able to receive the data from the transmitting terminal, and thus can not perform step S702.

[0118] The receiving terminal can perform a data reception operation. When the data is transmitted in a CG scheme, the receiving terminal can perform the data reception operation according to a period configured by the CG. When the link quality between the transmitting terminal and the relay terminal is poor and / or when the link quality between the receiving terminal and the relay terminal is poor, the receiving terminal can not be able to receive the data from the relay terminal. As the distance between the transmitting terminal and the relay terminal increases, the link quality between the transmitting terminal and the relay terminal can deteriorate. As the distance between the receiving terminal and the relay terminal increases, the link quality between the receiving terminal and the relay terminal can deteriorate. When no data is received from the relay terminal for a preset time, the receiving terminal can determine that condition #1 (for example, condition #1 defined in Table 3) is satisfied. When condition #1 is satisfied, the receiving terminal can declare an RLF (for example, an SL RLF) (S703). Condition #1 can refer to a first condition. The preset time can be set by the base station. For example, the preset time can be included in SL RLF configuration information. The preset time can be a multiple of a period configured by the CG.

[0119] After declaring the RLF, the receiving terminal can perform a connection procedure with a new relay terminal. For example, the receiving terminal can transmit a discovery message (for example, a discovery signal) to discover a neighboring relay terminal (S704). For example, the discovery message can include an identifier of the receiving terminal, an identifier of the transmitting terminal, and / or information requesting a connection configuration for SL relay communication. The discovery message can be transmitted in a broadcast scheme. The base station can pre-configure a communication node such that the communication node transmits the discovery message upon declaration of the RLF. Information indicating that a discovery message transmission operation is enabled upon declaration of the RLF can be included in the above-described SL RLF configuration information. The discovery message can be used to configure a temporary connection for RLF recovery. The base station can indicate that the discovery message is used to configure a temporary connection for RLF recovery.

[0120] The neighboring relay terminal can receive the discovery message from the reception terminal and identify the information elements included in the discovery message. The neighboring relay terminal can configure a connection (e.g., a PC5 connection, a PC5-S connection) with the transmission terminal indicated by the discovery message (S705). In step S705, the connection between the transmission terminal and one or more neighboring relay terminals can be configured. The neighboring relay terminal connected to the transmission terminal can transmit a connection complete message to the reception terminal (e.g., the reception terminal indicated by the discovery message) (S706). The connection complete message can indicate that the connection configuration between the transmission terminal and the neighboring relay terminal has been completed. In addition, the connection complete message can include an identifier of the neighboring relay terminal connected to the transmission terminal.

[0121] The reception terminal can receive the connection complete message from the neighboring relay terminal and can identify the information elements (e.g., the identifier of the neighboring relay terminal) included in the connection complete message. That is, the reception terminal can identify the neighboring relay terminal connected to the transmission terminal. The reception terminal can select one neighboring relay terminal (e.g., the best neighboring relay terminal) from among the neighboring relay terminals connected to the transmission terminal (S707). That is, the one neighboring relay terminal can be determined as a new relay terminal. The reception terminal can configure a connection with the new relay terminal. The reception terminal can transmit a message requesting disconnection from the transmission terminal (hereinafter referred to as a "connection disconnection request message") to the neighboring relay terminals other than the new relay terminal. When receiving the connection disconnection request message from the reception terminal, the remaining neighboring relay terminals can disconnect the connection with the transmission terminal.

[0122] The reception terminal can transmit an RLF recovery message to the transmission terminal through the new relay terminal (e.g., the selected neighboring relay terminal) (S708). The RLF recovery message can include an identifier of the new relay terminal and / or information indicating an attempt to perform RLF recovery through the new relay terminal. The RLF recovery message can be an RRC message (e.g., an RRCReconfigurationSidelink message). The new relay terminal can receive the RLF recovery message from the reception terminal. The new relay terminal can transmit the RLF recovery message to the transmission terminal (S709). The transmission terminal can receive the RLF recovery message from the new relay terminal and can identify the information elements included in the RLF recovery message. The transmission terminal can determine to perform SL relay communication through the new relay terminal based on the RLF recovery message. In this case, the transmission terminal can release the connection configuration with the existing relay terminal. Thereafter, SL relay communication between the transmission terminal, the new relay terminal, and the reception terminal can be performed (S710).

[0123] 2. RLF declaration method and / or RLF recovery method when using condition #2 (i.e., second condition)

[0124] Figure 8a and Figure 8b is a flowchart illustrating a first embodiment of an RLF declaration and recovery method when using condition #2.

[0125] Referring to Figure 8a and Figure 8b , a communication system can include a transmitting terminal, a receiving terminal, a relay terminal, and a neighboring relay terminal. Each of the transmitting terminal, the receiving terminal, the relay terminal, and the neighboring relay terminal can be configured identically or similarly to the communication node 300 illustrated in Figure 3 . The transmitting terminal, the receiving terminal, the relay terminal, and the neighboring relay terminal can support the protocol stack illustrated in Figures 4 to 6 . The transmitting terminal can be a terminal that transmits data (e.g., sidelink data), and the receiving terminal can be a terminal that receives data (e.g., sidelink data). The relay terminal can be a terminal that relays sidelink communication between the transmitting terminal and the receiving terminal.

[0126] A connection (e.g., a unicast link) between the transmitting terminal and the relay terminal can be configured, and sidelink communication between the transmitting terminal and the relay terminal can be performed in a unicast scheme. A connection (e.g., a unicast link) between the receiving terminal and the relay terminal can be configured, and sidelink communication between the receiving terminal and the relay terminal can be performed in a unicast scheme. The transmitting terminal, the relay terminal, and the receiving terminal can be in a state of receiving the above-described SL RLF configuration information from a base station.

[0127] The transmitting terminal can transmit data to the receiving terminal through the relay terminal (S801). The relay terminal can receive data from the transmitting terminal and can transmit a hybrid automatic repeat request (HARQ)-ACK of the data to the transmitting terminal (S802). The HARQ-ACK can be an acknowledgement (ACK) or a negative ACK (NACK). Step S802 can be omitted. The relay terminal can transmit the data of the transmitting terminal to the receiving terminal (S803). When link quality between the relay terminal and the receiving terminal is poor, the receiving terminal can not receive data from the relay terminal, and thus can not transmit a HARQ-ACK of the data to the relay terminal. Alternatively, the receiving terminal can receive data from the relay terminal and can transmit a HARQ-ACK of the data to the relay terminal (S804). When link quality between the relay terminal and the receiving terminal is poor, the relay terminal can not receive a HARQ-ACK from the receiving terminal.

[0128] When the number of reception failures (e.g., the number of DTX occurrences) of the HARQ-ACK of the data transmitted to the reception terminal is greater than or equal to a preset number, the relay terminal can determine that Condition #2 (e.g., Condition #2 defined in Table 3) is satisfied, and thus, can identify that the RLF has occurred (S805). Condition #2 can refer to a second condition. The number of reception failures can be the number of consecutive HARQ-ACK reception failures, and the number of DTX occurrences can be the number of consecutive DTX occurrences. The preset number can be preset by the base station. The preset number can be included in the SL RLF configuration information.

[0129] When it is identified that the RLF has occurred, the relay terminal can transmit an RLF occurrence message to the transmission terminal (S806). The RLF occurrence message can be an RRC message (e.g., RRCReconfigurationSidelink message). The RLF occurrence message can include information indicating that the RLF has occurred and / or information about a link (e.g., sidelink) in which the RLF has occurred. The information about the link in which the RLF has occurred can include an identifier of a communication node configured for the corresponding link. The transmission terminal can receive the RLF occurrence message from the relay terminal, and can identify that the RLF has occurred in the link between the reception terminal and the relay terminal based on the information elements included in the RLF occurrence message. Here, the RLF can be declared in the transmission terminal and / or the relay terminal.

[0130] When the RLF has occurred, the transmission terminal can perform a connection procedure with a new relay terminal. For example, the transmission terminal can transmit a discovery message (e.g., discovery signal) to discover a neighboring relay terminal (S807). For example, the discovery message can include an identifier of the reception terminal, an identifier of the transmission terminal, and / or information requesting a connection configuration for SL relay communication. The discovery message can be transmitted in a broadcast scheme. The base station can pre-configure the communication node to transmit the discovery message upon declaration of the RLF. Information indicating that the discovery message transmission operation is enabled upon declaration of the RLF can be included in the above-described SL RLF configuration information. The discovery message can be used to configure a temporary connection for RLF recovery. The base station can indicate that the discovery message is used to configure a temporary connection for RLF recovery.

[0131] The neighboring relay terminal can receive the discovery message from the transmitting terminal and identify the information elements included in the discovery message. The neighboring relay terminal can configure a connection (e.g., a PC5 connection, a PC5-S connection) with the receiving terminal indicated by the discovery message (S808). In step S808, the connection between the receiving terminal and one or more neighboring relay terminals can be configured. The neighboring relay terminal connected to the receiving terminal can transmit a connection complete message to the transmitting terminal (e.g., indicated by the discovery message) (S809). The connection complete message can indicate that the connection configuration between the receiving terminal and the neighboring relay terminal has been completed. In addition, the connection complete message can include an identifier of the neighboring relay terminal connected to the receiving terminal.

[0132] The transmitting terminal can receive the connection complete message from the neighboring relay terminal and can identify the information elements (e.g., the identifier of the neighboring relay terminal) included in the connection complete message. That is, the transmitting terminal can identify the neighboring relay terminal connected to the receiving terminal. The transmitting terminal can select one neighboring relay terminal (e.g., the best neighboring relay terminal) from among the neighboring relay terminals connected to the receiving terminal (S810). That is, the one neighboring relay terminal can be determined as a new relay terminal. The transmitting terminal can configure a connection with the new relay terminal. The transmitting terminal can transmit a message requesting disconnection from the receiving terminal (i.e., a connection disconnection request message) to the neighboring relay terminals other than the new relay terminal among the neighboring relay terminals. When receiving the connection disconnection request message from the transmitting terminal, the remaining neighboring relay terminals can disconnect the connection with the receiving terminal.

[0133] The transmitting terminal can transmit an RLF recovery message to the receiving terminal through the new relay terminal (e.g., the selected neighboring relay terminal) (S811). The RLF recovery message can include an identifier of the new relay terminal and / or information indicating an attempt to perform RLF recovery through the new relay terminal. The RLF recovery message can be an RRC message (e.g., an RRCReconfigurationSidelink message). The new relay terminal can receive the RLF recovery message from the transmitting terminal. The new relay terminal can transmit the RLF recovery message to the receiving terminal (S812). The receiving terminal can receive the RLF recovery message from the new relay terminal and can identify the information elements included in the RLF recovery message. The receiving terminal can determine to perform SL relay communication through the new relay terminal based on the RLF recovery message. In this case, the receiving terminal can release the connection configuration with the existing relay terminal. Thereafter, SL relay communication between the transmitting terminal, the new relay terminal, and the receiving terminal can be performed (S813).

[0134] 3. RLF declaration method and / or RLF recovery method when using condition #3 (i.e., third condition)

[0135] Figure 9a and Figure 9b is a flowchart illustrating a first embodiment of an RLF declaration and recovery method when condition #3 is used.

[0136] As shown in Figure 9a and Figure 9b , a communication system can include a transmitting terminal, a receiving terminal, a relay terminal, and a neighboring relay terminal. Each of the transmitting terminal, the receiving terminal, the relay terminal, and the neighboring relay terminal can be configured identically or similarly to the communication node 300 shown in Figure 3 . The transmitting terminal, the receiving terminal, the relay terminal, and the neighboring relay terminal can support the protocol stack shown in Figures 4 to 6 . The transmitting terminal can be a terminal that transmits data (e.g., sidelink data), and the receiving terminal can be a terminal that receives data (e.g., sidelink data). The relay terminal can be a terminal that relays sidelink communication between the transmitting terminal and the receiving terminal.

[0137] A connection (e.g., a unicast link) between the transmitting terminal and the relay terminal can be configured, and sidelink communication between the transmitting terminal and the relay terminal can be performed in a unicast scheme. A connection (e.g., a unicast link) between the receiving terminal and the relay terminal can be configured, and sidelink communication between the receiving terminal and the relay terminal can be performed in a unicast scheme. The transmitting terminal, the relay terminal, and the receiving terminal can be in a state of receiving the above-described SL RLF configuration information from a base station.

[0138] The transmitting terminal can transmit data to the receiving terminal through the relay terminal (S901). The relay terminal can receive data from the transmitting terminal and can transmit a HARQ-ACK of the data to the transmitting terminal (S902). The HARQ-ACK can be ACK or NACK. Step S902 can be omitted. The relay terminal can transmit data of the transmitting terminal to the receiving terminal (S903). The receiving terminal can receive data from the relay terminal and can transmit a HARQ-ACK of the data to the relay terminal (S904). When link quality between the relay terminal and the receiving terminal is poor, the receiving terminal can not receive data from the relay terminal. Accordingly, the relay terminal can not be able to transmit a HARQ-ACK of the receiving terminal to the transmitting terminal. That is, the transmitting terminal can not be able to receive a HARQ-ACK of the receiving terminal from the relay terminal. Alternatively, the relay terminal can receive a HARQ-ACK from the receiving terminal and can transmit the HARQ-ACK of the receiving terminal to the transmitting terminal (S905). However, when link quality between the transmitting terminal and the relay terminal is poor, the transmitting terminal can not be able to receive a HARQ-ACK of the receiving terminal from the relay terminal.

[0139] When the number of reception failures (e.g., the number of DTX occurrences) of the HARQ-ACK is greater than or equal to a preset number, the transmitting terminal can determine that Condition #3 (e.g., Condition #3 defined in Table 3) is satisfied, and thus, can declare RLF (S906). Condition #3 can refer to a third condition. The number of reception failures can be the number of consecutive HARQ-ACK reception failures, and the number of DTX occurrences can be the number of consecutive DTX occurrences. The preset number can be preset by the base station. The preset number can be included in the SL RLF configuration information.

[0140] After declaring RLF, the transmitting terminal can perform a connection procedure with a new relay terminal. For example, the transmitting terminal can transmit a discovery message (e.g., a discovery signal) to discover a neighboring relay terminal (S907). For example, the discovery message can include an identifier of the receiving terminal, an identifier of the transmitting terminal, and / or information requesting a connection configuration for SL relay communication. The discovery message can be transmitted in a broadcast scheme. The base station can pre-configure a communication node to transmit the discovery message upon declaration of RLF. Information indicating that the discovery message transmission operation is enabled upon declaration of RLF can be included in the above-described SL RLF configuration information. The discovery message can be used to configure a temporary connection for RLF recovery. The base station can indicate that the discovery message is used to configure a temporary connection for RLF recovery.

[0141] The neighboring relay terminal can receive the discovery message from the transmitting terminal and identify the information elements included in the discovery message. The neighboring relay terminal can configure a connection (e.g., a PC5 connection, a PC5-S connection) with the receiving terminal indicated by the discovery message (S908). In step S908, a connection between the receiving terminal and one or more neighboring relay terminals can be configured. The neighboring relay terminal connected to the receiving terminal can transmit a connection completion message to the transmitting terminal (e.g., the transmitting terminal indicated by the discovery message) (S909). The connection completion message can indicate that the connection configuration between the receiving terminal and the neighboring relay terminal has been completed. In addition, the connection completion message can include an identifier of the neighboring relay terminal connected to the receiving terminal.

[0142] The transmitting terminal can receive a connection complete message from the neighboring relay terminal, and can identify an information element (e.g., an identifier of the neighboring relay terminal) included in the connection complete message. That is, the transmitting terminal can identify the neighboring relay terminal connected to the receiving terminal. The transmitting terminal can select one of the neighboring relay terminals (e.g., the best neighboring relay terminal) from among the neighboring relay terminals connected to the receiving terminal (S910). That is, the one neighboring relay terminal can be determined as a new relay terminal. The transmitting terminal can configure a connection with the new relay terminal. The transmitting terminal can transmit a message requesting disconnection from the receiving terminal (i.e., a connection disconnection request message) to the neighboring relay terminals other than the new relay terminal. When receiving the connection disconnection request message from the transmitting terminal, the remaining neighboring relay terminals can disconnect the connection with the receiving terminal.

[0143] The transmitting terminal can transmit an RLF recovery message to the receiving terminal through the new relay terminal (e.g., the selected neighboring relay terminal) (S911). The RLF recovery message can include an identifier of the new relay terminal and / or information indicating an attempt to perform RLF recovery through the new relay terminal. The RLF recovery message can be an RRC message (e.g., an RRCReconfigurationSidelink message). The new relay terminal can receive the RLF recovery message from the transmitting terminal. The new relay terminal can transmit the RLF recovery message to the receiving terminal (S912). The receiving terminal can receive the RLF recovery message from the new relay terminal, and can identify information elements included in the RLF recovery message. The receiving terminal can determine to perform SL relay communication through the new relay terminal based on the RLF recovery message. In this case, the receiving terminal can release a connection configuration with the existing relay terminal. Thereafter, SL relay communication between the transmitting terminal, the new relay terminal, and the receiving terminal can be performed (S913).

[0144] 4. RLF declaration method and / or RLF recovery method when using condition #4 (i.e., fourth condition)

[0145] Figure 10 is a flowchart illustrating a first embodiment of an RLF declaration and recovery method when using condition #4.

[0146] Referring to Figure 10 , the communication system can include a transmitting terminal, a receiving terminal, a relay terminal, and a neighboring relay terminal. Each of the transmitting terminal, the receiving terminal, the relay terminal, and the neighboring relay terminal can be configured identically or similarly to the communication node 300 shown in Figure 3 . The transmitting terminal, the receiving terminal, the relay terminal, and the neighboring relay terminal can support Figures 4 to 6The illustrated protocol stack. The transmitting terminal can be a terminal that transmits data (e.g., sidelink data), and the receiving terminal can be a terminal that receives data (e.g., sidelink data). The relay terminal can be a terminal that relays sidelink communication between the transmitting terminal and the receiving terminal.

[0147] A connection (e.g., unicast link) between the transmitting terminal and the relay terminal can be configured, and sidelink communication between the transmitting terminal and the relay terminal can be performed in a unicast scheme. A connection (e.g., unicast link) between the receiving terminal and the relay terminal can be configured, and sidelink communication between the receiving terminal and the relay terminal can be performed in a unicast scheme. The transmitting terminal, the relay terminal, and the receiving terminal can be in a state of receiving the above-described SL RLF configuration information from the base station.

[0148] The transmitting terminal can transmit data to the receiving terminal through the relay terminal (S1001). The relay terminal can receive data from the transmitting terminal and can transmit HARQ-ACK of the data to the transmitting terminal (S1002). When the link quality between the transmitting terminal and the relay terminal is poor, the transmitting terminal can fail to receive the HARQ-ACK from the relay terminal. When the number of reception failures (e.g., the number of DTX occurrences) of the HARQ-ACK is greater than or equal to a preset number, the transmitting terminal can determine that Condition #4 (e.g., Condition #4 defined in Table 3) is satisfied, and thus, can declare RLF (S1003). Condition #4 can refer to a fourth condition. The number of reception failures can be the number of consecutive HARQ-ACK reception failures, and the number of DTX occurrences can be the number of consecutive DTX occurrences. The preset number can be preset by the base station. The preset number can be included in the SL RLF configuration information.

[0149] After declaring RLF, the transmitting terminal can perform a connection procedure with a new relay terminal. For example, the transmitting terminal can transmit a discovery message (e.g., a discovery signal) to discover a neighboring relay terminal (S1004). For example, the discovery message can include an identifier of the receiving terminal, an identifier of the transmitting terminal, and / or information requesting a connection configuration for SL relay communication. The discovery message can be transmitted in a broadcast scheme. The base station can pre-configure a communication node such that the communication node transmits the discovery message upon declaration of RLF. Information indicating that the discovery message transmission operation is enabled upon declaration of RLF can be included in the above-described SL RLF configuration information. The discovery message can be used to configure a temporary connection for RLF recovery. The base station can indicate that the discovery message is used to configure a temporary connection for RLF recovery.

[0150] The neighboring relay terminal can receive the discovery message from the transmission terminal and identify the information elements included in the discovery message. The neighboring relay terminal can configure a connection (e.g., a PC5 connection, a PC5-S connection) with the reception terminal indicated by the discovery message (S1005). In step S1005, the connection between the reception terminal and one or more neighboring relay terminals can be configured. The neighboring relay terminal connected to the reception terminal can transmit a connection completion message to the transmission terminal (e.g., indicated by the discovery message) (S1006). The connection completion message can indicate that the connection configuration between the reception terminal and the neighboring relay terminal has been completed. In addition, the connection completion message can include an identifier of the neighboring relay terminal connected to the reception terminal.

[0151] The transmission terminal can receive the connection completion message from the neighboring relay terminal and can identify the information elements (e.g., the identifier of the neighboring relay terminal) included in the connection completion message. That is, the transmission terminal can identify the neighboring relay terminal connected to the reception terminal. The transmission terminal can select one neighboring relay terminal (e.g., the best neighboring relay terminal) from among the neighboring relay terminals connected to the reception terminal (S1007). That is, the one neighboring relay terminal can be determined as a new relay terminal. The transmission terminal can configure a connection with the new relay terminal. The transmission terminal can transmit a message requesting disconnection from the reception terminal (hereinafter referred to as a "connection disconnection request message") to the neighboring relay terminals other than the new relay terminal. When receiving the connection disconnection request message from the transmission terminal, the remaining neighboring relay terminals can disconnect the connection with the reception terminal.

[0152] The transmission terminal can transmit an RLF recovery message to the reception terminal through the new relay terminal (e.g., the selected neighboring relay terminal) (S1008). The RLF recovery message can include an identifier of the new relay terminal and / or information indicating an attempt to perform RLF recovery through the new relay terminal. The RLF recovery message can be an RRC message (e.g., an RRCReconfigurationSidelink message). The new relay terminal can receive the RLF recovery message from the transmission terminal. The new relay terminal can transmit the RLF recovery message to the reception terminal (S1009). The reception terminal can receive the RLF recovery message from the new relay terminal and can identify the information elements included in the RLF recovery message. The reception terminal can determine to perform SL relay communication through the new relay terminal based on the RLF recovery message. In this case, the reception terminal can release the connection configuration with the existing relay terminal. Thereafter, SL relay communication between the transmission terminal, the new relay terminal, and the reception terminal can be performed (S1010).

[0153] 5. RLF declaration method and / or RLF recovery method when using condition #5 (i.e., fifth condition)

[0154] Figure 11 FIG. 7 is a flowchart illustrating a first embodiment of an RLF declaration and recovery method when a condition #5 is used.

[0155] Referring to Figure 11 , a communication system can include a transmitting terminal, a receiving terminal, a relay terminal, and a neighboring relay terminal. Each of the transmitting terminal, the receiving terminal, the relay terminal, and the neighboring relay terminal can be configured identically or similarly to the communication node 300 illustrated in FIG. 3. The transmitting terminal, the receiving terminal, the relay terminal, and the neighboring relay terminal can support the protocol stack illustrated in FIG. 4. The transmitting terminal can be a terminal that transmits data (e.g., sidelink data), and the receiving terminal can be a terminal that receives data (e.g., sidelink data). The relay terminal can be a terminal that relays sidelink communication between the transmitting terminal and the receiving terminal. Figure 3 Figures 4 to 6 A connection (e.g., a unicast link) between the transmitting terminal and the relay terminal can be configured, and sidelink communication between the transmitting terminal and the relay terminal can be performed in a unicast scheme. A connection (e.g., a unicast link) between the receiving terminal and the relay terminal can be configured, and sidelink communication between the receiving terminal and the relay terminal can be performed in a unicast scheme. The transmitting terminal, the relay terminal, and the receiving terminal can be in a state of receiving the SL RLF configuration information from the base station.

[0156] The transmitting terminal can transmit data to the receiving terminal through the relay terminal (S1101). The data can be periodic data. For example, the transmitting terminal can periodically transmit data to the relay terminal using a resource configured by a CG. When data is transmitted in a CG scheme, the relay terminal can perform a data reception operation according to a period configured by the CG. When link quality between the transmitting terminal and the relay terminal is poor, the relay terminal can not be able to receive data from the transmitting terminal. When data is not received from the transmitting terminal for a preset time, the relay terminal can determine that a condition #5 (e.g., the condition #5 defined in Table 3) is satisfied, and thus, can recognize that an RLF has occurred (S1102). The condition #5 can refer to a fifth condition. The preset time can be set by the base station. For example, the preset time can be included in the SL RLF configuration information. The preset time can be a multiple of a period configured by the CG.

[0157] The transmitting terminal can transmit data to the receiving terminal through the relay terminal (S1101). The data can be periodic data. For example, the transmitting terminal can periodically transmit data to the relay terminal using a resource configured by a CG. When data is transmitted in a CG scheme, the relay terminal can perform a data reception operation according to a period configured by the CG. When link quality between the transmitting terminal and the relay terminal is poor, the relay terminal can not be able to receive data from the transmitting terminal. When data is not received from the transmitting terminal for a preset time, the relay terminal can determine that a condition #5 (e.g., the condition #5 defined in Table 3) is satisfied, and thus, can recognize that an RLF has occurred (S1102). The condition #5 can refer to a fifth condition. The preset time can be set by the base station. For example, the preset time can be included in the SL RLF configuration information. The preset time can be a multiple of a period configured by the CG.

[0158] ​When it is identified that the RLF has occurred, the relay terminal can transmit an RLF occurrence message to the reception terminal (S1103). The RLF occurrence message can be an RRC message (e.g., RRCReconfigurationSidelink message). The RLF occurrence message can include information indicating that the RLF has occurred and / or information about a link (e.g., sidelink) in which the RLF has occurred. The information about the link in which the RLF has occurred can include an identifier of a communication node configured for the corresponding link. The reception terminal can receive the RLF occurrence message from the relay terminal, and can identify that the RLF has occurred in a link between the transmission terminal and the relay terminal based on the information elements included in the RLF occurrence message. Here, the RLF can be declared in the reception terminal and / or the relay terminal.

[0159] When the RLF has occurred, the reception terminal can perform a connection procedure with a new relay terminal. For example, the reception terminal can transmit a discovery message (e.g., discovery signal) to discover a neighboring relay terminal (S1104). For example, the discovery message can include an identifier of the reception terminal, an identifier of the transmission terminal, and / or information requesting a connection configuration for SL relay communication. The discovery message can be transmitted in a broadcast scheme. The base station can pre-configure the communication nodes so that the discovery message is transmitted upon declaration of the RLF. Information indicating that the discovery message transmission operation is enabled upon declaration of the RLF can be included in the SL RLF configuration information described above. The discovery message can be used to configure a temporary connection for RLF recovery. The base station can indicate that the discovery message is used to configure a temporary connection for RLF recovery.

[0160] The neighboring relay terminal can receive the discovery message from the reception terminal and identify the information elements included in the discovery message. The neighboring relay terminal can configure a connection (e.g., PC5 connection, PC5-S connection) with the transmission terminal indicated by the discovery message (S1105). In step S1105, a connection between the transmission terminal and one or more neighboring relay terminals can be configured. The neighboring relay terminal connected to the transmission terminal can transmit a connection completion message to the reception terminal (e.g., the reception terminal indicated by the discovery message) (S1106). The connection completion message can indicate that the connection configuration between the transmission terminal and the neighboring relay terminal has been completed. In addition, the connection completion message can include an identifier of the neighboring relay terminal connected to the transmission terminal.

[0161] The reception terminal can receive the connection complete message from the neighboring relay terminal, and can identify the information element (e.g., the identifier of the neighboring relay terminal) included in the connection complete message. That is, the reception terminal can identify the neighboring relay terminal connected to the transmission terminal. The reception terminal can select one of the neighboring relay terminals (e.g., the best neighboring relay terminal) from among the neighboring relay terminals connected to the transmission terminal (S1107). That is, the one neighboring relay terminal can be determined as a new relay terminal. The reception terminal can configure a connection with the new relay terminal. The reception terminal can transmit a message requesting disconnection from the transmission terminal (i.e., a connection disconnection request message) to the neighboring relay terminals except for the new relay terminal. When receiving the connection disconnection request message from the reception terminal, the remaining neighboring relay terminals can disconnect the connection with the transmission terminal.

[0162] The reception terminal can transmit an RLF recovery message to the transmission terminal through the new relay terminal (e.g., the selected neighboring relay terminal) (S1108). The RLF recovery message can include the identifier of the new relay terminal and / or information indicating an attempt to perform RLF recovery through the new relay terminal. The RLF recovery message can be an RRC message (e.g., an RRCReconfigurationSidelink message). The new relay terminal can receive the RLF recovery message from the reception terminal. The new relay terminal can transmit the RLF recovery message to the transmission terminal (S1109). The transmission terminal can receive the RLF recovery message from the new relay terminal, and can identify the information element included in the RLF recovery message. The transmission terminal can determine to perform SL relay communication through the new relay terminal based on the RLF recovery message. In this case, the transmission terminal can release the connection configuration with the existing relay terminal. Thereafter, SL relay communication between the transmission terminal, the new relay terminal, and the reception terminal can be performed (S1110).

[0163] The method of the disclosure can be implemented as program instructions executable by various computer devices and recorded on a computer readable medium. The computer readable medium can include program instructions, data files, data structures, or combinations thereof. The program instructions recorded on the computer readable medium can be specifically designed and configured for the disclosure, or can be known and available to those skilled in the computer software field.

[0164] Examples of the computer readable medium can include hardware devices specifically configured to store and run program instructions, such as ROM, RAM, and flash memory. Examples of the program instructions include machine code generated by a compiler, and high-level language code executable by a computer using an interpreter. The above-described exemplary hardware devices can be configured to operate as at least one software module to perform embodiments of the disclosure, and vice versa.

[0165] Although embodiments of the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined in the following claims.

Claims

1. An operation method of a transmitting terminal, comprising: performing, by a first relay terminal, first sidelink relay communication (SL relay communication) with a receiving terminal; declaring radio link failure (RLF) of a sidelink in which the first SL relay communication is performed, based on a number of times of failure in reception of a hybrid automatic repeat request-acknowledgement (HARQ-ACK) of data transmitted by the first relay terminal being greater than or equal to a threshold value; configuring connection with a second relay terminal; and performing, by the second relay terminal, second SL relay communication with the receiving terminal. 2.The operation method of a transmitting terminal according to claim 1, wherein, The operation method of the transmitting terminal further comprises receiving, from a base station, a first message including information indicating a preconfigured condition for declaring the RLF. 3.The method of Claim 2, wherein, The first message further includes information for determining whether the preconfigured condition is satisfied. 4.The operation method of a transmitting terminal according to claim 1, wherein, The preconfigured condition for declaring the RLF is configured independently for each terminal type, and the terminal type indicates a transmitting terminal, a relay terminal, or a receiving terminal. 5.The operation method of a transmitting terminal according to claim 1, wherein, Configuring connection with the second relay terminal includes: transmitting a discovery message in a broadcast scheme; and configuring connection with the second relay terminal determined based on the discovery message.

6. The operation method of a transmitting terminal according to claim 5, wherein The operation method of the transmitting terminal further comprises transmitting, to the receiving terminal through the second relay terminal, information of the second relay terminal connected to the transmitting terminal. 7.An operation method of a relay terminal, comprising: performing sidelink relay communication (SL relay communication) between a transmitting terminal and a receiving terminal; determining that radio link failure (RLF) of a sidelink in which first SL relay communication is performed has occurred, based on a number of times of failure in reception of a hybrid automatic repeat request-acknowledgement (HARQ-ACK) of data of the transmitting terminal transmitted from the relay terminal to the receiving terminal being greater than or equal to a threshold value; and transmitting a second message indicating that the RLF has occurred. 8.The method of claim 7, wherein, The operation method of the relay terminal further comprises receiving, from a base station, a first message including information indicating a preconfigured condition for declaring the RLF, wherein the first message further includes information for determining whether the preconfigured condition is satisfied. 9.The method of claim 7, wherein, The preconfigured condition for declaring the RLF is configured independently for each terminal type, and the terminal type indicates a transmitting terminal, a relay terminal, or a receiving terminal.

10. The operating method of claim 7, wherein, The second message further includes information of the sidelink in which the RLF has occurred.

Images