Communication control method, remote user equipment, relay user equipment, and processor

Abstract

To provide a communication control method, relay user equipment, remote user equipment, and a processor used in a cellular communication system.SOLUTION: A communication control method used in a cellular communication system includes remote user equipment receiving, from relay user equipment, a PC5-RRC message including information indicating a radio link failure (RLF) between the relay user equipment that relays data of the remote user equipment, and a network or other user equipment. The relay user equipment performs either UE-to-NW (U2N) relaying or UE-to-UE (U2U) relaying. The PC5-RRC message further includes information indicating whether the type of the information indicating the RLF is U2N relaying or U2U relaying.SELECTED DRAWING: Figure 16

Description

[Technical Field]

[0001] The present invention relates to a communication control method, a relay user equipment, a remote user equipment and a processor for use in a cellular communication system. [Background technology]

[0002] In recent years, fifth-generation (5G) cellular communication systems have been attracting attention. New Radio (NR), the radio access technology (RAT) of 5G systems, has introduced sidelink communication, which allows direct wireless communication between user devices. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] 3GPP technical specification "3GPP TS 38.300 V16.2.0 (2020-07)" Summary of the Invention

[0004] A communication control method according to a first aspect is a communication control method for use in a cellular communication system, comprising: a relay user equipment (UE) capable of relaying data of a remote user equipment (RUE) transmitting a message including information relating to the relay user equipment; the remote user equipment receiving the message; and the remote user equipment performing processing to establish a sidelink connection between the remote user equipment and the relay user equipment (RUE) based on the message.

[0005] A second aspect of the present invention provides a relay user equipment (REE) capable of relaying data of a remote user equipment (RUE) in a cellular communication system, the REE comprising: a transmitter configured to transmit a message to the RUE containing information about the REE, the message being used in a process for establishing a sidelink connection between the RUE and the RUE.

[0006] A remote user equipment according to a third aspect is a remote user equipment used in a cellular communication system, and comprises: a receiving unit that receives a message including information about the relay user equipment from a relay user equipment capable of relaying data of the remote user equipment; and a control unit that performs processing to establish a sidelink connection between the remote user equipment and the relay user equipment based on the message. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a diagram illustrating a configuration of a cellular communication system according to an embodiment. [Figure 2] FIG. 1 is a diagram illustrating a configuration of a UE (user equipment) according to an embodiment. [Figure 3] A diagram showing the configuration of a gNB (base station) according to one embodiment. [Figure 4] FIG. 10 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data. [Figure 5] FIG. 1 is a diagram showing the configuration of a protocol stack of a radio interface of a control plane that handles signaling (control signals). [Figure 6] FIG. 1 illustrates a sidelink relay according to an embodiment. [Figure 7] FIG. 1 illustrates an overall flow for sidelink relay according to an embodiment. [Figure 8] A diagram showing the configuration of a protocol stack for a side link of a user plane that handles data. [Figure 9] This figure shows the configuration of the protocol stack of the side link of the control plane that handles signaling (control signals). [Figure 10] FIG. 10 illustrates a relay UE message according to an embodiment. [Figure 11] FIG. 10 is a diagram showing a sequence of a relay UE selection procedure according to an embodiment. [Figure 12] FIG. 10 is a diagram showing a sequence of a relay UE reselection procedure according to an embodiment. [Figure 13] FIG. 10 is a diagram illustrating an operation pattern 1 of sidelink communication control by a remote UE according to an embodiment. [Figure 14] FIG. 10 is a diagram illustrating an operation pattern 2 of sidelink communication control by a remote UE according to an embodiment. [Figure 15] FIG. 10 is a diagram illustrating a modified example of operation pattern 2 of sidelink communication control by a remote UE according to an embodiment. [Figure 16] FIG. 1 illustrates a sidelink communication control by a relay UE according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0008] Sidelink relaying, in which a relay user equipment (UE) relays data from a remote user equipment (RUE) using sidelink communication, is being considered. A remote user equipment (RUE) can use sidelink relaying by selecting a relay user equipment (UE) and establishing a sidelink connection. However, if the remote user equipment (RUE) is unable to select an appropriate relay user equipment (UE), the sidelink communication cannot be performed properly.

[0009] Therefore, an object of the present disclosure is to enable sidelink communication to be performed appropriately.

[0010] A cellular communication system according to an embodiment will be described with reference to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0011] (Configuration of a cellular communication system) First, a configuration of a cellular communication system according to an embodiment will be described. Fig. 1 is a diagram showing the configuration of a cellular communication system according to an embodiment. This cellular communication system conforms to the 5th Generation System (5GS) of the 3GPP standard. In the following description, 5GS will be used as an example, but the LTE (Long Term Evolution) system may also be applied at least in part to the cellular communication system.

[0012] As shown in Fig. 1, the cellular communication system includes a user equipment (UE) 100, a 5G radio access network (NG-RAN: Next Generation Radio Access Network) 10, and a 5G core network (5GC: 5G Core Network) 20. The radio access network and the core network are collectively called a cellular communication network.

[0013] The UE 100 is a mobile wireless communication device. The UE 100 may be any device that is used by a user, and may be, for example, a mobile phone terminal (including a smartphone), a tablet terminal, a notebook PC, a communication module (including a communication card or a chipset), a sensor or a device provided in a sensor, a vehicle or a device provided in a vehicle (Vehicle UE), or an aircraft or a device provided in an aircraft (Aerial UE).

[0014] The NG-RAN 10 includes a base station (called a "gNB" in a 5G system) 200. The gNBs 200 are connected to each other via an Xn interface, which is an interface between base stations. The gNB 200 manages one or more cells. The gNB 200 performs wireless communication with a UE 100 that has established a connection with its own cell. The gNB 200 has a radio resource management (RRM) function, a routing function for user data (hereinafter simply referred to as "data"), a measurement control function for mobility control and scheduling, etc. The term "cell" is used to indicate the smallest unit of a wireless communication area. The term "cell" is also used to indicate a function or resource that performs wireless communication with a UE 100. One cell belongs to one carrier frequency.

[0015] In addition, gNBs can also connect to the Evolved Packet Core (EPC), which is the LTE core network. LTE base stations can also connect to 5GC. LTE base stations and gNBs can also be connected via a base station-to-base station interface.

[0016] The 5GC20 includes an Access and Mobility Management Function (AMF) and a User Plane Function (UPF) 300. The AMF performs various mobility controls for the UE 100. The AMF manages the mobility of the UE 100 by communicating with the UE 100 using NAS (Non-Access Stratum) signaling. The UPF controls data forwarding. The AMF and UPF are connected to the gNB 200 via an NG interface, which is an interface between a base station and a core network.

[0017] FIG. 2 is a diagram showing a configuration of a UE 100 (user equipment) according to an embodiment.

[0018] As shown in FIG. 2, the UE 100 includes a receiving unit 110, a transmitting unit 120, and a control unit .

[0019] The receiving unit 110 performs various types of reception under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 130.

[0020] The transmitting unit 120 performs various transmissions under the control of the control unit 130. The transmitting unit 120 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 130 into a radio signal and transmits it from the antenna.

[0021] The control unit 130 performs various controls in the UE 100. The control unit 130 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in processing by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0022] FIG. 3 is a diagram showing the configuration of a gNB200 (base station) according to one embodiment.

[0023] As shown in FIG. 3, the gNB 200 includes a transmitter 210, a receiver 220, a controller 230, and a backhaul communication unit 240.

[0024] The transmission unit 210 performs various transmissions under the control of the control unit 230. The transmission unit 210 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 230 into a radio signal and transmits it from the antenna.

[0025] The receiving unit 220 performs various types of reception under the control of the control unit 230. The receiving unit 220 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 230.

[0026] The control unit 230 performs various controls in the gNB 200. The control unit 230 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in processing by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0027] The backhaul communication unit 240 is connected to neighboring base stations via an inter-base station interface. The backhaul communication unit 240 is connected to the AMF / UPF 300 via a base station-core network interface. Note that the gNB is composed of a CU (Central Unit) and a DU (Distributed Unit) (i.e., functionally divided), and both units may be connected via an F1 interface.

[0028] FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data.

[0029] As shown in Figure 4, the user plane radio interface protocol includes a physical (PHY) layer, a medium access control (MAC) layer, a radio link control (RLC) layer, a packet data convergence protocol (PDCP) layer, and a service data adaptation protocol (SDAP) layer.

[0030] The PHY layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layer of the UE 100 and the PHY layer of the gNB 200 via a physical channel.

[0031] The MAC layer performs data priority control, retransmission processing using Hybrid ARQ (HARQ), random access procedures, etc. Data and control information are transmitted between the MAC layer of UE 100 and the MAC layer of gNB 200 via a transport channel. The MAC layer of gNB 200 includes a scheduler. The scheduler determines the uplink and downlink transport format (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be allocated to UE 100.

[0032] The RLC layer transmits data to the RLC layer on the receiving side using the functions of the MAC layer and PHY layer. Data and control information are transmitted between the RLC layer of the UE 100 and the RLC layer of the gNB 200 via logical channels.

[0033] The PDCP layer performs header compression / decompression and encryption / decryption.

[0034] The SDAP layer maps IP flows, which are the units for QoS control by the core network, to radio bearers, which are the units for QoS control by the AS (Access Stratum). Note that if the RAN is connected to the EPC, SDAP is not necessary.

[0035] FIG. 5 is a diagram showing the configuration of a protocol stack of a radio interface of a control plane that handles signaling (control signals).

[0036] As shown in FIG. 5, the protocol stack of the radio interface of the control plane has a Radio Resource Control (RRC) layer and a Non-Access Stratum (NAS) layer instead of the SDAP layer shown in FIG.

[0037] RRC signaling for various settings is transmitted between the RRC layer of UE100 and the RRC layer of gNB200. The RRC layer controls logical channels, transport channels, and physical channels according to the establishment, re-establishment, and release of radio bearers. When there is a connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC connected state. When there is no connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC idle state. When the connection between the RRC of UE100 and the RRC of gNB200 is suspended, UE100 is in an RRC inactive state.

[0038] The NAS layer, which is positioned above the RRC layer, performs session management, mobility management, etc. NAS signaling is transmitted between the NAS layer of the UE 100 and the NAS layer of the AMF 300.

[0039] The UE 100 has an application layer and the like in addition to the radio interface protocol.

[0040] (Sidelink Relay Overview) Next, an overview of sidelink relay according to an embodiment will be described. Fig. 6 is a diagram illustrating sidelink relay according to an embodiment.

[0041] As shown in FIG. 6, sidelink relaying has two modes: UE-to-NW relaying (U2N relaying) and UE-to-UE relaying (U2U relaying).

[0042] U2N relay is a mode in which the relay UE 100B relays between the remote UE 100A and the gNB 200 (cell). Here, a sidelink connection is established between the relay UE 100B and the remote UE 100A, and a network connection is established between the relay UE 100B and the gNB 200. Note that the sidelink is a direct link between the UEs 100 and is sometimes called a PC5 interface. The sidelink connection is sometimes called a PC5-S connection or a PC5-RRC connection. The network connection refers to a connection between the UE 100 and the cellular communication network (gNB 200) and is sometimes called a NAS connection, an RRC connection, or a Uu interface.

[0043] The relay UE 100B is located within the coverage of the cell of the gNB 200 (hereinafter simply referred to as "coverage"). On the other hand, the remote UE 100A is located outside the coverage. According to U2N relay, the remote UE 100A that is outside the coverage (so-called out of service area) can perform network communication via the relay UE 100B. However, the remote UE 100A may be located within the coverage.

[0044] U2U relay is a mode in which the relay UE 100B relays between the remote UE 100A and another UE 100C. Here, a sidelink connection is established between the relay UE 100B and the remote UE 100A and between the relay UE 100B and the other UE 100C. Each of the remote UE 100A, the relay UE 100B, and the other UE 100C may be located within a coverage area or outside of a coverage area. Note that a sidelink connection may be established indirectly between the remote UE 100A and the other UE 100C via the relay UE 100B.

[0045] According to U2U relay, even when direct sidelink communication between the remote UE 100A and another UE 100C is not possible, for example, outside the coverage area, the remote UE 100A can perform sidelink communication indirectly via the relay UE 100B.

[0046] 7 is a diagram showing an overall flow of sidelink relay according to an embodiment. In the following, it is assumed that the remote UE 100A has already established a network connection with the cellular communication network (gNB 200) or a sidelink connection with another UE 100C.

[0047] As shown in FIG. 7 , in step S1, the remote UE 100A and the relay UE 100B perform a relay UE discovery procedure. The relay UE discovery procedure includes a method in which the relay UE 100B notifies its own presence by transmitting a discovery message, and a method in which the remote UE 100A searches for the relay UE 100B by transmitting a discovery message. The transmitted discovery message may be a message dedicated to discovery, or may be an existing message for establishing a sidelink connection. Note that the remote UE 100A may transmit a discovery message indicating its own presence, or the relay UE 100B may transmit a discovery message for searching for the remote UE 100A. Through the relay UE discovery procedure, the remote UE 100A discovers the relay UE 100B. Here, the remote UE 100A may discover multiple candidate relay UEs 100B.

[0048] In step S2, the remote UE 100A performs a relay UE selection procedure to select a relay UE 100B to relay its own data. If only one relay UE 100B is found in step S1, the remote UE 100A may select the one relay UE 100B in the relay UE selection procedure depending on whether the discovered relay UE 100B satisfies a predetermined condition. On the other hand, if multiple relay UEs 100B are found in step S1, the remote UE 100A may select one relay UE 100B from the discovered relay UEs 100B in the relay UE selection procedure. Details of the relay UE selection procedure will be described later.

[0049] In step S3, the remote UE 100A and the relay UE 100B perform a sidelink connection establishment procedure to establish a sidelink connection. For example, the sidelink connection is established by the remote UE 100A and the relay UE 100B transmitting and receiving messages of a PC5-RRC layer (described later) (e.g., RRCReconfigurationSidelink, RRCReconfigurationCompleteSidelink). The sidelink connection establishment procedure may include transmitting and receiving messages of a PC5-S layer (described later) (e.g., DIRECT LINK ESTABLISHMENT REQUEST, DIRECT LINK ESTABLISHMENT ACCEPT).

[0050] In step S4, the remote UE 100A performs side link communication with the relay UE 100B, and transmits and receives data to and from the relay UE 100B. The relay UE 100B relays the data transmitted and received by the remote UE 100A (side link relay).

[0051] In step S5, the remote UE 100A may perform the relay UE selection procedure again. Specifically, the remote UE 100A may perform the relay UE reselection procedure to switch from the current relay UE 100B to another relay UE 100B.

[0052] (Sidelink protocol stack) Next, the configuration of a sidelink protocol stack according to one embodiment will be described.

[0053] Fig. 8 shows the configuration of a protocol stack for the user plane sidelink that handles data. As shown in Fig. 8, the user plane sidelink protocol has a PHY layer, a MAC layer, an RLC layer, a PDCP layer, and an SDAP layer.

[0054] The PHY layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layer of the remote UE 100A and the PHY layer of the relay UE 100B via a physical channel.

[0055] The MAC layer performs data priority control, retransmission processing using HARQ, etc. Data and control information are transmitted between the MAC layer of the remote UE 100A and the MAC layer of the relay UE 100B via a transport channel.

[0056] The RLC layer transmits data to the RLC layer on the receiving side using the functions of the MAC layer and PHY layer. Data and control information are transmitted between the RLC layer of the remote UE 100A and the RLC layer of the relay UE 100B via logical channels. The PDCP layer performs header compression / decompression and encryption / decryption. The SDAP layer performs mapping between IP flows, which are the units for QoS control by the core network, and radio bearers, which are the units for QoS control by the AS.

[0057] FIG. 9 is a diagram showing the configuration of a protocol stack for a side link of a control plane that handles signaling (control signals).

[0058] As shown in FIG. 9, the sidelink protocol stack of the control plane has a PC5-RRC layer and a PC5-S layer instead of the SDAP layer shown in FIG.

[0059] PC5-RRC messages for various settings are transmitted between the PC5-RRC layer of the remote UE 100A and the PC5-RRC layer of the relay UE 100B. When there is a connection (PC5-RRC connection) between the PC5-RRC of the remote UE 100A and the PC5-RRC of the relay UE 100B, the remote UE 100A may be in a PC5-RRC connected state. Note that the PC5-RRC layer is included in the AS layer.

[0060] The PC5-S layer is located above the PC5-RRC layer (AS layer). PC5-S messages such as discovery messages are transmitted between the PC5-S layer of the remote UE 100A and the PC5-S layer of the relay UE 100B.

[0061] (Relay UE selection procedure) Next, a relay UE selection procedure according to an embodiment will be described.

[0062] The remote UE 100A can use sidelink relay by selecting a relay UE 100B and establishing a sidelink connection, but if the remote UE 100A cannot select an appropriate relay UE 100B, the remote UE 100A cannot perform sidelink communication properly. A relay UE selection procedure according to one embodiment enables the remote UE 100A to select an appropriate relay UE 100B in the relay UE selection procedure.

[0063] The relay UE selection procedure according to one embodiment includes the steps of: a relay UE 100B, which has the capability to relay data of a remote UE 100A, transmitting a message including information about the relay UE 100B (hereinafter referred to as a "relay UE message"); a remote UE 100A receiving the relay UE message; and a remote UE 100A performing a process for establishing a sidelink connection between the remote UE 100A and the relay UE 100B based on the relay UE message. For example, the relay UE message is a PC5-RRC message of a PC5-RRC layer or a discovery message of a PC5-S layer.

[0064] When only one relay UE 100B is found, the remote UE 100A may select the one relay UE 100B in the relay UE selection procedure based on a relay UE message received from the one relay UE 100B. On the other hand, when multiple relay UEs 100B are found, the remote UE 100A may select one relay UE 100B in the relay UE selection procedure based on the relay UE messages of each of the multiple relay UEs 100B.

[0065] FIG. 10 is a diagram illustrating a relay UE message according to one embodiment.

[0066] 10, when the relay UE 100B has a network connection with the cellular communication network, the relay UE message includes information indicating a network communication state between the relay UE 100B and the cellular communication network, so that the remote UE 100A can select or reselect an appropriate relay UE 100B in consideration of the network communication state between the relay UE 100B and the cellular communication network.

[0067] When the relay UE 100B has a sidelink connection with another UE 100C, the relay UE message includes information indicating a sidelink communication state between the relay UE 100B and the other UE 100C. This allows the remote UE 100A to select or reselect an appropriate relay UE 100B in consideration of the sidelink communication state between the relay UE 100B and the other UE 100C.

[0068] The relay UE message includes information indicating the relay capability of the relay UE 100 B. This allows the remote UE 100 A to select or reselect an appropriate relay UE 100 B in consideration of the relay capability of the relay UE 100 B.

[0069] Specifically, the information indicating the network communication status between the relay UE 100B and the cellular communication network includes at least one of the following information elements: · An information element indicating the RRC state (either RRC connected, RRC inactive, or RRC idle) between the relay UE 100B and the cellular communication network; ·An information element indicating the coverage state of the relay UE 100B (either in coverage or out of coverage); ·An information element indicating a radio condition (e.g., any one of RSRP, RSRQ, and SINR) between the relay UE 100B and the cellular communication network; An information element indicating a frequency band used between the relay UE 100B and the cellular communication network; For example, the information element may indicate any one of licensed spectrum, which is a frequency band to which a license is granted, unlicensed spectrum, which is a frequency band that does not require a license, and shared spectrum, which is a frequency band that can be shared by multiple systems. The information element may also indicate any one of Frequency Range 1 (FR1) including frequencies below 6 GHz, Frequency Range 2 (FR2) including millimeter waves from 24.25 GHz to 52.6 GHz, which has a narrower communication range than FR1 but a wider frequency band, and other frequency ranges (FR3, etc.). The information element may also indicate the bandwidth (e.g., 100 MHz) of the frequency band used between the relay UE 100B and the cellular communication network; ·An information element indicating the link status (e.g., throughput (data rate), latency) between the relay UE 100B and the cellular communication network; An information element indicating the load status (for example, resource usage rate) between the relay UE 100B and the cellular communication network.

[0070] On the other hand, the information indicating the side link communication state between the relay UE 100B and the other UE 100C includes at least one of the following information elements: An information element indicating a frequency band used between the relay UE 100B and another UE 100C; · Information element indicating the radio state between the relay UE 100B and the other UE 100C; The information element may be an RSRP, an RSRQ, or an SINR for each other UE 100C (for each PC5-RRC connection). The information element may be an information element indicating a measurement result of a CBR (Channel Busy Ratio) indicating the degree of channel congestion between the relay UE 100B and the other UE 100C; · Information element indicating the link status between the relay UE 100B and another UE 100C; An information element indicating the load status between the relay UE 100B and another UE 100C.

[0071] On the other hand, the information indicating the relay capability of the relay UE 100B includes at least one of the following information elements. It is assumed that the upper layer (PC5-S layer) has this information: ·An information element indicating the relay mode (U2N, U2U, or both) of the relay UE 100B; The information element may be based on the mode authenticated by the network; An information element indicating the load status of the relay UE 100B (for example, the usage rate of the CPU or memory); An information element indicating the battery status of the relay UE 100B (for example, connected to a power source, using a battery, or low battery).

[0072] 11 is a diagram showing a sequence of a relay UE selection procedure according to an embodiment. Although an example in which there are two candidate relay UEs 100B is shown, there may be three or more candidate relay UEs 100B.

[0073] As shown in FIG. 11, in step S101, the relay UE 100B#1 transmits a relay UE message to the remote UE 100A.

[0074] If the relay UE message is a discovery message of the PC5-S layer, the AS layer in the relay UE 100B#1 notifies the upper layer (PC5-S layer) of its own network link state or the link state with the other side (with another UE 100C). The upper layer (PC5-S layer) generates a discovery message using the state information from the AS layer and its own relay capability information, and transmits the discovery message as a relay UE message.

[0075] On the other hand, if the relay UE message is a PC5-RRC message of the PC5-RRC layer, in the relay UE 100B#1, the upper layer (PC5-S layer) notifies the AS layer of its own relay capability state. The AS layer generates a PC5-RRC message using the relay capability information from the upper layer (PC5-S layer) and its own link state information, and transmits the PC5-RRC message as a relay UE message.

[0076] The PC5-RRC message may be a PC5-RRC message broadcast in the sidelink (e.g., an existing MasterInformationBlockSidelink or a new message). Alternatively, the PC5-RRC message may be a PC5-RRC message used for sidelink connection establishment (e.g., RRCReconfigurationSidelink, RRCReconfigurationCompleteSidelink), a PC5-RRC message used for sidelink inquiry and response (e.g., UECapabilityEnquirySidelink, UECapabilityInformationSidelink), and / or a new inquiry and response message dedicated to relaying. Regarding the inquiry and response messages, the Relay UE message is a response message, and a relay UE sends the Relay UE message (response message) in response to an inquiry from a remote UE.

[0077] In step S102, the relay UE 100B#2 transmits a relay UE message to the remote UE 100A.

[0078] In step S103, the remote UE 100A selects one relay UE 100B with which to establish a side link connection from among the multiple relay UEs 100B (relay UE 100B#1, relay UE 100B#2), based on the relay UE messages of each of the multiple relay UEs 100B.

[0079] In such a relay UE selection process, the remote UE 100A may extract selection candidates based on the radio conditions (e.g., RSRP of the side link) between the remote UE 100A and each relay UE 100B. The remote UE 100A may rank the candidates according to the radio conditions of the side link and select the top n relay UEs 100B (e.g., the top 5 UEs) as candidates. Alternatively, the remote UE 100A may select relay UEs 100B within a certain range (e.g., within −10 dB) based on the radio conditions of the relay UE 100B with the best radio conditions as candidates, or may select all relay UEs 100B with radio conditions equal to or higher than a threshold (e.g., equal to or higher than −100 dBm) as candidates.

[0080] In the relay UE selection process, the remote UE 100A selects one relay UE 100B by comparing information elements included in the relay UE messages of multiple candidate relay UEs 100B. Note that which information element is used for relay UE selection and the selection criteria (selection priority) may be set by the gNB 200, may be specified in advance (pre-configuration), or may be set by a user preference.

[0081] For example, the remote UE 100A may select a relay UE 100B in an RRC connected state with respect to the RRC states included in the network communication state, with priority over a relay UE 100B that is not in the RRC connected state. The remote UE 100A may perform relay UE selection according to the following priority ranking: RRC connected state > RRC inactive state > RRC idle state.

[0082] The remote UE 100A may exclude the in-coverage relay UE 100B or the out-of-coverage relay UE 100B from candidates for the coverage state included in the network communication state according to the respective uses of U2N and U2U. The remote UE 100A may preferentially select the in-coverage relay UE 100B over the out-of-coverage relay UE 100B.

[0083] The remote UE 100A may select the relay UE 100B having a good radio condition (RSRP, RSRQ, SINR) included in the network communication condition, preferentially over the relay UE 100B having a poor radio condition.

[0084] The remote UE 100A may preferentially select a relay UE 100B using licensed spectrum over a relay UE 100B not using licensed spectrum for a used frequency band included in the network communication state. The remote UE 100A may preferentially select a relay UE 100B using FR2 over a relay UE 100B not using FR2. The remote UE 100A may preferentially select a relay UE 100B using a wide bandwidth over a relay UE 100B using a narrow bandwidth. Note that these priority criteria may be reversed. For example, the remote UE 100A may preferentially select a relay UE 100B not using licensed spectrum or a relay UE 100B not using FR2.

[0085] The remote UE 100A may select a relay UE 100B having a high throughput with respect to a link state included in the network communication state, preferentially over a relay UE 100B having a low throughput. The remote UE 100A may select a relay UE 100B having a small latency with respect to a relay UE 100B having a large latency.

[0086] The remote UE 100A may select the relay UE 100B with a lighter load, with respect to the load status included in the network communication state, with priority over the relay UE 100B with a heavy load.

[0087] The remote UE 100A also performs relay UE selection for the sidelink communication state in the same manner as for the network communication state. However, for the radio state included in the sidelink communication state, the remote UE 100A may preferentially select a relay UE 100B having a good radio condition with respect to its communication counterpart (destination). The remote UE 100A may also preferentially select a relay UE 100B having a low CBR measurement value over a relay UE 100B having a high CBR measurement value.

[0088] Furthermore, the remote UE 100A may preferentially select a relay UE 100B having a relay mode suitable for the use of its own U2N and U2U, based on the relay capability information. The remote UE 100A may preferentially select a relay UE 100B having a small load status (such as a CPU or memory usage rate) over a relay UE 100B having a large load status. The remote UE 100A may preferentially select a relay UE 100B connected to a power source, based on the battery status.

[0089] When the remote UE 100A selects one relay UE 100B through the relay UE selection process, in step S104, the remote UE 100A performs a side link connection establishment process for the selected relay UE 100B (here, relay UE 100B#1).

[0090] 12 is a diagram showing a sequence of a relay UE reselection procedure according to an embodiment. Here, differences from the above-described relay UE selection procedure will be described.

[0091] 12, in step S151, the remote UE 100A is in a state of establishing a side link connection with the relay UE 100B#1. The remote UE 100A may trigger relay UE reselection in response to deterioration of the side link radio condition with the relay UE 100B#1.

[0092] In steps S152 and S153, the remote UE 100A receives a relay UE message from each of the relay UE 100B#1 and the relay UE 100B#2.

[0093] In step S154, the remote UE 100A determines whether to switch (reselect) to the relay UE 100B#2 based on the relay UE messages of the relay UE 100B#1 and the relay UE 100B#2. The criteria for such relay UE reselection are the same as the criteria for relay UE selection described above. Here, the description will proceed assuming that the remote UE 100A determines to reselect to the relay UE 100B#2.

[0094] In step S154, the remote UE 100A performs a process of establishing a side link connection with the reselected relay UE 100B#2. Note that the remote UE 100A releases the side link connection with the relay UE 100B#1.

[0095] (Sidelink communication control) Next, sidelink communication control in a sidelink relay procedure according to an embodiment will be described.

[0096] A remote UE 100A having a sidelink connection with a relay UE 100B can communicate with a cellular communication network or another UE 100C via the relay UE 100B. However, after such sidelink communication is initiated, a situation change, such as movement of the UE 100, may occur, making it difficult to properly perform the sidelink communication.

[0097] The sidelink communication control according to an embodiment includes a step in which a remote UE 100A having a sidelink connection with a relay UE 100B determines a network communication state between the remote UE 100A and a cellular communication network (gNB 200), and a step in which the remote UE 100A controls the sidelink communication between the remote UE 100A and the relay UE 100B based on the network communication state. This allows the sidelink communication between the remote UE 100A and the relay UE 100B to be appropriately controlled.

[0098] The sidelink communication control according to one embodiment includes a step in which a relay UE 100B having a sidelink connection with a remote UE 100A determines a communication state between the relay UE 100B and a cellular communication network (gNB 200) or a communication state between the relay UE 100B and another UE 100C, and a step in which the relay UE 100B controls the sidelink communication between the relay UE 100B and the remote UE 100A based on the determined communication state. This allows the sidelink communication between the remote UE 100A and the relay UE 100B to be appropriately controlled.

[0099] (1) Sidelink communication control by remote UE 100A

[0100] (1.1) Operation pattern 1 An operation pattern 1 of the side link communication control by the remote UE 100A according to one embodiment will be described.

[0101] In operation pattern 1 of the sidelink communication control by the remote UE 100A, the remote UE 100A having a sidelink connection with the relay UE 100B detects an improvement in the network communication state between the remote UE 100A and the cellular communication network. Then, in response to the detection of the improvement, the remote UE 100A performs a process to stop the sidelink communication between the remote UE 100A and the relay UE 100B.

[0102] As a result, if the network communication conditions between the remote UE 100A and the cellular communication network improve after the sidelink relay is started, the remote UE 100A can switch communication via sidelink relay to direct communication with the cellular communication network.

[0103] 13 is a diagram illustrating an operation pattern 1 of sidelink communication control by the remote UE 100A according to an embodiment. In FIG. 13, a case is mainly assumed in which the remote UE 100A having a sidelink connection with the relay UE 100B moves from outside the coverage area to inside the coverage area. Note that, in FIG. 13, non-essential steps are indicated by dashed lines.

[0104] 13, in step S201, the remote UE 100A is in a state where a side link connection is established with the relay UE 100B. The relay mode of the relay UE 100B may be either U2N relay or U2U relay.

[0105] In step S202, the remote UE 100A transmits and receives data to and from the relay UE 100B.

[0106] In step S203, the relay UE 100B relays data transmitted and received by the remote UE 100A. Although an example in which the relay UE 100B transmits and receives the relay data to and from the gNB 200 is shown in Fig. 13, the relay UE 100B may transmit and receive the relay data to and from another UE 100C.

[0107] In step S204, the remote UE 100A determines the network communication state between the remote UE 100A and the cellular communication network (gNB 200) and detects a change in the network communication state.

[0108] For example, in step S204, the remote UE 100A detects that the remote UE 100A has moved from out-of-coverage to in-coverage. The remote UE 100A may detect that it has moved into coverage in response to the received power (RSRP) of a reference signal received from the cellular communication network exceeding a threshold. The remote UE 100A may detect that it has moved into coverage in response to receiving a synchronization signal from the cellular communication network. Alternatively, in step S204, the remote UE 100A may detect that the remote UE 100A has transitioned from an RRC idle state or an RRC inactive state to an RRC connected state (step S205 described below).

[0109] Here, the threshold value to be compared with the RSRP and the network communication state to be detected by the remote UE 100A may be set by the gNB 200 using system information (SIB), etc., or may be a pre-defined threshold value (e.g., S-criteria) and / or a setting (pre-configuration) pre-written in a SIM (Subscriber Identity Module), etc.

[0110] In step S205, the remote UE 100A establishes a network connection with the gNB 200 and transitions to an RRC connected state. Note that step S205 may be performed after step S208 described later, for example, between step S208 and step S209 described later.

[0111] In response to detecting the change in the network communication state, the remote UE 100A performs a process for stopping the side link communication between the remote UE 100A and the relay UE 100B.

[0112] As such a process, the remote UE 100A may notify the relay UE 100B of a change in the network communication state (for example, transition to an RRC connected state) (step S206). Here, the remote UE 100A may notify the remote UE 100A of the received power (RSRP) of the reference signal received from the cellular communication network. Such notification may be performed by a PC5-RRC message or a PC5-S message.

[0113] Based on the notification from the remote UE 100A, the relay UE 100B may perform a process of releasing the sidelink connection with the remote UE 100A. For example, the relay UE 100B may discard the PC5-RRC entity and / or the PC5-S entity, or send a PC-RRC release indication to the remote UE 100A.

[0114] In response to detecting the change in the network communication state, the remote UE 100A may stop the sidelink communication with the relay UE 100B (step S207). For example, the remote UE 100A may cause a communication timeout by not responding to the relay UE 100B, and implicitly notify the relay UE 100B of the release of the sidelink connection. The remote UE 100A may discard its own PC5-RRC entity and / or PC5-S entity.

[0115] In step S208, the remote UE 100A performs a process of releasing the sidelink connection with the relay UE 100B. For example, the remote UE 100A may explicitly notify the relay UE 100B of the connection release by using a PC5-RRC message indicating the release of the sidelink connection.

[0116] In step S209, the remote UE 100A transmits and receives data through network communication with the gNB 200.

[0117] Although this operation pattern has been described with respect to U2N relaying, it may also be applied to U2U relaying. In this case, the gNB 200 in this operation pattern may be read as another UE (another remote UE) 100C, and the network communication state may be read as the communication state between the remote UE 100A and the other remote UE 100C. The remote UE 100A may detect that the other remote UE 100C is nearby (within a range where direct communication is possible) by, for example, a discovery procedure (step S204). Thereafter, the remote UE 100A performs an operation to release the side link connection with the relay UE 100B, as in this operation pattern.

[0118] (1.2) Operation pattern 2 An operation pattern 2 of the side link communication control by the remote UE 100A according to one embodiment will be described.

[0119] In operation pattern 2 of sidelink communication control by the remote UE 100A, the relay UE 100B performing U2N relay transfers first system information broadcast by a first cell of the cellular communication network to the remote UE 100A. The remote UE 100A detects an improvement in the network communication state with a second cell of the cellular communication network. Then, in response to the detection of the improvement, the remote UE 100A acquires second system information broadcast by the second cell instead of the first system information transferred from the relay UE 100B. As a result, when the remote UE 100A moves into the coverage of the second cell different from the first cell to which the remote UE 100A is connected, the remote UE 100A can acquire the system information of the second cell.

[0120] Fig. 14 is a diagram showing an operation pattern 2 of sidelink communication control by a remote UE 100A according to an embodiment. Fig. 14 shows an example in which cell #1 (first cell) is managed by gNB 200#1 and cell #2 (second cell) is managed by gNB 200#2, but cell #1 and cell #2 may be managed by a single gNB 200.

[0121] As shown in FIG. 14, in step S301, the relay UE 100B located within the coverage of the cell #1 receives (acquires) SIB #1 (first system information) that is system information of the cell #1 from the cell #1.

[0122] In step S302, the relay UE 100B forwards the SIB#1 to the remote UE 100A. The remote UE 100A, which is located out of coverage, receives and applies the SIB#1 from the relay UE 100B.

[0123] In step S303, the remote UE 100A determines the network communication state between the remote UE 100A and the cell #2, and detects a change in the network communication state.

[0124] For example, in step S303, the remote UE 100A detects that it has moved from outside the coverage to within the coverage of cell #2. The remote UE 100A detects that it has moved into the coverage of cell #2 in response to the received power (RSRP) of a reference signal received from cell #2 exceeding a threshold. The remote UE 100A may detect that it has moved into the coverage of cell #2 in response to receiving a synchronization signal from cell #2. Here, the threshold to be compared with the RSRP and the network communication state to be detected by the remote UE 100A may be set by the gNB 200 in system information (SIB) or the like, or may be a predefined threshold (e.g., S-criteria) and / or a setting (pre-configuration) pre-written in a SIM or the like.

[0125] In step S304, the remote UE 100A that has moved into the coverage of the cell #2 receives (acquires) SIB #2 (second system information) that is system information of the cell #2 from the cell #2. The remote UE 100A may discard the setting by SIB #1 transferred from the relay UE 100B.

[0126] 15 is a diagram illustrating a modified example of operation pattern 2 of the side link communication control by the remote UE 100A according to an embodiment. In this modified example, it is assumed that the remote UE 100A moves from inside the coverage area to outside the coverage area.

[0127] As shown in FIG. 15, in step S351, a remote UE 100A located within the coverage of the cell #1 receives (acquires) an SIB from the cell #1.

[0128] In step S352, the remote UE 100A determines the network communication state between the remote UE 100A and the cell #1, and detects a change in the network communication state.

[0129] For example, in step S352, the remote UE 100A detects that the remote UE 100A has moved from within the coverage of cell #1 to outside the coverage. The remote UE 100A may detect that the remote UE 100A has moved out of the coverage in response to the received power (RSRP) of a reference signal received from cell #1 falling below a threshold. Here, the threshold to be compared with the RSRP may be set by the gNB 200 in system information (SIB) or the like, or may be a predefined threshold (e.g., S-criteria) and / or a setting (pre-configuration) pre-written in a SIM or the like.

[0130] Here, the remote UE 100A may perform the relay UE discovery procedure or the above-described relay UE selection procedure to establish a side link connection with the relay UE 100B.

[0131] In step S353, the relay UE 100B located within the coverage of the cell #1 receives (acquires) the SIB from the cell #1.

[0132] In step S354, the relay UE 100B forwards the SIB to the remote UE 100A. The remote UE 100A, which is located out of coverage, receives and applies the SIB from the relay UE 100B.

[0133] 15, an example in which the relay UE 100B is located within the coverage of the cell #1 has been described, but the relay UE 100B may be located in another cell (cell #2). In this case, the relay UE 100B transfers the SIB of the cell #2 to the remote UE 100A that is outside the coverage. The remote UE 100A may discard the setting by the SIB of the cell #1.

[0134] (2) Communication control by relay UE 100B The side link communication control by the relaying UE 100B according to an embodiment will be described.

[0135] The relay UE 100B having a sidelink connection with the remote UE 100A detects deterioration of the communication state between the relay UE 100B and the cellular communication network (gNB 200) or the communication state between the relay UE 100B and another UE 100C. Then, in response to the detection of the deterioration, the relay UE 100B performs a predetermined process to stop the sidelink communication with the remote UE 100A.

[0136] The remote UE 100A cannot grasp the communication state between the relay UE 100B and the cellular communication network (gNB 200) or the communication state between the relay UE 100B and another UE 100C, and it is difficult for the remote UE 100A to determine whether the sidelink relay can be continued. Therefore, when the relay UE 100B detects a deterioration in the communication state, the relay UE 100B performs a predetermined process to stop the sidelink communication with the remote UE 100A, thereby enabling the remote UE 100A to perform a process such as a relay UE reselection process.

[0137] In a predetermined process, the relay UE 100B may transmit to the remote UE 100A a reselection instruction to reselect a relay UE 100B other than the relay UE 100B. When the relay UE 100B detects a deterioration in the communication state between the relay UE 100B and another UE 100C, the relay UE 100B may notify the remote UE 100A of the other UE.

[0138] 16 is a diagram illustrating side link communication control by the relay UE 100B according to an embodiment. Note that in FIG. 16, non-essential steps are indicated by dashed lines.

[0139] 16, in step S401, the remote UE 100A is in a state where a side link connection is established with the relay UE 100B. The relay mode of the relay UE 100B may be either U2N relay or U2U relay.

[0140] In step S402, the remote UE 100A transmits and receives data to and from the relay UE 100B.

[0141] In step S403, the relay UE 100B relays data transmitted and received by the remote UE 100A. Specifically, the relay UE 100B transmits and receives the relay data to and from the gNB 200 or another UE 100C.

[0142] In step S404, the relay UE 100B determines the communication state between the relay UE 100B and the cellular communication network (gNB 200) or the communication state between the relay UE 100B and another UE 100C, and detects deterioration of the communication state.

[0143] For example, in the case of U2N relaying, the relay UE 100B may detect at least one of a Radio Link Failure (RLF) in the network connection with the gNB 200, a failure in recovery of the RRC connection, and an RRC connection release. The relay UE 100B may detect that the received power (RSRP) of a reference signal received from the gNB 200 or the throughput of network communication with the gNB 200 has fallen below a threshold, or that the resource usage rate (occupancy rate) of network communication with the gNB 200 has exceeded a threshold.

[0144] On the other hand, in the case of U2U relay, the relay UE 100B may detect at least one of an RLF in the sidelink connection with the other UE 100C, a PC5-RRC connection release, a PC5-S connection release, a failure to establish a PC5-RRC connection, and a failure to establish a PC5-S connection. The relay UE 100B may detect that the received power (RSRP) of a reference signal received from the other UE 100C or the throughput of the sidelink communication with the other UE 100C has fallen below a threshold, or that the resource usage rate (occupancy rate) of the sidelink communication with the other UE 100C has exceeded a threshold.

[0145] The threshold may be set by gNB200 using system information (SIB), or may be a pre-defined threshold (e.g., S-criteria), or may be a setting (pre-configuration) pre-written in a SIM, etc.

[0146] In step S405, the relay UE 100B determines a deterioration in the communication state between the relay UE 100B and the cellular communication network (gNB 200) or a deterioration in the communication state between the relay UE 100B and another UE 100C, and transmits a message indicating the deterioration in the communication state to the remote UE 100A. The message may be a PC5-RRC message transmitted by unicast or a message transmitted by broadcast.

[0147] For example, the message in step S405 may be a reselection instruction instructing the remote UE 100A to perform a relay UE reselection process. In the case of U2U relay, the message may include a destination ID indicating another UE 100C whose side link communication state has deteriorated. The message may include information indicating whether the deterioration of the communication state is a deterioration of the network communication state or a deterioration of the side link communication state.

[0148] The message of step S405 may be a suspend instruction indicating a temporary suspension of the sidelink communication with the remote UE 100A. In this case, when the communication state between the relay UE 100B and the cellular communication network (gNB 200) or the communication state between the relay UE 100B and another UE 100C improves, the relay UE 100B may transmit a resume instruction indicating a resumption of the sidelink communication with the remote UE 100A to the remote UE 100A. These instructions may include information indicating a cause (based on the above detection result).

[0149] The message of step S405 may be a discovery message. The discovery message includes a list of identifiers of other UEs 100C with which the relay UE 100B has a sidelink connection. When the relay UE 100B detects deterioration of the communication state with any of the UEs 100C in the list, the relay UE 100B updates the list to delete the identifier of the UE 100C, and notifies the remote UE 100A of a discovery message including the updated list. This allows the remote UE 100A to correctly identify other UEs 100C with which communication is possible via sidelink relay. Note that if the list includes an identifier of the gNB 200 (i.e., in the case of U2N relay), the identifier (e.g., a cell ID or an identifier indicating a network connection) may be deleted from the list.

[0150] The message in step S405 may be the above-mentioned relay UE message. That is, the relay UE 200B triggers transmission of the above-mentioned relay UE message in response to a change (determination) in the communication state with the gNB 200 or another UE 100C.

[0151] In step S406, the relay UE 100B may perform a process to stop the sidelink communication with the remote UE 100A. For example, the relay UE 100B may cause a communication timeout by not responding to the remote UE 100A, thereby implicitly notifying the remote UE 100A of the release of the sidelink connection. The relay UE 100B may also explicitly notify the remote UE 100A of the release of the sidelink connection by using a PC5-RRC message indicating the release of the sidelink connection.

[0152] In step S407, the remote UE 100A may trigger a relay UE reselection process based on, for example, the message received from the relay UE 100B in step S405. Here, the remote UE 100A may perform the relay UE reselection process after excluding the relay UE 100B that sent the message from reselection candidates. In the remote UE 100A, the AS layer may notify a higher layer (such as PC5-S) that the message has been received (and the contents of the message). In this case, the higher layer may exclude the relay UE 100B that sent the message from discovery targets in the relay UE discovery procedure.

[0153] (Other embodiments) The above-described sequences are not limited to being performed independently, but may be performed by combining two or more steps of different sequences.

[0154] A program may be provided that causes a computer to execute each process performed by the UE 100 or the gNB 200. The program may be recorded on a computer-readable medium. Using the computer-readable medium, the program can be installed on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transitory recording medium. The non-transitory recording medium is not particularly limited, and may be, for example, a recording medium such as a CD-ROM or a DVD-ROM.

[0155] In addition, circuits that execute each process performed by UE100 or gNB200 may be integrated, and at least a part of UE100 or gNB200 may be configured as a semiconductor integrated circuit (chipset, SoC).

[0156] The above describes the embodiments in detail with reference to the drawings, but the specific configuration is not limited to that described above, and various design changes can be made within the scope that does not deviate from the gist of the invention.

[0157] This application claims priority to U.S. Provisional Application No. 63 / 086146 (filed October 1, 2020), the entire contents of which are incorporated herein by reference.

Claims

1. A communication control method for use in a cellular communication system, comprising: receiving, from a remote user equipment, a PC5-RRC message including information indicating a radio link failure (RLF) between a relay user equipment that relays data of the remote user equipment and a network or another user equipment; the relay user equipment performs either UE-to-NW (U2N) relaying or UE-to-UE (U2U) relaying; The PC5-RRC message further includes information indicating whether the type of information indicating the RLF is the case of the U2N relay or the U2U relay. Communication control method.

2. 1. A remote user equipment for use in a cellular communication system, comprising: a receiver for receiving, from a relay user equipment (REE) that relays data of the remote user equipment (UE), the REE performing either UE-to-NW (U2N) relaying or UE-to-UE (U2U) relaying, a PC5-RRC message including information indicating a radio link failure (RLF) between the REE and a network or another UE; The PC5-RRC message further includes information indicating whether the type of information indicating the RLF is the case of the U2N relay or the U2U relay. Remote user device.

3. 1. A processor for controlling a remote user device for use in a cellular communications system, comprising: receiving a PC5-RRC message from a relay user equipment (REE) that relays data of the remote user equipment, the relay user equipment performing either UE-to-NW (U2N) relaying or UE-to-UE (U2U) relaying, the PC5-RRC message including information indicating a radio link failure (RLF) between the REE and a network or another UE; The PC5-RRC message further includes information indicating whether the type of information indicating the RLF is the case of the U2N relay or the U2U relay. Processor.

4. A communication control method for use in a cellular communication system, comprising: a relay user equipment (REE) relaying data of a remote user equipment (RUE) sending a PC5-RRC message to the remote user equipment (RUE), the PC5-RRC message including information indicating a radio link failure (RLF) between the REE and a network or another user equipment; the relay user equipment performs either UE-to-NW (U2N) relaying or UE-to-UE (U2U) relaying; The PC5-RRC message further includes information indicating whether the type of information indicating the RLF is the case of the U2N relay or the U2U relay. Communication control method.

5. 1. A relay user equipment (UE) for relaying data of a remote user equipment (RUE) in a cellular communication system, comprising: a control unit that performs either UE-to-NW (U2N) relay or UE-to-UE (U2U) relay; a transmitter configured to transmit a PC5-RRC message to the remote user equipment, the PC5-RRC message including information indicating a radio link failure (RLF) between the relay user equipment and a network or another user equipment; The PC5-RRC message further includes information indicating whether the type of information indicating the RLF is the case of the U2N relay or the U2U relay, Relay user equipment.

6. 1. A processor for controlling a relay user equipment that relays data of a remote user equipment in a cellular communication system, comprising: A process of performing either UE-to-NW (U2N) relaying or UE-to-UE (U2U) relaying; transmitting a PC5-RRC message to the remote user equipment, the PC5-RRC message including information indicating a radio link failure (RLF) between the relay user equipment and a network or another user equipment; The PC5-RRC message further includes information indicating whether the type of information indicating the RLF is the case of the U2N relay or the U2U relay, Processor.

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