COMMUNICATION CONTROL METHOD

A communication control method using a relay user equipment relaying communication between a remote user equipment and a base station includes the steps of transmitting and receiving, by a first Radio Resource Control (RRC) layer of the remote user equipment, to and from the base station via the relay user equipment, an RRC message performing control of communication with the base station, transmitting and receiving, by a second RRC layer of the remote user equipment, to and from the relay user equipment, an RRC message performing control of communication with the relay user equipment, and notifying, by the second RRC layer, the first RRC layer of communication state information regarding a failure or disconnection of a first radio link between the remote user equipment and the relay user equipment.

TECHNICAL FIELD

The present invention relates to a communication control method used in a mobile communication system.

BACKGROUND ART

A sidelink relay technique in which a user equipment is used as a relay node in a mobile communication system based on the 3rd Generation Partnership Project (3GPP) standard has been studied. The sidelink relay is a technique in which a relay node referred to as a relay user equipment (Relay UE) intervenes in communication between a base station and a remote user equipment (Remote UE) and performs relay for the communication.

CITATION LIST

SUMMARY OF INVENTION

A communication control method according to a first aspect is a method using a relay user equipment relaying communication between a remote user equipment and a base station. The communication control method includes the steps of transmitting and receiving, by a first Radio Resource Control (RRC) layer of the remote user equipment, to and from the base station via the relay user equipment, an RRC message performing control of communication with the base station, transmitting and receiving, by a second RRC layer of the remote user equipment, to and from the relay user equipment, an RRC message performing control of communication with the relay user equipment, and by the second RRC layer, notifying the first RRC layer of communication state information regarding a failure or disconnection of a first radio link between the remote user equipment and the relay user equipment.

A communication control method according to a second aspect is a method using a relay user equipment relaying communication between a remote user equipment and a base station. The communication control method includes the steps of starting, by the remote user equipment, Radio Resource Control (RRC) reestablishment processing from the relay user equipment to a target base station in response to detection of occurrence of a communication failure, and transmitting, by the remote user equipment, an RRC reestablishment request message to the target base station in the RRC reestablishment processing. The RRC reestablishment request message includes a predetermined identifier identifying context information of the remote user equipment stored in the base station.

A communication control method according to a third aspect is a method using a relay user equipment relaying communication between a remote user equipment and a base station. The communication control method includes the steps of managing, by the base station, an identifier of the relay user equipment, context information of the relay user equipment, and context information of the remote user equipment in association with each other, transmitting, by the relay user equipment, when starting Radio Resource Control (RRC) reestablishment processing from the base station to a target base station is started, an RRC reestablishment request message including the identifier of the relay user equipment to the target base station, and acquiring, by the target base station, from the base station, the context information of the relay user equipment and the context information of the remote user equipment using the identifier included in the RRC reestablishment request message.

A communication control method according to a fourth aspect is a method using a relay user equipment relaying communication between a remote user equipment and a base station. The communication control method includes the steps of selecting, by the remote user equipment, a reconnection destination in accordance with a priority order where the relay user equipment has a highest priority order when Radio Resource Control (RRC) connection between the remote user equipment and the relay user equipment is released, and attempting, by the remote user equipment, processing of reconnection to the selected reconnection destination.

DESCRIPTION OF EMBODIMENTS

In the sidelink relay technique in Background Art, various failures (for example, a connection failure) in communication performed by a remote user equipment with a base stations via a relay user equipment are conceivable. Because the communication may be disconnected due to such failures, communication reliability may reduce.

Therefore, an object of the present disclosure is to improve reliability of communication using the sidelink relay technique.

A mobile 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 signs.

Configuration of Mobile Communication System

First, a configuration of a mobile communication system according to an embodiment will be described.FIG.1is a diagram illustrating a configuration of the mobile communication system according to an embodiment. This mobile communication system is based on the 5th Generation System (5GS) of the 3GPP standard. Although the following description will be given by exemplifying the 5GS, a Long Term Evolution (LTE) system may be at least partially applied to the mobile communication system.

As illustrated inFIG.1, the SGS1includes a user equipment (UE)100, a 5G radio access network (next generation radio access network (NG-RAN))10, and a 5G core network (5GC)20.

The UE100is a mobile wireless communication apparatus. The UE100may be any apparatus as long as the apparatus is used by a user. Examples of the UE100include a mobile phone terminal (including a smartphone), a tablet terminal, a laptop PC, a communication module (including a communication card or a chipset), a sensor or an apparatus provided in the sensor, a vehicle or an apparatus (Vehicle UE) provided in the vehicle, and/or a flying object or an apparatus (Aerial UE) provided in the flying object.

The NG-RAN10includes base stations (referred to as “gNBs” in the 5G system)200. The gNBs200are connected to each other via an Xn interface, which is an inter-base station interface. The gNB200manages one or a plurality of cells. The gNB200performs wireless communication with the UE100that has established connection with a cell of the gNB200. The gNB200has a radio resource management (RRM) function, a function of routing user data (hereinafter simply referred to as “data”), a measurement control function for mobility control and scheduling, and the like. The “cell” is used as a term denoting a minimum unit of a wireless communication area. The “cell” is also used as a term denoting a function or a resource for performing wireless communication with the UE100. One cell belongs to one carrier frequency.

Note that a gNB can also be connected to an Evolved Packet Core (EPC), which is a core network of the LTE. A base station of the LTE can also be connected to the 5GC. The base station of the LTE and a gNB can also be connected via an inter-base station interface.

The 5GC20includes an Access and Mobility Management Function (AMF) and a User Plane Function (UPF)300. The AMF performs various types of mobility control and the like for the UE100. The AMF manages mobility of the UE100through communication with the UE100using Non-Access Stratum (NAS) signaling. The UPF performs transfer control of data. The AMF and the UPF are connected to the gNB200via an NG interface being an interface between the base station and the core network.

FIG.2is a diagram illustrating a configuration of the UE100(user equipment).

As illustrated inFIG.2, the UE100includes a receiver110, a transmitter120, and a controller130.

The receiver110performs various kinds of reception under control of the controller130. The receiver110includes an antenna and a reception device. The reception device converts a radio signal received by the antenna into a baseband signal (reception signal) and outputs the baseband signal to the controller130.

The transmitter120performs various kinds of transmission under control of the controller130. The transmitter120includes an antenna and a transmission device. The transmission device converts a baseband signal (transmission signal) output by the controller130into a radio signal and transmits the radio signal from the antenna.

The controller130performs various kinds of control for the UE100. The controller130includes at least one processor and at least one memory. The memory stores programs to be executed by the processor and information to be used for processing performed by the processor. The processor may include a baseband processor and a Central Processing Unit (CPU). The baseband processor performs modulation and demodulation, and coding and decoding of a baseband signal, and the like. The CPU executes the programs stored in the memory to perform various kinds of processing.

FIG.3is a diagram illustrating a configuration of the gNB200(base station).

As illustrated inFIG.3, the gNB200includes a transmitter210, a receiver220, a controller230, and a backhaul communicator240.

The transmitter210performs various kinds of transmission under control of the controller230. The transmitter210includes an antenna and a transmission device. The transmission device converts a baseband signal (transmission signal) output by the controller230into a radio signal and transmits the radio signal from the antenna.

The receiver220performs various kinds of reception under control of the controller230. The receiver220includes an antenna and a reception device. The reception device converts a radio signal received by the antenna into a baseband signal (reception signal) and outputs the baseband signal to the controller230.

The controller230performs various kinds of control for the gNB200. The controller230includes at least one processor and at least one memory. The memory stores programs to be executed by the processor and information to be used for processing performed by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation and demodulation, and coding and decoding of a baseband signal, and the like. The CPU executes the programs stored in the memory to perform various kinds of processing.

The backhaul communicator240is connected to a neighboring base station via an inter-base station interface. The backhaul communicator240is connected to an AMF/UPF300via a base station-core network interface. Note that the gNB may be composed of (in other words, functionally split into) a Central Unit (CU) and a Distributed Unit (DU), and both the units may be connected by an F1 interface.

FIG.4is a diagram illustrating a configuration of a protocol stack of a radio interface in a user plane that handles data.

As illustrated inFIG.4, the radio interface protocol of the user plane 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.

The PHY layer performs coding and decoding, modulation and demodulation, antenna mapping and antenna demapping, and resource mapping and resource demapping. Between the PHY layer of the UE100and the PHY layer of the gNB200, data and control information are transmitted via a physical channel.

The MAC layer performs priority control of data, retransmission processing using a hybrid ARQ (HARQ), a random access procedure, and the like. Between the MAC layer of the UE100and the MAC layer of the gNB200, data and control information are transmitted via a transport channel. The MAC layer of the gNB200includes a scheduler. The scheduler determines transport formats (transport block sizes, modulation and coding schemes (MCSs)) in the uplink and the downlink and determines resource blocks to be allocated to the UE100.

The RLC layer transmits data to the RLC layer on the reception end by using functions of the MAC layer and the PHY layer. Between the RLC layer of the UE100and the RLC layer of the gNB200, data and control information are transmitted via a logical channel.

The PDCP layer performs header compression and header decompression, and encryption and decryption.

The SDAP layer maps an IP flow being a unit in which the core network performs QoS control onto a radio bearer being a unit in which the access stratum (AS) performs QoS control. Note that, when the RAN is connected to the EPC, the SDAP may not be provided.

FIG.5is a diagram illustrating a configuration of a protocol stack of a radio interface of a control plane that handles signaling (control signal).

As illustrated inFIG.5, the protocol stack of the radio interface of the control plane includes a Radio Resource Control (RRC) layer and a Non-Access Stratum (NAS) layer instead of the SDAP layer illustrated inFIG.4.

Between the RRC layer of the UE100and the RRC layer of the gNB200, RRC signaling for various configurations is transmitted. The RRC layer controls a logical channel, a transport channel, and a physical channel according to establishment, reestablishment, and release of a radio bearer. When there is connection (RRC connection) between the RRC of the UE100and the RRC of the gNB200, the UE100is in an RRC connected mode. When there is no connection (RRC connection) between the RRC of the UE100and the RRC of the gNB200, the UE100is in an RRC idle mode.

The NAS layer located in a layer higher than the RRC layer performs session management, mobility management, and the like. Between the NAS layer of the UE100and the NAS layer of the AMF300, NAS signaling is transmitted.

Note that the UE100includes an application layer and the like other than the protocol of the radio interface.

Conceivable Scenario

Next, a conceivable scenario in the mobile communication system 1 according to an embodiment will be described.FIG.6is a diagram illustrating the conceivable scenario.

As illustrated inFIG.6, a scenario in which relay UE100-2intervenes in communication between a gNB200-1and a remote UE100-1and sidelink relay relaying the communication is used will be conceived.

The remote UE100-1performs wireless communication (sidelink communication) with the relay UE100-2on a PC5 interface (sidelink), which is an inter-UE interface. The relay UE100-2performs wireless communication (Uu communication) with the gNB200-1on an NR Uu radio interface. As a result, the remote UE100-1indirectly communicates with the gNB200-1via the relay UE100-2. The Uu communication includes uplink communication and downlink communication.

FIG.7is a diagram illustrating an example of a protocol stack in a conceivable scenario. InFIG.7, illustration of the MAC layer and the PHY layer that are lower layers with respect to the RLC layer is omitted.

As illustrated inFIG.7, the gNB200-1may be split into the CU and the DU. An F1-C interface (Intra-donor F1-C) is established between the CU and the DU.

The PDCP layer of the CU of the gNB200-1and the PDCP layer of the remote UE100-1communicate with each other via the relay UE100-2. The RRC layer of the CU and the RRC layer of the remote UE100-1also communicate with each other via the relay UE100-2. In the DU, the relay UE100-2, and the remote UE100-1, an adaptation (Adapt) layer may be provided as an upper layer of the RLC layer.

Note that although illustration is omitted inFIG.7, the RRC layer of the CU and the RRC layer of the relay UE100-2communicate with each other. The PDCP layer of the CU and the PDCP layer of the relay UE100-2communicate with each other.

Also, each of the remote UE100-1and the relay UE100-2may include an RRC layer for the PC5. Such an RRC layer is referred to as a “PC5 RRC”. The PC5 RRC layer of the remote UE100-1and the PC5 RRC layer of the relay UE100-2communicate with each other.

FIG.8is a diagram illustrating an example of a protocol stack including a PC5 RRC layer.FIG.9is a diagram illustrating another example of the protocol stack including the PC5 RRC layer. AlthoughFIGS.8and9illustrate examples in which the gNB200-1is not split into a DU and a CU, the gNB200-1may be split into a DU and a CU.

As illustrated inFIG.8, the gNB200-1includes an RRC layer, a PDCP layer (Uu), an RLC layer (Uu), a MAC layer (Uu), and a PHY layer (Uu) used for communication on a Uu interface (Uu communication). Also, the gNB200-1includes an adaptation layer between the PDCP layer (Uu) and the RLC layer (Uu).

The relay UE100-2includes an RRC layer (not illustrated), an RLC layer (Uu), a MAC layer (Uu), and a PHY layer (Uu) used for communication on the Uu interface (Uu communication). Additionally, the relay UE100-2includes a PC5 RRC layer, a PDCP layer (PC5), an RLC layer (PC5), a MAC layer (PC5), and a PHY layer (PCS) used for communication on the PC5 interface (PC5 communication). Moreover, the relay UE100-2includes an adaptation layer as a layer that is higher than the PC5 RRC layer.

The remote UE100-1includes an RRC layer and a PDCP layer (Uu) used for communication on the Uu interface (Uu communication). Also, the remote UE100-1includes a PC5 RRC layer, a PDCP layer (PC5), an RLC layer (PC5), a MAC layer (PC5), and a PHY layer (PCS) used for communication on the PC5 interface (PC5 communication). Furthermore, the remote UE100-1includes an adaptation layer between the PDCP layer (Uu) and the PC5 RRC layer.

As illustrated inFIG.9, the remote UE100-1may not include the adaptation layer. In the example illustrated inFIG.9, the adaptation layer of the relay UE100-2is positioned as an upper layer with respect to the RLC layer (Uu).

Operations of Mobile Communication System

Operations of the mobile communication system 1 according to an embodiment will be described.

Notification Operation from PC5 RRC to Uu RRC in Remote UE

A notification operation from the PC5 RRC to the Uu RRC in the remote UE will be described.FIG.10is a diagram illustrating the notification operation.

As illustrated inFIG.10, the relay UE100-2relays communication between the remote UE100-1and the gNB200-1. The remote UE100-1includes an RRC layer (first RRC layer) and a PC5 RRC layer (second RRC layer).

The RRC layer of the remote UE100-1includes RRC connection with the RRC layer of the gNB200-1. The RRC layer of the remote UE100-1transmits and receives, to and from the RRC layer of the gNB200-1via the relay UE100-2, an RRC message performing control of communication with the gNB200-1.

The PC5 RRC layer of the remote UE100-1includes PC5 RRC connection with the PC5 RRC layer of the relay UE100-2. The PC5 RRC layer of the remote UE100-1transmits and receives, to and from the PC5 RRC layer of the relay UE100-2, an RRC message (PC5 RRC message) performing control of communication with the relay UE100-2. The PC5 RRC layer of the remote UE100-1includes a function of managing and monitoring a first radio link (sidelink) between the remote UE100-1and the relay UE100-2.

Note that the RRC layer and the PC5 RRC layer of the remote UE100-1may be separate RRC entities or may be separate functions in one RRC entity.

Also, the relay UE100-2includes an RRC layer of Uu and is in a state in which RRC connection with the RRC layer of the gNB200-1has been established (that is, an RRC connected mode).

In this notification operation, the PC5 RRC layer of the remote UE100-1notifies the RRC layer of the remote UE100-1of communication state information regarding a failure or disconnection of a radio link between the remote UE100-1and the relay UE100-2. In this manner, the RRC layer of the remote UE100-1can recognize and appropriately address the failure or the disconnection of the radio link between the remote UE100-1and the relay UE100-2.

For example, the RRC layer of the remote UE100-1performs processing (RRC reestablishment processing) of reestablishing RRC connection between the RRC layer of the remote UE100-1and the RRC layer of the gNB200-1on the basis of the communication state information notified. The RRC layer of the remote UE100-1may determine that a radio link failure (RLF) of the sidelink has been detected on the basis of the communication state information notified and perform the RRC reestablishment processing in response to the determination.

Here, the communication state information notified by the PC5 RRC layer of the remote UE100-1may include information regarding degradation of a sidelink communication state. For example, the communication state information includes at least one selected from the group consisting of information indicating occurrence of the RLF of the sidelink, information indicating that the reestablishment of the PC5 RRC connection has failed, and information indicating that a measurement result for the sidelink is below a threshold value.

In a case in which the reestablishment of the PC5 RRC connection has failed a predetermined number of times after detection of occurrence of the RLF of the sidelink, the PC5 RRC layer of the remote UE100-1may notify the RRC layer of the remote UE100-1of the information indicating that the reestablishment of the PC5 RRC connection has failed.

The measurement result for the sidelink is a measurement result for a sidelink reference signal transmitted by the relay UE100-2, for example (a measurement result of received power, for example). The threshold value compared with the measurement result may be configured by any of the RRC layer of the remote UE100-1, the PC5 RRC layer of the relay UE100-2, and the RRC layer of the gNB200-1. The measurement result for the sidelink may be a result of measuring a throughput in the sidelink, a result of measuring a communication delay time, or the like.

The communication state information may include information indicating that the PC5 RRC layer has received, from an upper layer, a release request requesting release of connection established by the PC5 RRC layer (specifically, PC5 RRC connection and/or PCS-S connection). The upper layer is a layer that is higher than the RRC layer and is, for example, the NAS layer.

The communication state information may include information regarding degradation of a communication state of the radio link (Uu link) between the relay UE100-2and the gNB200-1. The communication state information includes information indicating that the PC5 RRC layer of the remote UE100-1has received, from the relay UE100-2, a notification (Uu RLF Notification) indicating occurrence of a failure of the radio link between the relay UE100-2and the gNB200-1or a notification (buffer overflow notification) indicating that the amount of data in an uplink buffer of the relay UE100-2has exceeded a threshold value.

Here, description will be given in regard to the Uu RLF Notification. First, in a case in which the RLF of Uu has been detected or in a case in which reestablishment has failed, the RRC layer of the relay UE100-2notifies the PC5 RRC layer of the relay UE100-2of the fact. Second, when the PC5 RRC layer of the relay UE100-2receives such a notification, the PC5 RRC layer of the relay UE100-2transmits the Uu RLF Notification to the PC5 RRC layer of the remote UE100-1. Third, the PC5 RRC layer of the remote UE100-1notifies the RRC layer of the remote UE100-1of information indicating that the Uu RLF Notification has been received as communication state information in response to reception of the Uu RLF Notification.

Description will be given in regard to the buffer overflow notification. First, when the relay UE100-2receives data transmitted on the sidelink from the remote UE100-1, the relay UE100-2temporarily stores the received data in the uplink buffer. Second, the relay UE100-2monitors the amount of data in the uplink buffer and determines whether the amount of data has exceeded a threshold value. The threshold value may be set by the RRC layer of the gNB200-1. Third, if it is determined that the amount of data in the uplink buffer has exceeded the threshold value, the PC5 RRC layer of the relay UE100-2transmits the buffer overflow notification to the PC5 RRC layer of the remote UE100-1. Fourth, the PC5 RRC layer of the remote UE100-1notifies the RRC layer of the remote UE100-1of information indicating that the buffer overflow notification has been received as communication state information in response to the reception of the buffer overflow notification. Note that the Uu RLF Notification and/or the buffer overflow notification may be notified by control data of the adaptation layer in a case in which an adaptation layer (BAP or the like) link has been established between the relay UE100-2and the remote UE100-1.

Note that a wireless LAN or Bluetooth (trade name) may be used instead of the sidelink of the 3GPP standard as the wireless communication between the remote UE100-1and the relay UE100-2. In this case, the adaptation layer of the remote UE100-1may notify the RRC layer of the remote UE100-1of the communication state information.

The PC5 RRC layer or the RRC layer of the relay UE100-2may discard the context information of the remote UE100-1in the case in which the PC5 RRC connection with the remote UE100-1has been released. Also, in such a case, the RRC layer of the relay UE100-2may notify the RRC layer of the gNB200-1of the fact that the remote UE100-1is no longer a relay destination. The gNB200-1may reconfigure a Uu bearer on the basis of the notification.

RRC Reestablishment Operation of Remote UE from Relay UE to gNB

Next, operations of RRC reestablishment of the remote UE100-1from the relay UE100-2to the target gNB will be described.FIG.11is a diagram illustrating the operations.

As illustrated inFIG.11, the relay UE100-2and the gNB200-1are in a state in which RRC connection has been established (Step S101), the remote UE100-1and the relay UE100-2are in a state in which PC5 RRC connection has been established (Step S102), and the remote UE100-1and the gNB200-1are in a state in which RRC connection has been established via the relay UE100-2(Step S103).

Each of the remote UE100-1and the relay UE100-2is allocated the corresponding Cell-Radio Network Temporary Identifier (C-RNTI) by the gNB200-1. In this case, the gNB200-1may perform communication by associating the C-RNTI of the remote UE100-1with the C-RNTI of the relay UE100-2and performing conversion (interpretation) therebetween.

The remote UE100-1may acquire a cell identifier (physical cell identifier) of the gNB200-1via the relay UE100-2. The remote UE100-1acquires the cell identifier at the time of establishment of the PC5 RRC connection, at the time of a handover, or at the time of the RRC reestablishment. The cell identifier may be notified in a PC5 RRC message (for example, an RRC Reconfiguration Sidelink message or a Master Information Block Sidelink) from the relay UE100-2to the remote UE100-1. The cell identifier may be notified in an RRC message (for example, an RRC Reconfiguration message) from the gNB200-1to the remote UE100-1via the relay UE100-2.

In Step S104, the remote UE100-1starts RRC reestablishment processing from the relay UE100-2to a target gNB200-2in response to detection of occurrence of a communication failure. Here, Step S104may be a step corresponding to “(1) Notification Operation from PC5 RRC to Uu RRC in Remote UE” described above.

In Step S105, the remote UE100-1transmits an RRC reestablishment request message to the target gNB200-2in the RRC reestablishment processing. Note that the RRC reestablishment request message is transmitted from the remote UE100-1to the target gNB200-2without intervention of the relay UE100-2.

The RRC reestablishment request message includes a predetermined identifier to identify context information of the remote UE100-1stored in the gNB200-1.FIG.12is a diagram illustrating an example of the RRC reestablishment request (RRCReestablishmentRequest) message.

As illustrated inFIG.12, the RRC reestablishment request message includes ue-Identity (ReestabUE-Identity) corresponding to the predetermined identifier. The ue-Identity (ReestabUE-Identity) includes the C-RNTI (c-RNTI) allocated by the gNB200-1, the cell identifier (physCellId) acquired via the relay UE100-2, and shortMAC-I. The shortMAC-I is calculated by the PDCP layer of the remote UE100-1.

Returning toFIG.11, in Step S106, the target gNB200-2requests, by using the predetermined identifier included in the RRC reestablishment request message, the gNB200-1to derive the context information of the remote UE100-1. Specifically, the target gNB200-2transmits a context request message including the predetermined identifier to the gNB200-1.

The gNB200-1manages the context information of the remote UE100-1in association with the predetermined identifier. In Step S107, in response to the reception of the context request message from the target gNB200-2, the gNB200-1derives the context information corresponding to the predetermined identifier included in the context request message and transmits a context response message including the context information to the target gNB200-2.

In Step S108, the target gNB200-2transmits an RRC reestablishment message to the remote UE100-1in response to reception of the context response message from the gNB200-1. Note that the RRC reestablishment message is transmitted from the gNB200-1to the remote UE100-1without intervention of the relay UE100-2.

In Step S109, the remote UE100-1transmits an RRC reestablishment completion message to the target gNB200-2in response to reception of the RRC reestablishment message from the target gNB200-2. Note that the RRC reestablishment completion message is transmitted from the remote UE100-1to the target gNB200-2without intervention of the relay UE100-2. In this manner, the RRC reestablishment processing is completed. After the completion of the RRC reestablishment processing, the target gNB200-2or the gNB200-1may notify the relay UE100-2of the fact that the RRC reconnection processing of the remote UE100-1has been completed (that is, transition to Uu connection has been made).

In the operations, the example has been described in which the ue-Identity (ReestabUE-Identity) corresponding to the predetermined identifier includes the C-RNTI (c-RNTI) allocated by the gNB200-1, the cell identifier (physCellId) acquired via the relay UE100-2, and the shortMAC-I.

However, the ue-Identity (ReestabUE-Identity) corresponding to the predetermined identifier may include Destination ID (Destination Layer-2 ID) allocated to the remote UE100-1instead of the C-RNTI. The Destination ID corresponds to a transmission destination identifier for identifying the transmission destination in the sidelink communication. The Destination ID may be an identifier allocated by an entity (ProSe function) of the core network.

In such a case, the remote UE100-1notifies the gNB200-1of the Destination ID of the remote UE100-1in advance prior to a start of the RRC reestablishment processing. For example, the remote UE100-1notifies the gNB200-1of the Destination ID when the RRC connection via the relay UE100-2is established (Step S103).

RRC Reestablishment Operation of Relay UE

Next, an RRC reestablishment operation of the relay UE100-2will be described.FIG.13is a diagram illustrating an example of the RRC reestablishment operation of the relay UE100-2. AlthoughFIG.13illustrates the example in which a plurality of remote UEs100-1(a remote UE100-1aand a remote UE100-1b) are connected to the relay UE100-2, the number of remote UEs100-1connected to the relay UE100-2may be one.

As illustrated inFIG.13, the relay UE100-2and the gNB200-1are in a state in which RRC connection has been established (Step S201), the remote UE100-1aand the relay UE100-2are in a state in which PC5 RRC connection has been established (Step S202), and the remote UE100-lband the relay UE100-2are in a state in which PC5 RRC connection has been established (Step S203). Also, as illustrated inFIG.13, the remote UE100-1aand the gNB200-1are in a state in which RRC connection has been established via the relay UE100-2(Step S204), and the remote UE100-lband the gNB200-1are in a state in which RRC connection has been established via the relay UE100-2(Step S205).

In Step S206, the gNB200-1manages an identifier of the relay UE100-2, context information of the relay UE100-2, and context information of the remote UE100-1in association with each other. The identifier of the relay UE100-2includes at least one selected from the group consisting of the C-RNTI, the Destination ID, and the shortMAC-I.

In Step S207, the relay UE100-2starts RRC reestablishment processing from the gNB200-1to the target gNB200-2in response to detection of occurrence of a communication failure (an RLF of Uu, for example).

In Step S208, the relay UE100-2transmits an RRC reestablishment request message including the identifier of the relay UE100-2to the target gNB200-2. The RRC reestablishment request message may include information indicating that the remote UE100-1is present under the relay UE100-2(that the UE100-2is a relay UE). The RRC reestablishment request message may include information indicating that the plurality of remote UEs100-1are present under the relay UE100-2and/or the number of remote UEs100-1under the relay UE100-2.

In Step S209, the target gNB200-2requests, by using the identifier of the relay UE100-2included in the RRC reestablishment request message, the gNB200-1to derive the context information of each of the remote UE100-1and the relay UE100-2. Specifically, the target gNB200-2transmits a context request message including the identifier of the relay UE100-2to the gNB200-1.

In Step S210, in response to reception of the context request message from the target gNB200-2, the gNB200-1derives the context information (the context information of the relay UE100-2and the context information of the remote UE100-1) corresponding to the identifier of the relay UE100-2included in the context request message and transmits a context response message including these pieces of context information to the target gNB200-2.

In Step S211, the target gNB200-2transmits an RRC reestablishment message to the relay UE100-2in response to reception of the context response message from the gNB200-1. The RRC reestablishment message includes predetermined information for the remote UE100-1. The predetermined information is, for example, NextHopChainingCount (NCC) of each remote UE100-1. The NCC is information used for decoding or the like of encrypted data. Moreover, the RRC reestablishment message may include an NCC for the relay UE100-2.

In Step S212, the relay UE100-2transfers the NCC for the remote UE100-1aincluded in the RRC reestablishment message to the remote UE100-1a. In Step S213, the relay UE100-2transfers the NCC for the remote UE100-lbincluded in the RRC reestablishment message to the remote UE100-1b. Such a transferring operation may be performed by a PC5 RRC message. The remote UE100-1may determine that RRC reestablishment has been performed (completed) in the Uu link of the relay UE100-2on the basis of the transferring operation of the relay UE100-2in Step S212. Note that Step S212is not limited to the transferring operation and may be a notification indicating that the RRC reestablishment processing has been performed (completed).

In Step S214, the relay UE100-2transmits an RRC reestablishment completion message to the target gNB200-2. In this manner, the RRC reestablishment processing of each UE100is collectively completed.

FIG.14is a diagram illustrating another example of the RRC reestablishment operation of the relay UE100-2.

As illustrated inFIG.14, operations in Steps S201to S210are the same as the operations illustrated inFIG.13.

In Step S311, the target gNB200-2transmits an RRC reestablishment message including the NCC for the remote UE100-1ato the remote UE100-1avia the relay UE100-2. In Step S312, the target gNB200-2transmits an RRC reestablishment message including the NCC for the remote UE100-lbto the remote UE100-1B via the relay UE100-2.

In Step S313, the remote UE100-1atransmits an RRC reestablishment completion message to the target gNB200-2. In Step S314, the remote UE100-1btransmits an RRC reestablishment completion message to the target gNB200-2. Note that one remote UE100-1may transmit the RRC reestablishment completion message to the target gNB200-2as a representative of the remote UE100-1aand the remote UE100-lb.

Reconnection Destination Selecting Operation in RRC Reestablishment Processing of Remote UE

Next, a reconnection destination (reestablishment destination) selecting operation in the RRC reestablishment processing of the remote UE100-1will be described.

Typically, once the UE100starts the RRC reestablishment processing, the UE100selects a reconnection destination cell by a cell selecting operation and transmits an RRC reestablishment request message to the selected cell. However, because it is desirable that the context information of the remote UE100-1stored in the gNB200-1can be used in the case in which the remote UE100-1starts the RRC reestablishment processing, it is assumed that the relay UE100-2to which the remote UE100-1has been connected most recently is selected as the reconnection destination.

In other words, this reconnection destination selecting operation includes a step in which the remote UE100-1selects a reconnection destination in accordance with a priority order in which the relay UE100-2has the highest priority order in a case in which RRC connection (PC5 RRC connection) between the remote UE100-1and the relay UE100-2has been released and a step in which the remote UE100-1attempts processing for reconnection to the selected reconnection destination. Here, the release of the PC5 RRC connection means that the PC5 RRC connection has been unintentionally released due to occurrence of a communication failure.

Such an operation may be performed only at the time of release of the PC5 RRC connection when the remote UE100-1includes RRC connection to the gNB200-1. In a case in which the remote UE100-1does not include RRC connection to the gNB200-1, this is because the context information of the remote UE100-1is not stored in the gNB200-1.

FIG.15is a diagram illustrating an example of the reconnection destination selecting operation in the RRC reestablishment processing of the remote UE100-1.

As illustrated inFIG.15, the remote UE100-1attempts reconnection to the relay UE100-2to which the remote UE100-1had been originally connected (Steps S402and S403) in a case in which the PC5 RRC connection is disconnected unexpectedly (with no request from the upper layer) (Step S401). In a case in which the reconnection has been successfully performed (Step S404: YES), the remote UE100-1reestablishes PC5 RRC connection with the relay UE100-2(Step S405). Here, the PC5 RRC connection may be reestablished with a PC5 RRC message (for example, an RRC Reconfiguration Sidelink message) transmitted by the remote UE100-1or the relay UE100-2.

In a case in which the reconnection has failed (Step S404: NO), the remote UE100-1selects another relay UE as a reconnection destination (Step S406) and attempts reconnection to another relay UE (Step S407). Because the remote UE100-1is assumed to have a poor reception status from the cell originally, the remote UE100-1places higher priority on another relay UE than on the gNB. Note that the case in which the reconnection has failed means a case in which connection is not recovered even after the remote UE100-1attempts, N times (N ≥ 1), the reconnection to the relay UE100-2to which the remote UE100-1has originally been connected. For example, a case in which connection is not recovered even after N transmission opportunities (irrespective of whether actual transmission has been performed) or after an RRC Reconfiguration Sidelink message is transmitted N times corresponds thereto. Here, the transmission of the RRC Reconfiguration Sidelink message may be limited. For example, a next RRC Reconfiguration Sidelink message may not be able to be transmitted unless a specific period of time (Prohibit timer) elapses from previous transmission of the RRC Reconfiguration Sidelink message.

In a case in which reconnection to another relay UE has been successfully achieved (Step S408: YES), the remote UE100-1establishes PC5 RRC connection with another relay UE (Step S409). A method for determining such a success and a failure is similar to that in Step S404. In Step S409, the remote UE100-1may transmit an RRC reestablishment request message to the gNB via another relay UE concerned. The gNB can thus acquire the context information of the remote UE100-1.

In a case in which the reconnection to another relay UE has failed (Step S408: NO), the remote UE100-1selects the gNB as a reconnection destination (Step S410) and attempts reconnection to the gNB (Step S411). In a case in which the reconnection to the gNB has been successfully achieved (Step S412: YES), RRC connection with the gNB is established (Step S413). In Step S413, the remote UE100-1transmits an RRC reestablishment request message directly to the gNB.

Note that although this flowchart assumes that a state of reception from the cell is originally poor in the remote UE100-1, the gNB may be selected with a higher priority than another relay UE in a case in which the state of reception from the cell is not poor.

Other Embodiments

Although operations of the relay UE100-2have mainly been described in the aforementioned embodiment, the operations according to the aforementioned embodiment may be applied to an Integrated Access and Backhaul (IAB) node that is a radio relay node. Specifically, the IAB node may perform the operations of the relay UE100-2described in the aforementioned embodiment. In such an embodiment, the “relay UE” in the aforementioned embodiment is read as the “IAB node” instead and the “sidelink” in the aforementioned embodiment is read as an “access link” instead. Also, the PC5 RRC connection is read as RRC connection with the IAB node or RRC connection with an IAB donor instead.

A program causing a computer to execute each of the processing operations performed by the UE100or the gNB200may be provided. The program may be recorded in a computer readable medium. Use of the computer readable medium enables the program to 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, a DVD-ROM, or the like.

In addition, circuits for executing the processing operations to be performed by the UE100or the gNB200may be integrated, and at least part of the UE100or the gNB200may be configured as a semiconductor integrated circuit (a chipset or an SoC).

Embodiments have been described above in detail with reference to the drawings, but specific configurations are not limited to those described above, and various design modifications can be made without departing from the gist of the present embodiment.