Communication method, network node, user equipment, chipset, program, and mobile communication system

Abstract

To provide a communication method, a network node, user equipment, a chipset, a program, and a mobile communication system relating to a multicast broadcast service (MBS).SOLUTION: In a mobile communication system that provides an MBS, a communication method executed by UE 100 includes steps of receiving a multicast session from a network in an RRC idle state or an RRC inactive state, measuring reception quality from the network in a multicast reception state in which the multicast session is being received, and, in response to the measured reception quality being worse than a threshold, performing a process to transition to an RRC connected state for multicast reception.SELECTED DRAWING: Figure 10

Description

[Technical field]

[0001] The present disclosure relates to a communication method and user equipment for use in a mobile communication system. [Background technology]

[0002] The 3GPP (3rd Generation Partnership Project) (registered trademark; the same applies below) standard specifies the technical specifications of NR (New Radio), a fifth-generation (5G) wireless access technology. Compared with LTE (Long Term Evolution), a fourth-generation (4G) wireless access technology, NR has features such as high speed, large capacity, high reliability, and low latency. Discussions are underway in 3GPP to formulate technical specifications for 5G / NR multicast broadcast service (MBS) (see, for example, Non-Patent Document 1). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] 3GPP contribution: RP-201038, “WID revision: NR Multicast and Broadcast Services” Summary of the Invention

[0004] A communication method according to a first aspect is a method executed by a user device in a mobile communication system providing a multicast broadcast service (MBS). The communication method includes the steps of receiving a multicast session from a network in an RRC idle state or an RRC inactive state, measuring a reception quality from the network in a multicast reception state in which the multicast session is being received, and, when the measured reception quality is worse than a threshold, performing a process of transitioning to an RRC connected state for multicast reception.

[0005] A user equipment according to a second aspect is an equipment used in a mobile communication system providing a multicast broadcast service (MBS). The user equipment includes a receiving unit that receives a multicast session from a network in an RRC idle state or an RRC inactive state, and a control unit that measures a reception quality from the network in a multicast reception state in which the multicast session is being received. The control unit performs a process of transitioning to an RRC connected state for multicast reception in response to the measured reception quality being worse than a threshold. [Brief description of the drawings]

[0006] [Figure 1] 1 is a diagram showing a configuration of a mobile communication system according to an embodiment. [Diagram 2] FIG. 2 is a diagram illustrating a configuration of a UE (user equipment) according to an embodiment. [Diagram 3] A diagram showing the configuration of a gNB (base station) according to an embodiment. [Figure 4] A diagram showing the configuration of a protocol stack of a wireless interface of a user plane that handles data. [Diagram 5] FIG. 2 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. 2 is a diagram showing an overview of MBS traffic distribution according to an embodiment. [Figure 7] FIG. 13 is a diagram illustrating a distribution mode according to the embodiment. [Figure 8] FIG. 2 is a diagram showing an example of internal processing related to MBS reception in a UE according to an embodiment. [Figure 9] FIG. 11 is a diagram showing another example of internal processing related to MBS reception of the UE according to the embodiment. [Figure 10] FIG. 2 is a diagram illustrating an example of the operation of the mobile communication system according to the embodiment. [Figure 11] FIG. 11 is a diagram showing a first modified example of the operation of the mobile communication system according to the embodiment. [Figure 12] FIG. 11 is a diagram showing a second modified example of the operation of the mobile communication system according to the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] It is expected that 5G / NR multicast broadcast services will provide improved services over 4G / LTE multicast broadcast services.

[0008] Therefore, an object of the present disclosure is to make it possible to realize an improved multicast broadcast service.

[0009] 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 numerals.

[0010] (Configuration of a mobile communication system) FIG. 1 is a diagram showing a configuration of a mobile communication system according to an embodiment. The mobile communication system 1 complies with the 3GPP standard 5th Generation System (5GS). In the following description, 5GS is taken as an example, but the mobile communication system may be at least partially applied to an LTE (Long Term Evolution) system. Also, the mobile communication system may be at least partially applied to a 6th Generation (6G) system.

[0011] The mobile communication system 1 includes a user equipment (UE) 100, a 5G radio access network (NG-RAN) 10, and a 5G core network (5GC) 20. Hereinafter, the NG-RAN 10 may be simply referred to as the RAN 10. The 5GC 20 may be simply referred to as the core network (CN) 20.

[0012] The UE 100 is a mobile wireless communication device. The UE 100 may be any device that is used by a user. For example, the UE 100 is 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).

[0013] The NG-RAN 10 includes a base station (called "gNB" in the 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 the 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, and the like. 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 for performing wireless communication with the UE 100. One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

[0014] In addition, gNBs can also be connected to the Evolved Packet Core (EPC), which is the core network of LTE. LTE base stations can also be connected to 5GC. LTE base stations and gNBs can also be connected via an inter-base station interface.

[0015] 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 Non-Access Stratum (NAS) signaling. The UPF controls data forwarding. The AMF and the UPF are connected to the gNB 200 via an NG interface, which is an interface between a base station and a core network.

[0016] 2 is a diagram showing a configuration of a UE 100 (user equipment) according to an embodiment. The UE 100 includes a receiving unit 110, a transmitting unit 120, and a control unit 130. The receiving unit 110 and the transmitting unit 120 configure a wireless communication unit that performs wireless communication with the gNB 200.

[0017] The receiving unit 110 performs various receptions 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.

[0018] The transmission unit 120 performs various transmissions under the control of the control unit 130. The transmission 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.

[0019] The control unit 130 performs various controls and processes in the UE 100. Such processes include processes of each layer described below. 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 the processes 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.

[0020] 3 is a diagram showing a configuration of a gNB 200 (base station) according to an embodiment. The gNB 200 includes a transmitter 210, a receiver 220, a controller 230, and a backhaul communication unit 240. The transmitter 210 and the receiver 220 constitute a wireless communication unit that performs wireless communication with the UE 100. The backhaul communication unit 240 constitutes a network communication unit that performs communication with the CN 20.

[0021] 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.

[0022] 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.

[0023] The control unit 230 performs various controls and processes in the gNB 200. Such processes include processes of each layer described below. 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 the processes 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.

[0024] The backhaul communication unit 240 is connected to adjacent base stations via an Xn interface, which is an interface between base stations. The backhaul communication unit 240 is connected to the AMF / UPF 300 via an NG interface, which is an interface between a base station and a core network. Note that the gNB 200 may be configured (i.e., functionally divided) with a CU (Central Unit) and a DU (Distributed Unit), and both units may be connected to each other via an F1 interface, which is a fronthaul interface.

[0025] FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data.

[0026] 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.

[0027] The PHY layer performs coding / 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. The PHY layer of the UE 100 receives downlink control information (DCI) transmitted from the gNB 200 on a physical downlink control channel (PDCCH). Specifically, the UE 100 performs blind decoding of the PDCCH using a radio network temporary identifier (RNTI) and acquires the successfully decoded DCI as DCI addressed to the UE. The DCI transmitted from the gNB 200 has CRC parity bits scrambled by the RNTI added thereto.

[0028] The MAC layer performs data priority control, retransmission processing using Hybrid Automatic Repeat reQuest (HARQ), random access procedures, etc. Data and control information are transmitted between the MAC layer of UE100 and the MAC layer of gNB200 via a transport channel. The MAC layer of gNB200 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 assigned to UE100.

[0029] The RLC layer transmits data to the RLC layer on the receiving side by using the functions of the MAC layer and the 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 a logical channel.

[0030] The PDCP layer performs header compression / decompression, encryption / decryption, etc.

[0031] The SDAP layer maps IP flows, which are units for QoS (Quality of Service) control by the core network, to radio bearers, which are units for QoS control by the AS (Access Stratum). Note that if the RAN is connected to the EPC, the SDAP is not necessary.

[0032] 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).

[0033] 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.

[0034] Between the RRC layer of the UE 100 and the RRC layer of the gNB 200, RRC signaling for various settings is transmitted. The RRC layer controls logical channels, transport channels, and physical channels in response to establishment, re-establishment, and release of radio bearers. When there is a connection (RRC connection) between the RRC of the UE 100 and the RRC of the gNB 200, the UE 100 is in an RRC connected state. When there is no connection (RRC connection) between the RRC of the UE 100 and the RRC of the gNB 200, the UE 100 is in an RRC idle state. When the connection between the RRC of the UE 100 and the RRC of the gNB 200 is suspended, the UE 100 is in an RRC inactive state.

[0035] The NAS layer, which is located above the RRC layer, performs session management, mobility management, and the like. NAS signaling is transmitted between the NAS layer of the UE 100 and the NAS layer of the AMF 300A. The UE 100 has an application layer and the like in addition to the radio interface protocol. The layer below the NAS layer is called the AS layer.

[0036] (Overview of MBS) An overview of the MBS according to the embodiment will be described. The MBS is a service that enables broadcast or multicast, i.e., point-to-multipoint (PTM) data transmission from the NG-RAN 10 to the UE 100. Possible use cases (service types) of the MBS include public safety communication, mission-critical communication, V2X (Vehicle to Everything) communication, IPv4 or IPv6 multicast distribution, IPTV (Internet protocol television), group communication, and software distribution.

[0037] The broadcast service provides a service to all UEs 100 within a specific service area for applications that do not require highly reliable QoS. An MBS session used for the broadcast service is called a broadcast session.

[0038] The multicast service provides services not to all UEs 100 but to a group of UEs 100 participating in the multicast service (multicast session). The MBS session used in the multicast service is called a multicast session. The multicast service allows the same content to be provided to a group of UEs 100 in a more wirelessly efficient manner than the broadcast service.

[0039] FIG. 6 is a diagram showing an overview of MBS traffic delivery according to the embodiment.

[0040] MBS traffic (MBS data) is distributed from a single data source (application service provider) to multiple UEs. A 5G core network, 5G CN (5GC) 20, receives the MBS data from the application service provider, creates a copy of the MBS data (replication), and distributes it.

[0041] From the 5GC20 perspective, two multicast delivery methods are possible: 5GC Shared MBS Traffic delivery and 5GC Individual MBS Traffic delivery.

[0042] In the 5GC individual MBS traffic delivery method, the 5GC 20 receives a single copy of MBS data packets and delivers individual copies of those MBS data packets to individual UEs 100 via a PDU session for each UE 100. Therefore, one PDU session for each UE 100 needs to be associated with the multicast session.

[0043] In the 5GC shared MBS traffic delivery method, the 5GC 20 receives a single copy of MBS data packets and delivers the single copy of those MBS packets to a RAN node (i.e., the gNB 200). The gNB 200 receives the MBS data packets via an MBS tunnel connection and delivers them to one or more UEs 100.

[0044] From the perspective of the RAN (5G RAN) 10, there are two possible delivery methods for transmitting MBS data over the air in the 5GC shared MBS traffic delivery method: PTP (Point-to-Point) and PTM (Point-to-Multipoint). PTP stands for unicast, and PTM stands for multicast and broadcast.

[0045] In the PTP distribution method, the gNB 200 distributes individual copies of the MBS data packets wirelessly to each UE 100. On the other hand, in the PTM distribution method, the gNB 200 distributes a single copy of the MBS data packet wirelessly to a group of UEs 100. The gNB 200 can dynamically determine whether to use PTM or PTP as the distribution method for MBS data to one UE 100.

[0046] The PTP distribution method and the PTM distribution method are mainly related to the user plane. There are two control modes for MBS data distribution: a first distribution mode and a second distribution mode.

[0047] FIG. 7 is a diagram showing a distribution mode according to the embodiment.

[0048] The first delivery mode (Delivery mode 1: DM1) is a delivery mode that can be used by the UE 100 in the RRC connected state, and is a delivery mode for high QoS requirements. The first delivery mode is used for a multicast session among MBS sessions. However, the first delivery mode may also be used for a broadcast session. The first delivery mode may also be available to the UE 100 in the RRC idle state or the RRC inactive state.

[0049] The setting of MBS reception in the first distribution mode is performed by UE-dedicated signaling. For example, the setting of MBS reception in the first distribution mode is performed by an RRC Reconfiguration message (or an RRC Release message), which is an RRC message transmitted by unicast from the gNB 200 to the UE 100.

[0050] The MBS reception configuration includes MBS traffic channel configuration information (hereinafter referred to as "MTCH configuration information") related to the configuration of an MBS traffic channel that transmits MBS data. The MTCH configuration information includes MBS session information (including an MBS session identifier described later) related to an MBS session, and scheduling information of the MBS traffic channel corresponding to this MBS session. The scheduling information of the MBS traffic channel may include a discontinuous reception (DRX) configuration of the MBS traffic channel. The discontinuous reception configuration may include one or more parameters of a timer value (On Duration Timer) that defines an on-duration (On Duration: reception period), a timer value (Inactivity Timer) that extends the on-duration, a scheduling interval or DRX cycle (Scheduling Period, DRX Cycle), an offset value of the start subframe of the scheduling or DRX cycle (Start Offset, DRX Cycle Offset), a start delay slot value of the on-duration timer (Slot Offset), a timer value (Retransmission Timer) that defines the maximum time until retransmission, and a timer value (HARQ RTT Timer) that defines the minimum interval until DL allocation of HARQ retransmission.

[0051] The MBS traffic channel is a type of logical channel and may be referred to as an MTCH. The MBS traffic channel is mapped to a Down Link-Shared CHannel (DL-SCH), which is a type of transport channel.

[0052] The second delivery mode (Delivery mode 2: DM2) is a delivery mode that can be used by UE 100 in the RRC connected state as well as UE 100 in the RRC idle state or RRC inactive state, and is a delivery mode for low QoS requirements. The second delivery mode is used for a broadcast session among MBS sessions. However, the second delivery mode may also be applicable to a multicast session.

[0053] The setting of MBS reception in the second distribution mode is performed by broadcast signaling. For example, the setting of MBS reception in the second distribution mode is performed by a logical channel broadcast from the gNB 200 to the UE 100, such as a broadcast control channel (BCCH) and / or a multicast control channel (MCCH). The UE 100 can receive the BCCH and MCCH using a dedicated RNTI predefined in the technical specifications, for example. The RNTI for BCCH reception may be SI-RNTI, and the RNTI for MCCH reception may be MCCH-RNTI.

[0054] In the second distribution mode, UE 100 may receive MBS data in the following three procedures. First, UE 100 receives MCCH setting information by a SIB (MBS SIB) transmitted on a BCCH from gNB 200. Second, UE 100 receives an MCCH from gNB 200 based on the MCCH setting information. The MCCH transmits MTCH setting information. Third, UE 100 receives an MTCH (MBS data) based on the MTCH setting information. Hereinafter, MTCH setting information and / or MCCH setting information may be referred to as MBS reception setting.

[0055] In the first distribution mode and the second distribution mode, the UE 100 may receive the MTCH using a group RNTI (G-RNTI) assigned by the gNB 200. The G-RNTI corresponds to an RNTI for MTCH reception. The G-RNTI may be included in the MBS reception setting (MTCH setting information).

[0056] The network can provide different MBS services for each MBS session. An MBS session is identified by at least one of a Temporary Mobile Group Identity (TMGI), a source-specific IP multicast address (consisting of a source unicast IP address of an application function or application server, etc., and an IP multicast address indicating a destination address), a session identifier, and a G-RNTI. At least one of the TMGI, the source-specific IP multicast address, and the session identifier is called an MBS session identifier. The TMGI, the source-specific IP multicast address, the session identifier, and the G-RNTI are collectively called MBS session information.

[0057] Fig. 8 is a diagram showing an example of an internal process related to MBS reception of the UE 100 according to the embodiment. Fig. 9 is a diagram showing another example of an internal process related to MBS reception of the UE 100 according to the embodiment.

[0058] One MBS Radio Bearer (MRB) is one radio bearer that transmits a multicast session or a broadcast session. That is, there are cases where a multicast session is associated with an MRB and cases where a broadcast session is associated with an MRB.

[0059] The MRB and corresponding logical channels (e.g., MTCH) are configured in the UE 100 from the gNB 200 by RRC signaling. The MRB configuration procedure may be separated from the data radio bearer (DRB) configuration procedure. In the RRC signaling, one MRB can be configured with "PTM only", "PTP only", or "both PTM and PTP". The type of such an MRB can be changed by RRC signaling.

[0060] In FIG. 8, an example is shown in which a multicast session and a dedicated traffic channel (DTCH) are associated with MRB#1, a multicast session and MTCH#1 are associated with MRB#2, and a broadcast session and MTCH#2 are associated with MRB#3. That is, MRB#1 is an MRB with PTP only, MRB#2 is an MRB with PTM only, and MRB#3 is an MRB with PTM only. Note that DTCH is scheduled using the cell RNTI (C-RNTI). MTCH is scheduled using the G-RNTI.

[0061] The PHY layer of the UE 100 processes user data (received data) received on a PDSCH, which is one of the physical channels, and transmits the data to a downlink shared channel (DL-SCH), which is one of the transport channels. The MAC layer (MAC entity) of the UE 100 processes data received on the DL-SCH, and transmits the received data to a corresponding logical channel (corresponding RLC entity) based on a logical channel identifier (LCID) included in a header (MAC header) included in the received data.

[0062] In FIG. 9, an example is shown in which a DTCH and an MTCH are associated with an MRB associated with a multicast session. Specifically, one MRB is split into two legs, one leg is associated with a DTCH, and the other leg is associated with an MTCH. The two legs are combined in the PDCP layer (PDCP entity). That is, the MRB is an MRB for both PTM and PTP. Such an MRB is sometimes called a split MRB.

[0063] (Mobile communication system operation) The operation of the mobile communication system 1 according to the embodiment will be described. In the following, a case where multicast distribution is performed in the above-mentioned first distribution mode (DM1) is mainly assumed. As described above, the first distribution mode is a distribution mode that can be used by the UE 100 in the RRC connected state, and is a distribution mode for high QoS requirements.

[0064] In the embodiment, the UE 100 in the RRC idle state or the RRC inactive state receives a multicast session (i.e., multicast reception). For example, the gNB 200 starts multicast distribution to the UE 100 in the RRC connected state, and then continues the multicast distribution while transitioning the UE 100 to the RRC idle state or the RRC inactive state in response to an increase in the load of the gNB 200. After starting multicast reception in the RRC connected state, the UE 100 continues the multicast reception in the RRC idle state or the RRC inactive state.

[0065] Since the first delivery mode (DM1) may be required to guarantee high QoS requirements, it may be required to guarantee high QoS requirements for the UE 100 in the RRC idle state or the RRC inactive state. Here, if the UE 100 is kept in the RRC connected state, the QoS is satisfied, but the network side consumes more resources and the UE 100 consumes more power. By transitioning the UE 100 to the RRC idle state or the RRC inactive state, such a problem is solved, but there is a possibility that the QoS is not satisfied. Therefore, in the embodiment, the UE 100 that performs multicast reception in the RRC idle state or the RRC inactive state can transition to the RRC connected state depending on the deterioration of the reception quality of the multicast reception.

[0066] First, the UE 100 according to the embodiment receives a multicast session from a network (gNB 200) in an RRC idle state or an RRC inactive state. Second, the UE 100 measures the reception quality from the network (hereinafter also referred to as "multicast reception quality") in a multicast reception state in which the multicast session is being received. Third, the UE 100 performs a process of transitioning to an RRC connected state for multicast reception in response to the measured reception quality being worse than a threshold. As a result, by transitioning to an RRC connected state and continuing multicast reception, it becomes easier to guarantee the QoS required for multicast reception.

[0067] When the multicast reception quality is better than a threshold, the UE 100 does not transition to the RRC connected state for multicast reception. That is, when the multicast reception quality is better than a threshold, the UE 100 is prohibited from transitioning to the RRC connected state for multicast reception. This allows the UE 100 to maintain the RRC idle state or the RRC inactive state, thereby reducing the load on the network side.

[0068] The UE 100 may receive configuration information for setting a threshold from the network (gNB 200). The UE 100 may compare the measured reception quality with the threshold set according to the configuration information. In this manner, the threshold at which the UE 100 starts the process of transitioning to the RRC connected state may be set in the UE 100 from the network (gNB 200). This allows the UE 100 to transition to the RRC connected state under the management of the network.

[0069] FIG. 10 is a diagram showing an example of the operation of the mobile communication system 1 according to the embodiment.

[0070] In step S101, the UE 100 is in an RRC connected state in a cell of the gNB 200. The UE 100 is assumed to have an interest in a certain multicast session (hereinafter, referred to as a "target multicast session"). "Interested in a multicast session" may mean that an upper layer of the UE 100 requests or desires to receive the multicast session. Here, the upper layer includes a NAS layer. The upper layer may further include an application. The UE 100 (NAS entity) performs a multicast session join procedure for joining the target multicast session to the network. For example, the UE 100 joins the target multicast session by sending a first NAS message to the AMF 300A requesting to join the target multicast session and receiving a second NAS message from the AMF 300A approving the participation in the target multicast session. For example, the first NAS message is a PDU Session Modification Request, and the message may include an MBS session identifier and information of the join request. In the case where the approval of the first NAS message is implicitly notified by the MRB setting from the gNB 200, the second NAS message may be omitted. Also, "joining the target multicast session" may mean registering the UE 100 in the CN device as a member of a UE group (multicast group) that receives the multicast session. The CN device may authenticate the UE 100 at the time of the registration. Also, the CN device may give the UE 100 permission to receive the multicast session. Note that the participation in the multicast session may be performed when the multicast session is in a valid state (transmitting) or in an invalid state (waiting for transmission to start or transmission suspended).

[0071] In step S102, the UE 100 in the RRC connected state starts receiving the target multicast session. Prior to this multicast reception, the gNB 200 may perform multicast reception setting for receiving the target multicast session on the UE 100. The multicast reception setting may be performed by, for example, an RRC Reconfiguration message including an MRB setting.

[0072] In step S103, the UE 100 in the RRC connected state receives the target multicast session. Specifically, the UE 100 receives, from the gNB 200, MBS data belonging to the target multicast session.

[0073] In step S104, the gNB 200 determines to transition the UE 100 to an RRC idle state or an RRC inactive state, and transmits an RRC Release message to the UE 100 to transition the UE 100 to an RRC idle state or an RRC inactive state. The gNB 200 may determine to transition the UE 100 to an RRC idle state or an RRC inactive state in response to an increase in its own load (e.g., resource load). By transitioning the UE 100 to an RRC idle state or an RRC inactive state, the load of the gNB 200 is reduced.

[0074] In an embodiment, the gNB 200 transmits an RRC Release message including configuration information for setting the above-mentioned threshold to the UE 100. The threshold may be a radio quality threshold, for example, a Reference Signal Received Power (RSRP) threshold, a Reference Signal Received Quality (RSRQ) threshold, and / or a Signal-to-Interference-plus-Noise Ratio (SINR) threshold. The threshold may be a demodulation quality threshold, for example, a Bit Error Rate (BER) threshold, a Block Error Rate (BLER) threshold, and / or a Packet Error Rate (PER) threshold. The threshold may be set for each multicast session. That is, the threshold may be set in association with an MRB setting (MRB ID) or a multicast session ID (TMGI). Thus, when the UE 100 receives multiple multicast sessions, the UE 100 may hold multiple thresholds corresponding to the multiple multicast sessions. The gNB 200 may also transmit an RRC Release message including configuration information for setting a duration (first predetermined time) of the threshold condition to the UE 100. By including these setting information in the RRC Release message, it is possible to efficiently transmit UE-specific setting information to the UE 100. Note that such settings may be transmitted in the RRC Reconfiguration in step S102. When the settings are included in the RRC Reconfiguration, the UE 100 may determine that the multicast session can be received (reception is permitted) in the RRC idle or inactive state.

[0075] In step S105, the UE 100 that has received the RRC Release message transitions to an RRC idle state or an RRC inactive state. The UE 100 continues multicast reception in the RRC idle state or the RRC inactive state by using the multicast reception configuration that was set in the RRC connected state.

[0076] In step S107, the UE 100 measures a multicast reception quality corresponding to the threshold value set by the gNB 200. For example, the UE 100 may measure a radio quality, such as an RSRP, an RSRQ, and / or an SINR. The UE 100 may measure a demodulation quality, such as a BER, a BLER, and / or a PER.

[0077] In step S108, the UE 100 compares the multicast reception quality measured in step S107 with a threshold value set by the gNB 200. If the measured multicast reception quality is better than the threshold value (step S108: NO), the UE 100 returns the process to step S107. In this case, the UE 100 performs multicast reception while maintaining the RRC idle state or the RRC inactive state.

[0078] On the other hand, if the measured multicast reception quality is worse than the threshold (step S108: YES), in step S109, UE100 starts a process of transitioning to an RRC connected state, i.e., a random access procedure for gNB200.

[0079] In addition, the case where the multicast reception quality is worse than the threshold may be when the wireless quality (RSRP, RSRQ, and / or SINR) is lower than a threshold, or when the error rate (BER, BLER, and / or PER) corresponding to the demodulation quality is higher than a threshold.

[0080] In step S108, UE 100 may determine whether or not a state in which multicast reception quality is worse than a threshold has continued for a first predetermined time. The first predetermined time may be set in UE 100 by configuration information from gNB 200. Then, UE 100 may initiate a process of transitioning to an RRC connected state, that is, a random access procedure for gNB 200, when a state in which multicast reception quality is worse than a threshold has continued for the first predetermined time (step S109). This makes it easier to eliminate the effect of momentary deterioration of multicast reception quality.

[0081] In step S109, the UE 100 performs a random access procedure. Here, the AS layer of the UE 100 may start RRC Resume when in an RRC inactive state. On the other hand, when in an RRC idle state, the AS layer of the UE 100 may start RRC Setup. The AS layer of the UE 100 may notify the NAS layer that a threshold condition is satisfied. The NAS layer may instruct the AS layer to transition to an RRC connected state based on the notification.

[0082] The UE 100 transitions to the RRC connected state by performing the random access procedure in step S109. The UE 100 that transitioned to the RRC connected state continues multicast reception. In the RRC connected state, the gNB 200 can perform individual scheduling for the UE 100, so that high-quality services can be provided to the UE 100, but the load on the gNB 200 increases.

[0083] In addition, when a predetermined event other than multicast reception occurs during a period when UE 100 is in the RRC idle state or the RRC inactive state, UE 100 may be permitted to transition to the RRC connected state regardless of the above. The predetermined event may be reception of paging from the network, i.e., occurrence of MT (Mobile Terminated) traffic. The predetermined event may be necessity of uplink transmission, i.e., occurrence of MO (Mobile Originated) traffic.

[0084] In the above embodiment, an example in which the threshold is set from the gNB 200 to the UE 100 has been described, but the threshold may be preset in the UE 100. The UE 100 may hold a preset threshold for each QoS level (5QI). The UE 100 may compare the multicast reception quality with a preset threshold corresponding to the QoS level of the target multicast session.

[0085] (First modified example) As shown in FIG. 11, the gNB 200 may include the above-mentioned configuration information in a system information block (SIB) and broadcast it (step S106). The UE 100 in the RRC idle state or the RRC inactive state may receive the SIB including the configuration information from the gNB 200 and apply the threshold (and the first predetermined time) according to the received configuration information. Such an SIB may include individual configuration information for each bearer (MRB) (or each service) or for each QoS level (5QI). Also, such an SIB may include configuration information common to all bearers.

[0086] For example, the SIB may include a plurality of sets of bearer identifiers or 5QIs and configuration information associated with the bearer identifiers or 5QIs. The UE 100 that receives the SIB may obtain configuration information corresponding to the bearer or QoS level of the target multicast session from the SIB, and apply the threshold (and the first predetermined time) according to the obtained configuration information.

[0087] (Second modified example) As shown in FIG. 12, after transitioning to the RRC idle state or the RRC inactive state, the UE 100 may be prohibited from transitioning to the RRC connected state for multicast reception until a predetermined time (second predetermined time) has elapsed. This can prevent the UE 100 from transitioning to the RRC connected state immediately after transitioning to the RRC idle state or the RRC inactive state. The second predetermined time may be set by the gNB 200 to the UE 100 via an RRC Release message or SIB. The RRC Release message or SIB may include setting information for setting the second predetermined time. This allows the gNB 200 to set an appropriate second predetermined time depending on, for example, its own load situation.

[0088] In step S201, the UE 100 that has transitioned to the RRC idle state or the RRC inactive state starts a timer that is set to a second predetermined time (timer value). That is, the UE 100 measures the elapsed time from the transition to the RRC idle state or the RRC inactive state. The second predetermined time (timer value) is set by the gNB 200, for example, and may be specified by seconds, subframes, radio frames, or time.

[0089] The UE 100 avoids performing a process (random access procedure) of transitioning to the RRC connected state for multicast reception until the elapsed time from transitioning to the RRC idle state or the RRC inactive state exceeds a second predetermined time. Note that, when a predetermined event other than multicast reception occurs, the UE 100 may be permitted to transition to the RRC connected state regardless of the above. The predetermined event may be reception of paging from the network, i.e., occurrence of MT (Mobile Terminated) traffic. The predetermined event may be necessity of uplink transmission, i.e., occurrence of MO (Mobile Originated) traffic.

[0090] After determining that the reception quality is worse than the threshold (step S108: YES), in step S202, the UE 100 determines whether the timer started in step S201 has expired, that is, whether the elapsed time since transition to the RRC idle state or the RRC inactive state has exceeded a second predetermined time. If the timer has not expired (step S202: NO), the UE 100 returns the process to step S107. On the other hand, if the timer has expired (step S202: YES), in step S109, the UE 100 starts a process of transitioning to the RRC connected state, that is, a random access procedure for the gNB 200. In this way, the UE 100 performs a process of transitioning to the RRC connected state in response to the elapsed time since transition to the RRC idle state or the RRC inactive state exceeding the second predetermined time and the measured reception quality (multicast reception quality) being worse than the threshold.

[0091] (Other embodiments) The above-mentioned operational flows are not limited to being implemented separately and independently, but can be implemented by combining two or more operational flows. For example, some steps of one operational flow may be added to another operational flow, or some steps of one operational flow may be replaced with some steps of another operational flow.

[0092] In the above-mentioned embodiment and example, an example in which the base station is an NR base station (gNB) has been described, but the base station may be an LTE base station (eNB) or a 6G base station. The base station may be a relay node such as an IAB (Integrated Access and Backhaul) node. The base station may be a DU of the IAB node. The UE 100 may be an MT (Mobile Termination) of the IAB node.

[0093] A program may be provided that causes a computer to execute each process performed by UE100 or gNB200. The program may be recorded in a computer-readable medium. Using the computer-readable medium, it is possible to install the program in a computer. Here, the computer-readable medium on which the program is recorded may be a non-transient recording medium. The non-transient recording medium is not particularly limited, and may be, for example, a recording medium such as a CD-ROM or a DVD-ROM. In addition, a circuit that executes 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 (chip set, SoC: System on a chip).

[0094] As used in this disclosure, the terms "based on" and "depending on" do not mean "based only on" or "depending only on," unless otherwise specified. The term "based on" means both "based only on" and "based at least in part on." Similarly, the term "depending on" means both "based only on" and "based at least in part on." Additionally, "obtain / acquire" may mean obtaining information from stored information, from information received from other nodes, or by generating information. The terms "include," "comprise," and variations thereof do not mean including only the items listed, but may include only the items listed, or may include additional items in addition to the items listed. Additionally, the term "or" as used in this disclosure is not intended to be an exclusive or. Additionally, any reference to elements using designations such as "first," "second," etc., as used in this disclosure is not intended to generally limit the quantity or order of those elements. These designations may be used herein as a convenient method of distinguishing between two or more elements. Thus, reference to a first and a second element does not imply that only two elements may be employed therein or that the first element must precede the second element in some manner. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles are intended to include the plural unless the context clearly indicates otherwise.

[0095] 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, etc. are possible without departing from the spirit of the invention.

[0096] This application claims priority to Japanese Patent Application No. 2022-005614 (filed January 18, 2022), the entire contents of which are incorporated herein by reference. [Explanation of symbols]

[0097] 1: Mobile communication systems 10:RAN 20 :CN 100:UE 110: Receiving unit 120: Transmitter 130: Control section 200: gNB 210: Transmission unit 220: Receiving unit 230: Control section 240: Backhaul communication unit

Claims

1. A communication method in a mobile communication system providing a multicast broadcast service (MBS), comprising: a network node transmitting to a user equipment a message including a threshold value and information associating said threshold value with a multicast session; The network node transmits the multicast session to the user equipment in an RRC inactive state; The threshold value is used in a process for transitioning from the RRC inactive state to an RRC connected state in the user equipment receiving the multicast session. Communication methods.

2. The message is an RRC Release message for transitioning the user equipment from the RRC connected state to the RRC inactive state. The communication method according to claim 1 .

3. A network node for use in a mobile communication system providing a multicast broadcast service (MBS), comprising: A transmission unit that transmits a message including a threshold value and information associating the threshold value with a multicast session to a user device, The transmission unit transmits the multicast session to the user equipment in an RRC inactive state; The threshold value is used in a process for transitioning from the RRC inactive state to an RRC connected state in the user equipment receiving the multicast session. Network node.

4. A user equipment for use in a mobile communication system providing a multicast broadcast service (MBS), comprising: a receiving unit configured to receive a message including a threshold value and information associating the threshold value with a multicast session from a network node; The receiver receives the multicast session from the network node in an RRC inactive state; The threshold value is used in a process for transitioning from the RRC inactive state to an RRC connected state in the user equipment receiving the multicast session. User equipment.

5. A chipset for a user equipment for use in a mobile communication system providing a multicast broadcast service (MBS), comprising: receiving a message from a network node, the message including a threshold and information associating the threshold with a multicast session; receiving the multicast session from the network node in an RRC inactive state; The threshold value is used in a process for transitioning from the RRC inactive state to the RRC connected state in the user equipment receiving the multicast session. Chipset.

6. A user device for use in a mobile communication system providing a multicast broadcast service (MBS), receiving a message from a network node, the message including a threshold and information associating the threshold with a multicast session; receiving the multicast session from the network node in an RRC inactive state; The threshold value is used in a process for transitioning from the RRC inactive state to the RRC connected state in the user equipment receiving the multicast session. program.

7. A mobile communication system providing a multicast broadcast service (MBS), comprising: A network node; A user device, The network node: transmitting a message to the user device, the message including a threshold value and information associating the threshold value with a multicast session; Transmitting the multicast session to the user equipment in an RRC inactive state; The threshold value is used in a process for transitioning from the RRC inactive state to the RRC connected state in the user equipment receiving the multicast session. Mobile communication system.

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