Communication method, user equipment, and network node

Abstract

A communication method used in a mobile communication system for providing a multicast broadcast service (MBS) includes: a user device that receives a multicast session from a network node in a radio resource control (RRC) inactive state determining that the result of quality measurement of reception from the network node is worse than a threshold value; the user device starting an RRC connection resume procedure for the network node in response to the determination; and the user device transmitting a message including notification information on the result of reception quality measurement to the network node in the RRC connection resume procedure.

Description

COMMUNICATION METHOD, USER EQUIPMENT, AND NETWORK NODE

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

[0002] The 3rd Generation Partnership Project (3GPP) (registered trademark; the same applies hereinafter) defines the technical specifications for NR (New Radio), a fifth-generation (5G) wireless access technology. Compared to 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. 3GPP also defines the technical specifications for 5G / NR multicast / broadcast services (MBS).

[0003] In 3GPP Release 17, reception of MBS multicast (i.e., multicast reception) is possible only for user equipment in a radio resource control (RRC) connected state (see, for example, Non-Patent Document 1). In contrast, in 3GPP Release 18, the technical specifications are planned to be extended so that user equipment in an RRC inactive state can perform multicast reception.

[0004] 3GPP Technical Specification: TS 38.300 V17.5.0

[0005] A communication method according to a first aspect is a communication method used in a mobile communication system that provides a multicast broadcast service (MBS), and includes the steps of: a user equipment receiving a multicast session from a network node in a radio resource control (RRC) inactive state determining that a measurement result of reception quality from the network node is worse than a threshold; the user equipment initiating an RRC connection resume procedure with the network node in response to the determination; and the user equipment transmitting a message to the network node during the RRC connection resume procedure, the message including notification information regarding the measurement result of the reception quality.

[0006] A user equipment according to a second aspect is a user equipment used in a mobile communication system that provides a multicast broadcast service (MBS), and comprises: a control unit that, when receiving a multicast session from a network node in a radio resource control (RRC) inactive state, determines that a measurement result of reception quality from the network node is worse than a threshold, and initiates an RRC connection resume procedure with the network node in response to the determination; and a transmission unit that, during the RRC connection resume procedure, transmits a message to the network node that includes notification information regarding the measurement result of the reception quality.

[0007] A network node according to a third aspect is a network node used in a mobile communication system that provides a multicast broadcast service (MBS), and includes a receiving unit that receives an RRC connection resume procedure message from a user equipment that receives a multicast session from a network node in a radio resource control (RRC) inactive state in response to a measurement result of reception quality from the network node being worse than a threshold, and the message includes notification information regarding the measurement result of the reception quality.

[0008] 1 is a diagram illustrating an example of the configuration of a mobile communication system according to an embodiment. FIG. 2 is a diagram illustrating an example of the configuration of a UE (user equipment) according to an embodiment. FIG. 3 is a diagram illustrating an example of the configuration of a gNB (network node) according to an embodiment. FIG. 4 is a diagram illustrating a protocol stack configuration of a radio interface of a user plane that handles data. FIG. 5 is a diagram illustrating a protocol stack configuration of a radio interface of a control plane that handles signaling (control signals). FIG. 6 is a diagram illustrating an example of a ping-pong phenomenon. FIG. 7 is a diagram illustrating another example of the ping-pong phenomenon. FIG. 8 is a diagram illustrating an overview of the operation of a UE according to an embodiment. FIG. 9 is a diagram illustrating an example of a first operation pattern of the mobile communication system according to an embodiment. FIG. 10 is a diagram illustrating an example of a second operation pattern of the mobile communication system according to an embodiment.

[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] (1) System Configuration Example Fig. 1 is a diagram showing a configuration example of a mobile communication system 1 according to an embodiment. The mobile communication system 1 conforms to the 3GPP standard 5th Generation System (5GS). Although 5GS will be described below as an example, the mobile communication system may be at least partially based on an LTE (Long Term Evolution) system. The mobile communication system may be at least partially based on a 6th Generation (6G) system.

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

[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 may be 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 (referred to as "gNB" in the 5G system) 200, which is a type of network node. 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, etc. The term "cell" is used to indicate the smallest unit of a wireless communication area. The term "cell" is also used to indicate a function or resource that performs wireless communication with 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 AMF (Access and Mobility Management Function) and a UPF (User Plane Function) 300. The AMF performs various mobility controls for the UE 100. The AMF manages the mobility of the UE 100 by communicating with the UE 100 using NAS (Non-Access Stratum) signaling. The UPF controls data forwarding. The AMF and 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 illustrating an example 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 types of reception under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 130.

[0018] The transmitting unit 120 performs various transmissions under the control of the control unit 130. The transmitting unit 120 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 130 into a radio signal and transmits it from the antenna.

[0019] The control unit 130 performs various controls and processes in the UE 100. Such processes include processes of each layer described below. The operations of the UE 100 described above and below may be operations under the control of the control unit 230. 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 processing by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0020] 3 is a diagram showing an example configuration of a gNB 200 (network node) according to an embodiment. The gNB 200 has 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 communicates with the CN 20.

[0021] The transmitting unit 210 performs various transmissions under the control of the control unit 230. The transmitting 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 for each layer described below. The operations of the gNB 200 described above and below may be operations under the control of the control unit 230. 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 processing by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[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 is composed of a CU (Central Unit) and a DU (Distributed Unit) (i.e., functionally divided), and the two units may be connected by 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 encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layer of UE100 and the PHY layer of gNB200 via a physical channel. The PHY layer of UE100 receives downlink control information (DCI) transmitted from gNB200 on a physical downlink control channel (PDCCH). Specifically, UE100 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 gNB200 has a CRC parity bit scrambled by the RNTI added.

[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 the UE 100 and the MAC layer of the gNB 200 via a transport channel. The MAC layer of the gNB 200 includes a scheduler. The scheduler determines the uplink and downlink transport format (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be allocated to the UE 100.

[0029] The RLC layer transmits data to the receiving RLC layer using the functions of the MAC layer and PHY layer. Data and control information are transmitted between the RLC layer of the UE 100 and the RLC layer of the gNB 200 via a logical channel.

[0030] The PDCP layer performs header compression / decompression, encryption / decryption, and the like.

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

[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 an RRC (Radio Resource Control) layer and an NAS (Non-Access Stratum) layer instead of the SDAP layer shown in FIG.

[0034] RRC signaling for various settings is transmitted between the RRC layer of UE100 and the RRC layer of gNB200. The RRC layer controls logical channels, transport channels, and physical channels according to the establishment, re-establishment, and release of radio bearers. When there is a connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC connected state. When there is no connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC idle state. When the connection between the RRC of UE100 and the RRC of gNB200 is suspended, UE100 is in an RRC inactive state.

[0035] The NAS layer (also simply referred to as "NAS") located above the RRC layer performs session management, mobility management, etc. NAS signaling is transmitted between the NAS layer of the UE 100 and the NAS layer of the AMF 300A. Note that the UE 100 has an application layer and the like in addition to the radio interface protocol. Also, a layer lower than the NAS layer is referred to as the AS layer (also simply referred to as "AS").

[0036] (2) Overview of MBS The mobile communication system 1 can perform resource-efficient distribution using multicast / broadcast services (MBS).

[0037] (2.1) MBS Broadcast In the case of a broadcast communication service (also referred to as "MBS Broadcast"), the same service and the same specific content data are simultaneously provided to all UEs 100 in a geographical area. That is, all UEs 100 within the broadcast service area are permitted to receive the data. The broadcast communication service is delivered to the UEs 100 using a broadcast session, which is a type of MBS session. The UEs 100 can receive the broadcast session in any of the following states: RRC idle state, RRC inactive state, and RRC connected state.

[0038] Point-to-Multipoint (PTM) delivery is applied to broadcast communication services. In the case of PTM transmission, the gNB 200 delivers a single copy of an MBS packet to a set (group) of multiple UEs 100. For example, the gNB 200 schedules a group-common PDSCH scrambled by a G-RNTI (Group RNTI), which is a group-common RNTI, using a group-common PDCCH having a CRC (Cyclic Redundancy Code) scrambled by the G-RNTI.

[0039] In the case of a broadcast communication service, the UE 100 receives a broadcast session in the following procedure. First, the UE 100 receives a system information block type 20 (SIB20) from the gNB 200. The SIB20 includes a configuration of a multicast control channel (MCCH), which is a type of logical channel. Second, the UE 100 receives the MCCH from the gNB 200 based on the SIB20. The MCCH includes a PTM configuration. The PTM configuration transmits a configuration (MTCH configuration) for a multicast traffic channel (MTCH), which is a type of logical channel, and a broadcast MRB configuration, which is a multicast radio bearer (MRB) for the broadcast session. The information transmitted by the MCCH is sometimes referred to as MBS broadcast control information. Third, the UE 100 receives the MTCH based on the MCCH. The MTCH transmits the broadcast session (specifically, MBS data belonging to the broadcast session).

[0040] The MCCH is a PTM downlink channel for transmitting MBS broadcast control information associated with one or more MTCHs from the network 5 to the UE 100. The MTCH is a PTM downlink channel for transmitting MBS data of either a multicast session or a broadcast session from the network 5 to the UE 100.

[0041] (2.2) MBS Multicast In the case of a multicast communication service (also referred to as "MBS multicast"), the same service and the same specific content data are simultaneously provided to a specific set of UEs. That is, not all UEs 100 within a multicast service area are permitted to receive the data. The multicast communication service is delivered to the UEs 100 using a multicast session, which is a type of MBS session.

[0042] The UE 100 can receive the multicast session only after joining the multicast session (session join). Joining the multicast session may mean being registered in the network 5 (CN 20) as the UE 100 that can receive the multicast session.

[0043] In the case of a multicast communication service, only UEs 100 in an RRC connected state can receive a multicast session in 3GPP Release 17. On the other hand, in 3GPP Release 18, the standard is extended so that UEs 100 in an RRC inactive state can also receive a multicast session.

[0044] (2.2.1) Multicast reception in RRC connected state UE100 in the RRC connected state can receive a multicast session (specifically, MBS data belonging to the multicast session) using mechanisms such as PTP (Point-to-Point) and / or PTM (Point-to-Multipoint) delivery.

[0045] In the case of a multicast communication service, the UE 100 in the RRC connected state receives a multicast session in the following procedure. First, the UE 100 receives an RRC Reconfiguration message from the gNB 200. The RRC Reconfiguration message is a message transmitted on a dedicated control channel (DCCH). The RRC Reconfiguration message transmits a setting (MTCH setting) for the MTCH for receiving the multicast session and a setting of a multicast MRB, which is an MRB for the multicast session. Second, the UE 100 receives the MTCH based on the RRC Reconfiguration message. The MTCH transmits the multicast session (specifically, MBS data belonging to the multicast session).

[0046] (2.2.2) Multicast Reception in RRC Inactive State The UE 100 in the RRC inactive state can receive a multicast session (specifically, MBS data belonging to the multicast session) using the PTM distribution mechanism.

[0047] In the case of a multicast communication service, the UE 100 in the RRC inactive state can receive a multicast session in the following procedure. First, the UE 100 in the RRC inactive state receives a newly introduced system information block (also referred to as "SIBx") from the gNB 200. The SIBx includes a configuration of a newly introduced MCCH (also referred to as "multicast MCCH"). Second, the UE 100 in the RRC inactive state receives a multicast MCCH from the gNB 200 based on the SIBx. The multicast MCCH includes a PTM configuration. The PTM configuration transmits a configuration for an MTCH for receiving a multicast session (MTCH configuration) and a configuration of a multicast inactive MRB, which is an MRB for receiving a multicast session in the RRC inactive state (multicast inactive MRB configuration). The MTCH configuration may be included in the multicast inactive MRB configuration. Third, the UE 100 in the RRC inactive state receives the MTCH based on the multicast MCCH. The MTCH transmits the multicast session, specifically, the MBS data (i.e., multicast data) belonging to the multicast session.

[0048] When gNB200 configures UE100 to receive a multicast session in an RRC inactive state, it can send a PTM setting (multicast inactive MRB setting) to UE100 using an RRC Release message including a suspend setting. In this case, when UE100 receives an RRC Release message including a PTM setting from gNB200, it transitions to an RRC inactive state and receives the multicast session (multicast reception) in the RRC inactive state.

[0049] (2.2.3) Group Notification When there is temporarily no data to transmit to UE 100 in an active multicast session, gNB 200 may transition UE 100 to an RRC inactive state. When the multicast session is deactivated, gNB 200 may transition UE 100 to an RRC idle state or an RRC inactive state.

[0050] A gNB 200 that supports MBS uses a group notification mechanism to notify UEs 100 in an RRC idle state or an RRC inactive state when a multicast session is activated by CN 20. For example, a gNB 200 that supports MBS may use a group notification mechanism to notify UEs 100 in an RRC inactive state when a multicast session has been activated and there is multicast session data to be distributed in the gNB 200.

[0051] Upon receiving the group notification, the UE 100 reconnects or resumes connection to the network 5 and transitions to the RRC connected state. The group notification is processed in the paging RNTI (P-RNTI) on the PDCCH, and the paging channel is monitored by the UE 100.

[0052] The group notification paging message includes a session identifier (MBS Session ID) that is used to page all UEs 100 in RRC idle and RRC inactive states that have joined the associated MBS multicast session, i.e., the UEs 100 are not paged individually.

[0053] When UE 100 transitions to the RRC connected state, UE 100 may stop monitoring group notifications related to a particular multicast session. That is, UE 100 stops checking the MBS session ID in paging messages. UE 100 does not monitor group notifications in cases where UE 100 leaves this multicast session, network 5 requests UE 100 to leave, or network 5 releases the multicast session.

[0054] The group notification may be performed using the MCCH or may be performed using an MCCH Change Notification. When the MCCH is used, the determination may be made based on whether or not an MTCH configuration for the MBS session of interest exists in the MCCH. When the MCCH Change Notification is used, the group notification may be notified in a predetermined bit of the DCI.

[0055] (3) Operation of the Mobile Communication System The operation of the mobile communication system 1 according to the embodiment will be described.

[0056] (3.1) Operation Overview In the embodiment, a scenario is assumed in which UE 100 in an RRC inactive state starts an RRC connection resume (also referred to as "RRC resume") in relation to a multicast session (multicast reception). Here, the case in which UE 100 in an RRC inactive state starts an RRC connection resume in relation to a multicast session is when the reception quality during multicast reception at UE 100 is poor, and / or when UE 100 needs to acquire a multicast reception setting (PTM setting) from gNB 200. In the embodiment, UE 100 starts the RRC connection resume in response to poor reception quality during multicast reception at UE 100 in an RRC inactive state.

[0057] A case where the reception quality during multicast reception at UE 100 is poor is a case where the reference signal received power (RSRP) measured by UE 100 for the serving cell is below the RSRP threshold set in UE 100 by the network 5, and / or a case where the reference signal received quality (RSRQ) measured by UE 100 for the serving cell is below the RSRQ threshold set in UE 100 by the network 5. However, a case where the reception quality during multicast reception at UE 100 is poor may also be a case where the bit error rate (BER) measured by UE 100 for the multicast session exceeds the BER threshold set in UE 100 by the network 5, and / or a case where the block error rate (BLER) measured by UE 100 for the multicast session exceeds the BLER threshold set in UE 100 by the network 5.

[0058] The threshold value to be compared with the reception quality during such multicast reception can be set by the PTM setting (multicast reception setting) for each MBS session from the network 5 (gNB 200) to the UE 100 by an RRC Release message or a multicast MCCH (multicast MCCH message). In an embodiment, the UE 100 performing multicast reception in an RRC inactive state starts an RRC connection resume procedure when the reception quality of the serving cell is worse than the threshold value set by the network 5.

[0059] In addition, a case where UE100 needs to acquire the multicast reception setting from gNB200 is a case where UE100 has not received the multicast reception setting of the multicast session in which UE100 has joined in the RRC Release message, and the serving cell does not provide the multicast reception setting on the multicast MCCH, and UE100 may receive a paging message (group notification) indicating activation of the multicast session from the serving cell. A case where UE100 needs to acquire the multicast reception setting from gNB200 is a case where UE100 reselects a cell from the first cell to the second cell, and the second cell (reselected cell) does not provide the multicast reception setting of the multicast session in which UE100 has joined in the multicast MCCH.

[0060] In the embodiment, a scenario is assumed in which the reception quality of the UE 100 performing multicast reception in an RRC inactive state is worse than a threshold set by the network 5. In such a scenario, a ping-pong phenomenon may occur in which an RRC connection resume procedure and an RRC release procedure are repeated in a short period of time.

[0061] 6 is a diagram illustrating an example of the ping-pong phenomenon. In the following description of the embodiment, it is assumed that the UE 100 has already participated in a certain multicast session, and the multicast session is also referred to as a joined multicast session. The term "participating in a multicast session" may refer to a state in which the UE 100 is registered in the network 5 (CN 20) as the UE 100 that receives the multicast session.

[0062] In STEP 1, gNB200 transmits to UE100 an RRC Release message (i.e., an RRC Release message including a suspend setting) for transitioning UE100 to an RRC inactive state. UE100 transitions from an RRC connected state to an RRC inactive state in response to receiving the RRC Release message. Note that UE100 may start multicast reception in the RRC connected state. gNB200 may indicate to UE100 which multicast services can be received in the RRC inactive state in the suspend setting in the RRC Release message.

[0063] The RRC Release message transmitted by gNB200 includes multicast reception settings (PTM settings) for multicast sessions in which UE100 has joined, and the multicast reception settings may include thresholds (RSRP threshold and / or RSRQ threshold). Alternatively, the multicast MCCH transmitted (broadcast) by gNB200 includes multicast reception settings (PTM settings) for each multicast session, and the multicast reception settings may include thresholds (RSRP threshold and / or RSRQ threshold).

[0064] In STEP 2, UE 100, which performs multicast reception in an RRC inactive state, measures the reception quality of the serving cell and compares the measurement result (measured values ​​of RSRP and / or RSRQ) with a threshold. Here, if the reception quality at UE 100 is worse than the threshold set by network 5, UE 100 starts an RRC connection resume procedure for gNB 200 and transmits an RRC resume request (Resume Request) message to gNB 200. When the RRC connection resume procedure is completed, UE 100 transitions from the RRC inactive state to the RRC connected state.

[0065] For example, if the threshold setting by the gNB 200 is inappropriate and / or if the radio environment in which the UE 100 is located is poor, STEP 2 may occur immediately after STEP 1. In this case, an RRC connection resume procedure may occur immediately after an RRC release procedure, which may result in inefficient operation.

[0066] FIG. 7 is a diagram for explaining another example of the ping-pong phenomenon.

[0067] In STEP 1, UE 100, which performs multicast reception in an RRC inactive state, measures the reception quality of the serving cell and compares the measurement result (measured values ​​of RSRP and / or RSRQ) with a threshold. Here, if the reception quality at UE 100 is worse than the threshold set by network 5, UE 100 starts an RRC connection resume procedure with gNB 200 and transmits an RRC Resume Request message to gNB 200.

[0068] In STEP 2, the gNB 200 that has received the RRC Resume Request message transmits an RRC Release message including new multicast reception settings and suspend settings to the UE 100. That is, before the RRC connection resume procedure is completed, the gNB 200 provides the UE 100 with the new multicast reception settings and causes the UE 100 to maintain the RRC inactive state. The UE 100 applies the new multicast reception settings and performs multicast reception while maintaining the RRC inactive state. The new multicast reception settings may include a threshold setting.

[0069] In STEP 3, UE 100, which performs multicast reception in an RRC inactive state, measures the reception quality of the serving cell and compares the measurement result (measured values ​​of RSRP and / or RSRQ) with a threshold. Here, if the reception quality at UE 100 is worse than the threshold set by network 5, UE 100 starts an RRC connection resume procedure with gNB 200 and transmits an RRC Resume Request message to gNB 200.

[0070] For example, if the threshold setting by the gNB 200 is inappropriate and / or if the radio environment in which the UE 100 is located is poor, STEP 3 may occur immediately after STEP 2. In this case, an RRC connection resume procedure may occur immediately after an RRC release procedure, which may result in inefficient operation.

[0071] In the following embodiment, an operation that can suppress the occurrence of such an inefficient operation (ping-pong phenomenon) will be described. Fig. 8 is a diagram showing an outline of the operation of the UE 100 according to the embodiment.

[0072] In step S1, UE100 receives a multicast session from gNB200 in an RRC inactive state.

[0073] In step S2, UE100 determines that the measurement result of the reception quality from gNB200 is worse than a threshold (a threshold set by network 5).

[0074] In step S3, UE100 initiates an RRC connection resume procedure with gNB200 in response to the determination in step S2.

[0075] In step S4, UE100 transmits a message including notification information regarding the measurement results of reception quality to gNB200 in the RRC connection resume procedure.

[0076] Thus, in the embodiment, UE100 transmits a message including notification information regarding the measurement results of reception quality to gNB200 during the RRC connection resume procedure. This allows gNB200 to grasp the reception quality (multicast reception quality) at UE100 during the RRC connection resume procedure. As a result, when the radio environment in which UE100 is located is poor, gNB200 can transition UE100 to an RRC connected state, or increase the transmission error tolerance of multicast transmission and maintain UE100 in an RRC inactive state. gNB200 can also grasp the reception quality (multicast reception quality) at UE100 and maintain UE100 in an RRC inactive state while setting a new threshold appropriate for the reception quality in UE100.

[0077] 8, when receiving a multicast session from gNB200 in an RRC inactive state, the UE100 has a control unit 130 that determines that the measurement result of reception quality from gNB200 is worse than a threshold and starts an RRC connection resume procedure for gNB200 in response to the determination, and a transmission unit 120 that transmits a message to gNB200 in the RRC connection resume procedure, the message including notification information regarding the measurement result of reception quality. Meanwhile, the gNB200 has a reception unit 220 that receives an RRC connection resume procedure message from UE100 in response to the measurement result of reception quality from gNB200 being worse than the threshold, in UE100 receiving a multicast session from gNB200 in an RRC inactive state. The message includes notification information regarding the measurement result of reception quality.

[0078] In an embodiment, the message of step S4 may include a measurement value of the reception quality as notification information. This allows the gNB 200 to more accurately grasp the radio environment of the UE 100.

[0079] In an embodiment, the message of step S4 may include information indicating deterioration of reception quality as notification information. This allows the gNB 200 to understand that the radio environment of the UE 100 is poor.

[0080] In the first operation pattern of the embodiment, the message in step S4 (the message including the notification information) is an RRC Resume Request message. This allows the gNB200 to grasp the reception quality (multicast reception quality) of the UE100 at the UE100 earlier.

[0081] In the second operation pattern of the embodiment, the message in step S4 (the message including the notification information) is an RRC Resume Complete message. The RRC Resume Complete message can easily expand its message capacity compared to the RRC Resume Request message, and therefore can transmit more information as notification information.

[0082] In a second operation pattern of the embodiment, the UE 100 transmits an RRC Resume Request message in the RRC connection resume procedure. The RRC Resume Request message includes information indicating that an RRC Resume Complete message including notification information will be transmitted. That is, the UE 100 notifies the gNB 200 in advance by an RRC Resume Request message that it will transmit notification information in the RRC Resume Complete message. This allows the gNB 200 to wait for an RRC Resume Complete message including notification information without immediately transmitting an RRC Release message in response to the RRC Resume Request message.

[0083] In an embodiment, after transmitting the message of step S4, the UE 100 may receive an RRC Release message including a new threshold to be compared with the measurement result of the reception quality from the gNB 200. After transitioning to the RRC inactive state in response to receiving the RRC Release message, the UE 100 may determine whether the measurement result of the reception quality is worse than the new threshold when receiving a multicast session.

[0084] In an embodiment, gNB200, which receives a message including notification information from UE100, may determine, based on the notification information, in which state, RRC connected state or RRC inactive state, UE100 will receive the multicast session.

[0085] Furthermore, the gNB 200 that has received a message including notification information from the UE 100 may determine a new threshold to be compared with the measurement result of the reception quality based on the notification information. The gNB 200 may transmit an RRC Release message including the new threshold to the UE 100.

[0086] (3.2) Specific Operation Examples A first operation pattern and a second operation pattern will be described as specific examples of the operation of the mobile communication system 1. The first operation pattern and the second operation pattern may be implemented independently, or the two operation patterns may be implemented in combination.

[0087] (3.2.1) First Operation Pattern In the first operation pattern, the message including the notification information related to the measurement result of the reception quality is an RRC Resume Request message. Fig. 9 is a diagram showing an example of the first operation pattern of the mobile communication system 1 according to the embodiment.

[0088] In step S101, UE100 is in an RRC connected state in the cell of gNB200.

[0089] In step S102, the gNB 200 transmits to the UE 100 an RRC Release message including a suspend setting, i.e., an RRC Release message for transitioning the UE 100 to an RRC inactive state. The UE 100 receives the RRC Release message. The RRC Release message may include a multicast reception setting (PTM setting) for multicast reception in the RRC inactive state. The multicast reception setting may include a threshold value to be compared with the multicast reception quality.

[0090] In step S103, in response to the reception of the RRC Release message in step S102, the UE 100 transitions to an RRC inactive state.

[0091] In step S104, the gNB 200 may transmit a multicast MCCH including a multicast reception setting (PTM setting) for multicast reception in an RRC inactive state. The UE 100 may receive the multicast MCCH. The multicast reception setting may include a threshold value to be compared with the multicast reception quality.

[0092] In step S105, gNB200 transmits MBS data (multicast data) of the multicast session in which UE100 has joined on MTCH.

[0093] In step S106, UE100 applies the multicast reception setting received in step S102 or S104 and performs multicast reception in the RRC inactive state. Specifically, UE100 receives MBS data (multicast data) of the multicast session in which UE100 has joined from gNB200 on MTCH. UE100 also measures the reception quality (e.g., RSRP and / or RSRQ) for gNB200 (serving cell).

[0094] In step S107, the UE 100 compares the measurement result of the reception quality in step S106 with the threshold set in the multicast reception setting. If the measurement result of the reception quality in step S106 is not worse than the threshold (step S107: NO), the UE 100 returns the process to step S106.

[0095] On the other hand, if the measurement result of the reception quality in step S106 is worse than the threshold (step S107: YES), in step S108, the UE 100 starts an RRC connection resume procedure and transmits an RRC Resume Request message to the gNB 200, the RRC Resume Request message including the measurement result of the reception quality in step S106 (reception quality measurement value) as notification information. The gNB 200 receives the RRC Resume Request message. However, instead of such detailed measurement values, the notification information may be flag information indicating that the measurement result of the reception quality is worse than the threshold (information indicating deterioration of the reception quality). The RRC Resume Request message includes a Resume Cause, which is an information element regarding the cause of the RRC resume. The UE 100 may include, as a Resume Cause, information indicating deterioration of reception quality or information indicating RRC resume due to RSRP / RSRQ conditions in the RRC Resume Request message. The UE 100 may be permitted to include notification information in the RRC Resume Request message only when the RRC resume is due to satisfaction of the RSRP / RSRQ threshold conditions.

[0096] In step S109, the gNB 200 that has received the RRC Resume Request message of step S108 may determine whether to transition the UE 100 from the RRC inactive state to the RRC connected state based on the notification information in the RRC Resume Request message. For example, if the reception quality (radio environment) of the UE 100 is poor, the gNB 200 may decide to transition the UE 100 to the RRC connected state. In the RRC connected state, the gNB 200 can apply link adaptation and / or retransmission control specific to the UE 100, making it easier for the UE 100 to receive multicast data even in a poor radio environment. For example, if the gNB 200 determines that the reception quality (radio environment) at the UE 100 is slightly worse than the threshold but that multicast reception is possible in the RRC inactive state, it may determine to maintain the UE 100 in the RRC inactive state while determining a new threshold to set in the UE 100 (and / or determining an MCS change for the MTCH). Here, the gNB 200 may decide to change the MCS of the multicast data (MTCH) to a lower one (one with high error tolerance).

[0097] If it is determined that the UE 100 is to be maintained in the RRC inactive state (step S109: NO), in step S110, the gNB 200 may transmit an RRC Release message including a new multicast reception setting and a suspend setting to the UE 100. The new multicast reception setting may include a new threshold. The UE 100 that has received the RRC Release message maintains the RRC inactive state, applies the new multicast reception setting, and continues multicast reception in the RRC inactive state (step S106).

[0098] On the other hand, if it is determined that UE100 is to transition to the RRC connected state (step S109: YES), in step S111, gNB200 may transmit an RRC Resume message to UE100. UE100 may receive the RRC Resume message.

[0099] In step S112, in response to receiving the RRC Resume message in step S111, UE100 may transmit an RRC Resume Complete message to gNB200. gNB200 may receive the RRC Resume Complete message.

[0100] In step S113, the UE 100 may transition from the RRC inactive state to the RRC connected state. In this case, the RRC connection resume procedure is completed.

[0101] In step S114, gNB200 may transmit an RRC Reconfiguration message including new multicast reception settings to UE100. UE100 may receive the RRC Reconfiguration message.

[0102] In step S115, gNB200 may transmit MBS data (multicast data) of the multicast session in which UE100 has joined on MTCH.

[0103] In step S116, UE100 may apply the multicast reception setting of step S114 and perform multicast reception in the RRC connected state. Specifically, UE100 receives MBS data (multicast data) of the multicast session in which UE100 has joined from gNB200 on MTCH.

[0104] (3.2.2) Second Operation Pattern In the second operation pattern, the message including the notification information related to the measurement result of the reception quality is an RRC Resume Complete message. Fig. 10 is a diagram showing an example of the second operation pattern of the mobile communication system 1 according to the embodiment. Here, the differences from the first operation pattern will be mainly described.

[0105] The operations in steps S201 to S206 are the same as those in the first operation pattern.

[0106] In step S207, the UE 100 compares the measurement result of the reception quality in step S206 with the threshold set in the multicast reception setting. If the measurement result of the reception quality in step S206 is not worse than the threshold (step S207: NO), the UE 100 returns the process to step S206.

[0107] On the other hand, if the measurement result of the reception quality in step S206 is worse than the threshold (step S207: YES), in step S208, the UE 100 starts an RRC connection resume procedure and transmits an RRC Resume Request message to the gNB 200. The UE 100 may retain the measurement result of the reception quality in step S206 (the latest reception quality measurement value for the serving cell) in a memory.

[0108] The UE 100 may include information indicating that the reception quality measurement value is to be included as notification information in the RRC Resume Complete message in the RRC Resume Request message of step S208. The UE 100 may include information indicating deterioration of reception quality or information indicating that RRC resume is due to RSRP / RSRQ conditions in the RRC Resume Request message. The UE 100 may include information indicating that the gNB 200 should not transmit an RRC Release message in response to the RRC Resume Request message in the RRC Resume Request message. The UE 100 may include such information in the RRC Resume Request message as a Resume Cause.

[0109] In step S209, gNB200, which has received the RRC Resume Request message of step S208, determines, based on the information in the RRC Resume Request message, not to transmit an RRC Release message to UE100 at this time, and transmits an RRC Resume message instead of an RRC Release message to UE100. UE100 receives the RRC Resume message.

[0110] In step S210, in response to receiving the RRC Resume message in step S211, the UE 100 transmits to the gNB 200 an RRC Resume Complete message including the measurement result of the reception quality in step S206 (reception quality measurement value) as notification information. The gNB 200 receives the RRC Resume Complete message. However, instead of such detailed measurement values, the notification information may be flag information (information indicating deterioration of reception quality) indicating that the measurement result of the reception quality is worse than a threshold. The UE 100 may be permitted to include the notification information in the RRC Resume Complete message only when the RRC resume is due to satisfaction of the RSRP / RSRQ threshold condition.

[0111] In step S211, the UE 100 transitions from the RRC inactive state to the RRC connected state.

[0112] In step S212, the gNB 200 may determine whether to transition the UE 100 to the RRC connected state based on the notification information in the RRC Resume Complete message of step S210. For example, if the reception quality (radio environment) of the UE 100 is poor, the gNB 200 may decide to maintain the UE 100 in the RRC connected state. For example, if the reception quality (radio environment) of the UE 100 is slightly worse than a threshold but the gNB 200 determines that multicast reception is possible in the RRC inactive state, the gNB 200 may determine to transition the UE 100 to the RRC inactive state while determining a new threshold to be set for the UE 100 (and / or determine a change in the MCS of the MTCH). Here, the gNB 200 may decide to lower the MCS of the multicast data (MTCH) (to one with higher error resilience).

[0113] If it is determined that the UE 100 is to transition to the RRC inactive state (step S212: NO), in step S213, the gNB 200 may transmit an RRC Release message including a new multicast reception setting and a suspend setting to the UE 100. The new multicast reception setting may include a new threshold. The UE 100 that has received the RRC Release message transitions to the RRC inactive state (step S214), and applies the new multicast reception setting to perform multicast reception in the RRC inactive state (step S206).

[0114] On the other hand, if it is determined that UE100 is maintained in the RRC connected state (step S212: YES), in step S215, gNB200 may transmit an RRC Reconfiguration message including a new multicast reception setting to UE100. UE100 may receive the RRC Reconfiguration message. Then, in step S216, gNB200 may transmit MBS data (multicast data) of the multicast session in which UE100 has joined on an MTCH. In step S217, UE100 may apply the multicast reception setting of step S215 and perform multicast reception in the RRC connected state. Specifically, UE100 receives MBS data (multicast data) of the multicast session in which UE100 has joined from gNB200 on an MTCH.

[0115] (4) Other Embodiments In the above-described embodiment, multicast reception in the RRC inactive state has been mainly described, but the operation according to the above-described embodiment may be applied to multicast reception in the RRC idle state. In the RRC idle state, the above-described RRC resume (Resume) is replaced with RRC establishment (Establishment), and the above-described resume cause is replaced with establishment cause.

[0116] The above-described operational flows are not limited to being implemented 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. In each flow, it is not necessary to execute all steps, and only some steps may be executed. Furthermore, the order of steps in each flow may be changed as appropriate.

[0117] In the above-described embodiments and examples, 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 also be a relay node such as an IAB (Integrated Access and Backhaul) node. The base station may also be a DU of the IAB node. The UE 100 may also be an MT (Mobile Termination) of the IAB node.

[0118] That is, the UE 100 may be a terminal function unit (a type of communication module) for a base station to control a repeater that relays signals. Such a terminal function unit is referred to as an MT. Examples of the MT include, in addition to the IAB-MT, an NCR (Network Controlled Repeater)-MT and a RIS (Reconfigurable Intelligent Surface)-MT.

[0119] The term "network node" primarily refers to a base station, but may also refer to a core network device or a part of a base station (CU, DU, or RU). A network node may also be configured by a combination of at least a part of a core network device and at least a part of a base station.

[0120] A program may be provided that causes a computer to execute each process performed by the UE 100 or the gNB 200. The program may be recorded on a computer-readable medium. Using a computer-readable medium, the program can be installed on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transitory recording medium. The non-transitory recording medium is not particularly limited, and may be, for example, a recording medium such as a CD-ROM or a DVD-ROM. Furthermore, circuits that execute each process performed by the UE 100 or the gNB 200 may be integrated, and at least a portion of the UE 100 or the gNB 200 may be configured as a semiconductor integrated circuit (chip set, SoC: System on a chip).

[0121] The functions performed by the UE 100 or the gNB 200 (network node) may be implemented in circuitry or processing circuitry, including general-purpose processors, application-specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), CPUs (Central Processing Units), conventional circuits, and / or combinations thereof, programmed to perform the described functions. A processor includes transistors and other circuits and is considered to be circuitry or processing circuitry. A processor may also be a programmed processor that executes a program stored in memory. In this specification, circuitry, unit, or means refers to hardware that is programmed to perform the described functions or hardware that executes them. The hardware may be any hardware disclosed herein or any hardware known to be programmed or capable of performing the described functions. If the hardware is a processor, the circuitry, means, or unit is a combination of hardware and software used to configure the hardware and / or processor.

[0122] As used in this disclosure, the terms "based on" and "depending on / in response to" do not mean "based only on" or "depending only on," unless expressly stated otherwise. The term "based on" means both "based only on" and "based at least in part on." Similarly, the term "depending on" means both "depending only on" and "depending at least in part on." The terms "include," "comprise," and variations thereof do not mean including only the listed items, but may mean including only the listed items or may include additional items in addition to the listed items. Additionally, the term "or," as used in this disclosure, is not intended to mean an exclusive or. Furthermore, any reference to elements using designations such as "first," "second," etc., as used in this disclosure does not 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, a 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 way. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles shall include the plural unless the context clearly indicates otherwise.

[0123] The above describes the embodiments in detail with reference to the drawings, but the specific configuration is not limited to that described above, and various design changes can be made within the scope that does not deviate from the gist of the invention.

[0124] This application claims priority to U.S. Provisional Application No. 63 / 540,269 (filed September 25, 2023), the entire contents of which are incorporated herein by reference.

[0125] (5) First Supplementary Note The following is a supplementary note regarding the features of the above-described embodiment.

[0126] (Supplementary Note 1) A communication method used in a mobile communication system that provides a multicast broadcast service (MBS), comprising: a user equipment receiving a multicast session from a network node in a radio resource control (RRC) inactive state, determining that a measurement result of reception quality from the network node is worse than a threshold; the user equipment initiating an RRC connection resume procedure with the network node in response to the determination; and the user equipment transmitting a message including notification information regarding the measurement result of the reception quality to the network node during the RRC connection resume procedure.

[0127] (Supplementary Note 2) The communication method according to Supplementary Note 1, wherein the message includes a measurement value of the reception quality as the notification information.

[0128] (Supplementary Note 3) The communication method according to Supplementary Note 1 or 2, wherein the message includes, as the notification information, information indicating deterioration of the reception quality.

[0129] (Supplementary Note 4) The communication method according to any one of Supplementary Notes 1 to 3, wherein the message is an RRC resume request message.

[0130] (Supplementary Note 5) The communication method according to any one of Supplementary Notes 1 to 3, wherein the message is an RRC resume complete message.

[0131] (Supplementary Note 6) The communication method according to Supplementary Note 5, further comprising the user equipment transmitting an RRC resume request message in the RRC connection resume procedure, wherein the RRC resume request message includes information indicating that the RRC resume complete message including the notification information is to be transmitted.

[0132] (Supplementary Note 7) The communication method according to any one of Supplementary Notes 1 to 6, further comprising: the user equipment receiving from the network node an RRC release message including a new threshold to be compared with the measurement result of the reception quality; and the user equipment, after transitioning to an RRC inactive state in response to receiving the RRC release message, determining whether the measurement result of the reception quality is worse than the new threshold when receiving the multicast session.

[0133] (Supplementary Note 8) The communication method according to any one of Supplementary Notes 1 to 7, further comprising: the network node receiving the message including the notification information from the user equipment; and the network node determining, based on the notification information, in which state of an RRC connected state or an RRC inactive state the user equipment should receive the multicast session.

[0134] (Supplementary Note 9) The communication method according to any one of Supplementary Notes 1 to 8, further comprising: the network node receiving from the user equipment the message including the notification information; the network node determining, based on the notification information, a new threshold to be compared with a measurement result of the reception quality; and the network node transmitting to the user equipment an RRC release message including the new threshold.

[0135] (Supplementary Note 10) A user equipment used in a mobile communication system that provides a multicast broadcast service (MBS), comprising: a control unit that, when receiving a multicast session from a network node in a radio resource control (RRC) inactive state, determines that a measurement result of reception quality from the network node is worse than a threshold, and initiates an RRC connection resume procedure with the network node in response to the determination; and a transmission unit that, during the RRC connection resume procedure, transmits a message including notification information on the measurement result of the reception quality to the network node.

[0136] (Supplementary Note 11) A network node used in a mobile communication system providing a Multicast Broadcast Service (MBS), comprising: a receiving unit for receiving an RRC connection resume procedure message from a user equipment receiving a multicast session from a network node in a Radio Resource Control (RRC) inactive state in response to a measurement result of reception quality from the network node being worse than a threshold, the message including notification information regarding the measurement result of the reception quality.

[0137] (6) Supplementary Note 2 1. Introduction The work item on enhanced MBS (eMBS) aims to support multicast reception by UEs in inactive state as follows: - Specify support for multicast reception by UEs in RRC inactive state [RAN2, RAN3]. - PTM configuration for UEs receiving multicast in RRC inactive state [RAN2]. - Investigate the impact of mobility and state transitions for UEs receiving multicast in RRC inactive state (seamless / lossless mobility is not mandatory) [RAN2, RAN3].

[0138] This appendix discusses the details of RRC resume due to bad quality and the causes of the resume.

[0139] 2. Discussion 2.1. RRC Resume Due to Deterioration of Reception Quality RAN2#123 agreed to introduce RSRP / RSRQ-based RRC Resume as follows.

[0140] For a UE receiving multicast in an RRC inactive state, the UE resumes the RRC connection when the RSRP or RSRQ measured based on the existing measurement requirements of the serving cell (whichever is set by the network) falls below a threshold set by the network. Whether the ping-pong issue needs to be addressed is a matter for further study.

[0141] The threshold can be PTM configured per MBS session via RRC Release or multicast MCCH messages.

[0142] Based on the agreement, remaining issues are discussed in the following sections.

[0143] 2.1.1 Ping-pong issue The agreement captures the considerations of "whether or not the ping-pong issue needs to be addressed / how to address it." Possible scenarios for resuming ping-pong include the following:

[0144] 1. A UE in an inactive state with deteriorating radio quality will initiate RRC resume when the RSRP / RSRQ falls below the threshold. 2. In response to the RRC resume request, a gNB under network congestion will send an RRC release with a suspend config. that includes the RSRP / RSRQ threshold. 3. A UE whose radio quality remains deteriorating will transition to an inactive state again, and since the RSRP / RSRQ will soon fall below the threshold, RRC resume will be initiated again.

[0145] To avoid ping-pong in the above scenario, the gNB may keep the UE in a connected state or set appropriate RSRP / RSRQ thresholds for the UE.

[0146] To support such a decision, the gNB may need to know the radio conditions and / or the reason for RRC resume (in this case, due to deterioration in radio quality) as soon as the UE requests RRC resume.

[0147] Observation 1: In order to guarantee QoS requirements, the gNB may need to know the UE's radio conditions when the UE requests RRC resume and the reason for RRC resume.

[0148] It should be noted that, according to the RAN2 discussions in the early stages of Release 18, the two main motivations for supporting multicast reception in the inactive state are network congestion mitigation and UE power consumption. Therefore, it is not reasonable for the gNB to always keep the UE in the connected state and configure measurement reporting every time the UE requests RRC resume.

[0149] Observation 2: Since the motivation for supporting multicast reception in the inactive state is to reduce network congestion and save UE power, it is not reasonable to rely on measurement reports to guarantee QoS requirements.

[0150] Considering the above observations, when the UE initiates RRC resume due to a deterioration in reception quality during the RRC resume procedure, it is useful for the UE to report the measurement results in the RRC resume request or RRC resume complete. Another option is for the UE to indicate the deterioration in radio conditions in the resume cause. With such information, the gNB can make an appropriate decision on whether to keep the UE in connected state for QoS guarantees.

[0151] Proposal 1: RAN2 should agree that the measurement results are reported within the RRC resume procedure or a new resume cause "Multicast quality" is introduced.

[0152] 2.1.2 Validity of RSRP / RSRQ Thresholds Configured by RRC Release RAN2 agreed that "thresholds can be configured by RRC release or by PTM configuration per MBS session via multicast MCCH messages." For dedicated configurations provided by RRC release, many settings remain valid until the corresponding timer expires. For example, dedicated frequency priority remains valid until T320 expires. This occurs regardless of cell reselection; dedicated settings provided by the source cell remain valid even after the UE reselects the target cell.

[0153] Observation 3: According to the current specification, many dedicated settings remain valid until a timer expires, regardless of cell reselection.

[0154] Regarding the RSRP / RSRQ thresholds, if they are valid after cell reselection, problems may occur assuming the following conditions: - In the source cell, PTM is transmitted with a low MCS, so the UE sets a low RSRP threshold to ensure a certain reception quality. - In the target cell after cell reselection, PTM is transmitted with a higher MCS, but the UE still applies the old RSRP threshold. - As a result, the UE will experience MTCH reception errors even though the RSRP is still better than the old threshold.

[0155] To avoid this problem, the RSRP / RSRQ thresholds should be cell-specific, considering that the PTM transmission of NR MBS is cell-specific, i.e., if the UE reselects a different cell, the old RSRP / RSRQ thresholds should be discarded.

[0156] Proposal 2: RAN2 should agree to discard the RSRP / RSRQ thresholds upon cell reselection.

[0157] Another possible issue in handling RSRP / RSRQ thresholds is which threshold should the UE apply when thresholds are provided by both RRC release and multicast MCCH. In the current specification, dedicated settings provided by RRC release generally take precedence over settings provided by SIB. However, for inactive multicast reception, RAN2 agreed that "if the PTM setting needs to be changed, RAN2 will use the MCCH to indicate it," meaning that the PTM setting provided by RRC release may be changed by the PTM setting provided by multicast MCCH. This means that the RSRP / RSRQ thresholds cannot be UE-specific, i.e., TMGI-specific, because they are overwritten by the multicast MCCH.

[0158] Proposal 3: RAN2 should ensure that the PTM configuration including RSRP / RSRQ thresholds provided by RRC release is always overridden by the configuration provided by multicast MCCH, i.e. there is no UE-specific configuration for inactive multicast reception.

[0159] 2.1.3 Other Confirmations RAN2#123 agreed that "for a UE receiving multicast in an RRC inactive state, the UE resumes the RRC connection when the measured RSRP or RSRQ [...] of the serving cell falls below a threshold [...]." The RSRP / RSRQ threshold is introduced to guarantee QoS for multicast reception in an inactive state.

[0160] On the other hand, if the multicast session is deactivated by the network, a UE in an inactive state may not receive the MTCH (or temporarily not receive data) when the session is deactivated. In this case, the UE is considered the same as a legacy UE in an inactive state, so RRC resume due to quality deterioration (RSRP / RSRQ threshold) is not required. As already implied in the above agreement, a UE in an inactive state must ensure that it does not initiate RRC resume if the multicast session is deactivated (or there is temporarily no data), even if the RSRP / RSRQ deteriorates below the threshold.

[0161] Proposal 4: RAN2 needs to ensure that inactive UEs do not initiate RRC resume when the multicast session is disabled (or there is temporarily no data), even if the RSRP / RSRQ becomes worse than the threshold.

[0162] In RAN2#123, RAN2 discussed whether the threshold could be based on BLER, but ultimately RAN2 decided to adopt RSRP / RSRQ.

[0163] On the other hand, in Release 17, it was a common understanding in RAN2 that SFN (single frequency network) can only be deployed depending on the network implementation, e.g., within a DU. In inter-cell SFN, reception quality is stable even at the cell edge by combining reception from MTCHs of different cells. However, RSRP / RSRQ are cell-specific and degrade at the cell edge when moving away from SFN. Therefore, RSRP / RSRQ thresholds do not work well in SFN deployments, but BLER thresholds can avoid such issues.

[0164] Proposal 5: RAN2 should ensure that the NW implementation does not operate the SFN (Single Frequency Network) allowed in Release 17 at the RSRP / RSRQ threshold in Release 18.

[0165] RAN2#123 agreed to reuse existing resume causes for RRC resume due to several reasons other than the deterioration of reception quality discussed in the previous section in Release 18. However, it is still necessary to analyze which existing resume causes are applicable and whether new resume causes are really unnecessary.

[0166] No new resume causes are introduced for UEs receiving inactive MC when resuming due to quality degradation or incorrect SIBx / PTM configuration, unless a problem is identified with using one of the existing resume causes.

[0167] Currently, 11 resumes are defined, along with five spare fields: ResumeCause ::= ENUMERATED {emergency, highPriorityAccess, mt-Access, mo-Signalling, mo-Data, mo-VoiceCall, mo-VideoCall, mo-SMS, rna-Update, mps-PriorityAccess, mcs-PriorityAccess, spare1, spare2, spare3, spare4, spare5}

[0168] Similar to observation 2 above, one of the motivations for inactive multicast reception is to alleviate network congestion, so the analysis must consider the case when the network is congested.

[0169] Observation 4: The mapping of RRC resume and existing resume causes for multicast reception needs to take into account the congestion state of the network.

[0170] Typical gNB decisions when receiving an RRC Resume request during network congestion can be classified as follows: - Always accept emergency situations; - Prioritize highPriorityAccess, mps-PriorityAccess, and mcs-PriorityAccess; - Accept mt-Access and mo-Signalling (as much as possible); - May reject mo-Data, mo-VoiceCall, mo-VideoCall, and mo-SMS; - In the case of rna-Update, respond with an RRC release with a new RNA.

[0171] Observation 5: Resume allows the gNB to decide whether to accept, reject, or release with a new RNA in response to an RRC resume request, especially in the case of NW congestion.

[0172] RAN2 has agreed that the UE will initiate RRC resume in two cases: due to quality degradation or insufficient SIBx / PTM configuration. From the perspective of the impact on network load, these reasons are characterized as follows:

[0173] As discussed in Section 2.1.1, in the case of RRC resume due to quality deterioration, the UE must remain connected because if the UE is released again while in inactive, QoS requirements will no longer be guaranteed. Considering that the UE must remain connected, the UE does not consume additional downlink resources (since it only receives PTMs already sent to other UEs), but it does consume uplink resources (e.g., HARQ ACKs). In times of network congestion, this consumes slightly more resources compared to multicast reception in inactive state, but may still be prioritized over resume requests for unicast services. In other words, the gNB may prioritize RRC resume requests for multicast reception over RRC resume requests for unicast reception, but may still reject the RRC resume request due to high congestion.

[0174] In the case of RRC resume due to incomplete SIBx / PTM configuration, if the radio conditions are good, the UE is released with PTM configuration, i.e., RRC resume, and can receive the multicast session. Therefore, in terms of network congestion, it is "light," i.e., it consumes almost no downlink / uplink resources. In other words, the NW can immediately accept the RRC resume request, assuming the RRC release with PTM configuration of RRC resume.

[0175] Observation 6: In order to keep the UE connected when the network is congested, the gNB may reject RRC resume due to quality degradation, which may have some impact on the network congestion.

[0176] Observation 7: Due to SIBx / PTM configuration deficiency, the gNB can accept RRC resume and immediately release UEs with PTM configuration, as this does not affect network congestion.

[0177] Considering the above discussion, RRC resume due to bad quality may be mapped to either mo-Data, mo-VoiceCall, mo-VideoCall, or mo-SMS. However, since multicast reception in Connected does not require additional PDSCH resources (i.e., PTM is already provided for other UEs), current gNB implementations for these existing resume causes may be affected if a different type of communication (i.e., multicast reception only) is mapped to an existing resume cause.

[0178] For RRC resume due to lack of SIBx / PTM settings, there is no mapped resume cause because there is a new intent of RRC resume with PTM settings. For rna-Update, a similar gNB decision is assumed, but the gNB provides RNA instead of PTM settings for RRC resume. This means that the gNB cannot send an RRC release with PTM settings in response to an RRC resume request, and the gNB may mistakenly reject the UE's request even though the UE is only requesting a PTM setting update, which has no impact on network congestion.

[0179] Observation 8: There is no existing resume cause that matches the gNB's expected decision in the case of an RRC resume request, either due to quality degradation or insufficient SIBx / PTM configuration.

[0180] RAN2 has indeed identified new possible resume causes with the following discussion: Resume Causes: 1. Multicast Quality 2. Multicast Configuration

[0181] These two resume causes can easily solve all the problems discussed above without potentially impacting current gNB implementations. As discussed above, the two resume causes are useful for gNBs to appropriately guide different results. Therefore, two new resume causes should be introduced accordingly.

[0182] Since 3GPP is likely to begin considering 6G from Release 20, 5G NR is expected to evolve over only the next two releases (i.e., up to Release 20 at most). Given the history of additional resumes in past releases, three resumes in Release 18 will be sufficient.

[0183] Proposal 6: RAN2 should agree on a new resume cause "Multicast Quality" to be set when RRC resume is initiated due to quality degradation. Expected gNB decisions take priority over MO data, etc.

[0184] Proposal 7: RAN2 should agree on a new resume cause "multicast configuration" that is set when RRC resume is initiated due to SIBx / PTM misconfiguration. The expected gNB decision is to respond with an RRC release with PTM configuration.

[0185] 1: Mobile communication system 5: Network 10: RAN 20: CN 100: UE (user equipment) 110: Receiving unit 120: Transmitting unit 130: Control unit 200: gNB (base station) 210: Transmitting unit 220: Receiving unit 230: Control unit 240: Backhaul communication unit

Claims

1. A communication method used in a mobile communication system providing a multicast broadcast service (MBS), comprising: a user equipment receiving a multicast session from a network node in a radio resource control (RRC) inactive state determines that a measurement result of reception quality from the network node is worse than a threshold; the user equipment initiates an RRC connection resume procedure with respect to the network node in response to the determination; and the user equipment transmits a message including notification information regarding the measurement result of the reception quality to the network node during the RRC connection resume procedure.

2. The communication method according to claim 1, wherein the message includes a measurement value of the reception quality as the notification information.

3. The communication method according to claim 1, wherein the message includes information indicating the deterioration of the reception quality as the notification information.

4. The communication method according to claim 1, wherein the message is an RRC resume request message.

5. The communication method according to any one of claims 1 to 3, wherein the message is an RRC resume complete message.

6. The communication method according to claim 5, further comprising the user equipment transmitting an RRC resume request message in the RRC connection resume procedure, the RRC resume request message including information indicating that the RRC resume complete message including the notification information is to be transmitted.

7. The communication method according to claim 1, further comprising: the user equipment receiving from the network node an RRC release message including a new threshold for comparing with the measurement result of the reception quality; and the user equipment determining whether the measurement result of the reception quality is worse than the new threshold when receiving the multicast session after transitioning to an RRC inactive state in response to receiving the RRC release message.

8. The communication method according to claim 1, further comprising: the network node receiving the message including the notification information from the user equipment; and the network node determining, based on the notification information, in which state of an RRC connected state or an RRC inactive state the user equipment is to receive the multicast session.

9. The method of claim 1, further comprising: the network node receiving the message including the notification information from the user equipment; the network node determining a new threshold to be compared with the measurement result of the reception quality based on the notification information; and the network node transmitting an RRC release message including the new threshold to the user equipment.

10. A user equipment used in a mobile communication system providing a multicast broadcast service (MBS), comprising: a control unit that, when receiving a multicast session from a network node in a radio resource control (RRC) inactive state, determines that a measurement result of reception quality from the network node is worse than a threshold, and initiates an RRC connection resume procedure with respect to the network node in response to the determination; and a transmission unit that transmits a message including notification information regarding the measurement result of the reception quality to the network node in the RRC connection resume procedure.

11. A network node used in a mobile communication system providing a multicast broadcast service (MBS), comprising: a receiving unit that receives an RRC connection resume procedure message from a user equipment that receives a multicast session from a network node in a radio resource control (RRC) inactive state in response to a measurement result of reception quality from the network node being worse than a threshold value, the message including notification information regarding the measurement result of the reception quality.

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