Communication method and user device

Abstract

Provided is a communication method executed by UE 100 in a mobile communication system providing a multicast and broadcast service (MBS), the method comprising: a step S6 for receiving from a source cell C1 a radio resource control (RRC) reconfiguration message including a CHO configuration pertaining to a candidate cell C2 for conditional handover (CHO); and a step S9 for, in accordance with inclusion in the CHO configuration of an MBS reception configuration for receiving an MBS session the candidate cell provides by Point-To-Multipoint (PTM), starting MBS reception from the candidate cell C2 using the MBS reception configuration prior to executing the CHO.

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) standard defines the technical specifications for NR (New Radio), a fifth-generation (5G) radio access technology. Compared to LTE (Long Term Evolution), a fourth-generation (4G) radio access technology, NR offers higher speed, larger capacity, higher reliability, and lower latency. Discussions are underway within 3GPP to formulate technical specifications for 5G / NR multicast broadcast services (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] It is expected that 5G / NR multicast broadcast services will provide improved services compared to 4G / LTE multicast broadcast services.

[0005] Therefore, an object of the present disclosure is to make it possible to realize an improved multicast broadcast service.

[0006] A communication method according to a first aspect is a method executed by a user equipment in a mobile communication system providing a multicast broadcast service (MBS), the communication method including the steps of: receiving a radio resource control (RRC) reconfiguration message including a CHO configuration for a candidate cell for a conditional handover (CHO) from a source cell; and, in response to the CHO configuration including an MBS reception configuration for receiving an MBS session provided by the candidate cell in a point-to-multipoint (PTM) manner, starting MBS reception from the candidate cell using the MBS reception configuration before performing the CHO.

[0007] A user equipment according to a second aspect is an equipment used in a mobile communication system providing a multicast broadcast service (MBS), and includes: a receiver that receives, from a source cell, a radio resource control (RRC) reconfiguration message including a CHO configuration for a candidate cell for a conditional handover (CHO), and a controller that, in response to the CHO configuration including an MBS reception configuration for receiving an MBS session provided by the candidate cell via a point-to-multipoint (PTM) protocol, starts MBS reception from the candidate cell using the MBS reception configuration before performing the CHO. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram illustrating a configuration of a mobile communication system according to an embodiment. [Figure 2] 1 is a diagram illustrating a configuration of a UE (user equipment) according to an embodiment. [Figure 3] A diagram showing the configuration of a gNB (base station) according to an embodiment. [Figure 4] FIG. 10 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data. [Figure 5] FIG. 1 is a diagram showing the configuration of a protocol stack of a radio interface of a control plane that handles signaling (control signals). [Figure 6]FIG. 1 is a diagram illustrating an overview of MBS traffic distribution according to an embodiment. [Figure 7] FIG. 10 is a diagram illustrating a distribution mode according to the embodiment. [Figure 8] FIG. 10 is a diagram illustrating an example of internal processing related to MBS reception in a UE according to an embodiment. [Figure 9] FIG. 10 is a diagram illustrating another example of internal processing related to MBS reception in the UE according to the embodiment. [Figure 10] FIG. 2 is a diagram showing a first operation scenario of the mobile communication system according to the embodiment. [Figure 11] FIG. 10 is a diagram showing a second operation scenario of the mobile communication system according to the embodiment. [Figure 12] FIG. 1 is a diagram illustrating an example of operation of a mobile communication system according to an embodiment. [Figure 13] FIG. 1 is a diagram showing packets constituting MBS data, specifically, PDCP data PDUs (Protocol Data Units). [Figure 14] FIG. 10 is a diagram illustrating a modified example of the operation of the mobile communication system according to the embodiment. [Figure 15] FIG. 10 is a diagram illustrating an example of internal processing of a UE according to a modified example. [Figure 16] FIG. 10 is a diagram illustrating an example of internal processing of a UE according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[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) 1 is a diagram showing the configuration of a mobile communication system according to an embodiment. The mobile communication system 1 conforms to the 3GPP standard 5th Generation System (5GS). In the following description, 5GS is used as an example, but the mobile communication system may be at least partially applied to an LTE (Long Term Evolution) system or 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 may be a mobile phone terminal (including a smartphone), a tablet terminal, a laptop PC, a communication module (including a communication card or a chipset), a sensor or 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 a UE 100 that has established a connection with its own cell. The gNB 200 has a radio resource management (RRM) function, a routing function for user data (hereinafter simply referred to as "data"), a measurement control function for mobility control and scheduling, etc. The term "cell" is used to indicate the smallest unit of a wireless communication area. The term "cell" is also used to indicate a function or resource that performs wireless communication with a UE 100. One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

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

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

[0016] 2 is a diagram showing the configuration of a UE 100 (user equipment) according to the 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 constitute 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, which will be described later. 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 the 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 for each layer, which will be described later. 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 neighboring 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 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 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 successfully decoded DCI as DCI addressed to the UE. The DCI transmitted from gNB200 has CRC parity bits 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 UE100 and the MAC layer of gNB200 via transport channels. The MAC layer of gNB200 includes a scheduler, which determines the uplink and downlink transport format (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be allocated to UE100.

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

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

[0031] The SDAP layer maps IP flows, which are the units for Quality of Service (QoS) control by the core network, to radio bearers, which are the units for QoS control by the Access Stratum (AS). Note that if the RAN is connected to the EPC, 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] 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, which is 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, the layer below the NAS layer is called the AS layer.

[0036] (MBS Overview) 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 communications, mission-critical communications, V2X (Vehicle to Everything) communications, IPv4 or IPv6 multicast distribution, IPTV (Internet protocol television), group communications, and software distribution.

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

[0038] The multicast service provides a service to a group of UEs 100 participating in the multicast service (multicast session), rather than to all UEs 100. An MBS session used for the multicast service is called a multicast session.

[0039] FIG. 6 is a diagram illustrating an overview of MBS traffic distribution 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 (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, two delivery methods are possible 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 packet 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 for 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 distribution modes 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 for transmitting 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 for the MTCH corresponding to this MBS session. The MTCH scheduling information may include discontinuous reception (DRX) configuration for the MTCH. The discontinuous reception configuration may include one or more parameters: a timer value (On Duration Timer) defining an on-duration (on duration: reception period), a timer value (Inactivity Timer) extending the on-duration, a scheduling interval or DRX cycle (Scheduling Period, DRX Cycle), an offset value (Start Offset, DRX Cycle Offset) of the start subframe of the scheduling or DRX cycle, a start delay slot value (Slot Offset) of the on-duration timer, a timer value (Retransmission Timer) defining the maximum time until retransmission, and a timer value (HARQ RTT Timer) defining the minimum interval until DL allocation for HARQ retransmission. The MTCH (Multicast Traffic Channel) is a type of logical channel, and is mapped to a Down Link Shared Channel (DL-SCH), which is a type of transport channel.

[0051] The second delivery mode (Delivery mode 2: DM2) is a delivery mode that can be used not only by the UE 100 in the RRC connected state but also by the UE 100 in the RRC idle state or the 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.

[0052] 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 the MCCH using, for example, a dedicated RNTI predefined in a technical specification. The RNTI for BCCH reception may be SI-RNTI, and the RNTI for MCCH reception may be MCCH-RNTI.

[0053] In the second distribution mode, the UE 100 may receive MBS data in the following three procedures. First, the UE 100 receives MCCH configuration information from the MBS system information block (MBS SIB) transmitted on the BCCH from the gNB 200. Second, the UE 100 receives the MCCH from the gNB 200 based on the MCCH configuration information. The MCCH transmits MTCH configuration information. The MCCH may include neighboring cell information indicating whether the currently provided MBS session is also provided in a neighboring cell. Third, the UE 100 receives the MTCH (MBS data) based on the MTCH configuration information. Hereinafter, the MTCH configuration information and / or the MCCH configuration information may be referred to as the MBS reception configuration.

[0054] 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 configuration (MTCH configuration information).

[0055] 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, application server, etc., and an IP multicast address indicating the 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.

[0056] Fig. 8 is a diagram showing an example of internal processing related to MBS reception of the UE 100 according to the embodiment. Fig. 9 is a diagram showing another example of internal processing related to MBS reception of the UE 100 according to the embodiment.

[0057] An MBS Radio Bearer (MRB) is a radio bearer that carries a multicast session or a broadcast session. That is, an MRB may be associated with a multicast session or a broadcast session.

[0058] 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 RRC signaling, one MRB can be configured with "PTM only," "PTP only," or "both PTM and PTP." The bearer type of such an MRB can be changed by RRC signaling.

[0059] 8 shows an example 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 a PTP-only MRB, MRB#2 is a PTM-only MRB, and MRB#3 is a PTM-only MRB. Note that DTCH is scheduled using the cell RNTI (C-RNTI). MTCH is scheduled using the G-RNTI.

[0060] 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 the 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.

[0061] 9 shows an example 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 associated with a DTCH and the other leg 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.

[0062] (Mobile communication system operation) FIG. 10 is a diagram showing a first operation scenario of the mobile communication system 1 according to the embodiment.

[0063] gNB200A manages cell C1, and gNB200B, which is adjacent to gNB200A, manages cell C2. Cells C1 and C2 have at least partial overlapping coverage. gNB200A and gNB200B are interconnected via the Xn interface, which is an interface between base stations. Inter-base station communication between gNB200A and gNB200B is assumed to be performed over the Xn interface.

[0064] The gNB 200A provides an MBS session in the cell C1. Specifically, the gNB 200A receives MBS data belonging to the MBS session from the UPF 300B and transmits the MBS data in the cell C1 by PTM (multicast / broadcast). The UE 100 in the RRC connected state receives (MBS reception) the MBS data transmitted in the cell C1 by PTM. The reception of (MBS reception) the MBS data transmitted in the PTM is also referred to as PTM reception.

[0065] The gNB200B provides an MBS session in cell C2. Specifically, the gNB200B receives MBS data belonging to the MBS session from the UPF300B and transmits the MBS data in cell C2 using PTM. The gNB200B can provide the same MBS session in cell C2 as the MBS session provided in cell C1.

[0066] FIG. 11 is a diagram showing a second operation scenario of the mobile communication system 1 according to the embodiment.

[0067] The second operating scenario differs from the first operating scenario in that cells C1 and C2 are managed by a single gNB 200. The gNB 200 provides MBS sessions in each of cells C1 and C2. Specifically, the gNB 200 transmits MBS data in each of cells C1 and C2 by PTM (multicast / broadcast).

[0068] In the first and second operation scenarios, UE 100 in the RRC connected state moves from cell C1 to cell C2. For example, gNB 200 (gNB 200A) determines handover (HO) of UE 100 to cell C2 based on a measurement report from UE 100 and transmits an HO command to UE 100. UE 100 accesses cell C2 (gNB 200B) in response to receiving the HO command. During this HO procedure, there is a problem that PTM reception of UE 100 may be temporarily interrupted, resulting in loss of MBS data received by UE 100. Note that, assuming HO of UE 100 from cell C1 to cell C2, cell C1 and gNB 200A are referred to as the source cell and source gNB, respectively, and cell C2 and gNB 200B are referred to as the target cell and target gNB, respectively.

[0069] Furthermore, if the wireless quality between the UE 100 and the cell C1 (gNB 200A) suddenly deteriorates, the UE 100 may transition to an RRC idle state due to a radio link failure (RLF) or the like without receiving an HO command from the gNB 200A. In such a case, there is a problem that it may become difficult for the UE 100 to continue receiving an MBS session to which the first delivery mode is applied, for example.

[0070] In the embodiment, the above-mentioned problem can be solved by using a conditional handover (CHO). Note that, under the premise of performing HO of the UE 100 from the cell C1 to the cell C2, the cell C1 and the gNB 200A are referred to as the source cell and the source gNB, respectively, and the cell C2 and the gNB 200B are referred to as the candidate cell and the candidate gNB, respectively.

[0071] Here, a general overview of CHO will be described. CHO is defined as HO executed by UE 100 when HO execution conditions are satisfied. When UE 100 receives an RRC Reconfiguration message including CHO configuration from source cell C1 (source gNB 200A), it starts evaluating the execution conditions, and stops evaluating the execution conditions when HO is executed. The CHO configuration includes a candidate cell configuration generated by candidate gNB 200B and an execution condition generated by source gNB 200A. The candidate cell configuration and the execution condition are associated with each other. A set of candidate cell configuration and execution condition may be provided for each of multiple candidate cells. Such CHO configuration is also called Conditional Reconfiguration. The execution condition is information that sets the radio quality to be measured, a threshold to be compared with the radio quality, etc. During CHO execution, i.e., from the point when UE 100 starts synchronization with candidate cell C2, UE 100 does not monitor the source cell.

[0072] In an embodiment, the source cell C1 (source gNB 200A) includes, in the CHO configuration for the candidate cell C2 (specifically, the candidate cell configuration for the candidate cell C2), an MBS reception configuration (e.g., MTCH configuration information) for receiving an MBS session provided by the candidate cell C2 in PTM. The UE 100 receives an RRC Reconfiguration message including the CHO configuration (Conditional Reconfiguration) from the source cell C1. In response to the MBS reception configuration for receiving an MBS session provided by the candidate cell C2 in PTM being included in the CHO configuration, the UE 100 starts MBS reception from the candidate cell C2 using the MBS reception configuration before performing CHO. This makes it easy for the UE 100 to continue receiving the MBS session.

[0073] FIG. 12 is a diagram showing an example of the operation of the mobile communication system 1 according to the embodiment.

[0074] In step S1, the UE 100 in the RRC connected state receives an MBS session (MBS data) provided by the source cell C1 (source gNB 200A) via PTM. The MBS session may be a multicast session or a broadcast session. Such an MBS session is called a desired MBS session.

[0075] In step S2, UE100 may perform measurements for each cell based on the measurement configuration set from source cell C1 (source gNB200A) and send a measurement report including the measurement results to source cell C1 (source gNB200A).

[0076] In step S3, the source cell C1 (source gNB200A) decides to use CHO, for example, based on a Measurement Report.

[0077] In step S4, the source cell C1 (source gNB 200A) sends an HO Request message requesting CHO to the candidate cell C2 (candidate gNB 200B). The source cell C1 (source gNB 200A) may send the HO Request message to multiple candidate cells C2 (multiple candidate gNBs 200B).

[0078] In step S5, the candidate cell C2 (candidate gNB 200B) transmits an HO Request Acknowledge message to the source cell C1 (source gNB 200A) to accept the CHO. The HO response includes a candidate cell configuration of the candidate cell C2. The candidate cell configuration includes various settings required for the UE 100 to perform radio communication with the candidate cell C2. In the embodiment, the candidate cell configuration is a setting provided by the candidate cell C2 in the PTM. do Includes MBS reception settings (e.g., MTCH setting information) for receiving an MBS session.

[0079] In step S6, the source cell C1 (source gNB200A) transmits an RRC Reconfiguration message to the UE100, which includes the execution conditions determined by the source cell C1 and the candidate cell configuration received from the candidate cell C2 (candidate gNB200B) as a CHO configuration (Conditional Reconfiguration).

[0080] In step S7, UE 100 may transmit an RRC Reconfiguration Complete message to source cell C1 (source gNB 200A) in response to the RRC Reconfiguration message from source cell C1 (source gNB 200A). Note that step S7 may be performed after step S8 or step S9.

[0081] In step S8, UE 100 determines whether or not an MBS reception setting for receiving an MBS session provided by candidate cell C2 in PTM is included in CHO setting (Conditional Reconfiguration), specifically, in the candidate cell setting. When the MBS reception setting is included in the CHO setting (Conditional Reconfiguration), UE 100 may determine whether or not candidate cell C2 provides the same MBS session (desired MBS session) as the MBS session provided by source cell C1, based on the MBS reception setting. For example, when an MBS session identifier (e.g., TMGI) of the desired MBS session is included in the candidate cell setting, UE 100 may determine that candidate cell C2 provides the desired MBS session. Here, the description will proceed assuming that UE 100 has determined that candidate cell C2 provides the desired MBS session.

[0082] In step S9, UE 100 starts receiving an MBS from candidate cell C2 using the MBS reception setting. Specifically, UE 100 starts receiving an MBS session (desired MBS session) provided by candidate cell C2 in PTM using the MBS reception setting for the desired MBS session. Note that if there are multiple candidate cells that provide the desired MBS session, UE 100 may receive the MBS session from each of the multiple candidate cells, or may receive MBS only from a candidate cell with good radio conditions (RSRP, etc.).

[0083] In step S10, the UE 100 determines whether the execution condition for HO is satisfied for the candidate cell C2. Here, the description will proceed assuming that the UE 100 determines that the execution condition for HO is satisfied for the candidate cell C2.

[0084] In step S11, UE 100 performs HO to candidate cell C2. Specifically, UE 100 applies the candidate cell configuration of candidate cell C2, synchronizes with candidate cell C2, and transmits an RRC Reconfiguration Complete message to candidate cell C2. Upon receiving the RRC Reconfiguration Complete message, candidate cell C2 (candidate gNB 200B) may transmit an HO success (HANDOVER SUCCESS) message to source cell C1 (source gNB 200A).

[0085] In step S12, the UE 100 may continue receiving the MBS from the source cell C1 (source gNB 200A) for a certain period of time after starting synchronization with the candidate cell C2, and may end receiving the MBS from the source cell C1 (source gNB 200A) after the certain period has elapsed. The certain period may be set to the UE 100 by the source cell C1 (source gNB 200A).

[0086] (Example of change) In the above-described embodiment, UE 100 starts receiving an MBS from candidate cell C2 before performing CHO. Therefore, UE 100 receives packets belonging to the same MBS session from each of source cell C1 and candidate cell C2. As a result, overlapping may occur between a packet received from source cell C1 and a packet received from candidate cell C2. In this modification, when overlapping occurs between a packet received from source cell C1 and a packet received from candidate cell C2, UE 100 discards one of the overlapping packets.

[0087] Here, the packet may be a PDCP packet. Fig. 13 is a diagram showing packets constituting MBS data, specifically a PDCP data PDU (Protocol Data Unit). The PDCP data PDU has a PDCP sequence number (SN), data, and MAC-I. The PDCP SN is a sequence number that is sequentially assigned to the PDCP data PDU. The data corresponds to a PDCP SDU (Service Data Unit). The MAC-I corresponds to a message authentication code. The PDCP data PDU may not have a MAC-I.

[0088] In this modified example, it is assumed that the same sequence number (PDCP SN) is assigned to the same packet (IP packet) in the source cell C1 (source gNB 200A) and the candidate cell C2 (candidate gNB 200B). Since one PDCP PDU is generated per IP packet, as long as the PDCP SN of the first IP packet matches in both cells, the correspondence between each subsequent MBS packet and each PDCP SN will match regardless of the cell.

[0089] 14 is a diagram showing a modified example of the operation of the mobile communication system 1 according to the embodiment. Here, differences from the operation according to the embodiment described above (see FIG. 12) will be explained.

[0090] In step S101, the PDCP entity of UE 100 detects duplicate packets based on the PDCP sequence number included in each received packet. Specifically, when the PDCP entity of UE 100 receives packets belonging to the same MBS session from source cell C1 and candidate cell C2, if the PDCP SN of the packet received from source cell C1 matches the PDCP SN of the packet received from candidate cell C2, it recognizes these packets as duplicate packets and discards one of the packets.

[0091] FIG. 15 is a diagram showing an example of internal processing of the UE 100 according to this modification.

[0092] The UE 100 establishes one PDCP entity that terminates the first MBS transmission path from the source cell C1 to the UE 100 and the second MBS transmission path from the candidate cell C2 to the UE 100. That is, the PTM path (MTCH) from the source cell C1 and the PTM path (MTCH) from the candidate cell C2 terminate in the same PDCP entity.

[0093] For example, when UE100 starts receiving an MBS session (PTM) from candidate cell C2, it associates the MTCH of candidate cell C2 with the MRB (set by source cell C1) of the MBS session. In other words, from the perspective of UE100, a split MRB (a split MRB of PTM-leg and PTM-leg) is formed by source cell C1 and candidate cell C2. Such processing is performed only when the MBS session identifier associated with the MRB associated with the MTCH of candidate cell C2 matches the MBS session identifier of the MRB of source cell C1.

[0094] In such a configuration, the PDCP entity of UE 100, which receives packets (PDCP PDUs) from the source cell C1 and the candidate cell C2, discards one of the packets when it detects a duplicate packet. For example, the PDCP entity of UE 100 discards one of the duplicate packets that falls outside the reception window. As described above, since one PDCP PDU (i.e., one PDCP SN) is generated for one IP packet, even if the PDCP SNs transmitted by the source cell C1 and the candidate cell C2 at a certain point in time are different (not synchronized), the PDCP SN itself is associated with the same packet. Therefore, the PDCP entity of UE 100 can determine whether or not there is a duplicate IP packet by looking at the PDCP SN.

[0095] FIG. 16 is a diagram showing another example of the internal processing of the UE 100 according to this modification.

[0096] The UE 100 establishes a PDCP entity #1 (first PDCP entity) that terminates a first MBS transmission path from the source cell C1 to the UE 100, and a PDCP entity #2 (second PDCP entity) that terminates a second MBS transmission path from the candidate cell C2 to the UE 100. That is, the PTM path (MTCH) from the source cell C1 and the PTM path (MTCH) from the candidate cell C2 terminate in separate PDCP entities.

[0097] For example, when UE 100 starts receiving an MBS session (PTM) from candidate cell C2, it configures the corresponding MRB of candidate cell C2, i.e., an MRB (PDCP entity #2) separate from the MRB (PDCP entity #1) of source cell C1. Here, UE 100 identifies the MRB (PDCP entity #1) of source cell C1 that is linked to the same MBS session identifier as the generated MRB (PDCP entity #2).

[0098] In UE 100 receiving packets (PDCP PDUs) from each of source cell C1 and candidate cell C2, PDCP entity #2 of candidate cell C2 queries PDCP entity #1 of source cell C1 about the PDCP SN of the received packet, and discards the packet if it has already been received, or passes the packet to a higher layer if it has not yet been received. PDCP entity #1 of source cell C1 may consider the PDCP SN to have been received.

[0099] (Other embodiments) The above-mentioned operational flows are not limited to being implemented independently, but can also 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.

[0100] In the above-described 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 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 user equipment may also be an MT (Mobile Termination) of the IAB node.

[0101] A program may be provided that causes a computer to execute each process performed by UE100 or gNB200. The program may be recorded on a computer-readable medium. The computer-readable medium can be used to install the program 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 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).

[0102] As used in this disclosure, the terms "based on" and "depending on" 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 "based only on" and "at least in part on." Furthermore, "obtain" may mean obtaining information from stored information, obtaining information from information received from another node, or obtaining information by generating the information. The terms "include," "comprise," and variations thereof do not mean including only the listed items, but may also mean including only the listed items or including additional items in addition to the listed items. Furthermore, as used in this disclosure, the term "or" 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, reference to first and second elements 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 shall include the plural unless the context clearly indicates otherwise.

[0103] 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.

[0104] This application claims priority to Japanese Patent Application No. 2021-175942 (filed October 27, 2021), the entire contents of which are incorporated herein by reference. [Explanation of symbols]

[0105] 1: Mobile communication system 10:RAN 20 :CN 100:UE 110: Receiving unit 120: Transmitter 130: Control unit 200 :gNB 210: Transmission unit 220: Receiving unit 230: Control unit 240: Backhaul communication unit

Claims

1. A communication method performed by a user device in a mobile communication system providing a multicast broadcast service (MBS), comprising: receiving a radio resource control (RRC) reconfiguration message from a source cell, the radio resource control (RRC) reconfiguration message including a conditional handover (CHO) configuration for a candidate cell for the CHO; In response to the MBS reception setting for receiving an MBS session provided by the candidate cell in PTM (Point-To-Multipoint) being included in the CHO setting, starting MBS reception from the candidate cell using the MBS reception setting before performing the CHO. Communication method.

2. Starting MBS reception from the candidate cell before performing the CHO includes starting MBS reception from the candidate cell before determining whether a handover execution condition is satisfied for the candidate cell. The communication method according to claim 1 .

3. receiving an MBS session provided by the source cell in a PTM; and determining whether the candidate cell provides the same MBS session as the source cell based on the MBS reception configuration; The initiating includes, when it is determined that the candidate cell provides the same MBS session, initiating the MBS reception from the candidate cell using the MBS reception configuration before performing the CHO. The communication method according to claim 1 .

4. receiving packets belonging to the same MBS session from each of the source cell and the candidate cell; If an overlap occurs between the received packet from the source cell and the received packet from the candidate cell, discarding one of the overlapping packets. The communication method according to claim 3 .

5. The packet is a Packet Data Convergence Protocol (PDCP) packet, The discarding may be performed by a PDCP entity of the user equipment: detecting the duplicate packets based on a PDCP sequence number included in each received packet; discarding one of the duplicate packets. The communication method according to claim 4.

6. and establishing one PDCP entity terminating a first MBS transmission path from the source cell to the user equipment and a second MBS transmission path from the candidate cell to the user equipment. A communication method according to any one of claims 1 to 5.

7. and establishing a first PDCP entity terminating a first MBS transmission path from the source cell to the user equipment and a second PDCP entity terminating a second MBS transmission path from the candidate cell to the user equipment. A communication method according to any one of claims 1 to 5.

8. A user equipment for use in a mobile communication system providing a multicast broadcast service (MBS), comprising: a receiver for receiving a radio resource control (RRC) reconfiguration message from a source cell, the radio resource control (RRC) reconfiguration message including a conditional handover (CHO) configuration for a candidate cell for the CHO; A control unit that starts MBS reception from the candidate cell using the MBS reception setting before performing the CHO in response to the MBS reception setting for receiving an MBS session provided by the candidate cell in PTM (Point-To-Multipoint) being included in the CHO setting. User equipment.

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