Method and device for multicast transmission

Abstract

A method and device for multicast transmission is provided. [Solution] A method for multicast transmission performed by a first network device includes a step of receiving a first message requesting a handover from a second network device, and a step of transmitting a second message to the second network device in response to the first message.

Description

[Technical Field]

[0001] The present invention relates to wireless communication technology, and more particularly to a method and device for multicast transmission. [Background technology]

[0002] Efforts have been made to develop improved fifth-generation (5G) or pre-5G communication systems to meet the increased demand for wireless data traffic since the deployment of fourth-generation (4G) communication systems. 5G or pre-5G communication systems are also referred to as "beyond 4G networks" or "post-LTE (post long term evolution) systems." To achieve even higher data rates, 5G communication systems are expected to be implemented in higher frequency (mmWave) bands, such as the 60 GHz band. To reduce radio wave propagation loss and extend transmission distances, beamforming, massive MIMO (multiple-input multiple-output), full-dimensional MIMO (FD-MIMO), array antennas, analog beamforming, and large-scale antenna techniques are being discussed for 5G communication systems. Furthermore, in the 5G communication system, development is underway to improve the system network based on next-generation small cells, cloud radio access networks (RAN), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, mobile networks, cooperative communication, coordinated multi-points (CoMP), receiver interference cancellation, etc. In the 5G system, hybrid frequency shift keying (FSK), Feher's quadrature amplitude modulation (FQAM), and sliding window superposition coding (SWSC) have been developed as advanced coding modulation (ACM), and filter bank multicarrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) have been developed as advanced access technologies.

[0003] The Internet, a human-centered, connected network where humans generate and consume information, is evolving into the Internet of Everything (IoE), where distributed entities like things exchange and process information without human intervention. The Internet of Everything (IoE) has emerged as a combination of IoT technology and big data processing technology via connections to cloud servers. As technological elements like the technology-connected network where humans generate and consume information evolve into the Internet of Things (IoT), where cloud servers embody IoT, recent research has focused on sensor networks, machine-to-machine (M2M) communication, and machine-type communication (MTC). Such an IoT environment can provide intelligent Internet technology services that create new value in human life by collecting and analyzing data generated between connected things. Through the fusion and combination of existing information technology (IT) and various industrial applications, IoT can be applied to a variety of fields, including smart homes, smart buildings, smart cities, smart or connected cars, smart grids, healthcare, smart home appliances, and next-generation medical services.

[0004] Accordingly, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as sensor networks, MTC, and M2M communication are also implemented using beamforming, MIMO, and array antennas. The application of Cloud RAN as a big data processing technology mentioned above can be seen as an example of the fusion of 5G technology and IoT technology. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention provides a method and device for multicast transmission. [Means for solving the problem]

[0006] According to one aspect of the present disclosure, a method for multicast transmission performed by a first network device is provided, the method including: receiving a first message requesting a handover from a second network device; and transmitting a second message to the second network device in response to the first message.

[0007] According to one embodiment of the present disclosure, the first message includes information related to a sequence number (SN) of a multicast data packet sent by the second network device and / or information related to a forwarding proposal for the multicast service.

[0008] According to one embodiment of the present disclosure, the information related to the sequence number (SN) of the multicast data packet transmitted by the second network device includes packet data convergence protocol (PDCP) SN information and / or general packet radio service (GPRS) tunneling protocol user plane (GTP-U) SN information.

[0009] According to one embodiment of the present disclosure, the second message includes information related to the SN of the multicast data packet sent by the first network device and / or information related to an instruction to accept forwarding.

[0010] According to an embodiment of the present disclosure, the information related to the SN of the multicast data packet sent by the first network device includes PDCP SN information and / or GTP-U SN information.

[0011] According to one embodiment of the present disclosure, this further includes a step of receiving information related to the SN of the multicast data packet transmitted by the second network device from the second network device, where the information related to the SN of the multicast data packet transmitted by the second network device includes PDCP SN information and / or GTP-U SN information.

[0012] According to one embodiment of the present disclosure, this further includes a step of determining data that needs to be forwarded by referring to one or more of information related to the SN of the multicast data packet transmitted by the first network device and information related to the SN of the multicast data packet transmitted by the second network device.

[0013] According to one embodiment of the present disclosure, this further includes determining to discontinue data forwarding; and notifying the second network device of the discontinuation of data forwarding.

[0014] According to another aspect of the present disclosure, a method for multicast transmission performed by a second network device is provided, the method including: transmitting a first message requesting handover to a first network device; and receiving a second message from the first network device in response to the first message.

[0015] According to one embodiment of the present disclosure, the first message includes information related to the SN of the multicast data packet sent by the second network device and / or information related to a forwarding proposal for the multicast service.

[0016] According to an embodiment of the present disclosure, the information related to the SN of the multicast data packet sent by the second network device includes PDCP SN information and / or GTP-U SN information.

[0017] According to one embodiment of the present disclosure, the second message includes information related to the SN of the multicast data packet sent by the first network device and / or information related to an instruction to accept forwarding.

[0018] According to an embodiment of the present disclosure, the information related to the SN of the multicast data packet sent by the first network device includes PDCP SN information and / or GTP-U SN information.

[0019] According to one embodiment of the present disclosure, this further includes a step of determining data that needs to be forwarded by referring to one or more of information related to the SN of the multicast data packet transmitted by the first network device and information related to the SN of the multicast data packet transmitted by the second network device.

[0020] According to one embodiment of the present disclosure, this further includes deciding to discontinue data forwarding.

[0021] According to one embodiment of the present disclosure, this further includes a step of transmitting information related to the SN of the multicast data packet transmitted by the second network device to the first network device, where the information related to the SN of the multicast data packet transmitted by the second network device includes PDCP SN information and / or GTP-U SN information.

[0022] According to another aspect of the present disclosure, a first network device for multicast transmission is provided, the first network device including a transceiver configured to transmit and receive signals, and a control unit configured to perform a method according to each embodiment of the present disclosure.

[0023] According to another aspect of the present disclosure, a second network device for multicast transmission is provided, wherein a first network device includes a transceiver configured to transmit and receive signals, and a control unit configured to perform a method according to each embodiment of the present disclosure.

[0024] According to another aspect of the present disclosure, there is provided a method that includes setting SN information of a PDCP based on SN information of a GTP-U according to one or more predefined rules. [Brief explanation of the drawings]

[0025] For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals indicate like elements and in which: [Figure 1] Illustrates system architecture diagrams for system architecture evolution (SAE). [Figure 2] 1 illustrates a schematic diagram of the initial overall architecture of 5G. [Figure 3] 1 illustrates a schematic diagram of Method 1 according to various embodiments of the present disclosure. [Figure 4] 1 illustrates a schematic diagram of Method 2 according to various embodiments of the present disclosure. [Figure 5] 1 illustrates a schematic diagram of Example 1 according to various embodiments of the present disclosure. [Figure 6] 1 illustrates a schematic diagram of Example 2 according to various embodiments of the present disclosure. [Figure 7] 1 illustrates a schematic diagram of Example 3 according to various embodiments of the present disclosure. [Figure 8] 1 illustrates a schematic diagram of Example 4 according to various embodiments of the present disclosure. [Figure 9] 1 illustrates a schematic diagram of Example 5 according to various embodiments of the present disclosure. [Figure 10] 1 illustrates a block diagram of a network device in accordance with various embodiments of the present disclosure. [Figure 11] 10A-10C illustrate schematic diagrams according to various embodiments of Example 6 of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0026] Before proceeding with the detailed description below, it may be advantageous to provide definitions of certain words and phrases used throughout this patent document, as follows: the terms "include" and "comprise," and their derivatives, mean inclusive without limitation, the term "or" is inclusive and means "and / or," the terms "related to" and "related with," as well as their derivatives, mean "comprising," "comprised in," "coupled with," "including," "contained in," "coupled to or with," "communicable with," "cooperating with," "intervening through," "co-locating with," "proximate to," "coupled to or with," "having," "having the properties of," and the term "controller" means any device, system, or portion thereof that controls at least one operation, whether such device is embodied in hardware, firmware, or software, or some combination of at least two of them. It should be noted that the functionality associated with any particular controller may be locally or remotely, centralized or distributed.

[0027] Additionally, the various functions described below may be embodied or supported by one or more computer programs, each of which is formed from computer-readable program code and embodied in a computer-readable medium. The terms "application" and "program" refer to one or more computer programs, software components, sets of instructions, procedures, functions, objects, classes, instances, associated data, or portions thereof suitable for implementation in suitable computer-readable program code. The term "computer-readable program code" includes any type of computer code, including source code, object code, and executable code. The term "computer-readable medium" includes any type of medium that can be accessed by a computer, such as read-only memory (ROM), random access memory (RAM), hard disk drive, compact disc (CD), digital video disc (DVD), or any other type of memory. "Non-transitory" computer-readable medium excludes wired, wireless, optical, or other communication links that transmit transient electrical or other signals. Non-transitory computer-readable media include media on which data may be permanently stored and media on which data may be stored and subsequently overwritten, such as rewritable optical disks or erasable memory devices.

[0028] Definitions for certain words and phrases are provided throughout this patent document, and one of ordinary skill in the art should understand that in many, if not most, cases, such definitions apply to previous and future uses of the words and phrases so defined.

[0029] 1-11 discussed below and the various embodiments in this patent document illustrating the principles of the present disclosure are for illustrative purposes only and should not be construed as limiting the scope of the present disclosure in any way. Those of ordinary skill in the art will understand that the principles of the present disclosure may be embodied in any suitably arranged system or device.

[0030] After the deployment of 4G communication systems, efforts have been made to develop improved 5G or pre-5G communication systems to meet the increasing demand for wireless data communication services. Therefore, 5G or pre-5G communication systems are also referred to as "beyond 4G networks" or "post-LTE systems."

[0031] Wireless communication is currently one of the most successful innovations in history. The number of wireless communication service subscribers has recently exceeded 5 billion and is growing rapidly. The increasing popularity of smartphones and other mobile data devices (i.e., tablet computers, laptops, netbooks, e-book readers, and machine-type devices) among consumers and businesses is rapidly increasing the demand for wireless data services. To meet the rapid growth of mobile data services and support new applications and deployments, it is crucial to improve the efficiency and coverage of the air interface. When a user equipment (UE) moves, if the UE receives a multicast broadcast service (MBS), a data forwarding mechanism for the MBS service is required. The method and device for multicast transmission provided by the present disclosure can realize multicast transmission handover.

[0032] FIG. 1 illustrates an exemplary system architecture 100 of System Architecture Evolution (SAE). User equipment (UE) 101 is a terminal device for receiving data. E-UTRAN (evolved universal terrestrial radio access network) 102 is a radio access network, including a macro base station (eNodeB / NodeB) that provides the UE with an interface for accessing the radio network. Mobility management entity (MME) 103 is responsible for managing mobility context, UE session context, and security information. Serving gateway (SGW) 104 primarily provides user plane functions, and the MME 103 and SGW 104 may reside in the same physical entity. Packet data network gateway (PGW) 105 is responsible for functions such as charging and lawful interception, and may reside in the same physical entity as the SGW 104. The policy and charging rules function entity (PCRF) 106 provides quality of service (QoS) policies and charging criteria. The general packet radio service support node (SGSN) 108 is a network node device that provides routing for data transmission in the universal mobile telecommunications system (UMTS). The home subscriber server (HSS) 109 is the UE's home subsystem and is responsible for protecting user information, including the user equipment's current location, serving node address, user security information, and the user equipment's packet data context.

[0033] 2 illustrates an exemplary system architecture 200 according to various embodiments of the present disclosure. Other embodiments of the system architecture 200 may be used without departing from the scope of the present disclosure. The user equipment (UE) 201 is a terminal device for receiving data. The next generation radio access network (NG-RAN) 202 is a radio access network including a base station (a gNB or eNB connected to a 5G core network (5GC), where an eNB connected to a 5GC is also called an ng-gNB) that provides an interface for the UE to access the radio network. The access control and mobility management function entity (AMF) 203 is responsible for managing the UE's mobility context and security information. The user plane function entity (UPF) 204 mainly provides user plane functions. The session management function entity (SMF) 205 is responsible for session management. The data network (DN) 206 includes, for example, operator services, Internet access, and third-party services.

[0034] In the following embodiments, the description will be made by taking a 5G system as an example, a CU-CP (centralized unit-control plane) as an access network control plane, a CU-UP (centralized unit-user plane) as an example of an access network user plane, and a DU (distributed unit) as an example of a distributed unit, and the method can also be applied to the corresponding entities of other systems.

[0035] When a UE moves, if the UE is receiving a multicast broadcast service (hereinafter referred to as MBS), a source base station may transmit buffered data to a destination base station to reduce data loss. Compared with data forwarding for general services, data forwarding for MBS services requires a different mechanism.

[0036] The present invention provides a method and device for switching the reception of a multicast transmission. The method and device for multicast transmission provided by the present disclosure can ensure service continuity, avoid or reduce additional overhead for multicast data transmission, improve the utilization efficiency of access network resources and / or air interface resources, and reduce transmission delays, data loss, delays caused by switching, and unnecessary data transmissions when multicast transmissions are switched.

[0037] FIG. 3 illustrates a schematic diagram of Method 1 according to various embodiments of the present disclosure. In step 301, the source base station transmits a handover request to the destination base station, and the handover request message carries an MBS service identifier that the UE is receiving or is interested in, an identifier of the radio bearer of the MBS service, and / or a forwarding proposal for the MBS service, and the message may also include sequence number (SN) information contained in the source base station's MBS data packet.

[0038] In step 302, the destination base station determines whether data forwarding of the MBS is necessary. If data forwarding is necessary, a handover confirmation message transmitted by the destination base station to the source base station carries an instruction to accept the forwarding and carries SN information corresponding to data that the destination base station has transmitted and / or is transmitting, or carries SNs corresponding to forwarded data packets proposed by the destination base station.

[0039] In another embodiment, the destination base station may transmit a separate message to the source base station to convey SN information corresponding to data that the destination base station has transmitted and / or is transmitting, or to convey SN information corresponding to forwarded data proposed by the destination base station.

[0040] In step 303, the source base station determines when to stop data forwarding based on the SN included in the received MBS data packet currently being transmitted (already transmitted and / or being transmitted) by the destination base station.

[0041] According to the method of the present disclosure, when multicast transmission is switched, it is possible to ensure service continuity, avoid or reduce additional overhead of multicast data transmission, improve the utilization efficiency of access network resources and / or air interface resources, and reduce transmission delays, data loss, delays caused by switching, and unnecessary data transmissions.

[0042] FIG. 4 illustrates a schematic diagram of Method 2 according to various embodiments of the present disclosure. In step 401, the source base station transmits a handover request to the destination base station, and the handover request message carries an MBS service identifier that the UE has received or is interested in, an identifier of the radio bearer of the MBS service, and / or a forwarding offer for the MBS service.

[0043] In step 402, the destination base station determines whether data forwarding of the MBS is required. If data forwarding is required, an instruction to accept the forwarding is carried in a handover confirmation message transmitted by the destination base station to the source base station.

[0044] In step 403, when the destination base station determines that there is a need to suspend forwarding, the destination base station transmits a message to notify the source base station of the suspension of data forwarding. Alternatively, when the destination base station determines that there is a need to suspend forwarding, the destination base station discards forwarded data packets and starts transmitting data packets coming from the core network, and the source base station itself determines when to suspend data transmission.

[0045] In Method 1 and Method 2, the SN can be a GTP-u SN or a PDCP SN, and the PDCP SN and the GTP-U SN have a corresponding relationship, or a one-to-one correspondence, or the GTP-U SN is mapped to the PDCP SN according to a predetermined rule.

[0046] The method of the present disclosure can ensure service continuity when multicast transmission is switched, avoid or reduce additional overhead for multicast data transmission, improve the utilization efficiency of access network resources and / or air interface resources, and reduce transmission delays, data loss, delays caused by switching, and unnecessary data transmissions.

[0047] FIG. 5 illustrates a schematic diagram of Example 1 according to various embodiments of the present disclosure. FIG. 5 illustrates an embodiment of Method 1 for maintaining service continuity when a UE is handed over. When a UE moves from a source base station to a destination base station, if the UE receives an MBS from the source base station, the UE must continue to receive the MBS when the UE moves to the destination base station. It is preferable that the handover procedure does not result in loss of MBS data and allows the UE to continuously receive data. For unicast services, when a UE moves to a destination base station, the core network suspends data transmission to the source base station and notifies the source base station of the suspension of data transmission, identifying a specific data packet as the last data packet and indicating the suspension of data transmission. However, for MBS services, the destination base station has already transmitted data in a point-to-multipoint transmission mode, and the source base station must continue transmitting the MBS and not terminate data transmission simply because the UE is moving far away.

[0048] Therefore, the source base station needs to continue receiving data from the core network and cannot receive a data interruption instruction from the core network, so the source base station does not know when data forwarding should be terminated. According to this embodiment, when the source base station receives SN information transmitted by the destination base station, the source base station forwards data packets not yet transmitted by the destination base station to the destination base station according to the SN information and determines when to terminate data forwarding, which can reduce data forwarding and ensure continuous transmission and reception of data. Example 1 describes the procedure, and detailed descriptions of steps not related to the present disclosure will be omitted here. Example 1 includes the following steps.

[0049] In step 501, the source base station initiates a handover request message to the destination base station. The message carries a session identifier of each requested PDU session, information on the QoS flows included in the PDU session, DRB information, etc. If the UE is receiving an MBS, the message also includes an MBS identifier, a forwarding offer for the MBS service, or a forwarding offer configured for the radio bearer of each MBS, and the message also includes a session identifier corresponding to the MBS, information on the QoS flows corresponding to the MBS, configuration information of the DRB transmitting the MBS, etc., and mapping configuration information of the QoS flows to the MBS MRBs in the source base station. The message also includes SN information of the MBS data transmitted by the source base station, and the SN information may include the PDCP SN and / or the GTP-U SN.

[0050] In one embodiment, the SN information includes a PDCP SN, which may be one or more of the following information: ■ PDCP SN list corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ The highest PDCP SN corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■ a PDCP SN list corresponding to downlink PDCP data packets that have been successfully transmitted to the UE in sequence; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ a PDCP SN list corresponding to downlink PDCP data packets successfully transmitted to the UE; and / or ■ The highest PDCP SN corresponding to a downlink PDCP data packet successfully transmitted to the UE.

[0051] In one embodiment, the SN information may include a GTP-U SN. Specifically, the GTP-U SN may include one or more of the following information: ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the PDCP layer; ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the SDAP layer; ■ the SN of the received GTP-u data packet having the highest sequence number; a list of SNs of GTP-u data packets corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted in sequence to the UE; ■ the SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE; and / or ■ The SN in the header of the GTP-U data packet with the highest sequence number corresponding to a downlink PDCP data packet successfully transmitted to the UE.

[0052] Depending on whether the destination cell has already transmitted the MBS, the destination base station determines whether to transmit a message to the core network to request transmission of MBS data. If the destination cell receives the MBS from the core network, the destination base station transmits a response message to the source base station.

[0053] If the destination cell of the destination base station has not received the MBS data from the core network, the destination base station transmits a message to the core network to request the core network to transmit the MBS data. The procedure for establishing an MBS bearer is described in another embodiment. The destination base station transmits a response message to the source base station.

[0054] In step 502, the destination base station transmits a handover request response message, which includes a transparent transmitter from the destination to the source, which includes an RRC message transmitted by the destination base station to the UE.

[0055] The message also includes information about the successfully established PDU session. The PDU session information includes a PDU session identifier, an identifier of the successfully established DRB, an identifier of the successfully established MBS, and an identifier of the successfully established MRB. If the destination base station transmits an MBS service via point-to-point transmission or point-to-multipoint transmission, the message also includes indication information for the destination base station to determine that MBS data forwarding is necessary, and GTP-U SN and / or PDCP SN information corresponding to the forwarded data proposed by the destination base station. The destination base station may determine whether data forwarding is necessary by referring to various pieces of information. For example, in step 501, the destination base station may determine whether data forwarding is necessary by referring to the SN information of the MBS data transmitted by the received source base station. For example, the SN of the MBS data transmitted by the source base station indicates that the highest GTP-u SN being transmitted is 80 and the GTP-u SN currently being transmitted by the destination base station is 100, and the destination base station determines that data forwarding is required.

[0056] Depending on whether the mapping of QoS flows to MRBs (or DRBs) of an MBS service determined by the destination base station is the same as the mapping of QoS flows to MRBs (or DRBs) of the source base station, the destination base station may transmit different sequence numbers. For example, if the mapping of QoS flows to MRBs (or DRBs) of an MBS service determined by the destination base station is the same as the mapping of QoS flows to MRBs (or DRBs) of the source base station, and the contents of data packets corresponding to the same PDCP SN of the destination base station and the source base station are the same, the PDCP SN may be transmitted to the source base station. If the mapping configurations are different and the contents of data packets corresponding to the same PDCP SN of the destination base station and the source base station are different, the GTP-u SN may be transmitted. In an embodiment, for simplicity, if a point-to-multipoint radio data bearer MRB is also established for an MBS service, the mapping of the source base station and the destination base station is the same.

[0057] In one example, the PDCP SN information and / or the GTP-U SN information may be one or more of the following information: ■ PDCP SN list corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ The highest PDCP SN corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■ a PDCP SN list corresponding to downlink PDCP data packets that have been successfully transmitted to the UE in sequence; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ a PDCP SN list corresponding to downlink PDCP data packets successfully transmitted to the UE; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE; ■ a PDCP SN set by the base station indicating the highest PDCP SN corresponding to the downlink PDCP data packet that needs to be forwarded; and / or ■Indicates the PDCP SN list corresponding to the downlink PDCP data packets that need to be forwarded, and can also indicate a range of SNs, for example, a PDCP SN list set by the base station indicating the lowest and highest PDCP SNs.

[0058] In one example, the GTP-U SN specifically includes one or more of the following information: ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the PDCP layer; ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the SDAP layer; ■ the SN with the highest sequence number among the received GTP-u data packets; a list of SNs of GTP-u data packets corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted in sequence to the UE; ■ the SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted to the UE; ■ The GTP-U SN set by the base station indicating the highest GTP-U SN corresponding to the downlink data packet that needs to be forwarded; and / or ■ A GTP-U SN list set by the base station indicating the GTP-U SN list corresponding to the downlink data packets that need to be forwarded and also indicating the range of SNs, for example, the lowest and highest GTP-U SNs.

[0059] In step 503, the source base station transmits a handover execution command to the UE via an RRC reconfiguration message. The message carries configuration information for a PDU session that the UE needs to establish at the destination base station, and may also include an MBS service identifier and channel mode indication information for the MBS received at the destination base station, as well as point-to-point channel configuration information and / or point-to-multipoint channel configuration information for the MBS.

[0060] In step 504, the source base station sends an SN status transmission to the destination base station. If the PDCP SN information corresponding to the MBS data received by the UE from the source base station is not included in step 501, the PDCP SN and / or GTP-U SN corresponding to the MBS data may be transmitted via a separate message in step 503.

[0061] In one example, the PDCP SN specifically includes one or more of the following information: ■ PDCP SN list corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ The highest PDCP SN corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■ a PDCP SN list corresponding to downlink PDCP data packets that have been successfully transmitted to the UE in sequence; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ a PDCP SN list corresponding to downlink PDCP data packets successfully transmitted to the UE; and / or ■ The highest PDCP SN corresponding to a downlink PDCP data packet successfully transmitted to the UE.

[0062] In one example, the GTP-U SN specifically includes one or more of the following information: ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the PDCP layer; ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the SDAP layer; ■ the SN of the received GTP-u data packet having the highest sequence number; a list of SNs of GTP-u data packets corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted in sequence to the UE; ■ the SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE; and / or ■ The SN in the header of the GTP-U data packet with the highest sequence number corresponding to a downlink PDCP data packet successfully transmitted to the UE.

[0063] In step 505, the source base station forwards the MBS data to the destination base station. Based on the received SN information (SN information can be PDCP SN or GTP-u SN, as described above) corresponding to the forwarded data proposed by the destination base station, the source base station determines what data needs to be forwarded and when to end the forwarding. For example, if the destination base station proposes that the highest GTP-u SN of the forwarded data be set to 100 and the source base station is currently transmitting 80 data packets, the source base station forwards 80 to 100 data packets to the destination base station. To ensure continuous reception of the data, the destination base station may temporarily establish a point-to-point transmission mode for the UE and transmit the forwarded data to the UE via a point-to-point channel. The UE may then switch to point-to-multipoint mode to receive the data. When data forwarding ends, the source base station may indicate the end of data forwarding in the user plane or indicate that the forwarded data packet is the last data packet.

[0064] In step 506, the UE transmits an RRC reconfiguration complete message to the destination base station to indicate completion of the handover.

[0065] In step 507, the destination base station transmits a path switch request message to the core network. If the UE is the first user that needs to receive the MBS from the destination base station and the destination base station has not received MBS data from the core network, the destination base station can request transmission of MBS data via a message in step 507.

[0066] The message of step 507 carries information about the MBS, such as an identifier of the MBS, and requests the core network to transmit MBS data to the base station. In response to receiving the message of step 507, the core network may start transmitting a service start message for the MBS to the base station, and the base station may transmit a response message to the core network, after which the core network may start transmitting the MBS data to the base station. The destination base station first transmits the forwarded data received from the source base station, and then transmits new data received from the core network.

[0067] In step 508, the core network transmits a path switch response message to the destination base station.

[0068] In step 509, the destination base station transmits a UE context release message to the source base station to release the context information of the UE in the source base station.

[0069] In the first embodiment, the source base station transmits MBS data using a point-to-point channel or a point-to-multipoint channel, and the destination base station transmits MBS data using a point-to-multipoint channel. This can also be applied to other situations.

[0070] The method of the present disclosure can ensure service continuity when multicast transmission is switched, avoid or reduce additional overhead for multicast data transmission, improve the utilization efficiency of access network resources and / or air interface resources, and reduce transmission delays, data loss, delays caused by switching, and unnecessary data transmissions.

[0071] FIG. 6 illustrates a schematic diagram of Example 2 according to various embodiments of the present disclosure. FIG. 6 illustrates an embodiment of Method 1 for maintaining service continuity when a UE is handed over. When a UE moves from a source base station to a destination base station, if the UE receives an MBS from the source base station, the UE should continue to receive the MBS when the UE moves to the destination base station. It is more preferable that the handover procedure does not result in loss of MBS data and allows the UE to continuously receive data. Generally, in this disclosure, the destination base station transmits a forwarding suspension instruction to the source base station, and the source base station suspends data forwarding. Alternatively, the destination base station determines which data to discard based on the SN corresponding to the forwarded data packet, and the source base station independently determines when to suspend data forwarding, depending on the embodiment, for example, using a clock scheme. This reduces data forwarding and ensures continuous transmission and reception of data. Example 2 describes the procedure, and detailed descriptions of steps not related to this disclosure will be omitted here. This includes the following steps:

[0072] In step 601, the source base station initiates a handover request message to the destination base station. The message carries the session identifier of each requested PDU session, information on the QoS flows included in the PDU session, DRB information, etc. If the UE receives an MBS, the message also includes an MBS identifier, a forwarding offer for the MBS service, or a forwarding offer configured for the radio bearer of each MBS. The message also includes a session identifier corresponding to the MBS, information on the QoS flows corresponding to the MBS, configuration information of the DRB transmitting the MBS, etc., and mapping configuration information of the QoS flows to the MBS MRBs in the source base station. The message also includes SN information of the MBS data transmitted by the source base station, which may include the PDCP SN and / or the GTP-U SN. For specific information, see the description of Example 1 above.

[0073] Depending on whether the destination cell has already transmitted the MBS, the destination base station determines whether to transmit a message to the core network to request transmission of MBS data. If the destination cell receives the MBS from the core network, the destination base station transmits a response message to the source base station.

[0074] If the destination cell of the destination base station has not received MBS data from the core network, the destination base station transmits a message to the core network to request MBS data transmission from the core network. The procedure for establishing an MBS bearer has already been described in other embodiments, so repeated description will be omitted here. The destination base station transmits a response message to the source base station.

[0075] In step 602, the destination base station transmits a handover request response message, which includes a destination-to-source transparent sender, which includes an RRC message transmitted by the destination base station to the UE. The message also includes information about the successfully established PDU session, including a PDU session identifier, a successfully established DRB identifier, a successfully established MBS identifier, and a successfully established MRB identifier. The message includes indication information that the destination base station determines that MBS data forwarding is required.

[0076] In step 603, the source base station transmits a handover execution command to the UE via an RRC reconfiguration message. The message carries configuration information for a PDU session that the UE needs to establish at the destination base station, and may also include an MBS service identifier and channel mode indication information for the MBS received at the destination base station, as well as point-to-point channel configuration information and / or point-to-multipoint channel configuration information for the MBS.

[0077] In step 604, the source base station transmits an SN status transmission to the destination base station. If the MBS data received by the UE from the source base station does not include information about the PDCP SN corresponding to the MBS data in step 601, the PDCP SN and / or the GTP-U SN corresponding to the MBS data may be transmitted through a separate message in step 604. For specific information, refer to the description of embodiment 1 above. This step may also be omitted if necessary.

[0078] In step 605, the source base station forwards the MBS data to the destination base station. To transmit the forwarded data to the UE, the destination base station may establish a point-to-point transmission mode for the UE and transmit the forwarded data to the UE via a point-to-point channel. Then, after all the forwarded data has been transmitted to the UE, the UE may switch to point-to-multipoint mode or continue to receive the MBS data using the point-to-point channel, depending on the destination base station's further decision. When data forwarding ends, the source base station may indicate the end of data forwarding in the user plane or indicate that the forwarded data packet is the last data packet.

[0079] In step 606, the UE transmits an RRC reconfiguration complete message to the destination base station to indicate completion of the handover.

[0080] In step 607, the destination base station transmits a path switch request message to the core network. If the UE is the first user that needs to receive the MBS from the destination base station and the destination base station has not received MBS data from the core network, the destination base station may request transmission of MBS data via a message in step 607. The message in step 607 carries MBS information, such as an MBS identifier, and requests the core network to transmit the MBS data to the base station. In response to receiving the message in step 607, the core network may start transmitting an MBS service start message to the base station, and the base station may transmit a response message to the core network, after which the core network may start transmitting the MBS data to the base station. The destination base station first transmits the forwarded data received from the source base station, and then transmits new data received from the core network.

[0081] In step 608, the core network transmits a path switch response message to the destination base station.

[0082] In step 609, the destination base station transmits a data forwarding suspend message to the source base station.

[0083] After step 607, the core network starts transmitting data to the destination base station. After receiving the data transmitted by the core network, the destination base station obtains the GTP-U SN included in the header of the data packet, and can determine when forwarding is unnecessary based on the GTP-U SN transmitted by the core network as well as the SN included in the header of the forwarded data packet. Alternatively, when receiving a handover request message from the UE, the destination base station receives the MBS data packet transmitted by the core network, and can determine whether to suspend data forwarding based on the GTP-U SN received from the core network as well as the SN included in the header of the forwarded data packet. When the destination base station determines that forwarding needs to be suspended, the destination base station transmits a message notifying the source base station of the suspension of data forwarding, and the source base station suspends data forwarding to the destination base station. Alternatively, the destination base station transmits data forwarding suspension instruction information to the source base station via the user plane.

[0084] Alternatively, the destination base station determines which forwarded data packets to discard based on the GTP-U SN received from the core network, as well as the SN included in the header of the forwarded data packet, and starts transmitting the data packet out of the core network. According to an implementation method adopted by the source base station, the timing to suspend data forwarding is determined, and for example, the source base station suspends data forwarding when the forwarding clock times out based on a default data forwarding clock.

[0085] In step 610, the destination base station transmits a UE context release message to the source base station to release the UE context information at the source base station. Steps 609 and 610 may also be combined and transmitted in the same message.

[0086] In the second embodiment, the source base station transmits MBS data using a point-to-point channel or a point-to-multipoint channel, and the destination base station transmits MBS data using a point-to-multipoint channel. This can also be applied to other situations.

[0087] The method of the present disclosure can ensure service continuity when multicast transmission is switched, avoid or reduce additional overhead for multicast data transmission, improve the utilization efficiency of access network resources and / or air interface resources, and reduce transmission delays, data loss, delays caused by switching, and unnecessary data transmissions.

[0088] FIG. 7 illustrates a schematic diagram of Example 3 according to various embodiments of the present disclosure. The present invention also includes an embodiment of Method 2 relating to a method for maintaining service continuity when a UE is handed over, as shown in FIG. 7 (Embodiment 3). When a UE moves from a source base station to a destination base station, if the UE receives an MBS from the source base station, the UE should continue to receive the MBS when the UE moves to the destination base station. It is more preferable that there is no loss of MBS data during the handover procedure and that the UE continuously receives the data. Generally, this disclosure adopts two steps. In Step 1, the source base station notifies the destination base station of a sequence number (SN) corresponding to a package transmitted by the source base station, and the destination base station determines whether the destination base station needs to perform data forwarding based on the SN. In Step 2, the destination base station notifies the source base station of its acceptance of forwarding, carrying SN information corresponding to data that has been transmitted or is being transmitted by the destination base station, or carrying SN information corresponding to data forwarded by the destination base station, and based on the SN information, the source base station can determine which data needs to be forwarded or when to terminate data forwarding.

[0089] In the case of a unicast service, when a UE moves to a destination base station, the core network suspends data transmission to the source base station and notifies the source base station of the suspension of data transmission. A specific data packet is identified as the last data packet, and an instruction to suspend data transmission is issued. However, in the case of an MBS service, the destination base station has already transmitted data in a point-to-multipoint transmission mode, and the source base station must continue transmitting the MBS and must not terminate data transmission simply because the UE is moving far away. Therefore, the source base station continues to receive data from the core network and cannot receive a data suspension instruction from the core network. Therefore, the source base station does not know when data forwarding should be terminated. According to this embodiment, when the source base station receives the SN information transmitted by the destination base station, the source base station forwards data packets not yet transmitted by the destination base station to the destination base station according to the instruction and determines when to terminate data forwarding. This reduces data forwarding and ensures continuous transmission and reception of data. Example 3 describes the procedure applied to a different architecture, and detailed descriptions of steps not related to the present disclosure will be omitted here. Example 3 includes the following steps:

[0090] In step 701, the source base station CU-CP initiates a handover request message to the destination base station CU-CP. The message carries a session identifier of each requested PDU session, information on the QoS flows included in the PDU session, DRB information, etc. If the UE is transmitting an MBS, the message also includes an identifier of the MBS, a forwarding offer for the MBS service, or a forwarding offer configured for the radio bearer of each MBS, and the message also includes a session identifier corresponding to the MBS, information on the QoS flows corresponding to the MBS, mapping of the QoS flows to MRBs and / or QoS flows to DRBs, configuration information of the MRB transmitting the MBS and / or the DRB transmitting the MBS, etc.

[0091] Depending on whether the destination cell has already transmitted the MBS, the destination base station determines whether to transmit a message to the core network to request transmission of MBS data. If the destination cell receives the MBS from the core network, the destination base station transmits a response message to the source base station.

[0092] If the destination cell of the destination base station has not received the MBS data from the core network, the destination base station transmits a message to the core network to request the core network to transmit the MBS data. The procedure for establishing the MBS bearer is described in another embodiment. The destination base station transmits a response message to the source base station.

[0093] In step 702, the destination CU-CP initiates a bearer establishment request message to the destination CU-UP. Depending on whether the mapping of QoS flows to the MRB (or DRB) of the MBS service determined by the destination CU-CP is the same as the mapping of QoS flows to the MRB (or DRB) of the source base station, the destination CU-CP can indicate in the message the serial number that the CU-CP will send to the destination CU-CP. For example, if the mapping settings are the same and the contents of data packets corresponding to the same PDCP SN of the destination base station and the source base station are the same, the CU-CP instructs the CU-UP to transmit the PDCP SN to the CU-CP. If the mapping settings are different, the CU-CP instructs the CU-UP to transmit the GTP-u SN to the CU-CP.

[0094] The message also includes an MBS identifier, a forwarding proposal for the MBS service, or a forwarding proposal configured for the radio bearer of each MBS, and the message also includes a session identifier corresponding to the MBS, information on the QoS flow corresponding to the MBS, configuration information of the DRB transmitting the MBS, etc. The message may also include mapping configuration information of the QoS flow to the MRB of the MBS in the source base station. Depending on whether the mapping of the QoS flow to the MRB (or DRB) of the MBS service determined by the destination CU-CP is the same as the mapping of the QoS flow to the MRB (or DRB) of the source base station, the destination CU-UP can determine the sequence number to be transmitted to the destination CU-CP.

[0095] In step 703, the destination CU-UP initiates a bearer establishment response message to the destination CU-CP. The message carries information of the successfully established PDU session, the identifier of the successfully established DRB, the identifier of the successfully established MBS, and the identifier of the successfully established MRB. The message includes indication information indicating that the destination CU-UP determines that MBS data forwarding is required, and information of the PDCP SN or GTP-U SN corresponding to the forwarded data proposed by the destination CU-UP. For the SN identification information, see the next step 705.

[0096] In step 704, a UE context is set up between the destination CU-CP and the DU. This procedure is the same as that described in the previous embodiment and is omitted here.

[0097] In step 705, the destination CU-CP transmits a handover request confirmation message to the source CU-CP, the message including a destination-to-source transparent transmitter, which includes an RRC message transmitted by the destination base station to the UE. The message also includes information about the successfully established PDU session. The PDU session information includes a PDU session identifier, an identifier of the successfully established DRB, an identifier of the successfully established MBS, and an identifier of the successfully established MRB. If the destination base station is transmitting an MBS service in point-to-point transmission or point-to-multipoint transmission, the message includes indication information indicating that the destination base station has determined that MBS data forwarding is required, and GTP-U SN and / or PDCP SN information corresponding to the forwarded data proposed by the destination base station.

[0098] Specifically, the PDCP SN information and / or GTP-U SN information may be one or more of the following information: ■ PDCP SN list corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ The highest PDCP SN corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■ a PDCP SN list corresponding to downlink PDCP data packets that have been successfully transmitted to the UE in sequence; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ a PDCP SN list corresponding to downlink PDCP data packets successfully transmitted to the UE; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE; ■ a PDCP SN set by the base station indicating the highest PDCP SN corresponding to the downlink PDCP data packet that needs to be forwarded; and / or ■A PDCP SN list set by the base station that indicates the PDCP SN list corresponding to the downlink PDCP data packets that need to be forwarded and can also indicate a range of SNs, for example, the lowest and highest PDCP SNs.

[0099] In one example, the GTP-U SN specifically includes one or more of the following information: ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the PDCP layer; ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the SDAP layer; ■ the SN of the received GTP-u data packet having the highest sequence number; a list of SNs of GTP-u data packets corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted in sequence to the UE; ■ the SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted to the UE; ■ The GTP-U SN set by the base station indicating the highest GTP-U SN corresponding to the downlink data packet that needs to be forwarded; and / or ■ A GTP-U SN list set by the base station that indicates the GTP-U SN list corresponding to the downlink data packets that need to be forwarded and can also indicate a range of SNs, for example, the lowest and highest GTP-U SNs.

[0100] In step 706, a UE context modification procedure is performed between the source CU-CP and the DU.

[0101] In step 707, the source CU-CP transmits a bearer modification request message to the source CU-UP. The message carries PDCP SN information and / or GTP-U SN information corresponding to the forwarded data received from the destination CU-CP. For SN identification information, see step 705 above.

[0102] In step 708, the source CU-UP transmits a modify bearer response message to the source CU-CP. The message carries the PDCP SN information or GTP-U SN of the source base station, specifically as shown in step 705.

[0103] In step 709, the source CU-CP transmits an SN status transmission to the destination CU-CP.

[0104] In step 701, if the PDCP SN information corresponding to the MBS data received by the UE from the source base station is not included, the PDCP SN information and / or GTP-U SN information corresponding to the MBS data is transmitted via a separate message in step 709, and for SN identification information, refer to step 705 above.

[0105] In step 710, the source base station CU-CP transmits a handover execution command to the UE via an RRC reconfiguration message. The message carries configuration information for the PDU session that the UE needs to establish with the destination base station, and may include the MBS service identifier and MBS channel mode indication information received at the destination base station, and may also include point-to-point channel configuration information and / or point-to-multipoint channel configuration information for the MBS.

[0106] In step 711, the destination CU-CP transmits a Bearer Modify Request message to the destination CU-UP. The message may include the PDCP SN or GTP-u SN information received by the destination CP, for example, the information received in step 709.

[0107] In step 712, the destination CU-UP transmits a Modify Bearer Response message to the destination CU-CP.

[0108] In step 713, the UE transmits an RRC reconfiguration complete message to the destination base station to indicate completion of the handover.

[0109] In step 714, the destination CU-CP transmits a path switch request message to the core network. If the UE is the first user that needs to receive the MBS from the destination base station and the destination base station has not received MBS data from the core network, the destination base station may request transmission of MBS data via a message in step 714. The message in step 714 carries MBS information, such as an MBS identifier, and requests the core network to transmit the MBS data to the base station. In response to receiving the message in step 714, the core network may start transmitting an MBS service start message to the base station, and the base station may transmit a response message to the core network, after which the core network may start transmitting the MBS data to the base station. The destination base station first transmits the forwarded data received from the source base station, and then transmits new data received from the core network.

[0110] In step 715, the core network transmits a path switch response message to the destination CU-CP.

[0111] In step 716, the destination CU-CP transmits a UE context release message to the source CU-CP.

[0112] In step 717, the source CU-CP transmits a bearer release command to the source CU-UP.

[0113] In step 718, the source CU-CP initiates a UE context release procedure between the source CU-CP and the DU.

[0114] In step 719, the source CU-UP transmits a bearer release complete message to the source CU-CP. The method of the present disclosure can ensure service continuity when multicast transmission is switched, avoid or reduce additional overhead for multicast data transmission, improve the utilization efficiency of access network resources and / or air interface resources, and reduce transmission delays, data loss, delays caused by switching, and unnecessary data transmissions.

[0115] FIG. 8 illustrates a schematic diagram of Example 4 according to various embodiments of the present disclosure. The present invention also includes another embodiment of Method 2 relating to a method for maintaining service continuity when a UE is handed over, as shown in FIG. 8 (Embodiment 4). When a UE moves from a source base station to a destination base station, if the UE receives an MBS from the source base station, the UE should continue to receive the MBS when the UE moves to the destination base station. It is more preferable that there is no loss of MBS data during the handover procedure and the UE continuously receives the data. Typically, in this disclosure, the destination base station transmits a forwarding suspension instruction to the source base station, and the source base station suspends data forwarding. Alternatively, the destination base station determines which data to discard based on the SN corresponding to the forwarded data packet, and the source base station independently determines when to suspend data forwarding, for example, using a clock scheme, depending on the embodiment. This can reduce data forwarding and ensure continuous transmission and reception of data. Embodiment 4 describes the procedure, and detailed descriptions of steps not related to this disclosure will be omitted here. Embodiment 4 includes the following steps.

[0116] In step 801, the source base station CU-CP initiates a handover request message to the destination base station CU-CP. The message carries a session identifier of each requested PDU session, information on the QoS flows included in the PDU session, DRB information, etc. If the UE is transmitting an MBS, the message also includes an identifier of the MBS, a forwarding offer for the MBS service, or a forwarding offer configured for the radio bearer of each MBS, and the message also includes a session identifier corresponding to the MBS, information on the QoS flows corresponding to the MBS, mapping of the QoS flows to MRBs and / or QoS flows to DRBs, configuration information of the MRB transmitting the MBS and / or the DRB transmitting the MBS, etc.

[0117] Depending on whether the destination cell has already transmitted the MBS, the destination base station determines whether to transmit a message to the core network to request transmission of MBS data. If the destination cell receives the MBS from the core network, the destination base station transmits a response message to the source base station.

[0118] If the destination cell of the destination base station has not received the MBS data from the core network, the destination base station transmits a message to the core network to request the core network to transmit the MBS data. The procedure for establishing the MBS bearer is described in another embodiment. The destination base station transmits a response message to the source base station.

[0119] In step 802, the destination CU-CP initiates a bearer establishment request message to the destination CU-UP. Depending on whether the mapping of QoS flows to the MRB (or DRB) of the MBS service determined by the destination CU-CP is the same as the mapping of QoS flows to the MRB (or DRB) of the source base station, the destination CU-CP can indicate in the message the serial number that the CU-UP will send to the destination CU-CP. For example, if the mapping settings are the same and the contents of data packets corresponding to the same PDCP SN of the destination base station and the source base station are the same, the CU-CP instructs the CU-UP to transmit the PDCP SN to the CU-CP. If the mapping settings are different, the CU-CP instructs the CU-UP to transmit the GTP-u SN to the CU-CP.

[0120] The message also includes an MBS identifier, a forwarding proposal for the MBS service, or a forwarding proposal configured for the radio bearer of each MBS, and the message also includes a session identifier corresponding to the MBS, information on the QoS flow corresponding to the MBS, configuration information of the DRB transmitting the MBS, etc. The message may also include mapping configuration information of the QoS flow to the MRB of the MBS in the source base station. Depending on whether the mapping of the QoS flow to the MRB (or DRB) of the MBS service determined by the destination CU-CP is the same as the mapping of the QoS flow to the MRB (or DRB) of the source base station, the destination CU-UP can determine the sequence number to be transmitted to the destination CU-CP.

[0121] In step 803, the destination CU-UP initiates a bearer establishment response message to the destination CU-CP. The message carries information about the successfully established PDU session, the identifier of the successfully established DRB, the identifier of the successfully established MBS, and the identifier of the successfully established MRB. The message also includes an indication that the destination CU-UP has determined that MBS data forwarding is required.

[0122] In step 804, a UE context is set up between the destination CU-CP and the DU. This procedure is the same as that described in the previous embodiment and is omitted here.

[0123] In step 805, the destination CU-CP transmits a handover request response message to the source CU-CP, the message including a destination-to-source transparent sender, which includes an RRC message transmitted by the destination base station to the UE. The message also includes information about the successfully established PDU session, including a PDU session identifier, a successfully established DRB identifier, a successfully established MBS identifier, and a successfully established MRB identifier. The message includes indication information that the destination base station determines that MBS data forwarding is required.

[0124] In step 806, a UE context modification procedure is performed between the source CU-CP and the DU.

[0125] In step 807, the source CU-CP transmits a modify bearer request message to the source CU-UP. The message carries the identifier and indication information of the MBS required by the forwarded data of the MBS received from the destination CU-CP.

[0126] In step 808, the source CU-UP transmits a modify bearer response message to the source CU-CP. The message carries the PDCP SN or GTP-U SN information of the source base station. Specifically, the PDCP SN information and / or GTP-U SN information may be one or more of the following information: ■ PDCP SN list corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ The highest PDCP SN corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■ a PDCP SN list corresponding to downlink PDCP data packets that have been successfully transmitted to the UE in sequence; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ a PDCP SN list corresponding to downlink PDCP data packets successfully transmitted to the UE; and / or ■ The highest PDCP SN corresponding to a downlink PDCP data packet successfully transmitted to the UE.

[0127] In one example, the GTP-U SN specifically includes one or more of the following information: ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the PDCP layer; ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the SDAP layer; ■ SN corresponding to the highest sequence number in the received GTP-u data packet; a list of SNs of GTP-u data packets corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted in sequence to the UE; ■ the SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE; and / or ■ The SN in the header of the GTP-U data packet with the highest sequence number corresponding to a downlink PDCP data packet successfully transmitted to the UE.

[0128] In step 809, the source CU-CP transmits an SN status transmission to the destination CU-CP. If the PDCP SN information corresponding to the MBS data received by the UE at the source base station is not included in step 801, the PDCP SN information and / or GTP-U SN information corresponding to the MBS data is transmitted via a separate message in step 809, and for specific information on the SN, refer to step 808 above.

[0129] In step 810, the source base station CU-CP transmits a handover execution command to the UE via an RRC reconfiguration message. The message carries configuration information for the PDU session that the UE needs to establish with the destination base station, and may include the MBS service identifier and MBS channel mode indication information received at the destination base station, and may also include point-to-point channel configuration information and / or point-to-multipoint channel configuration information for the MBS.

[0130] In step 811, the destination CU-CP transmits a Bearer Modify Request message to the destination CU-UP. The message may also include the PDCP SN or GTP-u SN information received by the destination CP, for example, the information received in step 809.

[0131] In step 812, the destination CU-UP transmits a Modify Bearer Response message to the destination CU-CP.

[0132] In step 813, the UE transmits an RRC reconfiguration complete message to the destination base station to indicate completion of the handover.

[0133] In step 814, the destination CU-CP transmits a path switch request message to the core network. If the UE is the first user that needs to receive the MBS from the destination base station and the destination base station has not received MBS data from the core network, the destination base station may request transmission of MBS data via a message in step 814. The message in step 814 carries MBS information, such as an MBS identifier, and requests the core network to transmit the MBS data to the base station. In response to receiving the message in step 814, the core network may start transmitting an MBS service start message to the base station, and the base station may transmit a response message to the core network, after which the core network may start transmitting the MBS data to the base station. The destination base station first transmits the forwarded data received from the source base station, and then transmits new data received from the core network.

[0134] In step 815, the core network transmits a path switch response message to the destination CU-CP.

[0135] In step 816, the source CU-UP forwards the MBS data to the destination CU-UP. To transmit the forwarded data to the UE, the destination CU-CP establishes a point-to-point transmission mode for the UE and transmits the forwarded data to the UE via a point-to-point channel. Then, after all the forwarded data has been transmitted to the UE, the UE can switch to point-to-multipoint mode or continue to receive the MBS data using a point-to-point channel, depending on the destination base station's additional decision. When data forwarding ends, the source CU-CP can indicate the end of data forwarding in the user plane or indicate that the forwarded data packet is the last data packet.

[0136] In step 817, the destination CU-UP transmits a data forwarding suspend message to the destination CU-CP.

[0137] After step 814, the core network starts transmitting data to the destination CU-UP. After receiving the data transmitted by the core network, the destination base station obtains the GTP-U SN included in the header of the data packet, and the destination base station can determine when forwarding is unnecessary based on the GTP-U SN transmitted by the core network as well as the SN included in the header of the forwarded data packet. Alternatively, when receiving a bearer establishment message or bearer modification request message from the CU-CP, the destination CU-UP receives the MBS data packet transmitted by the core network, and the destination CU-UP can determine whether to suspend data forwarding based on the GTP-U SN received from the core network as well as the SN included in the header of the forwarded data packet.

[0138] When the destination CU-UP determines that forwarding needs to be suspended, the destination CU-UP transmits a message to notify the destination CU-CP of the suspension of data forwarding, and the destination CU-CP further notifies the source CU-CP, and the source CU-CP notifies the source CU-UP of the suspension of data forwarding, and then the source CU-UP suspends data forwarding to the destination CU-UP. Alternatively, the destination CU-UP transmits data forwarding suspension instruction information to the source CU-UP via the user plane. The user plane method is more direct and simple.

[0139] Alternatively, the destination CU-UP determines whether to discard a forwarded data packet based on the GTP-U SN received from the core network, in addition to the SN included in the header of the forwarded data packet, and starts transmitting the data packet out of the core network. According to an implementation method adopted by the source CU-UP, the timing to suspend data forwarding is determined, and for example, based on a default data forwarding clock, the source CU-UP suspends data forwarding when the forwarding clock times out.

[0140] In step 818, the destination CU-CP transmits a UE context release message to the source CU-CP. The message includes data forwarding suspension indication information. If the destination CU-CP detects that MBS data forwarding is occurring, the destination CU-CP waits for the destination CU-UP to notify it of the suspension of data forwarding before transmitting a UE context release message to the source CU-CP.

[0141] In step 819, the source CU-CP sends a bearer release command to the source CU-UP, the message including data forwarding suspension indication information.

[0142] In step 820, the source CU-CP initiates a UE context release procedure between the source CU-CP and the DU.

[0143] In step 821, the source CU-UP transmits a bearer release complete message to the source CU-CP.

[0144] The method of the present disclosure can ensure service continuity when multicast transmission is switched, avoid or reduce additional overhead for multicast data transmission, improve the utilization efficiency of access network resources and / or air interface resources, and reduce transmission delays, data loss, delays caused by switching, and unnecessary data transmissions.

[0145] FIG. 9 illustrates a schematic diagram of Example 5 according to various embodiments of the present disclosure. Another embodiment of Method 1 for maintaining service continuity when a UE is handed over is described as shown in FIG. 9 (Embodiment 5). When a UE moves from a source base station to a destination base station, if the UE receives an MBS at the source base station, the UE should continue to receive the MBS when the UE moves to the destination base station. It is more preferable that there is no loss of MBS data during the handover procedure and that the UE continuously receives data. In the case of a unicast service, when the UE moves to the destination base station, the core network suspends data transmission to the source base station and notifies the source base station of the suspension of data transmission, identifying a specific data packet as the last data packet and indicating the suspension of data transmission.

[0146] However, in the case of an MBS service, the destination base station has already transmitted data in the point-to-multipoint transmission mode, and the source base station must continue transmitting the MBS and must not terminate data transmission simply because the UE has moved far away. Therefore, the source base station continues to receive data from the core network and cannot obtain a data termination instruction from the core network, so the source base station does not know when data forwarding should be terminated. According to this embodiment, when the source base station receives the SN information transmitted by the destination base station, the source base station forwards data packets that have not been transmitted by the destination base station to the destination base station according to the information and determines when to terminate data forwarding, which can reduce data forwarding and ensure continuous transmission and reception of data. Example 5 describes the procedure, and detailed descriptions of steps not related to this disclosure will be omitted here. Example 5 includes the following steps.

[0147] In step 901, the source base station initiates a handover request message to the destination base station. The message carries a session identifier of each requested PDU session, information on the QoS flows included in the PDU session, DRB information, etc. If the UE is receiving an MBS, the message also includes an MBS identifier, a forwarding offer for the MBS service, or a forwarding offer configured for the radio bearer of each MBS, and the message also includes a session identifier corresponding to the MBS, information on the QoS flows corresponding to the MBS, configuration information of the DRB transmitting the MBS, etc., and mapping configuration information of the QoS flows to the MBS MRBs in the source base station. The message also includes SN information of the MBS data transmitted by the source base station, and the SN information can include the PDCP SN and / or the GTP-U SN.

[0148] In one example, the PDCP SN specifically includes one or more of the following information: ■ PDCP SN list corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ The highest PDCP SN corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■ a PDCP SN list corresponding to downlink PDCP data packets that have been successfully transmitted to the UE in sequence; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ a PDCP SN list corresponding to downlink PDCP data packets successfully transmitted to the UE; and / or ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE; In one example, the GTP-U SN specifically also includes one or more of the following information: ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the PDCP layer; ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the SDAP layer; ■ SN corresponding to the highest sequence number in the received GTP-u data packet; a list of SNs of GTP-u data packets corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted in sequence to the UE; ■ the SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE; and / or ■ The SN in the header of the GTP-U data packet with the highest sequence number corresponding to a downlink PDCP data packet successfully transmitted to the UE.

[0149] Depending on whether the destination cell has already transmitted the MBS, the destination base station determines whether to transmit a message to the core network to request transmission of MBS data. If the destination cell receives the MBS from the core network, the destination base station transmits a response message to the source base station. If the destination cell of the destination base station has not received MBS data from the core network, the destination base station transmits a message to the core network to request MBS data transmission from the core network. The procedure for establishing an MBS bearer has already been described in other embodiments. The destination base station transmits a response message to the source base station.

[0150] In step 902, the destination base station transmits a handover request response message, which includes a destination-to-source transparent sender, which includes an RRC message transmitted by the destination base station to the UE. The message also includes information about the successfully established PDU session. The PDU session information includes a PDU session identifier, an identifier of the successfully established DRB, an identifier of the successfully established MBS, and an identifier of the successfully established MRB. If the destination base station is transmitting an MBS service in point-to-point transmission or point-to-multipoint transmission, the message includes indication information indicating that the destination base station has determined that MBS data forwarding is required.

[0151] In step 903, the source base station transmits a handover execution command to the UE via an RRC reconfiguration message. The message carries configuration information for a PDU session that the UE needs to establish at the destination base station. The message may also include an MBS service identifier and channel mode indication information for the MBS received at the destination base station, and may also include point-to-point channel configuration information and / or point-to-multipoint channel configuration information for the MBS.

[0152] In step 904, the source base station transmits an SN status transmission to the destination base station. If the PDCP SN information corresponding to the MBS data received by the UE at the source base station is not included in step 901, the PDCP SN and / or GTP-U SN corresponding to the MBS data can be transmitted via a separate message in step 904.

[0153] In one example, the PDCP SN specifically includes one or more of the following information: ■ PDCP SN list corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ The highest PDCP SN corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■ a PDCP SN list corresponding to downlink PDCP data packets that have been successfully transmitted to the UE in sequence; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ a PDCP SN list corresponding to downlink PDCP data packets successfully transmitted to the UE; and / or ■ The highest PDCP SN corresponding to a downlink PDCP data packet successfully transmitted to the UE. In one example, the GTP-U SN specifically also includes one or more of the following information: ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the PDCP layer; ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the SDAP layer; ■ the SN of the received GTP-u data packet having the highest sequence number; a list of SNs of GTP-u data packets corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted in sequence to the UE; ■ the SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE; and / or ■ The SN in the header of the GTP-U data packet with the highest sequence number corresponding to a downlink PDCP data packet successfully transmitted to the UE.

[0154] In step 905, the destination base station transmits the SN status information to the source base station. The message may include SN information corresponding to the data currently being transmitted or being sent by the destination base station, or GTP-u SN and / or PDCP SN information corresponding to the forwarded data proposed by the destination base station. For example, if the GTP-u SN being transmitted by the destination base station is 100, the destination base station notifies the source base station that the transmitted GTP-u SN is 100, and the source base station forwards only data having a GTP-u SN less than 100. Alternatively, to ensure no data loss, the destination base station notifies the source base station that the highest GTP-u SN corresponding to the data proposed to be forwarded is 110, and then the source base station forwards only data having a GTP-u SN less than 110. The destination base station establishes a point-to-point channel for the UE to receive the MBS data and waits for all forwarded data to be transmitted to the UE. If necessary, the UE performs point-to-multipoint switching to receive the data.

[0155] Specifically, the PDCP SN information and / or the GTP-U SN information may be one or more of the following information: ■ PDCP SN list corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ The highest PDCP SN corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■ a PDCP SN list corresponding to downlink PDCP data packets that have been successfully transmitted to the UE in sequence; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ a PDCP SN list corresponding to downlink PDCP data packets successfully transmitted to the UE; ■ the highest PDCP SN corresponding to the downlink PDCP data packet successfully transmitted to the UE; ■ a PDCP SN set by the base station indicating the highest PDCP SN corresponding to the downlink PDCP data packet that needs to be forwarded; and / or ■ A PDCP SN list set by the base station indicating the PDCP SN list corresponding to the downlink PDCP data packets that need to be forwarded and also indicating the range of SNs, for example, indicating the lowest and highest PDCP SNs.

[0156] In one example, the GTP-U SN specifically includes one or more of the following information: ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the PDCP layer; ■ The highest SN of the GTP-u data packet corresponding to the data packet transmitted to the SDAP layer; ■ the SN of the received GTP-u data packet having the highest sequence number; a list of SNs of GTP-u data packets corresponding to downlink PDCP data packets transmitted to the lower RLC layer; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet transmitted to the lower RLC layer; ■SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE in sequence; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted in sequence to the UE; ■ the SN in the header of the GTP-U data packet corresponding to the downlink PDCP data packet successfully transmitted to the UE; ■ the SN in the header of the GTP-U data packet with the highest sequence number corresponding to the downlink PDCP data packet successfully transmitted to the UE; ■ The GTP-U SN set by the base station indicating the highest GTP-U SN corresponding to the downlink data packet that needs to be forwarded; and / or ■ A GTP-U SN list set by the base station indicating the GTP-U SN list corresponding to the downlink data packets that need to be forwarded and also indicating the range of SNs, for example, the lowest and highest GTP-U SNs.

[0157] Step 905 does not necessarily have to occur after step 904, but may occur before step 904. There is no absolute order between them.

[0158] In step 906, the source base station forwards the MBS data to the destination base station. Based on the received SN information (SN information can be PDCP SN or GTP-u SN, as described above) corresponding to the forwarded data proposed by the destination base station, the source base station determines which data needs to be forwarded and when to end the forwarding. For example, if the destination base station proposes that the highest GTP-u SN of the forwarded data be set to 100 and the source base station is currently transmitting a data packet GTP-u SN=80, the source base station forwards data packets with GTP-u SN=80 to SN=100 to the destination base station. To ensure continuous reception of the data, the destination base station may temporarily establish a point-to-point transmission mode for the UE and transmit the forwarded data to the UE via a point-to-point channel. The UE may then switch to point-to-multipoint mode to receive the data. When data forwarding ends, the source base station can indicate the end of data forwarding in the user plane or indicate that the forwarded data packet is the last data packet.

[0159] In step 907, the UE transmits an RRC reconfiguration complete message to the destination base station to indicate completion of the handover.

[0160] In step 908, the destination base station transmits a path switch request message to the core network. If the UE is the first user that needs to receive the MBS of the destination base station and the destination base station has not received MBS data from the core network, the destination base station may request transmission of MBS data via a message in step 908. The message in step 908 carries MBS information, such as an MBS identifier, and requests the core network to transmit the MBS data to the base station. In response to receiving the message in step 908, the core network may start transmitting an MBS service start message to the base station, and the base station may transmit a response message to the core network, after which the core network may start transmitting the MBS data to the base station. The destination base station first transmits the forwarded data received from the source base station, and then transmits new data received from the core network.

[0161] In step 909, the core network transmits a path switch response message to the destination base station.

[0162] In step 910, the destination base station transmits a UE context release message to the source base station to release the UE's context information at the source base station.

[0163] In this embodiment, the illustrated method can be used for a user separation architecture. In this case, the destination CU-UP needs to include, in the bearer establishment response or bearer modification response message, SN information corresponding to the data transmitted or being transmitted by the destination CU-UP, or PDCP SN information and / or GTP-U SN information corresponding to the forwarded data proposed by the destination base station. For specific information, see step 905. After receiving the information, the destination CU-CP transmits the SN information to the source CU-CP, and the source CU-CP transmits the SN information to the source CU-UP. According to the received SN information (which may be a PDCP SN or a GTP-u SN), the source CU-UP determines which data needs to be forwarded and when to terminate the forwarding. When data forwarding terminates, the source CU-UP can indicate the termination of data forwarding on the user plane or indicate that the forwarded data packet is the last data packet.

[0164] The above embodiments require improvements to messages / procedures between a source base station and a destination base station. In embodiment 6, a method using time-based data transmission without modifying messages / procedures between a source base station and a destination base station, or a method without adding messages, is proposed. The advantage of this method is that the impact on the protocol is minimized, and the disadvantage is that the forwarded data may be more or less than the data actually required. Specifically, embodiment 6 of FIG. 11 includes the following steps.

[0165] In step 1101, the source base station initiates a handover request message to the destination base station. Based on the UE's measurement report, the source base station determines to hand over the UE to the destination base station where the destination cell is located. When the source base station determines to initiate the handover procedure, it starts storing / buffering data that has not been transmitted to the UE and also stores / buffers newly received data from the core network. This data is all data to be forwarded to the destination base station. Buffering starts from the time when the handover is initiated and ends when the source base station receives a release request message transmitted by the destination base station, or at a specific time point after receiving the release request message transmitted by the destination base station, but this depends on the implementation.

[0166] The message carries the session identifier of each requested PDU session, information on the QoS flows included in the PDU session, DRB information, etc. If the UE is receiving an MBS, the message also includes an identifier of the MBS, a forwarding offer for the MBS service, or a forwarding offer configured for the radio bearer of each MBS, and the message also includes a session identifier corresponding to the MBS, information on the QoS flows corresponding to the MBS, configuration information for the DRB that sends the MBS, and configuration information for mapping the QoS flows to the MRBs of the MBS in the source base station.

[0167] The message also includes a PDCP SN corresponding to the MBS data transmission of the source base station and the corresponding GTP-U SN, and the specific contents are as described in Examples 1 to 6. The GTP-U SN is a sequence number included in a data packet transmitted to a base station by a core network and can be included in a GTP-U header or a GTP-U extended header. This SN can also be for a PDU session, a QoS flow, or multiple QoS flows, for example, multiple QoS flows mapped on the same radio bearer. The GTP-U SNs transmitted to different base stations for the same data packet are the same, and the destination base station knows the PDCP SN assigned by the source base station for data packets with the same GTP-U SN. Thus, the difference between the PDCP SN assigned by the destination base station and the PDCP SN assigned by the source base station is known, which is referred to as the PDCP SN difference.

[0168] Depending on whether the destination cell has already transmitted the MBS, the destination base station determines whether to transmit a message to the core network to request transmission of MBS data. A new message carrying MBS information, such as an MBS identifier, can be transmitted to the core network to request the core network to transmit MBS data to the base station. When the destination cell receives the MBS from the core network, the destination base station transmits a response message to the source base station.

[0169] If the destination cell of the destination base station has not received the MBS data from the core network, the destination base station transmits a message to the core network to request the core network to transmit the MBS data. The destination base station then transmits a response message to the source base station.

[0170] In step 1102, the destination base station transmits a handover request response message. This message includes a destination-to-source transparent sender. The destination-to-source transparent sender includes an RRC message transmitted by the destination base station to the UE. The RRC message may include the PDCP SN difference mentioned in step 1101 or the correspondence between the PDCP SN and GTP-U SN of the destination base station. However, because different PDCP SNs are assigned by the source base station and the destination base station for data packets corresponding to the same GTP-U SN, the PDCP SNs of the data received by the UE from the source base station and the data received by the UE from the destination base station during the handover procedure are discontinuous. The PDCP protocol layer on the UE side must transmit data packets to the upper layer protocol in SN order and must determine whether data is lost based on the PDCP SN. Once the UE obtains the PDCP SN difference, the UE can determine whether the data packets received from the destination base station are consecutive with the data packets received from the source base station based on the difference. The difference allows the data packets to be aligned. If the data packets are discontinuous, the UE can know whether any data packets have been lost, and can then request the destination base station to retransmit the lost data packets.

[0171] The message also includes information about the successfully established PDU session, including a PDU session identifier, a successfully established DRB identifier, a successfully established MBS identifier, a successfully established MRB identifier, and a tunnel address for receiving data forwarding.

[0172] In step 1103, the source base station transmits a handover execution command to the UE via an RRC reconfiguration message. The message carries configuration information for the PDU session that the UE needs to establish in the destination base station, and may also include an MBS service identifier and channel mode indication information for the MBS received at the destination base station, as well as point-to-point channel configuration information and / or point-to-multipoint channel configuration information for the MBS.

[0173] In step 1104, the source base station transmits an SN status transmission to the destination base station. The message includes the PDCP SN and / or GTP-U SN corresponding to the MBS data. The specific content is as described in the first to sixth embodiments.

[0174] In step 1105, the source base station forwards the MBS data to the destination base station. The source base station transmits the stored data to the destination base station. Optionally, when data forwarding ends, the source base station indicates the end of data forwarding in the user plane or indicates that the forwarded data packet is the last data packet.

[0175] In step 1106, the UE transmits an RRC reconfiguration complete message to the destination base station to indicate completion of the handover.

[0176] In step 1107, the destination base station transmits a path switch request message to the core network. If the UE is the first user that needs to receive the MBS from the destination base station and the destination base station has not received MBS data from the core network, the destination base station may request transmission of MBS data via a message in step 1107. The message in step 1107 carries MBS information, such as an MBS identifier, and requests the core network to transmit the MBS data to the base station. In response to receiving the message in step 1107, the core network may start transmitting an MBS service start message to the base station, and the base station may transmit a response message to the core network, after which the core network may start transmitting the MBS data to the base station. The destination base station first transmits the forwarded data received from the source base station, and then transmits new data received from the core network.

[0177] In step 1108, the core network transmits a path switch response message to the destination base station.

[0178] In step 1109, the destination base station transmits a UE context release message to the source base station to release the UE's context information at the source base station. This message may carry data forwarding suspension instruction information. The destination base station receives the data forwarded by the source base station, and the destination base station determines whether the source base station needs to continue data forwarding to the destination base station based on the data received from the core network and the status of the MBS data buffered by the destination base station. If the source base station does not need to suspend data forwarding, the destination base station transmits a UE context release message to the source base station. The UE context release message carries instruction information to suspend data forwarding or includes SN information for data forwarding. The SN information may be the PDCP SN and / or GTP-U SN corresponding to the MBS data, and the specific content is as described in Examples 1 to 6. Alternatively, the destination base station waits until the source base station no longer needs to continue data forwarding to the destination base station, and then transmits a UE context release message to the source base station. Upon receiving the UE context release message, the source base station suspends data forwarding. When data forwarding is interrupted, the forwarded data packet may indicate that the data forwarding is terminated or that the forwarded data packet is the last data packet.

[0179] If the source base station receives the data forwarding suspension instruction information included in the UE context release message, the source base station can suspend data forwarding to the destination base station. The forwarded data packet may indicate that data forwarding is ending or that the forwarded data packet is the last data packet.

[0180] When the source base station receives the SN information included in the UE context release message, the source base station determines when to stop data forwarding based on the SN information, and the forwarded data packet may indicate that data forwarding has ended and that the forwarded data packet is the last data packet.

[0181] In MBS transmission, to ensure data continuity and minimize data loss during handover, it is necessary to synchronize the PDCP SN among multiple base stations. There are two synchronization methods for synchronizing the PDCP SN and the GTP-U SN. In one synchronization method, the PDCP SN value is the same as the GTP-U SN value. In the other synchronization method, the PDCP SN value is different from the GTP-U SN value but is generated from the GTP-U SN according to a fixed rule. For example, the GTP-U SN corresponding to PDCP SN=0 is 10, and the GTP-U SN corresponding to PDCP SN=1 is 11.

[0182] Some base stations receive data from the core network late. For example, during a service procedure, a user who is receiving or needs to receive MBS moves to a cell that supports the MBS service. The base station where the cell is located is not receiving the MBS service from the core network, and the base station requests the core network to transmit the service. In this case, the core network transmits data to the base station. The SN included in the GTP-U header included in the first data packet received by this base station is 100. In this case, other base stations that receive the MBS service initially set their PDCP SNs according to the SN included in the GTP-U header. For a data packet with an SN included in the GTP-U header of 100, the PDCP SN is 100. For base stations that receive data later, the PDCP SN sequence number must be numbered starting from 1 according to current regulations. This results in a problem of PDCP SN out of sync.

[0183] Multiple base stations receive data of the same MBS service from the core network, and data loss may occur in the process of transmission from the core network to the base stations. For example, two base stations, i.e., base station 1 and base station 2, both receive data of the same MBS service from the core network, and data packets are lost in the process of being transmitted to base station 1, while data packets transmitted to base station 2 are not lost. In this case, if the PDCP SN of the base station is synchronized with the GTP-U SN, a situation occurs in which they become out of synchronization in the event of data loss.

[0184] If the above situation occurs, the relationship between the PDCP SN and the GTP-U SN will be broken. In this case, resynchronization will be performed in the following manner.

[0185] Method 1: The PDCP sequence number of the base station can start from the SN included in the header of the first received GTP-U data packet instead of 1. For example, if the SN carried in the header of the first GTP-U data packet received by the base station is 100, then PDCP SN=100. In this case, the base station needs to indicate to the UE that PDCP SN=100 is the first data packet, or notify the UE of the starting value of the PDCP SN through the PDCP header or PDCP control information.

[0186] Method 2: Through the GTP-U SN, the base station can know that there is data loss. The GTP-U SN also relates to a session or QoS flow and is persistently encoded in the core network. Therefore, the base station can know from the GTP-U SN whether there is data loss. If data loss is confirmed, the base station can generate a spoofed PDCP data packet and assign an assigned PDCP SN to the spoofed data packet to ensure synchronization between the PDCP SN and the GTP-U SN.

[0187] Method 3: Although the base station may be aware of data loss, it still assigns the PDCP SN according to the GTP-U SN. For example, if the first synchronization mode is adopted, i.e., if the GTP-U SN = PDCP SN, the base station receives a data packet with GTP-U SN = 9 and assigns PDCP SN = 9 to the data packet. If a data packet with GTP-U SN = 10 is transmitted from the core network to the base station during the transmission procedure, the base station receives a data packet with GTP-U SN = 11 instead of the data packet with GTP-U SN = 10. The PDCP SN immediately changes from 9 to 11. Therefore, the PDCP SNs included in the data packets transmitted by the base station are discontinuous. The UE side needs to sort the data according to the PDCP SN. If the UE cannot receive PDCP SN = 10, it will consider the data packet with PDCP SN = 10 to have been transmitted incorrectly and request a retransmission from the base station. To avoid this situation, the base station notifies the UE that PDCP SN 10 will no longer be transmitted, and then the continuously received PDCP SN is 11. The UE is notified through an RRC message transmitted by the base station, e.g., notified of the lost PDCP SN number, or the UE is informed through user plane control information, e.g., notified in the control information included in a data packet having PDCP SN=11 that the SN number of the data packet preceding the data packet was PDCP SN=9. In this way, the UE does not consider the data packet with PDCP SN=10 to be lost in order to request retransmission from the base station.

[0188] Method 4: When a base station generates a PDCP SN from a GTP-U SN according to a fixed rule, if there is data loss, the base station adopts a new rule and then notifies the UE of the new rule through an RRC message or other base stations through the Xn interface. For example, before data loss, the base station receives a data packet with GTP-U=10 and assigns PDCP SN=1. The rule is PDCP SN=GTP-U SN-9. If a data packet with GTP-U=11 is lost, the base station receives a data packet with GTP-U=12 and assigns PDCP SN=2 to the data packet. The new rule is PDCP SN=GTP-U SN-10. Neighboring base stations are notified of the new rule, for example, {PDCP SN=2, GTP-U SN=10}, according to the method of Example 6 or through a new message.

[0189] The method of the present disclosure can ensure service continuity when multicast transmission is switched, avoid or reduce additional overhead for multicast data transmission, improve the utilization efficiency of access network resources and / or air interface resources, and reduce transmission delays, data loss, delays caused by switching, and unnecessary data transmissions.

[0190] FIG. 10 illustrates a block diagram of a network device according to various embodiments of the present disclosure. The network device may be used to implement the DU, CU-UP, CU-CP, base station, source base station, destination base station, source DU, source CU-UP, source CU-CP, destination DU, destination CU-UP, destination CU-CP, etc. of the present disclosure. Referring to FIG. 10, the network device according to the present disclosure includes a transceiver unit 1010, a controller 1020 (or at least one processor), and a memory 1030. The transceiver unit 1010, controller 1020, and memory 1030 are configured to perform the operations of embodiments 1 to 5 of the present disclosure. Although the transceiver unit 1010, controller 1020, and memory 1030 are illustrated as separate entities, the transceiver unit 1010, controller 1020, and memory 1030 may be embodied as a single entity, such as a single chip. The transceiver unit 1010, controller 1020, and memory 1030 may be electrically connected or coupled to each other. The transceiver 1010 may transmit signals to and receive signals from other network devices, such as a UE, a base station, or a core network node. The controller 1020 may include one or more processing units and may control the network devices to perform operations and / or functions according to one of the above embodiments. The memory 1030 may store instructions for implementing one of the above embodiments.

[0191] Thus, the disclosed method and device for multicast transmission has been completed, which can ensure service continuity when multicast transmission is switched, avoid or reduce additional overhead of multicast data transmission, improve the utilization efficiency of access network resources and / or air interface resources, and reduce transmission delays, data loss, delays caused by switching, and unnecessary data transmissions.

[0192] While the present disclosure has been described with reference to various embodiments, various changes and modifications may be suggested to those of ordinary skill in the art, and the present disclosure is intended to include all such changes and modifications that fall within the scope of the appended claims. [Explanation of symbols]

[0193] 100, 200 System Architecture 101, 201 User Equipment (UE) 102 E-UTRAN 103 Mobility Management Entity (MME) 104 Serving Gateway (SGW) 105 Packet Data Network Gateway (PGW) 106 Charging Rules Functional Entity (PCRF) 108 General Packet Radio Service Support Node (SGSN) 109 Home Subscriber Server (HSS) 202 Next Generation Radio Access Network (NG-RAN) 203 Access Control and Mobility Management Functional Entity (AMF) 204 User Plane Functional Entity (UPF) 205 Session Management Function Entity (SMF) 206 Data Network (DN) 1010 Transmitter / Receiver 1020 control section 1030 memory

Claims

1. 1. A method for a multicast broadcast service (MBS) performed by a first network device, comprising: receiving a handover request message from a second network device; The handover request message includes a packet data convergence protocol (PDCP) sequence number (SN) of a first data packet for the MBS and identification information of a radio bearer for the MBS; determining whether to apply data forwarding for the MBS based on a PDCP SN of a first data packet for the MBS; sending a handover request response message to the second network device; The method, wherein the handover request response message includes a PDCP SN of a second data packet for the MBS.

2. The method of claim 1, wherein data forwarding for the MBS is performed based on the PDCP SN of the first data packet for the MBS.

3. 2. The method of claim 1, wherein the PDCP SN of the first data packet for the MBS includes the highest PDCP SN among data packets successfully transmitted to a user equipment (UE) for the MBS.

4. The handover request response message 2. The method of claim 1, further comprising radio bearer identification information for the MBS.

5. The method of claim 1 , wherein the PDCP SN of the second data packet for the MBS includes the PDCP SN of the data packet proposed by the first network device.

6. 2. The method of claim 1, wherein the second network device determines when to suspend the data forwarding based on a PDCP SN of a second data packet for the MBS.

7. 1. A method for a multicast broadcast service (MBS) performed by a second network device, comprising: sending a handover request message to a first network device; The handover request message includes a packet data convergence protocol (PDCP) sequence number (SN) of a first data packet for the MBS and identification information of a radio bearer for the MBS; receiving a handover request response message from the first network device; the handover request response message includes a PDCP SN of a second data packet for the MBS; determining when to suspend data forwarding based on the PDCP SN of a second data packet for the MBS.

8. The method of claim 7, wherein data forwarding for the MBS is performed based on the PDCP SN of the first data packet for the MBS.

9. 8. The method of claim 7, wherein the PDCP SN of the first data packet for the MBS includes the highest PDCP SN among data packets successfully transmitted to a user equipment (UE) for the MBS.

10. The method of claim 7, wherein the handover request response message further includes radio bearer identification information for the MBS.

11. The method of claim 7 , wherein the PDCP SN of the second data packet for the MBS includes the PDCP SN of the data packet proposed by the first network device.

12. The method of claim 7 , wherein the first network device determines whether to apply data forwarding for the MBS based on a PDCP SN of a first data packet for the MBS.

13. A first network device for a multicast broadcast service (MBS), comprising: The first network device a transmitter / receiver; at least one processor coupled to the transceiver; The at least one processor receiving a handover request message from a second network device; The handover request message includes a packet data convergence protocol (PDCP) sequence number (SN) of a first data packet for the MBS and identification information of a radio bearer for the MBS; determining whether to apply data forwarding for the MBS based on a PDCP SN of a first data packet for the MBS; A first network device configured to send a handover request response message to the second network device, the handover request response message including a PDCP SN of a second data packet for the MBS.

14. A second network device for a multicast broadcast service (MBS), the second network device comprising: a transmitter / receiver; at least one processor coupled to the transceiver; The at least one processor sending a handover request message to a first network device, the handover request message including a packet data convergence protocol (PDCP) sequence number (SN) of a first data packet for the MBS and identification information of a radio bearer for the MBS; receiving a handover request response message from the first network device, the handover request response message including a PDCP SN of a second data packet for the MBS; A second network device configured to determine when to suspend data forwarding based on a PDCP SN of a second data packet for the MBS.

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