New support for cell reselection of service cells for UE

Abstract

A method for supporting cell reselection for a user equipment (UE) operating in a Disconnected RRC state having one or more active data sessions is described. The method in the UE comprises: transmitting a reselection request message to a new serving cell to request cell reselection for receiving the one or more active data sessions from a target base station of the new serving cell; receiving a configuration message including configuration information associated with the target base station for the one or more active data sessions; and configuring the UE to receive data of the one or more active data sessions from the target base station in the new serving cell based on the configuration information. The active data sessions may include one or more Multicast Broadcast Service (MBS) sessions.

Description

Technical Field

[0001] The present invention generally relates to supporting cell reselection of a new serving cell for a user equipment (UE) in a wireless communication system, and more particularly, to methods performed at the UE, source base station, and target base station as part of the reselection process. Cell reselection by the UE can be used when handling the mobility of a UE receiving a multicast / broadcast service (MBS) in a 5G New Radio (NR) system.

Background Art

[0002] Wireless communication systems are mainly deployed to handle a wide range of applications, from mobile broadband, massive machine type communication to ultra-reliable low-latency communication (URLLC). Such systems enable multiple user equipments (UEs) or mobile terminals to share the wireless medium to exchange several types of data contents (such as video, audio, messaging, etc.) via a radio access network (RAN) through one or more base stations.

[0003] Examples of such wireless multi-connection communication systems include systems based on the 3rd Generation Partnership Project (3GPP-RTM) standards such as the 4th Generation (4G) Long Term Evolution (LTE) or the recent 5th Generation (5G) New Radio (NR) system, or systems based on the IEEE 802.11 standards such as Wi-Fi.

[0004] Among the requirements of 5G NR, there are service requirements related to multicast and broadcast services abbreviated as MBS.

[0005] In the case of a broadcast communication service, the same service and the same specific content data are delivered simultaneously to all UEs within a geographical area (i.e., all UEs within the broadcast service area are permitted to receive the data). Broadcast communication services are delivered to UEs using broadcast sessions. In the case of a multicast communication service, the same service and the same specific content data are delivered simultaneously to a dedicated set of UEs (i.e., not all UEs within the multicast service area are permitted to receive the data). Multicast communication services are delivered to UEs using multicast sessions.

[0006] Support for multicast / broadcast technology allows networks to operate in a more efficient way than unicast. Identified use cases that can benefit from this MBS capability include public safety and mission-critical, V2X applications, IPTV, live video, and software distribution over wireless and IoT applications. 3GPP began building functional support for MBS in 5G NR with Release 17.

[0007] In 5G NR, the Radio Resource Control (RRC) protocol operates in the control plane between the UE and the base station (gNB), providing the UE with three distinct states: RRC_CONNECTED, RRC_INACTIVE, and RRC_IDLE, as defined in the 3GPP standard TS 38.331. At startup, the UE is in the RRC_IDLE state, and changes to the RRC_CONNECTED state when establishing an RRC connection with the gNB. If the RRC connection is released, the UE returns to the RRC_IDLE state. While in the RRC_CONNECTED state, the RRC connection can be interrupted by the gNB, and the UE moves to the RRC_INACTIVE state. When the UE is in the RRC_INACTIVE state, it cannot communicate with the 5G system, but both the gNB and the UE track the RRC connection context. Therefore, the transition from the RRC_INACTIVE state to the RRC_CONNECTED state is faster than the establishment of an RRC connection from the RRC_IDLE state to the RRC_CONNECTED state.

[0008] In 3GPP Release 17, UEs are allowed to receive MBS broadcasts when they are in the RRC_IDLE, RRC_INACTIVE, or RRC_CONNECTED state, and are allowed to receive MBS multicast only when they are in the RRC_CONNECTED state. For Release 18, the plan is to further enable MBS multicast reception when UEs are in the RRC_INACTIVE state. See, for example, section 3 of 3GPP RP-213568 "New WID: Enhancements of NR Multicast and Broadcast Services" (3GPP TSG RAN Meeting #94-e, source CATT). The goal is to enable a higher density of UEs receiving multicast data from a single cell. In fact, having UEs in the RRC_INACTIVE state leads to a downsizing in the overall UE control plane footprint, thus allowing gNBs to handle more UEs. In addition, keeping UEs in the RRC_CONNECTED state at all times is not power efficient. As a result, the merit of also supporting multicast for UEs in the RRC_INACTIVE state has been recognized.

[0009] Establishing an MBS session in a UE can occur when the UE is in the RRC_CONNECTED state. The UE can then be set to the RRC_INACTIVE state by the serving gNB while still receiving MBS data. However, one issue with UEs receiving MBS multicast or broadcast in the RRC_INACTIVE state is handling mobility, where a mobile UE is moving from one cell managed by the source gNB to another managed by the target gNB, with the aim of maintaining continuity of service (or at least minimizing data loss). The mobility procedure for moving from one cell to another depends on the RRC state of the UE. In the RRC_CONNECTED state, the mobility procedure is called a handover and is triggered by the source gNB based on measurement reports provided by the UE. In the handover procedure, the network (i.e., the source gNB, the target gNB, and the 5G core network (5GC)) controls the procedure to ensure the delivery of MBS data through the target gNB. In the RRC_INACTIVE state (and RRC_IDLE state), the mobility procedure is called cell reselection and is managed by the UE itself. In the cell reselection process, the UE periodically evaluates the radio conditions and selects a suitable cell to camp on (stay).

[0010] Since there is no communication with the network during the cell reselection process, several issues can prevent the UE from continuing to receive MBS data after cell reselection. Firstly, the UE may select a target gNB that is outside the MBS service area, and therefore the target gNB is not a base station that can deliver the MBS services of interest to the UE. When the target gNB can provide the same MBS services, another issue for the UE is obtaining configuration information and receiving the MBS data. One solution might be to switch the UE in the RRC_CONNECTED state and apply a handover procedure. However, this negates the advantages of keeping the UE in the RRC_INACTIVE or RRC_IDLE state (as mentioned above, for example).

[0011] For MBS broadcasts, 3GPP Release 17 provides several solutions that allow UEs to remain in the RRC_INACTIVE or RRC_IDLE state and continue to receive MBS broadcast services after cell reselection. In practice, MBS broadcast configuration information is provided over the MBS Control Channel (MCCH). The MBSBroadcastConfiguration message indicates the MBS broadcast sessions provided within the cell and the corresponding scheduling-related information for these sessions. Optionally, the MBSBroadcastConfiguration message may also include a list of neighboring cells that provide the same broadcast MBS services as those provided in the current cell. MCCH information (i.e., information transmitted in messages sent over the MCCH) is transmitted periodically within a configured transmission window using a configurable repetition period. The configuration information required by UEs to receive the MCCH is provided in a System Information Block (SIB) broadcast to all UEs. Furthermore, another SIB broadcast to all UEs also provides information related to the continuity of MBS broadcast service, i.e., the mapping between frequency and MBS services. Based on the list of neighboring cells that provide the same broadcast MBS services, UEs can select the appropriate cell in the cell reselection process. Subsequently, based on the configuration information broadcast in the appropriate cell, the UE can configure its user plane to continue receiving MBS broadcast data. However, these solutions for MBS broadcast cannot be reused for MBS multicast. Since the MBS multicast service may be reserved to a limited set of UEs (e.g., specific multicast groups) that have permission to receive it, all configuration information for MBS multicast reception cannot be made available to all UEs as it is for MBS broadcast.Even if the target gNB is sending MBS multicast data from the same MBS session, the UE cannot receive the MBS multicast data from the target gNB because it does not know the proper configuration of the MBS radio bearer used by the target gNB.

[0012] Therefore, it is desirable to provide at least one solution for UEs receiving MBS multicast data in a disconnected RRC state, for example, to ensure that the continuity of MBS services is maintained or at least data loss is minimized when a UE performs cell reselection and switches to a new serving cell. [Overview of the Initiative]

[0013] According to a first aspect of the present invention, a method is provided for supporting cell reselection for a user device (UE), the UE operating in a disconnected RRC state with one or more active data sessions, the method comprising: the UE sending a reselection request message to a new serving cell to request cell reselection in order to receive one or more active data sessions from a target base station in a new serving cell; receiving a configuration message containing configuration information related to a target base station for one or more active data sessions; and configuring the UE to receive data from one or more active data sessions from a target base station in a new serving cell based on the configuration information.

[0014] The UE can switch to a new serving cell either before or after a reselection request is sent. After the UE is configured and after it has switched to a new serving cell (the switch may occur either before or after the reselection request is sent), the UE may receive data from the target base station for one or more active data sessions.

[0015] The UE may send a reselection request message to a RAN node, such as the source base station of the serving cell or the target base station of the new serving cell.

[0016] A second aspect of the present invention provides a method for a source base station of a serving cell of a user equipment (UE) that, in a UE operating in a disconnected RRC state with one or more active data sessions, receives a reselection request message from the UE requesting cell reselection to receive one or more active data sessions from a target base station of a new serving cell; transmits a migration request message to the target base station indicating that the UE is requesting cell reselection with one or more active data sessions, wherein the migration request message includes context information associated with the UE and one or more active data sessions; receives a migration confirmation message from the target base station including configuration information associated with one or more active data sessions to the target base station; and transmits a configuration message to the UE including configuration information.

[0017] A third aspect of the present invention provides a method at a target base station of a new serving cell of a user device (UE), wherein the UE is operating in a disconnected RRC state with one or more active data sessions, and the UE has been switched from a previous serving cell to a new serving cell, the method comprising: receiving a reselection request message from the UE to request cell reselection to the new serving cell with one or more active data sessions; sending a context request message to the source base station of the previous serving cell; receiving a context message from the source base station containing context information associated with the UE and one or more active data sessions; and sending a configuration message to the UE containing configuration information associated with the target base station for one or more active data sessions.

[0018] One or more active data sessions can be active MBS sessions (such as MBS multicast sessions). For example, a UE may be in the RRC_INACTIVE state.

[0019] By sending a reselection request message to the new serving cell to request cell reselection, and receiving configuration in response, the UE can obtain the configuration information necessary to receive data from one or more active data sessions via the target base station, ensuring that the continuity of data services (e.g., MBS services) is maintained, or that at least data loss is minimized when the UE performs cell reselection and switches to the new serving cell. This facilitates mobility handling for UEs receiving MBS multicast in the RRC_INACTIVE state. Furthermore, providing a method to be performed while the UE is in the disconnected RRC state means that the UE does not need to be switched to the RRC_CONNECTED state to apply handover procedures, and thus the benefits of keeping the UE in the disconnected RRC state (e.g., reduced control plane footprint) are maintained.

[0020] Therefore, a UE in a disconnected RRC state can switch to a new serving cell while still receiving one or more active data sessions (e.g., MBS services).

[0021] Furthermore, the target base station requests cell reselection to the target base station and notifies new UEs in an inconnected RRC state to receive one or more active data sessions (e.g., MBS services) from the target base station, so that the handling of mobility for UEs receiving MBS multicast in an RRC_INACTIVE state to establish one or more active sessions at the target base station can be properly managed, for example, by controlling whether the UE is allowed to receive MBS multicast and, if necessary, requesting the core network to provide MBS data to the target base station.

[0022] A fourth aspect of the present invention provides a method at a target base station of a target cell, wherein the target cell is selected as a new serving cell for a user device (UE), and the UE operates in a disconnected RRC state with one or more active data sessions via the serving cell, the method of claim 25.

[0023] A fifth aspect of the present invention provides a method at a source base station of a user device (UE)'s previous serving cell, wherein the UE operates in a disconnected RRC state with one or more active data sessions, and the UE switches from the previous serving cell to a new serving cell, the method of claim 31.

[0024] According to a sixth aspect of the present invention, the UE described in claim 34 is provided.

[0025] According to a seventh aspect of the present invention, the base station described in claim 35 is provided.

[0026] Further exemplary features of the present invention are described in other independent and dependent claims.

[0027] Any feature in one aspect of the present invention may be applied to other aspects of the present invention in any suitable combination. In particular, aspects of methods may be applied to aspects of apparatus / devices / units, and vice versa.

[0028] Furthermore, features implemented in hardware may be implemented in software, and vice versa. Any criteria to software and hardware features in this specification should be construed accordingly. For example, according to another aspect of the present invention, when a program is executed by a processing unit, there is provided a computer program comprising instructions for causing the processing unit to execute the method of any of the above aspects or examples, and a computer-readable storage medium carrying the computer program.

Brief Description of Drawings

[0029] Here, different aspects of the present invention will be described by way of example only, with reference to the following drawings. [Figure 1a] It is a schematic diagram showing a first exemplary wireless communication system capable of implementing the present invention according to one or more embodiments of the present invention. [Figure 1b] It is a schematic diagram showing a first exemplary wireless communication system capable of implementing the present invention according to one or more embodiments of the present invention. [Figure 1c] It is a schematic diagram showing a second exemplary wireless communication system capable of implementing the present invention according to one or more embodiments of the present invention. [Figure 1d] It is a schematic diagram showing a second exemplary wireless communication system capable of implementing the present invention according to one or more embodiments of the present invention. [Figure 2] It shows a block schematic diagram of an exemplary configuration of a UE capable of implementing the present invention according to one or more embodiments of the present invention. [Figure 3] It shows a block schematic diagram of an exemplary configuration of a base station capable of implementing the present invention according to one or more embodiments of the present invention. [Figure 4] It is a flowchart showing the RRC connection state and transition for a UE in a 5G NR system. [Figure 5]This is a simplified flowchart illustrating the handover procedure while the UE is receiving MBS multicast data in the RRC_CONNECTED state. [Figure 6] This flowchart shows an exemplary cell reselection procedure for a UE that is in the RRC_INACTIVE state and receiving MBS multicast data, according to a first embodiment of the present invention. [Figure 7] This flowchart shows an exemplary cell reselection procedure for a UE that is in the RRC_INACTIVE state and receiving MBS multicast data, according to a second embodiment of the present invention. [Figure 8a] This flowchart shows an exemplary method, according to embodiments of the present invention, performed in a UE that is in the RRC_INACTIVE state for cell reselection while receiving MBS multicast data. [Figure 8b] This flowchart shows an exemplary method, according to embodiments of the present invention, performed in a UE that is in the RRC_INACTIVE state for cell reselection while receiving MBS multicast data. [Figure 9a] This flowchart shows an exemplary method, according to embodiments of the present invention, that is performed at a source base station to support cell reselection of a UE in the RRC_INACTIVE state and to receive MBS multicast data. [Figure 9b] This flowchart shows an exemplary method, according to embodiments of the present invention, that is performed at a source base station to support cell reselection of a UE in the RRC_INACTIVE state and to receive MBS multicast data. [Figure 10a] This flowchart shows an exemplary method, according to embodiments of the present invention, for supporting cell reselection of a UE in the RRC_INACTIVE state and for being performed at a target base station to receive MBS multicast data. [Figure 10b]This flowchart shows an exemplary method, according to embodiments of the present invention, for supporting cell reselection of a UE in the RRC_INACTIVE state and for being performed at a target base station to receive MBS multicast data. [Figure 11] This is a flowchart of a method performed by a UE to support cell reselection for the UE, according to embodiments of the present invention. [Figure 12] This is a flowchart of a method performed by a source base station to support cell reselection for a UE operating in a disconnected RRC state with one or more active data sessions, according to an embodiment of the present invention. [Figure 13] This is a flowchart of a method performed by a target base station to support cell reselection for a UE operating in a disconnected RRC state with one or more active data sessions, according to another embodiment of the present invention. [Figure 14] This is a flowchart of a method performed by a target base station to support cell reselection for a UE operating in a disconnected RRC state using one or more active data sessions, according to an embodiment of the present invention. [Modes for carrying out the invention]

[0030] Figure 1a shows an exemplary wireless communication system 100, in particular a mobile wireless communication system such as a fifth-generation (5G) nu-radio (NR) system supporting multicast and broadcast services (MBS). While embodiments and examples of the present invention will be described in relation to a 5G NR system in the following description, it will be understood that the present invention is not limited to a 5G NR system and can be used in any wireless communication system supporting MBS or similar services.

[0031] System 100 comprises a user device (UE) 101, which may be, for example, inside or part of a vehicle, serviced by a base station 110 to communicate with a core network such as a 5G core network 102. The UE may be any wireless device, such as a wireless communication device or apparatus or terminal, an IoT device, a machine-type communication (MTC) device, a device-to-device (D2D) terminal, or a user device (e.g., a smartphone, laptop, mobile phone, tablet, camera, game console, wearable device), capable of wirelessly communicating with one or more core networks via one or more radio access networks. Base station 110 is a network node that provides an access point to the core network for the UE and is part of a radio access network (RAN) consisting of base stations 110 and 111. In NR, base stations are called next-generation node B (gNB), the RAN is next-generation (NG)RAN, and the core network is called 5GC. Hereafter, the terms RAN node, base station, and gNB are used interchangeably. Base stations 110 and 111 are interconnected by means of an Xn interface (as specified in 3GPP standard TS 38.423) implemented on wired or wireless link 130. Each base station is connected to the core network by an NG interface (as specified in 3GPP standard TS 38.413) implemented on wired or wireless links 140 and 141.

[0032] Each of these base stations controls one or more cells. For example, base station 110 controls cell 120, and base station 110 controls cell 121. A cell A is a geographical area of ​​a radio network defined by the frequencies used within the cell to transmit data. Cells can be uniquely identified by UEs from identification information broadcast across the geographical area. Each base station 110, 111 can serve several UEs, such as UE 101. When a UE establishes an RRC connection with a base station (discussed below), the base station to which the UE connects is called the UE's serving base station or source base station, and the cell controlled by the serving base station where the UE camps is called the serving cell. The interface between the gNB and the UE is a Uu interface that uses the protocol sublayers SDAP (Service Data Application Protocol), PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control), MAC (Medium Access Control), and PHY (Physical) in the user plane, and the protocol sublayers RRC (Radio Resource Control), PDCP, RLC, MAC, and PHY in the control plane.

[0033] Figure 2 shows a block diagram of a UE device 205, such as UE 101 in Figure 1a, in which the present invention may be implemented according to one or more embodiments of the present invention. The UE includes components for transmitting and receiving communications, including a UE communications manager 220, an I / O controller 255, a transceiver 235, a set of antennas 245, memory 225, and a processor (CPU: central processing unit) 215. All of these elements communicate with each other.

[0034] Memory 225 includes RAM (Random Access Memory), ROM (Read-Only Memory), or a combination of both, or, in non-exclusive examples, mass storage devices such as disks or solid-state drives. Basic Input / Output System (BIOS) instructions may be stored in memory 225.

[0035] The processor 215 is configured to execute machine-readable instructions. Execution of these machine-readable instructions causes the UE to perform various functions. These functions may involve or interact with peripheral devices such as a keyboard, screen, mouse, etc. (not shown in Figure 2). The processor can run an operating system such as iOS, Windows, or Android. The processor 215 may be a single processor or may include two or more processors that perform the processing necessary for the operation of the UE 205. The number of processors and the assignment of processing functions to the processors are design choices for those skilled in the art.

[0036] The I / O controller 255 provides the necessary hardware and enables these interactions with external peripherals by managing input and output signals.

[0037] The transceiver 235 is configured to provide bidirectional wireless communication with other wireless devices. For example, it provides the modem and frequency shifter necessary to connect to one or more wireless networks such as Wi-Fi, Bluetooth®, LTE, and 5G NR.

[0038] The wireless communication uses an antenna set 245 adapted to the spectrum of the frequency transposed signal issued from the baseband modem. The antenna set 245 may be limited to one antenna, but preferably includes several antennas to provide beamforming capability.

[0039] The UE communications manager 220 handles the establishment, control, and release of communications between the UE and the radio access network. The UE periodically receives indications from the base station of available slots for communication between the UE and the base station. The UE then knows where, temporally and frequency-wise, data is expected to arrive or where it must transmit its outgoing data, regardless of whether they belong to the control plane or the data plane. In an exemplary implementation, the UE communications manager 220 implements the Uu interface.

[0040] Figure 3 shows a block diagram of the base station or base station device 305 such as gNB110 and 111 in Figure 1a, and the present invention may be implemented according to one or more embodiments of the present invention. The base station device 305 includes components for transmitting and receiving communications, including a base station communications manager 320, a core network communications manager 355, a transceiver 335, a set of antennas 345, memory 325, a processor (CPU) 315, and an inter-station communications manager 365. All of these elements communicate with each other.

[0041] The base station communication manager 320 handles communication with multiple UEs. It is responsible for establishing, controlling, and releasing these communications. In an exemplary implementation, the base station communication manager 320 implements the Uu interface. The base station communication manager 320 includes a scheduler that assigns time-frequency slots to different UE communications. Information regarding the scheduling of these slots is periodically sent to the participating UEs.

[0042] The core network communication manager 355 manages communication between base stations and the core network. It can provide standardized NG interfaces, such as those defined by 3GPP standards, to support these communications.

[0043] The transceiver 335 is configured to provide bidirectional wireless communication with other wireless devices. These devices may be UEs or even other base stations. The transceiver 335 provides the modem and frequency shifter necessary to connect to multiple UEs simultaneously using different frequency carriers in time-division duplex (TDD) or frequency-division duplex (FDD). The transceiver 335 is connected to an antenna set 345 which may be limited to one antenna, but preferably includes several antennas to provide beamforming capability.

[0044] Memory 325 includes RAM, ROM, or a combination of both, or, in non-limiting examples, mass storage devices such as disks or solid-state drives. BIOS instructions may be stored within memory 325 to support the operating system.

[0045] The inter-station communication manager 365 manages communication with other base stations. To support these communications, the inter-station communication manager 365 can provide a standardized Xn interface, such as those defined by 3GPP standards.

[0046] Figure 4 is a flowchart 400 showing the RRC connection states and transitions of a UE in 5G NR. The RRC protocol operates between the UE and the base station (gNB) and is defined in the 3GPP specification TS 38.331 for 5G NR. The UE state name is prefixed with "NR" for New Radio. Other prefixes are used for other radio interfaces, such as the LTE radio interface. For simplicity, radio technology prefixes are omitted in the following description.

[0047] Radio Resource Control (RRC) is a layer within the 5G NR protocol stack. It exists only in the control plane, UE, and gNB. The behavior and functionality of base stations and UEs are governed by the current RRC state of the UE. In 5G NR, three distinct RRC states are assigned to the UE: RRC_IDLE state 401, RRC_CONNECTED state 402, and RRC_INACTIVE state 403.

[0048] At startup, the UE is in RRC_IDLE state 401, performs radio link quality measurements, and runs a cell selection evaluation process (as defined in 3GPP standard TS 38.304) to identify the target gNB to which it is connected. The UE state changes to RRC_CONNECTED state 402 when an RRC connection is established with the target gNB, which will become the source gNB providing services to the UE. Hereafter, the source base station (or source gNB) may also be referred to as the serving base station or serving gNB. If there is no radio activity for a period of time, the RRC connection may be released by the source gNB, and the UE's RRC state changes back to RRC_IDLE state 401.

[0049] Releasing RRC connections is interesting from the perspective of capacity utilization and power saving, but it is not ideal from the perspective of latency (delay time). The overhead of establishing an RRC connection requires extra signaling that introduces delay. To address this drawback, the RRC_INACTIVE state 403 has been introduced in 5G NR. When a UE is in the RRC_INACTIVE state 403, the UE cannot communicate with the 5G system, but both the source gNB (e.g., the last serving gNB) and the UE store the UE context or configuration. The stored UE context or configuration contains information to facilitate the rapid resumption of the connection. This information may include security context (e.g., security parameters such as security keys and UE security features), measurement configuration, radio configuration (e.g., UE radio features), bearer information, PDU session context, etc. Therefore, when in the RRC_CONNECTED state 402, the RRC connection may be suspended by the source gNB (release with suspension), and the UE moves to the RRC_INACTIVE state 403. From the RRC_INACTIVE state 403, the UE can be returned to the RRC_CONNECTED state 402 by the gNB (Resume), and the UE applies the stored UE context or configuration. The RRC resume message is sent by the gNB when it receives an RRC resume request message from the UE.

[0050] From either the RRC_CONNECTED state or the RRC_INNACTIVE state, the UE can transition to the RRC_IDLE state in response to an RRC release command received from the gNB.

[0051] The mobility procedure for migrating a UE from one cell to another depends on the UE's RRC state. In the RRC_CONNECTED state, a mobility procedure called handover is controlled by the network, and the source gNB decides to trigger the handover procedure based on the measurement report provided by the UE. In the RRC_INACTIVE and RRC_IDLE states (e.g., disconnected state), the mobility procedure is called cell reselection, and it is managed by the UE itself.

[0052] In the RRC_INACTIVE state, a UE may be configured by the network using a RAN notification area (RNA). For example, a message that transitions a UE to the RRC_INACTIVE state contains information indicating the RNA. The RNA is an area that a UE can move to without notifying the network. When a UE in the RRC_INACTIVE state moves to a cell that is not part of the RNA currently assigned to it, the UE performs a location update procedure that allows the RAN (e.g., a serving gNB) to update the RNA assigned to the UE. In other words, the UE may request an RNA update to signal an RNA modification. As part of the cell reselection process, when a UE selects a cell managed by a target gNB from the RNA, the UE sends a reactivation request to the target gNB, which has three options available to keep the UE in the RRC_INACTIVE state, set the UE to the RRC_IDLE state, or set the UE to the RRC_CONNECTED state.

[0053] In the RRC_IDLE state, the core network initiates a paging procedure to notify the UE that the connection must be resumed. In the RRC_INACTIVE state, the paging procedure is initiated by the NG RAN (i.e., the last gNB that set the UE to the RRC_INACTIVE state).

[0054] Returning to Figure 1a, assume that UE 101 is in the RRC_INACTIVE state and is receiving multicast data for one or more multicast MBS sessions generated by the multicast application server 103. The multicast data is provided to base station 110, the serving base station for UE 101, via link 140 through the core network 102 and transport bearer 104. The multicast data is then transmitted from base station 110 to UE 101 via the MBS Radio Bearer (MRB) 105. A radio bearer is a set of PHY (Layer 1) and MAC (Layer 2) parameters that enable higher-layer data connectivity between the UE and the gNB. In 5G NR, several types of radio bearers are defined: SRB (Signalling Radio Bearer) for the control plane, DRB (Data Radio Bearer) for point-to-point communication (unicast) with a single UE in the user plane, and MRB for point-to-point and point-to-multipoint communication (multicast / broadcast) with multiple UEs in the user plane.

[0055] The MBS session join procedure is used by the UE to notify the 5GC of its interest in joining a multicast MBS session, as specified in 3GPP TS 23.247. The first accepted UE join request triggers the establishment of a multicast MBS session to the NG RAN and the UE. Before sending a join request for a multicast MBS session, the UE must have established a PDU session that can be associated with the multicast session using the procedure specified in TS 23.502. The UE should also know the MBS session ID of the multicast group that the UE can join, at least through a service announcement broadcast by the network. To join a multicast group, the UE sends a PDU session change request to the associated PDU session, which includes one or more MBS session IDs and a join request. The MBS session ID indicates the multicast MBS session that the UE wishes to join.

[0056] To participate in an MBS session, the UE must be in the RRC_CONNECTED state. While receiving MBS multicast data in the RRC_CONNECTED state, UE 101 may have successively moved to different cells using the handover procedure shown in Figure 5.

[0057] Figure 5 is a simplified flowchart 500 illustrating the handover procedure while a UE, such as UE 101 in Figure 1a, is receiving MBS multicast data in the RRC_CONNECTED state. This figure shows UE 501 similar to UE 101, base station 510 similar to base station 110 which is a source or serving gNB in ​​the sense that it controls the cell (which may be called a serving cell) that the UE is currently camping, base station 511 similar to base station 111, and core network (5GC) 502 similar to core network 102 which controls a candidate cell that UE 501 may move to. The MBS multicast data is provided to the source gNB by 5GC (via bearer 540), and then the MBS multicast data is sent to UE 501 (via radio bearer 541) simultaneously to other UEs belonging to the same multicast group.

[0058] UE 501 is configured with measurements to be performed regularly on neighboring cells. The first step 520, which is performed sequentially when in the RRC_CONNECTED state, is to search for candidate cells. When a candidate cell is found, UE 501 measures one or more parameters of the signal received from the candidate cell. The parameters may include reference signal received power (RSRP) or reference signal received quality (RSRQ). For example, UE 501 measures reference signal received power (RSRP) or reference signal received quality (RSRQ), which are commonly performed on signal synchronization blocks (SSBs) transmitted in this candidate cell. Depending on a triggering event configured in the UE, the measurement may trigger a measurement report 531 (RRC protocol message) sent by UE 501 to the source gNB 510, using information about the candidate cell.

[0059] Upon receiving the measurement report 531, the source gNB 510 can determine in the handover determination step 521 whether to perform a handover. Additional information beyond the measurement report may be considered, such as whether the target gNB controlling the candidate or target cell is within the MBS service area and can provide the same MBS service that the UE is listening to (e.g., supporting one or more existing / ongoing MBS sessions for the UE), and / or whether there is sufficient capacity available in the target cell for the handover. The network may also decide to hand over the UE to another cell, even if no measurement report has been received, for example, for load balancing purposes. If the network decides to hand over the UE to another cell, such as a target cell controlled by the target gNB 511 (step 521), the source gNB 510 sends a handover request 532 (Xn protocol message) to the target gNB 511. If the source and target cells belong to the same gNB, this message is not necessary, as the situation in the target cell is already known to the gNB. The handover request message 532 includes a UE context containing information about the MBS session in which the UE is participating. The target gNB 511 then performs an acceptance control step 522 to determine whether to accept the handover request. For example, the request may be rejected if the load in the target cell is too high. In either case, the target gNB 511 sends a handover acknowledgment message 533 to the source gNB 510. If the target gNB 511 accepts the handover, the source gNB notifies the UE 501 to switch cells via an RRC reconfiguration message 534 containing the information necessary for the UE 501 to connect to the target cell, such as radio bearers and measurement configuration (information previously received by the source gNB from the target gNB in ​​the acknowledgment message 533). In the case of a conditional handover (CHO), the UE 501 is configured with trigger conditions that must be met before switching to a new cell (not shown in Figure 5).

[0060] In step 523, UE 501 switches to the target cell as the new serving cell and moves the RRC connection to the target gNB 511. Uplink synchronization is required to enable connection to the new serving cell, and therefore the UE typically performs random access 535 toward the target cell to obtain uplink synchronization. Once synchronization is established, the UE sends an RRC reconfiguration complete message 535 to the target gNB 511, which is now its new source or serving gNB managing the new serving cell.

[0061] On the other hand, target gNB 511 may perform a path switching handshake procedure 537 toward core network 502 to request delivery of MBS multicast data. Thus, MBS multicast data may be provided to target gNB 511 by core network 502 (via bearer 542), and then the MBS multicast data is simultaneously sent to UE 101 (via MBS radio bearer 543) to other UEs belonging to the same multicast group. To minimize or prevent data loss during the handover operation, any buffered data in source gNB 510 may be moved to target gNB 511 (not shown in Figure 5).

[0062] Returning to Figure 1a, UE 101 is currently camped in cell 120, which is UE 101's serving cell with base station 110 as the source or serving gNB. Apart from receiving MBS multicast data without radio activity, UE 101 is assumed to be set to the RRC_INACTIVE state by gNB 110. While UE 101 is still receiving MBS multicast data, it can move and arrive at the edge of serving cell 120, but it can also arrive at the edge of another cell, for example, cell 121. At some point, UE 101 may need to change serving cells as it could suddenly lose coverage of cell 120. When UE 101 needs to change its serving cell, unlike when the UE is in the RRC_connected state and the mobility procedure is handled by the network (RAN and core network) to ensure the continuity of MBS data delivery through the target gNB of the new serving cell, in the RRC_INACTIVE state (and the RRC_IDLE state, both of which are disconnected states), the mobility procedure is called cell reselection, and the UE manages it itself. When performing cell reselection, the UE performs a cell reselection evaluation process (e.g., as described in 3GPP standard TS 38.304 V16.7.0, section 5.2.4) and there is no communication with the network. Because there is no communication with the network in the cell reselection procedure, several problems can prevent the UE from continuing to receive MBS data after cell reselection. For example, when there is no communication with the network in the cell reselection process, the UE cannot obtain the configuration information necessary to receive MBS data through the target gNB, and the target gNB is not informed of a new UE in the RRC_INACTIVE state that wants to receive MBS services through the target gNB. Without configuration information, the UE cannot receive MBS data from the target gNB.Notifying the target gNB of a new UE in the RRC_INACTIVE state helps ensure that the UE is authorized to receive MBS services, and the target gNB helps provide the UE with configuration information for receiving MBS services in the new cell and, if necessary, request the core network to provide MBS data to the target gNB.

[0063] Referring now to Figure 11, which illustrates the steps of Method 1100 for supporting cell reselection for a UE of a wireless communication system according to an embodiment of the present invention, this method is performed by the UE. The wireless communication system may be, for example, the wireless communication system 100 in Figure 1a, the UE may be the UE 101 in Figure 1a, and the UE may comprise the UE 205 in Figure 2, with Method 1100 performed by the processor 215. The UE 101 is operating in a disconnected radio resource control (RRC) state (e.g., RRC_INACTIVE state) with one or more active data sessions (e.g., one or more active MBS sessions). An active MBS session may be an MBS multicast session or an MBS broadcast session. In other words, a data session is "active" in the sense that the UE is participating in or has participated in one or more data sessions. In the example shown in Figure 1a, the UE 101 is currently camp-on to serving cell 120 (the current serving cell), which is controlled by the source base station 110. The UE 101 may move to cell 121. Therefore, cell 121 is sometimes referred to as a candidate / target cell controlled by the target base station 111. A candidate / target cell can be a serving cell or a neighboring cell of the source cell (i.e., the current serving cell).

[0064] In short, in step 1101, UE 101 sends a reselection request message to the new serving cell to request cell reselection in order to receive one or more active data sessions from the target base station of the new serving cell (for example, to enable the UE to receive data from one or more active data sessions from the target base station). In the example in Figure 1a, the new serving call could be the UE 101 of the current serving cell or cell 121 selected by the source base station 110.

[0065] In step 1102, UE 101 receives a configuration message containing configuration information associated with a target base station for one or more active data sessions. For example, the configuration information includes radio configuration information indicating the radio configuration of the target base station for one or more active data sessions. In other words, UE 101 receives radio configuration information indicating the radio configuration used by the target base station to transmit data for one or more active data sessions (e.g., an MBS bearer configuration for one or more active data sessions in the new serving cell). Based on the received configuration information, UE 101 configures itself to receive data for one or more active data sessions from the target base station in the new serving cell (e.g., setting up the user plane in UE 101 according to the radio configuration of the target base station) (step 1103). UE 101 may switch to the new serving cell either before or after the reselection request is sent. After UE 101 is configured and after UE 101 switches to a new serving cell (the switch may occur either before or after a reselection request is sent), the UE receives data from the target base station for one or more active data sessions. Steps 1101, 1102, and 1103 are performed by UE 101 operating in a disconnected RRC state (e.g., RRC_INACTIVE state).

[0066] Prior to step 1101, UE 101 may identify one or more suitable cells for a new serving cell for UE 101 (step 1104, indicated by a dotted line). The new serving cell will be one of the identified suitable cells. For example, UE 101 may perform measurements on signals received by UE 101 from one or more candidate cells, as described below with reference to Figure 6 (step 620). UE 101 may select or identify one suitable cell or a list of suitable cells according to cell reselection evaluation criteria, such as frequency priority used in the candidate cells, radio link quality, and availability of MBS services in the candidate cells, as described below (for example, with reference to Figure 6). When UE selects one suitable cell, UE 101 identifies the one suitable cell selected as the new serving cell in a reselection request message sent by UE 101. Alternatively, UE 101 can select multiple suitable cells for the new serving cell, and thus multiple suitable cells (e.g., as a list of suitable cells) can be identified in the reselection request message sent by UE 101, and the selection of the new serving cell takes place after the reselection request message is sent (e.g., by the source base station receiving the reselection request message, or by the target base station if the target base station controls two or more of the suitable cells). In an example where the target base station selects the new serving cell, the source base station may send a request to the target base station (e.g., in a migration request message as described below) along with a list of suitable cells for the new serving cell for the UE. The target base station selects one of the suitable cells as the new serving cell (e.g., based on the load status of each cell), and in response to the source base station (e.g., in a migration confirmation message as described below), the target base station indicates to the source base station that the UE must switch the identification information (e.g., cell ID) of the selected cell.

[0067] UE 101 can send a reselection request message to a RAN node such as the source base station 110 of the serving cell or the target base station 111 of the new serving cell. An example of sending a reselection request message to the source base station 110 of the serving cell is described in more detail below with reference to Figures 6, 8a, 9a, and 10a. An example of sending a reselection request message to the target base station 111 of the new serving cell is described in more detail below with reference to Figures 7, 8b, 9b, and 10b.

[0068] In one example, when UE 101 sends a reselection request message to source base station 110, UE 101 receives a configuration message from the source base station. The reselection request message may contain identification information associated with or identifying one or more suitable cells for the new serving cell. When the reselection request message contains identification information for two or more suitable cells, source base station 110 selects one of the suitable cells as the new serving cell (or, if two or more of the suitable cells are controlled by the target base station, selects one of the suitable cells). Alternatively, UE 101 may select one suitable cell as the new serving cell for the UE, and the reselection request message sent by UE 101 will contain identification information associated with or identifying the one suitable cell selected as the new serving cell. The identification information for each selected or suitable cell may include the cell ID and base station (gNB) ID. 3GPP TS 38.413 sections 9.3.1.6 and 9.3.1.7 describe the gNB ID included in the cell ID.

[0069] UE 101 can switch to a new serving cell in response to receiving a configuration message from the source base station, and then receive data from one or more active data sessions from the target base station. When the source or target base station selects one of the appropriate cells as the new serving cell, the source base station may include identification information in the configuration message to identify the selected cell (e.g., cell ID) that the UE must switch to.

[0070] In one example, when UE 101 sends a reselection request message to target base station 111, UE 101 receives a configuration message from the target base station. In this case, UE 101 selects a suitable cell as the new serving cell for UE 101 according to cell reselection evaluation criteria, such as frequency priority used in the candidate cell, radio link quality, and availability of MBS services in the candidate cell, as described below (see, for example, Figure 6). UE 101 then sends a reselection request message to the target base station of the selected new serving cell. The reselection request message sent by UE 101 includes identification information associated with or identifying the source base station 110 of the serving cell (i.e., the current serving cell). Furthermore, the reselection request message may include session identification information, such as an MBS session identifier, that identifies one or more active data sessions. In this case, UE 101 switches to the new serving cell before sending the reselection request message to target base station 111 (i.e., before UE 101 receives the configuration information). After receiving the configuration information, the UE 101 can then receive data from one or more active data sessions from the target base station.

[0071] By sending a reselection request message to the new serving cell to request cell reselection, and receiving configuration in response, the UE 101 can obtain the configuration information necessary to receive data from one or more active data sessions via the target base station, ensuring that the continuity of data services (e.g., MBS services) is maintained, or that at least data loss is minimized when the UE performs cell reselection and switches to the new serving cell. Furthermore, the UE sending a reselection request message while in a disconnected RRC state means that the UE does not need to switch to the RRC_CONNECTED state to apply the handover procedure, thus preserving the benefits of keeping the UE in a disconnected RRC state (e.g., reduced control plane footprint).

[0072] When UE 101 sends a reselection request message to target base station 111, UE 101 can switch to the new serving cell sooner than if UE 101 were to send the reselection request message to source base station 110 and switch to the new serving cell after receiving configuration information. When UE 101 switches to the new serving cell sooner, the data connection to the serving cell is lost sooner. This can result in some data loss compared to when UE 101 switches to the new serving cell later after receiving configuration information. However, in cases where there is a communication problem with the source base station 110 (e.g., a radio link failure), an early switch to a new serving cell controlled by the target base station 111, and sending a reselection request message to the target base station 111, means that the UE 101 can receive configuration information associated with the target base station for one or more active data sessions and configure itself to receive one or more active data sessions from the target base station with minimal delay, while at the same time being in a disconnected RRC state, which helps to minimize data loss and ensure continuity of data services (e.g., MBS services) without having to switch to the RRC_CONNECTED state.

[0073] Referring now to Figure 12, which illustrates the steps of Method 1200 for supporting cell reselection for a UE of a wireless communication system according to an embodiment of the present invention, this method is performed by a source base station of the UE's serving cell (e.g., the current serving cell). The wireless communication system may be, for example, the wireless communication system 100 in Figure 1a, and the source base station may be a source base station 110 supporting UE 101. The source base station may comprise a base station 305 in Figure 3, where Method 1200 is performed by a processor 315. UE 101 is operating in a disconnected radio resource control (RRC) state (e.g., RRC_INACTIVE state) with one or more active data sessions (e.g., one or more active MBS sessions). Active MBS sessions may be MBS multicast sessions or MBS broadcast sessions. In the example shown in Figure 1a, UE 101 is currently staying in serving cell 120 (the current serving cell) controlled by the source base station 110. UE 101 may move to cell 121. Therefore, cell 121 is sometimes referred to as a candidate / target cell controlled by the target base station 111. A candidate / target cell can be a serving cell or a neighboring cell of the source cell (i.e., the current serving cell).

[0074] In short, in step 1201, source base station 110 receives a reselection request message from UE 101 to request cell reselection to a new serving cell in order to receive one or more active data sessions from the target base station for the new serving cell (for example, to enable the UE to receive data from one or more active data sessions from the target base station). The reselection request message may include identification information related to or identifying the selected new serving cell (when UE 101 has selected the new serving cell). Alternatively, the reselection request message may include identification information related to or identifying one or more suitable cells selected for the new serving cell. When the reselection request message includes identification information identifying two or more suitable cells for the new serving cell, source base station 110 may select one of the suitable cells as the new serving cell (or, as described below with reference to Figure 6, when two or more suitable cells are controlled by the target base station, the target base station selects one of the suitable cells). The identification information for each selected or suitable cell may include the cell ID and base station (gNB) ID of the cell. Sections 9.3.1.6 and 9.3.1.7 of 3GPP TS 38.413 describe the gNB IDs included in the cell ID.

[0075] In step 1202, the source base station 110 sends a migration request message to the target base station 111 of the new serving cell (for example, cell 121 selected by the UE 101 or by the source base station 110 as the new serving cell) indicating that the UE 101 is requesting cell reselection using one or more active data sessions. The migration request message includes context information associated with the UE and one or more active data sessions. The context information may include identification information for identifying the UE (for example, a UE ID) and status information indicating that the UE is requesting cell reselection using one or more active data sessions. The context information may further include session identification information for identifying one or more active data sessions (for example, an MBS session identifier). The source base station 110 then receives a migration approval message from the target base station 111 in step 1203, which includes configuration information associated with the target base station 111 for one or more active data sessions. The configuration information may include radio configuration information indicating the radio configuration of the target base station for one or more active data sessions (for example, an MBS radio bearer configuration in the new serving cell). The migration confirmation message may include a migration confirmation message indicating that the target base station accepts that it is a new serving cell for the UE for one or more active data sessions, and the migration confirmation message may include configuration information. In step 1204, the source base station 110 sends a configuration message containing configuration information to the UE 101, enabling the UE 101 to receive data (e.g., MBS multicast data) for one or more active data sessions from the target base station 111. The configuration message sent to the UE 101 notifies the UE 101 to switch to the new serving cell. The configuration message may include identification information (e.g., an identifier) ​​that identifies the new serving cell to which the UE should switch. The identification information may include the cell ID and base station (gNB) ID of the new serving cell.Sections 9.3.1.6 and 9.3.1.7 of 3GPP TS 38.413 describe the gNB IDs included in the cell ID.

[0076] Examples of steps performed at the source base station when it receives a reselection request message from the UE are described in more detail below with reference to Figures 6, 8a, 9a, and 10a.

[0077] By communicating with the target base station in response to receiving a reselection request message from the UE, the source base station supports cell reselection of a new serving cell for the UE, ensuring that the continuity of data services (e.g., MBS services) is maintained, or at least data loss is minimized, and enabling the UE 101 to obtain the configuration information necessary to receive data for one or more data sessions via the target base station.

[0078] Hereinafter, Figure 13 illustrates the steps of Method 1300 for supporting cell reselection for a UE of a wireless communication system according to an embodiment of the present invention. Method 1300 is performed by the target base station of the new serving cell of the UE, which has switched from the previous serving cell (e.g., the nearest serving cell) to the new serving cell. The wireless communication system may be, for example, the wireless communication system 100 in Figures 1a and 1b, and the target base station may be the target base station 111 supporting the UE 101. The target base station may comprise the base station 305 in Figure 3, and Method 1300 is performed by the processor 315. The UE 101 is operating in a disconnected radio resource control (RRC) state (e.g., RRC_INACTIVE state) with one or more active data sessions (e.g., one or more active MBS sessions). The active MBS sessions may be MBS multicast sessions or MBS broadcast sessions. In the example shown in Figures 1a and 1b, UE 101 was previously camped on a serving cell (or the last serving cell) 120 controlled by the source base station 110, and is being switched to stay in a new serving cell 121 (e.g., the current new serving cell) controlled by the target base station 111.

[0079] In short, in step 1301, the target base station 111 receives a reselection request message from the UE 101 requesting cell reselection to a new serving cell 121 using one or more active data sessions. The reselection request message may include identification information (e.g., gNB ID) that identifies the source base station 110 of the previous serving cell 120. The reselection request message may further include session identification information (e.g., MBS session identifier) ​​that identifies one or more active data sessions. In step 1302, the target base station 111 sends a context request message to the source base station 110 of the previous serving cell 120. The context request message may include identification information (e.g., UE ID) that identifies the UE. The context request message may further include information to request context information for one or more active data sessions.

[0080] The target base station 111 receives a context message from the source base station 110 that includes context information related to the UE and one or more active data sessions (step 1303). The context information may include information that identifies one or more active data sessions (for example, MBS session identifiers).

[0081] In step 1304, the target base station 111 sends a configuration message to the UE 101 containing configuration information related to the target base station for one or more active data sessions, in order to enable the UE 101 to receive data for one or more active data sessions from the target base station 111 (e.g., MBS multicast data). The configuration information may include radio configuration information indicating the radio configuration of the target base station for one or more active data sessions (e.g., MBS radio bearer configuration in a new serving cell). The target base station 111 may also set up radio resources for delivering data to the UE 101 for one or more active data sessions.

[0082] In one example, the target base station 111 determines whether to accept a cell reselection request for one or more active data sessions in response to receiving a context message. When the target base station 111 determines that it can accept a cell reselection request for one or more active data sessions (for example, the target base station has the ability and capacity to support MBS services for the UE), the target base station 111 sends a configuration message. When the target base station 111 determines that it cannot accept a cell reselection request for one or more active data sessions (for example, the target base station is overloaded and / or cannot support MBS services for the UE), the target base station 111 does not send a configuration message containing configuration information. To indicate that a cell reselection request for one or more active data sessions has been rejected, the target base station 111 may either not send a message in response to the cell reselection request, or it may send a rejection message indicating that a cell reselection request for one or more active data sessions has been rejected.

[0083] In one embodiment, upon receiving a context message, the target base station 111 can communicate with entities in the core network 102 to establish a packet data unit (PDU) session between the UE 101 and the target base station 111 for one or more active data sessions (for example, using a path switching handshake procedure such as the one described below (step 736 in Figure 7)). If the target base station 111 already supports one or more active data sessions for one or more other UEs, the target base station 111 has already received data for one or more active data sessions and therefore does not need to perform a path switching handshake procedure or similar procedure with the core network entities.

[0084] UE 101 has been switched to stay on the new serving cell 121, but in an example where two or more suitable cells have been identified according to the cell reselection evaluation criteria (as described below), the target base station 111 may receive a reselection request message from UE 101 to request cell reselection to the new serving cell 121 using one or more active data sessions, and the reselection request message may further include identification information that identifies two or more suitable cells (e.g., a list of suitable cells). The target base station 111 may then select one of the one or more suitable cells as the new serving cell (e.g., based on the load state in each cell) and may send a response to UE 101 (e.g., in a configuration message) that includes identification information (e.g., a cell ID) indicating the suitable cell selected by the target base station 111. If the selected suitable cell is not the new serving cell 121 where UE 101 is currently staying, UE 101 must switch the cell to the selected suitable cell.

[0085] Examples of steps performed at the target base station when it receives a reselection request message from the UE are described in more detail below with reference to Figures 7, 8b, 9b, and 10b.

[0086] By communicating with the source base station in response to receiving a reselection request message from the UE, the target base station supports cell reselection of a new serving cell for the UE, ensuring that the continuity of data services (e.g., MBS services) is maintained, or at least data loss is minimized, and enabling the UE 101 to obtain the configuration information necessary to receive data from one or more active data sessions via the target base station.

[0087] Hereinafter, we refer to Figure 14 illustrating the steps of Method 1400 for supporting cell reselection for a UE in a wireless communication system according to an embodiment of the present invention. Method 1400 is performed by a target base station that controls a target cell (or target serving cell) selected as a new serving cell for the UE (by the UE or source base station of the current serving cell). The wireless communication system may be, for example, the wireless communication system 100 in Figure 1a, and the target base station may be a target base station 111 controlling a target cell 121. The target base station may comprise a base station 305 in Figure 3, and Method 1400 is performed by a processor 315. UE 101 is operating in a disconnected radio resource control (RRC) state (e.g., RRC_INACTIVE state) with one or more active data sessions (e.g., one or more active MBS sessions) via the serving cell (current serving cell). The active MBS sessions may be MBS multicast sessions or MBS broadcast sessions. In the example shown in Figure 1a, UE 101 is staying in serving cell 120 (current serving cell) controlled by source base station 110. UE 101 has moved to the edge of serving cell 120 and as a result also has coverage of cell 121 (a serving cell or a neighboring cell of source cell 120), and cell 121 has been selected as the new serving cell (by source base station 110 or UE 101), but UE 101 has not yet switched to the new serving cell.

[0088] In short, in step 1401, the target base station 111 receives a migration request message from the source base station 110 of the serving cell 120, indicating that the UE is requesting a cell reselection from the serving cell 120 to the target cell 121 (which is the new serving cell) in one or more active data sessions. The migration request message includes context information associated with the UE and one or more active data sessions. The context information may include session identification information (e.g., MBS session identifiers) that identify one or more active data sessions. In step 1402, the target base station 111 sends a migration approval message to the source base station 110, which includes configuration information associated with the target base station for one or more active data sessions, in order to enable the UE 101 to receive data (e.g., MBS multicast data) from the target base station 111. The configuration information may include radio configuration information that shows the radio configuration of the target base station for one or more active data sessions (e.g., MBS radio bearer configuration in the new serving cell). The target base station 111 may also set up radio resources to deliver data to the UE 101 for one or more active data sessions.

[0089] In one example, the target base station 111, upon receiving a migration request message, decides whether to accept a cell reselection request for one or more active data sessions. If the target base station 111 determines that it can accept a cell reselection request for one or more active data sessions (for example, the target base station has the capacity and capability to support MBS services for the UE), the target base station 111 sends a migration confirmation message (for example, a migration acceptance message) containing configuration information. If the target base station 111 determines that it cannot accept a cell reselection request for one or more active data sessions (for example, the target base station is overloaded and / or cannot support MBS services for the UE), the target base station 111 may either not send a message in response to the migration request message to indicate that the cell reselection request for one or more active data sessions has been rejected, or it may send a migration confirmation message (for example, a migration rejection message) indicating that the cell reselection request for one or more active data sessions has been rejected.

[0090] If UE 101 selects several suitable cells for a new serving cell and identifies the selected suitable cells (e.g., as a list of suitable cells) in a reselection request message sent by UE 101 to source base station 110, target base station 111 may receive a request from source base station 110 (e.g., included in a migration request message) containing a list of suitable cells for the new serving cell for the UE. Target base station 111 then selects one of the suitable cells as the new serving cell (e.g., based on the load status in each cell) and sends a response to source base station 110 indicating the identification information of the selected cell (e.g., cell ID) that the UE needs to switch to (e.g., included in a migration confirmation message).

[0091] Examples of steps performed at the target base station when it receives a migration request message from the source base station are described in more detail below with reference to Figures 6, 8a, 9a, and 10a.

[0092] By communicating with the source base station in response to receiving a migration request message from the source base station, the target base station supports cell reselection for a new serving cell for the UE, ensuring that the continuity of data services (e.g., MBS services) is maintained, or at least data loss is minimized, and enabling the UE 101 to obtain the configuration information necessary to receive data for one or more data sessions via the target base station.

[0093] Next, an example of a cell reselection procedure according to an embodiment of the present invention will be described in more detail with reference to Figures 6 and 7 and Figures 8a, 8b, 9a, 9b, 10a, and 10b.

[0094] Figure 6 is a flowchart 600 illustrating a first example of a cell reselection procedure for a UE receiving MBS multicast data in the RRC_INACTIVE state, according to an embodiment of the present invention. A UE can be in the RRC_INACTIVE state if it has one or more active data sessions: that is, the UE is subscribed to or participating in one or more data sessions (such as one or more MBS sessions). The following description refers to one or more active MBS multicast sessions, or the MBS sessions may be MBS broadcast sessions.

[0095] This figure shows a UE 601 in the RRC_INACTIVE state, similar to UE 101 in Figure 1a; a base station 610, similar to base station 110, which is a source or serving gNB in ​​the sense that it controls the cell (i.e., serving cell) where UE 601 is currently located; a base station 611, similar to base station 111, which controls a target cell (which may also be called a candidate cell) to which UE 601 may move (the base station controlling the target cell may be called the target base station); and a core network (5GC) 602, similar to core network 102. MBS multicast data is provided to the source gNB by the 5GC (via bearer 640), and then transmitted to UE 601 (via MBS radio bearer 641) simultaneously to other UEs belonging to the same multicast group.

[0096] With respect to step 520 in the handover procedure (as described above with reference to Figure 5), UE 601 periodically performs measurements on signals received by UE 601 from the serving cell and one or more candidate cells, such as signal synchronization blocks (SSBs) transmitted in the serving cell and candidate cells (also called target cells) (step 620). Candidate cells may be neighboring cells of the serving cell or source cell (i.e., the current serving cell).

[0097] If UE 601 finds a specific SSB having a received power exceeding the currently received power of the SSB by a predetermined threshold, in step 621, it performs a cell reselection evaluation process. The cell reselection evaluation process is performed according to several criteria, which may include the frequency priority used in the candidate cell, radio link quality, and the availability of MBS services in the candidate cell. Input information for selection (priority frequency, MBS availability) may be provided by source gNB 610 via system information message 630. Measurements in step 620 provide radio link quality information for selection. Details of an exemplary cell reselection evaluation process performed by the UE are given in 3GPP TS 38.304 (V16.7.0), section 5.2.4.

[0098] System information is the name of all common (non-device-specific) information that a UE needs to function properly within a network. Generally, system information is carried within different System Information Blocks (SIBs), each containing different types of system information. The various SIBs are defined in the 3GPP standard TS 38.331. SIBs are delivered differently depending on whether the UE is connected to the network; if the device is connected to the network, a dedicated RRC signal is used, and if not, a broadcast signal is used. Among the different SIBs, SIB1 contains system information that the device needs to know before it can access the system (i.e., for initial random access). SIB1 is always broadcast periodically and also contains information about the mapping of the rest of the SIBs to the system information message, information about the transmission periodicity of each SIB, and information about whether it is broadcast or not. In practice, SIBs can be broadcast periodically as SIB1, but alternatively, these SIBs can be sent on demand to avoid periodic broadcasts (and thus power saving) in cells where the device is not currently camping. In this case, the UE must explicitly request the transmission of some SIB by a system information request message. When in the RRC_INACTIVE state, the UE can send such a request message through a random access procedure. See, for example, the description of the random access procedure in section 9.2.6 of 3GPP TS 38.300 (V16.8.0) and section 5.1 of 3GPP TS 38.321 (V16.7.0).

[0099] Based on the received system information 630 and the measurements performed in step 620, UE 601 may, in step 621 (cell reselection evaluation process), select a suitable cell to move to as the new serving cell for the UE. In step 621, UE 601 may identify one suitable cell or a list of suitable cells according to cell reselection evaluation criteria such as frequency priority used in the candidate cell, radio link quality, and availability of MBS services in the candidate cell, as described above. When UE 601 identifies multiple suitable cells, source gNB 610 may select one of the suitable cells as the new serving cell (or target gNB 611 may perform the selection). Taking the example in Figure 1a, UE 101 identifies target cell 121 (target cell 611) as a suitable cell to stay in.

[0100] Next, UE 601 sends a reselection request message 631 to source gNB 610 (the base station controlling the serving cell (i.e., the current serving cell)) indicating the identifier of the appropriate cell selected in step 621. The reselection request message sent by UE 601 is for requesting cell reselection to a new serving cell to receive one or more active data sessions from the target base station of the new serving cell, the new serving cell may have been selected by UE 601 before sending the reselection request message (the reselection request message contains information identifying the new serving cell that has been selected), or it may have been selected by source gNB 610 based on the information contained in the reselection request message (identification information for identifying multiple cells suitable for the new serving cell for the UE).

[0101] For example, the reselection request message 631 may be an RRCResumeRequest message or RRCResumeRequest1 message as specified in 3GPP standard TS 38.331 (v16.7.0), modified to include information indicating that the UE's intention is to perform cell reselection rather than switch to the RRC_CONNECTED state. For this purpose, the resumeCause Information Element (IE) may be set to a new value indicating that the cause is cell reselection. Furthermore, a new IE may be introduced to indicate the appropriate cell identifier selected in step 621. For example, the new IE may be introduced in an RRCResumeRequest message or RRCResumeRequest1 message to include identification information for identifying one or more appropriate cells. The identification information for identifying each of the one or more appropriate cells may include the cell ID and base station (gNB) ID of the cell. 3GPP TS 38.413 sections 9.3.1.6 and 9.3.1.7 describe the gNB ID included in the cell ID.

[0102] Upon receiving a reselection request message 631, source gNB 610 uses a cell identifier to determine the relevant target gNB. If UE 601 has provided several cell identifiers for several suitable cells, source gNB must select one target gNB according to predefined or specific criteria. For example, source gNB 610 could select target gNB 611, which provides MBS services with the lowest load. Otherwise, the cell identifiers provided in the reselection request message 631 could be the cell identifiers of suitable cells selected by UE 601 as the new serving cells, and source gNB 610 uses the cell identifiers of the selected cells to determine the relevant gNB, which is target gNB 611.

[0103] Next, the source gNB, referring to Figure 5, initiates a procedure similar to the handover procedure described above (for example, steps 521, 522, 533, and 534 in Figure 5). The source gNB 610 sends an MBS migration request message 632 to the target gNB 611. If the source and target cells belong to the same gNB, this message is not necessary because the situation within the target cell is already known to the gNB. The MBS migration request message 632 includes a UE context that contains information about the MBS sessions in which the UE has joined. The MBS migration request message 632 indicates that the UE is requesting cell reselection to the target cell using one or more active data sessions (i.e., the UE is subscribed to or participating in one or more MBS sessions) and may include context information related to the UE and the one or more active MBS sessions. In one example, the context information may include identification information to identify the UE and status information to indicate that the UE is requesting cell reselection using one or more active MBS sessions. The context information may also include information about the one or more active MBS sessions, such as the MBS session ID. In addition to indicating that the UE is requesting cell re-selection, the state information may also include information indicating that the UE is operating in a disconnected state (e.g., the RRC_INACTIVE state).

[0104] For example, MBS migration request message 632 could be a HANDOVER REQUEST message as specified in 3GPP standard TS 38.423 (v16.8.0), modified to include information indicating that the UE's intention is to perform cell reselection while receiving MBS multicast data (for example, while participating in or joining one or more MBS sessions). For this purpose, the Cause IE of the HANDOVER REQUEST message may be set to a new value indicating cell reselection by an active MBS session.

[0105] The target gNB 611 then performs admission control processing (step 622) to decide whether to accept the MBS migration request. For example, it may reject the request if the load in the target cell is too high. In either case, the target gNB 611 sends an MBS migration acknowledgment message 633 to the source gNB 610. For example, if the target gNB 611 accepts the request, the MBS migration acknowledgment message 633 is a HANDOVER REQUEST ACKNOWLEDGE message as defined in 3GPP standard TS 38.423 (v16.8.0), and includes information necessary for UE 601 to receive the MBS multicast data from the target cell (i.e., the MBS radio bearer configuration of the target cell). In other words, when target gNB 611 accepts the request, target gNB 611 sends a migration acceptance message (e.g., a HANDOVER REQUEST ACKNOWLEDGE message) as MBS migration acknowledgment message 633. The migration acceptance message includes configuration information associated with target gNB 611 for one or more active MBS sessions, enabling the UE to be properly configured to receive MBS data from target gNB 611 in the target cell. In another example, if target gNB 611 rejects the request, MBS migration acknowledgment message 633 may be a HANDOVER PREPARATION FAILURE message as specified in 3GPP standard TS 38.423 (v16.8.0), where Cause IE is set to an appropriate value to indicate the reason for the rejection. In other words, when target gNB 611 rejects the request, target gNB 611 sends a migration rejection message as MBS migration acknowledgment message 633.

[0106] If UE 601 selects several suitable cells for the new serving cell and identifies the selected suitable cells (e.g., as a list of suitable cells) in the reselection request message 631 sent by UE 601 to source gNB 110, source gNB 610 may send a request (e.g., included in migration request message 632) to target gNB 611 for UE 601 containing the list of suitable cells for the new serving cell. Target gNB 611 then selects one of the suitable cells as the new serving cell (e.g., based on the load status of each cell) and sends a response to source gNB 610 indicating the identification information of the selected cell (e.g., cell ID) (e.g., included in migration confirmation message 633 or migration acceptance message).

[0107] If target gNB 611 accepts an MBS migration request, source gNB 610 notifies UE 601 to switch cells through a configuration message, such as an MBS reconfiguration message 634, which contains the information necessary for UE 601 to receive MBS multicast data in the target cell (information previously received by source gNB from target gNB in ​​MBS acknowledgment message 633). The necessary information includes configuration information (e.g., configuration information received from target gNB 611 in the migration acceptance message) that enables UE to configure itself to receive MBS data from target gNB for one or more active MBS sessions in the target or serving cell. The configuration information may include radio configuration information indicating the radio configuration of target gNB for one or more active MBS sessions (i.e., the radio configuration set up for target gNB to transmit MBS data for one or more active MBS sessions). The radio configuration information may include information about the MBS radio bearer configuration used by target gNB for one or more active MBS sessions. Message 634 may include an identifier of the target cell that UE 601 should switch to. For example, this could be the case when source gNB 610 selects a target cell, or when target gNB 611 selects a target cell.

[0108] For example, MBS reconfiguration message 634 could be the RRCReconfiguration message specified in 3GPP standard TS 38.331(v16.7.0) as modified to allow this message to be sent to the UE in the RRC_INACTIVE state. The MBS radio bearer configuration may be included in the RadioBearerConfig IE specified in TS38.331 section 6.3.2. For another example, MBS reconfiguration message 634 could be the RRCRelease message or RRCRelease with suspend configuration message specified in 3GPP standard TS 38.331(v16.7.0). For yet another example, MBS reconfiguration message 634 could be a dedicated SIB message.

[0109] If the target gNB 611 rejects the reselection request, the source gNB 610 may notify the UE 601 of the decision. For example, message 634 may also be an RRCReconfiguration message, an RRCRelease message, an RRCRelease with a suspend configuration message, or an SIB. For example, the failure of UE 601 to receive message 634 at the expiration of the timer initialized when the reselection request message 631 was sent may indicate that the reselection request was rejected.

[0110] In step 623, UE 601 switches to a new serving cell controlled by target gNB 611 and configures its user plane to receive MBS multicast data from target gNB 611. For example, UE 601 configures itself based on the received configuration information to receive MBS data from target gNB 611 for one or more active MBS sessions.

[0111] On the other hand, target gNB 611 may perform path switching handshake procedure 637 toward core network 602 to request the delivery of MBS multicast data. The path switching handshake procedure is described in 3GPP TS 38.413v16.8.0 section 8.4.4. For example, target gNB 611 may communicate with core network 602 to establish or set up (configure) packet data unit (PDU) sessions between UE 601 and target base station 611 for one or more MBS sessions. If target gNB 611 has already provided MBS services to other UEs and has therefore already received MBS multicast data from core network 602, procedure 637 may not be necessary. Thus, the MBS multicast data is provided to target gNB 611 by core network 602 (via bearer 642), and then the MBS multicast data is simultaneously sent to UE 601 (via MBS radio bearer 643) to other UEs belonging to the same multicast group. To minimize or prevent data loss during MBS traffic migration, any buffered data in source gNB 610 may be moved to target gNB 611 and then sent to UE 601 (this step is not shown in Figure 6).

[0112] If target gNB 611 had not yet provided MBS services before the pass switch handshake procedure 637, the situation corresponds to that shown in Figure 1a. At the end of the cell reselection procedure in Figure 6, the resulting situation is shown in Figure 1b, where UE 101 switches to a new serving cell 121 controlled by target gNB 111 and configures its user plane to receive MBS multicast data from target gNB 111.

[0113] Figures 1c and 1d show the case where the target gNB 611 in Figure 6 was already providing MBS services when UE 601 switched cells in step 623.

[0114] Figure 1b shows the result of an application by UE 101 of the cell reselection procedure described with reference to Figure 6, or the cell reselection procedure described below with reference to Figure 7 (or Figure 11). It represents the same wireless communication system 100 as shown in Figure 1a (the same entities in Figure 1b are referred to by the same reference numerals as those used in Figure 1a), but here UE 101 has switched to a serving cell 121 controlled by base station 111, which is a new source gNB or serving gNB for UE 101.

[0115] MBS multicast data is provided from the multicast application server 103 to the base station 111 via the core network 102 and transport bearer 106, and then via link 141. The MBS multicast data is then transmitted from the base station 111 to the UE 101 via the MBS Radio Bearer (MRB) 107.

[0116] Here, base station 110 and cell 120 can be considered candidate / target gNB and target cell from the perspective of UE 101. If UE 101 was the only UE receiving the MBS multicast service in cell 120, then base station 110 may have released the radio resources associated with the MBS radio bearer 105. Therefore, the core network 102 is no longer providing base station 110 with the MBS multicast data associated with the MBS service that UE 101 was listening to. As a result, the transport bearer 104 is released.

[0117] Figure 1c represents the same wireless communication system 100 as shown in Figure 1a (the same entities in Figure 1c are referred to by the same reference numerals used in Figure 1a), but with the addition that there is another UE 151 that camps on cell 121 and stays on cell 120 controlled by gNB 110 and receives the same MBS multicast service as UE 101. MBS multicast data is provided to base station 111 via link 141 through the core network 102 and transport bearer 152. Then, the MBS multicast data is transmitted from base station 111 to UE 151 via MBS Radio Bearer (MRB) 153. UE 151 may be in the RRC_CONNECTED state or the RRC_INACTIVE state.

[0118] Figure 1d shows the result of the application of the cell reselection procedure by UE 101 as described in Figure 6 or Figure 7 (or Figure 11). It represents the same wireless communication system 100 as shown in Figure 1c (the same entities in Figure 1d are referred to by the same reference numerals as those used in Figure 1c), but here UE 101 has switched to a serving cell 121 controlled by base station 111, which is a new source gNB for UE 101.

[0119] The MBS multicast data is still provided to the base station 111 via link 141 through the core network 102 and transport bearer 106. The base station 111 then transmits it to UE 101 and UE 151 via the same MBS Radio Bearer (MRB) 153 (point-to-multipoint transmission).

[0120] Here, base station 110 and cell 120 can be considered candidate / target gNB and target cell from the perspective of UE 101 and UE 151. If UE 101 was the only UE receiving the MBS multicast service in cell 120, then base station 110 may have released the radio resources associated with the MBS radio bearer 105. Therefore, the core network 102 is no longer providing base station 110 with the MBS multicast data associated with the MBS service that UE 101 was listening to. As a result, the transport bearer 104 is released.

[0121] Figure 7 is a flowchart 700 showing a second example of a cell reselection procedure for a UE that is in the RRC_INACTIVE state and receiving MBS multicast data, according to an embodiment of the present invention. A UE can be in the RRC_INACTIVE state if it has one or more active data sessions: that is, the UE is participating in or joining one or more data sessions (such as one or more MBS sessions). The following description refers to one or more active MBS multicast sessions, or the MBS sessions may be MBS broadcast sessions. This procedure is intended to cover the case where the UE loses connection with the source gNB before the completion of the cell reselection procedure, as described with reference to Figure 6, in other words, the UE has switched to the target gNB before the completion of the cell reselection procedure, as described with reference to Figure 6.

[0122] Figure 7 shows a core network (5GC) 702, such as UE 701 in the RRC_INACTIVE state, as shown by UE 101 in Figure 1a; a source or serving gNB, such as base station 710 and base station 110, which control the cell (i.e., the serving cell) where UE 701 is currently staying; a base station, such as base station 111, which controls the target cell (which may also be called a candidate cell) where UE 701 may move; and a core network (5GC) 702, such as core network 102. MBS multicast data is provided to the source gNB by the 5GC (via bearer 740), and then transmitted to UE 701 (via MBS radio bearer 741) simultaneously to other UEs belonging to the same multicast group.

[0123] In step 520 of the handover procedure (as described above with reference to Figure 5), UE 701 periodically performs measurements on signals received by UE 701 from the serving cell and one or more candidate cells, such as signal synchronization blocks (SSBs) transmitted in the serving cell and candidate cells (also called target cells) (step 720). Candidate cells may be adjacent to the serving cell or source cell (i.e., the current serving cell). If UE 701 finds at least one particular SSB with received power exceeding the received power of the current SSB by a predetermined threshold, in step 721, it performs a cell reselection evaluation process. The cell reselection evaluation process is performed according to several criteria, which may include frequency priority used in the candidate cell, radio link quality, and availability of MBS services in the candidate cell. Input information for selection (priority frequency, MBS availability) may be provided by source gNB 710 via system information message 730. The measurements in step 720 provide radio link quality information for selection. Details of an exemplary cell reselection evaluation process performed by the UE are given in 3GPP TS 38.304 (V16.7.0), Section 5.2.4.

[0124] In some situations, UE 101 may fall outside the coverage of source gNB 710. This may be due to deterioration of the radio condition, which leads to a radio link failure. Based on the received system information 730 and the measurements performed in step 720, UE 701 selects one suitable cell as the new serving cell in step 721 and effectively switches to this cell in step 722. For example, in step 721, UE 701 may identify one suitable cell or a list of suitable cells according to criteria such as frequency priority used in the candidate cells, radio link quality, and availability of MBS services in the candidate cells, as described above. Once a suitable cell is selected by UE 701 as the new serving cell, UE 701 switches to this cell (step 722). Taking the example in Figure 1a, UE 101 identifies target cell 121 as a suitable cell to camp on and switches to this cell 121 as the new serving cell.

[0125] UE 701 receives system information 731 broadcast by target gNB 711, which helps UE 701 switch to target cell 121 as the new serving cell; however, the broadcasted system information is not sufficient to enable UE 701 to receive MBS multicast data that target gNB may be providing to other UEs. Therefore, UE 701 triggers random access processing and sends a reselection request 732 to target gNB 711. The reselection request message sent by UE 701 requests target cell 121 to reselect as the new serving cell to receive one or more active data sessions from the target base station of the new serving cell (for example, to enable UE 701 to receive data from one or more active MBS sessions from the target base station (e.g., gNB 711)). For example, the reselection request message 732 may be an RRCResumeRequest message or an RRCResumeRequest1 message as specified in 3GPP standard TS 38.331 (v16.7.0), and may be modified to include information indicating that the UE's intention is not to switch to the RRC_CONNECTED state or to perform an RNA update, but to perform cell reselection using the MBS multicast data to be received. For this purpose, the resumeCause IE may be set to a new value indicating that the cause is cell reselection. In addition, the new IE may also be introduced to indicate the MBS session ID of one or more active MBS sessions that the UE is listening to. Message 732 includes an identifier that enables the target gNB 711 to identify the source gNB 710. In other words, the reselection request message may include identification information (e.g., gNB ID or cell ID) to identify the base station 710 (source or serving gNB 710) of the serving cell (i.e., the previous serving cell which is the last serving cell that the UE switches to as the new serving cell, target cell 121).

[0126] Next, the target gNB 711 sends a UE context request message 733 to the source gNB 710 to retrieve information about UE 701 (for example, the context request message 733 is a request for context information). The context request message 733 may contain identification information (such as a UE ID) to identify UE 701. The context request message may further contain information to request context information for one or more active data sessions. Accordingly, the source gNB 710 sends a UE context message 734 which may contain information identifying one or more active MBS sessions (for example, the UE context message 734 may contain identifiers(s) of the MBS session(s) that UE 701 is listening to). For example, UE context request message 733 could be a RETRIEVE UE CONTEXT REQUEST message as specified in 3GPP standard document TS 38.423 (v16.8.0), and UE context message 734 could be a RETRIEVE UE CONTEXT RESPONSE message as specified in the same 3GPP standard document.

[0127] Next, the target gNB 711 performs an acceptance control step 723 to determine whether to accept the cell reselection request. For example, it may reject the request if the load in the target cell is too high. Alternatively, it may accept the UE 701's request but reject the delivery of the MBS multicast service. If the request to deliver the MBS multicast service is accepted, the target gNB 711 sends an MBS reconfiguration message 735 to the UE 701 containing the information necessary for the UE 701 to receive the MBS multicast data in the target cell (i.e., the MBS radio bearer configuration in the target cell). In other words, when the target gNB 711 accepts the cell reselection request and delivers the MBS service, it sends a configuration message 735 to the UE 701 containing configuration information associated with the target gNB 711 for one or more active MBS sessions, enabling the UE to be properly configured to receive MBS data from the target gNB 711 for one or more active MBS sessions in the target cell. The configuration information may include radio configuration information indicating the radio configuration of the target gNB for one or more active MBS sessions (i.e., the radio configuration set up for the target gNB to transmit MBS data for one or more active MBS sessions). The radio configuration information may include information about the MBS radio bearer configuration used by the target gNB for one or more active MBS sessions, etc. The UE 701 may then configure itself (e.g., its user plane) based on the received configuration information in order to receive MBS data from the target gNB 711 for one or more active MBS sessions.

[0128] Although UE 701 has been switched to stay on the new serving cell 121, in an example where two or more suitable cells have been identified according to the cell reselection evaluation criteria (as described below), target gNB 711 may receive a reselection request message from UE 701 to request cell reselection to the new serving cell using one or more active data sessions, the reselection request message may further include identification information that identifies two or more suitable cells (e.g., a list of suitable cells). Target gNB 711 may then select one of the one or more suitable cells as the new serving cell (e.g., based on the load status in each cell) and send a response to UE 701 (e.g., in configuration message 735) that includes identification information (e.g., cell ID) indicating the suitable cell selected by target gNB 711. If the selected suitable cell is not the new serving cell where UE 701 is currently staying, UE 701 must switch the cell to the selected suitable cell.

[0129] For example, the MBS reconfiguration message 735 could be the RRCReconfiguration message specified in 3GPP standard TS 38.331(v16.7.0) as modified to allow this message to be sent to the UE in the RRC_INACTIVE state. For another example, the MBS reconfiguration message 735 could be the RRCRelease message or RRCRelease with suspend configuration message specified in 3GPP standard TS 38.331(v16.7.0). For yet another example, message 735 could be a dedicated SIB message. For yet another example, non-receipt of message 735 at UE 701 upon expiration of the timer initialized when the reselection request message 732 was sent may indicate that the reselection request was rejected.

[0130] On the other hand, target gNB 711 may perform pass switch handshake procedure 736 toward core network 702 to request the delivery of MBS multicast data. For example, target gNB 711 may communicate with core network 702 to establish or set up packet data unit (PDU) sessions between UE 701 and target base station 711 for one or more MBS sessions. If target gNB 711 has already provided MBS services to other UEs and has therefore already received MBS multicast data from core network 702, procedure 736 may not be necessary. Thus, the MBS multicast data is provided to target gNB 711 by core network 702 (via bearer 742), and then the MBS multicast data is simultaneously sent to UE 701 (via MBS radio bearer 743) to other UEs belonging to the same multicast group.

[0131] The target gNB 711 can send a UE context release to the source gNB 710 so that the source gNB 710 can remove the UE context of UE 701. In addition, if UE 701 was the only UE receiving MBS multicast data in the cell controlled by the gNB 710, the source gNB 710 can release the radio resources associated with UE 701 and stop receiving MBS multicast data from the core network 702. To minimize or prevent data loss during MBS traffic migration, any buffered data in the source gNB 710 may be moved to the target gNB 711 and then sent to UE 701 (this step is not shown in Figure 7).

[0132] Figure 8a illustrates, using flowchart 800, an exemplary method performed in a UE to support cell reselection according to an embodiment of the present invention. The UE may be UE 101 in Figure 1a in a serving cell 120 controlled by a serving base station 110, and may comprise UE 205 in Figure 2, and the method is performed by processor 215. The UE is in the RRC_INACTIVE state and is receiving MBS multicast data (i.e., the UE has one or more active MBS sessions).

[0133] In step 801, UE 101 performs measurements on signals received at the UE from one or more candidate cells (e.g., neighboring cells such as cell 121). Measurements may also be performed on signals received at UE 101 from serving cell 120. For example, UE 101 measures one or more parameters of signals received from candidate or serving cells, such as reference signal received power (RSRP) or reference signal received quality (RSRQ). Measurements may be performed on a received signal synchronization block (SSB). Thus, UE 101 performs measurements on signals within the serving cell and on one or more neighboring candidate cells. Optionally, in step 802, UE 101 receives information from serving base station 110 regarding the ability of neighboring cells to provide MBS services (e.g., whether one or more candidate cells can support MBS services for the UE): this corresponds to message 630 in Figure 6. In step 803, UE 101 performs a cell reselection evaluation process (step 621 in Figure 6) based on the measurements performed in step 801 and, if applicable, the information received in step 802. In step 804, UE 101 sends a reselection request to serving base station 110 (corresponding to message 631 in Figure 6) along with identification information for the cell (e.g., if UE 101 has selected a new serving cell) or a list of cells (e.g., if a source or serving base station makes the selection of a new serving cell or target base station selected in step 803). In step 805, UE 101 receives an MBS reconfiguration message from serving base station 110 containing information for receiving MBS multicast data for one or more active MBS sessions in the new serving cell. The MBS reconfiguration message corresponds to message 634 in Figure 6. Finally, in step 806, UE 101 switches cells and stays in the new serving cell, configuring its user plane to receive MBS multicast data from the new serving cell based on the received configuration information.

[0134] Figure 8b illustrates another exemplary method performed in a UE to support cell reselection according to an embodiment of the present invention, using flowchart 810. The UE may be UE 101 in Figures 1a and 1b and may comprise UE 205 in Figure 2, and the method is performed by processor 215. The UE is in the RRC_INACTIVE state and is receiving MBS multicast data (i.e., the UE has one or more active MBS sessions).

[0135] In step 811, UE 101 performs measurements on signals received at the UE from one or more candidate cells (e.g., neighboring cells such as cell 121). Measurements may also be performed on signals received at UE 101 from serving cell 120. For example, UE 101 measures one or more parameters of signals received from candidate or serving cells, such as reference signal received power (RSRP) or reference signal received quality (RSRQ). Measurements may be performed on a received signal synchronization block (SSB). Thus, UE 101 performs measurements on signals within the serving cell and on one or more neighboring candidate cells. Optionally, in step 812, UE 101 receives information from serving base station 110 about the ability of neighboring cells to provide MBS services (e.g., whether one or more candidate cells can support MBS services for the UE), which corresponds to message 730 in Figure 7. In step 813, UE 101 performs a cell reselection evaluation process (step 721 in Figure 7) based on the measurements performed in step 811, and possibly based on the information received in step 812, to select a new serving cell. In step 814, UE 101 switches to and stays with the new serving cell selected in step 813 (step 722 in Figure 7). In step 815, UE 101 sends a reselection request to the target base station (or new serving base station) that controls the target cell selected as the new serving cell. This corresponds to message 732 in Figure 7, which may include identification information of the previous serving cell (i.e., the last serving cell before the UE switched to the new serving cell), such as a gNB ID or cell ID, and identification information of the MBS multicast services (one or more) (e.g., one or more active MBS sessions) that UE 101 is receiving.In step 816, UE 101 receives an MBS reconfiguration message from the target base station containing configuration information to receive MBS multicast data in the new serving cell, and configures its user plane to receive MBS multicast data from the new serving cell based on the received configuration information. The MBS reconfiguration message corresponds to message 735 in Figure 7.

[0136] Figure 9a, using flowchart 900, illustrates an exemplary method, according to an embodiment of the present invention, that supports cell reselection of a UE in the RRC_INACTIVE state and is performed at a source or serving base station of a serving cell to receive MBS multicast data (i.e., the UE has one or more active MBS sessions). The source base station may be gNB 110 in Figure 1a and may comprise base station / gNB 305 in Figure 3, and the method is performed by processor 315.

[0137] In step 901, base station 110 receives a cell reselection request from a UE that has identification of the appropriate cell or a list of appropriate cells (corresponding to message 631 in Figure 6). Optionally, in step 902, base station 110 selects a target base station to control a cell from among the multiple appropriate cells identified in step 901. In step 903, base station 110 sends an MBS migration request to the target base station that controls the appropriate cell (selected by the UE or base station 110). The MBS migration request corresponds to message 632 in Figure 6 and may include a UE context containing information about the MBS session the UE has joined. In step 904, base station 110 receives an MBS migration confirmation from the target base station that includes an MBS configuration for the UE to receive MBS multicast data from the target base station. This corresponds to message 633 in Figure 6 (for example, a migration acceptance message as described above). In step 905, base station 110 sends an MBS reconfiguration to the UE that includes the MBS configuration information received from the target base station. This corresponds to message 634 in Figure 6.

[0138] Figure 9b uses flowchart 910 to illustrate another exemplary method performed at the source base station of the UE's previous serving cell to support cell reselection of a UE operating in a disconnected RRC state using one or more active data sessions (e.g., the UE is in the RRC_INACTIVE state and receives MBS multicast data), according to an embodiment of the present invention. The UE is switching from the previous serving cell (or the immediately preceding serving cell) to a new serving cell controlled by the target base station. The source base station may be gNB 110 in Figures 1a and 1b, and may comprise base station / gNB 305 in Figure 3, and the method is performed by processor 315.

[0139] In step 911, base station 110 receives a UE context request message (e.g., a context request message) from the target base station of the new serving cell to request context information. The context request message includes identification information to identify the UE. The context request message may further include information requesting context information for one or more active data sessions. For example, this message corresponds to message 733 in Figure 7, which identifies a UE in the RRC_INACTIVE state that receives MBS multicast data (i.e., the UE has one or more active MBS sessions). In response to the context request message, source base station 110 sends a context message containing context information related to the UE and one or more data sessions (e.g., session identification information that identifies one or more active data sessions). For example, in step 912, base station 110 sends the UE context to the target base station. This corresponds to message 734 in Figure 7, which may include identifiers for the MBS sessions that UE 701 is listening to. In step 913, base station 110 can receive a UE context release from the target base station indicating that the base station can delete the UE context stored in the base station, and therefore can release the radio resources associated with the UE and stop receiving the associated MBS multicast data from the core network. The UE context release corresponds to message 737 in Figure 7.

[0140] Figure 10a illustrates, using flowchart 1000, an exemplary method performed at the target base station of a target cell to support cell reselection of a UE in the RRC_INACTIVE state and to receive MBS multicast data (i.e., the UE has one or more active MBS sessions) according to an embodiment of the present invention. The target cell is selected as a new serving cell for the UE. The target base station may be gNB 111 in Figure 1a and may comprise base station / gNB 305 in Figure 3, and the method is performed by processor 315.

[0141] In step 1001, base station 111 receives an MBS migration request from the source base station of the serving cell (i.e., the current serving cell). This corresponds to message 632 in Figure 6 and includes a UE context containing information about the MBS session in which the UE has joined. In step 1002, base station 111 performs acceptance control to determine whether to accept the MBS migration request. This corresponds to step 622 in Figure 6. If base station 111 accepts the request, in step 1003, base station 111 sends an MBS migration confirmation to the source base station containing the MBS configuration for the UE to receive the MBS multicast data. This corresponds to message 633 in Figure 6 (e.g., the migration acceptance message). Optionally, in step 1004, base station 111 may perform a pass switch handshake procedure toward core network 102 to request the UE to deliver the MBS multicast data to base station 111 that should be sent. This corresponds to procedure 637 in Figure 6. In step 1005, the base station 111 can set up the necessary radio resources and deliver MBS multicast data to the UE.

[0142] Figure 10b, using flowchart 1010, illustrates another exemplary method performed at a target base station to support cell reselection of a UE in the RRC_INACTIVE state and to receive MBS multicast data (i.e., the UE has one or more active MBS sessions) according to an embodiment of the present invention. The target base station may be gNB 111 in Figures 1a and 1b, and may comprise base station / gNB 305 in Figure 3, and the method is performed by processor 315.

[0143] In step 1011, base station 111 receives a reselection request from the UE. This corresponds to message 732 in Figure 7, which may include the identification of the previous serving cell and the identification of the MBS multicast service being received by the UE. In step 1012, base station 111 sends a UE context request message to the source base station that controls the serving cell identified in the reselection request. The UE context request corresponds to message 733 in Figure 7, which identifies the UE in the RRC_INACTIVE state and receives the MBS multicast data. In step 1013, base station 111 receives the UE context from the source base station. This corresponds to message 734 in Figure 7, which may include the identifier of the MBS session being listened to by the UE. In step 1014, base station 111 performs acceptance control to determine whether to accept the reselection request by the MBS multicast service. This corresponds to step 723 in Figure 7. If base station 111 accepts the request, in step 1015, base station 111 sends an MBS reconfiguration message to the UE containing information for receiving the MBS multicast data in the cell where the UE is staying. Optionally, in step 1016, base station 111 may perform a pass switch handshake procedure toward core network 102 to request the delivery of MBS multicast data to base station 111 to be sent to the UE. This corresponds to procedure 736 in Figure 7. In step 1017, base station 111 can set up the necessary radio resources and deliver the MBS multicast data to the UE. In step 1018, base station sends a release of the UE context to source base station. This corresponds to message 737 in Figure 7.

[0144] Although the present invention has been described above using embodiments, the present invention is not limited to these embodiments. It will be understood by those skilled in the art that various changes and modifications can be made without departing from the scope of the invention, as defined in the appended claims. All features disclosed in the specification (including any appended claims, abstract, and drawings) and / or all steps of any method or process disclosed so herein can be combined in any combination, except for combinations in which at least some of such features and / or steps are mutually exclusive. Each feature disclosed in the specification (including any appended claims, abstract, and drawings) may be replaced by an alternative feature that serves the same, equivalent, or similar purpose unless otherwise expressly stated. Thus, unless specifically stated otherwise, each disclosed feature is merely an example of a general set of equivalent or similar functions.

[0145] In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude the plural. The mere fact that different features are described in different dependent claims does not imply that combinations of these features cannot be used to one's advantage.

[0146] In the embodiments described above, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or codes on or transmitted through a computer-readable medium and executed by a hardware-based processing unit.

[0147] Computer-readable media may include computer-readable storage media corresponding to tangible media such as data storage media, or communication media including any media that facilitates the transfer of computer programs from one location to another in accordance with a communication protocol, for example. Thus, computer-readable media may generally correspond to (1) non-transient tangible computer-readable storage media, or (2) communication media such as signals or carrier waves. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code, and / or data structures for implementing the techniques described herein. Computer program products may include computer-readable media.

[0148] As an example, and not an limitation, such computer-readable storage media may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage devices, magnetic disk storage devices or other magnetic storage devices, flash memory, or any other media that can be used to store desired program code in the form of instructions or data structures and can be accessed by a computer. Also, any connection is appropriately called computer-readable media. For example, if instructions are transmitted from a website, server, or other remote source using coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of media. However, it should be understood that computer-readable storage media and data storage media do not include connections, carriers, signals, or other temporary media, but instead refer to non-temporary tangible storage media. The discs and discs used herein include compact discs (CDs), laser discs, optical discs, digital multipurpose discs (DVDs), floppy disks, and Blu-ray discs, where discs typically reproduce data magnetically, and discs reproduce data optically using a laser. Combinations of the above should also be included within the scope of computer-readable media.

Claims

1. A method for reselecting a cell in a user device (UE) operating in the RRC_INACTIVE state that is capable of participating in a multicast session of a multicast broadcast service (MBS) as defined in the Third Generation Partnership Project Standard, A transmission step of transmitting a request message to a first base station having a first cell selected by the UE, A receiving step of receiving a configuration message from the first base station that includes wireless configuration information relating to the MBS and configuration information relating to the wireless bearer of the MBS, A configuration step of configuring the first base station to receive data in the multicast session of the MBS based on the wireless configuration information relating to the MBS and the configuration information relating to the wireless bearer of the MBS, A receiving process from the first base station to receive data in the MBS multicast session, Methods that include...

2. The method according to claim 1, further comprising a switching step of switching from a second base station having a second cell to the first base station having the first cell.

3. The method according to claim 1, wherein, after configuration in the above-mentioned configuration step, data is received from the first base station in the MBS multicast session.

4. The method according to claim 1, wherein the request message is an RRCResumeRequest message or an RRCResumeRequest1 message, and the request message includes a ResumeCause information element.

5. The method according to claim 1, further comprising a selection step of selecting the first cell from among a plurality of cells.

6. The method according to claim 1, wherein the request message includes identification information that identifies the first cell.

7. The method according to claim 1, wherein the configuration message includes identification information for identifying the first base station.

8. The method according to claim 1, wherein the configuration message includes identification information for identifying the first cell.

9. The method according to claim 1, further comprising a receiving step of receiving information regarding the provision of the MBS in a neighboring cell, prior to the transmitting step of transmitting the request message to the first base station.

10. The method according to claim 1, wherein the request message includes session identification information that identifies the multicast session of the MBS.

11. The method according to claim 1, further comprising switching to the first base station before transmitting the request message to the first base station.

12. A method in a first base station having a first cell capable of providing multicast sessions for multicast broadcast services (MBS) as defined in the Third Generation Partnership Project Standard, A receiving process in which a request message is received from the user equipment (UE), A transmission step of sending a context request message to a second base station having a second cell, A receiving step of receiving a response message from the second base station that includes contextual information regarding the UE, A transmission step of sending a configuration message to the UE that includes wireless configuration information relating to the MBS and configuration information relating to the wireless bearer of the MBS, A transmission step in which data is sent to a UE operating in the RRC_INACTIVE state in the multicast session of the aforementioned MBS, Methods that include...

13. The method according to claim 12, wherein the request message includes identification information that identifies the second base station having the second cell.

14. The method according to claim 12, wherein the request message includes session identification information that identifies the multicast session of the MBS.

15. The method according to claim 12, wherein the request message of the context includes identification information for identifying the UE.

16. The method according to claim 12, further comprising a decision step of determining whether to accept a request in response to receiving a request message in the context described above.

17. The method according to claim 12, wherein the context information includes session identification information that identifies the multicast session of the MBS.

18. The method of claim 12, further comprising a configuration step of configuring wireless resources for transmitting data to the UE for the multicast session of the MBS.

19. The method according to claim 12, further comprising communicating with a core network entity to establish a packet data unit (PDU) session between the UE and the first base station for the MBS multicast session in response to receiving the response message.

20. The request message is an RRCResumeRequest message or an RRCResumeRequest1 message. The request message for the aforementioned context is a RETRIE UE CONTEXT REQUEST message, The method according to claim 12, wherein the response message is a RETRIE UE CONTEXT RESPONSE message.

21. User equipment (UE), A transceiver that provides wireless communication, A processor coupled to the transceiver and configured to perform the method described in any one of claims 1 to 11, User equipment equipped with the following features.

22. It is a base station, A transceiver that provides wireless communication, A processor coupled to the transceiver and configured to perform the method described in any one of claims 12 to 20, A base station equipped with the necessary equipment.

23. A computer program that, when executed by a computer, includes instructions causing the computer to perform the method described in any one of claims 1 to 11.

24. A computer program that, when executed by a computer, includes instructions causing the computer to perform the method described in any one of claims 12 to 20.

25. A computer-readable storage medium for storing the computer program described in claim 23.

26. A computer-readable storage medium for storing the computer program described in claim 24.

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