Supporting cell reselection to a new serving cell for a 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 relates generally to supporting cell reselection of a new serving cell for a user equipment (UE) of a wireless communication system, and in particular to a method performed in the UE, a source base station, and a target base station as part of the reselection process. The cell reselection by the UE may be used when handling mobility of a UE receiving a multicast / broadcast service (MBS) in a 5G New Radio (NR) system. [Background technology]

[0002] Wireless communication systems are primarily deployed to address a wide range of applications, from mobile broadband, large scale machine type communications, to ultra-reliable low latency communications (URLLC). Such systems enable multiple User Equipments (UEs) or mobile terminals to share a wireless medium to exchange several types of data content (e.g., video, voice, messaging, etc.) over a Radio Access Network (RAN) through one or more base stations.

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

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

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

[0006] Support for multicast / broadcast technologies allows networks to operate in a more efficient manner than unicast. Identified use cases that can benefit from this MBS capability include public safety and mission critical, V2X applications, IPTV, live video, software distribution over wireless and IoT applications. 3GPP has started to build out the 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) and provides the UE with three different states, namely RRC_CONNECTED, RRC_INACTIVE, and RRC_IDLE states, as defined in the 3GPP standard TS 38.331. At startup, the UE is in the RRC_IDLE state, and the UE changes to the RRC_CONNECTED state upon RRC connection establishment with the gNB. If the RRC connection is released, the UE returns to the RRC_IDLE state. When 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 RRC connection establishment from the RRC_IDLE state to the RRC_CONNECTED state.

[0008] In 3GPP Release 17, reception of MBS broadcast by a UE is allowed when the UE is in RRC_IDLE, RRC_INACTIVE, or RRC_CONNECTED state, and reception of MBS multicast is allowed only when the UE is in RRC_CONNECTED state. For Release 18, the plan is to further enable MBS multicast reception when the UE is in RRC_INACTIVE state. See, for example, 3GPP RP-213568 "New WID: Enhancements of NR Multicast and Broadcast Services" (3GPP TSG RAN Meeting #94-e, source CATT) section 3. The objective is to allow a larger density of UEs receiving multicast data from one cell. Indeed, having UEs in RRC_INACTIVE state leads to a downsize in the overall UE control plane footprint, thus allowing the gNB to handle more UEs. In addition, keeping UEs in RRC_CONNECTED state all the time is not power efficient. As a result, the advantage of also supporting multicast for UEs in RRC_INACTIVE state was recognized.

[0009] The establishment of the MBS session in the UE may be performed when the UE is in the RRC_CONNECTED state. The UE may then be set to the RRC_INACTIVE state by the serving gNB while still receiving the MBS data. However, one issue with a UE receiving an MBS multicast or broadcast in the RRC_INACTIVE state is the handling of mobility, where the mobile UE is moving from one cell managed by the source gNB to another cell managed by the target gNB with the aim of maintaining service continuity (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 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 that ensures the delivery of the MBS data through the target gNB. In the RRC_INACTIVE state (and the 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 on which to camp.

[0010] Since there is no communication with the network in the cell reselection process, several problems may prevent the UE from continuing to receive MBS data after cell reselection. First, 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 service that the UE is interested in. When the target gNB can provide the same MBS service, another problem for the UE is to obtain configuration information and receive the MBS data. One solution may be to switch the UE in the RRC_CONNECTED state and apply a handover procedure. However, this removes the advantage of keeping the UE in the RRC_INACTIVE or RRC_IDLE state (e.g., as described above).

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

[0012] Therefore, it is desirable to provide at least one solution, e.g., for a UE receiving MBS multicast data in a Disconnected RRC state, to ensure that MBS service continuity is maintained or at least data loss is minimized when the UE performs cell reselection and switches to a new serving cell. Summary of the Invention

[0013] According to a first aspect of the present invention, there is provided a method for supporting cell reselection for a user equipment (UE), the UE operating in a Disconnected RRC state having one or more active data sessions, the method comprising: transmitting, in the UE, a reselection request message to a new serving cell to request cell reselection to receive the one or more active data sessions from a target base station of the new serving cell; receiving a configuration message including configuration information related to 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.

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

[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] According to a second aspect of the present invention, there is provided a method, in a source base station of a serving cell of a user equipment (UE), in a UE operating in a Disconnected RRC state with one or more active data sessions, comprising: receiving a reselection request message from the UE to request cell reselection to a new serving cell for receiving the one or more active data sessions from a target base station of the new serving cell; transmitting a migration request message to the target base station to indicate that the UE requests cell reselection with the one or more active data sessions, the migration request message including context information associated with the UE and the one or more active data sessions; receiving a migration confirmation message from the target base station including configuration information associated with the target base station for the one or more active data sessions; and transmitting a configuration message including the configuration information to the UE.

[0017] According to a third aspect of the present invention, there is provided a method in a target base station of a new serving cell for a user equipment (UE), the UE operating in a Disconnected RRC state with one or more active data sessions, the UE being switched from a previous serving cell to the new serving cell, the method comprising: receiving a reselection request message from the UE to request cell reselection to the new serving cell with the one or more active data sessions; sending a context request message to a source base station of the previous serving cell; receiving from the source base station a context message including context information associated with the UE and the one or more active data sessions; and sending to the UE a configuration message including configuration information associated with the target base station for the one or more active data sessions.

[0018] The one or more active data sessions may be active MBS sessions (such as MBS multicast sessions). In one example, the UE may be in an RRC_INACTIVE state.

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

[0020] Thus, a UE in a Disconnected RRC state may switch to a new serving cell while still receiving one or more active data sessions (eg, MBS services).

[0021] Furthermore, since the target base station is notified to a new UE in an unconnected RRC state that receives one or more active data sessions (e.g., MBS services) from the target base station to request cell reselection to the target base station, mobility handling for a UE receiving an MBS multicast in an RRC_INACTIVE state to set up one or more active sessions at the target base station can be appropriately managed, e.g., controlling which UEs are allowed to receive MBS multicasts and, if necessary, requesting the core network to provide MBS data to the target base station.

[0022] According to a fourth aspect of the present invention, there is provided a method in a target base station of a target cell, the target cell being selected as a new serving cell for a user equipment (UE), the UE operating in a Disconnected RRC state with one or more active data sessions via the serving cell, as claimed in claim 25.

[0023] According to a fifth aspect of the present invention, there is provided a method in a source base station of a previous serving cell of a user equipment (UE), the UE operating in a non-connected RRC state with one or more active data sessions, and the UE has switched from the previous serving cell to a new serving cell, as claimed in claim 31.

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

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

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

[0027] Any feature in one aspect of the invention may be applied to other aspects of the invention in any appropriate combination, in particular method aspects may be applied to apparatus / device / unit aspects and vice versa.

[0028] Furthermore, features implemented in hardware may be implemented in software and vice versa. Any references in this specification to software and hardware features should be interpreted accordingly. For example, according to another aspect of the invention, there is provided a computer program comprising instructions that, when executed by a processing unit, cause the processing unit to perform the method of any aspect or example described above, and a computer readable storage medium carrying the computer program. [Brief description of the drawings]

[0029] Different aspects of the present invention will now be described, by way of example only, with reference to the following drawings, in which: [Figure 1a] 1 is a schematic diagram illustrating a first exemplary wireless communication system in which the present invention may be implemented in accordance with one or more embodiments of the present invention; [Figure 1b] 1 is a schematic diagram illustrating a first exemplary wireless communication system in which the present invention may be implemented in accordance with one or more embodiments of the present invention; [Figure 1c] 2 is a schematic diagram illustrating a second exemplary wireless communication system in which the present invention may be implemented in accordance with one or more embodiments of the present invention. [Figure 1d] 2 is a schematic diagram illustrating a second exemplary wireless communication system in which the present invention may be implemented in accordance with one or more embodiments of the present invention. [Diagram 2] 1 shows a block schematic diagram of an exemplary configuration of a UE capable of implementing the present invention in accordance with one or more embodiments of the present invention. [Diagram 3] 1 shows a block schematic diagram of an exemplary configuration of a base station capable of implementing the present invention in accordance with one or more embodiments of the present invention; [Figure 4] 1 is a flowchart illustrating RRC connection states and transitions for a UE in a 5G NR system. [Diagram 5]1 is a simplified flowchart illustrating a handover procedure while the UE is in RRC_CONNECTED state and receiving MBS multicast data. [Figure 6] 4 is a flowchart illustrating an exemplary cell reselection procedure for a UE in RRC_INACTIVE state and receiving MBS multicast data according to a first embodiment of the present invention. [Figure 7] 11 is a flowchart illustrating an exemplary cell reselection procedure for a UE in RRC_INACTIVE state and receiving MBS multicast data according to a second embodiment of the present invention. [Figure 8a] 4 is a flowchart illustrating an exemplary method performed in a UE in RRC_INACTIVE state for cell reselection while receiving MBS multicast data, according to an embodiment of the present invention. [Figure 8b] 4 is a flowchart illustrating an exemplary method performed in a UE in RRC_INACTIVE state for cell reselection while receiving MBS multicast data, according to an embodiment of the present invention. [Figure 9a] 4 is a flowchart illustrating an exemplary method performed at a source base station for supporting cell reselection for a UE in an RRC_INACTIVE state and receiving MBS multicast data, in accordance with an embodiment of the present invention. [Figure 9b] 4 is a flowchart illustrating an exemplary method performed at a source base station for supporting cell reselection for a UE in an RRC_INACTIVE state and receiving MBS multicast data, in accordance with an embodiment of the present invention. [Figure 10a] 5 is a flowchart illustrating an exemplary method performed in a target base station for supporting cell reselection for a UE in an RRC_INACTIVE state and receiving MBS multicast data, in accordance with an embodiment of the present invention. [Figure 10b]5 is a flowchart illustrating an exemplary method performed in a target base station for supporting cell reselection for a UE in an RRC_INACTIVE state and receiving MBS multicast data, in accordance with an embodiment of the present invention. [Figure 11] 4 is a flowchart of a method performed by a UE to support cell reselection for the UE, according to an embodiment of the present invention. [Figure 12] 4 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 having one or more active data sessions, according to an embodiment of the present invention. [Figure 13] 6 is a flowchart of a method performed by a target base station for supporting cell reselection for a UE operating in a Disconnected RRC state having one or more active data sessions, according to another embodiment of the present invention. [Figure 14] 4 is a flowchart of a method performed by a target base station for supporting 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. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] 1a illustrates an exemplary wireless communication system 100, in particular a mobile wireless communication system such as a fifth generation (5G) New Radio (NR) system supporting multicast and broadcast services (MBS). In the following description, embodiments and examples of the present invention are described with respect to a 5G NR system, however, it will be understood that the present invention is not limited to 5G NR systems and may be used in any wireless communication system supporting MBS or similar services.

[0031] The system 100 comprises a user equipment (UE) 101, which may be, for example, in or part of a vehicle, served 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, a user device (e.g., a smartphone, a laptop, a mobile phone, a tablet, a camera, a game console, a wearable device), capable of wirelessly communicating with one or more core networks via one or more radio access networks. The 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, the base station is called Next Generation Node B (gNB), the RAN is Next Generation (NG) RAN, and the core network is called 5GC. In the following, the terms RAN node, base station, and gNB are used interchangeably. The base stations 110 and 111 are interconnected by means of an Xn interface (specified in the 3GPP standard TS 38.423) implemented on a wired or wireless link 130. Each base station is connected to the core network by an NG interface (specified in the 3GPP standard TS 38.413) implemented on a wired or wireless link 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. Cell A is a geographical area of ​​the wireless network defined by the frequency used in the cell to transmit data. A cell can be uniquely identified by a UE from identification information broadcasted over 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 is connected is called the UE's serving base station or source base station, and the cell controlled by the serving base station and on which the UE is camped is called the serving cell. The interface between the gNB and the UE is the Uu interface, which uses protocol sublayers SDAP (Service Data Application Protocol), PDCP (Packet Data Convergence Protocol), RLC (Radio Link Control), MAC (Medium Access Control), PHY (Physical) in the user plane, and protocol sublayers RRC (Radio Resource Control), PDCP, RLC, MAC, PHY in the control plane.

[0033] Figure 2 shows a block diagram of a UE device 205, such as UE 101 of Figure 1a, in which the present invention may be implemented in accordance with 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, a memory 225, and a processor (CPU: central processing unit) 215. All of these elements communicate with each other.

[0034] The memory 225 may include a RAM (random access memory), a ROM (read only memory), or a combination of both, or a mass storage device such as a disk or solid state drive, as non-limiting examples. Basic input / output system (BIOS) instructions may be stored within the 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, for example, relate to or interact with peripheral devices such as a keyboard, screen, mouse, etc. (not shown in FIG. 2). The processor may, for example, execute an operating system such as iOS, Windows, Android, etc. 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 allocation of processing functions to the processors are a design choice for those skilled in the art.

[0036] The I / O controller 255 provides the necessary hardware and manages the input and output signals to enable these interactions with external peripherals.

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

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

[0039] The UE communications manager 220 handles the establishment, control, and release of communications of the UE to the radio access network. The UE periodically receives an indication from the base station of available slots for communications between the UE and the base station. The UE then knows where in time and frequency it expects data to arrive or where it has to transmit its outgoing data, 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] 3 shows a block diagram of a base station device 305, such as the base stations or gNBs 110 and 111 of FIG. 1a, which may be implemented in accordance with 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, a 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 communications with multiple UEs. It is responsible for the establishment, control, and release of 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 schedule of these slots is periodically transmitted to the involved UEs.

[0042] The core network communications manager 355 manages the base station's communications with the core network. It may provide a standardized NG interface, as defined by 3GPP standards, to support these communications.

[0043] The transceiver 335 is configured to provide two-way wireless communication with other wireless devices. These devices may be UEs, or even other base stations. The transceiver 335 provides the necessary modems and frequency shifters to simultaneously connect to multiple UEs 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 capabilities.

[0044] Memory 325 may include RAM, ROM, or a combination of both, or a mass storage device such as a disk or solid state drive, as a non-limiting example. BIOS instructions may be stored within memory 325 to support an operating system.

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

[0046] FIG. 4 is a flow chart 400 illustrating the RRC connection states and transitions of a UE in 5G NR. The RRC protocol operates between the UE and a base station (gNB) and is defined in 3GPP specification TS 38.331 for 5G NR. UE state names are 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 in the 5G NR protocol stack. It exists only in the control plane, the UE, and the gNB. The behavior and functionality of the base station and the UE are governed by the UE's current RRC state. In 5G NR, three distinct RRC states are specified for the UE: RRC_IDLE state 401, RRC_CONNECTED state 402, and RRC_INACTIVE state 403.

[0048] At start-up, the UE is in RRC_IDLE state 401 and performs radio link quality measurements and executes a cell selection evaluation process (as defined in 3GPP standard TS 38.304) to identify a target gNB to connect to. The UE state changes to RRC_CONNECTED state 402 upon RRC connection establishment with the target gNB, which becomes the source gNB serving the UE. In the following, 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 some 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 the RRC connection is interesting from a capacity utilization and power saving perspective, but is not ideal from a latency perspective. The overhead in establishing the RRC connection requires extra signaling that introduces delays. To address this drawback, the RRC_INACTIVE state 403 is introduced in 5G NR. When the UE is in the RRC_INNACTIVE 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 includes information to facilitate a quick resumption of the connection. The information may include security context (e.g., security parameters such as security keys, UE security capabilities), measurement configuration, radio configuration (e.g., UE radio capabilities), information about bearers, PDU session context, etc. Thus, when in the RRC_CONNECTED state 402, the RRC connection may be suspended by the source gNB (release with suspend) and the UE moves to the RRC_INACTIVE state 403. From the RRC_INACTIVE state 403, the UE may be switched back 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 upon receiving an RRC Resume Request message from the UE.

[0050] From either the RRC_CONNECTED or 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 to transition a UE from one cell to another depends on the RRC state of the UE. In RRC_CONNECTED state the mobility procedure called handover is controlled by the network and the source gNB decides to trigger the handover procedure based on measurement reports provided by the UE. In RRC_INACTIVE and RRC_IDLE states (e.g. not connected) the mobility procedure is called cell reselection and it is managed by the UE itself.

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

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

[0054] Returning to FIG. 1a, assume that the UE 101 is in an 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 the base station 110, which is the serving base station for the UE 101, via the core network 102 and the transport bearer 104 over the link 140. The multicast data is then transmitted from the base station 110 to the 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 connection between the UE and the gNB. In 5G NR, several types of radio bearers are defined: Signaling Radio Bearer (SRB) for the control plane, Data Radio Bearer (DRB) that enables point-to-point communication (unicast) with one UE in the user plane, and MRB that enables 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 inform 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 multicast MBS session establishment 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 at least the MBS session IDs of the multicast groups that the UE can join via the service announcements broadcast by the network. To join a multicast group, the UE sends a PDU session change request for the associated PDU session, including one or several MBS session IDs and the join request. The MBS session ID indicates the multicast MBS session that the UE wants to join.

[0056] To join an MBS session, the UE must be in the RRC_CONNECTED state. While receiving MBS multicast data in the RRC_CONNECTED state, the UE 101 may have moved successively to different cells using the handover procedure illustrated by FIG.

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

[0058] The UE 501 is configured with measurements to perform regularly on neighboring cells. The first step 520, which is performed continuously when in RRC_CONNECTED state, is to search for candidate cells. Once a candidate cell is found, the 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, the UE 501 measures Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ), which are typically performed on a signal synchronization block (SSB) transmitted in this candidate cell. Depending on a triggering event configured in the UE, the measurements may trigger a measurement report 531 (RRC protocol message) sent by the UE 501 to the source gNB 510 with information about the candidate cell.

[0059] Upon receiving the measurement report 531, the source gNB 510 can decide whether to perform a handover in a handover decision step 521. Additional information other than the measurement report may be taken into account, 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 handover the UE to another cell even if no measurement report has been received, e.g., for load balancing purposes. If the network decides to handover 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, there is no need for this message, since the situation in the target cell is already known by the gNB. The handover request message 532 includes the UE context, which includes information about the MBS sessions in which the UE has participated. The target gNB 511 then performs an admission control step 522 to decide whether to accept the handover request or not. For example, the request can be rejected if the load in the target cell is too high. In either case, the target gNB 511 sends a handover confirm message 533 to the source gNB 510. If the target gNB 511 accepts the handover, the source gNB informs the UE 501 to switch cells via an RRC reconfiguration message 534, which includes the information required for the UE 501 to connect to the target cell, e.g., radio bearers, measurement configurations (information previously received from the target gNB by the source gNB in ​​the acknowledgement message 533). In case of a conditional handover (CHO), the UE 501 is configured with trigger conditions to be met before switching to a new cell (not shown in FIG. 5).

[0060] In step 523, the UE 501 switches to the target cell as the new serving cell and moves the RRC connection to the target gNB 511. To be able to connect to the new serving cell, uplink synchronization is required, so the UE typically performs random access 535 towards the target cell to gain 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 that manages the new serving cell.

[0061] Meanwhile, the target gNB 511 may perform a path switch handshake procedure 537 towards the core network 502 to request delivery of the MBS multicast data. Thus, the MBS multicast data may be provided by the core network 502 (via bearer 542) to the target gNB 511, which is then simultaneously transmitted to the 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 data buffered in the source gNB 510 may be moved to the target gNB 511 (not shown in FIG. 5).

[0062] Returning again to FIG. 1a, it is assumed that UE 101 is currently camped in cell 120, which is the serving cell of UE 101 with base station 110 as the source or serving gNB, and that UE 101 has been set to RRC_INACTIVE state by gNB 110 with no radio activity apart from receiving MBS multicast data. While UE 101 is still receiving MBS multicast data, it may move and arrive at the edge of serving cell 120, but also at the edge of other cell 121, for example. At some point, UE 101 may need to change serving cell as it may soon fall out of coverage of cell 120. Unlike when the UE 101 needs to change its serving cell, 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 delivery of MBS data through the target gNB of the new serving cell, in the RRC_INACTIVE state (and the RRC_IDLE state, both of which are non-connected), the mobility procedure is called cell reselection, which is managed by the UE 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. Since there is no communication with the network in the cell reselection procedure, some problems may 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 necessary configuration information to receive MBS data through the target gNB, and the target gNB is not informed of the new UE in the RRC_INACTIVE state that wants to receive MBS services through the target gNB. Without the configuration information, the UE cannot receive MBS data from the target gNB.Informing the target gNB of the new UE in RRC_INACTIVE state helps to ensure that the UE is authorized to receive MBS services, and the target gNB helps to provide the UE with configuration information for receiving MBS services in the new cell and, if necessary, to request the core network to provide MBS data to the target gNB.

[0063] Now, referring to FIG. 11, which illustrates steps of a method 1100 for supporting cell reselection for a UE of a wireless communication system according to an embodiment of the present invention, the method being performed by a UE. The wireless communication system may be, for example, the wireless communication system 100 of FIG. 1a, and the UE may be the UE 101 of FIG. 1a, and may comprise the UE 205 of FIG. 2, in which the method 1100 is performed by a processor 215. The UE 101 is operating in an unconnected radio resource control (RRC) state (e.g., an RRC_INACTIVE state) having 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 other words, a data session is "active" in the sense that the UE is participating or has participated in one or more data sessions. In the example illustrated in FIG. 1a, the UE 101 is currently camped on a serving cell 120 (current serving cell) controlled by a source base station 110. The UE 101 may move to the cell 121. Therefore, the cell 121 may be referred to as a candidate / target cell controlled by the target base station 111. The candidate / target cell may be a serving cell or a neighboring cell of the source cell (i.e., the current serving cell).

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

[0065] In step 1102, the UE 101 receives a configuration message including configuration information associated with a target base station for one or more active data sessions. In one example, the configuration information includes radio configuration information indicating a radio configuration of the target base station for one or more active data sessions. In other words, the UE 101 receives radio configuration information indicating a radio configuration used by the target base station to transmit data for one or more active data sessions (e.g., MBS bearer configuration for one or more active data sessions in the new serving cell). Based on the received configuration information, the UE 101 configures itself to receive data of the one or more active data sessions from the target base station in the new serving cell (e.g., sets up a user plane in the UE 101 according to the radio configuration of the target base station) (step 1103). The 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 for one or more active data sessions from the target base station. Steps 1101, 1102, and 1103 are performed by UE 101 operating in a non-connected 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 shown by a dotted line). The new serving cell will be one of the identified one or more suitable cells. For example, UE 101 may perform measurements on signals received at UE 101 from one or more candidate cells (step 620), as described below with reference to FIG. 6. UE 101 may select or identify one suitable cell or a list of suitable cells according to cell reselection evaluation criteria, such as the priority of the frequency used in the candidate cell, the radio link quality, the availability of MBS service in the candidate cell, etc., as described below (e.g., with reference to FIG. 6). When UE selects one suitable cell, UE 101 identifies one suitable cell selected as the new serving cell in a reselection request message sent by UE 101. Alternatively, the UE 101 may select multiple suitable cells for the new serving cell and thus identify multiple suitable cells (e.g., as a list of suitable cells) in the reselection request message sent by the UE 101, with the selection of the new serving cell being made after sending the reselection request message (e.g., by the source base station receiving the reselection request message, or by the target base station if the target base station controls more than one 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) 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 conditions in each cell), and in a response to the source base station (e.g., in a migration confirm message as described below), the target base station indicates to the source base station that the UE should switch to an identity (e.g., a cell ID) of the selected cell.

[0067] The UE 101 may 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. Examples of sending a reselection request message to the source base station 110 of the serving cell are described in more detail below with respect to Figures 6, 8a, 9a, and 10a. Examples of sending a reselection request message to the target base station 111 of the new serving cell are described in more detail below with respect to Figures 7, 8b, 9b, and 10b.

[0068] In one example, when the UE 101 transmits a reselection request message to the source base station 110, the UE 101 receives a configuration message from the source base station. The reselection request message may include identification information associated with or identifying one or more suitable cells for the new serving cell. When the reselection request message includes identification information of two or more suitable cells, the source base station 110 selects one of the suitable cells as the new serving cell (or the target base station selects one of the suitable cells when two or more of the suitable cells are controlled by the target base station). Alternatively, the UE 101 may select one suitable cell as the new serving cell for the UE, and the reselection request message transmitted by the UE 101 includes identification information associated with or identifying the one suitable cell selected as the new serving cell. The identification information of each of the selected or suitable 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.

[0069] The UE 101 may switch to the new serving cell in response to receiving a configuration message from the source base station and then receive data of one or more active data sessions from the target base station. When the source base station or the 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., a cell ID) to which the UE should switch.

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

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

[0072] When the UE 101 sends a reselection request message to the target base station 111, the UE 101 can switch to the new serving cell sooner than when the UE 101 sends a reselection request message to the source base station 110 and switches to the new serving cell after receiving the configuration information. If the UE 101 switches to the new serving cell sooner, the data connection with the serving cell can be lost sooner. This can result in some data being lost compared to when the UE 101 switches to the new serving cell later after receiving the configuration information. However, in case there is a communication problem with the source base station 110 (e.g., 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 being in a Disconnected RRC state, which helps to ensure minimal data loss and continuity of data services (e.g., MBS services) without the need to switch to an RRC_CONNECTED state.

[0073] Now, referring to FIG. 12 illustrating steps of a method 1200 for supporting cell reselection for a UE of a wireless communication system according to an embodiment of the present invention, the method is performed by a source base station of a serving cell (e.g., a current serving cell) of the UE. The wireless communication system may be, for example, the wireless communication system 100 of FIG. 1a, and the source base station may be the source base station 110 supporting the UE 101. The source base station may comprise the base station 305 of FIG. 3, in which the method 1200 is performed by the processor 315. The UE 101 is operating in an unconnected radio resource control (RRC) state (e.g., an RRC_INACTIVE state) having 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 illustrated in FIG. 1a, the UE 101 is currently camped on a serving cell 120 (current serving cell) controlled by the source base station 110. The UE 101 may move to the cell 121. Therefore, the cell 121 may be referred to as a candidate / target cell controlled by the target base station 111. The candidate / target cell may be a serving cell or a neighboring cell of the source cell (i.e., the current serving cell).

[0074] Briefly, in step 1201, the source base station 110 receives a reselection request message from the UE 101 to request cell reselection to a new serving cell for receiving one or more active data sessions from the target base station of the new serving cell (e.g., to enable the UE to receive data of one or more active data sessions from the target base station). The reselection request message may include identification information associated with or identifying the selected new serving cell (when the UE 101 selects a new serving cell). Alternatively, the reselection request message may include identification information associated with 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, the source base station 110 may select one of the suitable cells as the new serving cell (or when two or more of the suitable cells are controlled by the target base station, the target base station selects one of the suitable cells, as described below with reference to FIG. 6). The identification information of each of the selected or suitable cells may include a cell ID and a 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.

[0075] In step 1202, the source base station 110 sends a migration request message to the target base station 111 of a new serving cell (e.g., a cell 121 selected by the UE 101 or by the source base station 110 as the new serving cell) to indicate that the UE 101 requests cell reselection with one or more active data sessions. The migration request message includes context information associated with the UE and the one or more active data sessions. The context information may include identification information (e.g., a UE ID) for identifying the UE and state information for indicating that the UE is requesting cell reselection with one or more active data sessions. The context information may further include session identification information (e.g., an MBS session identifier) ​​for identifying the one or more active data sessions. Then, the source base station 110 receives a migration accept message from the target base station 111 in step 1203, the migration request message including configuration information associated with the target base station 111 for the one or more active data sessions. The configuration information may include radio configuration information indicating the radio configuration of the target base station for the one or more active data sessions (e.g., an MBS radio bearer configuration in the new serving cell). The migration confirm message may comprise a migration confirm message indicating acceptance of the target base station to be the new serving cell for the UE for one or more active data sessions, the migration confirm message comprising configuration information. In step 1204, the source base station 110 sends a configuration message including the configuration information to the UE 101 to enable the UE 101 to receive data (e.g., MBS multicast data) of the one or more active data sessions from the target base station 111. The configuration message sent to the UE 101 informs 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 a cell ID and a base station (gNB) ID of the new serving cell.3GPP TS 38.413 sections 9.3.1.6 and 9.3.1.7 describe the gNB ID included in the Cell ID.

[0076] Example steps performed at a source base station upon receiving a reselection request message from a UE are described in more detail below with respect 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 and enables the UE 101 to obtain necessary configuration information for receiving data of one or more data sessions via the target base station, ensuring that continuity of data services (e.g., MBS services) is maintained or at least data loss is minimized.

[0078] Reference is now made to FIG. 13, which illustrates steps of a method 1300 for supporting cell reselection for a UE of a wireless communication system according to an embodiment of the present invention. The method 1300 is performed by a target base station of a new serving cell of a UE that has switched from a previous serving cell (e.g., a previous serving cell) to a new serving cell. The wireless communication system may be, for example, the wireless communication system 100 of FIG. 1a and FIG. 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 of FIG. 3, and the method 1300 is performed by the processor 315. The UE 101 is operating in an unconnected radio resource control (RRC) state (e.g., RRC_INACTIVE state) having 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, the UE 101 was previously camped on a previous serving cell (or last serving cell) 120 controlled by a source base station 110 and is switched to stay on a new serving cell 121 (e.g., a current new serving cell) controlled by a target base station 111.

[0079] Briefly, in step 1301, the target base station 111 receives a reselection request message from the UE 101 to request cell reselection to a new serving cell 121 with one or more active data sessions. The reselection request message may include identification information (e.g., gNB ID) identifying 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) ​​identifying the one or more active data sessions. In step 1302, the target base station 111 transmits 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) for identifying the UE. The context request message may further include information for requesting context information for the 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 and one or more active data sessions associated with the UE (step 1303). The context information may include information identifying the one or more active data sessions (e.g., an MBS session identifier).

[0081] In step 1304, the target base station 111 sends to the UE 101 a configuration message including configuration information related to the target base station for the one or more active data sessions to enable the UE 101 to receive data (e.g., MBS multicast data) of the one or more active data sessions from the target base station 111. The configuration information may include radio configuration information indicating the radio configuration of the target base station for the 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 for delivering data to the UE 101 for the one or more active data sessions.

[0082] In one example, the target base station 111 determines whether to accept the cell reselection request for one or more active data sessions in response to receiving the context message. When the target base station 111 determines that it can accept the cell reselection request for one or more active data sessions (e.g., the target base station has the capability and capacity to support MBS service for the UE), the target base station 111 transmits a configuration message. When the target base station 111 determines that it cannot accept the cell reselection request for one or more active data sessions (e.g., the target base station is overloaded and / or cannot support MBS service for the UE), the target base station 111 does not transmit a configuration message including configuration information. The target base station 111 may not transmit a message in response to the cell reselection request to indicate that the cell reselection request for one or more active data sessions has been rejected, or may transmit a rejection message indicating that the cell reselection request for one or more active data sessions has been rejected.

[0083] In one embodiment, in response to receiving the context message, the target base station 111 may communicate with entities of the core network 102 (e.g., using a path switch handshake procedure (step 736 of FIG. 7) as described below) 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. 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 the one or more active data sessions and therefore does not need to perform a path switch handshake procedure or a similar procedure with a core network entity.

[0084] In an example where the UE 101 has been switched to camp on the new serving cell 121 but has identified two or more suitable cells according to cell reselection evaluation criteria (as described below), the target base station 111 may receive a reselection request message from the UE 101 to request cell reselection to the new serving cell 121 with one or more active data sessions, and the reselection request message may further include identification information identifying the 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 a load condition in each cell) and send a response (e.g., in a configuration message) to the UE 101 including 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 the UE 101 is currently camped, the UE 101 needs to switch cells to the selected suitable cell.

[0085] Example steps performed at a target base station upon receiving a reselection request message from the UE are described in more detail below with respect 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 and enables the UE 101 to obtain necessary configuration information for receiving data of one or more active data sessions via the target base station, ensuring that continuity of data services (e.g., MBS services) is maintained or at least data loss is minimized.

[0087] Reference is now made to FIG. 14, which illustrates steps of a method 1400 for supporting cell reselection for a UE of a wireless communication system according to an embodiment of the present invention. The method 1400 is performed by a target base station that controls a target cell (or a target serving cell) selected (by the UE of the current serving cell or by a source base station) as a new serving cell for the UE. The wireless communication system may be, for example, the wireless communication system 100 of FIG. 1a, and the target base station may be the target base station 111 that controls the target cell 121. The target base station may comprise the base station 305 of FIG. 3, and the method 1400 is performed by the processor 315. The UE 101 is operating in an unconnected radio resource control (RRC) state (e.g., an RRC_INACTIVE state) having 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 illustrated in FIG. 1a, the UE 101 is camped on a serving cell 120 (current serving cell) controlled by the source base station 110. UE 101 has moved to the edge of serving cell 120 and is therefore also in the coverage of cell 121 (either a serving cell or a neighboring cell of source cell 120), and cell 121 has been selected (by source base station 110 or UE 101) as a new serving cell, but UE 101 has not yet switched to the new serving cell.

[0088] Briefly, in step 1401, the target base station 111 receives a migration request message from the source base station 110 of the serving cell 120 to indicate that the UE requests cell reselection from the serving cell 120 to the target cell 121 (which is a new serving cell) with one or more active data sessions. The migration request message includes context information associated with the UE and the one or more active data sessions. The context information may include session identification information (e.g., MBS session identifiers) that identify the one or more active data sessions. In step 1402, the target base station 111 transmits a migration accept message to the source base station 110 including configuration information associated with the target base station for the one or more active data sessions to enable the UE 101 to receive data (e.g., MBS multicast data) of the one or more active data sessions from the target base station 111. The configuration information may include radio configuration information indicating the radio configuration of the target base station for the 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 for delivering data to the UE 101 for one or more active data sessions.

[0089] In one example, the target base station 111 determines whether to accept the cell reselection request for one or more active data sessions in response to receiving the migration request message. When the target base station 111 determines that it can accept the cell reselection request for one or more active data sessions (e.g., the target base station has the capacity and capability to support MBS service for the UE), the target base station 111 transmits a migration confirmation message (e.g., a migration accept message) including configuration information. When the target base station 111 determines that it cannot accept the cell reselection request for one or more active data sessions (e.g., the target base station is overloaded and / or cannot support MBS service for the UE), the target base station 111 may not transmit 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 may transmit a migration confirmation message (e.g., a migration reject message) indicating that the cell reselection request for one or more active data sessions has been rejected.

[0090] If the UE 101 has selected multiple suitable cells for a new serving cell and identified the selected multiple suitable cells (e.g., as a list of suitable cells) in the reselection request message sent by the UE 101 to the source base station 110, the target base station 111 may receive a request (e.g., included in a migration request message) from the source base station 110 with a list of suitable cells for a new serving cell for the UE. The 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 the source base station 110 (e.g., included in a migration confirm message) indicating to the source base station 110 an identity of the selected cell (e.g., a cell ID) to which the UE should switch.

[0091] Example steps performed at a target base station upon receiving a migration request message from a source base station are described in more detail below with respect to Figures 6, 8a, 9a and 10a.

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

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

[0094] 6 is a flow chart 600 illustrating a first example of a cell reselection procedure for a UE in RRC_INACTIVE state receiving MBS multicast data, according to an embodiment of the present invention. A UE may be in RRC_INACTIVE state with one or more active data sessions: that is, the UE subscribes to or participates in one or more data sessions (e.g., one or more MBS sessions). The following description refers to one or more active MBS multicast sessions. Alternatively, an MBS session may be an MBS broadcast session.

[0095] This figure shows a UE 601 in RRC_INACTIVE state, similar to the UE 101 in Figure 1a, a base station 610 which is a source or serving gNB in ​​the sense that it controls the cell in which the UE 601 is currently camped (i.e. the serving cell), similar to the base station 110, a base station 611 which controls a target cell (which may also be called a candidate cell) to which the UE 601 may move (the base station controlling the target cell may be called the target base station), similar to the base station 111, and a core network (5GC) 602, similar to the core network 102. MBS multicast data are provided by the 5GC (via bearer 640) to the source gNB, which are then transmitted to the 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 (described above with reference to FIG. 5), the UE 601 periodically performs measurements (step 620) on signals received at the UE 601 from the serving cell and one or more candidate cells, such as signal synchronization blocks (SSBs) transmitted in the serving cell and the candidate cells (also called target cells). The candidate cells may be the serving cell or neighbor cells of the source cell (i.e., the current serving cell).

[0097] When the UE 601 finds at least one particular SSB with a received power that exceeds the received power of the current SSB by a predefined threshold, it performs a cell reselection evaluation process in step 621. The cell reselection evaluation process is performed according to several criteria that may include the priority of the frequency used in the candidate cell, the radio link quality, the availability of MBS services in the candidate cell. The input information for the selection (priority frequency, MBS availability) may be provided by the source gNB 610 via a system information message 630. The measurements in step 620 provide the radio link quality information for the 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 for all common (non-device specific) information that a UE needs to operate properly in the network. In general, system information is carried in different System Information Blocks (SIBs), each containing a different type of system information. The various SIBs are specified in the 3GPP specification TS 38.331. The delivery of the SIBs is done differently depending on whether the UE is connected to the network or not: if the device is connected to the network, a dedicated RRC signal is used, otherwise a broadcast signal is used. Among the different SIBs, SIB1 comprises the 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 on the mapping of the remaining SIBs to the system information message, on the transmission periodicity of each SIB, and on whether it is broadcast or not. In fact, the SIBs can be broadcast periodically as SIB1, but alternatively, these SIBs can be transmitted on demand, avoiding periodic broadcasts in cells where the device is not currently camped (hence power saving). In this case, the UE must explicitly request some SIB transmission by a system information request message. When in 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, the UE 601 can select a suitable cell to move to as a new serving cell for the UE in step 621 (cell reselection evaluation process). In step 621, the UE 601 may identify one suitable cell or a list of suitable cells according to cell reselection evaluation criteria such as the priority of the frequency used in the candidate cell, the radio link quality, the availability of MBS service in the candidate cell, as described above. When the UE 601 identifies multiple suitable cells, the source gNB 610 may select one of the suitable cells as the new serving cell (or the target gNB 611 may perform the selection). Taking the example of FIG. 1a, the UE 101 identifies the target cell 121 (target cell 611) as a suitable cell to stay on.

[0100] Then, the UE 601 sends a reselection request message 631 to the source gNB 610 (which is the base station controlling the serving cell (i.e., the current serving cell)) indicating an identifier of the suitable cell selected in step 621. The reselection request message sent by the UE 601 is to request cell reselection to a new serving cell for receiving one or more active data sessions from the target base station of the new serving cell, and the new serving cell may be selected by the UE 601 before sending the reselection request message (the reselection request message includes information identifying the selected new serving cell) or may be selected by the source gNB 610 based on information included in the reselection request message (identification information for identifying multiple cells suitable as new serving cells for the UE).

[0101] According to one example, the reselection request message 631 may be a RRCResumeRequest message or a RRCResumeRequest1 message specified in 3GPP specification TS 38.331 (v16.7.0) modified to include information indicating that the UE's intention is to perform cell reselection rather than switching to an 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, a new IE may be introduced in the RRCResumeRequest message or the 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 a cell ID and a 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 the reselection request message 631, the source gNB 610 uses the cell identifier to determine the associated target gNB. If the UE 601 provided several cell identifiers for several suitable cells, the source gNB should select one target gNB according to a predefined or specific criterion. For example, the source gNB 610 may select the target gNB 611 that provides MBS service with the lowest load. If not, the cell identifier provided in the reselection request message 631 may be the cell identifier of the suitable cell selected by the UE 601 as the new serving cell, and the source gNB 610 uses the cell identifier of the selected cell to determine the associated gNB that is the target gNB 611.

[0103] The source gNB then initiates a procedure similar to the handover procedure described above with reference to FIG. 5 (e.g., similar to steps 521, 522, 533, 534 of FIG. 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, there is no need for this message since the situation in the target cell is already known by the gNB. The MBS migration request message 632 includes a UE context including information about MBS sessions in which the UE has participated. The MBS migration request message 632 indicates that the UE is requesting cell reselection to the target cell with 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 includes identification information for identifying the UE and state information for indicating that the UE is requesting cell reselection with one or more active MBS sessions. The context information may also include information about one or more active MBS sessions, such as an MBS session ID. In addition to or instead of indicating that the UE is requesting cell reselection, the state information may include information indicating that the UE is operating in an unconnected state (eg, RRC_INACTIVE state).

[0104] According to one example, the MBS migration request message 632 may be a HANDOVER REQUEST message as specified in 3GPP specification TS 38.423 (v16.8.0) modified to include information indicating the UE's intention to perform cell reselection while receiving MBS multicast data (e.g., while participating or subscribing to 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 due to an active MBS session.

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

[0106] If the UE 601 has selected multiple suitable cells for a new serving cell and identified the selected multiple suitable cells (e.g., as a list of suitable cells) in the reselection request message 631 sent by the UE 601 to the source gNB 110, the source gNB 610 may send a request (e.g., included in a migration request message 632) to the target gNB 611 with the list of suitable cells for a new serving cell for the UE 601. The target gNB 611 then selects one of the suitable cells as the new serving cell (e.g., based on the load conditions in each cell) and sends a response to the source gNB 610 indicating the identity of the selected cell (e.g., a cell ID) to the source gNB 610 (e.g., included in a migration confirm message 633 or a migration accept message).

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

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

[0109] If the target gNB 611 rejects the reselection request, the source gNB 610 may inform the UE 601 of the decision. According to one example, the message 634 may also be an RRCReconfiguration message, an RRCRelease message, an RRCRelease with suspend configuration message, or a SIB. According to another example, non-reception of the message 634 at the UE 601 upon expiration of a timer initialized upon transmission of the reselection request message 631 may indicate that the reselection request has been rejected.

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

[0111] Meanwhile, the target gNB 611 may perform a path switch handshake procedure 637 towards the core network 602 to request delivery of the MBS multicast data. The path switch handshake procedure is described in 3GPP TS 38.413v16.8.0 section 8.4.4. For example, the target gNB 611 may communicate with the core network 602 to establish or set up a packet data unit (PDU) session between the UE 601 and the target base station 611 for one or more MBS sessions. If the target gNB 611 is already providing MBS services to other UEs and thus already receiving MBS multicast data from the core network 602, the procedure 637 may not be required. Thus, the MBS multicast data is provided by the core network 602 to the target gNB 611 (via bearer 642), and then the MBS multicast data is simultaneously transmitted to the 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 data buffered in the source gNB 610 may be moved to the target gNB 611 and then transmitted to the UE 601 (this step is not shown in FIG. 6).

[0112] If the target gNB 611 was not already providing MBS services before the path switch handshake procedure 637, this corresponds to the situation shown in Figure 1a. At the end of the cell reselection procedure of Figure 6, the resulting situation is shown in Figure 1b, where the UE 101 has switched to the new serving cell 121 controlled by the target gNB 111 and has configured its user plane to receive MBS multicast data from the 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 the UE 601 switched cells in step 623.

[0114] Figure 1b shows the result of application by a 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 numbers as used in Figure 1a), but now the UE 101 has switched to a serving cell 121 controlled by base station 111 which is the new source or serving gNB for the UE 101.

[0115] The MBS multicast data is provided from the multicast application server 103 via the core network 102 and the transport bearer 106 to the base station 111 via the 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, the base station 110 and the cell 120 can be considered as a candidate / target gNB and a target cell from the perspective of the UE 101. If the UE 101 was the only UE receiving the MBS multicast service in the cell 120, the base station 110 may have released radio resources associated with the MBS radio bearer 105. Thus, the core network 102 is no longer providing the base station 110 with MBS multicast data associated with the MBS service that the UE 101 was listening to. As a result, the transport bearer 104 is released.

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

[0118] Figure 1d shows the result of application by a UE 101 of the cell reselection procedure 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 used in Figure 1c) but now the UE 101 has switched to a serving cell 121 controlled by base station 111 which is the new source gNB for the UE 101.

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

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

[0121] FIG. 7 is a flow chart 700 illustrating a second example of a cell reselection procedure for a UE in RRC_INACTIVE state receiving MBS multicast data, according to an embodiment of the present invention. The UE may be in RRC_INACTIVE state with one or more active data sessions: i.e., the UE is participating or subscribed to one or more data sessions (e.g., one or more MBS sessions). The following description refers to one or more active MBS multicast sessions. Alternatively, the MBS session may be an MBS broadcast session. This procedure is intended to cover the case where the UE loses connection with the source gNB before completion of the cell reselection procedure, as described with reference to FIG. 6; in other words, the UE has switched to the target gNB before completion of the cell reselection procedure, as described with reference to FIG. 6.

[0122] Figure 7 shows a UE 701 in RRC_INACTIVE state, such as UE 101 in Figure 1a, a base station 710, a source or serving gNB in ​​the sense that it controls the cell in which UE 701 is currently camped (i.e., the serving cell), such as base station 110, a base station 711, such as base station 111, a base station that controls a target cell (which may also be called a candidate cell) to which UE 701 may move, and a core network (5GC) 702, such as core network 102. MBS multicast data are provided by the 5GC (via bearer 740) to the source gNB, which is then transmitted (via MBS radio bearer 741) to UE 701 simultaneously to other UEs that belong to the same multicast group.

[0123] With respect to step 520 in the handover procedure (described above with reference to FIG. 5), the UE 701 periodically performs measurements on signals received at the UE 701 from the serving cell and one or more candidate cells, such as signal synchronization blocks (SSBs) transmitted in the serving cell and the candidate cell (also called target cell) (step 720). The candidate cells may be serving cells or cells neighboring the source cell (i.e., the current serving cell). When the UE 701 finds at least one SSB with a received power that exceeds the received power of the current SSB by a predefined threshold, the UE 701 performs a cell reselection evaluation process in step 721. The cell reselection evaluation process is performed according to several criteria that may include the priority of the frequency used in the candidate cell, the radio link quality, the availability of MBS services in the candidate cell. The input information for the selection (priority frequency, MBS availability) may be provided by the source gNB 710 via a system information message 730. The measurements in step 720 provide radio link quality information for the 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, the UE 101 may be out of coverage of the source gNB 710. This may be due to degraded radio conditions leading to radio link failure. Based on the received system information 730 and the measurements performed in step 720, the UE 701 selects one suitable cell as a new serving cell in step 721 and effectively switches to this cell in step 722. For example, in step 721, the UE 701 may identify one suitable cell or a list of suitable cells according to criteria such as the priority of the frequency used in the candidate cell, the radio link quality, the availability of MBS services in the candidate cell, etc., as explained above. Once a suitable cell is selected by the UE 701 as the new serving cell, the UE 701 switches to this cell (step 722). Taking the example of FIG. 1a, the UE 101 identifies the target cell 121 as a suitable cell to camp on and switches to this cell 121 as the new serving cell.

[0125] The UE 701 receives system information 731 broadcast by the target gNB 711, which helps the UE 701 switch to the target cell 121 as a new serving cell, but the broadcasted system information is not sufficient to enable the UE 701 to receive MBS multicast data that the target gNB may be providing to other UEs. Thus, the UE 701 triggers a random access process and sends a reselection request 732 to the target gNB 711. The reselection request message sent by the UE 701 is to request cell reselection to the target cell 121 as a new serving cell for receiving one or more active data sessions from the target base station of the new serving cell (e.g., to enable the UE 701 to receive data of one or more active MBS sessions from the target base station (e.g., gNB 711)). According to an example, the reselection request message 732 may be an RRC Resume Request message or an RRC Resume Request1 message 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, not to perform an RNA update, but to perform cell reselection with 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, a new IE may also be introduced to indicate the MBS session IDs of one or more active MBS sessions to which the UE is listening. The message 732 includes an identifier that allows the target gNB 711 to identify the source gNB 710. In other words, the reselection request message may include an identification (e.g., gNB ID or cell ID) for identifying the base station 710 (source or serving gNB 710) of the serving cell (i.e., the previous serving cell that is the last serving cell from which the UE is switched to the target cell 121 as the new serving cell).

[0126] The target gNB 711 then sends a UE context request message 733 to the source gNB 710 to retrieve information about the UE 701 (e.g., the context request message 733 is a request for context information). The context request message 733 may include identification information (e.g., a UE ID) for identifying the UE 701. The context request message may further include information for requesting context information for one or more active data sessions. In response, the source gNB 710 sends a UE context message 734 that may include information identifying one or more active MBS sessions (e.g., the UE context message 734 may include identifier(s) of the MBS session(s) to which the UE 701 is listening). According to one example, the UE context request message 733 may be a RETRIEVE UE CONTEXT REQUEST message specified in the 3GPP specification TS 38.423 (v16.8.0), and the UE context message 734 may be a RETRIEVE UE CONTEXT RESPONSE message specified in the same 3GPP specification.

[0127] Next, the target gNB 711 performs an admission control step 723 to determine whether to accept the cell reselection request. For example, if the load in the target cell is too high, the request can be rejected. It may also accept the admission of the UE 701 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 to the UE 701 an MBS reconfiguration message 735 including information required for the UE 701 to receive MBS multicast data in the target cell (i.e., 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, the target gNB 711 sends to the UE 701 a configuration message 735 including 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 comprise radio configuration information indicating a radio configuration of the target gNB for one or more active MBS sessions (i.e., a radio configuration set for the target gNB to transmit MBS data for the one or more active MBS sessions). The radio configuration information may include information regarding an MBS radio bearer configuration used by the target gNB for the 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 to receive MBS data from the target gNB 711 for the one or more active MBS sessions.

[0128] In an example where the UE 701 has been switched to camp on the new serving cell 121 but has identified two or more suitable cells according to cell reselection criteria (as described below), the target gNB 711 may receive a reselection request message from the UE 701 to request cell reselection to the new serving cell with one or more active data sessions, the reselection request message may further include identification information identifying the two or more suitable cells (e.g., a list of suitable cells). The 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 conditions in each cell) and send a response to the UE 701 (e.g., in a configuration message 735) including identification information (e.g., a cell ID) indicating the suitable cell selected by the target gNB 711. If the selected suitable cell is not the new serving cell on which the UE 701 is currently camped, the UE 701 should switch cells to the selected suitable cell.

[0129] According to one example, the MBS reconfiguration message 735 may be an RRCReconfiguration message as specified in 3GPP specification TS 38.331 (v16.7.0) modified to allow this message to be sent to the UE in the RRC_INACTIVE state. According to another example, the MBS reconfiguration message 735 may be an RRCRelease message or an RRCRelease with suspend configuration message as specified in 3GPP specification TS 38.331 (v16.7.0). According to another example, the message 735 may be a dedicated SIB message. According to another example, non-reception of the message 735 at the UE 701 upon expiration of a timer initialized upon transmission of the reselection request message 732 may indicate that the reselection request has been rejected.

[0130] Meanwhile, the target gNB 711 may perform a path switch handshake procedure 736 towards the core network 702 to request delivery of the MBS multicast data. For example, the target gNB 711 may communicate with the core network 702 to establish or set up a packet data unit (PDU) session between the UE 701 and the target base station 711 for one or more MBS sessions. If the target gNB 711 is already providing MBS services to other UEs and thus already receiving MBS multicast data from the core network 702, the procedure 736 may not be required. Thus, the MBS multicast data is provided by the core network 702 to the target gNB 711 (via bearer 742), and then the MBS multicast data is simultaneously transmitted to the UE 701 (via MBS radio bearer 743) to other UEs belonging to the same multicast group.

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

[0132] Fig. 8a illustrates, by way of a flowchart 800, an exemplary method performed in a UE for supporting cell reselection according to an embodiment of the present invention. The UE may be the UE 101 of Fig. 1a in a serving cell 120 controlled by a serving base station 110 and may comprise the UE 205 of Fig. 2, and the method is performed by a processor 215. The UE is in an 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, such as Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ), of signals received from candidate or serving cells. Measurements may be performed on a Received Signal Synchronization Block (SSB). Thus, UE 101 performs measurements on signals in the serving cell and neighboring candidate cell(s). Optionally, in step 802, UE 101 receives information from serving base station 110 about the capabilities of neighboring cells providing MBS services (e.g., whether one or more candidate cells can support MBS services for the UE): this corresponds to message 630 in FIG. 6. In step 803, the UE 101 performs a cell reselection evaluation process (step 621 in FIG. 6 ) based on the measurements performed in step 801 and possibly on the information received in step 802. In step 804, the UE 101 sends a reselection request (corresponding to message 631 in FIG. 6 ) to the serving base station 110 with an identification of a cell (e.g., if the UE 101 selected a new serving cell) or a list of cells (e.g., if the source or serving base station makes the selection of the new serving cell or the target base station selected in step 803). In step 805, the UE 101 receives an MBS reconfiguration message from the serving base station 110, which includes 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 FIG. 6 . Finally, in step 806, the UE 101 switches cells, camps on 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.

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

[0135] In step 811, the 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 the UE 101 from the serving cell 120. For example, the UE 101 measures one or more parameters, such as Reference Signal Received Power (RSRP) or Reference Signal Received Quality (RSRQ), of signals received from the candidate or serving cells. The measurements may be performed on a Received Signal Synchronization Block (SSB). Thus, the UE 101 performs measurements on signals in the serving cell and neighboring candidate cell(s). Optionally, in step 812, the UE 101 receives information from the serving base station 110 about the capabilities 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 FIG. 7. In step 813, the UE 101 performs a cell reselection evaluation process (step 721 in FIG. 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, the UE 101 switches cells and stays on the new serving cell selected in step 813 (step 722 in FIG. 7 ). In step 815, the UE 101 transmits a reselection request to the target base station (or the new serving base station) that controls the target cell selected as the new serving cell. It corresponds to message 732 in FIG. 7 , which may include an identification of the previous serving cell (i.e., the last serving cell before the UE is switched to the new serving cell), such as a gNB ID or cell ID, and an identification of the MBS multicast service(s) that the UE 101 is receiving (e.g., one or more active MBS sessions).In step 816, UE 101 receives an MBS reconfiguration message from the target base station including configuration information for receiving 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 FIG. 7.

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

[0137] In step 901, the base station 110 receives a request for cell reselection from the UE with an identification of a suitable cell or a list of suitable cells (corresponding to message 631 in FIG. 6). Optionally, in step 902, the base station 110 selects a target base station that controls a cell from among a plurality of suitable cells identified in step 901. In step 903, the base station 110 transmits an MBS migration request to the target base station that controls the suitable cell (selected by the UE or the base station 110). The MBS migration request corresponds to message 632 in FIG. 6 and may include a UE context that includes information about the MBS sessions that the UE has joined. In step 904, the 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 FIG. 6 (e.g., a migration accept message as described above). In step 905, the base station 110 transmits an MBS reconfiguration to the UE that includes the MBS configuration information received from the target base station. This corresponds to message 634 in FIG. 6.

[0138] 9b illustrates, using a flowchart 910, another exemplary method performed in a source base station of a UE's previous serving cell to support cell reselection of a UE operating in a disconnected RRC state with one or more active data sessions (e.g., the UE is in an 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 previous serving cell) to a new serving cell controlled by a target base station. The source base station may be the gNB 110 of FIGS. 1a and 1b and may comprise the base station / gNB 305 of FIG. 3, and the method is performed by a processor 315.

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

[0140] 10a illustrates, using a flowchart 1000, an exemplary method performed in a target base station of a target cell to support cell reselection of a UE in an RRC_INACTIVE state and 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 has been selected as a new serving cell for the UE. The target base station may be the gNB 111 of FIG. 1a and may comprise the base station / gNB 305 of FIG. 3, and the method is performed by the processor 315.

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

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

[0143] In step 1011, the base station 111 receives a reselection request from the UE. It corresponds to message 732 in FIG. 7, which may include an identification of the previous serving cell and an identification of the MBS multicast service that the UE is receiving. In step 1012, the 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 FIG. 7 and identifies the UE in RRC_INACTIVE state and receives MBS multicast data. In step 1013, the base station 111 receives a UE context from the source base station. It corresponds to message 734 in FIG. 7, which may include an identifier of the MBS session that the UE is listening to. In step 1014, the base station 111 performs admission control to decide whether to accept the request for reselection by the MBS multicast service. This corresponds to step 723 in FIG. 7. If the base station 111 accepts the request, in step 1015, the base station 111 sends an MBS reconfiguration message to the UE, which includes information for receiving MBS multicast data in the cell where the UE is camped. Optionally, in step 1016, the base station 111 may perform a path switch handshake procedure towards the core network 102 to request delivery of MBS multicast data to the base station 111 to be sent to the UE. This corresponds to step 736 in FIG. 7. In step 1017, the base station 111 can set up the necessary radio resources and deliver the MBS multicast data to the UE. In step 1018, the base station sends a release of UE context to the source base station. This corresponds to message 737 in FIG. 7.

[0144] Although the present invention has been described above using the 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 present invention, as defined in the appended claims. All features disclosed in the specification (including any accompanying claims, abstract, and drawings) and / or all steps of any method or process so disclosed can be combined in any combination, except combinations in which at least some of such features and / or steps are mutually exclusive. Each feature disclosed in the specification (including any accompanying claims, abstract, and drawings) may be replaced by an alternative feature serving the same, equivalent, or similar purpose, unless expressly stated otherwise. Thus, unless otherwise stated, each feature disclosed is merely one example of a generic series of equivalent or similar functions.

[0145] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage.

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

[0147] Computer-readable media may include computer-readable storage media, which correspond to tangible media such as data storage media, or communication media, which include any medium that facilitates transfer of a computer program from one place to another, for example, according to a communication protocol. As such, computer-readable media may generally correspond to (1) tangible computer-readable storage media that is non-transitory, or (2) a communication medium, such as a signal or carrier wave. Data storage media may be any available medium 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 in this disclosure. A computer program product may include a computer-readable medium.

[0148] By way of example, and not limitation, such computer-readable storage media may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, flash memory, or any other medium that may be used to store desired program code in the form of instructions or data structures and that may be accessed by a computer. Also, any connection is properly referred to as a computer-readable medium. For example, if the 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, the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of the medium. However, it should be understood that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but instead cover non-transitory tangible storage media. Disks and discs as used herein include compact discs (CDs), laser discs, optical discs, digital versatile discs (DVDs), floppy disks, and Blu-ray discs, where disks typically reproduce data magnetically and disks reproduce data optically with a laser. Combinations of the above should also be included within the scope of computer readable media.

Claims

1. 1. A method for supporting cell reselection for a user equipment (UE), the UE operating in a non-connected RRC state with one or more active data sessions, the method in the UE comprising: sending a reselection request message to a new serving cell to request cell reselection for receiving 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; 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; A method comprising:

2. The method of claim 1 , further comprising switching to the new serving cell.

3. 3. The method of claim 2, further comprising receiving data for the one or more active data sessions from the target base station after configuring the UE and after switching to the new serving cell.

4. The method of claim 1 , wherein the configuration information comprises radio configuration information indicating the radio configuration of the target base station for the one or more active data sessions.

5. The method of claim 4 , wherein configuring comprises updating the radio configuration of the UE according to the radio configuration of the target base station.

6. 6. The method of claim 1, further comprising: selecting one or more suitable cells for a new serving cell for the UE, the new serving cell being one of the selected one or more suitable cells.

7. Transmitting a reselection request message includes transmitting the reselection request message to a source base station of a serving cell for the UE; The method of claim 1 , wherein receiving a configuration message comprises receiving the configuration message from the source base station.

8. The method of claim 6 or 7, wherein the reselection request message includes identification information that identifies the one or more suitable cells for the new serving cell.

9. The selecting includes selecting one suitable cell as the new serving cell for the UE; The method of claim 6 or 7, wherein the reselection request message includes identification information identifying the one suitable cell selected as the new serving cell.

10. The method of claim 7 , further comprising switching to the new serving cell in response to receiving the configuration message.

11. The selecting further includes selecting one suitable cell as the new serving cell for the UE, and the target base station is a base station of the one suitable cell selected as the new serving cell; Transmitting a reselection request message includes sending the reselection request message to the target base station; The method of claim 6 , wherein receiving a configuration message comprises receiving the configuration message from the target base station.

12. The method of claim 11 , wherein the reselection request message includes an identification that identifies a source base station of a serving cell for the UE.

13. The method of claim 11 or 12, wherein the reselection request message includes session identification information that identifies the one or more active data sessions.

14. The method of claim 11 , further comprising switching to the new serving cell before sending the reselection request message to the target base station.

15. 1. A method in a source base station of a serving cell of a user equipment (UE), the UE operating in a non-connected RRC state having one or more active data sessions, the method comprising: receiving a reselection request message from the UE to request cell reselection to a new serving cell for receiving one or more active data sessions from a target base station of the new serving cell; sending a migration request message to the target base station to indicate that the UE requests cell reselection in one or more active data sessions, the migration request message including context information associated with the UE and the one or more active data sessions; receiving, from the target base station, a migration confirmation message including configuration information associated with the target base station for the one or more active data sessions; sending a configuration message to the UE including the configuration information; A method comprising:

16. The method of claim 15 , wherein the reselection request message includes identification information that identifies the new serving cell.

17. the reselection request message includes identification information identifying one or more suitable cells for the new serving cell; The method of claim 15 , further comprising selecting one of the one or more suitable cells as the new serving cell for the UE.

18. 18. The method of claim 15, wherein the context information comprises identity information for identifying the UE and state information for indicating that the UE is requesting cell reselection in one or more active data sessions.

19. The method of claim 15 , wherein the configuration information comprises radio configuration information indicating the radio configuration of the target base station for the one or more active data sessions.

20. 1. A method in a target base station of a new serving cell for a user equipment (UE), the UE operating in a non-connected RRC state having one or more active data sessions, the UE switching from a previous serving cell to the new serving cell, the method comprising: receiving a reselection request message from the UE to request cell reselection to a new serving cell for one or more active data sessions; sending a context request message to a source base station of the previous serving cell; receiving, from the source base station, a context message including context information associated with the UE and the one or more active data sessions; sending a configuration message to a UE including configuration information associated with the target base station for the one or more active data sessions; A method comprising:

21. The method of claim 20 , wherein the reselection request message includes an identification that identifies the source base station of the previous serving cell.

22. The method of claim 20 or claim 21, wherein the reselection request message includes session identification information that identifies the one or more active data sessions.

23. The method according to claim 20 , wherein the context request message includes identification information for identifying the UE.

24. determining whether to accept the cell reselection request in response to receiving the context message; 24. The method of claim 20, wherein transmitting a configuration message comprises transmitting the configuration message in response to deciding to accept the cell reselection request.

25. 1. A method in a target base station of a target cell, the target cell being selected as a new serving cell for a user equipment (UE), the UE operating in a Disconnected RRC state with one or more active data sessions via the serving cell, the method comprising: receiving, from a source base station of the serving cell, a migration request message for indicating that the UE requests cell reselection to the target cell in one or more active data sessions, the migration request message including context information associated with the UE and the one or more active data sessions; sending, to the source base station, a migration confirmation message including configuration information associated with the target base station for the one or more active data sessions; A method comprising:

26. determining whether to accept the cell reselection request; 26. The method of claim 25, wherein transmitting the migration confirmation message comprises transmitting a migration accept message including the configuration information in response to deciding to accept the cell reselection request.

27. 27. A method according to any one of claims 20 to 26, wherein the context information comprises session identification information which identifies the one or more active data sessions.

28. 28. The method of claim 20, further comprising setting up radio resources for delivering data to the UE for the one or more active data sessions.

29. 29. The method of claim 20, further comprising: in response to receiving the context message, communicating with a core network entity for establishing a packet data unit (PDU) session between the UE and the target base station for the one or more active data sessions.

30. 30. The method of claim 20, wherein the configuration information comprises radio configuration information indicating the radio configuration of the target base station for the one or more active data sessions.

31. 1. A method in a source base station of a previous serving cell of a user equipment (UE), the UE operating in a non-connected RRC state having one or more active data sessions, the UE switching from the previous serving cell to a new serving cell, the method comprising: receiving a context request message from a target base station of the new serving cell, the context request message including identification information for identifying the UE, the context request message being for requesting context information; sending a context message to a target base station in response to the context request message, the context message including context information associated with the UE and one or more active data sessions; The method includes:

32. receiving a context release message from the target base station; In response to the context release message, deleting the context information associated with the UE stored in the source base station; 32. The method of claim 31 , further comprising:

33. 33. The method of claim 23, 31 or 32, wherein the context request message further comprises information for requesting context information for the one or more active data sessions.

34. A user equipment, a transceiver for providing wireless communication; A processor coupled to the transceiver and configured to perform the method of any one of claims 1 to 14; A user equipment comprising:

35. A base station, a transceiver for providing wireless communication; A processor coupled to the transceiver and configured to perform the method of any one of claims 15 to 33; A base station comprising:

36. A computer program comprising instructions which, when executed by a computer, cause the computer to carry out the method of any one of claims 1 to 33.

37. 37. A computer readable medium carrying a computer program according to claim 36.

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