Communication method and network node

Abstract

This communication method, which is executed by a network node in a mobile communication system, comprises: communicating wirelessly with user equipment that is in an RRC connected state in a first cell of the network node; and transmitting, to another network node, a request message for requesting a change of a serving cell of the user equipment from the first cell to a second cell of the other network node. If the change of serving cell is performed by means of LTM, the network node transmits a request message indicating that the change is a serving cell change using said LTM. Said LTM is a procedure whereby a cell switching instruction is provided from the network node to the user equipment by means of a MAC CE.

Description

Communication method and network node

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

[0002] The 3rd Generation Partnership Project (3GPP) (registered trademark; the same applies hereinafter) defines technical specifications for NR (New Radio), a fifth-generation (5G) radio access technology. In a 3GPP mobile communication system, a serving cell change for a user equipment in a radio resource control (RRC) connected state is instructed by transmitting an RRC layer message (so-called handover command), which corresponds to Layer 3 (L3), from a network node to the user equipment.

[0003] Meanwhile, in Release 18 of the 3GPP standard (3GPP Release 18), technical specifications for LTM (L1 / L2-Triggered Mobility) are being developed. LTM is a procedure in which a network node receives a Layer 1 (L1) measurement report from a user equipment, and based on the report, the network node signals a cell switch command to the user equipment via a medium access control (MAC) control element (CE), causing the network node to change the serving cell of the user equipment.

[0004] In 3GPP Release 18, LTM is limited to serving cell changes between cells belonging to the same network node and does not support serving cell changes between cells belonging to different network nodes (i.e., inter-network node LTM).

[0005] 3GPP contribution: R2-2309335

[0006] The present disclosure relates to a communication method and a network node for realizing inter-network node LTM.

[0007] A communication method according to a first aspect of the present disclosure is a communication method executed by a network node in a mobile communication system, the method including: performing wireless communication with a user equipment (UE) in a Radio Resource Control (RRC) Connected state in a first cell of the network node; and transmitting a request message to another network node to request a change of serving cell of the user equipment from the first cell to a second cell of another network node. If the serving cell change is to be performed by L1 / L2-Triggered Mobility (LTM), the network node transmits the request message indicating that the serving cell change is due to LTM. The LTM is a procedure for instructing a cell change from the network node to the user equipment via a Medium Access Control (MAC) Control Element (CE).

[0008] A network node according to a second aspect of the present disclosure is a network node used in a mobile communication system, comprising: a first communication unit configured to perform wireless communication with a user equipment (UE) in a Radio Resource Control (RRC) Connected state in a first cell of the network node; and a second communication unit configured to transmit a request message to the other network node to request a change of serving cell of the user equipment from the first cell to a second cell of another network node. When the serving cell change is performed by LTM (L1 / L2-Triggered Mobility), the second communication unit transmits the request message indicating that the serving cell change is performed by LTM. The LTM is a procedure for instructing a cell change from the network node to the user equipment by a Medium Access Control (MAC) control element (CE).

[0009] 1 is a diagram illustrating an example of the configuration of a mobile communication system according to an embodiment. FIG. 2 is a diagram illustrating an example of the configuration of a UE (user equipment) according to an embodiment. FIG. 3 is a diagram illustrating an example of the configuration of a gNB (network node) according to an embodiment. FIG. 4 is a diagram illustrating a protocol stack configuration of a radio interface of a user plane that handles data. FIG. 5 is a diagram illustrating a protocol stack configuration of a radio interface of a control plane that handles signaling (control signals). FIG. 6 is a diagram illustrating an example of an LTM procedure whose specifications are being developed in 3GPP Release 18. FIG. 7 is a diagram for explaining an operation scenario of a mobile communication system according to an embodiment. FIG. 8 is a diagram illustrating a specific example of the operation of a mobile communication system according to an embodiment.

[0010] A mobile communication system according to an embodiment will be described with reference to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0011] (1) Configuration of a Mobile Communication System FIG. 1 is a diagram showing an example of the configuration of a mobile communication system 1 according to an embodiment. The mobile communication system 1 conforms to the 3GPP standard 5th Generation System (5GS). While the following description uses 5GS as an example, the mobile communication system may also be at least partially based on an LTE (Long Term Evolution) system. The mobile communication system may also be at least partially based on a 6th Generation (6G) system.

[0012] The mobile communication system 1 includes a user equipment (UE) 100, a 5G radio access network (NG-RAN: Next Generation Radio Access Network) 10, and a 5G core network (5GC: 5G Core Network) 20. Hereinafter, the NG-RAN 10 may be simply referred to as the RAN 10. Furthermore, the 5GC 20 may be simply referred to as the core network (CN) 20. The RAN 10 and the CN 20 constitute a network 5 of the mobile communication system 1.

[0013] The UE 100 is a mobile wireless communication device. The UE 100 may be any device that is used by a user. For example, the UE 100 may be a mobile phone terminal (including a smartphone), a tablet terminal, a notebook PC, a communication module (including a communication card or a chipset), a sensor or a device provided in a sensor, a vehicle or a device provided in a vehicle (Vehicle UE), or an aircraft or a device provided in an aircraft (Aerial UE).

[0014] The NG-RAN 10 includes a base station (referred to as "gNB" in the 5G system) 200, which is a type of network node. The gNBs 200 are connected to each other via an Xn interface, which is an interface between base stations. The gNB 200 manages one or more cells. The gNB 200 performs wireless communication with the UE 100 that has established a connection with its own cell. The gNB 200 has a radio resource management (RRM) function, a routing function for user data (hereinafter simply referred to as "data"), a measurement control function for mobility control and scheduling, etc. The term "cell" is used to indicate the smallest unit of a wireless communication area. The term "cell" is also used to indicate a function or resource that performs wireless communication with the UE 100. One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

[0015] In addition, gNBs can also be connected to the Evolved Packet Core (EPC), which is the core network of LTE. LTE base stations can also be connected to 5GC. LTE base stations and gNBs can also be connected via an inter-base station interface.

[0016] The 5GC20 includes an AMF (Access and Mobility Management Function) and a UPF (User Plane Function) 300. The AMF performs various mobility controls for the UE 100. The AMF manages the mobility of the UE 100 by communicating with the UE 100 using NAS (Non-Access Stratum) signaling. The UPF controls data forwarding. The AMF and the UPF are connected to the gNB 200 via an NG interface, which is an interface between a base station and a core network.

[0017] 2 is a diagram illustrating an example configuration of a UE 100 (user equipment) according to an embodiment. The UE 100 includes a receiving unit 110, a transmitting unit 120, and a control unit 130. The receiving unit 110 and the transmitting unit 120 configure a wireless communication unit that performs wireless communication with the gNB 200.

[0018] The receiving unit 110 performs various types of reception under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 130.

[0019] The transmitting unit 120 performs various transmissions under the control of the control unit 130. The transmitting unit 120 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 130 into a radio signal and transmits it from the antenna.

[0020] The control unit 130 performs various controls and processes in the UE 100. Such processes include processes of each layer described below. The operations of the UE 100 described above and below may be operations controlled by the control unit 230. The control unit 130 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processing by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0021] 3 is a diagram showing an example configuration of a gNB 200 (network node) according to an embodiment. The gNB 200 has a transmitter 210, a receiver 220, a controller 230, and a backhaul communication unit 240. The transmitter 210 and the receiver 220 constitute a wireless communication unit that performs wireless communication with the UE 100. The backhaul communication unit 240 constitutes a network communication unit that communicates with the CN 20.

[0022] The transmitting unit 210 performs various transmissions under the control of the control unit 230. The transmitting unit 210 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 230 into a radio signal and transmits it from the antenna.

[0023] The receiving unit 220 performs various types of reception under the control of the control unit 230. The receiving unit 220 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 230.

[0024] The control unit 230 performs various controls and processes in the gNB 200. Such processes include processes for each layer described below. The operations of the gNB 200 described above and below may be operations under the control of the control unit 230. The control unit 230 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processing by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0025] The backhaul communication unit 240 is connected to adjacent base stations via an Xn interface, which is an interface between base stations. The backhaul communication unit 240 is connected to the AMF / UPF 300 via an NG interface, which is an interface between a base station and a core network. Note that the gNB 200 is composed of a CU (Central Unit) and a DU (Distributed Unit) (i.e., functionally divided), and the two units may be connected by an F1 interface, which is a fronthaul interface.

[0026] FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data.

[0027] The user plane radio interface protocol includes a physical (PHY) layer, a medium access control (MAC) layer, a radio link control (RLC) layer, a packet data convergence protocol (PDCP) layer, and a service data adaptation protocol (SDAP) layer.

[0028] The PHY layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layer of UE100 and the PHY layer of gNB200 via a physical channel. The PHY layer of UE100 receives downlink control information (DCI) transmitted from gNB200 on a physical downlink control channel (PDCCH). Specifically, UE100 performs blind decoding of the PDCCH using a radio network temporary identifier (RNTI) and acquires the successfully decoded DCI as DCI addressed to the UE. The DCI transmitted from gNB200 has a CRC parity bit scrambled by the RNTI added.

[0029] The MAC layer performs data priority control, retransmission processing using Hybrid Automatic Repeat reQuest (HARQ), random access procedures, etc. Data and control information are transmitted between the MAC layer of the UE 100 and the MAC layer of the gNB 200 via a transport channel. The MAC layer of the gNB 200 includes a scheduler. The scheduler determines the uplink and downlink transport format (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be allocated to the UE 100.

[0030] The RLC layer transmits data to the receiving RLC layer using the functions of the MAC layer and PHY layer. Data and control information are transmitted between the RLC layer of the UE 100 and the RLC layer of the gNB 200 via a logical channel.

[0031] The PDCP layer performs header compression / decompression, encryption / decryption, and the like.

[0032] The SDAP layer maps IP flows, which are units for Quality of Service (QoS) control by the core network, to radio bearers, which are units for QoS control by the Access Stratum (AS). Note that if the RAN is connected to the EPC, SDAP may not be required.

[0033] FIG. 5 is a diagram showing the configuration of a protocol stack of a radio interface of a control plane that handles signaling (control signals).

[0034] The protocol stack of the radio interface of the control plane has an RRC (Radio Resource Control) layer and an NAS (Non-Access Stratum) layer instead of the SDAP layer shown in FIG.

[0035] RRC signaling for various settings is transmitted between the RRC layer of UE100 and the RRC layer of gNB200. The RRC layer controls logical channels, transport channels, and physical channels according to the establishment, re-establishment, and release of radio bearers. When there is a connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC connected state. When there is no connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC idle state. When the connection between the RRC of UE100 and the RRC of gNB200 is suspended, UE100 is in an RRC inactive state.

[0036] The NAS layer (also simply referred to as "NAS") located above the RRC layer performs session management, mobility management, etc. NAS signaling is transmitted between the NAS layer of the UE 100 and the NAS layer of the AMF 300A. Note that the UE 100 has an application layer and the like in addition to the radio interface protocol. Also, a layer lower than the NAS layer is referred to as the AS layer (also simply referred to as "AS").

[0037] (2) Overview of LTM The mobile communication system according to the embodiment supports LTM (L1 / L2-triggered mobility).

[0038] LTM is a technology for shortening mobility delay (specifically, delay in serving cell change) compared to a general handover procedure by triggering cell switching by signaling of a lower layer, Layer 1 (L1) and / or Layer 2 (L2). In a general handover procedure, a measurement report message, which is an RRC message, is transmitted from the UE 100 to the gNB 200, and the gNB 200 determines a handover of the UE 100 based on the Measurement Report message. The handover is instructed by transmitting a handover command (specifically, an RRC Reconfiguration message) which is an RRC message from the gNB 200 to the UE 100.

[0039] In contrast, in LTM, first, the gNB 200 prepares an LTM candidate cell configuration for a candidate cell to be switched to and provides the LTM candidate cell configuration to the UE 100 by RRC signaling. Second, the UE 100 performs synchronization processing with the candidate cell by early synchronization (Early sync). Third, the gNB 200 receives an L1 measurement report from the UE 100, determines cell switching to the target cell based on the L1 measurement report, and transmits a cell switch command (Cell Switch Command) indicating the target cell (LTM candidate cell configuration) to the UE 100 by MAC CE. The cell switch trigger is conveyed in a MAC CE that includes at least a candidate configuration index together with a beam indicator. Fourth, the UE 100 changes the serving cell in response to the cell switch command from the gNB 200 (source cell). In this way, cell switching is triggered by gNB200 by selecting the LTM candidate cell setting as the target setting. The LTM candidate cell setting can be added, changed, and released by gNB200 via RRC signaling.

[0040] The following principles apply to LTM:

[0041] Each LTM candidate cell configuration can be provided as a differential configuration (delta configuration) relative to the reference configuration used to form the complete candidate cell configuration.

[0042] If a full candidate cell configuration is applied, it replaces the current UE configuration at cell switch. A reconfiguration procedure does the replacement but does not necessarily reset the MAC, RLC or PDCP layers.

[0043] The user plane continues without a reset if configured in RRC signaling to avoid additional delays in data recovery.

[0044] - Security is not updated in LTM.

[0045] LTM between subsequent LTM candidate cell configurations can be performed without RRC reconfiguration, i.e., the UE 100 does not release other LTM candidate cell configurations after LTM is triggered.

[0046] 6 is a diagram showing an example of an LTM procedure being specified in 3GPP Release 18. In the illustrated example, UE 100 performs cell switching from a first cell (source cell) of gNB 200 to a second cell. Here, the first cell and the second cell may be configured with different TRPs (Transmission and Reception Points). Until cell switching by LTM is determined, the second cell is also referred to as a "candidate cell (or LTM candidate cell)", and after cell switching by LTM is determined, the second cell is also referred to as a "target cell".

[0047] In step S1, UE100 is in an RRC connected state in the cell of gNB200.

[0048] In step S2, UE100 transmits a Measurement Report message, which is an RRC message, to gNB200.

[0049] In step S3, gNB200 decides to use LTM based on the Measurement Report message and starts preparing the candidate cell.

[0050] In step S4, gNB200 sends an RRC Reconfiguration message to UE100, which includes LTM candidate cell configurations (LTM Candidate Configurations) for one or more candidate cells.

[0051] In step S5, UE100 saves the LTM candidate cell setting and sends an RRC Reconfiguration Complete message to gNB200.

[0052] In step S6, the UE 100 may perform synchronization processing with the candidate cell before receiving the cell switch command. Such synchronization processing is called early synchronization (Early sync). Here, the UE 100 may perform early timing advance (TA) acquisition in the candidate cell requested by the gNB 200 (source cell) before receiving the cell switch command of step S9. This is performed by contention-free random access (CFRA) triggered by a PDCCH command (PDCCH order) from the source cell. Note that when DCI Format 1_0 is used and all "Frequency domain resource assignment" fields in the DCI are set to "1", the DCI is treated as a PDCCH order. The UE 100 transmits a random access preamble (RA preamble) to the specified candidate cell. In order to minimize communication interruption of the source cell due to CFRA for the candidate cell, in early synchronization, the UE 100 does not receive a random access response (RAR) for the purpose of acquiring a TA value from the candidate cell. The TA value of the candidate cell (target cell) is indicated in the cell switching command in step S9. The TA value is a value for adjusting the uplink transmission timing of the UE 100.

[0053] In step S7, the UE 100 performs layer 1 (L1) measurement in the configured candidate cell and transmits a physical layer measurement report (L1 Measurement Report) to the gNB 200. The L1 Measurement Report is transmitted and received at L1, which is the PHY layer. For example, the UE 100 transmits L1-RSRP and / or L1-SINR to the gNB 200 via a PUCCH (Physical Up-link Control Channel) and / or a PUSCH.

[0054] In step S8, gNB200 decides to perform cell switching to the target cell (second cell).

[0055] In step S9, gNB200 transmits a Cell Switch Command (MAC CE) including a candidate configuration index of the target cell to UE100. The Cell Switch Command may include a TA value obtained by early synchronization.

[0056] In step S10, the UE 100 switches to the configuration of the target cell. Specifically, the UE 100 detaches from the source cell (first cell) and applies the configuration of the target cell.

[0057] In step S11, if the cell switch needs to include the execution of a random access procedure (for example, if the Cell Switch Command does not include a valid TA value), the UE 100 executes the random access procedure for the target cell. Note that, if the UE 100 does not need to acquire the TA of the target cell at the time of cell switch (for example, if the Cell Switch Command includes a valid TA value), the random access procedure can be skipped.

[0058] In step S12, the UE 100 indicates that the cell switch to the target cell has been successfully completed. Thereafter, the UE 100 may perform steps S6 to S12 multiple times for subsequent LTM cell switches based on the configuration provided in step S4.

[0059] (3) Operation of the mobile communication system The LTM of 3GPP Release 18 is limited to serving cell changes between cells belonging to the same gNB 200 (same CU). Therefore, there is a problem that serving cell changes between cells belonging to different gNBs 200 (different CUs) cannot be realized by LTM. Note that such LTM may be referred to as inter-network node LTM, specifically, inter-gNB LTM (inter-gNB LTM) or inter-CU LTM (inter-CU LTM). In the following embodiment, an operation for realizing inter-network node LTM will be described.

[0060] FIG. 7 is a diagram for explaining an operation scenario of the mobile communication system 1 according to the embodiment.

[0061] The UE 100 changes the serving cell from the first cell (source cell) of the gNB 200a, which is the source gNB, to the second cell of the gNB 200b. Until the cell switching by LTM is determined, the second cell is also referred to as a "candidate cell (or LTM candidate cell)", and after the cell switching by LTM is determined, the second cell is also referred to as a "target cell". An Xn interface is established between the gNB 200a and the gNB 200b. Communication between the gNB 200a and the gNB 200b is assumed to be performed on the Xn interface.

[0062] In an embodiment, the gNB 200a performs wireless communication with the UE 100 in an RRC connected state in a first cell of the gNB 200a. The gNB 200a transmits to the gNB 200b a request message for requesting a change of the serving cell of the UE 100 from the first cell of the gNB 200a to the second cell of the gNB 200b. Here, when the serving cell change is performed by LTM, the gNB 200a transmits to the gNB 200b a request message indicating that the serving cell change is by LTM. Note that LTM is a procedure in which the gNB 200a instructs the UE 100 to switch cells by MAC CE.

[0063] This allows gNB200b to understand, based on the request message from gNB200a, that a serving cell change using LTM is being requested rather than a general handover.

[0064] The request message may be a handover request (HO Request) message that can be used in a handover procedure that instructs a handover from the gNB 200a to the UE 100 by an RRC message. A request message indicating a serving cell change by LTM may be an HO Request message including an LTM indicator. This makes it possible to use the HO Request message used in general handovers in LTM between network nodes, making it easier to minimize changes to technical specifications.

[0065] Alternatively, the request message indicating a serving cell change due to LTM may be a request message for LTM that is different from the HO Request message. The request message for LTM may be a request message that is used exclusively for LTM.

[0066] The request message indicating the serving cell change by LTM may include information indicating whether the gNB 200b needs to configure a contention-free random access (CFRA) resource. The CFRA resource is used in early synchronization performed by the UE 100 with respect to the second cell before the cell switch instruction by the MAC CE. This allows the gNB 200b to determine whether to configure the CFRA resource based on the request message.

[0067] The gNB 200a may send another request message to the gNB 200b requesting that the gNB 200b configure or activate CFRA resources for early synchronization, thereby allowing the gNB 200b to appropriately configure or activate CFRA resources for early synchronization.

[0068] The gNB 200a may transmit a PDCCH command (PDCCH order) instructing the UE 100 to perform CFRA (i.e., transmit an RA preamble), to the UE 100. The PDCCH order may include information for identifying the second cell. This makes it easy for the UE 100 to identify whether the target of the RA preamble transmission is the first cell or the second cell.

[0069] The gNB 200a may receive from the gNB 200b a notification indicating that the early synchronization performed by the UE 100 with respect to the second cell has been successful. This allows the gNB 200a to understand whether the early synchronization performed by the UE 100 with respect to the second cell has been successful.

[0070] Alternatively, gNB200a may receive a notification from UE100 indicating that UE100 has successfully performed early synchronization with the second cell.

[0071] Fig. 8 is a diagram showing a specific example of the operation of the mobile communication system 1 according to the embodiment. In Fig. 8, steps that can be omitted are indicated by dashed lines. Note that, although a duplicated explanation of the operation described in Fig. 6 will be omitted, the operation described in Fig. 6 may be applied as appropriate.

[0072] In step S101, the UE 100 transmits an L3 (RRC) Measurement Report to the gNB 200a. The gNB 200a receives the L3 (RRC) Measurement Report.

[0073] In step S102, gNB200a determines to use inter-gNB LTM based on the L3 (RRC) Measurement Report in step S101 and starts preparing a candidate cell. Here, it is assumed that the second cell of gNB200b is determined as the candidate cell.

[0074] In step S103, the gNB 200a transmits a request message (LTM HO Request) indicating a serving cell change by LTM to the gNB 200b. The gNB 200b receives the request message (LTM HO Request). The request message (LTM HO Request) includes an LTM indicator and may be a Handover Request message used in a general handover. Alternatively, the request message (LTM HO Request) may be a new message different from the Handover Request message, for example, an LTM Handover Request message. The request message (LTM HO Request) may include information indicating whether or not early synchronization configuration, i.e., configuration of CFRA resources for early synchronization is necessary (or may be information proposing configuration of early synchronization). Note that the request message (LTM HO Request) may include RRC configuration information of the UE 100 and a cell identifier indicating the second cell, as in a general handover.

[0075] In step S104, the gNB200b determines whether or not to accept the request of step S103 (Admission control). Here, the description will proceed assuming that the request of step S103 is accepted. In this case, the gNB200b may configure a CFRA resource for early synchronization in the second cell. Note that, if the request of step S103 is rejected, the gNB200b may send a rejection message to the gNB200a. The rejection message may include information indicating that inter-gNB LTM cannot be used.

[0076] In step S105, the gNB200b transmits an acknowledgement message (LTM HO Request Ack) indicating acceptance of the request of step S103 to the gNB200a. The gNB200a receives the acknowledgement message (LTM HO Request Ack). The acknowledgement message (LTM HO Request Ack) includes an LTM indicator and may be a Handover Request Ack message used in a general handover. Alternatively, the acknowledgement message (LTM HO Request Ack) may be a new message different from the Handover Request Ack message, for example, an LTM Handover Request Ack message. The acknowledgement message (LTM HO Request Ack) may include information indicating an early synchronization CFRA resource (e.g., an RA preamble and / or a PRACH (Physical Random Access Channel) resource) configured by the gNB200b for the second cell. Note that the acknowledgement message (LTM HO Request Ack) may include RRC reconfiguration information (RRC Reconfiguration) of the UE100 to be applied in the second cell, as in a general handover.

[0077] In step S106, the gNB 200a transmits an RRC Reconfiguration message including an LTM candidate cell configuration (LTM Candidate Configuration) of the second cell to the UE 100. The UE 100 receives the RRC Reconfiguration message. The RRC Reconfiguration message may include information indicating the early synchronization CFRA resource configured by the gNB 200b for the second cell.

[0078] In step S107, the UE 100 saves the LTM candidate cell setting and transmits an RRC Reconfiguration Complete message to the gNB 200a. The gNB 200a receives the RRC Reconfiguration Complete message.

[0079] In step S108, the UE 100 may transmit an L1 measurement report (or an L3 measurement report) to the gNB 200a for the gNB 200a to determine early synchronization. The gNB 200a may receive the L1 measurement report (or the L3 measurement report).

[0080] In step S109, gNB200a may make a decision to perform early synchronization.

[0081] In step S110, the gNB 200a may transmit an Early sync CFRA Request message to the gNB 200b, which is a request message requesting preparation of a CFRA resource for early synchronization, specifically, configuration and / or activation (validation) of a CFRA resource for early synchronization. The gNB 200b may receive the request message (Early sync CFRA Request message). The request message (Early sync CFRA Request message) may include an identifier (Xn-AP UE ID) for identifying the UE 100 and / or an identifier (cell ID) for identifying the second cell.

[0082] In step S111, gNB200b may prepare CFRA resources for early synchronization.

[0083] In step S112, the gNB 200b may transmit a notification message indicating that the CFRA resource for early synchronization has been prepared, for example, an Early sync CFRA Request Ack message, to the gNB 200a. The gNB 200a may receive the notification message (Early sync CFRA Request Ack message).

[0084] In step S113, the gNB 200a transmits a PDCCH order to the UE 100 and instructs the UE 100 to perform CFRA for early synchronization. The UE 100 receives the PDCCH order. The PDCCH order may include information (Target cell indicator) for identifying the second cell as a target of the CFRA. The information may be the cell ID (or cell index) of the second cell. The information may be an index in the list of LTM candidate cell settings in step S106. The information may be information (index) that specifies the TRP corresponding to the second cell.

[0085] In step S114, the UE 100 may perform early synchronization of the downlink (DL) with the second cell. For example, the UE 100 performs timing synchronization using the SSB (PSS / SSS) of the second cell. Note that the UE 100 may have performed DL synchronization before this point.

[0086] In step S115, the UE 100 transmits a CFRA, specifically, an RA preamble on a PRACH, to the second cell specified in the PDCCH order in order to perform early synchronization of the uplink (UL) with the second cell. The gNB 200b receives the RA preamble. The UE 100 identifies the CFRA resource (for example, an RA preamble and / or a PRACH resource) based on information set in the SIB or the like and information such as a "Random Access Preamble index" and a "PRACH Mask Index" in the PDCCH order.

[0087] In step S116, the gNB 200b may transmit an RAR including a TA value derived based on the RA preamble to the UE 100. The UE 100 may receive the RAR. Step S116 may be an optional step that is executed only if there is a setting from the gNB 200a (e.g., the setting in step S106). The UE 100 may transmit a notification (Early Sync Complete) indicating that UL early synchronization with the second cell has been completed to the gNB 200a (step S117). The notification (Early Sync Complete) may include the TA value notified in the RAR.

[0088] In step S118, the gNB 200b may transmit to the gNB 200a a notification message (Early Sync Complete) indicating that UL early synchronization with the UE 100 has been completed. The gNB 200a may receive the notification message (Early Sync Complete). The notification message (Early Sync Complete) may include the TA value derived based on the RA preamble of step S115.

[0089] In step S119, the UE 100 transmits an L1 measurement report to the gNB 200a. The gNB 200a receives the L1 measurement report.

[0090] In step S120, when gNB200a determines that the possibility of LTM execution has increased, for example, based on the L1 measurement report of step S119, it may transmit a UL resource request message to gNB200b. gNB200b may receive the request message. The UL resource request may be a request for preparation or activation of CFRA resources. The UL resource request may be a request for preparation or execution of UL grant transmission to UE100. The UL resource request may be a request for preparation or activation of UL configured grant (CG) resources. Note that the transmission of the request message in step S120 may be simultaneous with the LTM execution decision in step S121. The transmission may be after the LTM execution decision in step S121.

[0091] In step S121, gNB200a decides to perform LTM based on the L1 measurement report of step S119.

[0092] In step S122, in response to the LTM execution decision, the gNB 200a transmits a Cell switch command (MAC CE) to the UE 100. The UE 100 receives the Cell switch command. The Cell switch command may include the TA value notified to the gNB 200a in step S117 or S118.

[0093] In step S123, in response to the reception of the Cell switch command, the UE 100 detaches from the first cell (source cell) and applies the LTM candidate cell setting of the second cell (target cell).

[0094] In step S124, if the Cell switch command does not include a TA value (a valid TA value), the UE 100 may perform a random access procedure for the second cell.

[0095] In step S125, the UE 100 transmits an RRC Reconfiguration Complete message to the second cell. The gNB 200b receives the RRC Reconfiguration Complete message.

[0096] In step S126, the gNB 200b transmits DCI including a CRC (Cyclic Redundancy Code) scrambled with the C-RNTI assigned to the UE 100 to the UE 100 on the PDCCH, and may transmit a Contention Resolution MAC CE to the UE 100 on the PDSCH assigned by the DCI. The UE 100 may receive the DCI and the Contention Resolution MAC CE.

[0097] In step S127, gNB200b may transmit a notification message (LTM HO Success) to gNB200a indicating that the inter-network node LTM to the second cell has been completed. gNB200a may receive the notification message (LTM HO Success).

[0098] (4) Other Embodiments The above-described operational flows are not limited to being implemented independently, but can be implemented by combining two or more operational flows. For example, some steps of one operational flow may be added to another operational flow, or some steps of one operational flow may be replaced with some steps of another operational flow. In each flow, it is not necessary to execute all steps, and only some steps may be executed. Furthermore, the order of steps in each flow may be changed as appropriate.

[0099] In the above-described embodiments and examples, an example in which the base station is an NR base station (gNB) has been described, but the base station may be an LTE base station (eNB) or a 6G base station. The base station may also be a relay node such as an IAB (Integrated Access and Backhaul) node. The base station may also be a DU of the IAB node. The UE 100 may also be an MT (Mobile Termination) of the IAB node.

[0100] That is, the UE 100 may be a terminal function unit (a type of communication module) for a base station to control a repeater that relays signals. Such a terminal function unit is referred to as an MT. Examples of the MT include, in addition to the IAB-MT, an NCR (Network Controlled Repeater)-MT and a RIS (Reconfigurable Intelligent Surface)-MT.

[0101] The term "network node" primarily refers to a base station, but may also refer to a core network device or a part of a base station (CU, DU, or RU). A network node may also be configured by a combination of at least a part of a core network device and at least a part of a base station.

[0102] A program may be provided that causes a computer to execute each process performed by the UE 100 or the gNB 200. The program may be recorded on a computer-readable medium. Using a computer-readable medium, the program can be installed on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transitory recording medium. The non-transitory recording medium is not particularly limited, and may be, for example, a recording medium such as a CD-ROM or a DVD-ROM. Furthermore, circuits that execute each process performed by the UE 100 or the gNB 200 may be integrated, and at least a portion of the UE 100 or the gNB 200 may be configured as a semiconductor integrated circuit (chip set, SoC: System on a chip).

[0103] The functions performed by the UE 100 or the gNB 200 (network node) may be implemented in circuitry or processing circuitry, including general-purpose processors, application-specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), CPUs (Central Processing Units), conventional circuits, and / or combinations thereof, programmed to perform the described functions. A processor includes transistors and other circuits and is considered to be circuitry or processing circuitry. A processor may also be a programmed processor that executes a program stored in memory. In this specification, circuitry, unit, or means refers to hardware that is programmed to perform the described functions or hardware that executes them. The hardware may be any hardware disclosed herein or any hardware known to be programmed or capable of performing the described functions. If the hardware is a processor, the circuitry, means, or unit is a combination of hardware and software used to configure the hardware and / or processor.

[0104] As used in this disclosure, the terms "based on" and "depending on / in response to" do not mean "based only on" or "depending only on," unless expressly stated otherwise. The term "based on" means both "based only on" and "based at least in part on." Similarly, the term "depending on" means both "depending only on" and "depending at least in part on." The terms "include," "comprise," and variations thereof do not mean including only the listed items, but may mean including only the listed items or may include additional items in addition to the listed items. Additionally, the term "or," as used in this disclosure, is not intended to mean an exclusive or. Furthermore, any reference to elements using designations such as "first," "second," etc., as used in this disclosure does not generally limit the quantity or order of those elements. These designations may be used herein as a convenient method of distinguishing between two or more elements. Thus, a reference to a first and a second element does not imply that only two elements may be employed therein or that the first element must precede the second element in some way. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles shall include the plural unless the context clearly indicates otherwise.

[0105] The above describes the embodiments in detail with reference to the drawings, but the specific configuration is not limited to that described above, and various design changes can be made within the scope that does not deviate from the gist of the invention.

[0106] This application claims priority from Japanese Patent Application No. 2023-170459 (filed September 29, 2023), the entire contents of which are incorporated herein by reference.

[0107] (5) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0108] (Supplementary Note 1) A communication method executed by a network node in a mobile communication system, comprising: performing wireless communication with a user equipment in a Radio Resource Control (RRC) Connected state in a first cell of the network node; and transmitting a request message to another network node to request a change of a serving cell of the user equipment from the first cell to a second cell of the other network node, wherein, when the serving cell change is performed by LTM (L1 / L2-Triggered Mobility), the network node transmits the request message indicating that the serving cell change is by LTM, and the LTM is a procedure for instructing a cell change from the network node to the user equipment by a Medium Access Control (MAC) control element (CE).

[0109] (Supplementary Note 2) The communication method according to Supplementary Note 1, wherein the request message is a handover request message usable in a handover procedure for instructing a handover from the network node to the user equipment by an RRC message, and the request message indicating that the serving cell change is due to LTM is the handover request message including an LTM indicator.

[0110] (Supplementary Note 3) The communication method according to Supplementary Note 1, wherein the request message indicating the serving cell change by the LTM is a request message for LTM that is different from a handover request message that can be used in a handover procedure that instructs a handover from the network node to the user equipment by an RRC message.

[0111] (Supplementary Note 4) The communication method according to any one of Supplementary Notes 1 to 3, wherein the request message indicating the serving cell change by the LTM includes information indicating whether the other network node needs to configure a contention-free random access (CFRA) resource, and the CFRA resource is used in early synchronization performed by the user equipment to the second cell before a cell switch instruction is issued by the MAC CE.

[0112] (Supplementary Note 5) The communication method according to any one of Supplementary Notes 1 to 4, further comprising transmitting another request message to the other network node requesting that the other network node configure or activate a contention-free random access (CFRA) resource, the CFRA resource being used in early synchronization performed by the user equipment to the second cell before a cell switch indication by the MAC CE.

[0113] (Supplementary Note 6) The communication method according to any one of Supplementary Notes 1 to 5, further comprising transmitting a Physical Random Access Channel (PDCCH) command to the user equipment instructing the user equipment to perform Contention Free Random Access (CFRA), wherein the PDCCH command includes information for identifying the second cell.

[0114] (Supplementary Note 7) The communication method according to any one of Supplementary Notes 1 to 6, further comprising receiving a notification from the other network node indicating that early synchronization performed by the user equipment to the second cell has been successful.

[0115] (Supplementary Note 8) The communication method according to any one of Supplementary Notes 1 to 6, further comprising receiving a notification from the user equipment indicating that early synchronization performed by the user equipment with the second cell has been successful.

[0116] (Supplementary Note 9) A network node used in a mobile communication system, comprising: a first communication unit that performs wireless communication with a user equipment in a Radio Resource Control (RRC) Connected state in a first cell of the network node; and a second communication unit that transmits a request message to another network node to request a change of a serving cell of the user equipment from the first cell to a second cell of another network node, wherein, when the serving cell change is performed by LTM (L1 / L2-Triggered Mobility), the second communication unit transmits the request message indicating that the serving cell change is by LTM, and the LTM is a procedure for instructing a cell change from the network node to the user equipment by a Medium Access Control (MAC) Control Element (CE).

[0117] 1: Mobile communication system 5: Network 10: RAN 20: CN 100: UE 110: Receiving unit 120: Transmitting unit 130: Control unit 200: gNB 210: Transmitting unit 220: Receiving unit 230: Control unit 240: Backhaul communication unit

Claims

1. A communication method executed by a network node in a mobile communication system, comprising: performing wireless communication with a user equipment in a radio resource control (RRC) connected state in a first cell of the network node; and transmitting a request message to another network node to request a change of a serving cell of the user equipment from the first cell to a second cell of another network node, wherein, when the serving cell change is performed by LTM (L1 / L2-Triggered Mobility), the network node transmits the request message indicating that the serving cell change is by LTM, and the LTM is a procedure for instructing a cell change from the network node to the user equipment by a medium access control (MAC) control element (CE).

2. The communication method according to claim 1, wherein the request message is a handover request message usable in a handover procedure for instructing a handover from the network node to the user equipment by an RRC message, and the request message indicating that a serving cell change is due to the LTM is the handover request message including an LTM indicator.

3. The communication method according to claim 1, wherein the request message indicating the serving cell change by the LTM is a request message for LTM that is different from a handover request message that can be used in a handover procedure that instructs a handover from the network node to the user equipment by an RRC message.

4. The communication method according to any one of claims 1 to 3, wherein the request message indicating the serving cell change by the LTM includes information indicating whether or not the other network node needs to configure a contention-free random access (CFRA) resource, and the CFRA resource is used in early synchronization performed by the user equipment to the second cell before a cell switch instruction is issued by the MAC CE.

5. The communication method according to claim 1, further comprising: transmitting another request message to the other network node requesting that the other network node configure or activate a Contention Free Random Access (CFRA) resource, the CFRA resource being used in early synchronization performed by the user equipment to the second cell before a cell switch indication by the MAC CE.

6. The communication method according to any one of claims 1 to 3, further comprising transmitting a physical random access channel (PDCCH) command to the user equipment instructing the user equipment to execute a contention-free random access (CFRA), the PDCCH command including information for identifying the second cell.

7. A method according to any one of claims 1 to 3, further comprising receiving from the further network node a notification indicating successful early synchronization by the user equipment to the second cell.

8. The method of any one of claims 1 to 3, further comprising receiving a notification from the user equipment indicating that early synchronization performed by the user equipment to the second cell has been successful.

9. A network node used in a mobile communication system, comprising: a first communication unit that performs wireless communication with a user equipment in a radio resource control (RRC) connected state in a first cell of the network node; and a second communication unit that transmits a request message to the other network node to request a change of a serving cell of the user equipment from the first cell to a second cell of another network node, wherein when the serving cell change is performed by LTM (L1 / L2-Triggered Mobility), the second communication unit transmits the request message indicating that the serving cell change is by LTM, and the LTM is a procedure for instructing a cell change from the network node to the user equipment by a medium access control (MAC) control element (CE).

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