Communication methods, network nodes, mobile communication systems, programs, and chipsets

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

Technical Field

[0001] The present disclosure relates to a communication method and a user device used in a mobile communication system.

Background Art

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

[0003] On the other hand, in 3GPP Release 18 (3GPP Release 18), the formulation of the technical specifications of LTM (L1 / L2-Triggered Mobility) is in progress. In LTM, a network node receives a layer 1 (L1) measurement report from a user equipment, and based on this, the network node changes the serving cell of the user equipment by a cell switching command that signals the user equipment by a media access control (MAC) control element (CE).

[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).

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

Summary of the Invention

[0006] This disclosure relates to a communication method and network nodes for realizing LTM between network nodes.

[0007] A first aspect of the present disclosure is a communication method performed by a network node in a mobile communication system, comprising: performing wireless communication with a user device in a radio resource control (RRC) connected state in a first cell of the network node; and transmitting a request message from the first cell to a second cell of another network node to request a change in the serving cell of the user device. When the network node changes the serving cell by LTM (L1 / L2-Triggered Mobility), it transmits the request message indicating that the serving cell change is by LTM. LTM is a procedure in which the network node instructs the user device to switch cells using a media access control (MAC) control element (CE).

[0008] A network node according to a second aspect of this disclosure is a network node used in a mobile communication system, comprising: a first communication unit that performs wireless communication with a user device 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 from the first cell to a second cell of another network node to request a change in the serving cell of the user device. The second communication unit transmits the request message indicating that the serving cell change is performed by LTM (L1 / L2-Triggered Mobility) when the serving cell change is performed by LTM. The LTM is a procedure that instructs the user device to switch cells via a media access control (MAC) control element (CE) from the network node. [Brief explanation of the drawing]

[0009] [Figure 1] This is a diagram showing an example configuration of a mobile communication system according to the embodiment. [Figure 2] This figure shows an example configuration of a UE (User Equipment) according to the embodiment. [Figure 3] This figure shows an example configuration of a gNB (Network Node) according to the embodiment. [Figure 4] This diagram shows the protocol stack configuration of the user plane wireless interface that handles data. [Figure 5] This diagram shows the protocol stack configuration of the wireless interface of the control plane that handles signaling (control signals). [Figure 6] This figure shows an example of a procedure for the Long-Term Manual (LTM) currently under specification development in 3GPP Release 18. [Figure 7] This is a diagram illustrating the operation scenario of the mobile communication system according to the embodiment. [Figure 8] This figure shows a specific example of the operation of the mobile communication system according to the embodiment. [Modes for carrying out the invention]

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

[0011] (1) Configuration of the mobile communication system Figure 1 shows an example configuration of a mobile communication system 1 according to an embodiment. The mobile communication system 1 conforms to the 5th Generation System (5GS) of the 3GPP standard. In the following explanation, 5GS will be used as an example, but the mobile communication system may also incorporate an LTE (Long Term Evolution) system at least partially. The mobile communication system may also incorporate a 6th Generation (6G) system at least partially.

[0012] The mobile communication system 1 comprises User Equipment (UE) 100, a 5G radio access network (NG-RAN) 10, and a 5G core network (5GC) 20. Hereinafter, NG-RAN 10 may be simply referred to as RAN 10, and 5GC 20 may be simply referred to as the core network (CN) 20. RAN 10 and CN 20 constitute the network 5 of the mobile communication system 1.

[0013] UE100 is a mobile wireless communication device. UE100 can be any device used by a user. For example, UE100 can be a mobile phone terminal (including a smartphone) or tablet terminal, a notebook PC, a communication module (including a communication card or chipset), a sensor or device attached to a sensor, a vehicle or device attached to a vehicle (Vehicle UE), or an aircraft or device attached to an aircraft (Aerial UE).

[0014] NG-RAN10 includes base stations (referred to as "gNBs" in 5G systems) 200, which are a type of network node. The gNBs 200 are interconnected via the Xn interface, which is an inter-base station interface. Each gNB 200 manages one or more cells. The gNB 200 performs wireless communication with UEs 100 that have established a connection with its own cell. The gNB 200 has radio resource management (RRM) functions, user data routing functions (hereinafter simply referred to as "data"), measurement and control functions for mobility control and scheduling, etc. "Cell" is used as a term to indicate the smallest unit of a wireless communication area. "Cell" is also used as a term to indicate a function or resource that performs wireless communication with a UE 100. One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

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

[0016] The 5GC 20 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 performs data transfer control. The AMF and the UPF are connected to the gNB 200 via the NG interface, which is an interface between the base station and the core network.

[0017] Figure 2 is a diagram showing a configuration example of the 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 constitute a wireless communication unit that performs wireless communication with the gNB 200.

[0018] The receiving unit 110 performs various receptions under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts the wireless signal received by the antenna into a baseband signal (received signal) and outputs it 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 the baseband signal (transmitted signal) output by the control unit 130 into a wireless signal and transmits it from the antenna.

[0020] The control unit 130 performs various controls and processes in the UE 100. Such processes include the processes of each layer described later. The operations of the UE 100 described above and below may be operations under the control of 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 for the processes by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals, etc. The CPU executes programs stored in the memory to perform various processes.

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

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

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

[0024] The control unit 230 performs various control and processing operations in the gNB200. Such processing includes processing in each layer described later. The operation of the gNB200 described above and below may also be controlled by 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 for processing by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation, demodulation, encoding, and decoding of baseband signals. The CPU executes programs stored in memory and performs various processing operations.

[0025] The backhaul communication unit 240 is connected to an adjacent base station via the Xn interface, which is an inter-base station interface. The backhaul communication unit 240 is connected to the AMF / UPF300 via the NG interface, which is an inter-base station-core network interface. The gNB200 may consist of a CU (Central Unit) and a DU (Distributed Unit) (i.e., functionally separated), and the two units may be connected by the F1 interface, which is a fronthaul interface.

[0026] Figure 4 shows the configuration of the protocol stack for the user plane's wireless interface that handles data.

[0027] The user plane radio interface protocol consists of a physical (PHY) layer, a MAC (Medium Access Control) layer, an RLC (Radio Link Control) layer, a PDCP (Packet Data Convergence Protocol) layer, and an SDAP (Service Data Adaptation Protocol) 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 UE100's PHY layer and the gNB200's PHY layer via a physical channel. The UE100's PHY layer receives downlink control information (DCI) transmitted from the gNB200 over the physical downlink control channel (PDCCH). Specifically, the UE100 performs blind decoding of the PDCCH using a Radio Network Temporary Identifier (RNTI) and acquires the successfully decoded DCI as the DCI addressed to its own UE. The DCI transmitted from the gNB200 has a CRC parity bit added, which is scrambled by the RNTI.

[0029] The MAC layer performs data priority control, retransmission processing using Hybrid Automatic Repeat request (HARQ), and random access procedures. Data and control information are transmitted between the MAC layer of the UE100 and the MAC layer of the gNB200 via the transport channel. The MAC layer of the gNB200 includes a scheduler. The scheduler determines the transport format for the up and down links (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be allocated to the UE100.

[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 UE100's RLC layer and the gNB200's RLC layer via a logical channel.

[0031] The PDCP layer performs header compression / decompression, encryption / decryption, etc.

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

[0033] Figure 5 shows the configuration of the protocol stack of the wireless interface of the control plane that handles signaling (control signals).

[0034] The control plane's wireless interface protocol stack includes an RRC (Radio Resource Control) layer and a NAS (Non-Access Stratum) layer, instead of the SDAP layer shown in Figure 4.

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

[0036] The NAS layer (also simply referred to as "NAS"), located above the RRC layer, handles session management and mobility management, among other things. NAS signaling is transmitted between the UE100's NAS layer and the AMF300A's NAS layer. The UE100 also has application layers and other components in addition to its wireless interface protocol. Furthermore, the layer below the NAS layer is called the AS layer (also simply referred to as "AS").

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

[0038] LTM is a technique for reducing mobility delays (specifically, serving cell change delays) compared to typical handover procedures by triggering cell switching with signaling at lower layers, specifically Layer 1 (L1) and / or Layer 2 (L2). In a typical handover procedure, a Measurement Report message, which is an RRC message, is sent from UE100 to gNB200. Based on this Measurement Report message, gNB200 decides to hand over UE100 and instructs the handover by sending a Handover Command message (specifically, an RRC Reconfiguration message) from gNB200 to UE100.

[0039] In contrast, with LTM, firstly, the gNB200 prepares an LTM candidate cell setting for the target cell and provides this LTM candidate cell setting to the UE100 via RRC signaling. Secondly, the UE100 performs synchronization with the candidate cell using early sync. Thirdly, the gNB200 receives an L1 measurement report from the UE100, decides to switch to the target cell based on the L1 measurement report, and sends a Cell Switch Command indicating the target cell (LTM candidate cell setting) to the UE100 via MAC CE. The cell switch trigger is transmitted via MAC CE, which includes at least the candidate setting index along with the beam indicator. Fourthly, the UE100 changes the serving cell in response to the cell switch command from the gNB200 (source cell). In this way, cell switching is triggered by the gNB200 selecting the LTM candidate cell setting as the target setting. LTM candidate cell settings can be added, modified, and released by the gNB200 via RRC signaling.

[0040] The following principles apply to LTM:

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

[0042] • When the complete candidate cell settings are applied, the current UE settings are replaced when switching cells. The reset procedure performs the replacement, but does not necessarily reset the MAC, RLC, or PDCP layers.

[0043] • The user plane will continue without resetting if configured with RRC signaling, in order to avoid additional delays in data recovery.

[0044] • Security updates are not provided in LTM versions.

[0045] • LTM can be performed between subsequent LTM candidate cell settings without RRC reconfiguration. In other words, UE100 does not release other LTM candidate cell settings after an LTM has been triggered.

[0046] Figure 6 shows an example of an LTM procedure currently being specified in 3GPP Release 18. In the example shown, UE100 performs a cell switch from the first cell (source cell) to the second cell of gNB200. Here, the first and second cells may be composed of different TRPs (Transmission and Reception Points). Until the cell switch by LTM is decided, the second cell is also referred to as the "candidate cell (or LTM candidate cell)," and after the cell switch by LTM is decided, the second cell is also referred to as the "target cell."

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

[0048] In step S2, UE100 sends 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 begins preparing candidate cells.

[0050] In step S4, gNB200 sends an RRC Reconfiguration message to UE100 that includes the LTM Candidate Configuration for one or more candidate cells.

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

[0052] In step S6, UE100 may perform synchronization with the candidate cell before receiving the cell switching command. Such synchronization is called early sync. Here, UE100 may perform early timing advance (TA) acquisition on the candidate cell requested by gNB200 (source cell) before receiving the cell switching command in step S9. This is done by competition-free random access (CFRA) triggered by a PDCCH order from the source cell. Note that if 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. UE100 sends a random access preamble (RA preamble) to the specified candidate cell. In order to minimize the interruption of source cell communication to the candidate cell by CFRA, in early sync, UE100 does not receive a random access response (RAR) from the candidate cell for the purpose of acquiring the TA value. The TA value of the candidate cell (target cell) is indicated by the cell switching command in step S9. The TA value is used to adjust the uplink transmission timing of UE100.

[0053] In step S7, UE100 performs Layer 1 (L1) measurements at the configured candidate cells and sends a physical layer measurement report (L1 Measurement Report) to gNB200. The L1 Measurement Report is transmitted and received at L1, which is the PHY layer. For example, UE100 sends L1-RSRP and / or L1-SINR to gNB200 via PUCCH (Physical Up-link Control Channel) and / or PUSCH.

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

[0055] In step S9, the gNB200 sends a Cell Switch Command (MAC CE) to the UE100, which includes the candidate setting index for the target cell. The Cell Switch Command may include the TA value obtained through early synchronization.

[0056] In step S10, UE100 switches to the target cell settings. Specifically, UE100 detaches from the source cell (first cell) and applies the target cell settings.

[0057] In step S11, if the cell switch requires the execution of a random access procedure (for example, if the Cell Switch Command does not contain a valid TA value), UE100 executes the random access procedure on the target cell. Note that UE100 executes the TA of the target cell when switching cells. value If it is not necessary to obtain the TA value (for example, if the Cell Switch Command contains a valid TA value), the random access procedure can be skipped.

[0058] In step S12, UE100 indicates that the cell switch to the target cell has been successfully completed. Subsequently, UE100 may execute steps S6 through S12 multiple times for subsequent LTM cell switches based on the settings provided in step S4.

[0059] (3) Operation of the mobile communication system 3GPP Release 18's LTM (Live Term Change) is limited to serving cell changes between cells belonging to the same gNB200 (same CU). Therefore, there is a problem in that serving cell changes between cells belonging to different gNB200s (different CUs) cannot be achieved by LTM. Such LTM may be referred to as inter-network node LTM, specifically inter-gNB LTM or inter-CU LTM. The following embodiment describes the operation for realizing inter-network node LTM.

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

[0061] UE100 performs a serving cell change from the first cell (source cell) of the source gNB, gNB200a, to the second cell of gNB200b. Until the cell switchover by LTM is decided, the second cell is also referred to as the "candidate cell (or LTM candidate cell)," and after the cell switchover by LTM is decided, the second cell is also referred to as the "target cell." An Xn interface is established between gNB200a and gNB200b. Communication between gNB200a and gNB200b is assumed to take place over the Xn interface.

[0062] In this embodiment, gNB200a communicates wirelessly with UE100 in an RRC-connected state in its first cell. gNB200a sends a request message from its first cell to gNB200b's second cell to request a change in the serving cell of UE100. If the serving cell change is performed by LTM, gNB200a sends a request message to gNB200b indicating that the serving cell change is performed by LTM. LTM is a procedure in which gNB200a instructs UE100 to switch cells via MAC CE.

[0063] This allows gNB200b to understand, based on the request message from gNB200a, that a serving cell change via LTM (Long-Term Modification) is requested, rather than a general handover.

[0064] The request message may be a Handover Request (HO Request) message usable in a handover procedure that instructs a handover from gNB200a to UE100 via an RRC message. A request message indicating a serving cell change by LTM may be an HO Request message containing an LTM indicator. This makes it possible to reuse the HO Request message used in general handovers for inter-network node LTM, thus minimizing changes to the technical specifications.

[0065] Alternatively, a request message indicating a serving cell change by LTM may be a different LTM-specific request message than the HO Request message. The LTM-specific request message may be a request message used exclusively for LTM.

[0066] A request message indicating a serving cell change via LTM may include information indicating whether gNB200b should configure a competition-free random access (CFRA) resource. This CFRA resource is used in the early synchronization performed by UE100 on the second cell before the MAC CE's cell switchover instruction. This allows gNB200b to determine, based on the request message, whether or not to configure the CFRA resource.

[0067] gNB200a may send another request message to gNB200b requesting that gNB200b configure or activate the CFRA resources for early synchronization. This allows gNB200b to properly configure or activate the CFRA resources for early synchronization.

[0068] The gNB200a may send a PDCCH order to the UE100 instructing it to perform a CFRA (i.e., send an RA preamble). The PDCCH order may include information to identify the second cell. This makes it easier for the UE100 to determine whether the RA preamble transmission is for the first cell or the second cell.

[0069] gNB200a may receive a notification from gNB200b indicating that the early synchronization performed by UE100 with the second cell was successful. This allows gNB200a to determine whether or not the early synchronization performed by UE100 with the second cell was successful.

[0070] Alternatively, gNB200a may receive a notification from UE100 indicating that the early synchronization performed by UE100 for the second cell was successful.

[0071] Figure 8 shows a specific example of the operation of the mobile communication system 1 according to the embodiment. Steps that can be omitted in Figure 8 are indicated by dashed lines. Although redundant explanations of the operations described in Figure 6 are omitted, the operations described in Figure 6 may be applied as appropriate.

[0072] In step S101, UE100 sends the L3(RRC) Measurement Report to gNB200a. gNB200a receives the L3(RRC) Measurement Report.

[0073] In step S102, gNB200a decides to use inter-gNB LTM based on the L3(RRC) Measurement Report from step S101 and begins preparing candidate cells. Here, it is assumed that the second cell of gNB200b has been selected as the candidate cell.

[0074] In step S103, gNB200a sends an LTM HO Request message to gNB200b indicating that the serving cell change is due to LTM. gNB200b receives the LTM HO Request message. The LTM HO Request message includes an LTM Indicator and may be a Handover Request message used in a typical handover. Alternatively, the LTM HO Request message may be a new message different from the Handover Request message, such as an LTM Handover Request message. The LTM HO Request message may also include information indicating whether early synchronization settings are required, i.e., whether CFRA resources for early synchronization need to be configured (or information suggesting early synchronization settings). The LTM HO Request message may also include RRC configuration information for UE100 and a cell identifier indicating the second cell, similar to a typical handover.

[0075] In step S104, gNB200b determines whether to accept the request in step S103 (Admission control). Here, we will proceed assuming that the request in step S103 is accepted. In this case, gNB200b may configure the CFRA resource for early synchronization in the second cell. If the request in step S103 is rejected, gNB200b may send a rejection message to gNB200a. The rejection message may include information indicating that inter-gNB LTM is unavailable.

[0076] In step S105, gNB200b sends an acknowledgment message (LTM HO Request Ack) to gNB200a indicating acceptance of the request in step S103. gNB200a receives the acknowledgment message (LTM HO Request Ack). The acknowledgment message (LTM HO Request Ack) may include an LTM Indicator and may be a Handover Request Ack message used in a typical handover. Alternatively, the acknowledgment 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 acknowledgment message (LTM HO Request Ack) may include information indicating the early synchronization CFRA resources (e.g., RA preamble and / or PRACH (Physical Random Access Channel) resources) that gNB200b has set up for the second cell. The acknowledgment message (LTM HO Request Ack) may also include RRC reconfiguration information for the UE100 to be applied in the second cell, similar to a typical handover.

[0077] In step S106, gNB200a sends an RRC Reconfiguration message to UE100 that includes the LTM Candidate Configuration for the second cell. UE100 receives the RRC Reconfiguration message. The RRC Reconfiguration message may also include information indicating the early synchronization CFRA resources configured by gNB200b for the second cell.

[0078] In step S107, UE100 saves the LTM candidate cell configuration and sends an RRC Reconfiguration Complete message to gNB200a. gNB200a receives the RRC Reconfiguration Complete message.

[0079] In step S108, UE100 may send an L1 measurement report (or L3 measurement report) to gNB200a for gNB200a to make a decision about early synchronization. gNB200a may receive an L1 measurement report (or L3 measurement report).

[0080] In step S109, gNB200a may make an early synchronization decision.

[0081] In step S110, gNB200a may send an Early sync CFRA Request message to gNB200b, which is a request message requesting the preparation of CFRA resources for early synchronization, specifically, the configuration and / or activation of CFRA resources for early synchronization. gNB200b may receive the request message (Early sync CFRA Request message). The request message (Early sync CFRA Request message) may include an identifier for identifying UE100 (Xn-AP UE ID) and / or an identifier for identifying the second cell (cell ID).

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

[0083] In step S112, gNB200b may send a notification message to gNB200a indicating that the CFRA resources for early synchronization are ready, for example, an Early sync CFRA Request Ack message. gNB200a may receive the notification message (Early sync CFRA Request Ack message).

[0084] In step S113, gNB200a sends a PDCCH order to UE100, instructing UE100 to perform a CFRA for early synchronization. UE100 receives the PDCCH order. The PDCCH order may include information for identifying a second cell as the target of the CFRA (Target cell indicator). This information may be the cell ID (or cell index) of the second cell. This information may be an index in the list of LTM candidate cell settings in step S106. This information may also be information (an index) specifying the TRP corresponding to the second cell.

[0085] In step S114, UE100 may perform early synchronization of the downlink (DL) with the second cell. For example, UE100 may perform timing synchronization using the second cell's SSB (PSS / SSS). Note that UE100 may have performed DL synchronization earlier than this point.

[0086] In step S115, UE100 sends a CFRA, specifically an RA preamble on PRACH, to the second cell specified in the PDCCH order in order to perform early synchronization of the uplink (UL) with the second cell. gNB200b receives the RA preamble. UE100 identifies the CFRA resources (e.g., RA preamble and / or PRACH resources) using information set in the SIB, etc., and information such as the "Random Access Preamble index" and "PRACH Mask Index" in the PDCCH order.

[0087] In step S116, gNB200b may send a RAR to UE100 that includes the TA value derived based on the RA preamble. UE100 may receive the RAR. Step S116 may be an optional step that is performed only if there is a setting from gNB200a (for example, the setting in step S106). UE100 may send an Early Sync Complete notification to gNB200a indicating that the UL early synchronization with the second cell is complete (step S117). The Early Sync Complete notification may include the TA value notified in the RAR.

[0088] In step S118, gNB200b may send an Early Sync Complete notification message to gNB200a indicating that the UL early synchronization with UE100 has been completed. gNB200a may receive the Early Sync Complete notification message. The Early Sync Complete notification message may include the TA value derived based on the RA preamble in step S115.

[0089] In step S119, UE100 sends the L1 measurement report to gNB200a. gNB200a receives the L1 measurement report.

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

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

[0092] In step S122, gNB200a sends a Cell switch command (MAC CE) to UE100 in response to the LTM execution decision. UE100 receives the Cell switch command. The Cell switch command may include the TA value notified to gNB200a in step S117 or S118.

[0093] In step S123, upon receiving the Cell switch command, UE100 detaches from the first cell (source cell) and applies the LTM candidate cell settings of the second cell (target cell).

[0094] In step S124, if the Cell switch command does not contain a TA value (a valid TA value), UE100 may execute a random access procedure on the second cell.

[0095] In step S125, UE100 sends an RRC Reconfiguration Complete message to the second cell. gNB200b receives the RRC Reconfiguration Complete message.

[0096] In step S126, gNB200b may send a DCI containing a CRC (Cyclic Redundancy Code) scrambled with the C-RNTI assigned to UE100 to UE100 on the PDCCH, and send a Contention Resolution MAC CE to UE100 on the PDSCH assigned with the DCI. UE100 may receive the DCI and Contention Resolution MAC CE.

[0097] In step S127, gNB200b may send 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 operation flows can be performed not only independently, but also in combination of two or more operation flows. For example, some steps of one operation flow may be added to another operation flow, or some steps of one operation flow may be replaced with some steps of another operation flow. It is not necessary to execute all steps in each flow; only some steps may be executed. Furthermore, the order of steps in each flow may be changed as appropriate.

[0099] In the embodiments and examples described above, an example in which the base station is an NR base station (gNB) was described, but the base station may also be an LTE base station (eNB) or a 6G base station. Furthermore, 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 an IAB node. Furthermore, UE100 may be an MT (Mobile Termination) of an IAB node.

[0100] In other words, UE100 may be a terminal function unit (a type of communication module) for a base station to control a repeater that performs signal relay. Such a terminal function unit is called an MT. Examples of MTs other than IAB-MT include NCR (Network Controlled Repeater)-MT and RIS (Reconfigurable Intelligent Surface)-MT.

[0101] Furthermore, 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). Additionally, a network node may consist of a combination of at least a part of the 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 UE100 or gNB200. The program may be recorded on a computer-readable medium. Using a computer-readable medium, it is possible to install the program on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transient recording medium. The non-transient recording medium is not particularly limited, but may be a recording medium such as a CD-ROM or DVD-ROM. Alternatively, the circuits that execute each process performed by the UE100 or gNB200 may be integrated, and at least a part of the UE100 or gNB200 may be configured as a semiconductor integrated circuit (chipset, SoC: System on a chip).

[0103] The functions realized by UE100 or gNB200 (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 (a Central Processing Unit), conventional circuits, and / or combinations thereof, programmed to realize the described functions. A processor, including transistors and other circuits, is considered circuitry or processing circuitry. A processor may be a programmed processor that executes a program stored in memory. In this specification, circuitry, unit, and means are hardware programmed to realize or perform the described functions. Such hardware may be any hardware disclosed herein, or any hardware known to be programmed to realize or perform the described functions. If such hardware is a processor that is considered to be a type of circuitry, then such circuitry, means, or unit is a combination of hardware and software used to constitute such hardware and / or processor.

[0104] The terms "based on" and "depending on / in response to" used in this disclosure do not mean "based solely on" or "depending solely on" unless otherwise specified. "Based on" means both "based solely on" and "at least partially on." Similarly, "depending on" means both "at least partially on" and "at least partially on." The terms "include," "comprise," and their variations do not mean to include only the listed items, but may include only the listed items or may include additional items in addition to the listed items. Furthermore, the term "or" used in this disclosure is not intended to mean exclusive OR. Additionally, any reference to elements using designations such as "first," "second," etc., used in this disclosure does not generally limit the quantity or order of those elements. These designations may be used herein as a convenient way to distinguish between two or more elements. Therefore, references to the first and second elements do not imply that only two elements may be adopted therein, or that the first element must precede the second element in any way. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles shall be plural unless it is clearly indicated by the context that they are not.

[0105] Although the embodiments have been described in detail above with reference to the drawings, the specific configuration is not limited to those described above, and various design changes can be made without departing from the gist of the invention.

[0106] This application claims priority to Japanese Patent Application No. 2023-170459 (filed September 29, 2023), and all of its contents are incorporated into the specification of this application.

[0107] (5) Note The features of the above-described embodiment are noted below.

[0108] (Note 1) A communication method performed by a network node in a mobile communication system, The first cell of the network node performs wireless communication with a user device in a wireless resource control (RRC) connected state, The first cell sends a request message to the second cell of another network node to request a change in the serving cell of the user device, When the network node modifies the serving cell using LTM (L1 / L2-Triggered Mobility), it sends the request message indicating that the serving cell modification is performed by LTM. The aforementioned LTM is a procedure that instructs the user device to switch cells via a media access control (MAC) control element (CE) from the network node. Communication method.

[0109] (Note 2) The aforementioned request message is a handover request message that can be used in a handover procedure that instructs the network node to perform a handover to the user device via an RRC message. The request message indicating that the serving cell change is due to the aforementioned LTM is the handover request message that includes the LTM indicator. The communication method described in Appendix 1.

[0110] (Note 3) The request message indicating that the serving cell change is due to the aforementioned LTM is a request message for the LTM that is different from the handover request message that can be used in the handover procedure that instructs the handover from the network node to the user device via an RRC message. The communication method described in Appendix 1.

[0111] (Note 4) The request message indicating that the serving cell change is due to the aforementioned LTM includes information indicating whether the other network node needs to configure a conflicted free random access (CFRA) resource, The CFRA resource is used in the early synchronization that the user device performs with respect to the second cell before the MAC CE issues a cell switching instruction. The communication method described in any of the appendices 1 to 3.

[0112] (Note 5) The further comprises sending another request message to the other network node requesting that the other network node configure or activate a conflict-free random access (CFRA) resource, The CFRA resource is used in the early synchronization that the user device performs with respect to the second cell before the MAC CE issues a cell switching instruction. The communication method described in any of the appendices 1 to 4.

[0113] (Note 6) The system further includes transmitting a physical random access channel (PDCCH) command to the user device instructing it to perform a competition-free random access (CFRA), The PDCCH instruction includes information for identifying the second cell. The communication method described in any of the appendices 1 to 5.

[0114] (Note 7) The user device further receives a notification from another network node indicating that the early synchronization performed by the user device with respect to the second cell was successful. The communication method described in any of the appendices 1 to 6.

[0115] (Note 8) The system further includes receiving a notification from the user device indicating that the early synchronization performed by the user device on the second cell was successful. The communication method described in any of the appendices 1 to 6.

[0116] (Note 9) A network node used in a mobile communication system, A first communication unit that performs wireless communication with a user device in a wireless resource control (RRC) connected state in the first cell of the network node, The system includes a second communication unit that sends a request message from the first cell to the second cell of another network node to the other network node requesting a change in the serving cell of the user device, When the second communication unit changes the serving cell by LTM (L1 / L2-Triggered Mobility), it sends the request message indicating that the serving cell change is by LTM. The aforementioned LTM is a procedure that instructs the user device to switch cells via a media access control (MAC) control element (CE) from the network node. Network node. [Explanation of Symbols]

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

Claims

1. A communication method performed by a first network node in a mobile communication system, The first network node performs wireless communication with a user device in a wireless resource control (RRC) connected state in the first cell, The first network node transmits a handover request message to the second network node via LTM (L1 / L2-Triggered Mobility) requesting a change in the serving cell of the user device from the first cell to the second cell of the second network node. The aforementioned handover request message includes an LTM indicator. Communication method.

2. The handover request message is an LTM request message that is different from the handover request message that can be used in the handover procedure that instructs the first network node to perform a handover to the user device by an RRC message. The communication method according to claim 1.

3. The handover request message includes information indicating whether the second network node is required to configure a conflicted free random access (CFRA) resource, The CFRA resource is used in early synchronization performed by the user device with respect to the second cell before a cell switching instruction is issued by the Media Access Control (MAC) control element (CE). The communication method according to claim 1.

4. The handover request message includes information proposing that the user device set up early synchronization for the second cell. The communication method according to claim 1.

5. The first network node further comprises receiving an acknowledgment message for the handover request message from the second network node, The acknowledgment message includes information indicating the early synchronization CFRA resource set up by the second network node for the second cell. The communication method according to claim 1.

6. The first network node further transmits a physical downlink control channel (PDCCH) command to the user device instructing it to perform a competition-free random access (CFRA), The PDCCH command includes information for identifying the second cell. The communication method according to any one of claims 1 to 5.

7. The first network node further receives from the second network node a timing advance (TA) value derived from the early synchronization performed by the user device on the second cell. The communication method according to any one of claims 1 to 5.

8. The first network node further receives from the user device a timing advance (TA) value derived from the early synchronization performed by the user device on the second cell. The communication method according to any one of claims 1 to 5.

9. The first network node further comprises receiving a notification message from the second network node indicating that the user device has successfully handed over to the second cell. The communication method according to any one of claims 1 to 5.

10. A network node used in a mobile communication system, A first communication unit that performs wireless communication with a user device in a wireless resource control (RRC) connected state in the first cell of the network node, The system includes a second communication unit that transmits a handover request message to the second cell of another network node, requesting a change in the serving cell of the user device, from the first cell to the second cell of another network node, using LTM (L1 / L2-Triggered Mobility), The aforementioned handover request message includes an LTM indicator. Network node.

11. comprising a user device and a network node connectable to the user device, The network node communicates wirelessly with the user device in the first cell, which is in a wireless resource control (RRC) connected state. The network node sends a handover request message to the other network node requesting a change of the serving cell of the user device from the first cell to the second cell of the other network node using LTM (L1 / L2-Triggered Mobility). The aforementioned handover request message includes an LTM indicator. Mobile communication system.

12. A network node used in a mobile communication system, The first cell performs wireless communication with a user device in a Wireless Resource Control (RRC) connected state, The process involves sending a handover request message to the other network node via LTM (L1 / L2-Triggered Mobility) requesting a change in the serving cell of the user device from the first cell to the second cell of the other network node, The process of including the LTM indicator in the aforementioned handover request message is executed. program.

13. A chipset for a network node used in a mobile communication system, The first cell performs wireless communication with a user device in a Wireless Resource Control (RRC) connected state, The process involves sending a handover request message to the other network node via LTM (L1 / L2-Triggered Mobility) requesting a change in the serving cell of the user device from the first cell to the second cell of the other network node, The process of including the LTM indicator in the aforementioned handover request message and executing Chipset.

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