Communication method and network node

Abstract

A communication method performed by a network node in a mobile communication system includes the following steps: performing wireless communication with a user equipment (UE) in an RRC (Relational Radio Communication) connection state in a first cell of the network node; and sending a request message to another network node requesting that the UE's serving cell be changed from the first cell to a second cell of the other network node. When changing the serving cell via LTM (Local Time Management), the network node sends a request message indicating the change of serving cell via LTM. LTM is the process of instructing the UE to perform cell handover via MAC CE (Machine Interface CE).

Description

Technical Field

[0001] This disclosure relates to a communication method and user equipment used in a mobile communication system. Background Technology

[0002] The 3rd Generation Partnership Project (3GPP; trademark name, hereinafter the same) has defined the technical specifications for New Radio (NR) as the fifth-generation (5G) radio access technology. In mobile communication systems within 3GPP, a change of serving cell for a user equipment in a Radio Resource Control (RRC) connection state is indicated by sending a message (so-called a handover command) from the network node to the user equipment at the RRC layer corresponding to Layer 3 (L3).

[0003] On the other hand, in 3GPP Release 18, the standardization of the L1 / L2 Triggered Mobility (LTM) specification is underway. LTM is a process in which a network node receives a Layer 1 (L1) measurement report from a user equipment (UE), and based on the L1 measurement report, the network node changes the UE's serving cell via a cell handover command, wherein the cell handover command is sent to the UE by the network node through a Media Access Control (MAC) control element (CE) signaling.

[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., LTM between network nodes).

[0005] Reference List

[0006] Non-patent literature

[0007] Non-patent literature 1: 3GPP contribution R2-2309335 Summary of the Invention

[0008] This disclosure relates to a communication method and network node for implementing LTM between network nodes.

[0009] According to a first aspect of this disclosure, a communication method is a communication method performed by a network node in a mobile communication system, comprising the steps of: performing wireless communication with a user equipment (UE) in a radio resource control (RRC) connection state in a first cell of the network node; and sending a request message to another network node requesting that the UE's serving cell be changed from the first cell to a second cell of the other network node. The network node is configured to send a request message indicating a change of serving cell via LTM when a mobility transfer (LTM) change of serving cell is triggered via L1 / L2. LTM is a process of instructing the UE to hand over a cell via a media access control (MAC) control element (CE).

[0010] According to a second aspect of this disclosure, a network node used in a mobile communication system includes: a first communicator configured to perform wireless communication with a user equipment (UE) in a radio resource control (RRC) connection state in a first cell of the network node; and a second communicator configured to send a request message to another network node requesting that the UE's serving cell be changed from the first cell to a second cell of the other network node. The second communicator is configured to send a request message indicating a change of serving cell via LTM when a change of serving cell is triggered via L1 / L2-triggered mobility (LTM). LTM is a process of instructing a cell handover from the network node to the UE via a media access control (MAC) control element (CE). Attached Figure Description

[0011] Figure 1 This is a configuration example diagram of a mobile communication system according to an embodiment.

[0012] Figure 2 This is a configuration example diagram of a UE (User Equipment) according to an embodiment.

[0013] Figure 3 This is a diagram illustrating a configuration example of a gNB (network node) according to an embodiment.

[0014] Figure 4 This is a configuration diagram showing the protocol stack of the radio interface for the user plane that processes data.

[0015] Figure 5 This is a configuration diagram of the protocol stack of the radio interface of the control plane that processes signaling (control signals).

[0016] Figure 6 This is an example diagram illustrating the LTM process, which is being developed as a specification in 3GPP Release 18.

[0017] Figure 7 This is a diagram illustrating the operational scenario of a mobile communication system according to an embodiment.

[0018] Figure 8 This is a specific example diagram illustrating the operation of a mobile communication system according to an embodiment. Detailed Implementation

[0019] According to an embodiment, a mobile communication system is described herein with reference to the accompanying drawings. In the description of the drawings, the same or similar reference numerals denote the same or similar parts.

[0020] (1) Configuration of mobile communication system

[0021] Figure 1 This diagram illustrates a configuration example of a mobile communication system 1 according to an embodiment. The mobile communication system 1 conforms to the 3GPP standard for a fifth-generation system (5GS). The following description uses 5GS as an example, but a Long Term Evolution (LTE) system can be applied at least partially to the mobile communication system. Alternatively, a sixth-generation (6G) system can be applied at least partially to the mobile communication system.

[0022] Mobile communication system 1 includes user equipment (UE) 100, a 5G radio access network (Next Generation Radio Access Network (NG-RAN)) 10, and a 5G core network (5GC) 20. In the following text, NG-RAN 10 may be simply referred to as RAN 10. 5GC 20 may be simply referred to as core network (CN) 20. RAN 10 and CN 20 constitute network 5 of mobile communication system 1.

[0023] UE 100 is a mobile wireless communication device. UE 100 can be any device as long as it is used by a user. Examples of UE 100 include mobile phone terminals (including smartphones) or tablet terminals, laptop PCs, communication modules (including communication cards or chipsets), sensors or devices mounted on sensors, vehicles or devices mounted on vehicles (vehicle UE), and flying objects or devices mounted on flying objects (airborne UE).

[0024] NG-RAN 10 includes base stations (referred to as "gNBs" in 5G systems) 200 as network nodes. gNBs 200 are interconnected via an Xn interface, which serves as an inter-base station interface. Each gNB 200 manages one or more cells. gNBs 200 perform wireless communication with UE 100, which has established a connection with a cell of the gNB 200. gNBs 200 have radio resource management (RRM) functions, functions for routing user data (hereinafter referred to as "data"), measurement and control functions for mobility control and scheduling, etc. "Cell" is used as a term to represent the smallest unit of a wireless communication area. "Cell" is also used as a term to represent the functions or resources used to perform wireless communication with UE 100. A cell belongs to a carrier frequency (hereinafter referred to as "frequency").

[0025] Note that a gNB can also connect to the Evolved Packet Core (EPC) corresponding to the LTE core network. LTE base stations can also connect to the 5GC. LTE base stations and gNBs can also connect via an inter-base station interface.

[0026] The 5GC 20 includes Access and Mobility Management Functions (AMF) and User Plane Functions (UPF) 300. The AMF performs various types of mobility control for the UE 100. The AMF manages the mobility of the UE 100 by communicating with it using Non-Access Stratum (NAS) signaling. The UPF controls data transmission. The AMF and UPF are connected to the gNB 200 via the NG interface, which serves as the interface between the base station and the core network.

[0027] Figure 2 This diagram illustrates a configuration example of a UE 100 (User Equipment) according to an embodiment. UE 100 includes a receiver 110, a transmitter 120, and a controller 130. The receiver 110 and transmitter 120 constitute a wireless communication device that performs wireless communication with the gNB 200.

[0028] Receiver 110 performs various receiving operations under the control of controller 130. Receiver 110 includes an antenna and receiving equipment. The receiving equipment converts the radio signals received through the antenna into baseband signals (received signals) and outputs the resulting signals to controller 130.

[0029] Transmitter 120 performs various transmissions under the control of controller 130. Transmitter 120 includes an antenna and a transmitting device. The transmitting device converts the baseband signal (transmit signal) output by controller 130 into a radio signal and transmits the obtained signal through the antenna.

[0030] Controller 130 performs various control and processing operations within UE 100. This processing includes the processing of the various layers described below. The operation of UE 100 described above and below can be an operation controlled by controller 230. Controller 130 includes at least one processor and at least one memory. The memory stores programs to be executed by the processor and information to be processed in the processor. The processor may include a baseband processor and a central processing unit (CPU). The baseband processor performs modulation and demodulation, encoding and decoding of baseband signals, etc. The CPU executes programs stored in the memory, thereby performing various types of processing.

[0031] Figure 3 This diagram illustrates a configuration example of gNB 200 (network node) according to an embodiment. gNB 200 includes a transmitter 210, a receiver 220, a controller 230, and a backhaul communicator 240. Transmitter 210 and receiver 220 constitute a wireless communication device for performing wireless communication with UE 100. Backhaul communicator 240 constitutes a network communicator for performing communication with CN 20.

[0032] Transmitter 210 performs various transmissions under the control of controller 230. Transmitter 210 includes an antenna and a transmitting device. The transmitting device converts the baseband signal (transmit signal) output by controller 230 into a radio signal and transmits the obtained signal through the antenna.

[0033] Receiver 220 performs various types of reception under the control of controller 230. Receiver 220 includes an antenna and receiving equipment. The receiving equipment converts the radio signals received through the antenna into baseband signals (received signals) and outputs the resulting signals to controller 230.

[0034] Controller 230 performs various types of control and processing within gNB 200. This processing includes the processing of the various layers described below. The operations of gNB 200 described above and below can also be performed under the control of controller 230. Controller 230 includes at least one processor and at least one memory. The memory stores programs to be executed by the processor and information to be processed in the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation and demodulation, encoding and decoding of baseband signals, etc. The CPU executes programs stored in the memory, thereby performing various types of processing.

[0035] The backhaul communicator 240 is connected to an adjacent base station via the Xn interface, which serves as an inter-base station interface. The backhaul communicator 240 is connected to the AMF / UPF300 via the NG interface, which is the interface between the base station and the core network. Note that the gNB 200 may include (i.e., its functions are divided into) a central unit (CU) and a distributed unit (DU), and these two units can be connected via the F1 interface, which serves as a fronthaul interface.

[0036] Figure 4 This is a configuration diagram showing the protocol stack of the radio interface for the user plane that processes data.

[0037] The user plane radio interface protocol includes the physical (PHY) layer, media access control (MAC) layer, radio link control (RLC) layer, packet data convergence protocol (PDCP) layer, and service data adaptation protocol (SDAP) layer.

[0038] The PHY layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layers of UE 100 and gNB 200 via physical channels. Note that the PHY layer of UE 100 receives downlink control information (DCI) transmitted from gNB 200 via the Physical Downlink Control Channel (PDCCH). Specifically, UE 100 performs blind decoding of the PDCCH using the Radio Network Temporary Identifier (RNTI) and obtains the successfully decoded DCI as the DCI addressed to the UE. CRC parity bits scrambled by the RNTI are added to the DCI transmitted from gNB 200.

[0039] The MAC layer performs data priority control, retransmission processing via Hybrid ARQ (HARQ: Hybrid Automatic Repeat Request), and random access procedures. Data and control information are transmitted between the MAC layer of UE 100 and the MAC layer of gNB 200 via the transport channel. The MAC layer of gNB 200 includes a scheduler. The scheduler determines the transmission format (transmission block size, modulation and coding scheme (MCS)) in the uplink and downlink, as well as the resource blocks to be allocated to UE 100.

[0040] 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 UE 100 and the RLC layer of gNB 200 via logical channels.

[0041] The PDCP layer performs header compression / decompression, encryption / decryption, etc.

[0042] The SDAP layer performs the mapping between IP flows (as a unit of QoS (Quality of Service) control performed by the core network) and radio bearers (as a unit of QoS control performed by the access layer (AS)). Note that SDAP is not required when the RAN is connected to the EPC.

[0043] Figure 5 This is a configuration diagram of the protocol stack of the radio interface of the control plane that processes signaling (control signals).

[0044] The protocol stack for the control plane's radio interface includes a Radio Resource Control (RRC) layer and a Non-Access Stratum (NAS) layer, rather than... Figure 4 The SDAP layer is shown.

[0045] RRC signaling for various configurations is transmitted between the RRC layer of UE 100 and the RRC layer of gNB 200. The RRC layer controls logical channels, transport channels, and physical channels based on the establishment, reconstruction, and release of radio bearers. UE 100 is in an RRC connected state when a connection (RRC connection) is established between the RRC layers of UE 100 and gNB 200. UE 100 is in an RRC idle state when no connection (RRC connection) is established between the RRC layers of UE 100 and gNB 200. UE 100 is in an RRC inactive state when the connection between the RRC layers of UE 100 and gNB 200 is suspended.

[0046] The NAS layer (also simply "NAS"), located above the RRC layer, performs session management, mobility management, etc. NAS signaling is transmitted between the NAS layer of UE100 and the NAS layer of AMF300A. Note that UE100 includes the application layer in addition to the radio interface protocol. Layers lower than the NAS layer are called AS layers (also simply "AS").

[0047] (2) Overview of LTM

[0048] The mobile communication system according to the embodiment supports L1 / L2 triggered mobility (LTM).

[0049] LTM is a technique for reducing mobility latency (specifically, serving cell change latency) (compared to a typical handover process) by triggering cell handover via signaling at the lower layers (Layer 1 (L1) and / or Layer 2 (L2)). In a typical handover process, UE 100 sends a measurement report message as an RRC message to gNB 200. gNB 200 determines the handover of UE 100 based on this measurement report message and sends a handover command as an RRC message (specifically, an RRC reconfiguration message) to UE 100, thereby indicating the handover.

[0050] In contrast, in LTM, the gNB 200 first prepares the LTM candidate cell configuration related to the handover destination cell candidate and provides it to the UE 100 via RRC signaling. Second, the UE 100 performs synchronization with the candidate cells via early synchronization. Third, the gNB 200 receives the L1 measurement report from the UE 100, determines the cell handover to the target cell based on the L1 measurement report, and sends a cell handover command indicating the target cell (LTM candidate cell configuration) to the UE 100 via MAC CE. The cell handover triggering condition is transmitted via MAC CE, which includes at least the candidate configuration index and beam indicator. Fourth, the UE 100 changes its serving cell in response to the cell handover command from the gNB 200 (source cell). As described above, cell handover is triggered when the gNB 200 selects the LTM candidate cell configuration as the target configuration. The gNB 200 can add, modify, and release LTM candidate cell configurations via RRC signaling.

[0051] The following principles apply to LTM:

[0052] • Each LTM candidate cell configuration can be provided as a differential configuration (incremental configuration) relative to the reference configuration to form the complete candidate cell configuration.

[0053] • When applying the full candidate cell configuration, the current UE configuration is replaced during cell handover. Although the replacement is performed during reconfiguration, the MAC, RLC, or PDCP layers are not necessarily reset.

[0054] • To avoid additional delays in data recovery, if configured via RRC signaling, the user plane will continue without being reset.

[0055] • In LTM, security is not updated.

[0056] • LTM can be performed between subsequent LTM candidate cell configurations without requiring RRC reconfiguration. That is, after triggering LTM, UE 100 does not release other LTM candidate cell configurations.

[0057] Figure 6This is an example diagram illustrating the LTM process, which is being standardized in 3GPP Release 18. In the example shown, it is assumed that UE 100 performs a cell handover from a first cell (source cell) to a second cell in gNB 200. Here, the first and second cells can be configured with different Transmit and Receive Points (TRPs). Before determining the cell handover via LTM, the second cell will always be referred to as the "candidate cell (or LTM candidate cell)," and after determining the cell handover via LTM, the second cell will also be referred to as the "target cell."

[0058] In step S1, UE 100 is in RRC connection state in the cell of gNB 200.

[0059] In step S2, UE 100 sends a measurement report message as an RRC message to gNB 200.

[0060] In step S3, gNB 200 determines to use LTM based on the measurement report message and begins preparing candidate cells.

[0061] In step S4, gNB 200 sends an RRC reconfiguration message to UE 100, which includes an LTM candidate cell configuration (LTM candidate configuration) of one or more candidate cells.

[0062] In step S5, UE 100 saves the LTM candidate cell configuration and sends an RRC reconfiguration complete message to gNB 200.

[0063] In step S6, UE 100 may perform synchronization processing with the candidate cell before receiving the cell handover command. This synchronization processing is called early synchronization. Here, UE 100 may perform early timing advance (TA) acquisition in the candidate cell requested by gNB 200 (source cell) before receiving the cell handover command in step S9. This early timing advance acquisition is performed via contention-free random access (CFRA) triggered by a PDCCH command from the source cell. Note that when using DCI format 1_0 and setting all "Frequency Domain Resource Assignment" fields in the DCI to "1", the DCI is processed as a PDCCH command. UE 100 sends a random access preamble (RA preamble) to the designated candidate cell. To minimize source cell communication interruption due to the CFRA to the candidate cell, UE 100 does not receive a random access response (RAR) from the candidate cell to obtain the TA value during early synchronization. The TA value of the candidate cell (target cell) is indicated by the cell handover command in step S9. Note that the TA value is used to adjust the uplink transmission timing of UE 100.

[0064] In step S7, UE 100 performs Layer 1 (L1) measurements on the configured candidate cells and sends a physical layer measurement report (L1 measurement report) to gNB 200. The L1 measurement report is sent and received by L1, which is the PHY layer. For example, UE 100 sends L1-RSRP and / or L1-SINR to gNB 200 via the Physical Uplink Control Channel (PUCCH) and / or PUSCH.

[0065] In step S8, gNB 200 determines to perform a cell handover to the target cell (second cell).

[0066] In step S9, gNB 200 sends a cell handover command (MAC CE) to UE 100, which includes the candidate configuration index of the target cell. The cell handover command may include the TA value obtained through early synchronization.

[0067] In step S10, UE 100 switches to the configuration of the target cell. Specifically, UE 100 leaves the source cell (first cell) and applies the configuration of the target cell.

[0068] In step S11, if cell handover requires the execution of a random access procedure (e.g., if the cell handover command does not include a valid TA value), then UE 100 performs a random access procedure for the target cell. Note that if obtaining the TA of the target cell is not required during cell handover (e.g., if the cell handover command includes a valid TA value), then UE 100 may skip the random access procedure.

[0069] In step S12, UE 100 indicates that the cell handover to the target cell has been successfully completed. Thereafter, UE 100 can execute steps S6 to S12 multiple times to perform subsequent LTM cell handovers based on the configuration provided in step S4.

[0070] (3) Operation of mobile communication systems

[0071] 3GPP Release 18's LTM is limited to serving cell changes between cells belonging to the same gNB 200 (same CU). Therefore, there is a problem that LTM cannot be used to achieve serving cell changes between cells belonging to different gNB 200s (different CUs). Note that this type of LTM can be referred to as inter-network node LTM, specifically, gNB LTM or CU LTM. The following embodiments will describe the operations used to implement inter-network node LTM.

[0072] Figure 7 This is a diagram illustrating the operational scenario of the mobile communication system 1 according to an embodiment.

[0073] UE 100 performs a serving cell change from the first cell (source cell) of gNB 200a (the source gNB) to the second cell of gNB 200b. The second cell will be referred to as the "candidate cell (or LTM candidate cell)" until a cell handover via LTM is determined, and will also be referred to as the "target cell" after the LTM handover is determined. An Xn station interface is established between gNB 200a and gNB 200b. It is assumed that communication between gNB 200a and gNB 200b is performed on the Xn interface.

[0074] In this embodiment, gNB 200a performs wireless communication with UE 100, which is in an RRC connection state, in gNB 200a's first cell. gNB 200a sends a request message to gNB 200b requesting that the serving cell of UE 100 be changed from gNB 200a's first cell to gNB 200b's second cell. Here, when gNB 200a changes the serving cell via LTM, gNB 200a sends a request message to gNB 200b indicating the change of serving cell via LTM. Note that LTM is the process of instructing UE 100 to handover from gNB 200a via MACCE.

[0075] Therefore, gNB 200b can identify, based on the request message from gNB 200a, that it is not a request for a general handover but a request for a change of serving cell via LTM.

[0076] The request message can be a handover request (HO request) message that indicates a handover from gNB 200a to UE 100 via an RRC message, which can be used during the handover process. A request message that indicates a serving cell change via LTM can be a HO request message that includes an LTM indicator. Therefore, HO request messages used for general handover can be used for inter-network node LTM, making it easy to minimize changes to technical specifications.

[0077] Alternatively, the request message indicating a change in serving cell via LTM can be an LTM request message, distinct from the HO request message. The LTM request message can be a request message specifically designed for LTM.

[0078] The request message indicating a serving cell change via LTM may include information indicating whether the gNB 200b needs to configure contention-free random access (CFRA) resources. CFRA resources are used by the UE 100 during early synchronization of the second cell prior to the cell handover indication via MAC CE. Therefore, the gNB 200b can determine whether CFRA resources need to be configured based on the request message.

[0079] gNB 200a can send another request message to gNB 200b to request gNB 200b to configure or activate the Early Synchronization CFRA resource. Therefore, gNB 200b can appropriately configure or activate the Early Synchronization CFRA resource.

[0080] The gNB 200a can send a PDCCH command to the UE 100 to instruct the execution of CFRA (i.e., the transmission of the RA preamble). The PDCCH command may include information for identifying the second cell. Therefore, the UE 100 can identify whether the RA preamble transmission target is the first cell or the second cell.

[0081] gNB 200a can receive a notification from gNB 200b indicating that the early synchronization performed by UE 100 on the second cell has been successful. Therefore, gNB 200a can identify whether the early synchronization performed by UE 100 on the second cell has been successful.

[0082] Alternatively, gNB 200a can receive a notification from UE 100 indicating that the early synchronization performed by UE 100 on the second cell has been successful.

[0083] Figure 8 This is a specific example diagram illustrating the operation of a mobile communication system 1 according to an embodiment. Figure 8 In the diagram, steps that can be omitted are indicated by dashed lines. Note that although references are omitted... Figure 6 The described operations are repeated, but references may be applied appropriately. Figure 6 The described operation.

[0084] In step S101, UE 100 sends an L3 (RRC) measurement report to gNB 200a. gNB 200a receives the L3 (RRC) measurement report.

[0085] In step S102, gNB 200a determines to use inter-gNB LTM and begins preparing candidate cells based on the L3 (RRC) measurement report in step S101. Here, it is assumed that the second cell of gNB 200b is determined as a candidate cell.

[0086] In step S103, gNB 200a sends a request message (LTM HO request) to gNB 200b indicating a change in the serving cell via LTM. gNB 200b receives the request message (LTM HO request). The request message (LTM HO request) can be a handover request message that includes an LTM indicator and is used for general handover. Alternatively, the request message (LTM HO request) can be a new message different from the handover request message, such as an LTM handover request message. The request message (LTM HO request) can include information indicating whether early synchronization needs to be configured (i.e., whether early synchronization CFRA resources need to be configured) (this request message can be information used to suggest configuring early synchronization). Note that the request message (LTM HO request) can include the RRC configuration information of UE 100 and a cell identifier indicating a second cell, as with general handover.

[0087] In step S104, gNB 200b determines whether the request in step S103 (admission control) can be accepted. This description is based on the assumption that the request in step S103 has already been approved. In this case, gNB 200b can configure early synchronization CFRA resources in the second cell. Note that if gNB 200b rejects the request in step S103, gNB 200b can send a rejection message to gNB 200a. The rejection message may include information indicating that inter-gNB LTM cannot be used.

[0088] In step S105, gNB 200b sends a response message (LTM HO request response) to gNB 200a indicating acceptance of the request in step S103. gNB 200a receives the response message (LTM HO request response). The response message (LTM HO request response) may be a handover request response message including an LTM indicator and used for general handover. Alternatively, the response message (LTM HO request response) may be a new message different from the handover request response message, such as an LTM handover request response message. The response message (LTM HO request response) may include information indicating the early synchronization CFRA resources (e.g., RA preamble and / or Physical Random Access Channel (PRACH) resources) configured by gNB 200b for the second cell. Note that the response message (LTM HO request response) may include RRC reconfiguration information (RRC reconfiguration) to be applied to UE 100 in the second cell, as in general handover.

[0089] In step S106, gNB 200a sends an RRC reconfiguration message to UE 100, including the LTM candidate cell configuration of the second cell. UE 100 receives the RRC reconfiguration message. The RRC reconfiguration message may include information indicating the early synchronization CFRA resources configured by gNB 200b for the second cell.

[0090] In step S107, UE 100 saves the LTM candidate cell configuration and sends an RRC reconfiguration complete message to gNB 200a. gNB 200a receives the RRC reconfiguration complete message.

[0091] In step S108, UE 100 may send an L1 measurement report (or L3 measurement report) to gNB 200a so that gNB 200a can make a determination regarding early synchronization. gNB 200a may receive the L1 measurement report (or L3 measurement report).

[0092] In step S109, gNB 200a can determine early synchronization.

[0093] In step S110, gNB 200a may send an Early Synchronization CFRA Request message to gNB 200b. This Early Synchronization CFRA Request message is a request message for preparing Early Synchronization CFRA resources (specifically, configuring and / or activating (verifying) Early Synchronization CFRA resources). gNB 200b may receive the request message (Early Synchronization CFRA Request message). The request message (Early Synchronization CFRA Request message) may include an identifier (Xn-AP UE ID) for specifying UE 100 and / or an identifier (cell ID) for identifying the second cell.

[0094] In step S111, gNB 200b can prepare early synchronization CFRA resources.

[0095] In step S112, gNB 200b may send a notification message to gNB 200a indicating that preparation for the Early Synchronization CFRA resource has been completed, such as an Early Synchronization CFRA Request Response message. gNB 200a may receive the notification message (Early Synchronization CFRA Request Response message).

[0096] In step S113, gNB 200a sends a PDCCH command to UE 100 to instruct the execution of CFRA for early synchronization with UE 100. UE 100 receives the PDCCH command. The PDCCH command may include information (target cell indicator) for identifying the second cell as the CFRA target. This information may be the cell ID (or cell index) of the second cell. This information may be an index of the LTM candidate cell configuration list in step S106. This information may be an index for specifying the TRP corresponding to the second cell.

[0097] In step S114, UE 100 may perform early downlink (DL) synchronization with the second cell. For example, UE 100 may use the SSB (PSS / SSS) of the second cell to perform timed synchronization. Note that UE 100 may perform DL synchronization before this time point.

[0098] In step S115, UE 100 sends a CFRA (specifically, an RA preamble) on the PRACH to the second cell specified by the PDCCH command to perform uplink (UL) early synchronization with the second cell. gNB 200b receives the RA preamble. Note that UE 100 identifies the CFRA resource (e.g., the RA preamble and / or PRACH resource) based on information configured with SIBs, etc., and information such as the "random access preamble index" and "PRACH mask index" in the PDCCH command.

[0099] In step S116, gNB 200b may send a RAR including a TA value derived based on the RA preamble to UE 100. UE 100 may receive the RAR. Step S116 may be an optional step performed only if a configuration from gNB 200a exists (e.g., the configuration in step S106). UE 100 may send a notification (early synchronization complete) to gNB 200a indicating that early UL synchronization with the second cell has been completed (step S117). The notification (early synchronization complete) may include a TA value notified using the RAR.

[0100] In step S118, gNB 200b may send a notification message (early synchronization complete) to gNB 200a indicating that early UL synchronization with UE 100 has been completed. gNB 200a may receive the notification message (early synchronization complete). The notification message (early synchronization complete) may include the TA value derived based on the RA preamble in step S115.

[0101] In step S119, UE 100 sends an L1 measurement report to gNB 200a. gNB 200a receives the L1 measurement report.

[0102] In step S120, when it is determined, for example, based on the L1 measurement report in step S119 that the probability of performing LTM has increased, gNB 200a may send a request message for UL resources to gNB 200b. gNB 200b may receive the request message. The request for UL resources may be a request for preparing or verifying CFRA resources. The request for UL resources may be a request for preparing or performing UL authorization to UE 100. The request for UL resources may be a request for preparing or verifying UL Configuration Authorization (CG) resources. Note that sending the request message in step S120 may be performed simultaneously with determining to perform LTM in step S121. Alternatively, the sending may be performed after determining to perform LTM in step S121.

[0103] In step S121, gNB 200a determines to perform LTM based on the L1 measurement report in step S119.

[0104] In step S122, gNB 200a sends a cell handover command (MACCE) to UE 100 in response to determining that LTM will be performed. UE 100 receives the cell handover command. The cell handover command may include the TA value notified to gNB 200a in step S117 or S118.

[0105] In step S123, UE 100 responds to receiving a cell handover command by leaving the first cell (source cell) and applies the LTM candidate cell configuration of the second cell (target cell).

[0106] In step S124, if the cell handover command does not include a TA value (valid TA value), then UE 100 may perform a random access procedure for the second cell.

[0107] In step S125, UE 100 sends an RRC reconfiguration complete message to the second cell. gNB 200b receives the RRC reconfiguration complete message.

[0108] In step S126, gNB 200b may send a DCI (which includes a cyclic redundancy code (CRC) scrambled using a C-RNTI allocated to UE 100) to UE 100 on the PDCCH, and send a contention-resolving MAC CE to UE 100 on the PDSCH allocated using the DCI. UE 100 may receive the DCI and the contention-resolving MAC CE.

[0109] In step S127, gNB 200b can send a notification message to gNB 200a indicating that LTM between network nodes in the second cell has been completed (LTM HO successful). gNB 200a can receive the notification message (LTM HO successful).

[0110] (4) Other embodiments

[0111] The above operational procedures can be implemented separately and independently, or they can be implemented as a combination of two or more operational procedures. For example, some steps in one operational procedure can be added to another, or some steps in one operational procedure can be replaced by steps in another. In each procedure, not all steps must be executed; only some steps can be executed. The order of steps in each procedure can be changed as needed.

[0112] Although the example of an NR base station (gNB) has been described in the above embodiments and examples, the base station can also be an LTE base station (eNB) or a 6G base station. The base station can be a relay node, such as an Integrated Access and Backhaul (IAB) node. The base station can be a DU of an IAB node. UE 100 can be a mobile terminal (MT) of an IAB node.

[0113] That is, UE 100 can be a terminal functional unit (a communication module) of a repeater that enables base station control to perform signal relay. This terminal functional unit is called MT. In addition to IAB-MT, examples of MT include Network Control Repeater (NCR)-MT and Reconfigurable Smart Surface (RIS)-MT.

[0114] The term "network node" primarily refers to a base station, but can also refer to core network equipment or a portion of a base station (CU, DU, or RU). A network node can include a combination of at least a portion of a core network equipment and at least a portion of a base station.

[0115] A program may be provided that enables a computer to perform each process executed by UE 100 or gNB 200. The program may be recorded on a computer-readable medium. The computer-readable medium allows the program to 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 specifically limited and may, for example, be a recording medium such as a CD-ROM or DVD-ROM. Circuitry for performing the processes executed by UE 100 or gNB 200 may be integrated, and at least a portion of UE 100 or gNB 200 may be configured as a semiconductor integrated circuit (chipset, system-on-chip (SoC)).

[0116] The functions implemented by UE 100 or gNB 200 (network node) can be implemented in circuitry or processing circuitry programmed to perform the functions, including general-purpose processors, application-specific processors, integrated circuits, application-specific integrated circuits (ASICs), central processing units (CPUs), conventional circuitry, and / or combinations thereof. A processor may include transistors and other circuitry and may be considered as a circuit or processing circuitry. A processor may be a programmed processor that executes a program stored in memory. As used herein, circuits, units, and devices are hardware programmed to perform the functions or hardware that performs the functions. Hardware may be any hardware disclosed herein, or any hardware programmed to perform the functions or known to perform the functions. When hardware is a processor considered as a type of circuit, the circuit, device, or unit is a combination of hardware and software used to configure the hardware and / or processor.

[0117] Unless otherwise expressly stated, the phrases “based on” and “depending on / responding to” as used in this disclosure do not mean “based on only” and “depending on / responding to only”. The phrase “based on” means “based on only” and “at least partially based on” both. The phrase “depending on / responding to” means “depending on / responding to only” and “at least partially dependent on / responding to” both. The terms “comprising,” “including,” and variations thereof do not mean “including only the said items,” but rather mean “may include only the said items” or “may include not only the said items but also other items.” The term “or” as used in this disclosure is not intended to be “exclusive or.” Any reference to elements in this disclosure using names such as “first” and “second” does not generally limit the number or order of those elements. These names may be used herein as a convenient way to distinguish two or more elements. Therefore, references to a first element and a second element do not imply that only the two elements may be used there or that the first element needs to precede the second element in some way. For example, when English articles such as “a,” “one,” and “the” are added in this disclosure by translation, these articles include plural unless the context clearly indicates otherwise.

[0118] The embodiments have been described in detail above with reference to the accompanying drawings, but the specific configurations are not limited to those described above, and various design changes can be made without departing from the spirit of this disclosure.

[0119] This application claims priority to Japanese Patent Application No. 2023-170459 (filed on September 29, 2023), the entire contents of which are incorporated herein by reference.

[0120] (5) Supplement

[0121] The features related to the above embodiments are described below as supplementary notes.

[0122] Supplementary Note 1

[0123] A communication method performed by a network node in a mobile communication system includes the following steps:

[0124] Perform wireless communication with user equipment in Radio Resource Control (RRC) connection state in the first cell of the network node; and

[0125] A request message is sent to another network node to request that the serving cell of the user equipment be changed from the first cell to the second cell of the other network node, wherein...

[0126] The network node is configured to: when a mobility (LTM) change of serving cell is triggered via L1 / L2, send a request message indicating the change of serving cell via LTM, and

[0127] LTM is the process by which a network node instructs a user equipment to perform a cell handover through a Media Access Control (MAC) control element (CE).

[0128] Supplementary Note 2

[0129] According to the communication method described in Supplementary Note 1, wherein,

[0130] A handover request message is a handover request message used during the handover process, which is sent from a network node to a user equipment via an RRC message to indicate the handover.

[0131] A request message to change the serving cell via LTM indication is a handover request message that includes an LTM indicator.

[0132] Supplementary Note 3

[0133] According to the communication method described in Supplementary Note 1, the request message indicating a change of serving cell via LTM is a different LTM request message from the handover request message that indicates the handover from the network node to the user equipment via RRC message, which can be used in the handover process.

[0134] Supplementary Note 4

[0135] According to any one of Supplementary Notes 1 to 3, in the communication method, wherein,

[0136] The LTM-indicated request message for changing the serving cell includes: information indicating whether another network node needs to configure contention-free random access (CFRA) resources, and

[0137] CFRA resources are used by the user equipment during early synchronization of the second cell prior to the cell handover indication via MAC CE.

[0138] Supplementary Note 5

[0139] The communication method according to any one of Supplementary Notes 1 to 4 further includes sending another request message to another network node to request that the other network node to configure or activate contention-free random access (CFRA) resources.

[0140] Specifically, CFRA resources are used by the user equipment during early synchronization of the second cell prior to the cell handover indication via MAC CE.

[0141] Supplementary Note 6

[0142] The communication method according to any one of Supplementary Notes 1 to 5 further includes sending a Physical Random Access Channel (PDCCH) command to the user equipment to instruct the execution of Contention-Free Random Access (CFRA).

[0143] The PDCCH command includes information used to identify the second cell.

[0144] Supplementary Note 7

[0145] The communication method according to any one of Supplementary Notes 1 to 6 further includes receiving a notification from another network node indicating that the early synchronization performed by the user equipment on the second cell has been successful.

[0146] Supplementary Note 8

[0147] The communication method according to any one of Supplementary Notes 1 to 6 further includes receiving a notification from the user equipment indicating that the early synchronization performed by the user equipment on the second cell has been successful.

[0148] Supplementary Note 9

[0149] A network node used in a mobile communication system includes:

[0150] A first communicator is configured to perform wireless communication with a user equipment in a Radio Resource Control (RRC) connection state in a first cell of a network node; and

[0151] A second communicator is configured to send a request message to another network node, requesting that the serving cell of the user equipment be changed from the first cell to the second cell of the other network node, wherein...

[0152] The second communicator is configured to send a request message indicating a change of serving cell via LTM when a mobility (LTM) change of serving cell is triggered via L1 / L2, and

[0153] LTM is the process by which a network node instructs a user equipment to perform a cell handover through a Media Access Control (MAC) control element (CE).

[0154] Figure Labels

[0155] 1: Mobile communication system

[0156] 5: Network

[0157] 10: RAN

[0158] 20:CN

[0159] 100:UE

[0160] 110: Receiver

[0161] 120: Transmitter

[0162] 130: Controller

[0163] 200: gNB

[0164] 210: Transmitter

[0165] 220: Receiver

[0166] 230: Controller

[0167] 240: Backhaul communicator.

Claims

1. A communication method performed by a network node in a mobile communication system, the communication method comprising the following steps: In the first cell of the network node, wireless communication is performed with user equipment that is in a Radio Resource Control (RRC) connection state. as well as A request message is sent to another network node to change the serving cell of the user equipment from the first cell to the second cell of the other network node, wherein... The network node is configured to: when a mobility LTM change is triggered via L1 / L2 to change the serving cell, send a request message indicating a change to the serving cell via the LTM, and The LTM is the process by which the network node instructs the user equipment to perform a cell handover through the Media Access Control (MAC) control element (CE).

2. The communication method according to claim 1, wherein, The request message is a handover request message that can be used during the handover process to indicate the handover from the network node to the user equipment via an RRC message, and The request message to change the serving cell via the LTM indicator is a handover request message that includes the LTM indicator.

3. The communication method according to claim 1, wherein, The LTM request message that indicates a change of the serving cell is a different LTM request message from the network node to the user equipment via an RRC message that can be used during handover.

4. The communication method according to any one of claims 1 to 3, wherein, The LTM-indicated request message for changing the serving cell includes information indicating whether the other network node needs to configure contention-free random access (CFRA) resources, and The CFRA resources are used during the early synchronization performed by the user equipment on the second cell prior to the cell handover indication via the MAC CE.

5. The communication method according to any one of claims 1 to 3, further comprising: Send another request message to the other network node, requesting the other network node to configure or activate contention-free random access (CFRA) resources, wherein, The CFRA resources are used during the early synchronization performed by the user equipment on the second cell prior to the cell handover indication via the MAC CE.

6. The communication method according to any one of claims 1 to 3, further comprising: Send a Physical Random Access Channel (PDCCH) command to the user equipment instructing it to perform contention-free random access (CFRA), wherein, The PDCCH command includes information for identifying the second cell.

7. The communication method according to any one of claims 1 to 3, further comprising: Receive a notification from the other network node indicating that the early synchronization performed by the user equipment on the second cell has been successful.

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

9. A network node used in a mobile communication system, the network node comprising: A first communicator is configured to perform wireless communication with a user equipment in a Radio Resource Control (RRC) connection state in a first cell of the network node; as well as A second communicator is configured to send a request message to another network node requesting that the serving cell of the user equipment be changed from the first cell to the second cell of the other network node, wherein... The second communicator is configured to: when a mobility LTM change is triggered via L1 / L2 to change the serving cell, send a request message indicating a change to the serving cell via the LTM, and The LTM is the process by which the network node instructs the user equipment to perform a cell handover through the Media Access Control (MAC) control element (CE).

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