Communication method and user device

Abstract

Provided is a communication method that is executed by a user device in a mobile communication system supporting L1 / L2 Triggered Mobility (LTM), said method involving: in a state in which the user device uses a first cell as a serving cell, receiving, from the first cell, a first candidate setting for setting the first cell as an LTM candidate cell and a second candidate setting for setting a second cell as an LTM candidate cell; using a serving cell TA timer to manage the validity of a first timing advance (TA) value to be applied to uplink communication with the first cell; executing first LTM cell switching to the second cell on the basis of the second candidate setting; and maintaining the first TA value even if the first LTM cell switching is executed when the first candidate setting is in effect and the serving cell TA timer is in operation.

Description

Communication Method and User Equipment

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

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

[0003] On the other hand, in Release 18 of the 3GPP standard (3GPP Release 18), the technical specifications of LTM (L1 / L2-Triggered Mobility), which is a new procedure for serving cell switching, have been established. LTM is a procedure in which a network node receives a layer 1 (L1) measurement report from user equipment, and based thereon, the network node changes the serving cell of the user equipment by means of a cell switching command signaled to the user equipment through a medium access control (MAC) control element (CE).

[0004] 3GPP Technical Specification "3GPP TS 38.300 V18.2.0"

[0005] The present disclosure provides a technique for improving LTM.

[0006] A communication method according to a first aspect of the present disclosure is a communication method performed by a user device in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: receiving from the first cell a first candidate setting that sets the first cell as an LTM candidate cell and a second candidate setting that sets the second cell as an LTM candidate cell, while the user device is using the first cell as a serving cell; managing the validity of a first timing advance (TA) value to be applied to uplink communication with the first cell using a serving cell TA timer; performing a first LTM cell switch to the second cell based on the second candidate setting; and retaining the first TA value even when performing the first LTM cell switch when the first candidate setting exists and the serving cell TA timer is operating.

[0007] A user device according to a second aspect of the present disclosure is a user device used in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), and includes a receiving unit that receives from the first cell a first candidate setting that sets the first cell as an LTM candidate cell and a second candidate setting that sets the second cell as an LTM candidate cell when the user device is connected to the first cell as a serving cell, and a control unit that manages the validity of a first timing advance (TA) value applied to uplink communication with the first cell using a serving cell TA timer. The control unit performs a first LTM cell switch to the second cell based on the second candidate setting, and maintains the first TA value even when performing the first LTM cell switch if the first candidate setting exists and the serving cell TA timer is operating.

[0008] This figure shows an example configuration of a mobile communication system according to an embodiment. This figure shows an example configuration of a UE (User Equipment) according to an embodiment. This figure shows an example configuration of a gNB (Network Node) according to an embodiment. This figure shows the configuration of the protocol stack of the wireless interface of the user plane that handles data. This figure shows the configuration of the protocol stack of the wireless interface of the control plane that handles signaling (control signals). This figure shows an example of a cell switching procedure by LTM in an intraCU (i.e., within the same gNB) according to an embodiment. This figure is for explaining LTM cell switching of an interCU according to an embodiment. This figure is for explaining the operation of C-LTM cell switching according to an embodiment. This figure shows an example configuration of an RRC Reconfiguration message for setting C-LTM to UE100 according to an embodiment. This figure shows the operation of UE100 according to the first embodiment. This figure shows an embodiment of the first embodiment. This figure shows the operation of UE100 according to the second embodiment. This figure shows an embodiment of the second embodiment. This figure shows the operation of UE100 according to the third embodiment. This figure shows an embodiment 1 of the third embodiment. This figure shows an embodiment 2 of the third embodiment. This figure shows an embodiment 3 of the third embodiment. This figure shows a scenario in which the validity of the TA value is not guaranteed. This diagram shows the case where a source cell is set as an LTM candidate cell.

[0009] 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.

[0010] (1) The mobile communication system configuration diagram 1 is a diagram showing an example of the configuration of the mobile communication system 1 according to this embodiment. The mobile communication system 1 conforms to the 5th generation system (5GS) of the 3GPP standard. In the following description, 5GS will be used as an example, but the mobile communication system may also have an LTE (Long Term Evolution) system applied to it at least partially, or a 6th generation (6G) system applied at least partially.

[0011] The mobile communication system 1 comprises a user equipment (UE) 100, a 5G radio access network (NG-RAN: Next Generation Radio Access Network) 10, and a 5G core network (5GC: 5G Core Network) 20. Hereinafter, 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.

[0012] 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) and / or a tablet terminal, a notebook PC, a communication module (including a communication card or chipset (System on a chip)), a sensor or a device attached to a sensor, a vehicle or a device attached to a vehicle (Vehicle UE), or an aircraft or a device attached to an aircraft (Aerial UE). The link in the transmission direction from UE100 to network 5 is called the uplink (UL), and the link in the transmission direction from network 5 to UE100 is called the downlink (DL).

[0013] NG-RAN10 includes a base station (referred to as "gNB" in the 5G system) 200, which is a type of network node. The gNBs 200 are interconnected via the Xn interface, which is an inter-base station interface. The gNBs 200 manage one or more cells. The gNBs 200 perform wireless communication with UEs 100 that have established a connection with their own cell. The gNBs 200 have radio resource management (RRM) functions, user data (hereinafter simply referred to as "data") routing functions, 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. "Cell" is identified by a cell identifier (cell ID). One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

[0014] A single cell may be formed by one or more TRPs (Transmission and Reception Points). Each TRP may form one or more beams. Each beam is associated with a reference signal. The reference signal is either an SSB (SS: Synchronization Signal / PBCH Block) or a CSI-RS (Channel State Information-Reference Signal). The SSB includes a primary synchronization signal (PSS), a secondary synchronization signal (SSS), a PBCH (Physical Broadcast Channel), and a demodulation reference signal (DMRS). For example, the SSB may consist of four consecutive OFDM (Orthogonal Frequency Division Multiplex) symbols in the time domain. Alternatively, the SSB may consist of 240 consecutive subcarriers (i.e., 20 resource blocks) in the frequency domain. The PBCH is the physical channel carrying the Master Information Block (MIB). The CSI-RS is a reference signal transmitted for the UE100 to measure the state of the radio channel.

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

[0016] The 5GC20 includes an AMF (Access and Mobility Management Function) and a UPF (User Plane Function) 300. The AMF performs various mobility controls for the UE100. The AMF manages the mobility of the UE100 by communicating with the UE100 using NAS (Non-Access Stratum) signaling. The UPF controls data transfer. The AMF and UPF are connected to the gNB200 via the NG interface, which is the base station-core network interface.

[0017] Figure 2 shows an example configuration of UE100 (user device) according to this embodiment. UE100 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 140 that performs wireless communication with gNB200.

[0018] The receiving unit 110 performs various types of reception under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts the radio 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 types of 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 (transmission signal) output by the control unit 130 into a wireless signal and transmits it from the antenna.

[0020] The control unit 130 performs various control and processing operations in the UE 100. Such processing includes processing in each layer described later. The operation of the UE 100 described above and later may also be controlled by the control unit 130. 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 processing by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation, demodulation, encoding, and decoding of baseband signals. The CPU executes programs stored in memory and performs various processing operations.

[0021] Figure 3 shows an example configuration of a gNB200 (network node) according to this embodiment. The gNB200 includes a transmitting unit 210, a receiving unit 220, a control unit 230, and a network communication unit 240. The transmitting unit 210 and the receiving unit 220 constitute a wireless communication unit 250 that performs wireless communication with the UE100. The network communication unit 240 includes a transmitting unit 241 that performs transmission and a receiving unit 242 that performs reception.

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

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

[0024] The control unit 230 performs various control and processing operations in the gNB 200. Such processing includes processing in each layer described later. The operation of the gNB 200 described above and later 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, decoding, etc. of the baseband signal. The CPU executes programs stored in memory and performs various processing operations.

[0025] The network communication unit 240 is connected to an adjacent base station via the Xn interface, which is an inter-base station interface. The network communication unit 240 is connected to the AMF / UPF 300 via the NG interface, which is an inter-base station-core network interface. The gNB 200 may consist of a central unit (CU) and a distributed unit (DU) (i.e., functionally divided), and the two units may be connected by the F1 interface, which is a front-haul interface. In this case, each of the gNB-DU and gNB-CU may have a functional block configuration similar to the one shown in Figure 3. However, the gNB-CU is assumed not to have a wireless communication unit 250.

[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 comprises 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 PHY layer of UE100 and the PHY layer of gNB200 via a physical channel. The PHY layer of UE100 receives Downlink Control Information (DCI) transmitted from gNB200 over the Physical Downlink Control Channel (PDCCH). Specifically, UE100 performs blind decoding of the PDCCH using the 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 includes a CRC (Cyclic Redundancy Check) parity bit that has been scrambled by RNTI.

[0029] The MAC layer performs data priority control, retransmission processing using Hybrid ARQ (HARQ: Hybrid Automatic Repeat reQuest), and random access procedures. Data and control information are transmitted between the MAC layer of UE100 and the MAC layer of gNB200 via the transport channel. The MAC layer of gNB200 includes a scheduler. The scheduler determines the transport format for the up and down links (transport block size, modulation and coding scheme (MCS: Modulation and Coding Scheme)) and the resource blocks to be allocated to 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 RLC layer of UE100 and the RLC layer of gNB200 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 protocol stack of the control plane's wireless interface 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 NAS layer of the UE100 and the NAS layer of the AMF300A. The UE100 also has an application layer in addition to the 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 1 according to this embodiment supports LTM (L1 / L2-triggered mobility).

[0038] In a typical handover procedure, the serving cell switchover is triggered by signaling at the higher layer, L3, specifically the RRC layer. This type of typical handover is also called an L3 handover. In an L3 handover, the UE100 sends an L3 measurement report message, which is an RRC message, to the gNB200. Based on this measurement report message, the gNB200 decides to hand over the UE100 and instructs the serving cell switchover by sending a handover command (specifically, an RRC Reconfiguration message) from the gNB200 to the UE100.

[0039] On the other hand, LTM is a technique for reducing mobility delays (specifically, serving cell switching delays) compared to general handover procedures by triggering serving cell switching through signaling at the lower layers, Layer 1 (L1) and / or Layer 2 (L2). In LTM, the gNB200 receives an L1 measurement report from the UE100, and based on this, the gNB200 instructs the UE100 to switch serving cells by signaling a cell switching command via MAC CE.

[0040] Specifically, in LTM, firstly, gNB200 prepares LTM candidate settings for candidate cells to be switched to, and provides LTM candidate settings to UE100 via RRC signaling.

[0041] Secondly, UE100 performs synchronization with LTM candidate cells through early synchronization.

[0042] Thirdly, the gNB 200 receives an L1 measurement report from the UE 100, determines to switch the serving cell to a target cell based on the L1 measurement report, and transmits a Cell Switch Command indicating the target cell (LTM candidate configuration) to the UE 100 via a MAC Control Element (CE). The serving cell switch trigger is transmitted in a MAC CE that includes at least a candidate configuration index (configuration ID) along with a beam indicator.

[0043] Fourthly, the UE 100 switches the serving cell in response to the cell switch command MAC CE from the gNB 200 (source cell).

[0044] As described above, serving cell switching is triggered by the gNB 200 selecting an LTM candidate configuration as a target configuration. The LTM candidate configuration can be added, modified, and released by the gNB 200 via RRC signaling.

[0045] The following principles apply to LTM.

[0046] ・Each LTM candidate configuration may be provided as a differential configuration (delta configuration) relative to a reference configuration used to form a complete LTM candidate configuration.

[0047] ・When the complete LTM candidate configuration is applied, the current UE configuration is replaced at the time of serving cell switching. The replacement is performed in the reconfiguration procedure, but the MAC, RLC, or PDCP layers are not necessarily reset.

[0048] ・For the purpose of avoiding additional delay of data recovery, the user plane continues without being reset if configured by RRC signaling.

[0049] ・Security is not updated in LTM.

[0050] ・Subsequent LTM between subsequent LTM candidate configurations can be performed without RRC reconfiguration. That is, the UE 100 does not release other LTM candidate configurations after LTM is triggered. (2.1) Intra-CU LTM

[0051] Figure 6 shows an example of a cell switching procedure using LTM within an intraCU (i.e., within the same gNB200). In the illustrated example, UE100 performs an LTM-based serving cell switch (i.e., LTM cell switch) from the first cell to the second cell of the gNB200. Hereafter, the first cell may be referred to as "cell a" and the second cell as "cell b".

[0052] Here, the first cell and the second cell may be formed from different TRPs. In the following description of the embodiments, the second cell will also be referred to as the "LTM candidate cell (or candidate cell)" until the LTM decides to switch the serving cell, and the second cell will also be referred to as the "target cell" after the LTM decides to switch the serving cell. The first cell will also be referred to as the "source cell" or the "(current) serving cell".

[0053] In step S1, UE100 is in an RRC connected state in the cell (first cell) of gNB200.

[0054] In step S2, UE100 sends a Measurement Report message, which is an RRC message, to gNB200 (first cell). The measurement report, which includes the L3 measurement result, is also called the L3 Measurement Report (L3 MR).

[0055] In step S3, gNB200 decides to use LTM based on the L3 measurement report and begins preparing LTM candidate cells.

[0056] In step S4, gNB200 (first cell) sends an LTM configuration, including the LTM Candidate Configuration / LTM Candidate for one or more LTM candidate cells, to UE100 in an RRC message, specifically an RRC Reconfiguration message. The LTM Candidate Configuration may include a Random Access Channel (RACH) configuration used for sending RA (Random Access) preambles to the corresponding LTM candidate cells, such as a Conflict-Free Random Access (CFRA) configuration. Such a RACH configuration may be referred to as an Early Ul Sync Configuration. CFRA is a random access procedure in which each UE100 is assigned dedicated RACH resources (e.g., dedicated preamble sequences and / or dedicated time and frequency resources), and no RACH contention occurs between UE100s.

[0057] In step S5, UE100 saves the LTM settings (LTM candidate settings) and sends an RRC Reconfiguration Complete message to gNB200 (first cell).

[0058] In step S6, UE100 may perform synchronization with the LTM candidate cell (second cell) before receiving the cell switching command MAC CE from the first cell. Such synchronization may be called Early Sync. Here, UE100 may perform downlink synchronization (DL synchronization) with the LTM candidate cell and then perform timing advance (TA) acquisition with the LTM candidate cell requested by gNB200 (serving cell). This is done by a CFRA triggered by a PDCCH order from the first 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. Furthermore, if Early Ul Sync Config is set on UE100, the PDCCH order may include a cell indicator that indicates the corresponding RACH transmission cell, i.e., which LTM candidate cell UE100 should send a random access preamble (RA preamble) to.

[0059] UE100 transmits an RA preamble to the designated LTM candidate cell (second cell). To minimize the interruption of serving cell communication by CFRA to the LTM candidate cell, during early synchronization, UE100 does not receive a Random Access Response (RAR) from the LTM candidate cell for the purpose of obtaining the Timing Advance (TA) value. The TA value of the LTM candidate cell (target cell) is indicated by the cell switching command MAC CE in step S9. The TA value is used to adjust the uplink transmission timing of UE100.

[0060] In step S7, UE100 performs a Layer 1 (L1) measurement on the configured LTM candidate cell and transmits a measurement report (also referred to as "L1 measurement report (L1 MR)") including the L1 measurement result to gNB200 (first cell). The L1 measurement result may be, for example, L1-RSRP and / or L1-SINR.

[0061] In step S8, the gNB200 decides to switch the serving cell to the target cell (second cell).

[0062] In step S9, the gNB200 (first cell) sends a cell switching command MAC CE to the UE100, which includes the candidate setting index (setting ID) of the target cell. The cell switching command MAC CE may include the TA value obtained by UL early synchronization (i.e., the TA value derived based on the RA preamble).

[0063] In step S10, UE100 switches to the settings for the target cell (second cell). Specifically, UE100 detaches from the first cell and applies the settings for the target cell (second cell).

[0064] In step S11, if the serving cell switch requires the execution of a random access procedure (for example, if the cell switch command MAC CE does not contain a valid TA value), UE 100 executes a random access procedure on the target cell (RACH-based LTM cell switch). However, if UE 100 does not need to obtain the TA of the target cell during the serving cell switch (for example, if the cell switch command MAC CE contains a valid TA value), the random access procedure can be skipped (RACH-less LTM cell switch).

[0065] In step S12, UE100 indicates that the serving cell switch to the target cell has been successfully completed by sending, for example, an RRC Reconfiguration Complete message to the target cell (second cell). Subsequently, UE100 may perform steps S6 to S12 multiple times for subsequent LTM serving cell switches based on the settings provided in step S4. Such subsequent LTM cell switches may be referred to as "Subsequent LTMs".

[0066] Although not shown in the diagram, the UE100 itself may measure the TA value corresponding to the LTM candidate cell. Such measurement of the TA value may be called UE-based TA measurement. UE-based TA measurement may be set in the UE100 by the LTM candidate setting. Even if the cell switching command MAC CE received in step S9 does not contain a TA value, if the UE100 has a valid TA value obtained through UE-based TA measurement, it may apply that TA value in step S11 and perform RACH-less LTM cell switching.

[0067] (2.2) InterCU LTM The LTM in 3GPP Release 18 supports intraCU cell switching, but does not support interCU cell switching. In 3GPP Release 19, an extension to the LTM specification is being considered to support interCU cell switching.

[0068] Figure 7 is a diagram illustrating the LTM cell switching of the interCU according to the embodiment.

[0069] As shown in Figure 7, gNB200a has gNB-CU201a and gNB-DU202a. gNB-CU201a and gNB-DU202a are interconnected via the F1 interface. gNB200b has gNB-CU201b and gNB-DU202b. gNB-CU201b and gNB-DU202b are interconnected via the F1 interface. In addition, gNB-CU201a and gNB-CU201b are interconnected via the Xn interface. gNB-DU202a manages one or more cells a, and gNB-DU202b manages one or more cells b. UE100 is in an RRC connected state in cell a and is performing lower-layer communication with gNB-DU202a and upper-layer communication with gNB-CU201a. In this scenario, gNB200a performs an LTM cell switchover to switch the serving cell of UE100 from cell a to cell b.

[0070] In the interCU's LTM cell switching, the LTM cell switching may be performed by a procedure such as the following: 1) UE100 performs an L1 measurement on the configured LTM candidate cell and sends an L1 measurement report (L1 MR) including the L1 measurement result to gNB-DU202a (cell a). 2) gNB-DU202a decides to switch the LTM cell to cell b based on the L1 measurement result from UE100. 3) gNB-DU202a sends a cell switching command MAC CE to UE100 instructing the LTM cell switching to cell b, and UE100 performs the cell switching from cell a to cell b upon receiving the cell switching command MAC CE.

[0071] (2.3) Overview of C-LTM In 3GPP Release 19, the introduction of a Conditional LTM (C-LTM) procedure is planned. In a Conditional LTM, the gNB200 includes information indicating the conditions for executing LTM cell switching (e.g., radio quality conditions) for each LTM candidate cell in the RRC Reconfiguration message, for example in step S4 of Figure 6.

[0072] Instead of receiving the MAC CE cell switching command from the gNB200, the UE100 performs LTM cell switching on LTM candidate cells that meet the set execution conditions (wireless quality conditions). This eliminates the need to send and receive L1 measurement reports and MAC CE cell switching commands, enabling faster LTM cell switching. The execution conditions for LTM cell switching may also be called trigger conditions.

[0073] Figure 8 is a diagram illustrating the operation of C-LTM. Note that the following operation scenario assumes cell switching (C-LTM) between cells within the same gNB (same CU), i.e., intra-gNB C-LTM. However, it may also be applied to cell switching (C-LTM) between cells in different gNBs (different CUs), i.e., inter-gNB C-LTM.

[0074] In step S101, UE100 is in the RRC Connected state in the first cell (serving cell) of gNB200. UE100 may transmit capability information (UE capability) indicating that it supports C-LTM to gNB200. gNB200 may receive said capability information (UE capability).

[0075] In step S102, UE100 sends a Measurement Report message, which is an RRC message, to gNB200 (serving cell). gNB200 receives the Measurement Report message. The Measurement Report message includes a measurement of at least one of the radio quality of the first cell (serving cell) and the radio quality of the second cell (LTM candidate cell).

[0076] In step S103, gNB200 decides to set C-LTM to UE100 and identifies candidate LTM cells.

[0077] In step S104, gNB200 transmits configuration information (C-LTM settings) for C-LTM cell switching to UE100. gNB200 may also transmit an RRC message (specifically, an RRC Reconfiguration message) containing the C-LTM settings to UE100. UE100 receives the C-LTM settings from gNB200 (serving cell). UE100 stores the received C-LTM settings in memory (memory in the control unit 130).

[0078] Figure 9 shows an example of the configuration of an RRC Reconfiguration message that includes a C-LTM configuration. As shown in Figure 9, each LTM candidate configuration (LTM Candidate) in the LTM configuration and each LTM candidate configuration (LTM Candidate) in the C-LTM configuration are merged into a single list (LTM Candidate ToAddModList), and this list is included in the LTM configuration (LTM Config).

[0079] Furthermore, the LTM setting includes a list (cltm-ServingCellExecutionConditions) that shows the execution conditions (trigger conditions) for each of the one or more LTM candidate settings. This list includes as entries a set of the setting ID (LTM Candidate ID) and the execution condition (ExecutionCondition) of the LTM candidate setting. Here, the execution conditions are L1 trigger conditions (L1-Conditions) and / or L3 trigger conditions (L3-Conditions). The L1 trigger condition is a trigger condition that uses the L1 measurement result. The trigger evaluation of whether the L1 trigger condition is met is performed by the MAC layer of UE100. The L3 trigger condition is a trigger condition that uses the L3 measurement result. The trigger evaluation of whether the L3 trigger condition is met is performed by the RRC layer of UE100.

[0080] In this configuration example, UE100 identifies LTM candidate cells with an LTM Candidate ID in the list of execution conditions (cltm-ServingCellExecutionConditions) as C-LTM candidate cells. The execution conditions (cltm-ExecutionConditions) in the LTM candidate settings are used by UE100 to perform subsequent LTM cell switching after switching to the corresponding LTM candidate cell.

[0081] The LTM Candidate setting for C-LTM may include at least one of the following pieces of information: a1) to a5).

[0082] a1) LTM candidate setting ID: Information that identifies a single LTM candidate setting. When gNB200 changes some of the settings of an LTM candidate setting (for example, one of a2) to a4) below), it only needs to notify UE100 of the identifier and the set of settings to be changed. In response to this notification, UE100 only needs to change the notified setting while maintaining the other settings of the LTM candidate setting.

[0083] a2) Cell identifier of LTM candidate cell: This is information that identifies an LTM candidate cell. One example is PCI (Physical Cell Identity).

[0084] a3) Cell switching execution conditions: These are the execution conditions used by UE100 to perform a subsequent LTM cell switching after a cell switching to the corresponding LTM candidate cell. The information in a3) indicates the radio quality conditions that must be met for at least one of the serving cell and the LTM candidate cell in order to trigger a cell switching.

[0085] a4) Timer value of candidate cell TA timer: This is the TA timer setting value, which indicates the setting time of the candidate cell TA timer, a timer that manages the validity of the TA value corresponding to the LTM candidate cell. The candidate cell TA timer may also be called CLTM TAT (Conditional LTM Time Alignment Timer). Details of the candidate cell TA timer will be described later. Upon receiving the timer value in a4), UE100 stores the timer value in memory (memory in the control unit 130) and sets the timer value in the candidate cell TA timer associated with the LTM candidate ID.

[0086] a5) Serving Cell TA Timer Value: This is the TA timer setting value that indicates the timer setting time for which the validity of the TA value applied to a serving cell is managed when an LTM candidate cell becomes a serving cell of UE100. The timer value in a5) is one of the Information Elements (IE) associated with PTAG (Primary Timing Advance Group) within the RRC setting (RRC Reconfiguration) corresponding to the LTM candidate cell. PTAG is a group consisting of one or more serving cells that include the primary cell of UE100. The same TA value is applied to all serving cells in that group. Details of the serving cell TA timer will be described later.

[0087] Returning to Figure 8, in step S105, UE100 sends an RRC Reconfiguration Complete message to gNB200 (serving cell) indicating that the configuration by the RRC Reconfiguration message in step S104 is complete. gNB200 receives the RRC Reconfiguration Complete message.

[0088] In step S106, UE100 performs early synchronization with each LTM candidate cell set in step S104. Hereafter, the terms "LTM candidate cell" and "candidate cell" may be used interchangeably. "C-LTM candidate cell" is also used with the same meaning as these terms.

[0089] Specifically, UE100 sends an RA preamble to each LTM candidate cell. gNB200 receives the RA preamble sent from UE100 at each LTM candidate cell. Based on the RA preamble, gNB200 determines the TA value to apply to each candidate cell.

[0090] In step S107, gNB200 sends a MAC CE containing the TA value (candidate cell TA value) to be applied to the candidate cell to UE100. The MAC CE includes a set of the LTM candidate ID and TA value corresponding to the candidate cell. UE100 receives the MAC CE containing the candidate cell TA value from gNB200 (serving cell).

[0091] Although not shown in the diagram, UE100 may measure the candidate cell TA value itself using UE-based TA measurement. UE-based TA measurement may also be set in UE100 for each candidate cell by the LTM candidate setting (LTM Candidate) in the C-LTM setting.

[0092] In step S108, UE100 starts the candidate cell TA timer associated with the LTM candidate ID contained in the MAC CE received in step S107. UE100 maintains the TA value associated with the candidate cell TA timer while it is operating. When the candidate cell TA timer expires or stops, UE100 discards the TA value associated with the candidate cell TA timer.

[0093] If the candidate cell TA timer expires before UE100 performs LTM cell switching in C-LTM, UE100 may obtain a new TA value from gNB200 by performing the operation in step S106 (Early sync) again for the candidate cell.

[0094] Furthermore, gNB200 may start the candidate cell TA timer, similar to UE100. gNB200 may send a PDCCH order to UE100 when the candidate cell TA timer expires. UE100 may perform an Early sync using CFRA triggered by the PDCCH order. This allows UE100 to obtain a new TA value from gNB200.

[0095] In the case of the inter-gNB C-LTM, the gNB200a managing the serving cell may transmit the timer value of the candidate cell TA timer set in UE100 to the gNB200b managing the candidate cell via the Xn interface. Also, when the gNB200a starts the candidate cell TA timer, it may transmit information indicating the remaining time of the candidate cell TA timer to the gNB200b.

[0096] In step S109, UE100 evaluates whether the conditions for executing cell switching by C-LTM have been met based on the C-LTM settings received in step S104, and detects that the conditions have been met. For example, UE100 detects that Event LTM3 has been met. Event LTM3 is a trigger condition that indicates when the radio quality of the candidate cell's beam has improved by an offset amount compared to the radio quality of the serving cell's beam.

[0097] Here, detection of the fulfillment of the execution conditions may be performed by the MAC layer of UE100. This detection may also be performed by the RRC layer of UE100. For example, if the execution condition is an L1 trigger condition (L1-Conditions), the MAC layer of UE100 detects that the execution condition has been fulfilled. If the execution condition is an L3 trigger condition (L3-Conditions), the RRC layer of UE100 detects that the execution condition has been fulfilled. When the RRC layer of UE100 detects that the execution condition has been fulfilled, the RRC layer of UE100 may indicate to the MAC layer of UE100 that LTM cell switching should be performed.

[0098] In step S110, UE100 detaches from the serving cell and applies the RRC settings of the candidate cell corresponding to the satisfied execution conditions. In the following, the candidate cell corresponding to the satisfied execution conditions may also be called the target cell.

[0099] Although not shown in the diagram, UE100 may perform the operation in step S110 in response to receiving the cell switching command MAC CE from gNB200.

[0100] In step S111, UE100 determines whether a valid TA value exists for the target cell. Specifically, UE100 determines that a valid TA value exists for the target cell if the candidate cell TA timer associated with the LTM candidate ID corresponding to the target cell (candidate cell) is operating. However, UE100 may also determine that a valid TA value exists for the target cell if it has a TA value obtained by UE-based TA measurement, even if the candidate cell TA timer is not operating.

[0101] On the other hand, if the candidate cell TA timer is not operating and there is no TA value obtained by UE-based TA measurement, the UE100 determines that there is no valid TA value for that target cell.

[0102] If UE100 determines that a valid TA value exists for the target cell (S111: YES), it proceeds to step S112. On the other hand, if UE100 determines that no valid TA value exists for the target cell (S111: NO), it proceeds to step S113.

[0103] In step S112, UE100 applies a valid TA value and performs uplink communication with the target cell. At this point, since the target cell becomes the serving cell of UE100, UE100 starts the serving cell TA timer corresponding to the target cell.

[0104] One example of uplink communication is the transmission of an RRC Reconfiguration Complete message. UE100 may also transmit the RRC Reconfiguration Complete message using a CG (configured grant). For example, if a CG was configured in step S104, UE100 will use the CG resource to perform a PUSCH (Physical Uplink Shared Channel) transmission.

[0105] Uplink communication may also be the transmission of user data. Such uplink communication may also be the transmission of other control signaling (such as MAC signaling or RRC signaling).

[0106] Meanwhile, in step S113, UE100 performs a random access procedure on the target cell. In the random access procedure, UE100 can obtain a valid TA value. Upon obtaining a valid TA value, UE100 activates the serving cell TA timer corresponding to the target cell.

[0107] In step S114, UE100 completes the LTM cell switching. Specifically, if UE100 has performed the operation in step S112, UE100 may consider the LTM cell switching to have been successfully completed if it determines that gNB200 has successfully received the uplink communication in step S112. For example, after performing the operation in step S112, UE100 may consider the LTM cell switching to have been successfully completed if it receives an uplink grant or downlink assignment for a new transmission on the PDCCH. Receiving such an uplink grant or downlink assignment for a new transmission on the PDCCH suggests that gNB200 has successfully received the uplink communication in step S112. Alternatively, if UE100 has performed the operation in step S113, it may consider the LTM cell switching to have been successfully completed if the random access procedure has been successfully completed.

[0108] The operations in steps S110 to S114 may be collectively referred to as the LTM cell switching operation in C-LTM. The LTM cell switching operation when the operation in step S112 is performed may be referred to as the RACH-Less LTM cell switching operation. The LTM cell switching operation when the operation in step S113 is performed may be referred to as the RACH-based LTM cell switching operation.

[0109] In step S115, UE100 performs a subsequent LTM cell switchover. Specifically, in step S115, UE100 may perform steps S106 to S114 for the subsequent LTM cell switchover operation in C-LTM based on the settings provided in step S104. UE100 may perform the subsequent LTM cell switchover operation multiple times.

[0110] (2.4) Timers for managing the validity of TA values ​​In the C-LTM cell switching described above, there are two types of timers for managing the validity of TA values: a timer for managing the validity of the TA value applied to the current serving cell, and a timer for managing the validity of the TA value corresponding to the LTM candidate cell. Hereinafter, the former timer will be referred to as the "serving cell TA timer," and the latter timer will be referred to as the "candidate cell TA timer."

[0111] (2.4.1) Serving Cell TA Timer The serving cell TA timer manages the validity of the TA value that UE100 is applying to the current serving cell.

[0112] UE100 manages the serving cell TA timer in association with the PTAG. One PTAG is configured in UE100. One TA value is applied to the PTAG. UE100 starts or restarts the serving cell TA timer whenever the TA value applied to the PTAG is updated. The update of the TA value applied to the PTAG is performed, for example, by a Timing Advance Command MAC CE sent from the serving cell. UE100 considers the TA value applied to the PTAG to be invalid if the serving cell TA timer has expired.

[0113] The serving cell TA timer may also be called "TimeAlignmentTimer" (without prefix), "TAT" (without prefix), or "PTAG TAT".

[0114] After UE100 performs an LTM cell switch to an LTM candidate cell, that LTM candidate cell becomes the primary cell of UE100, and the TA value corresponding to that LTM candidate cell is applied to the PTAG. As a result, the TA value applied to the PTAG is updated, and the serving cell TA timer is restarted.

[0115] (2.4.2) Candidate Cell TA Timer The candidate cell TA timer manages the validity of the TA value of the LTM candidate cell held by UE100. Note that the validity of the TA value obtained by UE-based TA measurement is not managed by the candidate cell TA timer.

[0116] The UE100 manages one candidate cell TA timer for each LTM candidate cell.

[0117] UE100 starts or restarts the candidate cell TA timer whenever the TA value corresponding to the LTM candidate cell is updated.

[0118] UE100 invalidates the TA value corresponding to a candidate cell if the candidate cell TA timer corresponding to that candidate cell expires.

[0119] (3) First Embodiment Based on the above-described configuration and operation, the first embodiment will be described.

[0120] UE100 can perform an LTM cell switch to the target cell (cell b) when it has applied TA value #1 to the serving cell (cell a) and the serving cell TA timer is running. As a result, the target cell (cell b) becomes the serving cell (primary cell) of UE100, and UE100 applies TA value #2 to the target cell (cell b) as the TA value to be applied to the PTAG. At this time, since the serving cell TA timer is running, TA value #1 is still valid. However, because there is only one TA value to be applied to the PTAG, UE100 no longer retains TA value #1.

[0121] If cell a is set as an LTM candidate cell in UE100, after UE100 performs an LTM cell switch to the target cell (cell b) (first LTM cell switch), UE100 may perform an LTM cell switch to cell a (second LTM cell switch) via Subsequent LTM. In this case, UE100, which does not hold the TA value #1 for cell a, needs to perform early synchronization to receive a MAC CE containing the TA value for cell a from the current serving cell (cell b) in order to obtain the TA value for cell a. Alternatively, UE100 can perform a RACH-based LTM cell switch for cell a. Such signaling presents the problem of wasted radio resources and user plane interruptions.

[0122] In this embodiment, an operation that solves these problems will be described.

[0123] (3.1) UE Operation Diagram 10 is a diagram showing the operation of UE 100 according to the first embodiment.

[0124] In step S201, UE100, with the first cell (cell a) set as the serving cell, receives a first candidate setting (first LTM candidate setting) that sets the first cell as the LTM candidate cell, and a second candidate setting (second LTM candidate setting) that sets the second cell (cell b) as the LTM candidate cell. This operation corresponds to step S4 in Figure 6.

[0125] In step S202, the UE100 manages the validity of the first TA value applied to the uplink communication of the first cell using the serving cell TA timer.

[0126] In step S203, UE100 performs an LTM cell switch to the second cell (cell b) (first LTM cell switch) based on the second candidate setting.

[0127] In step S204, UE100 retains the first TA value even when performing an LTM cell switch to the second cell, provided that a first candidate setting exists and the serving cell TA timer is operating.

[0128] Thus, in this embodiment, it becomes possible to continuously maintain the first TA value of the first cell, which is still valid.

[0129] In this embodiment, UE100 may retain the first TA value even when performing a first LTM cell switchover if there is a first candidate setting having the same cell identifier (e.g., physical cell identifier (PCI)) as the cell identifier of the first cell (cell a), and the serving cell TA timer is operating. This makes it possible to associate the first TA value with the correct cell and retain it.

[0130] In this embodiment, UE100 may receive a first candidate setting from the first cell (cell a) that sets the first cell (cell a) as a candidate cell for conditional LTM. This makes it possible to continuously maintain the TA value for the candidate cell of C-LTM.

[0131] In this embodiment, UE100 may receive a second TA value from the first cell to be applied to uplink communication with the second cell (cell b). When UE100 performs an LTM cell switch to the second cell, it may associate the first TA value with the first candidate setting and apply the second TA value to uplink communication with the second cell. This makes it possible to apply an appropriate TA value to uplink communication with the second cell while still maintaining a valid TA value.

[0132] In this embodiment, UE100 maintains TA value #1 in association with the first candidate setting and manages the validity of TA value #1 using the candidate cell TA timer. UE100 may also set the remaining time of the serving cell TA timer to the candidate cell TA timer. This makes it possible to guarantee the validity of TA value #1, which is to be used as the candidate cell TA value.

[0133] In this embodiment, when performing an LTM cell switch to the first cell (specifically, the LTM cell switch described later above) after an LTM cell switch to the second cell, the held TA value #1 may be applied to the uplink communication with the first cell, depending on whether the candidate cell TA timer corresponding to the first cell is operating. This makes it possible to perform a subsequent RACH-less LTM cell switch to the first cell and reduce delays caused by cell switching.

[0134] (3.2) Figure 11 of the first embodiment is a diagram showing an embodiment of the first embodiment. The basic operation is the same as in Figure 8, and some of the steps shown in Figure 8 are omitted from the illustration and explanation.

[0135] In step S301, UE100 is in the RRC Connected state in the first cell (cell a). The serving cell of UE100 is cell a, the TA value applied to cell a is TA value #1, and the serving cell TA timer that manages the validity of TA value #1 is operating.

[0136] UE100 stores LTM Candidate setting #1 and LTM Candidate setting #2 as C-LTM settings. Cell a is set as the candidate cell in LTM Candidate setting #1. The second cell (cell b) is set as the candidate cell in LTM Candidate setting #2. Here, cells a and b may be managed by the same gNB200, or they may be managed by different gNB200s.

[0137] LTM Candidate #1 contains at least the following information: • Candidate cell's cell identifier (CandidatePCI): PCI #1, which is the PCI of cell a. • Candidate cell TA timer's timer value (CLTM TAT): Timer value #1. • Serving cell TA timer's timer value (PTAG TAT): Timer value #2.

[0138] The LTM Candidate setting (LTM Candidate) #2 contains at least the following information: • Candidate cell's cell identifier (CandidatePCI): PCI #2, which is the PCI of cell b. • Candidate cell TA timer's timer value (cltm-TAT): Timer value #3. • Serving cell TA timer's timer value (PTAG TAT): Timer value #4.

[0139] In step S302, UE100 receives a MAC CE from cell a that includes TA value #2, which is the TA value of cell b. TA value #2 is obtained, for example, by early synchronization performed by UE100 on cell b in a manner similar to step S106 in Figure 8.

[0140] In step S303, UE100 starts the candidate cell TA timer corresponding to cell b. Here, UE100 sets the stored timer value #3 to the candidate cell TA timer.

[0141] In step S304, while the candidate cell TA timer corresponding to cell b is operating, UE100 detects that the conditions for executing the C-LTM cell switch to cell b have been met. This detection operation is the same as the operation in step S109 in Figure 8.

[0142] In step S305, UE100 applies the retained TA value #2 to PTAG and retains TA value #1 without discarding it. UE100 associates TA value #1 with LTM candidate setting #1 which has PCI #1, the cell identifier of cell a.

[0143] UE100 performs uplink communication with cell b (new serving cell) applying TA value #2. UE100 stores the remaining time of the serving cell TA timer in association with LTM candidate setting #1. Then UE100 sets timer value #4 to the serving cell TA timer and restarts the serving cell TA timer.

[0144] The specific details of the uplink communication in step S305 are the same as those of the uplink communication in step S112 in Figure 8.

[0145] In step S306, UE100 completes the LTM cell switch to cell b. The specific operation is the same as in step S114 in Figure 8.

[0146] In step S307, UE100 sets the remaining time stored in step S305 into the candidate cell TA timer corresponding to cell a, and starts the candidate cell TA timer.

[0147] Furthermore, UE100 may set a time less than the stored remaining time to the candidate cell TA timer corresponding to cell a. For example, UE100 may set a time obtained by applying a negative offset value to the stored remaining time.

[0148] In step S308, while the candidate cell TA timer corresponding to cell a is operating, the UE100 detects that the conditions for executing a C-LTM cell switch to cell a for the subsequent LTM have been met.

[0149] In step S309, UE100 applies the held TA value #1 to the PTAG and performs uplink communication with cell a (new serving cell) using the applied TA value #1.

[0150] In step S310, UE100 completes the Subsequent LTM cell switch to cell a.

[0151] (4) Second Embodiment The second embodiment will be described mainly in terms of the differences from the first embodiment. The second embodiment may be implemented in combination with the first embodiment.

[0152] When UE100 performs a C-LTM cell switch to a candidate cell, if the candidate cell TA timer is running, it applies the TA value held for that candidate cell to the PTAG. To manage the validity of the TA value applied to the PTAG, UE100 sets the timer value (PTAG TAT value) of the serving cell TA timer stored in association with that candidate cell to the serving cell TA timer and restarts the serving cell TA timer.

[0153] In this case, the TA value is valid for the remaining time of the candidate cell TA timer, but if a timer value longer than that remaining time is set for the serving cell TA timer, the TA value that should be invalid may still be considered valid.

[0154] In this embodiment, an operation that solves these problems will be described.

[0155] (4.1) UE Operation Diagram 12 is a diagram showing the operation of UE 100 according to the second embodiment.

[0156] In step S401, UE100 receives a TA value from the first cell to be applied to uplink communication with the second cell, while the first cell (cell a) is connected as a serving cell and the second cell (cell b) is set as a candidate cell for C-LTM.

[0157] In step S402, UE100 holds the TA value and manages the validity of the TA value using the candidate cell TA timer.

[0158] In step S403, when UE100 performs an LTM cell switching to switch the serving cell to the second cell (cell b), it applies the TA value to the uplink communication with the second cell, depending on whether the candidate cell TA timer is operating.

[0159] In step S404, UE100 determines the setting time of the serving cell TA timer based on the remaining time of the candidate cell TA timer in order to manage the validity of the TA value applied to uplink communication with the second cell using the serving cell TA timer.

[0160] Thus, in this embodiment, it is possible to avoid the TA value that should be invalid being considered valid, and to guarantee the validity of the TA value applied to the serving cell.

[0161] In this embodiment, UE100 may set the remaining time of the candidate cell TA timer as the setting time of the serving cell TA timer. This makes it possible to guarantee the validity of the TA value applied to the serving cell.

[0162] Alternatively, UE100 may set the serving cell TA timer to a time less than the remaining time of the candidate cell TA timer. For example, UE100 may set the serving cell TA timer to a time obtained by applying a negative offset value to the remaining time of the candidate cell TA timer. This makes it possible to guarantee the validity of the TA value applied to the serving cell.

[0163] (4.2) Figure 13 of the second embodiment shows an example of the second embodiment. The basic operation is the same as in Figure 8, and some of the steps shown in Figure 8 are omitted from the illustration and explanation.

[0164] In step S501, UE100 is in the RRC Connected state in the first cell (cell a). The serving cell of UE100 is cell a, the TA value applied to cell a is TA value #1, and the serving cell TA timer that manages the validity of TA value #1 is operating.

[0165] UE100 stores LTM Candidate #1 as a C-LTM setting. Cell b is set as a candidate cell in LTM Candidate #1. Here, cells a and b may be managed by the same gNB200, or they may be managed by different gNB200s.

[0166] LTM Candidate #1 contains at least the following information: • Candidate cell's cell identifier (CandidatePCI): PCI #2, which is the PCI of cell b. • Candidate cell TA timer (CLTM TAT) timer value: Timer value #1. • Serving cell TA timer (PTAG TAT) timer value: Timer value #2.

[0167] In step S502, UE100 receives a MAC CE from cell a that includes TA value #2, which is the TA value of cell b. TA value #2 is obtained, for example, by early synchronization performed by UE100 on cell b in a manner similar to step S106 in Figure 8.

[0168] In step S503, UE100 starts the candidate cell TA timer corresponding to cell b. Here, UE100 sets the stored timer value #1 to the candidate cell TA timer.

[0169] In step S504, while the candidate cell TA timer corresponding to cell b is operating, UE100 detects that the conditions for executing the C-LTM cell switch to cell b have been met. This detection operation is the same as the operation in step S109 in Figure 8.

[0170] In step S505, UE100 applies the held TA value #2 to the PTAG. UE100 also performs uplink communication with cell b (the new serving cell) applying TA value #2. UE100 sets the remaining time of the candidate cell TA timer to the serving cell TA timer and restarts the serving cell TA timer.

[0171] Furthermore, UE100 may set the serving cell TA timer to a time less than the remaining time of the candidate cell TA timer. For example, UE100 may set the serving cell TA timer to a time obtained by applying a negative offset value to the remaining time of the candidate cell TA timer.

[0172] Alternatively, UE100 may compare the remaining time of the candidate cell TA timer with the timer value (PTAG TAT) (timer value #2) of the serving cell TA timer stored as an LTM candidate setting. If the remaining time of the candidate cell TA timer is less, UE100 may set the remaining time of the candidate cell TA timer to the serving cell TA timer and restart the serving cell TA timer. In other words, if the timer value (timer value #2) of the serving cell TA timer stored as an LTM candidate setting is less, UE100 may set the timer value (timer value #2) of the serving cell TA timer to the serving cell TA timer and restart the serving cell TA timer.

[0173] In step S506, UE100 completes the LTM cell switch to cell b.

[0174] Subsequently, UE100 discards TA value #2 in accordance with the expiration of the serving cell TA timer.

[0175] (5) Third Embodiment The differences between the third embodiment and the first and second embodiments will be described primarily. The third embodiment may be implemented in combination with at least one embodiment of the first and second embodiments.

[0176] After the UE100 performs an LTM cell switch to an LTM candidate cell, it manages the validity of the TA value applied to the serving cell after the LTM cell switch using the serving cell TA timer. The timer value set in the TA timer is the TA timer setting value that the UE100 stored when the LTM setting was configured in the UE100 before the LTM cell switch. Once the TA timer setting value is stored, it will not be changed unless it is reset by the RRC. Therefore, there is a possibility that this TA timer setting value may not match the current state of the UE100.

[0177] Specifically, the TA value depends on the physical distance between the cell (e.g., TRP) and UE100. Therefore, the timer value of the TA timer that manages the effectiveness of the TA value is set depending on the movement speed of UE100, for example. For example, when UE100 is moving slowly, the distance traveled per unit time is short, so the timer value can be set to be long. On the other hand, when UE100 is moving fast, the distance traveled per unit time is long, so the timer value should be shortened. Here, since the movement speed of UE100 can change, there is a problem that the previously set timer value may not be consistent with the current UE movement speed. This problem may become more apparent in Subsequent LTM.

[0178] In this embodiment, an operation that solves these problems will be described.

[0179] (5.1) UE Operation Diagram 14 is a diagram showing the operation of UE 100 according to the third embodiment.

[0180] In step S601, UE100 stores a TA timer setting value that indicates the setting time for the TA timer to manage the validity of the TA value.

[0181] In step S602, UE100 receives a MAC CE from the serving cell to update the TA timer setting value.

[0182] In step S603, UE100 updates the stored TA timer setting value in response to the MAC CE reception.

[0183] Thus, in this embodiment, the TA timer setting value stored in UE100 can be appropriately updated at the MAC layer instead of RRC. Therefore, the TA timer setting value can be adaptively updated without RRC resetting. The validity of the TA value managed by the TA timer set to such a TA timer setting value can be guaranteed. In this embodiment, the "TA timer" may be the serving cell TA timer described above, or the candidate cell TA timer described above.

[0184] In this embodiment, MAC CE may include information specifying the new TA timer setting value after the update. UE 100 may update the stored TA timer setting value to overwrite it with the new TA timer setting value specified by MAC CE. This makes it possible to easily update the TA timer setting value.

[0185] In this embodiment, MAC CE may include an offset value for the TA timer setting value stored in UE 100. UE 100 may update the TA timer setting value by applying this offset value. This makes it possible to update the TA timer setting value with less signaling information than signaling the TA timer setting value itself.

[0186] In this embodiment, the TA timer setting value is included in the setting of the LTM candidate cell. This makes it possible to appropriately update the timer setting value associated with the LTM candidate cell.

[0187] In this embodiment, UE100 applies the updated TA timer setting value in response to the execution of an LTM cell switch from a serving cell to an LTM candidate cell. This ensures the validity of the TA value applied to the new serving cell after the LTM cell switch.

[0188] In this embodiment, the TA timer setting value is included in the setting of the serving cell. This makes it possible to guarantee the effectiveness of the TA value applied to the current serving cell of UE100.

[0189] In this embodiment, when an LTM cell switch is performed from a serving cell to an LTM candidate cell, the UE 100 applies the updated TA timer setting value included in the settings of the serving cell before the LTM cell switch as the TA timer setting value for the LTM candidate cell. This eliminates the need for MAC CE to update the TA timer setting value for the LTM candidate cell, reducing signaling while ensuring the validity of the TA value for the LTM candidate cell.

[0190] (5.2) Example 1 of the Third Embodiment Figure 15 shows Example 1 of the Third Embodiment. Example 1 of the Third Embodiment is an example relating to updating the TA timer setting value of a serving cell TA timer set in an LTM candidate cell. Hereinafter, the terms "TA timer setting value" and "timer value" may be used interchangeably.

[0191] In step S701, UE100 is in the RRC Connected state in the first cell (cell a). The serving cell of UE100 is cell a, the TA value applied to cell a is TA value #1, and the serving cell TA timer that manages the validity of TA value #1 is operating.

[0192] UE100 stores LTM candidate setting #1 as an LTM setting. Cell b is set as a candidate cell in LTM candidate setting #1. Here, cells a and b may be managed by the same gNB200, or they may be managed by different gNB200s.

[0193] LTM candidate setting #1 contains at least the following information: • Candidate cell identifier (CandidatePCI): PCI #2, which is the PCI of cell b. • Serving cell TA timer (PTAG TAT) timer value: Timer value #1

[0194] In step S702, UE100 receives a MAC CE from cell a, which includes timer value #2 for the serving cell TA timer for cell b. This MAC CE may be an existing MAC CE (e.g., a Timing Advance Command MAC CE). This MAC CE may be a newly introduced MAC CE for updating the timer setting value for the serving cell TA timer for a candidate cell. This MAC CE includes a set of LTM candidate ID corresponding to LTM candidate setting #1 and timer value #2. This allows UE100 to understand that timer value #2 is for cell b.

[0195] In step S703, UE100 overwrites the setting value of the serving cell TA timer stored in correspondence with LTM candidate setting #1 with timer value #2.

[0196] In step S704, UE100 performs an LTM cell switch to cell b, applies TA value #2 for cell b, and performs uplink communication to cell b. Here, the LTM cell switch to cell b may be performed in response to the reception of a Cell Switch Command. The LTM cell switch may also be performed in response to the fulfillment of the execution conditions for C-LTM set for cell b. TA value #2 for cell b may be obtained by a Cell Switch Command. TA value #2 may also be obtained by early synchronization to cell b. TA value #2 may also be obtained by UE-based TA measurement.

[0197] UE100 sets timer value #2 to the serving cell TA timer and restarts the serving cell TA timer upon executing the LTM cell switch to cell b.

[0198] In step S705, UE100 completes the LTM cell switch to cell b.

[0199] (5.3) Example 2 of the Third Embodiment Figure 16 shows Example 2 of the Third Embodiment. Example 2 is an example relating to updating the setting value of the serving cell TA timer set in the current serving cell of UE100. The differences between Example 2 and Example 1 will be mainly explained.

[0200] In step S801, UE100 is in the RRC Connected state in the first cell (cell a). The serving cell of UE100 is cell a, the TA value applied to cell a is TA value #1, and the serving cell TA timer that manages the validity of TA value #1 is operating.

[0201] UE100 stores LTM candidate setting #1 as its LTM setting. In LTM candidate setting #1, cell b is set as the candidate cell.

[0202] LTM candidate setting #1 contains at least the following information: • Candidate cell identifier (CandidatePCI): PCI#2, which is the PCI of cell b. • Serving cell TA timer (PTAG TAT) timer value: Timer value #2

[0203] Furthermore, UE100 stores the serving cell settings for the current serving cell (cell a). Such serving cell settings include at least a timer value #1, which is the timer value of the serving cell TA timer (PTAG TAT).

[0204] In step S802, UE100 receives a MAC CE from cell a, which includes timer value #3, the timer value of the serving cell TA timer for the current serving cell (cell a). This MAC CE may be an existing MAC CE in the specifications (e.g., Timing Advance Command MAC CE). This MAC CE may be a newly introduced MAC CE for updating the timer setting value of the serving cell TA timer for the current serving cell.

[0205] In step S803, UE100 overwrites the PTAG TAT value in the stored serving cell settings with timer value #3.

[0206] In step S804, UE100 performs an LTM cell switch to cell b, applies TA value #2 for cell b, and performs uplink communication to cell b. UE100 restarts the serving cell TA timer by performing the LTM cell switch to cell b. Here, UE100 sets the serving cell TA timer with timer value #3, which is updated in step S803, instead of the stored timer value #2 corresponding to cell b.

[0207] In step S805, UE100 completes the LTM cell switch to cell b.

[0208] In step S803, UE100 may update the timer value of the serving cell TA timer stored in association with LTM Candidate #1 by overwriting it with timer value #3.

[0209] (5.4) Example 3 of the Third Embodiment Figure 17 shows Example 3 of the Third Embodiment. Example 3 is an example relating to updating the TA timer setting value of the candidate cell TA timer set in the LTM candidate cell. Example 3 will be explained mainly in terms of the differences from Example 1.

[0210] In step S901, UE100 is in the RRC Connected state in the first cell (cell a). The serving cell of UE100 is cell a, the TA value applied to cell a is TA value #1, and the serving cell TA timer that manages the validity of TA value #1 is operating.

[0211] UE100 stores LTM candidate setting #1 as the C-LTM setting. Cell b is set as the candidate cell in LTM candidate setting #1.

[0212] LTM candidate setting #1 contains at least the following information: • Candidate cell's cell identifier (CandidatePCI): PCI#2, which is the PCI of cell b. • Candidate cell TA timer (CLTM TAT) timer value: Timer value #1

[0213] In step S902, UE100 receives a MAC CE from cell a that includes the timer value #2 of the candidate cell TA timer for cell b. This MAC CE may be an existing MAC CE (e.g., Timing Advance Command MAC CE). This MAC CE may be a newly introduced MAC CE for updating the TA timer setting value of the candidate cell TA timer for the candidate cell. This MAC CE includes a set of LTM candidate ID corresponding to LTM candidate setting #1 and timer value #2. This allows UE100 to understand that timer value #2 is for cell b.

[0214] In step S903, UE100 overwrites the timer value of the candidate cell TA timer stored in correspondence with LTM candidate setting #1 with timer value #2.

[0215] In step S904, UE100 receives a MAC CE from cell a that includes TA value #2, which is the TA value of cell b.

[0216] In step S905, UE100 starts the candidate cell TA timer corresponding to cell b. Here, UE100 sets the timer value #2, which is updated in step S903, to the candidate cell TA timer.

[0217] In step S906, while the candidate cell TA timer corresponding to cell b is operating, UE100 detects that the conditions for executing the C-LTM cell switch to cell b have been met.

[0218] In step S907, UE100 performs an LTM cell switch to cell b, applies TA value #2 for cell b, and performs uplink communication to cell b.

[0219] In step S908, UE100 completes the LTM cell switch to cell b.

[0220] (6) Other Embodiments Each of the above-described operation flows can be implemented not only separately and 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, and only some steps may be executed. In addition, the order of steps in each flow may be changed as appropriate.

[0221] 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. That is, UE100 may be a terminal function unit (a type of communication module) for the base station to control a relay device that performs signal relay. Such a terminal function unit is referred to as an MT. Examples of multi-transmission architectures (MTs) include IAB-MT, NCR (Network Controlled Repeater)-MT, and RIS (Reconfigurable Intelligent Surface)-MT.

[0222] 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.

[0223] A program is provided that causes a computer to execute each of the processes according to the above embodiment. 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 storage medium. The non-transient storage medium is not particularly limited, but may be a recording medium such as a CD-ROM and / or DVD-ROM. Furthermore, the circuits that execute each of the processes performed by the apparatus according to the above embodiment may be integrated, and at least a part of the apparatus may be configured as a semiconductor integrated circuit (chipset (System on a chip), SoC (System on a chip)).

[0224] The functions realized by the apparatus according to the above-described embodiment may be implemented in a circuit or processing circuit, including a general-purpose processor, application-specific processor, integrated circuit, ASICs (Application Specific Integrated Circuits), CPU (Central Processing Unit), conventional circuits, and / or combinations thereof, which are programmed to realize the described functions. The processor includes transistors and / or other circuits and is considered a circuit or processing circuit. The processor may be a programmed processor that executes a program stored in memory. In this specification, circuit, unit, and means are hardware programmed to realize or execute the described functions. The hardware may be any hardware disclosed herein, or any hardware known to be programmed to perform or execute the functions described herein. If the hardware is a processor that is considered to be of the type of circuit, the circuit, means, or unit is a combination of hardware and software used to constitute the hardware and / or processor.

[0225] The phrases “based on” and “depending on / in response to” as used in this disclosure do not mean “based solely on” or “in response solely” unless otherwise specified. “Based on” means both “based solely on” and “at least partially on.” Similarly, “depending” means both “at least partially on” and “in at least partially on.” The terms “include,” “comprise,” and variations thereof do not mean that only the listed items are included, but that they may include only the listed items or may include additional items in addition to the listed items. Furthermore, the term “or” as used in this disclosure is not intended to mean exclusive OR. Moreover, any reference to elements using designations such as “first,” “second,” etc., as used in this disclosure does not generally limit the quantity or order of those elements. These designations may be used herein as a convenient 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 from the context that they are not.

[0226] 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.

[0227] This application claims priority to U.S. Provisional Application No. 63 / 777931 (filed March 26, 2025), the entirety of which is incorporated into the specification of this application.

[0228] (7) First Supplementary Notes The features of the above-described embodiment are described below.

[0229] - Appendix 1 A communication method to be performed by a user device in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: receiving from the first cell a first candidate setting that sets the first cell as an LTM candidate cell and a second candidate setting that sets the second cell as an LTM candidate cell, while the user device is using the first cell as a serving cell; managing the validity of a first timing advance (TA) value to be applied to uplink communication with the first cell using a serving cell TA timer; performing a first LTM cell switch to the second cell based on the second candidate setting; and retaining the first TA value even when performing the first LTM cell switch when the first candidate setting exists and the serving cell TA timer is operating.

[0230] - Appendix 2 The communication method according to Appendix 1, wherein the user device retains the first TA value even when the first LTM cell switching is performed when the first candidate setting having the same cell identifier as the first cell exists and the serving cell TA timer is operating.

[0231] - Appendix 3 The communication method described in Appendix 1 or Appendix 2, wherein the user device receives the first candidate setting from the first cell, setting the first cell as a candidate cell for conditional LTM.

[0232] - Appendix 4: A communication method according to any one of Appendix 1 to 3, further comprising: receiving a second TA value from the first cell to apply to uplink communication with the second cell; and, when performing the first LTM cell switching, holding the first TA value in association with the first candidate setting and applying the second TA value to uplink communication with the second cell.

[0233] - Appendix 5 The communication method according to Appendix 4, further comprising: holding the first TA value in association with the first candidate setting, and managing the validity of the held first TA value by the candidate cell TA timer, wherein the user device sets the remaining time of the serving cell TA timer to the candidate cell TA timer.

[0234] - Appendix 6 The communication method according to Appendix 5, further comprising: performing a second LTM cell switch from the second cell to the first cell based on the first candidate setting after the first LTM cell switch; and applying the held first TA value to the uplink communication with the first cell, depending on whether the candidate cell TA timer is operating when the second LTM cell switch is performed.

[0235] - Appendix 7 A user device for use in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: a receiving unit that receives from the first cell a first candidate setting that sets the first cell as an LTM candidate cell and a second candidate setting that sets the second cell as an LTM candidate cell when the user device is connected to the first cell as a serving cell; and a control unit that manages the validity of a first timing advance (TA) value applied to uplink communication with the first cell using a serving cell TA timer, wherein the control unit performs a first LTM cell switch to the second cell based on the second candidate setting, and maintains the first TA value even when performing the first LTM cell switch when the first candidate setting exists and the serving cell TA timer is operating.

[0236] (8) Second Addendum 1. Introduction In RAN2#129, the following matters were agreed upon regarding the MAC issue: Agreements regarding the MAC issue:

[0237] CLTM TAT is started or restarted when the UE receives a TA value for a candidate cell sent by the MAC CE from the current serving cell.

[0238] If the CLTM TAT associated with a candidate cell is active, the UE considers the TA for that candidate cell to be valid. If the CLTM TAT associated with a candidate cell has expired, the UE considers the TA for that candidate cell to be invalid.

[0239] If the UE holds a valid TA value when CLTM is executed, the UE will execute RACH-less CLTM; otherwise, it will execute RACH-based CLTM.

[0240] CLTM TAT is set per candidate configuration.

[0241] Whether received CLTM TAs from other candidate cells are released during CLTM execution, or whether they remain valid on the UE side, will be considered as an additional matter.

[0242] MAC CE for early TA acquisition in CLTM includes absolute TA values.

[0243] For obtaining early TA in CLTM, the MAC CE includes candidate configuration IDs. Furthermore, each MAC CE contains one configuration ID for each TA.

[0244] This addendum discusses the handling of the new timer, namely CLTM TAT.

[0245] 2. Discussion 2.1. Handling of CLTM TAT and Legacy TAT in CLTM Execution In the conditional LTM (CLTM) of Release 19 (Rel-19), the TA value acquired in the early synchronization phase is notified via a new MAC CE that is different from the cell switching command MAC CE of Release 18 (Rel-18). Therefore, there is a time lag between the receipt of the new MAC CE with the TA value in the UE and the occurrence of CLTM execution. Consequently, after the early synchronization phase, the UE needs to check whether the TA value it holds is valid before executing RACH-less CLTM. For this purpose, it was agreed at RAN2#128 meeting that "the TA of candidate cells will be maintained by a new timer." In this note, this new timer will be referred to as CLTM TAT.

[0246] Before considering the CLTM TAT for the TA of LTM candidate cells, we will summarize the current Time Alignment Timer (TAT) for serving cell TA management. According to TS38.321, the UE uses the TAT to manage the validity of the TA value for uplink (UL) transmission to the serving cell. Upon TAT expiration, the UE's MAC sequentially flushes the HARQ buffer, instructs the RRC to delete the PUCCH / SRS, and cancels any downlink allocations / uplink grants that have already been received. The TAT is set for each Timing Advance Group (TAG) associated with multiple serving cells.

[0247] timeAlignmentTimer (per TAG): A MAC entity controls the period over which a serving cell for an associated TAG considers the uplink to be time-synchronized for that TAG.

[0248] On the other hand, according to the latest MAC running CR, the UE uses CLTM TAT to manage the validity of retained TA values ​​for CLTM candidate cells acquired in the early synchronization phase. Each CLTM TAT is associated with one candidate setting for CLTM.

[0249] ltm-Candidate-TimeAlignmentTimer: The MAC entity controls the period during which the CLTM candidate cell associated with this timer is considered time-synchronized on the uplink. Each ltm-Candidate-TimeAlignmentTimer is associated with one candidate configuration for CLTM.

[0250] Observation 1: It should be noted that CLTM TAT operates on TA values ​​retained for CLTM candidate cells, and not on TA values ​​applied for serving cells handled by legacy TAT.

[0251] Furthermore, we will consider the behavior of the UE when the CLTM TAT expires. In legacy TAT, the UE operates to stop UL / DL transmission when the timer expires. On the other hand, when the CLTM TAT expires, the UE does not need to stop UL / DL transmission to the serving cell (i.e., source cell), but it should consider the corresponding TA value invalid and / or discard the retained TA value.

[0252] Proposal 1: RAN2 should agree that upon the expiration of the CLTM TAT, the UE should invalidate any retained TA values ​​associated with the timer, and / or discard any retained TA values.

[0253] According to the latest MAC running CR, after CLTM execution begins, the TA value corresponding to the CLTM candidate cell (i.e., target cell) on which LTM was executed is applied to the PTAG, and the legacy TAT associated with the PTAG is started or restarted using the timeAlignmentTimer provided within the applied LTM candidate setting.

[0254] 1> If a conditional LTM cell switching procedure is triggered for an LTM candidate cell as described in Section 5.y.3 or as instructed by a higher layer, and the ltm-Candidate-TimeAlignmentTimer associated with the LTM candidate cell is running: 2> Apply the retained TA value associated with the LTM target cell to the PTAG as specified in Section 6.1.3.4x. 2> Start or restart the timeAlignmentTimer associated with the PTAG.

[0255] Finding 2: When the CLTM cell switching procedure is triggered, the legacy TAT is started / restarted using the TAT value provided in the LTM candidate configuration.

[0256] However, there are doubts about applying timeAlignmentTimer within the LTM candidate setting because the validity of the TA value may not be guaranteed. The following three steps illustrate the scenario in which the validity of the TA value may not be guaranteed.

[0257] Step 1: When the execution of CLTM is triggered, the remaining time for the CLTM TAT is 300ms. This means that the retained TA value will be valid for the next 300ms.

[0258] Step 2: The legacy TAT value (i.e., timeAlignmentTimer) in the corresponding CLTM candidate setting is 500ms. Therefore, when the retained TA value is applied to the target cell, the UE starts the legacy TAT at 500ms.

[0259] Step 3: The applied TA value is actually invalid for 200ms after 300ms has elapsed, and this is the problem.

[0260] The challenge in this scenario can be avoided by updating the TA value in the Timing Advance Command MAC CE each time the target cell detects a UL transmission from the UE. Alternatively, another way to avoid this challenge is for the UE to set the remaining time of the CLTM TAT to the legacy TAT timer value when applying the retained TA value of the CLTM candidate cell (i.e., the target cell).

[0261] Proposal 2: When the UE applies the retained TA value of a CLTM candidate cell during CLTM execution, RAN2 should discuss whether the UE sets the remaining time of the CLTM TAT associated with the CLTM candidate cell (i.e., the target cell) to the timer value of the legacy TAT associated with the PTAG.

[0262] Furthermore, it remains unclear how CLTM TATs associated with other TA values ​​for other CLTM candidate cells are handled during CLTM execution.

[0263] In conventional conditional handover (CHO), the TA value is always obtained at the time of CHO execution, so there is no TA value held in advance for CHO candidate cells. It should also be noted that CHO does not support subsequent (subsequent) CHO executions.

[0264] In Rel-18's LTM, subsequent LTM executions are introduced, allowing for multiple LTM executions without RRC reconfiguration. According to TS38.300, the steps of early synchronization, LTM cell switching, and completion of LTM cell switching can be executed multiple times for subsequent LTMs using the LTM candidate configuration provided in the LTM preparation phase. It should be noted that executing the early synchronization phase for subsequent LTMs is not mandatory, and the TA values ​​for LTM candidate cells are managed by the network. Therefore, for more efficient subsequent LTMs, it is possible for gNB implementations to notify the UE of TA values ​​acquired before an LTM execution for the next LTM execution (i.e., TA values ​​are acquired in a previous serving cell, not the current cell, or something like an "early synchronization-less, RACH-less subsequent LTM").

[0265] For subsequent CLTMs, the same principles as Rel-18 apply; that is, it is natural for subsequent CLTMs to run without an LTM preparation phase (i.e., the UE maintains the LTM candidate configuration). In addition, as a smart implementation in the aforementioned Rel-18 LTM, an early synchronization phase is not essential even for RACH-less subsequent CLTMs.

[0266] Therefore, even after CLTM has been performed on a candidate cell, it is desirable to maintain the TA values ​​for other LTM candidate cells as long as they are valid. To manage the validity of TA values ​​after CLTM execution, CLTM TAT should not be stopped after LTM execution. This mechanism allows the UE to perform RACH-less CLTM even if the early synchronization phase does not necessarily occur after CLTM execution.

[0267] Furthermore, if all TA values ​​are discarded after each CLTM execution, and the conditions for subsequent CLTM executions are met before the UE acquires TA values ​​in the early synchronization phase, a RACH-based CLTM must be executed, which is inefficient. It may be possible to reduce RACH-based CLTMs by having the UE continue to retain TA values ​​and keep the corresponding CLTM TAT running.

[0268] Proposal 3: In order to support more efficient subsequent CLTMs, RAN2 should agree that even after the CLTM has been executed, the UE should continue to operate the CLTM TAT and maintain the TA value.

[0269] In the case of a subsequent CLTM, the source cell may be set as a CLTM candidate cell while the UE is still being serviced by the source cell. If the UE performs a CLTM to a target cell (which will become the new source cell) and then turns around to the original source cell, the UE may perform a CLTM from the new source cell (i.e., the original target cell) back to the original source cell. In this case, early synchronization will be required again to perform a RACH-less CLTM back to the original source cell.

[0270] Observation 3: Due to subsequent CLTMs, source cells may also be set as CLTM candidate cells.

[0271] However, as long as legacy TAT is running when CLTM is executed on the target cell, the TA value applied to the source cell immediately before CLTM execution remains valid after CLTM execution. However, because UE applies the retained TA value of the CLTM candidate cell to PTAG, that value is lost when CLTM is executed. Therefore, when RACH-less CLTM is executed on the source cell again, early synchronization needs to be performed again.

[0272] If legacy TAT is running during CLTM execution, the UE can support more efficient subsequent CLTMs by storing the TA value currently applied to the source cell in the CLTM candidate configuration corresponding to the source cell before applying the retained TA value to the target cell's PTAG. This behavior allows for the retention of a valid TA value for the source cell, enabling RACH-less CLTM without unnecessary early synchronization procedures.

[0273] Proposal 4: If a candidate CLTM setting exists for a source cell, and legacy TAT is still running when CLTM is executed, RAN2 should discuss whether it is permissible for the UE to retain the TA value that was applied to the source cell after CLTM is executed on the target cell.

[0274] 1: Mobile communication system 5: Network 10: RAN 20: CN 100: UE 110: Receiving unit 120: Transmitting unit 130: Control unit 140: Wireless communication unit 200: gNB 210: Transmitting unit 220: Receiving unit 230: Control unit 240: Network communication unit 241: Transmitting unit 242: Receiving unit 250: Wireless communication unit 300: AMF / UPF

Claims

1. A communication method performed by a user device in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: receiving from the first cell a first candidate setting that sets the first cell as an LTM candidate cell and a second candidate setting that sets the second cell as an LTM candidate cell, while the user device is using the first cell as a serving cell; managing the validity of a first timing advance (TA) value to be applied to uplink communication with the first cell using a serving cell TA timer; performing a first LTM cell switch to the second cell based on the second candidate setting; and retaining the first TA value even when performing the first LTM cell switch when the first candidate setting exists and the serving cell TA timer is operating.

2. The communication method according to claim 1, wherein the user device retains the first TA value even when the first LTM cell switching is performed, when there is a first candidate setting having the same cell identifier as the first cell and the serving cell TA timer is operating.

3. The communication method according to claim 1, wherein the user device receives a first candidate setting from the first cell, which sets the first cell as a candidate cell for a conditional LTM.

4. The communication method according to claim 1, further comprising: receiving a second TA value from the first cell to be applied to uplink communication with the second cell; and, when performing the first LTM cell switching, holding the first TA value in association with the first candidate setting and applying the second TA value to uplink communication with the second cell.

5. The communication method according to claim 4, further comprising: holding the first TA value in association with the first candidate setting; and managing the validity of the held first TA value by the candidate cell TA timer, wherein the user device sets the remaining time of the serving cell TA timer in the candidate cell TA timer.

6. The communication method according to claim 5, further comprising: performing a second LTM cell switch from the second cell to the first cell based on the first candidate setting after the first LTM cell switch; and applying the held first TA value to the uplink communication with the first cell, depending on whether the candidate cell TA timer is operating when the second LTM cell switch is performed.

7. A user device for use in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: a receiving unit that receives from the first cell a first candidate setting that sets the first cell as an LTM candidate cell and a second candidate setting that sets the second cell as an LTM candidate cell when the user device is connected to the first cell as a serving cell; and a control unit that manages the validity of a first timing advance (TA) value applied to uplink communication with the first cell using a serving cell TA timer, wherein the control unit performs a first LTM cell switch to the second cell based on the second candidate setting, and maintains the first TA value even when performing the first LTM cell switch when the first candidate setting exists and the serving cell TA timer is operating.

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