Communication method and user equipment

Abstract

This communication method is used in a mobile communication system that supports LTM, the method including: a user device, in which conditional LTM cell switching to one or more LTM candidate cells is configured, receives, from a network node, a timing advance value used for uplink communication after LTM cell switching; the user device holding the timing advance value in association with a timer that determines a period in which the timing advance value is valid; and the user device applying the held timing advance value to the uplink communication if executing the LTM cell switching during operation of the timer.

Description

Communication method and user equipment

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

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

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

[0004] 3GPP technical specification "3GPP TS 38.300 V18.2.0"

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

[0006] A communication method according to a first aspect of the present disclosure is a communication method for use in a mobile communication system that supports an LTM, comprising: a user device configured to conditionally switch to one or more LTM candidate cells receiving a timing advance value from a network node to be used for uplink communication after the LTM cell switch; the user device holding the timing advance value in association with a timer that defines the period during which the timing advance value is valid; and, if the user device performs the LTM cell switch while the timer is operating, applying the held timing advance value to the uplink communication.

[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, comprising: a receiving unit that receives a timing advance value from a network node for use in uplink communication between the user device and the LTM candidate cell when conditional LTM cell switching to the LTM candidate cell is set in the user device; and a control unit that holds the timing advance value in association with a timer that defines the period during which the timing advance value is valid, wherein the control unit applies the held timing advance value to the uplink communication when the conditional LTM cell switching is performed while the timer is operating.

[0008] This figure shows an example configuration of a mobile communication system according to the embodiment. This figure shows an example configuration of a UE (User Equipment) according to the embodiment. This figure shows an example configuration of a gNB (Network Node) according to the 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 the embodiment. This figure is for explaining LTM cell switching of an interCU according to the embodiment. This figure shows an example configuration of an RRC Reconfiguration message for setting C-LTM to UE100 according to the embodiment. This figure is for explaining the operation according to the embodiment.

[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. The mobile communication system may also have a 6th generation (6G) system applied to it 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), 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 from UE100 to network 5 in the transmission direction is called the uplink (UL), and the link from network 5 to UE100 in the transmission direction 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 Transmission and Reception Points (TRPs). 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 on the physical downlink control channel (PDCCH). Specifically, UE100 performs blind decoding of the PDCCH using a Radio Network Temporary Identifier (RNTI) and acquires the successfully decoded DCI as the DCI addressed to its own UE. The DCI transmitted from gNB200 has a CRC (Cyclic Redundancy Check) parity bit added, which is scrambled by the 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)) 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 control plane's wireless interface protocol stack includes an RRC (Radio Resource Control) layer and a NAS (Non-Access Stratum) layer, instead of the SDAP layer shown in Figure 4.

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

[0036] The NAS layer (also simply referred to as "NAS"), located above the RRC layer, handles session management and mobility management, among other things. NAS signaling is transmitted between the 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 switch 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 cell switch 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 an LTM candidate cell setting for the candidate cell to be switched to, and provides the LTM candidate cell setting 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 a serving cell switch to a target cell based on the L1 measurement report, and transmits a cell switch command (Cell Switch Command) indicating the target cell (LTM candidate cell setting) to the UE 100 by means of a MAC control element (CE). The serving cell switch trigger is transmitted in a MAC CE including at least a candidate setting index (setting ID) together with a beam indicator.

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

[0044] Thus, by selecting the LTM candidate cell setting as a target setting by the gNB 200, a serving cell switch is triggered. The LTM candidate cell setting can be added, changed, and released by the gNB 200 via RRC signaling.

[0045] The following principles apply to LTM.

[0046] - Each LTM candidate cell setting can be provided as a difference setting (delta setting) with respect to a reference setting used to form a complete LTM candidate cell setting.

[0047] - When a complete LTM candidate cell setting is applied, the current UE setting is replaced at the time of serving cell switch. Replacement is performed in the reconfiguration procedure, but the MAC, RLC, or PDCP layer is not necessarily reset.

[0048] - The user plane continues without reset if set by RRC signaling for the purpose of avoiding additional delay in data recovery. <L

[0049] - In LTM, the security is not updated.

[0050] - Subsequent LTM (Subsequent LTM) between LTM candidate cell settings can be executed without RRC reconfiguration. That is, the UE 100 does not release other LTM candidate cell settings 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.

[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 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 cell 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 Early Timing Advance (TA) acquisition (i.e., UL Early Sync) with the LTM candidate cell requested by gNB200 (serving cell). This is performed 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 (Reference Signal Received Power) and / or L1-SINR (Signal-to-Interference-plus-Noise Ratio).

[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.

[0066] (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.

[0067] Figure 7 is a diagram illustrating the LTM cell switching of the interCU according to the embodiment.

[0068] 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.

[0069] 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.

[0070] (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.

[0071] 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.

[0072] Figure 8 shows an example of the configuration of an RRC Reconfiguration message for setting a C-LTM to UE100 according to the embodiment. As shown in Figure 8, each LTM candidate cell setting (LTM Candidate) in the LTM setting and each LTM candidate cell setting (LTM Candidate) in the C-LTM setting are merged into a single list (LTM Candidate ToAddModList), and this list is included in the LTM setting (LTM Config).

[0073] 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 cell settings. This list includes as entries a set of the setting ID (LTM Candidate ID) and the execution condition (ExecutionCondition) of the LTM candidate cell setting. Here, the execution conditions are L1 trigger conditions (L1-Conditions) and / or L3 trigger conditions (L3-Conditions). Note that the L1 trigger conditions are trigger conditions that use the L1 measurement results. The trigger evaluation of whether or not the L1 trigger conditions are met is performed by the MAC layer of UE100. The L3 trigger conditions are trigger conditions that use the L3 measurement results. The trigger evaluation of whether or not the L3 trigger conditions are met is performed by the RRC layer of UE100.

[0074] 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 cell settings are used by UE100 to perform subsequent LTM cell switching after switching to the corresponding LTM candidate cell.

[0075] (3) Operation according to the embodiment As described above, in conventional LTM, UE100 receives the TA value from the gNB200 (first cell) via the LTM cell switching command MAC CE, and performs uplink communication applying the TA value to the target cell in conjunction with the LTM cell switching. However, in C-LTM, the LTM cell switching command MAC CE is not used. In other words, UE100 performs LTM cell switching when the execution conditions for LTM cell switching are met. Therefore, in C-LTM, UE100 cannot receive the TA value via the LTM cell switching command MAC CE.

[0076] Therefore, a method is being considered to introduce a new MAC CE from gNB200 (first cell) to UE100 and to transmit the TA value to UE100 using the new MAC CE. Such a new MAC CE is transmitted from gNB200 (first cell) to UE100 before UE100 performs the LTM cell switchover. When the conditions for performing the LTM cell switchover are met, UE100 performs the LTM cell switchover and performs uplink communication applying the TA value notified by the new MAC CE.

[0077] However, the TA value is an adjustment value to compensate for the propagation delay between UE100 and gNB200, and the optimal value varies depending on the distance between UE100 and gNB200 and / or the propagation environment. Therefore, the optimal TA value for UE100 can change moment by moment. For example, if UE100 travels a long distance after receiving the TA value from gNB200, the received TA value may no longer be appropriate.

[0078] Therefore, in this embodiment, the UE 100, which is configured with C-LTM, stores the TA value in association with a timer that determines the period during which the TA value notified by the gNB 200 is valid. When the UE 100 performs an LTM cell switchover while the timer is operating, it applies the stored TA value to the uplink communication after the LTM cell switchover. On the other hand, when the timer expires, the UE 100 considers the TA value notified by the gNB 200 to be invalid and discards the TA value. This makes it possible for the UE 100 to perform uplink communication with an appropriate TA value applied.

[0079] In this embodiment, the timer is managed by UE100 on a per-LTM candidate cell basis. UE100 may store an identifier indicating the LTM candidate cell and a TA value associated with the timer. In this case, UE100 manages the timer and TA value for each LTM candidate cell. This allows for appropriate management of the timer and TA value for each LTM candidate cell.

[0080] Alternatively, in this embodiment, the timer is managed by UE100 on a per-beam basis for LTM candidate cells. UE100 may store a TA value associated with a beam identifier and timer. In this case, UE100 manages the timer and TA value for each beam. This allows for appropriate management of the timer and TA value for each beam. Here, the beam identifier may be an SSB identifier or a CSI-RS (Channel State Information-Reference Signal) resource identifier. Alternatively, the beam identifier may be a TCI (Transmission Configuration Indication) status identifier.

[0081] In this embodiment, a UE 100 with a conditional LTM set may continuously manage the timer even after performing an LTM cell switchover. If the UE 100 performs a subsequent LTM cell switchover following an LTM cell switchover while the timer is running, it may apply the retained TA value to the uplink communication after the subsequent LTM cell switchover. In this case, the UE 100 can continuously retain the TA value even after performing an LTM cell switchover in the C-LTM. This allows the UE 100 to apply the TA value when performing a subsequent LTM cell switchover in the C-LTM, eliminating the need for signaling to obtain the TA value again from the network.

[0082] In this embodiment, if the timer is operating when an LTM cell switchover is performed, the UE 100 may continuously manage the timer and maintain the TA value associated with the timer. In this case, the UE 100 can continuously maintain the TA value even after an LTM cell switchover in C-LTM, and signaling to obtain the TA value again from the network can be omitted.

[0083] Figure 9 is a diagram illustrating the operation according to the embodiment. 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.

[0084] 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).

[0085] 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 (candidate cell).

[0086] In step S103, gNB200 decides to set C-LTM to UE100 and identifies candidate cells.

[0087] In step S104, gNB200 sends configuration information (C-LTM settings) for setting the C-LTM cell switching to UE100. gNB200 may also send 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).

[0088] The C-LTM setting includes a) one or more LTM candidate cell settings (LTM Candidate), and b) setting a timer value for a timer that determines the period during which the TA value is valid.

[0089] The LTM candidate cell settings in a) may include at least one of the following pieces of information a1) through a7).

[0090] a1) LTM candidate cell setting setting ID (LTM candidate ID): Information that identifies a single LTM candidate cell setting. When gNB200 changes some of the settings of an LTM candidate cell setting (for example, one of a2) to a7) 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 cell setting.

[0091] a2) Cell identifier of candidate cell: This is information that identifies the LTM candidate cell. One example is PCI (Physical Cell Identity).

[0092] a3) Cell switching execution conditions: This is information indicating the radio quality conditions that must be met for at least one of the serving cell and LTM candidate cells in order to trigger cell switching.

[0093] a4) RRC settings: These are the RRC settings applied to LTM candidate cells.

[0094] a5) CG (configured grant) setting: This is configuration information for setting up the uplink radio resources (e.g., PUSCH (Physical uplink shared channel) resources) available for the first uplink transmission at the LTM candidate cell.

[0095] a6) SSB settings (ltm-SSB-Config): These are the settings required for UE100 to receive SSB in LTM candidate cells. SSB settings include, for example, the SSB frequency (ssb-Frequency), subcarrier spacing (subcarrier Spacing), and period (ssb-Periodity).

[0096] a7) TCI Information (ltm-TCI-Info): This information sets TCI-related information for LTM candidate cell settings used during the activation of the TCI (Transmission Configuration Indication) state.

[0097] TCI status refers to information about signal / channel pseudo-collocation (QCL), and may also be called spatial reception parameters or spatial relation information (SRI). TCI status may be set for each channel or each signal.

[0098] The downlink channel on which the TCI state is set may be, for example, at least one of a Physical Downlink Shared Channel (PDSCH) or a PDCCH. The downlink reference signal that has a QCL relationship with the downlink channel may be, for example, at least one of SSB and CSI-RS. The TCI information in a7) may include one or more TCI state identifiers (TCI-stateId).

[0099] On the other hand, the setting of the timer value of the timer that determines the period during which the TA value is valid may include one of the following pieces of information: The unit of the timer value is milliseconds (ms), slots, or subframes.

[0100] b1) Setting information for setting cell-specific (cell-specific) timer values: This is a single timer value associated with one LTM candidate cell.

[0101] The optimal TA value applied to a candidate cell may vary depending on the positional relationship between UE100 and the candidate cell (e.g., propagation distance). Specifically, the degree of change in the positional relationship between UE100 and each candidate cell over time differs due to factors such as the movement speed of UE100. As a result, the validity period of the TA value may also differ for each candidate cell. Therefore, gNB200 sets a timer value individually for each candidate cell. This allows UE100 to manage the TA value more appropriately.

[0102] The setting information in b1) may include a set of the cell identifier (e.g., PCI) of the candidate cell and a timer value for that candidate cell. For example, if there are three candidate cells, the setting information in b1) includes a set of cell identifier #1 and timer value #1 that identifies candidate cell #1, a set of cell identifier #2 and timer value #2 that identifies candidate cell #2, and a set of cell identifier #3 and timer value #3 that identifies candidate cell #3.

[0103] The setting information in b1) may include a set of the setting ID for the LTM candidate cell setting (LTM candidate ID) and the timer value for the candidate cell.

[0104] The setting information in b1) may be the information included in one of the LTM candidate cell settings in a) above. In this case, UE100 can understand that the timer value indicated by the setting information in b1) is associated with the candidate cell identified by a2) in the LTM candidate cell setting.

[0105] b2) Setting information for setting beam-specific (beam-specific) timer values: This is a single timer value associated with one beam in a candidate cell.

[0106] In some cases, a single candidate cell may be formed by multiple TRPs, each forming a different beam. In this case, the degree of change in the positional relationship between the UE100 and each beam (TRP) over time differs due to factors such as the movement speed of the UE100. As a result, the validity period of the TA value may also differ for each beam. Therefore, the gNB200 sets a timer value individually for each beam. This makes it possible to manage the TA value more appropriately, even when multiple beams are provided in a single candidate cell.

[0107] The setting information in b2) may include a set of a beam identifier indicating a beam and a timer value for that beam. For example, if there are three beams (beam #1, beam #2, and beam #3) in one candidate cell, the setting information in b2) includes a set of beam identifier #1 and timer value #1 to identify beam #1, a set of beam identifier #2 and timer value #2 to identify beam #2, and a set of beam identifier #3 and timer value #3 to identify beam #3.

[0108] Here, the beam identifier is one of the following: an SSB identifier indicating an SSB, a CSI-RS resource identifier indicating a CSI-RS, and a TCI status identifier indicating a TCI status.

[0109] b3) Setting information for setting a timer value per UE (UE-specific): This is one timer value associated with one UE100.

[0110] For example, if multiple candidate cells are concentrated in a relatively small geographical area, and the UE100 is moving at a relatively fast speed, the rate of change in the positional relationship between the UE100 and each candidate cell over time may be roughly equal. In this case, the validity period of the TA for each candidate cell will be roughly the same, so it is more convenient for management to set a single timer value for the UE100 rather than setting a timer value for each candidate cell or beam.

[0111] The setting information in b3) may be a single timer value common to multiple LTM candidate cell settings in the C-LTM setting.

[0112] In step S104, UE100 sets the timer value received in the timer and manages the timer.

[0113] If the timer value received in step S104 is a cell-level timer value, UE100 manages the timers on a cell-by-cell basis to which that timer value is set. Specifically, UE100 manages the timers to which that timer is set, in association with a cell identifier indicating a candidate cell associated with that timer value.

[0114] If the timer value received in step S104 is a beam-specific timer value, the UE 100 manages the timers on a beam-by-beam basis for which that timer value is set. Specifically, the UE 100 manages the timers on which that timer is set, in association with the cell identifier that indicates the beam associated with that timer value.

[0115] If the timer value received in step S104 is a timer value in UE units, UE 100 manages the timers to which that timer value is set on a cell-by-cell basis. That is, UE 100 manages the timers to which a common timer value is set for each LTM candidate cell.

[0116] 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.

[0117] In step S106, UE100 performs early synchronization with each LTM candidate cell set in step S104.

[0118] Specifically, UE100 transmits an RA preamble to each LTM candidate cell. gNB200 receives the RA preamble transmitted by UE at the TRP in each LTM candidate cell. Based on the RA preamble, gNB200 determines the TA value to apply to each candidate cell. If a candidate cell has multiple beams, gNB200 may determine the TA value for each beam.

[0119] In step S107, gNB200 sends setting information (TA value setting) for setting the TA value to UE100. gNB200 may also send MAC CE including the TA value setting to UE100. UE100 receives the TA value setting from gNB200 (serving cell).

[0120] The TA value setting may include the following information c1) or c2).

[0121] c1) Setting information for setting cell-specific (cell-specific) TA values: One TA value associated with one candidate cell.

[0122] The configuration information for c1 may include a set of identifiers indicating a candidate cell (e.g., PCI) and a TA value for that candidate cell.

[0123] The setting information for c1 may be the TA value included in a field provided for each candidate cell. The correspondence between candidate cells and fields may be set by signaling in the RRC layer (for example, RRC Reconfiguration in step S104). This eliminates the need to include an identifier indicating the candidate cell in MACCE, thereby reducing the amount of information in MACCE.

[0124] c2) Setting information for setting beam-specific (beam-specific) TA values: This is a single TA value associated with one beam in a candidate cell.

[0125] If a candidate cell has multiple TRPs, each TRP can receive the RA preamble transmitted from UE100. Therefore, gNB200 sets the TA value individually for each TRP (beam). This makes it possible to set a more appropriate TA value even when a candidate cell has multiple TRPs.

[0126] The configuration information for c2 may include a set of identifiers indicating the beam in the candidate cell and the TA value for that beam.

[0127] The setting information for c2 may be the TA value included in a field provided for each beam. The correspondence between beams and fields may be set by signaling of the RRC layer (for example, RRC Reconfiguration in step S104). This eliminates the need to include a beam identifier in MACCE, thereby reducing the amount of information in MACCE.

[0128] In step S108, UE100 stores the TA value received in step S107 in association with the timer it manages.

[0129] Here, if the timer is managed on a cell-by-cell basis and the TA value is a cell-by-cell TA value, the UE100 stores the TA value in association with the cell identifier and the timer.

[0130] UE100 stores the TA value in association with the beam identifier and the timer, when the timer is managed on a per-beam basis and the TA value is a per-beam TA value.

[0131] When the timer is managed on a cell basis and the TA value is a beam-based TA value, the UE100 stores the TA value in association with the cell identifier of the cell to which the associated beam belongs and the timer. In other words, in this case, the UE100 stores multiple TA values ​​corresponding to multiple beams installed in a candidate cell, in association with one timer corresponding to that candidate cell.

[0132] UE100 stores the TA value (one or more TA values) in association with one timer of the UE unit, when the timer is managed on a UE unit basis and the TA value is a beam unit TA value or a cell unit TA value.

[0133] UE100 stores the TA value (one or more TA values) in association with multiple timers that have set the timer value for the UE unit, when the timer value is set in units of UE, the timer is managed in units of cell, and the TA value is a beam-unit TA value or a cell-unit TA value.

[0134] In step S109, UE100 starts the timer associated with the TA value. UE100 holds the TA value associated with the timer while it is operating. When the timer expires or stops, UE100 discards the TA value associated with that timer. The conditions for stopping the timer will be described later.

[0135] Here, if the timer expires before the LTM cell switching in C-LTM is performed, UE100 may obtain a new TA value from gNB200 by performing the operation in step 106 (Early sync) again for the LTM candidate cell corresponding to that timer.

[0136] In step S109, gNB200 may start a timer, similar to UE100. gNB200 may send a PDCCH order to UE100 when the 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.

[0137] In the case of the inter-gNB C-LTM, the gNB200a, which manages the serving cell, may transmit the timer value set in UE100 to the gNB200b, which manages the candidate cell, via the Xn interface. Also, when the gNB200a starts the timer, it may transmit information indicating the remaining time of the timer to the gNB200b.

[0138] In step S110, 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.

[0139] 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.

[0140] In step S111, UE100 detaches from the serving cell and applies the settings of the candidate cell corresponding to the satisfied execution conditions (for example, the RRC settings in a4 above). In the following, the candidate cell corresponding to the satisfied execution conditions may also be called the target cell.

[0141] Although not shown in the diagram, UE100 may perform the operation in step S111 in response to receiving a cell switching command from gNB200.

[0142] In step S112, UE100 determines whether a valid TA value exists for the target cell.

[0143] Specifically, UE100 determines that a valid TA value exists for a target cell if the timer associated with the TA value for that target cell is operating. The "TA value for the target cell" is either the TA value associated with the target cell (the TA value of c1 above) or the TA value associated with the beam installed in the target cell (the TA value of c2 above). Furthermore, if UE100 manages multiple timers for a target cell, UE100 determines that a valid TA value exists for that target cell if at least one of those timers is operating.

[0144] On the other hand, if the timer associated with the TA value for the target cell is not operating (i.e., the timer has stopped or expired), the UE100 determines that there is no valid TA value for that target cell. Also, if the UE100 manages multiple timers for a target cell, and none of those timers are operating, the UE100 determines that there is no valid TA value for that target cell.

[0145] If UE100 determines that a valid TA value exists for the target cell (S112: YES), it proceeds to step S113. On the other hand, if UE100 determines that no valid TA value exists for the target cell (S112: NO), it proceeds to step S114.

[0146] In step S113, UE100 applies a valid TA value and performs uplink communication with the target cell.

[0147] In this case, there may be multiple valid TA values ​​for the target cell. In this case, UE100 may apply the TA value corresponding to the beam with the highest radio quality. Alternatively, a priority may be set for each beam (for example, set in the RRC Reconfiguration in step S104), and UE100 may apply the TA value corresponding to the beam with the highest priority.

[0148] An example of uplink communication is the transmission of the RRC Reconfiguration Complete message. UE100 may also transmit RRC Reconfiguration Complete using CG (configured grant). For example, if CG settings were made in step S104, UE100 will use the CG resource to perform a PUSCH transmission.

[0149] 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).

[0150] In step S114, UE100 performs a random access procedure on the target cell. In the random access procedure, UE100 can obtain a valid TA value.

[0151] In step S115, UE100 completes the LTM cell switchover. After the LTM cell switchover is complete, the target cell may be called the new serving cell.

[0152] If UE100 has performed the operation in step S113, 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 S113. For example, UE100 may consider the LTM cell switching to have been successfully completed if, after performing the operation in step S113, 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 S113.

[0153] If UE100 has performed the operation in step S114, and the random access procedure has been successfully completed, it may be considered that the LTM cell switching has been successfully completed.

[0154] The operations in steps S111 to S115 may be collectively referred to as the LTM cell switching operation in C-LTM. The LTM cell switching operation when step S113 is executed may be referred to as the RACH-Less LTM cell switching operation. The LTM cell switching operation when step S114 is executed may be referred to as the RACH-based LTM cell switching operation.

[0155] UE100 may continue to manage the timers it manages for each candidate cell after completing the LTM cell switching in C-LTM. While UE100 is continuously managing the timers, if the timers are operating, UE100 will keep them running without stopping them. Also, if the timers expire while UE100 is continuously managing them, UE100 may restart the timers in response to notification of a new TA value, rather than discarding them.

[0156] In step S116, UE100 performs a subsequent LTM cell switchover. Specifically, in step S116, UE100 may perform steps S106 to S115 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. In the subsequent LTM cell switchover, UE100 may apply a TA value held by a continuously managed timer.

[0157] The above-described operation scenario assumes that the TA value is set to a value other than "0". The case in which the TA value is set to "0" will be explained in the operation modification example 2 of the embodiment described below.

[0158] (4) Example of operation modification according to the embodiment 1 In C-LTM, gNB200 may change or remove some of the C-LTM settings set in UE100. This modification example is an example of operation regarding the handling of the timer in this case.

[0159] Firstly, gNB200 sends an RRC reset message to UE100 to change or remove some of the one or more LTM candidate cell settings (LTM candidate cell settings) configured in UE100.

[0160] For example, when gNB200 changes the setting of one LTM candidate cell, it includes a set of the LTM candidate ID that identifies the LTM candidate cell setting and at least one of the changed settings (e.g., a2) to a7) in the RRC reset message. When gNB200 removes one LTM candidate cell setting, it includes a release list (ReleaseList) containing the LTM candidate ID that identifies the LTM candidate cell setting in the RRC reset message.

[0161] The gNB200 can send such RRC reset messages to the UE100 at the following timings.

[0162] 1) Before the LTM cell switchover is performed in C-LTM: In this case, UE100 receives the RRC reset message from the source cell.

[0163] 2) After the LTM cell switchover in C-LTM: In this case, UE100 receives the RRC reset message from the new serving cell after the LTM execution.

[0164] 3) After the subsequent LTM cell switchover in C-LTM: In this case, UE100 receives the RRC reset message from the new serving cell after the subsequent LTM cell switchover.

[0165] Secondly, UE100 changes the timer management state in response to the received RRC reset message.

[0166] Specifically, if the settings of an LTM candidate cell are not changed by the received RRC reset message, the UE100 continuously manages the timer associated with that LTM candidate cell.

[0167] If the setting for an LTM candidate cell is removed by an RRC reset message, UE100 will consider the timer associated with that LTM candidate cell to have stopped or expired.

[0168] If a specific setting parameter in the settings of an LTM candidate cell is changed by an RRC reset message, UE100 will consider the timer associated with the LTM candidate cell to be stopped or expired. The specific setting parameter is the identifier indicating the LTM candidate cell and the beam setting of the LTM candidate cell. The specific setting parameter is, for example, at least one of the above-mentioned a1) LTM candidate cell setting identifier, a2) candidate cell identifier, a6) SSB setting (ltm-SSB-Config), and a7) TCI information. Note that "stop the timer" may be read as "discard the timer". When UE100 discards a timer, it discards the TA value associated with that timer.

[0169] Here, a change in a specific configuration parameter may result from a change in the physical location of the TRP. In this case, the TA value held by UE100 may also become inappropriate. Therefore, UE100 considers the timer to be stopped or expired and discards the TA value associated with that timer. This prevents communication failures caused by the application of an inappropriate TA value.

[0170] (5) Example of Operation Modification According to the Embodiment This modification example concerns the handling of the timer when the TA value is set to "0". In C-LTM, if the TRP forming the LTM candidate cell is physically close to UE100, gNB200 may set the TA value associated with the LTM candidate cell to "0". The TA value should always be valid while it is set to "0". However, if the timer is started for a TA value set to "0", the TA value becomes invalid when the timer expires or stops. In this case, unnecessary signaling may occur in UE100 to acquire a new TA value that is still set to "0". Also, in this case, UE100 may execute a RACH-based LTM cell switching operation, resulting in unnecessary signaling. Therefore, in the modified operation example 2 of the embodiment, when UE100 receives setting information (new MAC CE) from gNB200 that sets the TA value to "0", it controls not to start the timer corresponding to the TA value. That is, if the TA value received from gNB200 is "0", UE100 does not start the timer corresponding to the TA value (i.e., it skips the procedure for starting the timer corresponding to the TA value). If the TA value received from gNB200 is not "0", UE100 starts the timer corresponding to the TA value. Alternatively, if the TA value received from gNB200 is "0", UE100 starts the timer corresponding to the TA value, but may ignore the expiration of the timer. In this way, UE100 considers the TA value to be valid for LTM candidate cells associated with a TA value set to "0".

[0171] (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.

[0172] In the embodiments and examples described above, an example in which the base station is an NR base station (gNB) was explained, 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. Additionally, 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 repeater that performs signal relay. Such a terminal function unit is referred to as an MT. Examples of MTs other than IAB-MT include, for example, NCR (Network Controlled Repeater)-MT and RIS (Reconfigurable Intelligent Surface)-MT.

[0173] 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.

[0174] A program may be provided that causes a computer to execute each process performed by the UE100 or gNB200. The program may be recorded on a computer-readable medium. Using a computer-readable medium, it is possible to install the program on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transient recording medium. The non-transient recording medium is not particularly limited, but may be a recording medium such as a CD-ROM and / or DVD-ROM. Alternatively, the circuits that execute each process performed by the UE100 or gNB200 may be integrated, and at least a part of the UE100 or gNB200 may be configured as a semiconductor integrated circuit (chipset, SoC: System on a chip).

[0175] The functions realized by UE100 or gNB200 may be implemented in a circuit or processing circuit, including general-purpose processors, application-specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), CPUs (Central Processing Units), conventional circuits, and / or combinations thereof, programmed to realize the described functions. A processor, including transistors and / or other circuits, is considered a circuit or processing circuit. A 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 perform the described functions. Such hardware may be any hardware disclosed herein, or any hardware known to be programmed to realize or perform the described functions. If the hardware is a processor that is considered to be of the type of circuit, then the circuit, means, or unit is a combination of hardware and software used to constitute the hardware and / or processor.

[0176] 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.

[0177] 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.

[0178] This application claims priority to U.S. Provisional Application No. 63 / 754110 (filed February 5, 2025), the entirety of which is incorporated into the specification of this application.

[0179] (7) First Addendum The features relating to the above-described embodiment are added below. Addendum 1 A communication method for use in a mobile communication system that supports LTM, comprising: a user device which is set to conditionally switch to one or more LTM candidate cells, receiving a timing advance value from a network node to be used for uplink communication after LTM cell switching; the user device storing the timing advance value in association with a timer which determines the period for which the timing advance value is valid; and when the user device performs the LTM cell switching while the timer is operating, applying the stored timing advance value to the uplink communication.

[0180] - Appendix 2 The communication method according to Appendix 1, wherein the user device further receives setting information for setting the conditional LTM cell switching from the network node, and the setting information includes a timer value indicating the duration of the timer.

[0181] - Appendix 3 The timer is a timer managed by the user device on an LTM candidate cell basis, and the user device holds the timing advance value in association with an identifier indicating the LTM candidate cell and the timer, as described in Appendix 1 or Appendix 2.

[0182] - Appendix 4 The timer is a timer managed by the user device on a beam-by-beam basis for LTM candidate cells, and the user device holds the timing advance value in association with the identifier representing the beam and the timer, as described in Appendix 1 or Appendix 2.

[0183] - Appendix 5 The communication method described in Appendix 4, wherein the identifier indicating the beam is an SSB (SS: Synchronization Signal / PBCH Block) identifier or a CSI-RS (Channel State Information Reference Signal) resource identifier.

[0184] - Appendix 6 The communication method described in Appendix 4, wherein the identifier relating to the beam is a TCI (Transmission Configuration Indication) status identifier.

[0185] - Appendix 7: The communication method according to any one of Appendix 1 to Appendix 6, further comprising: the user device continuously managing the timer even when the LTM cell switching is performed; and, when the user device performs a subsequent LTM cell switching that follows the LTM cell switching while the timer is operating, applying the held timing advance value to the uplink communication after the subsequent LTM cell switching.

[0186] - Appendix 8 The communication method according to any one of Appendix 1 to Appendix 7, wherein the user device continuously manages the timer and maintains the timing advance value associated with the timer when the timer is operating when the LTM cell switching is performed.

[0187] - Appendix 9 The communication method according to any one of Appendix 1 to Appendix 8, wherein the user device further receives an RRC reset message, and if the settings of the LTM candidate cell are not changed by the RRC reset message, the user device continuously manages the timer associated with the LTM candidate cell.

[0188] - Appendix 10 The communication method described in Appendix 9, wherein the user device stops or deems the timer associated with the LTM candidate cell to have expired when the setting of the LTM candidate cell is removed by the RRC reset message.

[0189] - Appendix 11 The communication method described in Appendix 9, wherein if a specific setting parameter among the settings of the LTM candidate cell is changed by the RRC reset message, the user device stops or considers the timer associated with the LTM candidate cell to have expired, and the specific setting parameter is an identifier indicating the LTM candidate cell and a setting relating to the beam of the LTM candidate cell.

[0190] - Appendix 12 The communication method described in Appendix 11, wherein the user device continuously manages the timer associated with the LTM candidate cell when a setting parameter different from the specific setting parameter among the settings of the LTM candidate cell is changed by the RRC reset message.

[0191] - Appendix 13 A user device for use in a mobile communication system that supports LTM, comprising: a receiving unit that receives a timing advance value from a network node for use in uplink communication between the user device and the LTM candidate cell when conditional LTM cell switching to the LTM candidate cell is set in the user device; and a control unit that holds the timing advance value in association with a timer that determines the period during which the timing advance value is valid, wherein the control unit applies the held timing advance value to the uplink communication when the conditional LTM cell switching is performed while the timer is operating.

[0192] (8) Second Addendum 1. Introduction In RAN2#128, the following was agreed upon regarding the early sync phase.

[0193] Agreements regarding C-LTM: "7. For CLTM, the "Candidate Cell TCI State Activation / Deactivation MAC CE" will be reused for early activation / deactivation of the TCI state of the CLTM candidate setting." "8. Early TA is signaled from the source cell to the UE (i.e., not directly from the candidate cell to the UE). This agreement will be included in the LS (Liaison Statement) to RAN1 / 3 / 4." "9. The network can notify the UE of the candidate cell's TA information, which is the TA value when the UE switches to that candidate cell during CLTM, via the new MAC CE." "10. The candidate cell's TA is maintained by the new timer." "11. In the case of L1-based conditional LTM, condition evaluation is performed at the MAC layer, and in the case of L3-based conditional LTM, condition evaluation is performed at the RRC layer."

[0194] This addendum will discuss the handling of the new timer.

[0195] 2. Discussion 2.1. New Timer for TA Values ​​of C-LTM Candidate Cells In Rel-18 LTM, the serving gNB manages the validity of the TA acquired in the early synchronization phase for each LTM candidate cell and always includes a valid TA value in the "Cell Switching Command MAC CE" and sends it. The UE applies the TA value and simultaneously performs the LTM on the target cell.

[0196] However, in the Conditional LTM (C-LTM) of Rel-19, the TA value acquired in the early synchronization phase is indicated via a new MAC CE that is different from the "cell switching command MAC CE" of Rel-18. Therefore, there is a time lag between the reception of the new MAC CE with the TA value in the UE and the occurrence of the C-LTM execution. In this sense, it is clear that the TA value indicated by the new MAC CE needs to be retained by the UE. In addition, since different LTM candidate cells have different TA values, the UE should maintain associations between multiple TA values ​​and each LTM candidate cell.

[0197] Proposal 1: RAN2 should agree that UE should maintain multiple TA values ​​associated with each LTM candidate cell.

[0198] After the early synchronization phase, the UE needs to verify whether its TA values ​​are valid before executing a RACH-less C-LTM. For this purpose, it was agreed at the previous meeting that "the candidate cell's TA will be maintained by a new timer." However, RAN2 has not considered the details of the new validity timer or the corresponding UE behavior.

[0199] Before considering the new expiration timer for the TA of LTM candidate cells, we will summarize the current Timing Alignment Timer (TAT) for TA management of serving cells. According to TS38.321, the UE uses the TAT to manage the validity of TA values ​​for UL transmission to serving cells. Upon TAT expiration, the UE's MAC proceeds to flush the HARQ buffer, notify the RRC to remove PUCCH / SRS, and cancel any downlink allocations / uplink grants already received. The TAT is set for each Timing Advance Group (TAG) associated with multiple serving cells.

[0200] - "timeAlignmentTimer (per TAG): Controls the period during which a MAC entity considers a serving cell for an associated TAG to be uplink time aligned for that TAG."

[0201] In C-LTM, the UE uses a new timer to manage the validity of the TA values ​​of C-LTM candidate cells acquired in the early synchronization phase. It should be noted that this new timer operates for C-LTM candidate cells, not for serving cells like the existing TAT. Considering that different early synchronizations for different C-LTM candidate cells do not occur simultaneously, while the existing TAT is set per TAG, multiple expiration timers should be able to be started for different C-LTM candidate cells. Further consideration is needed as to whether the timer value setting is common to all individual timers or set individually for each C-LTM candidate cell.

[0202] Proposal 2: RAN2 should agree that a new expiration timer should be set for each C-LTM candidate cell.

[0203] Furthermore, the behavior of the UE upon the expiration of the new expiration timer should be considered. In the existing TAT, the UE operates to stop UL / DL transmission upon timer expiration. On the other hand, upon the expiration of the new expiration timer, the UE does not need to stop UL / DL transmission to the serving cell (i.e., source cell), but the corresponding TA value should be considered invalid and / or any held TA value should be discarded.

[0204] Proposal 3: RAN2 should agree that upon the expiration of a new timer, the UE should consider any held TA values ​​associated with that timer to be invalid and / or discard such held TA values.

[0205] According to the current MAC Running CR, after a C-LTM cell switchover is triggered, the TA value corresponding to the C-LTM candidate cell (i.e., target cell) on which the LTM cell switchover was performed is applied to the PTAG, and the existing TAT associated with the PTAG is started or restarted. "1> If a conditional LTM cell switchover procedure is triggered for an LTM candidate cell, as described in Section 5.y.2, and the ltm-Candidate-TimeAlignmentTimer associated with that LTM candidate cell is operating: 2> Apply the retained TA value associated with the LTM target cell to the PTAG, as specified in Section 6.1.3.75. 2> Start or restart the timeAlignmentTimer associated with the PTAG, as specified in Section 6.1.3.75."

[0206] However, it remains unclear how to handle new expiration timers associated with other TA values ​​for other C-LTM candidate cells when a C-LTM cell switchover is performed. In conventional conditional handovers (CHOs), TA values ​​are always acquired at the time of the CHO execution, so TA values ​​for CHO candidate cells are not held in advance. Also note that CHOs do not support subsequent CHO (subsequent CHO) executions. LTMs in Rel-18 introduce subsequent LTM (subsequent LTM) executions, which allow for multiple LTM executions without RRC reconfiguration. According to TS38.300, the steps of early synchronization, LTM cell switchover execution, and LTM cell switchover completion can be performed multiple times for subsequent LTMs using the LTM candidate settings provided in the LTM preparation phase. It should be noted that the execution of the early synchronization phase is not mandatory in subsequent LTMs, and the TA values ​​for LTM candidate cells are managed by the network. Therefore, for more efficient subsequent LTMs, it is possible in the gNB implementation to instruct the UE for the next LTM cell switchover using TA values ​​acquired before the LTM cell switchover (i.e., the TA value is acquired from the previous serving cell, not the current cell, or something like a "RACH-less subsequent LTM without early synchronization").

[0207] For subsequent C-LTMs, the same principles as Rel-18 apply; that is, it is natural for subsequent C-LTMs to be executed without an LTM preparation phase, or in other words, for the UE to maintain the LTM candidate settings. In addition, as a smart implementation in the Rel-18 LTM described above, an early synchronization phase is not essential even for RACH-less subsequent C-LTMs. Therefore, even after a C-LTM has been executed for 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 a C-LTM cell switch, new expiration timers should not be stopped after an LTM cell switch either. This mechanism allows the UE to execute RACH-less C-LTMs even if an early synchronization phase does not always occur after a C-LTM cell switch. Furthermore, if all TA values ​​are discarded each time a C-LTM cell is switched, and if the execution conditions for subsequent C-LTMs are met before the UE acquires TA values ​​in the early synchronization phase, a RACH-based C-LTM must be executed, which is inefficient. The RACH-based C-LTM can potentially be reduced if the UE continues to retain TA values ​​and the corresponding expiration timers continue to operate.

[0208] Proposal 4: RAN2 should agree that, in order to support more efficient subsequent C-LTMs, the UE should be able to continue operating the new expiration timer and retain the TA value even after a C-LTM cell switchover.

[0209] 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 201a, 201b: gNB-CU 202a, 202b: gNB-DU 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 for use in a mobile communication system that supports LTM (L1 / L2-Triggered Mobility), comprising: a user device configured to conditionally switch to one or more LTM candidate cells receiving a timing advance value from a network node to be used for uplink communication after LTM cell switching; the user device holding the timing advance value in association with a timer that defines the period for which the timing advance value is valid; and, if the user device performs the LTM cell switching while the timer is operating, applying the held timing advance value to the uplink communication.

2. The communication method according to claim 1, wherein the user device further receives setting information for setting the conditional LTM cell switching from the network node, and the setting information includes a timer value indicating the duration of the timer.

3. The communication method according to claim 1, wherein the timer is associated with an LTM candidate cell, and the user device holds the timing advance value in association with an identifier indicating the LTM candidate cell and the timer.

4. The communication method according to claim 1, wherein the timer is managed by the user device on a beam-by-beam basis for LTM candidate cells, and the user device holds the timing advance value in association with an identifier representing the beam and the timer.

5. The communication method according to claim 4, wherein the identifier indicating the beam is an SSB (SS: Synchronization Signal / PBCH Block) identifier or a CSI-RS (Channel State Information Reference Signal) resource identifier.

6. The communication method according to claim 4, wherein the identifier relating to the beam is a TCI (Transmission Configuration Indication) status identifier.

7. The communication method according to claim 1, further comprising: the user device continuously managing the timer even when the LTM cell switching is performed; and, when the user device performs a subsequent LTM cell switching that follows the LTM cell switching while the timer is operating, applying the held timing advance value to the uplink communication after the subsequent LTM cell switching.

8. The communication method according to claim 1, wherein the user device, when performing the LTM cell switching, holds the timing advance value associated with the timer, and when the timer is operating, continues to operate the timer without stopping it.

9. The communication method according to claim 7, further comprising the user device receiving an RRC reset message, wherein the user device continuously manages the timer associated with the LTM candidate cell if the RRC reset message does not change the settings of the LTM candidate cell.

10. The communication method according to claim 9, wherein the user device stops or deems the timer associated with the LTM candidate cell to have expired when the setting of the LTM candidate cell is removed by the RRC reset message.

11. The communication method according to claim 9, wherein if a specific setting parameter among the settings of the LTM candidate cell is changed by the RRC reset message, the user device stops or deems the timer associated with the LTM candidate cell to have expired, and the specific setting parameter is an identifier indicating the LTM candidate cell and a setting relating to the beam of the LTM candidate cell.

12. The communication method according to claim 11, wherein the user device continuously manages the timer associated with the LTM candidate cell when a setting parameter in the settings of the LTM candidate cell that is different from the specific setting parameter is changed by the RRC reset message.

13. A user device for use in a mobile communication system that supports LTM (L1 / L2-Triggered Mobility), comprising: a receiving unit that receives a timing advance value from a network node for use in uplink communication between the user device and the LTM candidate cell when conditional LTM cell switching to the LTM candidate cell is set in the user device; and a control unit that holds the timing advance value in association with a timer that determines the period during which the timing advance value is valid, wherein the control unit applies the held timing advance value to the uplink communication when the conditional LTM cell switching is performed while the timer is operating.

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