Communication method, user equipment, and network node

Abstract

This communication method is executed by user equipment of a mobile communication system which supports L1 / L2 Triggered Mobility (LTM), the method including: receiving, from a first cell, a Radio Resource Control (RRC) message which includes information indicating a radio quality condition to be satisfied in order to perform an LTM cell switch for switching a serving cell of the user equipment from a first cell to a second cell by means of LTM, and information indicating a timing advance value to be applied to the second cell; evaluating whether or not the radio quality condition is satisfied for the second cell; and performing the LTM cell switch by applying the timing advance value notified by the RRC message, in response to the evaluation that the radio quality condition is satisfied.

Description

COMMUNICATION METHOD, USER EQUIPMENT, AND NETWORK NODE

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

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

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

[0004] 3GPP Technical Specification "3GPP TS 38.300 V18.0.0 (2023-12)"

[0005] The present disclosure relates to a communication method that enables conditional LTM to be performed appropriately.

[0006] A communication method according to a first aspect of the present disclosure is a communication method executed by a user equipment in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), the communication method including: receiving, from a first cell, a Radio Resource Control (RRC) message including information indicating a radio quality condition to be satisfied in order to perform an LTM cell switch, which switches a serving cell of the user equipment from a first cell to a second cell by the LTM, and information indicating a timing advance value to be applied to the second cell; evaluating whether the radio quality condition is satisfied for the second cell; and, in response to the evaluation that the radio quality condition is satisfied, performing the LTM cell switch by applying the timing advance value notified in the RRC message.

[0007] A user equipment according to a second aspect of the present disclosure is a user equipment of a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), and includes: a receiving unit that receives, from the first cell, an RRC (Radio Resource Control) message including information indicating a radio quality condition that must be satisfied in order to perform an LTM cell switch that switches a serving cell of the user equipment from a first cell to a second cell by the LTM, and information indicating a timing advance value that should be applied to the second cell; and a control unit that evaluates whether the radio quality condition is satisfied for the second cell, and, if it is determined that the radio quality condition is satisfied, performs the LTM cell switch by applying the timing advance value notified in the RRC message.

[0008] A network node according to a third aspect of the present disclosure is a network node that manages a first cell in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), and includes a transmitter that transmits, to the user equipment, an RRC (Radio Resource Control) message including information indicating a radio quality condition that should be satisfied to perform LTM cell switch, which switches a serving cell of a user equipment from the first cell to a second cell by the LTM, and information indicating a timing advance value that should be applied to the second cell.

[0009] A communication method according to a fourth aspect of the present disclosure is a communication method executed by a user equipment in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), the communication method including: receiving, from the first cell, an RRC (Radio Resource Control) message including information indicating a radio quality condition that must be satisfied in order to perform an LTM cell switch, which switches a serving cell of the user equipment from a first cell to a second cell by the LTM; and a user equipment measurement configuration in which the user equipment measures a timing advance value to be applied to the second cell by itself; evaluating whether the radio quality condition is satisfied for the second cell; performing the user equipment measurement based on the configuration in response to the evaluation that the radio quality condition is satisfied; and performing the LTM cell switch by applying the timing advance value measured by the user equipment measurement.

[0010] A user equipment according to a fifth aspect of the present disclosure is a user equipment of a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), and includes: a receiver that receives, from the first cell, an RRC (Radio Resource Control) message including information indicating a radio quality condition that must be satisfied in order to perform an LTM cell switch that switches a serving cell of the user equipment from a first cell to a second cell by the LTM, and a user equipment measurement configuration that causes the user equipment to measure a timing advance value to be applied to the second cell; and a controller that evaluates whether the radio quality condition is satisfied for the second cell, and, in response to the evaluation that the radio quality condition is satisfied, performs the user equipment measurement based on the configuration, and performs the LTM cell switch by applying the timing advance value measured by the user equipment measurement.

[0011] A network node according to a sixth aspect of the present disclosure is a network node that manages a first cell in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), and includes a transmitter that transmits, to a user equipment, an RRC (Radio Resource Control) message including information indicating a radio quality condition that must be satisfied to perform an LTM cell switch that switches a serving cell of a user equipment from a first cell to a second cell by the LTM, and user equipment measurement configuration that causes the user equipment to measure, by itself, a timing advance value that should be applied to the second cell.

[0012] 1 is a diagram showing an example of the configuration of a mobile communication system according to an embodiment. FIG. 2 is a diagram showing an example of the configuration of a UE (user equipment) according to an embodiment. FIG. 3 is a diagram showing an example of the configuration of a gNB (network node) according to an embodiment. FIG. 4 is a diagram showing a protocol stack configuration of a radio interface of a user plane that handles data. FIG. 5 is a diagram showing a protocol stack configuration of a radio interface of a control plane that handles signaling (control signals). FIG. 6 is a diagram showing an example of an LTM procedure. FIG. 7 is a diagram showing an operation of a UE according to the first embodiment. FIG. 8 is a diagram showing an example of the configuration of an RRC Reconfiguration message according to the first embodiment. FIG. 9 is a diagram for explaining an example of UE-based TA measurement according to a second embodiment. FIG. 10 is a diagram showing an operation of a UE according to the second embodiment. FIG. 11 is a diagram showing an example of the configuration of an RRC Reconfiguration message according to the second embodiment.

[0013] A mobile communication system according to an embodiment will be described with reference to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0014] (1) First Embodiment The first embodiment will be described.

[0015] (1.1) Configuration of a Mobile Communication System FIG. 1 is a diagram showing an example of the configuration of a mobile communication system 1 according to this embodiment. The mobile communication system 1 conforms to the 3GPP standard 5th Generation System (5GS). While the following description uses 5GS as an example, the mobile communication system may also be at least partially based on an LTE (Long Term Evolution) system. The mobile communication system may also be at least partially based on a 6th Generation (6G) system.

[0016] The mobile communication system 1 includes a user equipment (UE) 100, a 5G radio access network (NG-RAN: Next Generation Radio Access Network) 10, and a 5G core network (5GC: 5G Core Network) 20. Hereinafter, the NG-RAN 10 may be simply referred to as the RAN 10. Furthermore, the 5GC 20 may be simply referred to as the core network (CN) 20. The RAN 10 and the CN 20 constitute a network 5 of the mobile communication system 1.

[0017] The UE 100 is a mobile wireless communication device. The UE 100 may be any device that is used by a user, but for example, the UE 100 is a mobile phone terminal (including a smartphone) and / or a tablet terminal, a notebook PC, a communication module (including a communication card or a chipset), a sensor or a device provided in a sensor, a vehicle or a device provided in a vehicle (Vehicle UE), or an aircraft or a device provided in an aircraft (Aerial UE).

[0018] The NG-RAN 10 includes a base station (referred to as "gNB" in the 5G system) 200, which is a type of network node. The gNBs 200 are connected to each other via an Xn interface, which is an interface between base stations. The gNB 200 manages one or more cells. The gNB 200 performs wireless communication with the UE 100 that has established a connection with its own cell. The gNB 200 has a radio resource management (RRM) function, a routing function for user data (hereinafter simply referred to as "data"), a measurement control function for mobility control and scheduling, etc. The term "cell" is used to indicate the smallest unit of a wireless communication area. The term "cell" is also used to indicate a function or resource that performs wireless communication with the UE 100. One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

[0019] In addition, gNBs can also be connected to the Evolved Packet Core (EPC), which is the core network of LTE. LTE base stations can also be connected to 5GC. LTE base stations and gNBs can also be connected via an inter-base station interface.

[0020] The 5GC20 includes an AMF (Access and Mobility Management Function) and a UPF (User Plane Function) 300. The AMF performs various mobility controls for the UE 100. The AMF manages the mobility of the UE 100 by communicating with the UE 100 using NAS (Non-Access Stratum) signaling. The UPF controls data forwarding. The AMF and the UPF are connected to the gNB 200 via an NG interface, which is an interface between a base station and a core network.

[0021] 2 is a diagram showing an example of the configuration of a UE 100 (user equipment) according to this embodiment. The UE 100 has 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 the gNB 200.

[0022] The receiving unit 110 performs various types of reception under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 130.

[0023] The transmitting unit 120 performs various transmissions under the control of the control unit 130. The transmitting unit 120 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 130 into a radio signal and transmits it from the antenna.

[0024] The control unit 130 performs various controls and processes in the UE 100. Such processes include processes of each layer described below. The operations of the UE 100 described above and below may be operations controlled by the control unit 230. The control unit 130 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processing by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0025] 3 is a diagram showing an example configuration of a gNB 200 (network node) according to this embodiment. The gNB 200 has a transmitter 210, a receiver 220, a controller 230, and a network communication unit 240. The transmitter 210 and the receiver 220 constitute a wireless communication unit 250 that performs wireless communication with the UE 100. The network communication unit 240 has a transmitter 241 that transmits and a receiver 242 that receives.

[0026] The transmitting unit 210 performs various transmissions under the control of the control unit 230. The transmitting unit 210 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 230 into a radio signal and transmits it from the antenna.

[0027] The receiving unit 220 performs various types of reception under the control of the control unit 230. The receiving unit 220 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 230.

[0028] The control unit 230 performs various controls and processes in the gNB 200. Such processes include processes for each layer described below. The operations of the gNB 200 described above and below may be operations under the control of the control unit 230. The control unit 230 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processing by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0029] The network communication unit 240 is connected to adjacent base stations via an Xn interface, which is an interface between base stations. The network communication unit 240 is connected to the AMF / UPF 300 via an NG interface, which is an interface between a base station and a core network. The gNB 200 is composed of a CU (Central Unit) and a DU (Distributed Unit) (i.e., functionally divided), and the two units may be connected by an F1 interface, which is a fronthaul interface.

[0030] FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data.

[0031] The user plane radio interface protocol includes a physical (PHY) layer, a medium access control (MAC) layer, a radio link control (RLC) layer, a packet data convergence protocol (PDCP) layer, and a service data adaptation protocol (SDAP) layer.

[0032] The PHY layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layer of UE100 and the PHY layer of gNB200 via a physical channel. The PHY layer of UE100 receives downlink control information (DCI) transmitted from gNB200 on a physical downlink control channel (PDCCH). Specifically, UE100 performs blind decoding of the PDCCH using a radio network temporary identifier (RNTI) and acquires successfully decoded DCI as DCI addressed to the UE. The DCI transmitted from gNB200 has a CRC (Cyclic Redundancy Code) parity bit scrambled by the RNTI added.

[0033] The MAC layer performs data priority control, retransmission processing using Hybrid Automatic Repeat reQuest (HARQ), random access procedures, etc. Data and control information are transmitted between the MAC layer of the UE 100 and the MAC layer of the gNB 200 via a transport channel. The MAC layer of the gNB 200 includes a scheduler. The scheduler determines the uplink and downlink transport format (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be allocated to the UE 100.

[0034] The RLC layer transmits data to the receiving RLC layer using the functions of the MAC layer and PHY layer. Data and control information are transmitted between the RLC layer of the UE 100 and the RLC layer of the gNB 200 via a logical channel.

[0035] The PDCP layer performs header compression / decompression, encryption / decryption, and the like.

[0036] The SDAP layer maps IP flows, which are units for Quality of Service (QoS) control by the core network, to radio bearers, which are units for QoS control by the Access Stratum (AS). Note that if the RAN is connected to the EPC, SDAP may not be required.

[0037] FIG. 5 is a diagram showing the configuration of a protocol stack of a radio interface of a control plane that handles signaling (control signals).

[0038] The protocol stack of the radio interface of the control plane has an RRC (Radio Resource Control) layer and an NAS (Non-Access Stratum) layer instead of the SDAP layer shown in FIG.

[0039] RRC signaling for various settings is transmitted between the RRC layer of UE100 and the RRC layer of gNB200. The RRC layer controls logical channels, transport channels, and physical channels according to the establishment, re-establishment, and release of radio bearers. When there is a connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC connected state. When there is no connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC idle state. When the connection between the RRC of UE100 and the RRC of gNB200 is suspended, UE100 is in an RRC inactive state.

[0040] The NAS layer (also simply referred to as "NAS") located above the RRC layer performs session management, mobility management, etc. NAS signaling is transmitted between the NAS layer of the UE 100 and the NAS layer of the AMF 300A. Note that the UE 100 has an application layer and the like in addition to the radio interface protocol. Also, a layer lower than the NAS layer is referred to as the AS layer (also simply referred to as "AS").

[0041] (1.2) Overview of LTM The mobile communication system according to this embodiment supports LTM (L1 / L2-triggered mobility).

[0042] In a typical handover procedure, a serving cell switch is triggered by signaling in the upper layer L3, specifically the RRC layer. Hereinafter, such a typical handover is also referred to as an L3 handover. In an L3 handover, an L3 measurement report message, which is an RRC message, is transmitted from the UE 100 to the gNB 200. The gNB 200 determines a handover of the UE 100 based on the Measurement Report message, and instructs the cell switch by transmitting a handover command, which is an RRC message (specifically, an RRC Reconfiguration message), from the gNB 200 to the UE 100.

[0043] On the other hand, LTM is a technology for shortening mobility delay (specifically, serving cell switching delay) compared to a typical handover procedure by triggering a serving cell switch by signaling of a lower layer, Layer 1 (L1) and / or Layer 2 (L2). In LTM, the gNB 200 receives an L1 measurement report from the UE 100, and based on the L1 measurement report, the gNB 200 signals the UE 100 via a MAC CE to instruct the serving cell switch by a cell switch command.

[0044] Specifically, in LTM, first, gNB200 prepares an LTM candidate cell configuration for a candidate cell to be switched to, and provides the LTM candidate cell configuration to UE100 via RRC signaling.

[0045] Secondly, the UE 100 performs a synchronization process with the candidate cell by early synchronization (Early sync).

[0046] Third, the gNB 200 receives an L1 measurement report from the UE 100, determines a serving cell switch to the target cell based on the L1 measurement report, and transmits a cell switch command (Cell Switch Command) indicating the target cell (LTM candidate cell setting) to the UE 100 by MAC CE. The serving cell switch trigger is conveyed in a MAC CE including at least a candidate setting index together with a beam indicator.

[0047] Fourth, UE100 switches the serving cell in response to a cell switching command from gNB200 (source cell).

[0048] In this way, a serving cell switch is triggered by selecting an LTM candidate cell setting as a target setting by gNB200. An LTM candidate cell setting can be added, changed, and released by gNB200 via RRC signaling.

[0049] The following principles apply to LTM:

[0050] Each LTM candidate cell configuration can be provided as a differential configuration (delta configuration) relative to the reference configuration used to form the complete candidate cell configuration.

[0051] If a full candidate cell configuration is applied, it replaces the current UE configuration upon a serving cell switch. The reconfiguration procedure does the replacement but does not necessarily reset the MAC, RLC or PDCP layers.

[0052] The user plane continues without a reset if configured in RRC signaling to avoid additional delays in data recovery.

[0053] - Security is not updated in LTM.

[0054] LTM between subsequent LTM candidate cell configurations can be performed without RRC reconfiguration, i.e., the UE 100 does not release other LTM candidate cell configurations after LTM is triggered.

[0055] 6 is a diagram showing an example of an LTM procedure. In the illustrated example, it is assumed that the UE 100 performs a serving cell switch from the first cell of the gNB 200 to the second cell.

[0056] Here, the first cell and the second cell may be formed by different TRPs (Transmission and Reception Points). In the following description of the embodiment, the second cell is also referred to as a "candidate cell (or LTM candidate cell)" until a serving cell switch by LTM is determined, and after a serving cell switch by LTM is determined, the second cell is also referred to as a "target cell". The first cell is also referred to as a "source cell".

[0057] In step S1, UE100 is in an RRC connected state in the cell (first cell, source cell) of gNB200.

[0058] In step S2, UE100 transmits a Measurement Report message, which is an RRC message, to gNB200.

[0059] In step S3, gNB200 decides to use LTM based on the Measurement Report message and starts preparing the candidate cell.

[0060] In step S4, gNB200 sends an RRC Reconfiguration message to UE100, which includes LTM candidate cell configurations (LTM Candidate Configurations) for one or more candidate cells.

[0061] In step S5, UE100 saves the LTM candidate cell setting and sends an RRC Reconfiguration Complete message to gNB200.

[0062] In step S6, the UE 100 may perform synchronization processing with the candidate cell before receiving the cell switch command. Such synchronization processing is called early synchronization (Early sync). Here, the UE 100 may perform early timing advance (TA) acquisition in the candidate cell requested by the gNB 200 (source cell) before receiving the cell switch command of step S9. This is performed by contention-free random access (CFRA) triggered by a PDCCH command (PDCCH order) from the source cell. Note that when DCI Format 1_0 is used and all "Frequency domain resource assignment" fields in the DCI are set to "1", the DCI is treated as a PDCCH order. The UE 100 transmits a random access preamble (RA preamble) to the specified candidate cell. In order to minimize communication interruption of the source cell due to CFRA for the candidate cell, in early synchronization, the UE 100 does not receive a random access response (RAR) for the purpose of acquiring a TA value from the candidate cell. The TA value of the candidate cell (target cell) is indicated in the cell switching command in step S9. The TA value is a value for adjusting the uplink transmission timing of the UE 100.

[0063] In step S7, the UE 100 performs layer 1 (L1) measurement in the configured candidate cell and transmits a physical layer measurement report (L1 Measurement Report) to the gNB 200. The L1 Measurement Report is transmitted and received at L1, which is the PHY layer. For example, the UE 100 transmits L1-RSRP and / or L1-SINR to the gNB 200 via a PUCCH (Physical Uplink Control Channel) and / or a PUSCH (Physical Uplink Shared Channel).

[0064] In step S8, gNB200 decides to switch the serving cell to the target cell (second cell).

[0065] In step S9, gNB200 transmits a Cell Switch Command (MAC CE) including a candidate configuration index of the target cell to UE100. The Cell Switch Command may include a TA value obtained by early synchronization.

[0066] In step S10, the UE 100 switches to the configuration of the target cell. Specifically, the UE 100 detaches from the source cell (first cell) and applies the configuration of the target cell.

[0067] In step S11, if the serving cell switch needs to include the execution of a random access procedure (for example, if the Cell Switch Command does not include a valid TA value), the UE 100 executes the random access procedure for the target cell. Note that, if the UE 100 does not need to acquire the TA of the target cell at the time of the serving cell switch (for example, if the Cell Switch Command includes a valid TA value), the random access procedure can be skipped.

[0068] In step S12, the UE 100 indicates that the serving cell switch to the target cell has been successfully completed. Thereafter, the UE 100 may perform steps S6 to S12 multiple times for subsequent LTM serving cell switches based on the configuration provided in step S4.

[0069] (1.3) Operation According to First Embodiment Introduction of a conditional LTM (C-LTM) procedure is planned for consideration in 3GPP Release 19. The cell switching procedure using LTM as shown in Fig. 6 can be applied to the C-LTM procedure, similar to a conditional L3 handover (CHO).

[0070] In this embodiment, the gNB 200 sets in advance in the UE 100 an execution condition (trigger condition, CondEvent), which is a radio quality condition for performing cell switching, in the RRC Reconfiguration message of step S4. The UE 100 performs cell switching by LTM (also referred to as "LTM cell switching") when the radio quality condition is satisfied. This makes it possible to skip the operations from the L1 Measurement Report in step S7 to the Cell Switch Command (MAC CE) in step S9. As a result, it is possible to speed up cell switching and minimize interruptions in data communication.

[0071] Here, in conventional LTM, the TA value to be applied to the second cell (candidate cell, target cell) is notified from the gNB 200 to the UE 100 in the Cell Switch Command (MAC CE) of step S9. On the other hand, in C-LTM, the Cell Switch Command (MAC CE) of step S9 is skipped, so there is a problem that the UE 100 cannot obtain the TA value to be applied to the second cell from the gNB 200. As a result, the UE 100 may need to perform a random access procedure for the second cell when the radio quality condition (trigger condition, CondEvent) is satisfied. Therefore, it is difficult to speed up cell switching.

[0072] In the present embodiment, such a problem can be solved by notifying (setting) the TA value to be applied to the second cell to the UE 100 in advance by the RRC Reconfiguration message in step S4.

[0073] FIG. 7 is a diagram showing the operation of the UE 100 according to this embodiment.

[0074] In step S21, the UE 100 receives an RRC message from the first cell, the RRC message including information indicating a radio quality condition that must be satisfied to perform an LTM cell switch, which switches the serving cell of the UE 100 from a first cell (current serving cell) to a second cell (candidate cell, target cell) by LTM, and information indicating a TA value that should be applied to the second cell. The RRC message may be the RRC Reconfiguration message of step S4. At this point, the UE 100 holds the TA value but does not apply it.

[0075] The radio quality may be at least one measurement value of a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal-to-interference-and-noise ratio (SINR), and a received signal strength indicator (RSSI). The reference signal may be an SSB (SS / PBCH block) or a CSI (Channel State Information) reference signal. The radio quality condition may be a threshold value to be compared with these measurements.

[0076] The information indicating the radio quality condition may be information indicating a radio quality condition that should be satisfied for the first cell (current serving cell) and / or the second cell (candidate cell, target cell) to trigger an LTM cell switch. For example, the information indicating the radio quality condition may be an event (condition) such as "Neighbor becomes offset better than SpCell," i.e., a trigger condition indicating that the radio quality of the second cell has improved by an offset amount relative to the radio quality of the first cell. In this case, the information indicating the radio quality condition may include an offset value. Alternatively, the information indicating the radio quality condition may be information indicating an event (condition) such as "Neighbor / SpCell becomes better / worse than threshold." In this case, the information indicating the radio quality condition may include a threshold.

[0077] The information indicating the TA value to be applied to the second cell may be information indicating that the TA value to be applied to the second cell is zero, or information indicating that the TA value to be applied to the second cell is the same as the TA value of the first cell.

[0078] In step S22, the UE 100 evaluates whether or not the radio quality condition for the second cell is satisfied. For example, the UE 100 measures the radio quality of the first cell and / or the radio quality of the second cell, and evaluates (determines) whether or not the measured value satisfies the condition set in step S21.

[0079] In step S23, in response to the evaluation that the radio quality condition is satisfied, the UE 100 performs the LTM cell switch by applying the TA value notified in the RRC message of step S21. Here, the UE 100 performs the LTM cell switch by applying the TA value notified in the RRC message of step S21 without performing a random access procedure for the second cell. For example, the UE 100 transmits an RRC Reconfiguration Complete message to the second cell. The UE 100 may consider that the execution of the LTM cell switch has been completed successfully when it determines that the network has successfully received the first uplink data.

[0080] According to this operation, the UE 100 skips the operations from the L1 Measurement Report in step S7 to the Cell Switch Command (MAC CE) in step S9, and can quickly perform LTM cell switching when the radio quality condition set by the gNB 200 is satisfied. In addition, when the radio quality condition is satisfied, the UE 100 does not need to perform a random access procedure for the second cell, and cell switching can be accelerated.

[0081] UE 100 that performs such operations has a receiving unit 110 that receives an RRC message from a first cell, the RRC message including information indicating radio quality conditions that must be satisfied in order to perform LTM cell switching, which switches the serving cell of UE 100 from a first cell to a second cell by LTM, and information indicating a TA value that should be applied to the second cell, and a control unit 130 that evaluates whether the radio quality conditions are satisfied for the second cell, and, if it is determined that the radio quality conditions are satisfied, performs LTM cell switching by applying the TA value notified in the RRC message.

[0082] On the other hand, the gNB200 that manages the first cell has a transmitter 210 that transmits to the UE100 an RRC message including information indicating radio quality conditions that must be satisfied to perform LTM cell switching, which switches the serving cell of the UE100 from the first cell to the second cell by LTM, and information indicating a TA value that should be applied to the second cell. The gNB200 may manage the first cell as well as the second cell. Alternatively, the second cell may be a cell managed by another gNB200. In this case, when the UE100 accesses the second cell, the other gNB200 that manages the second cell may notify the gNB200 that manages the first cell of the access (e.g., notify on the Xn interface).

[0083] Here, the case where there is one second cell (candidate cell) has been described as an example, but multiple candidate cells may be set in the UE 100. In this case, the RRC message in step S21 may include information indicating multiple TA values ​​corresponding to the multiple second cells. In step S22, the UE 100 may perform evaluation for each of the multiple second cells. In step S23, the UE 100 may apply the TA value corresponding to the one second cell in response to evaluating that the radio quality condition is satisfied for any one second cell among the multiple second cells, and perform LTM cell switching for the one second cell.

[0084] The RRC message of step S21 may include a set of information indicating a radio quality condition and information indicating a TA value for each of the plurality of second cells. In step S22, the UE 100 may evaluate whether or not the corresponding radio quality condition for each of the plurality of second cells is satisfied.

[0085] FIG. 8 is a diagram showing an example of the configuration of the RRC Reconfiguration message (RRC message of step S21) according to this embodiment.

[0086] The RRC Reconfiguration message includes, as an information element, a conditional LTM setting (C-LTM config) for setting the conditional LTM cell switching in the UE 100. The conditional LTM setting includes, as an information element, a candidate cell setting list (C-LTM Config List) for adding the setting of the conditional LTM cell switching for each candidate cell (second cell).

[0087] The candidate cell setting list includes, as entries, candidate cell settings that are conditional LTM cell switching settings for each candidate cell (second cell). The candidate cell setting includes a setting ID for identifying the setting, a cell ID for identifying the corresponding candidate cell (second cell), information indicating a cell switching execution condition (radio quality condition), an RRC setting to be applied to communication with the corresponding candidate cell (second cell), and information indicating a TA value to be applied to the corresponding candidate cell (second cell). When the candidate cell setting includes information indicating the TA value, the UE 100 may consider that early TA acquisition is not required for the corresponding candidate cell.

[0088] In the illustrated example, a conditional LTM configuration is newly provided instead of LTM Config, which is an information element introduced in 3GPP Release 18. However, as another configuration example, 3GPP Release 18 may be utilized, and a cell switching execution condition and a TA value may be added to each entry of a candidate cell configuration list (LTM Candidate To Add Mod List in 3GPP Release 18) in the LTM Config.

[0089] The candidate cell configuration for a candidate cell (second cell) for which early TA acquisition is required may include information on early TA acquisition instead of information indicating the TA value. The information on early TA acquisition may include at least one of information specifying execution of early TA acquisition (preamble transmission by CFRA) and configuration of a physical random access channel (PRACH) used for preamble transmission.

[0090] Furthermore, the candidate cell configuration for a candidate cell (second cell) to which UE-based TA measurement (user equipment measurement) described later can be applied may include information on the UE-based TA measurement instead of information indicating the TA value. The information on the UE-based TA measurement may include at least one of information specifying the execution of the UE-based TA measurement and measurement parameters used in the UE-based TA measurement.

[0091] (2) Second Embodiment The second embodiment will be described, focusing mainly on the differences from the first embodiment.

[0092] In the conventional LTM procedure of FIG. 6 , in step S6, the UE 100 triggers a CFRA by a PDCCH order from the first cell (current serving cell, source cell), thereby realizing early TA for the second cell (candidate cell, target cell).

[0093] Instead of such CFRA, the UE 100 may acquire the TA value for the second cell by UE-based TA measurement. In the UE-based TA measurement, the UE 100 itself measures the TA value to be applied to the second cell.

[0094] (2.1) UE-Based TA Measurement FIG. 9 is a diagram illustrating an example of UE-based TA measurement according to this embodiment. In the illustrated example, it is assumed that the frame timing between the first cell and the second cell is asynchronous, and the frame timing difference between the first cell and the second cell is also referred to as "Tdiff_s-t_nw." In the illustrated example, "Tdiff_s-t_nw" is the time from time t1 to time t3. Note that if the first cell and the second cell are perfectly synchronized, the frame timing difference "Tdiff_s-t_nw" is zero. As a synchronization method, for example, synchronization is achieved using GNSS (Global Navigation Satellite System) and / or IEEE 1588.

[0095] The UE-based TA measurement includes, for example, the following procedures.

[0096] STEP 1: UE100 is in an RRC connected state in the first cell and knows the TA value being applied in the first cell. The TA value being applied to UE100 in the first cell is also referred to as "TA_s." "TA_s" is the time by which the UL frame timing precedes the DL frame timing in UE100. In the illustrated example, the TA value "TA_s" being applied in the first cell is the time from time t6 to time t7. Note that the TA value is used to control the UL transmission timing of each UE100 so that UL transmissions from all UE100 are synchronized when received by the serving cell (gNB200). UE100 closer to the TRP of the cell has a short propagation delay, so the TA value is small. On the other hand, UE100 farther from the TRP of the cell has a long propagation delay, so the TA value is large. Generally, the TA value "TA_s" is set to the UE 100 by the serving cell (gNB 200) in an RA response during the RA procedure, and then adjusted by a TA command (MAC CE) transmitted from the serving cell (gNB 200) to the UE 100. Therefore, the serving cell (gNB 200) also knows the TA value "TA_s".

[0097] STEP 2: The UE 100 performs RSTD (Reference Signal Timing Difference) measurement and generates reference signal time difference information "Tdiff_s-t_ue" relating to the reception timing difference between the DL reference signal of the first cell and the DL reference signal of the second cell. The RSTD measurement measures the reception timing difference between the DL reference signal of the first cell and the DL reference signal of the second cell, and determines the timing difference "Tdiff_s-t_ue" between the DL radio frames of the first cell and the second cell at the receiving end of the UE 100. In the illustrated example, the timing difference "Tdiff_s-t_ue" between the DL radio frames of the first cell and the second cell is the time from time t3 to time t4.

[0098] STEP 3: The second cell (gNB200) receives an UL reference signal (e.g., SRS (Sounding Reference Signal)) transmitted from UE100 to the first cell, and generates timing error information "TA_temp_t" regarding the error in the reception timing of the UL reference signal relative to the frame timing of the second cell. Here, "TA_s" is applied to the UL reference signal transmitted from UE100 to the first cell. The second cell (gNB200) grasps the reception error "TA_temp_t" between the second cell's own UL radio frame and the UL reference signal from UE100. In the illustrated example, the reception timing error "TA_temp_t" of the UL reference signal relative to the frame timing of the second cell is the time from time t1 to time t5. Note that STEP 3 may be performed before STEP 2. STEP 3 may be performed simultaneously with STEP 2.

[0099] STEP 4: From "TA_s", "Tdiff_s-t_ue", and "TA_temp_t", a TA value "TA_t" to be applied by UE 100 in the second cell is calculated by the following equation (1): TA_t = (TA_temp_t + TA_s) - Tdiff_s-t_ue (1) However, the TA value "TA_t" to be applied by UE 100 in the second cell may also be calculated by the following equation (2) that further takes into account the inter-cell synchronization error "Tdiff_s-t_nw": TA_t = (TA_temp_t + TA_s) - (Tdiff_s-t_ue + Tdiif_s-t_nw) (2) Alternatively, "Tdiff_s-t_nw" may be used only for the second cell to receive the UL reference signal from the UE 100, i.e., to calculate "TA_temp_t".

[0100] Therefore, by calculating equation (1) based on parameters (variables) such as "TA_s", "Tdiff_s-t_ue", and "TA_temp_t", the TA value "TA_t" that UE100 should apply in the second cell can be derived without UE100 performing an RA procedure to the second cell.

[0101] (2.2) Operation according to the second embodiment In the conventional LTM procedure, when the TA value of the second cell is provided in the Cell Switch Command (MAC CE), the UE 100 applies the TA value to perform cell switching to the second cell. On the other hand, if the UE-based TA measurement is configured in the RRC from the gNB 200, the UE 100 performs UE-based TA measurement. If the UE 100 cannot derive the TA of the target second cell, a random access procedure (RACH) with the second cell is required.

[0102] However, in the case of C-LTM, the UE 100 configured with the UE-based TA measurement has a problem as to when to perform the UE-based TA measurement. For example, it is inefficient for the UE 100 to perform the UE-based TA measurement for all of the candidate cells (multiple second cells) immediately after the UE 100 is configured with these candidate cells, and the processing load of the UE 100 may be increased.

[0103] In the present embodiment, when the radio quality condition (trigger condition, CondEvent) for the second cell is satisfied, the UE 100 performs the UE-based TA measurement, thereby making it possible to solve such a problem.

[0104] FIG. 10 is a diagram showing the operation of the UE 100 according to this embodiment.

[0105] In step S31, the UE 100 receives, from the first cell, an RRC message including information indicating a radio quality condition that must be satisfied to perform an LTM cell switch that switches the serving cell of the UE 100 from a first cell (current serving cell) to a second cell (candidate cell, target cell) by LTM, and a UE-based TA measurement configuration in which the UE 100 itself measures a TA value that should be applied to the second cell. The RRC message may be the RRC Reconfiguration message of step S4.

[0106] The UE-based TA measurement configuration (UE-based TA configuration) includes information regarding the UE-based TA measurement, and may include, for example, at least one of information specifying the execution of the UE-based TA measurement and measurement parameters used in the UE-based TA measurement.

[0107] In step S32, the UE 100 evaluates whether or not the radio quality condition for the second cell is satisfied. For example, the UE 100 measures the radio quality of the first cell and / or the radio quality of the second cell, and evaluates (determines) whether or not the measured value satisfies the condition set in step S31.

[0108] In step S33, the UE 100 performs UE-based TA measurement based on the UE-based TA measurement configuration notified in the RRC message in step S31 in response to the evaluation that the radio quality condition is satisfied. For example, the UE 100 starts UE-based TA measurement for the second cell in response to the evaluation that the radio quality condition for the second cell is satisfied. As a result, the UE 100 acquires a TA value for the second cell.

[0109] In step S34, the UE 100 performs the LTM cell switch by applying the TA value measured (acquired) in the UE-based TA measurement in step S33. Here, the UE 100 performs the LTM cell switch by applying the TA value measured (acquired) in the UE-based TA measurement in step S33 without performing a random access procedure for the second cell. For example, the UE 100 transmits an RRC Reconfiguration Complete message to the second cell. The UE 100 may consider that the execution of the LTM cell switch has been completed successfully when it determines that the network has successfully received the first uplink data.

[0110] According to this operation, the UE 100 skips the operations from the L1 Measurement Report in step S7 to the Cell Switch Command (MAC CE) in step S9, and can quickly perform LTM cell switching when the radio quality condition set by the gNB 200 is satisfied. In addition, when the radio quality condition is satisfied, the UE 100 does not need to perform a random access procedure for the second cell, and cell switching can be accelerated.

[0111] UE100 that performs such operations has a receiver 110 that receives an RRC message from a first cell, the RRC message including information indicating the radio quality conditions that must be satisfied in order to perform LTM cell switching, which switches UE100's serving cell from a first cell to a second cell by LTM, and a UE-based TA measurement setting in which UE100 itself measures the TA value to be applied to the second cell, and a control unit 130 that evaluates whether the radio quality conditions are satisfied for the second cell, and, if it is determined that the radio quality conditions are satisfied, performs UE-based TA measurement based on the setting, and performs LTM cell switching by applying the TA value measured by the UE-based TA measurement.

[0112] On the other hand, the gNB200 that manages the first cell has a transmitter 210 that transmits to the UE100 an RRC message including information indicating radio quality conditions that must be satisfied to perform LTM cell switching, which switches the serving cell of the UE100 from the first cell to the second cell by LTM, and a UE-based TA measurement configuration in which the UE100 itself measures the TA value to be applied to the second cell. The gNB200 may manage the first cell as well as the second cell. Alternatively, the second cell may be a cell managed by another gNB200. In this case, when the UE100 accesses the second cell, the other gNB200 that manages the second cell may notify the gNB200 that manages the first cell of the access (e.g., notify on the Xn interface).

[0113] FIG. 11 is a diagram showing an example of the configuration of the RRC Reconfiguration message (RRC message of step S31) according to this embodiment.

[0114] The RRC Reconfiguration message includes, as an information element, a conditional LTM setting (C-LTM config) for setting the conditional LTM cell switching in the UE 100. The conditional LTM setting includes, as an information element, a candidate cell setting list (C-LTM Config List) for adding the setting of the conditional LTM cell switching for each candidate cell (second cell).

[0115] The candidate cell configuration list includes, as entries, candidate cell configurations that are conditional LTM cell switching configurations for each candidate cell (second cell). The candidate cell configuration includes a configuration ID for identifying the configuration, a cell ID for identifying the corresponding candidate cell (second cell), information indicating a cell switching execution condition (radio quality condition), an RRC configuration to be applied to communication with the corresponding candidate cell (second cell), and a UE-based TA configuration for the corresponding candidate cell (second cell). When the UE-based TA configuration is included in the candidate cell configuration, the UE 100 may determine to perform UE-based TA configuration for the corresponding candidate cell.

[0116] In the illustrated example, a conditional LTM configuration is newly provided instead of LTM Config, which is an information element introduced in 3GPP Release 18. However, as another configuration example, 3GPP Release 18 may be utilized, and a cell switching execution condition and a UE-based TA configuration may be added to each entry of a candidate cell configuration list (LTM Candidate To Add Mod List in 3GPP Release 18) in the LTM Config.

[0117] (3) Other Embodiments The above-described operational flows are not limited to being implemented independently, but can be implemented by combining two or more operational flows. For example, some steps of one operational flow may be added to another operational flow, or some steps of one operational flow may be replaced with some steps of another operational flow. In each flow, it is not necessary to execute all steps, and only some steps may be executed. Furthermore, the order of steps in each flow may be changed as appropriate.

[0118] In the above-described embodiments and examples, an example in which the base station is an NR base station (gNB) has been described, but the base station may be an LTE base station (eNB) or a 6G base station. The base station may also be a relay node such as an IAB (Integrated Access and Backhaul) node. The base station may also be a DU of the IAB node. The UE 100 may also be an MT (Mobile Termination) of the IAB node. That is, the UE 100 may be a terminal function unit (a type of communication module) for the base station to control a relay that relays signals. Such a terminal function unit is referred to as an MT. Examples of MTs include, in addition to IAB-MT, NCR (Network Controlled Repeater)-MT and RIS (Reconfigurable Intelligent Surface)-MT.

[0119] The term "network node" primarily refers to a base station, but may also refer to a core network device or a part of a base station (CU, DU, or RU). A network node may also be configured by a combination of at least a part of a core network device and at least a part of a base station.

[0120] A program may be provided that causes a computer to execute each process performed by the UE 100 or the gNB 200. The program may be recorded on a computer-readable medium. Using a computer-readable medium, the program can be installed on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transitory recording medium. The non-transitory recording medium is not particularly limited, and may be, for example, a recording medium such as a CD-ROM or a DVD-ROM. Furthermore, circuits that execute each process performed by the UE 100 or the gNB 200 may be integrated, and at least a portion of the UE 100 or the gNB 200 may be configured as a semiconductor integrated circuit (chip set, SoC: System on a chip).

[0121] The functions performed by the UE 100 or the gNB 200 may be implemented in circuitry or processing circuitry, including general-purpose processors, application-specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), a CPU (a Central Processing Unit), conventional circuits, and / or combinations thereof, programmed to perform the described functions. A processor includes transistors and other circuits and is considered to be circuitry or processing circuitry. A processor may also be a programmed processor that executes a program stored in memory. In this specification, circuitry, unit, or means refers to hardware that is programmed to perform the described functions or hardware that executes them. The hardware may be any hardware disclosed herein or any hardware known to be programmed or capable of performing the described functions. If the hardware is a processor, the circuitry, means, or unit is a combination of hardware and software used to configure the hardware and / or processor.

[0122] As used in this disclosure, the terms "based on" and "depending on / in response to" do not mean "based only on" or "depending only on," unless expressly stated otherwise. The term "based on" means both "based only on" and "based at least in part on." Similarly, the term "depending on" means both "depending only on" and "depending at least in part on." The terms "include," "comprise," and variations thereof do not mean including only the listed items, but may mean including only the listed items or may include additional items in addition to the listed items. Additionally, the term "or," as used in this disclosure, is not intended to mean an exclusive or. Furthermore, any reference to elements using designations such as "first," "second," etc., as used in this disclosure does not generally limit the quantity or order of those elements. These designations may be used herein as a convenient method of distinguishing between two or more elements. Thus, a reference to a first and a second element does not imply that only two elements may be employed therein or that the first element must precede the second element in some way. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles shall include the plural unless the context clearly indicates otherwise.

[0123] The above describes the embodiments in detail with reference to the drawings, but the specific configuration is not limited to that described above, and various design changes can be made within the scope that does not deviate from the gist of the invention.

[0124] This application claims priority from Japanese Patent Application No. 2024-015183 (filed February 2, 2024), the entire contents of which are incorporated herein by reference.

[0125] (4) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0126] Supplementary Note 1: A communication method executed by a user equipment in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), the communication method comprising: receiving, from the first cell, an RRC (Radio Resource Control) message including information indicating a radio quality condition that should be satisfied in order to perform an LTM cell switch, which switches a serving cell of the user equipment from a first cell to a second cell by the LTM, and information indicating a timing advance value that should be applied to the second cell; evaluating whether the radio quality condition is satisfied for the second cell; and, in response to the evaluation that the radio quality condition is satisfied, performing the LTM cell switch by applying the timing advance value notified in the RRC message.

[0127] Supplementary Note 2: The communication method according to Supplementary Note 1, wherein performing the LTM cell switch includes performing the LTM cell switch by applying the timing advance value notified in the RRC message without performing a random access procedure for the second cell.

[0128] Supplementary Note 3: The communication method according to Supplementary Note 1 or 2, wherein the information indicating the timing advance value is information indicating that the timing advance value to be applied to the second cell is zero, or information indicating that the timing advance value to be applied to the second cell is the same as the timing advance value of the first cell.

[0129] Supplementary Note 4: A user equipment in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a receiver that receives, from the first cell, an RRC (Radio Resource Control) message including information indicating a radio quality condition that should be satisfied in order to perform an LTM cell switch that switches a serving cell of the user equipment from a first cell to a second cell by the LTM, and information indicating a timing advance value that should be applied to the second cell; and a controller that evaluates whether the radio quality condition is satisfied for the second cell, and, in response to evaluation that the radio quality condition is satisfied, performs the LTM cell switch by applying the timing advance value notified in the RRC message.

[0130] Supplementary Note 5: A network node that manages a first cell in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: a transmitter that transmits to the user equipment an RRC (Radio Resource Control) message, the RRC message including information indicating a radio quality condition that should be satisfied to perform an LTM cell switch that switches a serving cell of a user equipment from the first cell to a second cell by the LTM, and information indicating a timing advance value that should be applied to the second cell.

[0131] Supplementary Note 6: A communication method executed by a user equipment in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: receiving, from the first cell, an RRC (Radio Resource Control) message including information indicating a radio quality condition that must be satisfied in order to perform an LTM cell switch, which switches a serving cell of the user equipment from a first cell to a second cell by the LTM, and a user equipment measurement configuration in which the user equipment measures a timing advance value to be applied to the second cell by itself; evaluating whether the radio quality condition is satisfied for the second cell; performing the user equipment measurement based on the configuration in response to evaluating that the radio quality condition is satisfied; and performing the LTM cell switch by applying the timing advance value measured by the user equipment measurement.

[0132] Supplementary Note 7: The communication method according to Supplementary Note 6, wherein performing the user equipment measurements includes starting the user equipment measurements when it is determined that the radio quality condition is satisfied.

[0133] Supplementary Note 8: The communication method according to Supplementary Note 6 or 7, wherein performing the LTM cell switch includes performing the LTM cell switch by applying the timing advance value measured by the user equipment measurements without performing a random access procedure for the second cell.

[0134] Supplementary Note 9: A user equipment in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a receiver that receives, from the first cell, an RRC (Radio Resource Control) message including information indicating a radio quality condition that must be satisfied in order to perform an LTM cell switch that switches a serving cell of the user equipment from a first cell to a second cell by the LTM, and a user equipment measurement configuration that measures a timing advance value to be applied to the second cell by the user equipment itself; and a controller that evaluates whether the radio quality condition is satisfied for the second cell, and, in response to evaluation that the radio quality condition is satisfied, performs the user equipment measurement based on the configuration, and performs the LTM cell switch by applying the timing advance value measured by the user equipment measurement.

[0135] Supplementary Note 10: A network node managing a first cell in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a transmitter configured to transmit to a user equipment an RRC (Radio Resource Control) message including information indicating a radio quality condition to be satisfied in order to perform an LTM cell switch for switching a serving cell of a user equipment from a first cell to a second cell by the LTM, and a user equipment measurement configuration for measuring a timing advance value to be applied to the second cell by the user equipment itself.

[0136] 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 executed by a user equipment in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: receiving, from the first cell, an RRC (Radio Resource Control) message including information indicating a radio quality condition that must be satisfied in order to perform an LTM cell switch that switches the serving cell of the user equipment from a first cell to a second cell using the LTM, and information indicating a timing advance value that should be applied to the second cell; evaluating whether the radio quality condition is satisfied for the second cell; and, in response to the evaluation that the radio quality condition is satisfied, performing the LTM cell switch by applying the timing advance value notified in the RRC message.

2. The communication method according to claim 1, wherein performing the LTM cell switch includes performing the LTM cell switch by applying the timing advance value notified in the RRC message without performing a random access procedure for the second cell.

3. The communication method described in claim 1, wherein the information indicating the timing advance value is information indicating that the timing advance value to be applied to the second cell is zero, or information indicating that the timing advance value to be applied to the second cell is the same as the timing advance value of the first cell.

4. A user equipment of a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: a receiver that receives from the first cell an RRC (Radio Resource Control) message including information indicating a radio quality condition that must be satisfied in order to perform an LTM cell switch that switches the serving cell of the user equipment from a first cell to a second cell using the LTM, and information indicating a timing advance value that should be applied to the second cell; and a controller that evaluates whether the radio quality condition is satisfied for the second cell, and, if it is determined that the radio quality condition is satisfied, performs the LTM cell switch by applying the timing advance value notified in the RRC message.

5. A network node that manages a first cell in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), the network node having a transmitter that transmits to the user equipment an RRC (Radio Resource Control) message that includes information indicating a radio quality condition that must be satisfied to perform an LTM cell switch that switches a serving cell of a user equipment from the first cell to a second cell by the LTM, and information indicating a timing advance value that should be applied to the second cell.

6. A communication method executed by a user equipment in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: receiving, from the first cell, an RRC (Radio Resource Control) message including information indicating radio quality conditions that must be satisfied in order to perform an LTM cell switch, which switches the serving cell of the user equipment from a first cell to a second cell using the LTM, and a user equipment measurement configuration in which the user equipment measures a timing advance value to be applied to the second cell; evaluating whether the radio quality conditions are satisfied for the second cell; performing the user equipment measurement based on the configuration in response to the evaluation that the radio quality conditions are satisfied; and performing the LTM cell switch by applying the timing advance value measured by the user equipment measurement.

7. The communication method according to claim 6, wherein performing the user equipment measurements includes starting the user equipment measurements when the radio quality condition is evaluated as being satisfied.

8. The communication method according to claim 6, wherein performing the LTM cell switch includes performing the LTM cell switch by applying the timing advance value measured in the user equipment measurements without performing a random access procedure for the second cell.

9. A user equipment of a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: a receiver that receives from the first cell an RRC (Radio Resource Control) message including information indicating radio quality conditions that must be satisfied in order to perform an LTM cell switch that switches the serving cell of the user equipment from a first cell to a second cell using the LTM, and a user equipment measurement configuration that causes the user equipment to measure a timing advance value to be applied to the second cell; and a controller that evaluates whether the radio quality conditions are satisfied for the second cell, and, in response to the evaluation that the radio quality conditions are satisfied, performs the user equipment measurement based on the configuration, and performs the LTM cell switch by applying the timing advance value measured by the user equipment measurement.

10. A network node that manages a first cell in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), the network node having a transmitter that transmits to a user equipment an RRC (Radio Resource Control) message that includes information indicating a radio quality condition that must be satisfied to perform an LTM cell switch that switches a serving cell of a user equipment from a first cell to a second cell using the LTM, and a user equipment measurement configuration that causes the user equipment to measure a timing advance value that should be applied to the second cell by itself.

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