Communication method, user equipment, chip set, program, and mobile communication system

Abstract

A communication method executed by user equipment in a mobile communication system supporting L1 / L2 triggered mobility (LTM), the method comprises: receiving, from a first cell, an LTM cell switching configuration for switching a serving cell of the user equipment from a first cell to a second cell by the LTM, and a user equipment measurement configuration for measuring, by the user equipment itself, a timing advance value to be applied to the second cell; starting the user equipment measurement on the basis of the user equipment measurement configuration; receiving a cell switching command instructing execution of the LTM cell switching from the first cell; holding the LTM cell switching when the cell switching command has been received and the user equipment measurement is being executed; and executing the LTM cell switching in response to completion of the user equipment measurement.

Description

COMMUNICATION METHOD, USER EQUIPMENT, CHIPSET, PROGRAM, AND MOBILE COMMUNICATION SYSTEM

[0001] The present disclosure relates to a communication method, a user device, a chipset, a program, and 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 technology of the present disclosure aims to provide a communication method, a user equipment, and a network node that can improve LTM.

[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 method including: receiving, from the first cell, an LTM cell switch configuration for switching a serving cell of the user equipment from a first cell to a second cell using the LTM; and a user equipment measurement configuration for the user equipment itself to measure a timing advance value to be applied to the second cell; starting the user equipment measurement based on the user equipment measurement configuration; receiving from the first cell a cell switch command instructing to execute the LTM cell switch; suspending the LTM cell switch when the cell switch command is received and the user equipment measurement is being executed; and executing the LTM cell switch in response to completion of the user equipment measurement.

[0007] A user equipment according to a second aspect of the present disclosure is a user equipment used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), and includes: a receiver that receives from the first cell an LTM cell switch configuration for switching a serving cell of the user equipment from a first cell to a second cell using the LTM; and a user equipment measurement configuration for the user equipment itself to measure a timing advance value to be applied to the second cell; and a controller that starts the user equipment measurement based on the user equipment measurement configuration, wherein the receiver receives a cell switch command from the first cell instructing to execute the LTM cell switch, and when the controller receives the cell switch command and the user equipment measurement is being executed, suspends the LTM cell switch and executes the LTM cell switch in response to completion of the user equipment measurement.

[0008] A chipset according to a third aspect of the present disclosure includes a circuit for performing the communication method according to the first aspect.

[0009] A program according to a fourth aspect of the present disclosure causes a user device to execute the communication method according to the first aspect.

[0010] A mobile communication system according to a fifth aspect of the present disclosure includes a user equipment according to the second aspect and a network node.

[0011] 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 for explaining an example of UE-based TA measurement according to an embodiment; FIG. 8 is a diagram showing an example of the operation of a UE according to an embodiment; FIG. 9 is a diagram showing an example of the configuration of an RRC Reconfiguration message according to a first operation pattern of an embodiment;

[0012] 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.

[0013] (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.

[0014] 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.

[0015] The UE 100 is a mobile wireless communication device. The UE 100 may be any device that is used by a user. For example, the UE 100 may be 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).

[0016] 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").

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] The PDCP layer performs header compression / decompression, encryption / decryption, and the like.

[0034] 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.

[0035] 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).

[0036] 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.

[0037] 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.

[0038] 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").

[0039] (2) Overview of LTM The mobile communication system 1 according to this embodiment supports LTM (L1 / L2-triggered mobility).

[0040] In a typical handover procedure, a serving cell switch is triggered by signaling in the upper layer L3, specifically, the RRC layer. 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 the handover of the UE 100 based on the Measurement Report message, and instructs the cell switch by transmitting a handover command (specifically, an RRC Reconfiguration message) which is an RRC message from the gNB 200 to the UE 100.

[0041] 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.

[0042] 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.

[0043] Secondly, the UE 100 performs a synchronization process with the candidate cell by early synchronization (Early sync).

[0044] 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.

[0045] Fourth, UE100 switches the serving cell in response to a cell switching command from gNB200 (source cell).

[0046] 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.

[0047] The following principles apply to LTM:

[0048] 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.

[0049] 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.

[0050] The user plane continues without a reset if configured in RRC signaling to avoid additional delays in data recovery.

[0051] - Security is not updated in LTM.

[0052] 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.

[0053] 6 is a diagram showing an example of a cell switching procedure using LTM. In the illustrated example, it is assumed that UE 100 performs serving cell switching from a first cell of gNB 200 to a second cell.

[0054] 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".

[0055] In step S1, UE100 is in an RRC connected state in the cell (first cell, source cell) of gNB200.

[0056] In step S2, UE100 transmits a Measurement Report message, which is an RRC message, to gNB200.

[0057] In step S3, gNB200 decides to use LTM based on the Measurement Report message and starts preparing the candidate cell.

[0058] In step S4, the gNB 200 transmits to the UE 100 an RRC Reconfiguration message including LTM candidate cell configurations (LTM Candidate Configurations) of one or more candidate cells. The candidate cell configurations may include a random access channel (RACH) configuration used for RA preamble transmission to the corresponding candidate cells, such as a contention-free random access (CFRA) configuration. Such a RACH configuration may be referred to as an early UL synchronization configuration (EarlyUlSyncConfig). CFRA is a random access procedure in which a dedicated RACH resource (e.g., a dedicated preamble sequence and / or a dedicated time-frequency resource) is assigned to the UE 100, and no RACH contention occurs between the UEs 100.

[0059] In step S5, UE100 saves the LTM candidate cell setting and sends an RRC Reconfiguration Complete message to gNB200.

[0060] In step S6, the UE 100 may perform synchronization processing with the candidate cell before receiving the cell switching command. Such synchronization processing may be referred to as early synchronization (Early sync). Here, the UE 100 may perform downlink synchronization processing (DL synchronization processing) for the candidate cell, and then perform early timing advance (TA) acquisition in the candidate cell requested by the gNB 200 (source cell). This is performed by a 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. In addition, when early UL synchronization setting (EarlyUlSyncConfig) is configured in UE 100, the PDCCH order may include a cell indicator indicating a corresponding RACH transmission cell, i.e., a candidate cell to which UE 100 should transmit a random access preamble (RA preamble).

[0061] The UE 100 transmits an RA preamble to the designated candidate cell. In order to minimize communication interruption of the source cell due to the 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. Note that the TA value is a value for adjusting the uplink transmission timing of the UE 100.

[0062] 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).

[0063] In step S8, gNB200 decides to switch the serving cell to the target cell (second cell).

[0064] In step S9, the gNB 200 transmits a cell switch command (MAC CE) including a candidate configuration index of the target cell to the UE 100. The cell switch command may include the TA value obtained by early synchronization.

[0065] 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.

[0066] 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 (RACH-based LTM cell switch). 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 (RACH-less LTM cell switch).

[0067] 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.

[0068] (3) UE-based TA Measurement Depending on the configuration by the gNB 200, the UE 100 can initiate an uplink TA acquisition (referred to as "early TA") procedure for one or more candidate cells (second cells) different from the current serving cell (first cell). If the candidate cell has the same TA value as the current serving cell or the TA value = 0, the early TA acquisition procedure is not necessary. The gNB 200 can request the UE 100 to perform early TA acquisition of the candidate cell before cell switching. The early TA acquisition procedure is realized by CFRA triggered by a PDCCH order as described above, or by UE-based TA measurement configured by the RRC.

[0069] In the case of CFRA triggered by a PDCCH order, the gNB 200 to which the candidate cell belongs calculates the TA value and transmits it to the gNB 200 to which the serving cell belongs. When the serving cell triggers an LTM cell switch, it transmits the TA value in a cell switch command (MAC CE).

[0070] In the case of UE-based TA measurement, the UE 100 performs TA measurement of the candidate cell after being configured by the RRC, but the exact time at which the UE 100 performs the TA measurement depends on the implementation of the UE 100. When the UE 100 receives a cell switch command, it applies the TA value it has measured and performs RACH-less LTM.

[0071] Depending on whether a valid TA value is available, the UE 100 performs either a RACH-less LTM cell switch or a RACH-based LTM cell switch. If a TA value is specified in the cell switch command, the UE 100 applies the TA value according to the specification. If UE-based TA measurement is configured but a TA value is not specified in the cell switch command, the UE 100 applies its own measured TA value, if available. If a valid TA value is not available, the UE 100 performs a RACH-based LTM cell switch.

[0072] 7 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.

[0073] The UE-based TA measurement includes, for example, the following procedures.

[0074] 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".

[0075] 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.

[0076] 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 or may be performed simultaneously with STEP 2.

[0077] 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".

[0078] 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.

[0079] (4) Operation According to the present 3GPP technical specifications, in a UE 100 configured by RRC for UE-based TA measurement (user equipment measurement) of a candidate cell (second cell), the exact time to perform the TA measurement is left up to the UE implementation. When the UE 100 configured for UE-based TA measurement receives a cell switch command from a serving cell (first cell), if a TA value is not specified in the received cell switch command, the UE 100 applies its own measured TA value if available, and performs a RACH-based LTM cell switch if a valid TA value is not available.

[0080] Furthermore, according to the current 3GPP technical specifications, when UE 100 receives a cell switch command, it performs an LTM cell switch. However, UE 100 may receive a cell switch command that does not specify a TA value while performing UE-based TA measurement. In this case, UE 100 cannot use a valid TA value because UE-based TA measurement is incomplete, and performs RACH-based LTM cell switch. RACH-based LTM cell switch takes a longer time to complete than RACH-less LTM cell switch. Therefore, it is desirable to perform RACH-less LTM cell switch after UE-based TA measurement is complete.

[0081] Therefore, in this embodiment, when the UE 100 receives a cell switch command that does not specify a TA value while the UE-based TA measurement is being performed, the UE 100 performs the LTM cell switch after the UE-based TA measurement is completed. That is, when the UE 100 receives a cell switch command that does not specify a TA value and the UE-based TA measurement is being performed, the UE 100 suspends the LTM cell switch. This makes it possible to apply the RACH-less LTM cell switch, thereby speeding up the cell switch.

[0082] Furthermore, a UE 100 configured for UE-based TA measurement may not necessarily be able to derive a TA value for a candidate cell after starting the UE-based TA measurement. For example, if an error occurs during the execution of the UE-based TA measurement, the UE 100 may fail the UE-based TA measurement. However, the current 3GPP technical specifications do not take into consideration the failure of the UE-based TA measurement. As described above, if an LTM cell switch is performed after the UE-based TA measurement is completed, there is a concern that the UE-based TA measurement may not be completed and the LTM cell switch may not be performed. Therefore, in this embodiment, the UE 100 determines that the UE-based TA measurement has failed if the TA value cannot be derived within a predetermined time by the UE-based TA measurement. This allows the UE-based TA measurement failure to be appropriately determined.

[0083] In addition, when the UE 100 receives a cell switching command that does not specify a TA value and the UE-based TA measurement fails, the UE 100 performs a RACH-based LTM cell switching. Here, since the UE 100 configured for the UE-based TA measurement is considered not to perform early TA by CFRA triggered by the PDCCH order, it is highly likely that the gNB 200 has not provided the CFRA setting to the UE 100. Therefore, in this embodiment, the UE 100 performs an LTM cell switching with contention-based random access (CBRA) to the second cell in response to the failure of the UE-based TA measurement.

[0084] Fig. 8 is a diagram showing an example of the operation of the UE 100 according to this embodiment. Fig. 9 is a diagram showing a configuration example of an RRC Reconfiguration message (the RRC message of step S4 in Fig. 6) according to the first operation pattern of this embodiment. The following operation is based on the conventional LTM procedure of Fig. 6, but redundant explanations of operations similar to the conventional LTM procedure will be omitted.

[0085] As shown in Figure 8, in step S101, UE100 receives from the first cell (gNB200) the LTM cell switching configuration for switching the serving cell of UE100 from the first cell (current serving cell) to the second cell (candidate cell, target cell) by LTM, and the UE-based TA measurement configuration for measuring the TA value to be applied to the second cell by UE100 itself. UE100 receives the LTM cell switching configuration and the UE-based TA measurement configuration from the first cell in the RRC Reconfiguration message (see step S4 of Figure 6).

[0086] As shown in Fig. 9, the RRC Reconfiguration message includes, as an information element, an LTM setting (LTM config) for configuring LTM cell switching in the UE 100. The LTM setting includes, as an information element, a candidate cell setting list (LTM Config List) for adding the setting of LTM cell switching for each candidate cell (second cell).

[0087] The candidate cell configuration list includes, as entries, candidate cell configurations, which are 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), an RRC configuration to be applied to communication with the corresponding candidate cell (second cell), and a UE-based TA measurement configuration to be applied to the corresponding candidate cell (second cell). The UE-based TA measurement configuration may include a timer value setting for a timer for determining whether the UE-based TA measurement is successful (i.e., a timer that determines an upper limit on the time for performing the UE-based TA measurement). Here, it is assumed that the candidate cell configuration including the UE-based TA measurement configuration does not include a CFRA configuration to be applied to the corresponding candidate cell (second cell).

[0088] 8, in step S102, the UE 100 starts UE-based TA measurement for the second cell based on the UE-based TA measurement configuration. The UE 100 may start the UE-based TA measurement in response to receiving a cell switch command from the first cell that does not include a valid TA value. The UE 100 may perform UE-based TA measurement only for the second cell for which the UE-based TA measurement is configured in the candidate cell configuration, among the multiple second cells.

[0089] In step S103, the UE 100 receives a cell switch command (MAC CE) from the first cell (gNB 200) instructing the execution of an LTM cell switch. The cell switch command specifies a target cell determined by the gNB 200 based on the L1 measurement report. Specifically, the cell switch command includes a candidate configuration index (configuration ID) of the target cell. The cell switch command may include a TA value (i.e., may specify a TA value).

[0090] In step S104, the UE 100 determines whether the received cell switch command specifies a TA value. If the received cell switch command specifies a TA value (step S104: YES), in step S108, the UE 100 applies the TA value specified in the cell switch command to the target cell (second cell) specified in the cell switch command to perform LTM cell switch (specifically, RACH-less LTM cell switch without a random access procedure). When performing LTM cell switch, the UE 100 transmits an RRC Reconfiguration Complete message to the second cell. The UE 100 may consider that the execution of LTM cell switch has been successfully completed when it is determined that the second cell (gNB 200) has successfully received the first uplink data.

[0091] On the other hand, if the received cell switch command does not specify a TA value (step S104: NO), in step S105, UE 100 determines whether UE-based TA measurement has been completed or is currently being performed for the target cell (second cell) specified in the cell switch command. If UE-based TA measurement is currently being performed (step S105: NO), in step S106, UE 100 suspends LTM cell switch and waits for the completion of UE-based TA measurement. Note that if UE-based TA measurement is not configured for the target cell (second cell), UE 100 cannot obtain a valid TA value, and therefore performs RACH-based LTM cell switch involving a random access procedure (step S109). Here, it is assumed that UE-based TA measurement is configured for the target cell (second cell).

[0092] The UE-based TA measurement being completed in step S105 means that the UE-based TA measurement has been completed successfully (i.e., the TA value has been derived by the UE-based TA measurement), or that the UE-based TA measurement has failed. Here, the UE 100 determines that the UE-based TA measurement has failed in response to the UE being unable to derive a TA value by the UE-based TA measurement within a predetermined time.

[0093] For example, the UE-based TA measurement configuration includes a timer value for a timer that determines a predetermined time. The UE 100 starts a timer to which the timer value is set, and if the TA value cannot be derived by the UE-based TA measurement before the timer expires, the UE 100 determines that the UE-based TA measurement has failed.

[0094] Specifically, the UE 100 starts a timer at the timing of receiving the above-mentioned RRC Reconfiguration message, the timing of transmitting an RRC Reconfiguration Complete message in response to the RRC Reconfiguration message, the timing of starting UE-based TA measurement, or the timing of receiving a cell switching command. If the UE 100 succeeds in measuring the TA value of the second cell while the timer is running, the UE 100 stops the timer. If the UE 100 does not succeed in measuring the TA value of the second cell and the timer expires, the UE 100 determines that the UE-based TA measurement has failed.

[0095] When the UE-based TA measurement is completed (step S105: YES), in step S107, the UE 100 determines whether a valid TA value is available. For example, when the UE-based TA measurement is successfully completed, the UE 100 determines that a valid TA value is available. On the other hand, when the UE-based TA measurement fails, the UE 100 determines that a valid TA value is unavailable.

[0096] If a valid TA value is available (step S107: YES), in step S108, the UE 100 applies the TA value derived by the UE-based TA measurement to the target cell (second cell) specified in the cell switch command to perform LTM cell switch (specifically, RACH-less LTM cell switch without a random access procedure). In this way, the UE 100 performs LTM cell switch without a random access procedure to the second cell in response to successful completion of the UE-based TA measurement.

[0097] On the other hand, if a valid TA value is unavailable (step S107: NO), in step S109, the UE 100 performs a RACH (Random Access Channel)-based LTM cell switch with a random access procedure for the target cell (second cell) specified in the cell switch command. In this way, the UE 100 performs an LTM cell switch with a random access procedure for the second cell in response to a failure of the UE-based TA measurement. Here, the random access procedure for the second cell may be a contention-based random access (CBRA) random access procedure. When a special PRACH (Physical Random Access Channel) resource (PRACH partition) for TA acquisition during LTM execution is configured in an RRC Reconfiguration message (LTM Config) or a system information block (SIB), the UE 100 selects the PRACH resource and transmits a PRACH (i.e., an RA preamble).

[0098] In response to the failure of the UE-based TA measurement, the UE 100 may transmit a notification indicating that the UE-based TA measurement has failed to the first cell (gNB 200). This allows the gNB 200 to know that the UE-based TA measurement has failed. The notification may be transmitted by being included in a MAC CE, an RRC message, an Uplink Control Information (UCI), a Scheduling Request (SR), or an L1 measurement report. The notification may include the cell ID (or configuration ID) of the second cell for which the UE-based TA measurement has failed. This allows the gNB 200 to know for which second cell the UE-based TA measurement has failed.

[0099] In step S107, the UE 100 performs the LTM cell switch by applying the TA value derived by the UE-based TA measurement, and even if the LTM cell switch fails (for example, if the UE 100 determines that the transmission of the RRC Reconfiguration Complete message to the second cell has failed, or if the second cell has failed to receive the message), the UE 100 may determine that the UE-based TA measurement has failed. In this case, the UE 100 may execute the process of step S109.

[0100] In the above-described operation example, the UE 100 transmits a notification indicating that the UE-based TA measurement has failed to the first cell (gNB 200). However, the UE 100 may transmit a notification indicating that the UE-based TA measurement has failed to a cell other than the first cell (e.g., a cell to which the UE 100 is connected and which has requested the transmission of the notification). The UE 100 may transmit the notification in an RRC message (e.g., a UE Information Response message). The UE 100 in which the UE-based TA measurement has failed may retain information indicating that the UE-based TA measurement has failed, and may transmit a notification including the information at a later point in time (e.g., when the cell requests the transmission of the notification). The information may include at least one of the following: that a RACH-based LTM cell switch has been performed; that a LTM cell switch involving CBRA has been performed; and the cause thereof, e.g., UE-based TA measurement failure, no valid TA value was available when the LTM cell switch was performed, or the first transmission (RRC Reconfiguration Complete) to the target cell applying the TA value could not be sent (or delivery confirmation could not be obtained).

[0101] (5) 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.

[0102] 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.

[0103] 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.

[0104] A program that causes a computer to execute each process performed by the UE 100 or the gNB 200 may be provided. 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 and / 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).

[0105] The functions performed by UE100 or gNB200 may be implemented in circuitry or processing circuitry, 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 perform the described functions. A processor includes transistors and / or 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.

[0106] 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.

[0107] 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.

[0108] This application claims priority from Japanese Patent Application No. 2024-017704 (filed February 8, 2024), the entire contents of which are incorporated herein by reference.

[0109] (6) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0110] Supplementary Note 1: 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 LTM cell switch configuration for switching a serving cell of the user equipment from a first cell to a second cell by the LTM, and a user equipment measurement configuration for the user equipment to measure, by itself, a timing advance value to be applied to the second cell; starting the user equipment measurement based on the user equipment measurement configuration; receiving, from the first cell, a cell switch command instructing to execute the LTM cell switch; suspending the LTM cell switch when the cell switch command is received and the user equipment measurement is being executed; and executing the LTM cell switch in response to completion of the user equipment measurement.

[0111] Supplementary Note 2: The end of the user equipment measurement is a successful completion of the user equipment measurement, and the user equipment performs the LTM cell switch without a random access procedure to the second cell in response to the successful completion of the user equipment measurement. The communication method according to Supplementary Note 1.

[0112] Supplementary Note 3: The end of the user equipment measurement is a failure of the user equipment measurement, and the user equipment performs the LTM cell switch involving a random access procedure to the second cell in response to the failure of the user equipment measurement. The communication method according to Supplementary Note 1.

[0113] Supplementary Note 4: The communication method according to Supplementary Note 3, wherein the user equipment determines that the user equipment measurements have failed in response to the user equipment measurements not being able to derive the timing advance value within a predetermined time.

[0114] Supplementary Note 5: The communication method according to Supplementary Note 4, wherein the user equipment measurement configuration includes a timer value of a timer that determines the predetermined time, the user equipment starts the timer to which the timer value is set, and if the timing advance value cannot be derived by the user equipment measurement before the timer expires, determines that the user equipment measurement has failed.

[0115] Supplementary Note 6: The communication method according to any one of Supplementary Notes 3 to 5, wherein the user equipment performs the LTM cell switch involving contention-based random access (CBRA) to the second cell in response to the user equipment measurement failure.

[0116] Supplementary Note 7: The communication method according to any one of Supplementary Notes 3 to 5, wherein the user equipment transmits a notification indicating that the user equipment measurement has failed to the first cell in response to the user equipment measurement having failed.

[0117] Supplementary Note 8: A user equipment used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a receiver that receives, from the first cell, an LTM cell switch configuration for switching a serving cell of the user equipment from a first cell to a second cell by the LTM, and a user equipment measurement configuration for the user equipment itself to measure a timing advance value to be applied to the second cell; and a controller that starts the user equipment measurement based on the user equipment measurement configuration, wherein the receiver receives a cell switch command from the first cell instructing to execute the LTM cell switch, and the controller suspends the LTM cell switch when the cell switch command is received and the user equipment measurement is being executed, and executes the LTM cell switch in response to completion of the user equipment measurement.

[0118] Supplementary Note 9: A chipset comprising a circuit for executing the communication method according to Supplementary Note 1.

[0119] Supplementary Note 10: A program that causes a user device to execute the communication method described in Supplementary Note 1.

[0120] Supplementary Note 11: A mobile communication system comprising a user equipment according to Supplementary Note 8 and a network node.

[0121] 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 supporting LTM (L1 / L2 Triggered Mobility), comprising: receiving from the first cell an LTM cell switch configuration for switching the serving cell of the user equipment from a first cell to a second cell using the LTM, and a user equipment measurement configuration for the user equipment to measure a timing advance value to be applied to the second cell by itself; starting the user equipment measurement based on the user equipment measurement configuration; receiving from the first cell a cell switch command instructing the execution of the LTM cell switch; suspending the LTM cell switch when the cell switch command is received and the user equipment measurement is being executed; and executing the LTM cell switch in response to the end of the user equipment measurement.

2. The communication method according to claim 1, wherein the end of the user equipment measurement is a successful completion of the user equipment measurement, and the user equipment performs the LTM cell switch without a random access procedure to the second cell in response to the successful completion of the user equipment measurement.

3. The communication method according to claim 1, wherein the end of the user equipment measurement is a failure of the user equipment measurement, and the user equipment performs the LTM cell switch involving a random access procedure to the second cell in response to the failure of the user equipment measurement.

4. The communication method according to claim 3, wherein the user equipment determines that the user equipment measurements have failed in response to the user equipment measurements not being able to derive the timing advance value within a predetermined time.

5. The communication method according to claim 4, wherein the user equipment measurement settings include a timer value of a timer that determines the predetermined time, the user equipment starts the timer to which the timer value is set, and if the timing advance value cannot be derived by the user equipment measurement before the timer expires, determines that the user equipment measurement has failed.

6. The communication method according to claim 3, wherein the user equipment performs the LTM cell switch involving contention-based random access (CBRA) to the second cell in response to the user equipment measurement failure.

7. The communication method according to any one of claims 3 to 5, wherein the user equipment transmits a notification indicating that the user equipment measurement has failed to the first cell in response to the user equipment measurement having failed.

8. A user equipment used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a receiver that receives from the first cell an LTM cell switch configuration for switching the serving cell of the user equipment from a first cell to a second cell using the LTM, and a user equipment measurement configuration for the user equipment itself to measure a timing advance value to be applied to the second cell; and a controller that starts the user equipment measurement based on the user equipment measurement configuration, wherein the receiver receives a cell switch command from the first cell instructing execution of the LTM cell switch, and the controller suspends the LTM cell switch when the cell switch command is received and the user equipment measurement is being executed, and executes the LTM cell switch in response to completion of the user equipment measurement.

9. A chipset comprising a circuit for performing the communication method of claim 1.

10. A program that causes a user device to execute the communication method according to claim 1.

11. A mobile communication system comprising a user equipment according to claim 8 and a network node.

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