Communication method and user device

Abstract

Provided is a communication method executed by a user device that switches a serving cell from a first cell to a second cell via L1 / L2 triggered mobility (LTM), said communication method comprising: executing early timing advance (TA) acquisition with respect to the second cell in accordance with an LTM-related instruction from the first cell; determining whether or not a prescribed time has elapsed since a timing relating to execution of the early TA acquisition; and executing the early TA acquisition again in response to a determination that the prescribed time has elapsed since the timing.

Description

Communication method and user device

[0001] The present disclosure relates to a communication method 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 provides a technique that can reduce the occurrence of LTM cell switching failures caused by inappropriate timing advance (TA) values.

[0006] A communication method according to a first aspect is a communication method executed by a user equipment that switches a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the communication method including: performing early timing advance (TA) acquisition for the second cell in response to an LTM-related instruction from the first cell; determining whether a predetermined time has elapsed since a timing related to the execution of the early TA acquisition; and performing the early TA acquisition again in response to determining that the predetermined time has elapsed since the timing.

[0007] A user equipment according to a second aspect is a user equipment that switches a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), and includes a control unit that performs early timing advance (TA) acquisition for the second cell in response to an LTM-related instruction from the first cell, determines whether a predetermined time has elapsed since a timing related to the execution of the early TA acquisition, and, in response to determining that the predetermined time has elapsed since the timing, executes the early TA acquisition again.

[0008] A communication method according to a third aspect is a communication method executed by a user equipment for switching a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the method comprising: receiving LTM candidate cell configurations of a plurality of second cells from the first cell; performing early Timing Advance (TA) acquisition for each of the plurality of second cells; performing an LTM cell switch for one of the plurality of second cells while retaining the LTM candidate cell configuration and a TA value obtained using the early TA acquisition; and, after performing the LTM cell switch, performing a subsequent LTM cell switch from the one cell to another second cell using the retained LTM candidate cell configuration and the retained TA value.

[0009] A user equipment according to a fourth aspect is a user equipment that switches a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), and includes a controller that receives LTM candidate cell configurations of a plurality of second cells from the first cell, performs early timing advance (TA) acquisition for each of the plurality of second cells, performs LTM cell switch for one of the plurality of second cells, retains the LTM candidate cell configuration and a TA value obtained using the early TA acquisition, and, after performing the LTM cell switch, performs a subsequent LTM cell switch from the one cell to another second cell using the retained LTM candidate cell configuration and the retained TA value.

[0010] 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 for intra-CU. FIG. 7 is a diagram for explaining an example of UE-based TA measurement according to an embodiment. FIG. 8 is a diagram for explaining an example of an operating environment of a mobile communication system according to an embodiment. FIG. 9 is a diagram showing an overview of the operation of a UE according to an embodiment. FIG. 10 is a diagram for explaining a specific example of the operation of a UE 100 according to an embodiment. FIG. 11 is a diagram for explaining a specific example of period designation information and operation designation information according to an embodiment. FIG. 12 is a diagram showing the operation of a UE according to another embodiment. FIG. 13 is a diagram showing the signaling procedure of LTM. FIG. 14 is a diagram showing LTM between gNB and DU.

[0011] 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.

[0012] (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.

[0013] 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.

[0014] The UE 100 is a mobile wireless communication device. The UE 100 may be any device 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 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). A link in the transmission direction from the UE 100 to the network 5 is referred to as an uplink (UL), and a link in the transmission direction from the network 5 to the UE 100 is referred to as a downlink (DL).

[0015] 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").

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] The PDCP layer performs header compression / decompression, encryption / decryption, and the like.

[0033] 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.

[0034] 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).

[0035] 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.

[0036] 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.

[0037] 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").

[0038] (2) Overview of LTM The mobile communication system 1 according to this embodiment supports LTM (L1 / L2-triggered mobility).

[0039] 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.

[0040] 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.

[0041] 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.

[0042] Secondly, the UE 100 performs a synchronization process with the LTM candidate cell by early synchronization (Early sync).

[0043] 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 via a MAC control element (CE). The serving cell switch trigger is conveyed in a MAC CE including at least a candidate setting index together with a beam indicator.

[0044] Fourth, the UE 100 switches the serving cell in response to a cell switch command MAC CE from the gNB 200 (source cell).

[0045] 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.

[0046] The following principles apply to LTM:

[0047] Each LTM candidate cell configuration can be provided as a differential configuration (delta configuration) relative to a reference configuration used to form the complete LTM candidate cell configuration.

[0048] If a full LTM candidate cell configuration is applied, the current UE configuration is replaced upon a serving cell switch. The reconfiguration procedure does this but does not necessarily reset the MAC, RLC or PDCP layers.

[0049] The user plane continues without a reset if configured in RRC signaling to avoid additional delays in data recovery.

[0050] - Security is not updated in LTM.

[0051] LTM between subsequent LTM candidate cell configurations (subsequent LTM cell switching) can be performed without RRC reconfiguration, i.e., the UE 100 does not release other LTM candidate cell configurations after LTM is triggered.

[0052] 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.

[0053] 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 an "LTM 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".

[0054] In step S1, UE100 is in an RRC connected state in the cell (first cell, source cell) of gNB200.

[0055] In step S2, UE100 transmits a measurement report (L3 Measurement Report) message, which is an RRC message, to gNB200.

[0056] In step S3, gNB200 decides to use LTM based on the Measurement Report message and starts preparing an LTM candidate cell.

[0057] In step S4, the gNB 200 transmits to the UE 100 an RRC Reconfiguration message including an LTM candidate cell configuration (LTM Candidate Configuration) of one or more LTM candidate cells. The LTM candidate cell configuration may include a random access channel (RACH) configuration used for RA preamble transmission to the corresponding LTM candidate cell, for example, 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.

[0058] In step S5, UE100 saves the LTM candidate cell setting and sends an RRC Reconfiguration Complete message to gNB200.

[0059] In step S6, the UE 100 may perform synchronization processing with the LTM candidate cell before receiving the cell switch command MAC CE. 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 LTM candidate cell, and then perform early timing advance (TA) acquisition in the LTM candidate cell requested by the gNB 200 (source cell). This is performed by CFRA triggered by a PDCCH order (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 UE100, the PDCCH order may include a cell indicator indicating the corresponding RACH transmission cell, i.e., to which LTM candidate cell UE100 should transmit a random access preamble (RA preamble).

[0060] The UE 100 transmits an RA preamble to the designated LTM candidate cell. In order to minimize communication interruption of the source cell due to CFRA for the LTM 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 LTM candidate cell. The TA value of the LTM candidate cell (target cell) is indicated in the cell switching command MAC CE in step S9. Note that the TA value is a value for adjusting the uplink transmission timing of the UE 100 taking into account propagation delay.

[0061] In step S7, the UE 100 performs layer 1 (L1) measurement in the configured LTM 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).

[0062] In step S8, gNB200 decides to switch the serving cell to the target cell (second cell).

[0063] 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 MAC CE may include a TA value obtained by early synchronization.

[0064] 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.

[0065] In step S11, if the serving cell switch needs to include execution of a random access procedure (for example, if the cell switch command MAC CE 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 serving cell switch (for example, if the cell switch command MAC CE includes a valid TA value), it can skip the random access procedure (RACH-less LTM cell switch).

[0066] 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.

[0067] (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" or "early TA acquisition") procedure for one or more LTM candidate cells (second cells) different from the current serving cell (first cell). If the LTM 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 LTM candidate cell before the LTM cell switch. The early TA acquisition procedure is realized by a CFRA triggered by a PDCCH order as described above, or by a UE-based TA measurement configured by the RRC.

[0068] In the case of CFRA triggered by a PDCCH order, the gNB 200 to which the LTM candidate cell belongs calculates the TA value and transmits it to the gNB 200 to which the serving cell belongs. When triggering an LTM cell switch, the serving cell notifies the UE 100 of the TA value in the cell switch command MAC CE.

[0069] In the case of UE-based TA measurement, the UE 100 performs TA measurement of the LTM 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 the cell switch command MAC CE, it applies the TA value it has measured and performs RACH-less LTM.

[0070] 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 MAC CE, the UE 100 applies the TA value according to the specification. If UE-based TA measurement is configured but no TA value is specified in the cell switch command MAC CE, 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.

[0071] 7 is a diagram illustrating an example of UE-based TA measurement according to an 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.

[0072] The UE-based TA measurement includes, for example, the following procedures.

[0073] 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".

[0074] 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.

[0075] 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.

[0076] 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".

[0077] 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.

[0078] (4) Inter-CU LTM The LTM specified in Release 18 of the 3GPP standard is an intra-CU (Central Unit) LTM and does not support inter-CU LTM. In other words, in conventional LTM, LTM cell switching is possible between cells under the same CU (same gNB 200), but LTM cell switching cannot be performed between cells under different CUs (different gNB 200). On the other hand, in Release 19 of the 3GPP standard, support for inter-CU LTM is being considered.

[0079] FIG. 8 is a diagram for explaining an example of an operating environment of the mobile communication system 1 according to the embodiment.

[0080] The UE100 is in an RRC connected state in the cell of the gNB200S. The gNB200S is an example of a first network node. The cell of the gNB200S is an example of a first cell. The cell of the gNB200S is also referred to as a source cell or a current serving cell. The gNB200S is also referred to as a source gNB200S or a serving gNB200S.

[0081] The gNB200C manages the cells adjacent to the cell of gNB200S. In the illustrated example, the cells adjacent to the cell of gNB200S are managed by gNB200C1 and gNB200C2. gNB200C1 is an example of a second network node, and gNB200C2 is an example of a third network node. The cell of gNB200C1 is also referred to as LTM candidate cell #1, and the cell of gNB200C2 is also referred to as LTM candidate cell #2. Each of LTM candidate cell #1 and LTM candidate cell #2 is an example of a second cell.

[0082] When gNB200C1 and gNB200C2 are not particularly distinguished, they are referred to as candidate gNB200C, and when LTM candidate cell #1 and LTM candidate cell #2 are not distinguished, they are simply referred to as LTM candidate cells. However, after a cell switch by LTM is determined, the determined LTM candidate cell is also referred to as a target cell.

[0083] In the illustrated example, there are two candidate gNB200C and two LTM candidate cells, but there may be one candidate gNB200C or three or more. There may also be one LTM candidate cell or three or more. Furthermore, one candidate gNB200C may manage multiple LTM candidate cells.

[0084] 8 are connected to each other by an Xn interface, which is an interface between network nodes (or, from another perspective, an interface between CUs). Communication between the gNBs 200 is assumed to be performed over the Xn interface.

[0085] Each candidate gNB200C provides RRC Reconfiguration (with sync) including its own LTM candidate cell configuration (LTM candidate cell configuration) to UE100 via gNB200S. In the illustrated example, gNB200C1 provides RRC Reconfiguration (with sync) including LTM candidate cell configuration of LTM candidate cell #1 to UE100 via gNB200S. Also, gNB200C2 provides RRC Reconfiguration (with sync) including LTM candidate cell configuration of LTM candidate cell #2 to UE100 via gNB200S. The LTM candidate cell configuration may include early UL synchronization configuration (CFRA configuration), as described above. The LTM candidate cell configuration may include configuration of a PUSCH resource to be used when performing the first UL transmission (for example, transmission of an RRC Reconfiguration Complete message to the target cell) with the corresponding LTM candidate cell as the target cell.

[0086] After storing the LTM candidate cell setting of each LTM candidate cell, the UE 100 performs early UL synchronization with one or more LTM candidate cells, and performs LTM cell switching with the LTM candidate cell specified in the cell switching command MAC CE from the source cell as the target cell. However, if the TA value of the LTM candidate cell is the same as the TA value of the source cell (TA = same as source), or if the TA value of the LTM candidate cell is zero (TA = 0), the UE 100 may not perform early UL synchronization with the LTM candidate cell.

[0087] (5) Operations Related to Early TA Acquisition In the LTM procedure, the UE 100 performs early TA acquisition for an LTM candidate cell, except when the LTM candidate cell has the same TA value as the current serving cell (source cell) or the TA value = 0. Early TA acquisition is achieved by CFRA triggered by a PDCCH order or UE-based TA measurement configured (instructed) by the RRC, as described above.

[0088] The TA value obtained using early TA acquisition is valid as long as the location of UE 100 does not change. However, when UE 100 moves, the distance (i.e., propagation delay) between UE 100 and the LTM candidate cell changes, and the TA value may become inappropriate. If UE 100 performs LTM cell switching using such an inappropriate TA value, there is a concern that the LTM candidate cell (target cell) may not be able to properly receive UL transmission from UE 100, resulting in failure of LTM cell switching.

[0089] Therefore, in the embodiment, a validity period is set for the TA value obtained using the early TA acquisition, thereby preventing the LTM cell switching from being performed using a TA value that has expired, thereby suppressing the occurrence of LTM cell switching failures caused by an inappropriate TA value.

[0090] 9 is a diagram showing an overview of the operation of the UE 100 according to the embodiment. The operation may be applied to the LTM of an intra-CU. The operation may be applied to the LTM of an inter-CU.

[0091] In step S11, UE100 performs early TA acquisition for the LTM candidate cell in response to an LTM-related instruction from the source cell (gNB200).

[0092] The LTM-related indication may be a PDCCH order that triggers a CFRA, in which case early TA acquisition is achieved by transmitting a CFRA RA preamble to the LTM candidate cell.

[0093] Alternatively, the LTM-related instruction may be an RRC message (RRC Reconfiguration message) that configures (instructs) UE-based TA measurement. In this case, early TA acquisition is achieved by UE-based TA measurement for LTM candidate cells.

[0094] In step S12, the UE 100 determines whether a predetermined time (i.e., a validity period of the TA value) has elapsed since the timing of performing the early TA acquisition. The timing of performing the early TA acquisition may be the transmission timing of transmitting the RA preamble of the CFRA to the LTM candidate cell. Alternatively, the timing of performing the early TA acquisition may be the acquisition timing of acquiring the TA value by UE-based TA measurement for the LTM candidate cell.

[0095] In step S13, the UE 100 performs early TA acquisition again in response to determining that a predetermined time (validity period) has elapsed. By performing early TA acquisition again, the TA value is updated. Thereafter, the UE 100 applies the TA value obtained using early TA acquisition to perform LTM cell switching to the LTM candidate cell.

[0096] According to this operation, when the validity period of the TA value obtained using early TA acquisition expires, the TA value is updated by the UE 100 performing early TA acquisition again, so that it is possible to suppress the occurrence of LTM cell switching failures due to inappropriate TA values.

[0097] In addition, as a conventional technology, there is a mechanism using a timer called TAT (Timing Alignment Timer). Specifically, when the UE 100 receives a Timing Advance Command (MAC CE) from the gNB 200, the UE 100 starts the TAT, and when the TAT expires, all UL transmissions other than the PRACH (Physical Random Access Channel) are suspended. Such an operation differs from the operation of the UE 100 according to the embodiment. Furthermore, the TAT indicates the validity period of the TA value for the current serving cell, and does not indicate the validity period of the TA value for the LTM candidate cell, which is a non-serving cell, and is not a means for solving the above-mentioned problem.

[0098] In an embodiment, the UE 100 may receive designation information specifying the validity period from the gNB 200 that manages the source cell. In step S12, the UE 100 may determine whether the validity period specified by the designation information has elapsed. This allows the network 5 (gNB 200) to control the validity period, making it possible to use a more appropriate validity period.

[0099] When the source cell and the LTM candidate cell are managed by different gNB200 (i.e., inter-CU LTM), UE100 may receive designation information indicating the validity period determined by the gNB200 (source gNB200S) that manages the source cell. Since the source gNB200S can easily grasp the status of UE100, such as the movement speed, the source gNB200S can determine the validity period and set a validity period appropriate for the status of UE100.

[0100] Alternatively, when the source cell and the LTM candidate cell are managed by different gNB200 (i.e., inter-CU LTM), UE100 may receive designation information indicating the validity period determined by the gNB200 (candidate gNB200C) that manages the LTM candidate cell. For example, when the candidate gNB200C is a mobile network node (e.g., a Mobile IAB (Integrated Access and Backhaul) node), the candidate gNB200C can determine the validity period taking into account the status of the candidate gNB200C, such as its movement speed, thereby setting an appropriate validity period. In this case, the candidate gNB200C transmits information indicating the determined validity period to the source gNB200S, and the source gNB200S transmits the information to UE100. For example, the candidate gNB200C may store information indicating the validity period in an RRC container (RRC Reconfiguration message) to be included in a Handover Request Acknowledge message or other Xn message. The RRC container (RRC Reconfiguration message) transparently passes through the source gNB200S and is transmitted to the UE100. Alternatively, the candidate gNB200C may store an information element other than the transparent container in the Handover Request Acknowledge message or other Xn message as information indicating the validity period. In this case, the source gNB200S can easily grasp the information indicating the validity period, and can, for example, include the information indicating the validity period in a PDCCH order and transmit it to the UE100.

[0101] In the embodiment, in step S13, the UE 100 may perform UE-based TA measurement in response to determining that the validity period has elapsed. Thereby, when the validity period of the TA value has elapsed, the UE 100 can update the TA value by the UE-based TA measurement.

[0102] Alternatively, in step S13, the UE 100 may request the source cell (gNB 200) to transmit a PDCCH order in response to determining that the validity period has elapsed. The request may be made by a MAC CE, an RRC message, or a PUCCH (UCI: Uplink Control Information). Then, the UE 100 may transmit an RA preamble of a CFRA to the LTM candidate cell in response to receiving the PDCCH order from the source cell (gNB 200). As a result, the UE 100 can update the TA value by CFRA when the validity period of the TA value has elapsed. Note that the TA value is notified to the UE 100 in the cell switch command MAC CE, as described above.

[0103] FIG. 10 is a diagram showing a specific example of the operation of the UE 100 according to the embodiment, based on the above-described outline of the operation.

[0104] In step S101, the UE 100 receives designation information (hereinafter referred to as "period designation information") that designates the validity period of the TA value from the source cell (gNB 200). The designation information may be a timer value (timer setting value) that indicates the validity period of the TA value. The designation information may be the value of n when the validity period is specified as n times the unit time (e.g., frame, subframe, or slot). In the case of inter-CU LTM, the period designation information may be determined by the source gNB 200S. The period designation information may also be determined by the candidate gNB 200C.

[0105] Furthermore, UE100 may further receive from the source cell (gNB200) designation information (hereinafter referred to as "operation designation information") that designates the operation of UE100 when the validity period elapses. The operation designated by the operation designation information may be an operation in which UE100 transmits a CFRA request (i.e., a transmission request of a PDCCH order for UE100) to the source cell. The operation designated by the operation designation information may be an operation in which UE100 performs UE-based TA measurement. In the case of inter-CU LTM, the operation designation information may be determined by the source gNB200S. The operation designation information may be determined by the candidate gNB200C.

[0106] FIG. 11 is a diagram for explaining specific examples of period designation information and action designation information according to the embodiment.

[0107] In the illustrated example, the RRC Reconfiguration message sent by the source cell (gNB 200) to the UE 100 includes an LTM candidate cell configuration list (ltm-CandidateToAddModList). The LTM candidate cell configuration list (ltm-CandidateToAddModList) includes one or more LTM candidate cell configurations (LTM-Candidate) as entries. Each LTM candidate cell configuration (LTM-Candidate) may include a configuration ID (LTM-CandidateId), a cell ID (PhysCellId), an RRC configuration (RRCReconfiguration), an early UL synchronization configuration (EarlyUL-SyncConfig), an SSB configuration (ltm-SSB-Config), a CSI-RS resource configuration (NZP-CSI-RS-Resource), a TCI state configuration (TCI-State), a UE-based TA measurement configuration (ltm-UE-MeasuredTA), period specification information, and operation specification information, etc. At least one of the parameters other than the RRC configuration (RRCReconfiguration) may be included in the RRC configuration (RRCReconfiguration).

[0108] According to such a message configuration example, since the period designation information can be set individually (independently) for each LTM candidate cell, the validity period of the TA value can be optimized for each LTM candidate cell. Also, since the operation designation information can be set individually (independently) for each LTM candidate cell, the operation of UE 100 when the validity period has elapsed can be optimized for each LTM candidate cell.

[0109] The period designation information and / or operation designation information may be included in a PDCCH order that triggers a CFRA. That is, the source cell (gNB200) may transmit a PDCCH order including the period designation information and / or operation designation information to the UE100. The PDCCH order includes information indicating the cell to which the RA preamble of the CFRA is to be transmitted. Therefore, the validity period of the TA value (and / or the operation of the UE100 when the validity period has elapsed) can be optimized for each LTM candidate cell.

[0110] However, assuming that the amount of information that can be stored in the DCI that constitutes the PDCCH order is small, the correspondence between the validity period and the index value may be transmitted from the source cell (gNB200) to the UE100 in an RRC Reconfiguration message, and then the PDCCH order (DCI) including the index value may be transmitted from the source cell (gNB200) to the UE100.

[0111] 10 , in step S102, UE 100 performs early TA acquisition. When UE 100 receives a PDCCH order that triggers CFRA for an LTM candidate cell from the source cell (gNB 200), UE 100 performs RA preamble transmission of CFRA for the LTM candidate cell as early TA acquisition. When UE-based TA measurement for an LTM candidate cell is configured by UE-based TA measurement setting (ltm-UE-MeasuredTA) in the RRC Reconfiguration message, UE 100 performs UE-based TA measurement for the LTM candidate cell as early TA acquisition.

[0112] Here, the UE 100 starts a timer corresponding to the validity period of the TA value. In the case of CFRA, the UE 100 may start the timer when it receives a PDCCH order or when it transmits an RA preamble to the LTM candidate cell. In the case of UE-based TA measurement, the UE 100 may start the timer when it starts the UE-based TA measurement or when it completes the UE-based TA measurement (when it acquires the TA value).

[0113] In step S103, the UE 100 determines whether a trigger condition for LTM cell switching to the LTM candidate cell is satisfied. The UE 100 may determine that the trigger condition for LTM cell switching is satisfied when receiving a cell switch command MAC CE specifying the LTM candidate cell from the source cell (gNB 200). As described above, the cell switch command MAC CE may include a TA value (specifically, a TA value derived by CFRA) to be applied to the LTM candidate cell. Alternatively, in the case of conditional LTM, a radio quality condition for executing LTM cell switching is set in advance by the gNB 200, and when the radio quality condition is satisfied, it may be determined that the trigger condition for LTM cell switching is satisfied.

[0114] When it is determined that the trigger condition for the LTM cell switching is satisfied (step S103: YES), in step S104, the UE 100 performs the LTM cell switching with the LTM candidate cell for which the trigger condition is satisfied as the target cell. Specifically, the UE 100 completes the LTM cell switching by transmitting an RRC Reconfiguration Complete message to the LTM candidate cell (target cell) while applying the TA value for the LTM candidate cell (target cell).

[0115] On the other hand, if it is determined that the trigger condition for LTM cell switching is not satisfied (step S103: NO), in step S105, the UE 100 determines whether or not the timer started in step S102 has expired (i.e., whether or not the validity period of the TA value has elapsed). If the timer has not expired (step S105: NO), the UE 100 returns the process to step S103.

[0116] If the timer has expired (step S105: YES), in step S106, the UE 100 performs early TA acquisition again. This updates the TA value. Thereafter, the UE 100 returns to the process of step S103.

[0117] The operation of early TA acquisition in step S106 (CFRA or UE-based TA measurement) may be the operation specified by the operation specification information.

[0118] If no operation is specified by the operation specification information, in step S106, UE 100 may perform the same operation as the early TA acquisition operation performed in step S102. For example, if UE 100 performed CFRA in step S102, UE 100 may also perform CFRA in step S106. If UE 100 performed UE-based TA measurement in step S102, UE 100 may also perform UE-based TA measurement in step S106.

[0119] Alternatively, in step S106, the UE 100 may perform an operation different from the early TA acquisition operation performed in step S102. For example, if the UE 100 performed CFRA in step S102, the UE 100 may perform UE-based TA measurement in step S106. If the UE 100 performed UE-based TA measurement in step S102, the UE 100 may perform CFRA in step S106.

[0120] When CFRA is performed in step S106, UE100 may request the source cell (gNB200) to transmit a PDCCH order. Then, UE100 may transmit a CFRA RA preamble to the LTM candidate cell in response to receiving the PDCCH order from the source cell (gNB200).

[0121] (6) Other embodiments The LTM defined in Release 18 of the 3GPP standard supports subsequent LTM (Subsequent LTM) cell switching. In subsequent LTM cell switching, the UE 100 receives an LTM candidate cell configuration (LTM candidate cell configuration list) of multiple LTM candidate cells from a source cell by RRC signaling, and then performs LTM cell switching from the source cell to one LTM candidate cell. The UE 100 reuses the LTM candidate cell configuration without releasing it, and performs LTM cell switching to another LTM candidate cell. Therefore, according to subsequent LTM cell switching, even when the UE 100 performs multiple LTM cell switches, only one RRC signaling (RRC configuration) is required, which enables signaling reduction and faster operation.

[0122] Here, it is assumed that in the operating environment of Fig. 8 , UE 100 receives the LTM candidate cell settings of LTM candidate cell #1 and LTM candidate cell #2 from the source cell, performs early TA acquisition for each LTM candidate cell, and then receives a cell switch command MAC CE that specifies LTM candidate cell #1, and performs LTM cell switch from the source cell to LTM candidate cell #1. Here, it is assumed that UE 100 performs UE-based TA measurement as early TA acquisition for LTM candidate cell #2, and has already acquired the TA value of LTM candidate cell #2.

[0123] UE 100 retains the LTM candidate cell setting even after performing LTM cell switching. Therefore, after performing LTM cell switching to LTM candidate cell #1, UE 100 can perform subsequent LTM cell switching from LTM candidate cell #1 to LTM candidate cell #2 using the retained LTM candidate cell setting by receiving a cell switch command MAC CE specifying LTM candidate cell #2 from LTM candidate cell #1. However, if the cell switch command MAC CE received from LTM candidate cell #1 does not include a TA value, UE 100 does not have a valid TA value for LTM candidate cell #2 and needs to perform a random access procedure for LTM candidate cell #2 at the time of subsequent LTM cell switching.

[0124] However, if the UE 100 holds the TA value of the LTM candidate cell #2 that has been acquired before the LTM cell switching to the LTM candidate cell #1 even after the LTM cell switching, the random access procedure can be eliminated at the time of the subsequent LTM cell switching to the LTM candidate cell #2. Eliminating the need for the random access procedure can realize faster subsequent LTM cell switching.

[0125] As described above, in another embodiment, when UE 100 acquires a TA value for LTM candidate cell #2 through UE-based TA measurement and then performs LTM cell switching to LTM candidate cell #1, not only the LTM candidate cell setting for LTM candidate cell #2 but also the TA value acquired through UE-based TA measurement is retained. As a result, UE 100 can use the retained TA value at the time of subsequent LTM cell switching from LTM candidate cell #1 to LTM candidate cell #2, thereby making it unnecessary to perform a random access procedure to LTM candidate cell #2.

[0126] 12 is a diagram showing an operation of the UE 100 according to another embodiment. The operation may be applied to the LTM of an intra-CU. The operation may be applied to the LTM of an inter-CU. Note that the operation may be performed in combination with the operation according to the above-described embodiment.

[0127] In step S21, UE100 receives LTM candidate cell settings (LTM candidate cell setting list) of multiple LTM candidate cells from the source cell (gNB200).

[0128] In step S22, the UE 100 performs early TA acquisition for each of the plurality of LTM candidate cells. The UE 100 performs UE-based TA measurement as early TA acquisition for at least one of the plurality of LTM candidate cells.

[0129] In step S23, the UE 100 performs LTM cell switching to one cell among a plurality of LTM candidate cells. For example, the UE 100 performs LTM cell switching using an LTM candidate cell designated in a cell switching command MAC CE received from the source cell as a target cell. Here, the UE 100 holds an LTM candidate cell configuration (LTM candidate cell configuration list) and a TA value obtained using early TA acquisition for an LTM candidate cell other than the target cell. Specifically, the UE 100 holds the TA value obtained by performing UE-based TA measurement on an LTM candidate cell other than the target cell even after performing LTM cell switching.

[0130] In step S24, after performing the LTM cell switching in step S23, UE100 uses the LTM candidate cell setting and the TA value stored in step S23 to perform a subsequent LTM cell switching from the one cell (current serving cell) to another LTM candidate cell.

[0131] Such an operation can increase the possibility that the random access procedure can be omitted at the time of the subsequent LTM cell switching. That is, the UE 100 can perform the subsequent LTM cell switching without the random access procedure to another LTM candidate cell using the TA value stored in step S23.

[0132] Such an operation may be performed according to a setting from the gNB200 (source cell). For example, the UE100 may receive information from the source cell for setting TA value retention. In response to the setting of TA value retention, the UE100 may perform the above-mentioned TA value retention after performing LTM cell switching. If TA value retention is not set, the UE100 may discard the TA value at the time of LTM cell switching without performing the above-mentioned TA value retention. Such TA value retention setting may be individually configurable for each LTM candidate cell setting, along with the UE-based TA measurement setting of FIG. 11.

[0133] When the validity period of the TA value is set as in the above-described embodiment, the UE 100 may discard the retained TA value when the validity period has elapsed. The UE 100 may continue the operation of the timer described above for the retained TA value, and discard the TA value when the timer expires.

[0134] In addition, if UE100 receives an instruction from gNB200 to delete an LTM candidate cell setting and has retained the TA value of the LTM candidate cell corresponding to the LTM candidate cell setting, it may discard the TA value along with the LTM candidate cell setting.

[0135] In the above-described embodiments, the inter-CU LTM has been mainly described as an example, but this is not limited thereto. It may also be applied to intra-CU LTM. It may also be applied to conditional LTM. In conditional LTM, instead of a cell switching command transmitted from the gNB 200, the UE 100 performs LTM cell switching to a cell that satisfies a preset radio quality condition. In addition, AI (Artificial Intelligence) / ML (Machine Learning) inference using a UE side model (for example, Handover LTM execution event prediction) may be applied.

[0136] 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.

[0137] 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.

[0138] 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.

[0139] 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 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).

[0140] 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.

[0141] 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.

[0142] 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.

[0143] This application claims priority to U.S. Provisional Application No. 63 / 573,633, filed April 3, 2024, the entire contents of which are incorporated herein by reference.

[0144] (7) First Supplementary Note The following is a supplementary note regarding the features of the above-described embodiment.

[0145] Supplementary Note 1: A communication method executed by a user equipment that switches a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the communication method comprising: performing early Timing Advance (TA) acquisition for the second cell in response to an LTM-related instruction from the first cell; determining whether a predetermined time has elapsed since a timing related to the execution of the early TA acquisition; and performing the early TA acquisition again in response to determining that the predetermined time has elapsed since the timing.

[0146] Supplementary Note 2: The communication method of Supplementary Note 1, further comprising: performing a long-term mobile station (LTM) cell switch to the second cell by applying a TA value obtained using the early TA acquisition.

[0147] Supplementary Note 3: The communication method according to Supplementary Note 1 or 2, further comprising receiving, from a network node that manages the first cell, designation information that designates the predetermined time, wherein the user equipment determines whether the predetermined time designated by the designation information has elapsed from the timing.

[0148] Supplementary Note 4: When the first cell and the second cell are managed by different network nodes, the user equipment receives the designation information indicating the predetermined time determined by a network node that manages the first cell. The communication method according to Supplementary Note 3.

[0149] Supplementary Note 5: When the first cell and the second cell are managed by different network nodes, the user equipment receives the designation information indicating the predetermined time determined by a network node that manages the second cell. The communication method according to Supplementary Note 3.

[0150] Supplementary Note 6: The communication method according to any one of Supplementary Notes 1 to 5, wherein re-performing the early TA acquisition includes performing user equipment-based TA measurement in which the user equipment itself measures a TA value to be applied to the second cell in response to determining that the predetermined time has elapsed from the timing.

[0151] Supplementary Note 7: The communication method according to any one of Supplementary Notes 1 to 5, wherein re-performing the early TA acquisition includes requesting the first cell to transmit a Physical Downlink Control Channel (PDCCH) order in response to determining that the predetermined time has elapsed from the timing, and the user equipment transmits a Contention Free Random Access (CFRA) RA preamble to the second cell in response to receiving the PDCCH order from the first cell.

[0152] Supplementary Note 8: The communication method according to any one of Supplementary Notes 1 to 7, wherein the timing is a transmission timing of an RA preamble of Contention Free Random Access (CFRA) transmitted to the second cell.

[0153] Supplementary Note 9: The communication method according to any one of Supplementary Notes 1 to 7, wherein the timing is a timing at which a TA value to be applied to the second cell is obtained by user equipment-based TA measurement in which the user equipment measures the TA value to be applied to the second cell by itself.

[0154] Supplementary Note 10: A user equipment that switches a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), comprising: a controller that performs early Timing Advance (TA) acquisition for the second cell in response to an LTM-related instruction from the first cell; determines whether a predetermined time has elapsed since a timing related to the execution of the early TA acquisition; and, in response to determining that the predetermined time has elapsed since the timing, performs the early TA acquisition again.

[0155] Supplementary Note 11: A communication method executed by a user equipment for switching a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), comprising: receiving LTM candidate cell configurations of a plurality of second cells from the first cell; performing early Timing Advance (TA) acquisition for each of the plurality of second cells; performing an LTM cell switch for one of the plurality of second cells, and retaining the LTM candidate cell configuration and a TA value obtained using the early TA acquisition; and after performing the LTM cell switch, performing a subsequent LTM cell switch from the one cell to another second cell using the retained LTM candidate cell configuration and the retained TA value.

[0156] Supplementary Note 12: The communication method according to Supplementary Note 11, wherein the user equipment performs the subsequent LTM cell switch to the different second cell without a random access procedure using the stored TA value.

[0157] Supplementary Note 13: The communication method according to Supplementary Note 11 or 12, wherein the user equipment retains the TA value obtained by performing user equipment-based TA measurement as the early TA acquisition even after performing the LTM cell switch.

[0158] Supplementary Note 14: The communication method according to any one of Supplementary Notes 11 to 13, further comprising receiving information from the first cell for setting retention of the TA value, wherein the user equipment retains the TA value even after performing the LTM cell switch in response to the setting of retention of the TA value.

[0159] Supplementary Note 15: The communication method according to any one of Supplementary Notes 11 to 14, wherein the user equipment discards the stored TA value after a predetermined time has elapsed.

[0160] Supplementary Note 16: A user equipment configured to switch a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), comprising: a controller configured to receive from the first cell LTM candidate cell configurations of a plurality of second cells; perform early Timing Advance (TA) acquisition for each of the plurality of second cells; perform LTM cell switch for one of the plurality of second cells, and retain the LTM candidate cell configuration and a TA value obtained using the early TA acquisition; and, after performing the LTM cell switch, perform a subsequent LTM cell switch from the one cell to another second cell using the retained LTM candidate cell configuration and the retained TA value.

[0161] (8) Appendix 2 1. Introduction RAN#102 approved a new work item for NR Mobility Enhancement Phase 4. The WID lists inter-CU LTM, event-triggered L1 measurement reporting, and conditional LTM as enhancements to Rel-19 led by RAN2. Regarding event-triggered L1 measurement reporting, detailed goals are as follows:

[0162] Measurement-related enhancements to support L1 measurement reporting: [RAN2, RAN1] Measurement-related enhancements apply to intra-CU MCG / SCG LTM and inter-CU MCG / SCG LTM and specify the components required to support event-triggered L1 measurement reporting [RAN2, RAN1].

[0163] RAN1 and RAN2 will proceed independently with the events triggered by the measurement objectives of their respective MIMO and mobility enhancement work orders. They will review the progress of RAN#105 to see if any modifications to the objectives are necessary to avoid / manage overlapping efforts.

[0164] Specifies support for CSI-RS measurements for LTM procedures, enables CSI-RS based beam management, and performs other required physical layer operations for pre-LTM candidate cells. [RAN1]

[0165] This appendix provides an initial look at potential issues with event-triggered L1 measurement reporting.

[0166] 2. Discussion The Rel-18 LTM procedure is shown in Figure 13 below. In step 5, since the current L1 measurement report only supports periodic reporting, the purpose of the Rel-19 extension is to support event-triggered L1 measurement reporting to improve radio resource efficiency and UE power consumption. The L1 measurement report (step 5) is used by the gNB for LTM decision, after which a cell switch command MAC CE is sent to the UE (step 6).

[0167] Therefore, a promising use case for event-triggered L1 measurement reporting is the LTM decision in gNBs. RAN2 needs to confirm this use case first. Other use cases (e.g., application to Step 1) can be discussed if sufficient justification is provided.

[0168] Proposal 1: RAN2 should ensure that the gNB uses event-triggered L1 measurement reports for LTM decisions, i.e., replacing the periodic L1 measurement reports in step 5 of the LTM procedure of TS38.300.

[0169] Considering the use case of Proposal 1 above, RAN2 should first discuss what "events" need to be defined for L1 measurement reporting, since currently, many event trigger conditions are defined for L3 measurement reporting.

[0170] In existing L3 handovers, event A3 is commonly used for handover decisions, so it is quite easy to define an event like A3 for L1 measurement reports, without excluding other events if justified as beneficial for the use case of event-triggered L1 measurement reports.

[0171] Proposal 2: RAN2 should agree that a trigger such as event A3 is defined for L1 measurement reporting.

[0172] Since LTM is also considered to be beneficial for aircraft or equipment installed in aircraft (Aerial UE) that requires low latency mobility, it is worth discussing whether events such as H1 / H2 should be defined to support vertical LTM. In this case, other events such as A3H1 are also considered to be beneficial. Therefore, RAN2 should discuss whether these AxHx events should be supported in L1 measurement reports.

[0173] Proposal 3: RAN2 should discuss whether triggers such as Event AxHx for aircraft or aircraft-mounted equipment (Aerial UE) should also be supported in L1 measurement reports.

[0174] Considering that events are defined for L1 measurement reports, the question then arises: which layer should handle the trigger for the measurement report, i.e. the entry / exit conditions? The following options are possible:

[0175] L1 (Physical Layer): This is a viable option since L1 measurement reporting is performed within the physical layer. The drawback is that it would require significant changes to RAN1, such as implementing beam merging functionality (cell-level measurements) and triggering measurement reporting.

[0176] L2 (MAC layer): This is also a strong option since the LTM decision is made in the source DU. The MAC of the source DU sends a cell switch command to the UE. The drawbacks are the increased MAC specification and the interaction between layers. For example, the MAC needs to control the PHY to start / stop L1 measurement reports.

[0177] L3 (RRC layer): This is a promising option as all functionality for event-triggered L3 measurement reporting is available at the RRC layer and can potentially be reused with relatively little effort / modification. The drawback of this option is that it requires interaction between layers, e.g., RRC instructing the PHY to start / stop L1 measurement reporting.

[0178] Since each of the three options has its advantages and disadvantages, RAN2 should discuss at which layer the triggering of L1 measurement reports should be handled. RAN2 is the lead WG for this WI.

[0179] Proposal 4: RAN2 should discuss which layer handles triggering of L1 measurement reports.

[0180] (9) Appendix 3 1. Introduction RAN#102 approved a new work item for NR mobility enhancement phase 4. WID lists inter-CU LTM, event-triggered L1 measurement reports, and conditional LTM as enhancement items in RAN2-led Rel-19. The detailed goals for inter-CU LTM are as follows:

[0181] ・Specify support for inter-CU Layer 2 mobility (LTM) [RAN2, RAN3] If DC is not configured, priority is given to the case where the CU acts as MN Secondary priority is to support the case where NR-DC is configured, CU acts as SN, and MCG is not changed Secondary priority is to support the case where NR-DC is configured, CU acts as MN, and SCG is not changed or SCG is released Note: This does not apply when LTM is configured for both MCG and SCG Specify support for subsequent LTM mobility procedures, which aim to avoid RRC setup between cell switches, in accordance with Rel-18 LTM Handling requires coordination with SA3 Note: The Rel. 18 intra-CU LTM procedures are considered as the baseline for adding inter-CU support

[0182] This appendix provides initial insight into potential issues with inter-CU LTM.

[0183] 2. Discussion As stated in WID, "the Rel. 18 same-CU LTM procedure is considered the baseline for adding different-CU support," so a representative Rel-18 LTM procedure is shown in Figure 13 below.

[0184] While enhancements to the network interfaces (F1-AP and Xn-AP) are the subject of RAN3, RAN2 should be aware that there is significant latency at the network interfaces, especially in the case of non-ideal backhaul as referenced in the following table.

[0185] The classification of non-ideal backhaul is as follows:

[0186] Since the LTM signaling procedure includes F1-AP signaling and Xn-AP signaling, the latency of the backhaul link is several tens of milliseconds according to the above table. Therefore, in the case of inter-CU LTM, it becomes more difficult to synchronize the UL transmission timing at the UE (PRACH for completing RRC reconfiguration after CFRA and cell switch command) with the UL reception preparation timing at the target cell (because the target cell does not know or learns late the timing when the source cell sent the PDCCH command and cell switch command).

[0187] If an UL transmission occurs before the UL reception preparation, the UL transmission will not be received, and the UE will not only have to retransmit, but unnecessary UL interference will occur. If the UL transmission is transmitted significantly later than the UL reception preparation, UL resources will be wasted for a long time until the gNB receives the UL transmission. Therefore, RAN2 should discuss ways to minimize UE power consumption and UL resource waste.

[0188] Proposal 1: RAN2 should discuss ways to minimize UE power consumption and UL resource waste caused by timing mismatch between the source cell and the target cell when performing early TA and / or LTM cell switch due to backhaul delay.

[0189] The WID states that "specifies support for subsequent LTM mobility procedures aimed at avoiding RRC setup between cell switches in accordance with Rel-18 LTM."

[0190] According to the current L3 handover or current conditional handover procedure, the UE context is shared with the target gNB only once in the Xn Handover Request message. This is because subsequent (conditional) handover execution is not supported. The UE context is usually not updated during the handover procedure. This ensures that the target gNB has the latest UE context. In the case of a conditional handover, subsequent CHO is not supported. That is, since the UE performs a process to delete all conditional reconfigurations during CHO, there is no problem in reusing the same procedure for UE context management. That is, the UE context is shared only once via the Xn Handover Request.

[0191] Observation 1: In the conventional L3 handover mechanism, the UE context is shared with the target cell only once by the Xn handover request message.

[0192] In Rel-18, for subsequent LTM within a CU, since the source cell and target cell belong to the same CU, the gNB always knows the latest UE context, regardless of whether LTM is performed or whether the LTM configuration is updated (e.g., a new LTM candidate is added).

[0193] Observation 2: In intra-CU LTM in Rel-18, the UE context is always managed by the same CU to which both the source and target cells belong.

[0194] In the case of subsequent LTM between CUs, even after an LTM candidate is configured in the UE, other candidate cells may be added or removed by the LTM configuration without the knowledge of the already configured candidate gNB. Therefore, when the UE is reconfigured to add a new LTM candidate (or when it is changed or removed), the UE context shared with the candidate gNB becomes invalid (i.e., is not synchronized with the UE's current configuration). Subsequently, when LTM is performed, i.e., the UE moves to a new source cell (i.e., the same as the previous target cell), the new source cell is unaware of this change in the UE's LTM configuration. Therefore, the source cell cannot decide whether to configure the UE for LTM first or to send a Cell Switch Command MAC CE to the UE for subsequent LTM execution for the LTM candidate. Therefore, the UE context (i.e., the LTM configuration) must always be synchronized between the serving gNB and the LTM candidate gNB. Additional signaling via the Xn-AP may be required, and RAN2 may need to inform RAN3 of this issue.

[0195] Observation 3: In subsequent LTM between CUs, if the LTM configuration is updated (i.e., adding / changing / deleting an LTM candidate), the source gNB needs to share the latest UE context with the LTM candidate gNB.

[0196] Observation 4: RAN2 may send an LS to RAN3 informing it that for subsequent LTM of inter-CU, the UE context (i.e., LTM configuration) must always be synchronized between the source gNB and neighboring gNBs providing the LTM candidate cells.

[0197] Another issue, as pointed out in WID, is how to handle security keys. In Rel-17, this was not a critical issue because only intra-CU LTM was supported, but this is no longer true in Rel-18, which does not support inter-CU LTM. RAN2 may need to discuss this issue with SA3.

[0198] Observation 5: RAN2 may negotiate with SA3 on how to handle security keys in case of subsequent LTM between CUs.

[0199] After a successful LTM, the UE discards the source cell configuration. However, since it was the UE's serving cell for the last few seconds, the source cell is still considered a promising LTM candidate. For the next LTM, the UE retains the configuration for all other LTM candidate cells, but not the source cell. This may be considered a ping-pong problem because the UE immediately returns to the source cell, but this is not always the case. This is because the UE's direction of movement is unpredictable, meaning that the UE may return to the source cell even if the network has optimized handover parameters. In this case, the target cell will configure the source cell as an additional LTM candidate as soon as the LTM is successfully completed. This is clearly inefficient.

[0200] Observation 6: After the successful completion of LTM execution in the target cell, the source cell is still a promising LTM candidate, but the source cell configuration has already been discarded by the UE.

[0201] Therefore, to maximize the benefit of subsequent LTM, the source cell configuration should be maintained after the LTM execution is successfully completed. RAN2 should discuss whether and how to maintain the source cell configuration after the LTM execution.

[0202] Proposal 2: RAN2 should discuss whether and how the UE should retain the source cell configuration after performing LTM.

[0203] 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 that switches a serving cell from a first cell to a second cell using LTM (L1 / L2 Triggered Mobility), comprising: performing early timing advance (TA) acquisition for the second cell in response to an LTM-related instruction from the first cell; determining whether a predetermined time has elapsed since the timing related to the execution of the early TA acquisition; and performing the early TA acquisition again in response to determining that the predetermined time has elapsed since the timing.

2. The communication method according to claim 1, further comprising: performing a long-term mobile station (LTM) cell switch to the second cell by applying a TA value obtained using the early TA acquisition.

3. The communication method according to claim 1, further comprising receiving designation information specifying the predetermined time from a network node that manages the first cell, and the user equipment determining whether the predetermined time specified by the designation information has elapsed from the timing.

4. The communication method according to claim 3, wherein, when the first cell and the second cell are managed by different network nodes, the user equipment receives the designation information indicating the predetermined time determined by the network node that manages the first cell.

5. The communication method according to claim 3, wherein, when the first cell and the second cell are managed by different network nodes, the user equipment receives the designation information indicating the predetermined time determined by a network node that manages the second cell.

6. The communication method according to any one of claims 1 to 5, wherein re-performing the early TA acquisition includes performing user equipment-based TA measurement in which the user equipment itself measures a TA value to be applied to the second cell in response to determining that the predetermined time has elapsed from the timing.

7. The communication method according to any one of claims 1 to 5, wherein re-performing the early TA acquisition includes requesting the first cell to transmit a Physical Downlink Control Channel (PDCCH) order in response to determining that the predetermined time has elapsed from the timing, and the user equipment transmits a Contention Free Random Access (CFRA) RA preamble to the second cell in response to receiving the PDCCH order from the first cell.

8. The communication method according to any one of claims 1 to 5, wherein the timing is a transmission timing of a contention-free random access (CFRA) RA preamble transmitted to the second cell.

9. A communication method according to any one of claims 1 to 5, wherein the timing is an acquisition timing at which a TA value is acquired by user equipment-based TA measurement in which the user equipment itself measures the TA value to be applied to the second cell.

10. A user equipment that switches a serving cell from a first cell to a second cell using LTM (L1 / L2 Triggered Mobility), comprising: a control unit that performs early timing advance (TA) acquisition for the second cell in response to an LTM-related instruction from the first cell; determines whether a predetermined time has elapsed since the timing related to the execution of the early TA acquisition; and, in response to determining that the predetermined time has elapsed since the timing, performs the early TA acquisition again.

11. A communication method executed by a user equipment for switching a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), comprising: receiving LTM candidate cell configurations of a plurality of second cells from the first cell; performing early timing advance (TA) acquisition for each of the plurality of second cells; performing an LTM cell switch for one of the plurality of second cells, and retaining the LTM candidate cell configuration and a TA value obtained using the early TA acquisition; and after performing the LTM cell switch, performing a subsequent LTM cell switch from the one cell to another second cell using the retained LTM candidate cell configuration and the retained TA value.

12. The communication method according to claim 11, wherein the user equipment performs the subsequent LTM cell switch to the different second cell without a random access procedure using the stored TA value.

13. The communication method according to claim 11, wherein the user equipment retains the TA value obtained by performing user equipment-based TA measurement as the early TA acquisition even after performing the LTM cell switch.

14. The communication method according to any one of claims 11 to 13, further comprising receiving information from the first cell to set retention of the TA value, wherein the user equipment, in response to the setting of retention of the TA value, retains the TA value even after performing the LTM cell switch.

15. A communication method according to any one of claims 11 to 13, wherein the user equipment discards the stored TA value after a predetermined time has elapsed.

16. A user equipment that switches a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), comprising: a controller that receives LTM candidate cell configurations of multiple second cells from the first cell; performs early timing advance (TA) acquisition for each of the multiple second cells; performs LTM cell switch to one of the multiple second cells while retaining the LTM candidate cell configuration and a TA value obtained using the early TA acquisition; and, after performing the LTM cell switch, performs a subsequent LTM cell switch from the one cell to another second cell using the retained LTM candidate cell configuration and the retained TA value.

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