Communication method, user device, and network node

Abstract

Provided is a communication method for switching the serving cell of a user device from a first cell to a second cell by means of L1 / L2 Triggered Mobility (LTM), said method involving: the user device receiving, from the first cell, a gap period configuration for performing, on the second cell, contention-based random access (CBRA) which is used for uplink early synchronization in the LTM; and the user device performing the CBRA on the second cell during the gap period on the basis of the configuration. Performing the CBRA entails transmitting an RA preamble of the CBRA to the second cell.

Description

COMMUNICATION METHOD, USER EQUIPMENT, AND NETWORK NODE

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

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

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

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

[0005] This disclosure provides techniques for improving LTM.

[0006] A first aspect of the present disclosure provides a communication method for switching a serving cell of a user equipment (UE) from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), the communication method comprising: receiving, by the first network node, a HANDOVER REQUEST ACKNOWLEDGE message from the second network node, the HANDOVER REQUEST ACKNOWLEDGE message including an LTM candidate cell configuration, the HANDOVER REQUEST ACKNOWLEDGE message including information regarding uplink early synchronization configuration as information separate from the LTM candidate cell configuration.

[0007] A communication method according to a second aspect of the present disclosure is a communication method for switching a serving cell of a user equipment (UE) from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the communication method including: receiving, from the first cell, a configuration of a gap period for performing contention-based random access (CBRA) used for uplink early synchronization in the LTM with the second cell; and performing, by the user equipment, the CBRA with the second cell during the gap period based on the configuration. Performing the CBRA includes transmitting an RA preamble for the CBRA to the second cell.

[0008] A user equipment according to a third aspect of the present disclosure 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 receiving unit that receives, from the first cell, a setting of a gap period for performing contention-based random access (CBRA) for uplink early synchronization in the LTM on the second cell; a control unit that performs the CBRA on the second cell during the gap period based on the setting; and a transmitting unit that transmits an RA preamble to the second cell in the CBRA.

[0009] A network node according to a fourth aspect of the present disclosure is a network node that manages a first cell in a mobile communication system in which a serving cell of a user equipment is switched from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), and includes a transmission unit that transmits to the user equipment a setting of a gap period for the user equipment to perform contention-based random access (CBRA) with the second cell, which is used for uplink early synchronization in the LTM.

[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 a cell switching procedure by LTM in intra-CU (i.e., within the same gNB). FIG. 7 is a diagram showing an example of operation when CFRA-based early TA is used as early synchronization (early TA) in inter-CU LTM. FIG. 8 is a diagram showing an example of an operation sequence of a mobile communication system according to a first operation pattern of an embodiment. FIG. 9 is a diagram showing an example of gap setting according to the first operation pattern of an embodiment. FIG. 10 is a diagram showing an example of gap setting according to a second operation pattern of an embodiment. FIG. 11 is a diagram showing an example of gap setting according to a third operation pattern of an embodiment. FIG. 12 is a diagram for explaining activation and deactivation of an early synchronization gap according to the third operation pattern of an embodiment. FIG. 13 is a diagram for explaining an overview of a two-step RA procedure.

[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 reception operations 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 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 LTM settings for candidate cells to be switched to, and provides the LTM settings 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 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 (ID) 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 the LTM configuration as the target configuration by gNB200. The LTM configuration can be added, changed, and released by gNB200 via RRC signaling.

[0046] The following principles apply to LTM:

[0047] Each LTM setting can be provided as a delta setting relative to a reference setting that is used to form the complete LTM setting.

[0048] If a full LTM 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 configurations can be performed without RRC reconfiguration, i.e., the UE 100 does not release other LTM configurations after an LTM is triggered.

[0052] 6 is a diagram showing an example of a cell switching procedure by LTM in intra-CU (i.e., within the same gNB 200). 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" until a serving cell switch by LTM is determined, and the second cell is also referred to as a "target cell" after a serving cell switch by LTM is determined. The first cell is also referred to as a "source cell" or a "(current) serving cell".

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

[0055] In step S2, the UE 100 transmits a Measurement Report message, which is an RRC message, to the gNB 200 (first cell). The measurement report transmitted by the RRC message is also referred to as an "L3 measurement report."

[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 (first cell) transmits an RRC message, specifically an RRC Reconfiguration message, to the UE 100, including LTM configurations (LTM Candidate Configurations) of one or more LTM candidate cells. The LTM configurations may include a random access channel (RACH) configuration used for transmitting RA preambles to the corresponding LTM 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.

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

[0059] In step S6, the UE 100 may perform synchronization processing with the LTM candidate cell (second cell) before receiving the cell switch command MAC CE from the first cell. Such synchronization processing may be referred to as early synchronization (Early sync) or early timing advance (Early TA). 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 (i.e., UL early synchronization) in the LTM candidate cell requested by the gNB 200 (serving cell). This is performed by a CFRA triggered by a PDCCH order (PDCCH order) from the first 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. Note that, when early UL synchronization configuration (EarlyUlSyncConfig) is configured in the UE 100, the PDCCH order may include a corresponding RACH transmission cell, i.e., a cell indicator indicating to which LTM candidate cell the UE 100 should transmit a random access preamble (RA preamble).

[0060] The UE 100 transmits an RA preamble to the designated LTM candidate cell (second cell). In order to minimize communication interruption of the serving 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.

[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 (also referred to as an "L1 measurement report") to the gNB 200 (first cell). 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 (first cell) 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 UL early synchronization (i.e., a TA value derived based on the RA preamble).

[0064] In step S10, the UE 100 switches to the configuration of the target cell (second cell). Specifically, the UE 100 detaches from the first cell and applies the configuration of the target cell (second 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, for example, by sending an RRC Reconfiguration Complete message to the target cell (second cell). 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) Inter-CU LTM The LTM introduced in 3GPP Release 18 only supports intra-CU and does not support inter-CU (i.e., between different gNBs 200) LTM. That is, in conventional LTM, it is possible to perform LTM cell switching between cells under the same CU (same gNB 200), but it is not possible to perform LTM cell switching between cells under different CUs (different gNBs 200).

[0068] Meanwhile, Inter-CU LTM is scheduled to be newly introduced in 3GPP Release 19. In Inter-CU LTM, UE100 performs LTM cell switching from a first cell of one gNB200 (also referred to as "serving gNB200S") to a second cell of another gNB200 (also referred to as "LTM candidate gNB200C"). The serving gNB200S manages the serving cell (first cell) of UE100, and the LTM candidate gNB200C manages the LTM candidate cell (second cell) of UE100.

[0069] 7 is a diagram showing an example of operation when CFRA-based early TA is used as early synchronization (early TA) in inter-CU LTM. In the following embodiment, communication between the serving gNB200S and the LTM candidate gNB200C is performed over the Xn interface, which is an interface between gNBs.

[0070] In step S51, UE100 in an RRC connected state in the serving cell of the serving gNB200S transmits an L3 measurement report to the serving gNB200S. The serving gNB200S that receives the L3 measurement report decides to use LTM based on the L3 measurement report.

[0071] In step S52, the serving gNB200S transmits a HANDOVER REQUEST message to the LTM candidate gNB200C that manages the LTM candidate cell to request preparation for LTM cell switching. The LTM candidate gNB200C that receives the HANDOVER REQUEST message allocates (reserves) dedicated CFRA resources in the LTM candidate cell for the UE100.

[0072] In step S53, the LTM candidate gNB200C sends a HANDOVER REQUEST ACKNOWLEDGE message to the serving gNB200S, including an LTM candidate cell setting including a CFRA setting.

[0073] In step S54, the serving gNB200S sends an RRC Reconfiguration message to the UE100, which includes the LTM candidate cell setting from the LTM candidate gNB200C as the LTM setting.

[0074] In step S55, UE100, which has received the RRC Reconfiguration message, saves the LTM setting and sends an RRC Reconfiguration Complete message to the serving gNB200S.

[0075] In step S56, the serving gNB200S decides, for example based on an L1 measurement report from UE100, to have UE100 perform early synchronization to the LTM candidate cell, and sends a PDCCH order to UE100 instructing it to transmit a CFRA RA preamble to the LTM candidate cell.

[0076] In step S57, the UE 100 that has received the PDCCH order transmits a CFRA RA preamble to the LTM candidate cell on the PRACH (Physical Random Access Channel). The LTM candidate gNB 200C that has received the CFRA RA preamble derives a TA value based on the RA preamble, and uniquely identifies the UE 100 based on the RA preamble (CFRA resource), and can provide the TA value together with the identification information of the UE 100 to the serving gNB 200S. As a result, the serving gNB 200S can transmit a cell switching command including the TA value for the UE 100 to the UE 100.

[0077] In such an operation, the LTM candidate gNB200C does not know the timing when the serving gNB200S transmits a PDCCH order to the UE100. Therefore, the LTM candidate gNB200C needs to reserve CFRA resources for the UE100 from the time it transmits a HANDOVER REQUEST ACKNOWLEDGE message to the serving gNB200S until it receives an RA preamble from the UE100, which may result in a waste of radio resources (specifically, CFRA resources). Also, although multiple LTM candidate cells are typically configured for the UE100, there may be LTM candidate cells among them that the UE100 does not access, which may result in a further waste of radio resources.

[0078] One possible solution to this further waste of radio resources (CFRA resources) is to introduce contention-based random access (CBRA) for early synchronization (early TA). When CBRA is applied to early TA acquisition, although RA preamble contention may occur, each LTM candidate cell does not need to reserve CFRA resources.

[0079] However, while UE100 is performing CBRA on the LTM candidate cell, it suspends communication with the serving gNB200S (serving cell), but a typical CBRA is initiated at a timing arbitrarily determined by UE100. Therefore, if the serving gNB200S cannot determine that UE100 has initiated CBRA, it is unclear when to stop DL transmission. Therefore, the serving gNB200S may continue DL transmission even when UE100 cannot receive DL, which may result in waste of radio resources.

[0080] In order to solve such problems, in an embodiment, the serving gNB200S transmits to the UE100 a gap period setting (also referred to as a "gap setting") for the UE100 to perform CBRA for UL early synchronization (early TA) in LTM with respect to an LTM candidate cell. The UE100 receives the setting from the serving cell (serving gNB200S). Based on the gap setting, the UE100 performs CBRA with the LTM candidate gNB200C (LTM candidate cell) during the gap period. Specifically, the UE100 transmits a CBRA RA preamble to the LTM candidate gNB200C (LTM candidate cell) during the gap period. The UE100 may receive a response to the RA preamble from the LTM candidate cell during the gap period.

[0081] In this way, by the serving gNB200S setting a gap period for the UE100 to perform CBRA for UL early synchronization (early TA) on the LTM candidate cell, the serving gNB200S can suspend communication (including DL transmission) with the UE100 during the gap period. Therefore, it is possible to prevent the serving gNB200S from continuing DL transmission when the UE100 cannot receive DL. As a result, it is possible to suppress the waste of radio resources as described above.

[0082] (3.1) First Operation Pattern In the first operation pattern of the embodiment, a measurement gap mechanism is used as a gap period during which the UE 100 performs CBRA, which is used for UL early synchronization (early TA), on an LTM candidate cell. The measurement gap is originally a period during which the UE 100 performs measurements (e.g., radio quality measurements of reference signals) on a cell different from the serving cell (e.g., a neighboring cell using a different frequency).

[0083] That is, the gap configuration received by the UE 100 from the serving cell is a measurement gap configuration for performing measurements on a cell different from the serving cell. In the first operation pattern, the measurement gap configuration includes notification information indicating that the measurement gap can be used for UL early synchronization. Based on the notification information, the UE 100 uses the configured measurement gap as a gap period for UL early synchronization (CBRA).

[0084] 8 is a diagram showing an example of an operation sequence of the mobile communication system 1 according to the first operation pattern of the embodiment. In this operation example, steps S101 to S105 are an LTM preparation phase, steps S106 to S109 are an early synchronization phase, steps S110 to S111 are an LTM cell switching execution phase, and step S112 is an LTM cell switching completion phase.

[0085] In step S101, UE100 in an RRC connected state in the serving cell of the serving gNB200S transmits an L3 measurement report to the serving gNB200S. The serving gNB200S that receives the L3 measurement report decides to use LTM based on the L3 measurement report.

[0086] In step S102, the serving gNB200S sends a HANDOVER REQUEST message to the LTM candidate gNB200C that manages the LTM candidate cell to request preparation for LTM cell switching.

[0087] In step S103, the LTM candidate gNB200C sends a HANDOVER REQUEST ACKNOWLEDGE message including the LTM candidate cell setting (LTM setting) to the serving gNB200S.

[0088] Here, the LTM candidate gNB200C may configure early TA of CBRA in the LTM candidate cell configuration (LTM configuration). The LTM candidate cell configuration (LTM configuration) may include information instructing the UE100 to perform early TA of CBRA. The LTM candidate gNB200C may configure radio quality conditions under which the UE100 starts CBRA in the LTM candidate cell configuration (LTM configuration). For example, the LTM candidate cell configuration (LTM configuration) may include a radio quality threshold (such as an RSRP threshold) that triggers the execution of early TA of CBRA for the corresponding LTM candidate cell.

[0089] The LTM candidate gNB200C may prepare a CBRA resource dedicated to early TA. The LTM candidate gNB200C may include information about the CBRA resource in the LTM candidate cell configuration (LTM configuration).

[0090] The LTM candidate cell setting (LTM setting) is notified to UE100 from the LTM candidate gNB200C via the serving gNB200S, but it is assumed that the serving gNB200S transmits the LTM candidate cell setting (LTM setting) to UE100 without deciphering the LTM candidate cell setting (LTM setting) from the LTM candidate gNB200C. Therefore, the LTM candidate gNB200C may notify the serving gNB200S that it has configured early TA for CBRA. For example, the LTM candidate gNB200C may include information indicating that it has configured early TA for CBRA in the HANDOVER REQUEST ACKNOWLEDGE message as information separate from the LTM candidate cell setting (LTM setting). For similar reasons, the LTM candidate gNB200C may notify the serving gNB200S of the timing of the CBRA resource dedicated to early TA. For example, the LTM candidate gNB200C may include information indicating the time resource of CBRA in the HANDOVER REQUEST ACKNOWLEDGE message as information separate from the LTM candidate cell setting (LTM setting).

[0091] In step S104, the serving gNB200S transmits an RRC Reconfiguration message including an LTM candidate cell setting (LTM setting) from the LTM candidate gNB200C to the UE100. Here, the serving gNB200S may transmit to the UE100 a gap setting in the RRC Reconfiguration message including a notification that early TA may be performed. That is, the serving gNB200S may notify the UE100 that early TA may be performed in the measurement gap.

[0092] In step S105, UE100, which has received the RRC Reconfiguration message, saves the LTM setting and sends an RRC Reconfiguration Complete message to the serving gNB200S.

[0093] In step S106, UE100 determines to perform early TA of CBRA to the configured LTM candidate cell, and transmits a CBRA RA preamble to the LTM candidate cell on the PRACH during the configured measurement gap. The serving gNB200S suspends communication with UE100 (particularly, DL transmission) during the measurement gap.

[0094] The UE 100 may perform PRACH transmission (RA preamble transmission) only if PRACH transmission (RA preamble transmission) has been triggered in the set measurement gap. Note that the UE 100 may perform RA preamble transmission in the set measurement gap when the set radio quality condition is satisfied. The LTM candidate gNB 200C that has received the RA preamble derives a TA value based on the RA preamble.

[0095] In step S107, the LTM candidate gNB200C may transmit an RA response including the derived TA value to the UE100. The UE100 receives the RA response from the LTM candidate cell in the configured measurement gap. The UE100 associates the TA value notified in the RA response with the LTM candidate cell and stores it. Then, in step S108, the UE100 may transmit a notification message indicating the completion of early synchronization, the notification message including the TA value, to the serving cell (serving gNB200S).

[0096] Alternatively, in step S109, the LTM candidate gNB200C may send a notification message to the serving gNB200S indicating the completion of early synchronization, the notification message including the TA value.

[0097] In step S110, the UE 100 transmits an L1 measurement report to the serving cell.

[0098] In step S111, the serving gNB200S determines an LTM cell switch to the LTM candidate cell based on the L1 measurement report from the UE100, and transmits a cell switch command MAC CE instructing the UE100 to switch the LTM cell to the LTM candidate cell. The cell switch command MAC CE includes an index (candidate setting index) corresponding to the LTM candidate cell and a TA value obtained by UL early synchronization (early TA). Since the cell switch command MAC CE includes a valid TA value, the UE100 determines to skip the RA procedure to the LTM candidate cell (RACH-less LTM cell switch).

[0099] In step S112, the UE 100 applies the TA value to the LTM candidate cell (LTM candidate gNB 200C) and transmits an RRC Reconfiguration Complete message. This completes the LTM cell switching successfully.

[0100] 9 is a diagram showing an example of gap setting according to the first operation pattern of the embodiment. The gap setting according to the first operation pattern is the setting of a measurement gap.

[0101] In Example 1 of gap configuration according to the first operation pattern, the RRC Reconfiguration message in step S104 includes the LTM configuration, and the LTM configuration includes RRC Reconfiguration for the LTM candidate cell. Since the measurement gap is configured in the LTM configuration, it is possible to configure the measurement gap for each LTM candidate cell.

[0102] The RRC Reconfiguration for the LTM candidate cell includes a measurement configuration (Meas Config) that is a configuration related to measurements. The measurement configuration (Meas Config) includes a measurement gap configuration (Meas gap Config) that is a configuration related to measurement gaps. The measurement gap configuration (Meas gap Config) includes a list (gapToAddModList-r17) that has a measurement gap configuration (Gap Config-r17) as an entry.

[0103] Each entry (Gap Config-r17) is configured with a measurement gap ID (measGapId), a measurement gap length (mgl), a measurement gap repetition frequency (mgrp), and a measurement gap TA (mgta). For a measurement gap in which early TA may be performed, each entry (Gap Config-r17) includes notification information (Early TA) indicating that the measurement gap can be used for early TA. The UE 100 determines that the measurement gap configuration (Gap Config-r17) including the notification information (Early TA) can be used for early TA. This makes it possible to set whether or not each measurement gap configuration (Gap Config-r17) can be used for early TA.

[0104] On the other hand, Example 2 of gap configuration according to the first operation pattern differs from Example 1 in that the measurement gap configuration (Meas gap Config) includes a list (EarlysyncList) of measurement gap IDs of measurement gap configurations that can be used for early TA, in addition to a list having measurement gap configurations (Gap Config-r17) as entries. Also, Example 2 differs from Example 1 in that the measurement gap configuration (Gap Config-r17) does not include notification information (Early TA). The UE 100 determines that the measurement gap configuration (Gap Config-r17) associated with the measurement gap ID included in the measurement gap ID list (EarlysyncList) can be used for early TA.

[0105] (3.2) Second Operation Pattern The second operation pattern of the embodiment will be described, focusing mainly on the differences from the first operation pattern described above.

[0106] In the second operation pattern, a MUSIM (Multi-Universal Subscriber Identity Module) gap mechanism is used as a gap period for the UE 100 to perform CBRA used for UL early synchronization (early TA) on an LTM candidate cell. The MUSIM gap is originally a period used for the UE 100 to perform MUSIM operation for switching between different networks.

[0107] That is, the gap configuration that the UE 100 receives from the serving cell is a MUSIM gap configuration for MUSIM operation. In the second operation pattern, the MUSIM gap configuration includes notification information indicating that the MUSIM gap can be used for UL early synchronization. Based on the notification information, the UE 100 uses the MUSIM gap as a gap period for UL early synchronization (CBRA).

[0108] The operation sequence of the mobile communication system 1 according to the second operation pattern is the same as the operation sequence shown in Fig. 8. In the second operation pattern, the "measurement gap" in the operation described with reference to Fig. 8 can be read as a "MUSIM gap."

[0109] 10 is a diagram showing an example of gap setting according to the second operation pattern of the embodiment. The gap setting according to the second operation pattern is a MUSIM gap setting.

[0110] In Example 1 of gap setting according to the second operation pattern, the RRC Reconfiguration message in step S104 includes a MUSIM gap setting (MUSIM-Gap Config), the MUSIM gap setting (MUSIM-Gap Config) includes a MUSIM gap (MUSIM-Gap), and the MUSIM gap (MUSIM-Gap) includes MUSIM gap information (MUSIM-Gap Info).

[0111] The MUSIM gap information (MUSIM-Gap Info) is set with the length of the MUSIM gap and the repetition frequency of the MUSIM gap. In the case of a MUSIM gap for which early TA may be performed, the MUSIM gap information (MUSIM-Gap Info) includes notification information (Early TA). The UE 100 determines that the MUSIM gap information (MUSIM-Gap Info) including the notification information (Early TA) can be used for early TA. This makes it possible to set whether or not each MUSIM gap information (MUSIM-Gap Info) can be used for early TA.

[0112] On the other hand, Example 2 of gap setting according to the second operation pattern differs from Example 1 in that the MUSIM gap setting (MUSIM-Gap Config) includes a list (EarlysyncList) of MUSIM gap IDs (musim-GapId) of MUSIM gaps (MUSIM-Gap) that can be used for early TA. Also, Example 2 differs from Example 1 in that the MUSIM gap information (MUSIM-Gap Info) does not include notification information (Early TA). The UE 100 determines that the MUSIM gap (MUSIM-Gap) associated with the MUSIM gap ID (musim-GapId) included in the list (EarlysyncList) can be used for early TA.

[0113] (3.3) Third Operation Pattern The third operation pattern of the embodiment will be described, focusing mainly on the differences from the first and second operation patterns described above.

[0114] In the third operation pattern, the early synchronization gap is used as a gap period for the UE 100 to perform CBRA used for UL early synchronization (early TA) on the LTM candidate cell. The early synchronization gap is a new gap mechanism used exclusively for UL early synchronization.

[0115] That is, the gap configuration that the UE 100 receives from the serving cell is a configuration of an early synchronization gap that is used exclusively for UL early synchronization. In the third operation pattern, the UE 100 uses the set early synchronization gap as a gap period for UL early synchronization (CBRA).

[0116] The operation sequence of the mobile communication system 1 according to the third operation pattern is the same as the operation sequence shown in Fig. 8. In the third operation pattern, the "measurement gap" in the operation described with reference to Fig. 8 can be read as an "early synchronization gap."

[0117] 11 is a diagram illustrating an example of gap setting according to the third operation pattern of the embodiment. The gap setting according to the third operation pattern is an early synchronization gap setting. Because the early synchronization gap is a gap period dedicated to early TA, the early synchronization gap setting does not need to include notification information like the first and second operation patterns.

[0118] In an example of gap setting according to the third operation pattern, the RRC Reconfiguration message in step S104 includes an LTM setting (LTM Config), which includes an early synchronization gap setting (Early sync gap Config), which includes a gap setting (Gap Config-r17). Since the early synchronization gap is set in the LTM setting, it is possible to set an early synchronization gap for each LTM candidate cell.

[0119] The early sync gap configuration (Early sync gap Config) includes a list (earlygapToAddModList-r17) having the gap configuration of the early sync gap (Gap Config-r17) as an entry.

[0120] Each entry (Gap Config-r17) has a gap ID (measGapId), gap length (mgl), gap repetition frequency (mgrp), and gap TA (mgta) set therein.

[0121] In the early synchronization gap setting (Early sync gap Config) in the RRC Reconfiguration message in step S104, the serving gNB 200S sets the gap ID (measGapId) for early TA as a list (earlygapToAddModList-r17) and transmits it to the UE 100. The list (earlygapToAddModList-r17) may include an LTM candidate ID that identifies an LTM candidate cell. This allows an early synchronization gap to be set for each LTM candidate cell.

[0122] 12 is a diagram illustrating activation and deactivation of an early synchronization gap according to a third operation pattern of the embodiment. Deactivation of an early synchronization gap may be skipping or canceling the early synchronization gap.

[0123] In the third operation pattern, the UE 100 may transmit a notification of early synchronization gap activation and / or a notification of early synchronization gap deactivation to the serving cell (serving gNB 200S). The notification may be transmitted in an RRC message (e.g., a UE Assistance Information message), a MAC CE, or uplink control information (UCI).

[0124] The serving gNB200S may activate / deactivate the early synchronization gap based on a notification from the UE100. The early synchronization gap does not need to be activated at all times because it is not required before the PRACH transmission (preamble transmission) of the CBRA is triggered or after the end of the early TA. If the early synchronization gap is deactivated, the serving gNB200S can continue communication (particularly DL transmission) with the UE100 during the early synchronization gap. The serving gNB200S may suspend communication (particularly DL transmission) with the UE100 during the early synchronization gap only if the early synchronization gap is activated.

[0125] Under the assumption that the early synchronization gap is always activated, the UE100 sends a notification to the serving gNB200S to skip or cancel the early synchronization gap if the CBRA is not triggered.

[0126] In step S210, the UE 100 transmits a notification to the serving gNB 200S indicating that the most recent early synchronization gap will be skipped or canceled. In this case, only the most recent early synchronization gap is skipped or canceled.

[0127] On the other hand, in step S220, the UE 100 transmits a notification to the serving gNB 200S indicating that all subsequent early synchronization gaps will be skipped or canceled. In this case, all subsequent early synchronization gaps are skipped or canceled.

[0128] Alternatively, under the assumption that the early synchronization gap is always deactivated, the UE100 sends a notification to the serving gNB200S to activate the early synchronization gap when a CBRA is triggered.

[0129] In step S230, the UE 100 sends a notification to the serving gNB 200S that the early synchronization gap will be activated. In this case, all subsequent early synchronization gaps are activated. Then, in step S240, the UE 100 sends a notification to the serving gNB 200S that the early synchronization gap will be deactivated after the early TA (CBRA) ends. In this case, all subsequent early synchronization gaps are deactivated.

[0130] (4) First Modification Example In the above-described embodiment, particularly the first and third operation patterns, an example in which gap settings are included in the LTM settings has been described. In this case, it is primarily assumed that the LTM candidate gNB200C determines the contents of the gap settings (gap period). However, the serving gNB200S may determine the contents of the gap settings (gap period) without including the gap settings in the LTM settings. The serving gNB200S may notify the UE100 of the gap settings (gap period) determined by itself using an RRC Reconfiguration message.

[0131] Under the assumption that the serving gNB200S determines the content of the gap setting (gap period), the UE100 may send a gap setting request to the serving gNB200S. The request may be sent in an RRC message, a MAC CE, or a UCI. The request may include information indicating the setting content of the gap period requested by the UE100. If the serving gNB200S accepts the request, it may configure the UE100 with a gap according to the request from the UE100 in an RRC Reconfiguration message.

[0132] (5) Second Modification Example As in step S109 of Figure 8, when the LTM candidate gNB200C notifies the serving gNB200S of the TA value, it is required that the LTM candidate gNB200C be able to uniquely identify the UE100. However, since the CBRA RA preamble is randomly selected by the UE100, the LTM candidate gNB200C that receives the CBRA RA preamble cannot uniquely identify the UE100 based on the received RA preamble. Therefore, there is a concern that the LTM candidate gNB200C will not be able to provide the serving gNB200S with the TA value along with the identification information of the UE100.

[0133] Therefore, in this modified example, first, UE100 receives a cell radio network temporary identifier (C-RNTI) assigned to UE100 for the LTM candidate cell from the serving cell (serving gNB200S). Specifically, UE100 receives the C-RNTI (i.e., the C-RNTI for the LTM candidate cell) included in the LTM configuration corresponding to the LTM candidate cell. Second, UE100 transmits a CBRA preamble (RA preamble) used for UL early synchronization in LTM and the C-RNTI to the LTM candidate cell (LTM candidate gNB200C) by message A (MSGA) in a two-step RA procedure (also referred to as "two-step RACH").

[0134] Thus, in this modified example, UE100 uses MSGA in the two-step RA procedure to transmit the CBRA RA preamble to the LTM candidate cell and transmits the C-RNTI for the LTM candidate cell. As a result, the LTM candidate gNB200C (LTM candidate cell) can derive a TA value using the RA preamble and can uniquely identify UE100 using the C-RNTI. Specifically, the LTM candidate gNB200C receives an MSGA including the RA preamble and C-RNTI from UE100, derives a TA value based on the RA preamble, and associates the derived TA value with the C-RNTI.

[0135] Then, the LTM candidate gNB200C can provide the serving gNB200S with the TA value together with the identification information of UE100 (step S109 in FIG. 8). As a result, the serving gNB200S can include the TA value for UE100 in the cell switching command sent to UE100. UE100 receives a cell switching command from the serving gNB200S (serving cell) that includes the TA value and instructs LTM cell switching to the LTM candidate cell.

[0136] Here, an overview of a general two-step RA procedure will be described with reference to Figure 13. The general two-step RA procedure consists of two steps: transmission of message A (MSGA) from UE 100 to gNB 200, and transmission of message B (MSGB) from gNB 200 to UE 100. The two-step RA procedure supports contention-based random access (CBRA) and contention-free random access (CFRA), but this modification uses CBRA.

[0137] MSGA combines the transmission of message 1 (MSG1) and message 3 (MSG3) in the four-step RA procedure into one step. MSG1 is the transmission of an RA preamble (Random Access Preamble) on the PRACH. MSG3 is the transmission of a PUSCH payload (PUSCH payload), specifically, an RRC message. The RRC message is, for example, an RRC Setup Request message. However, in the embodiment, the MSGA transmitted by the UE 100 to the LTM candidate gNB 200C does not necessarily include an RRC message as a PUSCH payload. In the embodiment, the MSGA transmitted by the UE 100 to the LTM candidate gNB 200C only needs to include at least a C-RNTI as a PUSCH payload.

[0138] MSGB is a response message to MSGA. MSGB may include a contention resolution response and / or a fallback instruction. In a typical two-step RA procedure, after transmitting MSGA, UE100 monitors a response from gNB200 within a set window. In the case of CBRA, if contention resolution is successful upon receiving a network response (MSGB), UE100 terminates the random access procedure. On the other hand, if a fallback instruction is received in MSGB, UE100 transmits MSG3 using the UL grant scheduled in the fallback instruction and monitors contention resolution. If contention resolution is not successful after retransmission of MSG3, UE100 returns to MSGA transmission.

[0139] In this modified example, MSGA is used for early TA of LTM. Therefore, UE100 that transmitted MSGA to LTM candidate gNB200C does not need to monitor MSGB (contention resolution) from LTM candidate gNB200C. If the LTM candidate gNB200C can acquire the C-RNTI in the PUSCH payload, it can determine that it has been received without contention, so contention resolution is not required. After transmitting MSGA to the LTM candidate gNB200C (LTM candidate cell), UE100 resumes communication with the serving gNB200S (serving cell) without waiting for the reception of a response message (MSGB) from the LTM candidate cell.

[0140] (6) Other embodiments The operation of the inter-CU LTM according to the above-described embodiments may be applied to intra-CU LTM. In this case, the serving gNB200S and the LTM candidate gNB200C in FIG. 8 become a single gNB200, and the single gNB200 manages the first cell and the second cell.

[0141] The LTM in the above-described embodiments may be interpreted as a conditional LTM. For example, the above-described MCG LTM of an inter-CU or intra-CU may be a conditional LTM of the MCG of an inter-CU or intra-CU. Furthermore, the above-described SCG LTM of an intra-CU may be a conditional LTM of the SCG of an intra-CU. In conditional LTM, for example, the RRC Reconfiguration message in step S4 of FIG. 6 includes information indicating the execution conditions (e.g., radio quality conditions) of LTM cell switching for each LTM candidate cell. The UE 100 performs LTM cell switching to an LTM candidate cell that satisfies the preset execution conditions (radio quality conditions) instead of the cell switching command MAC transmitted from the gNB 200. This eliminates the need to transmit and receive an L1 measurement report and a cell switching command MAC CE, enabling faster LTM cell switching. Under such a premise, when UE100 starts UL early synchronization (early TA) in response to satisfying a preset execution condition (radio quality condition), it transmits a CBRA RA preamble to the LTM candidate cell during the set gap period.

[0142] 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.

[0143] 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.

[0144] 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.

[0145] 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).

[0146] The functions performed by the above-described communication device (such as the UE 100 or the gNB 200) may be implemented in circuitry or processing circuitry, including general-purpose processors, application-specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), a CPU (Central Processing Unit), 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 be a programmed processor that executes a program stored in a memory. In this specification, a circuitry, unit, or means is hardware that is programmed to realize or executes a described function. The hardware may be any hardware disclosed herein or any hardware known to be programmed to realize or execute the described function. If the hardware is a processor, which is considered to be a type of circuitry, the circuitry, means, or unit is a combination of hardware and software used to configure the hardware and / or processor.

[0147] 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, as used in this disclosure, the term "or" is not intended to mean an exclusive or. Furthermore, as used in this disclosure, any reference to an element using a designation such as "first," "second," etc. 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 are intended to include the plural unless the context clearly indicates otherwise.

[0148] 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.

[0149] This application claims priority from Japanese Patent Application No. 2024-132052 (filed August 8, 2024), the entire contents of which are incorporated herein by reference.

[0150] (7) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0151] Supplementary Note 1: A communication method for switching a serving cell of a user equipment from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the communication method comprising: receiving, from the first cell, a configuration of a gap period for performing, with the second cell, a contention-based random access (CBRA) used for uplink early synchronization in the LTM; and performing, with the second cell, the CBRA in the gap period based on the configuration, the user equipment; and performing the CBRA including transmitting an RA preamble for the CBRA to the second cell.

[0152] Supplementary Note 2: The communication method according to Supplementary Note 1, wherein performing the CBRA further includes receiving a response to the RA preamble from the second cell during the gap period.

[0153] Supplementary Note 3: The communication method according to Supplementary Note 1 or 2, wherein the configuration is a configuration of a measurement gap for performing measurements on a cell different from the first cell, the configuration of the measurement gap includes notification information indicating that the measurement gap can be used for the uplink early synchronization, and the user equipment uses the measurement gap as the gap period based on the notification information.

[0154] Supplementary Note 4: The communication method according to Supplementary Note 1 or 2, wherein the setting is a setting of a MUSIM gap for a MUSIM (Multi-Universal Subscriber Identity Module) operation, the setting of the MUSIM gap includes notification information indicating that the MUSIM gap can be used for the uplink early synchronization, and the user equipment uses the MUSIM gap as the gap period based on the notification information.

[0155] Supplementary Note 5: The communication method according to Supplementary Note 1 or 2, wherein the setting is a setting of an early synchronization gap used exclusively for the uplink early synchronization, and the user equipment uses the early synchronization gap as the gap duration.

[0156] Supplementary Note 6. The communication method according to Supplementary Note 5, wherein the user equipment transmits a notification of activation of the early synchronization gap and / or a notification of deactivation of the early synchronization gap to the first cell.

[0157] Supplementary Note 7: A user equipment that switches a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), comprising: a receiver that receives, from the first cell, a setting of a gap period for performing, for the second cell, a contention-based random access (CBRA) method used for uplink early synchronization in the LTM; a controller that performs the CBRA method on the second cell during the gap period based on the setting; and a transmitter that transmits an RA preamble to the second cell during the CBRA method.

[0158] Supplementary Note 8: A network node that manages a first cell in a mobile communication system in which a serving cell of a user equipment is switched from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the network node comprising: a transmitter that transmits to the user equipment a setting of a gap period in which the user equipment performs contention-based random access (CBRA) used for uplink early synchronization in the LTM with the second cell.

[0159] 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 for switching a serving cell of a user equipment from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), comprising: the first network node receiving a HANDOVER REQUEST ACKNOWLEDGE message from the second network node, the HANDOVER REQUEST ACKNOWLEDGE message including an LTM candidate cell configuration; and the HANDOVER REQUEST ACKNOWLEDGE message including information regarding uplink early synchronization configuration as information separate from the LTM candidate cell configuration.

2. A communication method for switching a serving cell of a user equipment from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the communication method comprising: the user equipment receiving, from the first cell, a setting of a gap period for performing contention-based random access (CBRA) for uplink early synchronization in the LTM with the second cell; and the user equipment performing the CBRA with the second cell during the gap period based on the setting, wherein performing the CBRA includes transmitting an RA preamble for the CBRA to the second cell.

3. The communication method according to claim 2, wherein performing the CBRA further comprises receiving a response to the RA preamble from the second cell during the gap period.

4. The communication method described in claim 2, wherein the setting is a measurement gap setting for performing measurements on a cell different from the first cell, the measurement gap setting includes notification information indicating that the measurement gap can be used for the uplink early synchronization, and the user equipment uses the measurement gap as the gap period based on the notification information.

5. The communication method according to claim 2, wherein the setting is a MUSIM gap setting for MUSIM (Multi-Universal Subscriber Identity Module) operation, the MUSIM gap setting includes notification information indicating that the MUSIM gap can be used for the uplink early synchronization, and the user equipment uses the MUSIM gap as the gap period based on the notification information.

6. The communication method according to claim 2, wherein the setting is a setting of an early synchronization gap used exclusively for the uplink early synchronization, and the user equipment uses the early synchronization gap as the gap duration.

7. The communication method of claim 6, wherein the user equipment transmits a notification of activation of the early synchronization gap and / or a notification of deactivation of the early synchronization gap to the first cell.

8. A user equipment that switches a serving cell from a first cell to a second cell using LTM (L1 / L2 Triggered Mobility), comprising: a receiving unit that receives, from the first cell, a setting of a gap period for performing contention-based random access (CBRA) for uplink early synchronization in the LTM on the second cell; a control unit that performs the CBRA on the second cell during the gap period based on the setting; and a transmitting unit that transmits an RA preamble to the second cell during the CBRA.

9. A network node that manages a first cell in a mobile communication system in which a serving cell of a user equipment is switched from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the network node having a transmitter that transmits to the user equipment a setting of a gap period for the user equipment to perform contention-based random access (CBRA) to the second cell, the gap period being used for uplink early synchronization in the LTM.

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