Communication method, user equipment, and network node

Abstract

This communication method comprises: user equipment, which is in a radio resource control (RRC) connected state in a first cell, interrupting communication with the first cell, and transmitting a random access (RA) preamble for uplink early synchronization with a second cell serving as a candidate for cell switching in LTM to the second cell; the user equipment receiving, from the second cell, a random access response (RAR) including information relating to a timing advance (TA) value to be applied to uplink transmission to the second cell; and the user equipment transmitting, in response to the reception of the RAR, uplink signaling for resuming the communication with the first cell to the first cell using a physical uplink shared channel (PUSCH) resource, a physical uplink control channel (PUCCH) resource, or a physical random access channel (PRACH) resource.

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 V18.1.0 (2024-03)"

[0005] This disclosure provides techniques for improving LTM.

[0006] A communication method according to a first aspect of the present disclosure is a communication method used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), the method including: a user equipment in a Radio Resource Control (RRC) Connected state in a first cell suspending communication with the first cell and transmitting, to the second cell, a random access (RA) preamble for uplink early synchronization with a second cell that is a candidate for cell switching of the LTM; and, in response to communication with the first cell being enabled, transmitting uplink signaling to the first cell for resuming communication with the first cell, using a Physical Uplink Shared Channel (PUSCH) resource, a Physical Uplink Control Channel (PUCCH) resource, or a Physical Random Access Channel (PRACH) resource.

[0007] A user equipment according to a second aspect of the present disclosure is a user equipment used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), and includes: a control unit that, when in a Radio Resource Control (RRC) Connected state in a first cell, suspends communication with the first cell and transmits, to the second cell, a random access (RA) preamble for uplink early synchronization with a second cell that is a candidate for cell switching of the LTM; and a transmission unit that, when communication with the first cell becomes possible, transmits uplink signaling for resuming communication with the first cell to the first cell using a Physical Uplink Shared Channel (PUSCH) resource, a Physical Uplink Control Channel (PUCCH) resource, or a Physical Random Access Channel (PRACH) resource.

[0008] A network node according to a third aspect of the present disclosure is a network node that manages a first cell in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), and includes: a control unit that suspends communication with a first cell when a user equipment in a Radio Resource Control (RRC) Connected state in the first cell transmits a random access (RA) preamble for uplink early synchronization with a second cell that is a candidate for cell switching of the LTM to the second cell; and a receiving unit that receives, from the user equipment, uplink signaling for resuming communication with the first cell, using a physical uplink shared channel (PUSCH) resource, a physical uplink control channel (PUCCH) resource, or a physical random access channel (PRACH) resource in response to the user equipment being able to communicate with the first cell.

[0009] 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 procedure for intra-CU LTM. FIG. 7 is a diagram showing a CFRA procedure in early synchronization executed in step S6 of FIG. 6. FIG. 8 is a diagram showing an example of a procedure for performing UL early synchronization by CFRA in inter-CU LTM. FIG. 9 is a diagram showing an example of a procedure for performing UL early synchronization by CFRA in inter-CU LTM. FIG. 10 is a diagram showing an operation of a UE according to the first embodiment. FIG. 11 is a diagram showing an example of information included in a PDCCH order (DCI Format 1_0) used in a first operation pattern of the first embodiment. FIG. 12 is a diagram showing an example of information included in an RRC message (RRC Reconfiguration message) used in a second operation pattern of the first embodiment. FIG. 13 is a diagram showing an example of an operation according to the first operation pattern of the second embodiment. FIG. 10 is a diagram showing an operation example according to a third operation pattern of the second embodiment. FIG. 11 is a diagram showing an operation example according to a first operation pattern of the third embodiment. FIG. 12 is a diagram showing an operation example according to a second operation pattern of the third embodiment. FIG. 13 is a diagram showing an operation example according to a third operation pattern of the third embodiment. FIG. 14 is a diagram showing an operation example according to a fourth operation pattern of the third embodiment. FIG. 15 is a diagram showing an operation example of a conditional LTM according to the embodiment. FIG. 16 is a diagram showing an operation example of a conditional LTM according to the embodiment.

[0010] 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.

[0011] (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.

[0012] 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.

[0013] 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).

[0014] 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").

[0015] 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.

[0016] The 5GC20 includes an AMF (Access and Mobility Management Function) and a UPF (User Plane Function) 300. The AMF performs various mobility controls for the 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.

[0017] 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.

[0018] The receiving unit 110 performs various types of reception under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 130.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] The receiving unit 220 performs various types of reception under the control of the control unit 230. The receiving unit 220 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 230.

[0024] 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.

[0025] 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.

[0026] FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data.

[0027] 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.

[0028] The PHY layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layer of UE100 and the PHY layer of gNB200 via a physical channel. The PHY layer of UE100 receives downlink control information (DCI) transmitted from gNB200 on 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.

[0029] 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.

[0030] The RLC layer transmits data to the receiving RLC layer using the functions of the MAC layer and PHY layer. Data and control information are transmitted between the RLC layer of the UE 100 and the RLC layer of the gNB 200 via a logical channel.

[0031] The PDCP layer performs header compression / decompression, encryption / decryption, and the like.

[0032] 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.

[0033] 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).

[0034] 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.

[0035] 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.

[0036] 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").

[0037] (2) Overview of LTM The mobile communication system 1 according to this embodiment supports LTM (L1 / L2-triggered mobility).

[0038] In a typical handover procedure, 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.

[0039] 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.

[0040] 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.

[0041] Secondly, the UE 100 performs a synchronization process with the LTM candidate cell by early synchronization (Early sync).

[0042] 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.

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

[0044] 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.

[0045] The following principles apply to LTM:

[0046] 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.

[0047] 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.

[0048] The user plane continues without a reset if configured in RRC signaling to avoid additional delays in data recovery.

[0049] - Security is not updated in LTM.

[0050] LTM between subsequent LTM candidate cell configurations can be performed without RRC reconfiguration, i.e., the UE 100 does not release other LTM candidate cell configurations after LTM is triggered.

[0051] 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.

[0052] 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".

[0053] In step S1, UE100 is in an RRC connected state in the cell (first cell) of gNB200.

[0054] 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.

[0055] In step S3, gNB200 decides to use LTM based on the Measurement Report message and starts preparing an LTM candidate cell.

[0056] In step S4, the gNB 200 (first cell) transmits an RRC message, specifically an RRC Reconfiguration message, to the UE 100, 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, such as a contention-free random access (CFRA) configuration, used to transmit an RA preamble to the corresponding LTM candidate cell. 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.

[0057] In step S5, UE100 saves the LTM candidate cell setting and sends an RRC Reconfiguration Complete message to gNB200 (first cell).

[0058] 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. 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 CFRA triggered by a 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. 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).

[0059] 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.

[0060] 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).

[0061] In step S8, gNB200 decides to switch the serving cell to the target cell (second cell).

[0062] 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).

[0063] 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).

[0064] 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).

[0065] 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.

[0066] (3) Operation of the Mobile Communication System FIG. 7 is a diagram showing the CFRA procedure in early synchronization executed in step S6 of FIG.

[0067] In step S21, gNB200 (first cell, Serving cell) transmits a PDCCH order to UE100 instructing the second cell (LTM candidate cell, Candidate cell) to transmit a CFRA RA preamble. UE100 receives the PDCCH order from the first cell of gNB200. The PDCCH order is also referred to as an RA Preamble assignment.

[0068] In step S22, in response to receiving the PDCCH order from the first cell of the gNB 200, the UE 100 transmits an RA preamble to the second cell of the gNB 200. The second cell of the gNB 200 receives the RA preamble. The gNB 200 derives a TA value that the UE 100 should apply to the second cell based on the received RA preamble. The gNB 200 notifies the UE 100 of the derived TA value using the cell switching command MAC CE in step S9 of FIG. 6.

[0069] Thus, in the CFRA random access procedure in LTM UL early synchronization, the gNB 200 does not transmit a random access response (RAR) to the UE 100 in response to the RA preamble transmission. However, the LTM introduced in 3GPP Release 18 only supports intra-CU and does not support inter-CU (i.e., between different gNBs 200) LTM. In other words, 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).

[0070] Meanwhile, inter-CU LTM is scheduled to be newly introduced in 3GPP Release 19. In inter-CU LTM, the UE 100 performs LTM cell switching from a first cell of one gNB 200 to a second cell of another gNB 200.

[0071] Figure 8 is a diagram showing an example of a procedure for performing UL early synchronization using CFRA in inter-CU LTM. Each gNB200 (gNB200a, gNB200b) shown in Figure 8 is connected to each other by an Xn interface, which is an interface between network nodes (or, from another perspective, an interface between CUs). In the following embodiment, communication between gNBs 200 is assumed to be performed over the Xn interface. Each gNB200 (gNB200a, gNB200b) shown in Figure 8 is functionally divided into a CU and a DU. In the following embodiment, communication between a CU and a DU within the same gNB200 is assumed to be performed over an F1 interface, which is a fronthaul interface.

[0072] In step S21a, the DU (first cell, Serving cell) of gNB200a transmits a PDCCH order (RA Preamble assignment) to the UE 100, instructing the UE 100 to transmit a CFRA RA preamble to the second cell (LTM candidate cell, Candidate cell) of gNB200b. The UE 100 receives the PDCCH order from the first cell of the DU of gNB200a.

[0073] In step S22a, UE100 transmits an RA preamble to the second cell of the DU of gNB200b in response to receiving a PDCCH order from the first cell of the DU of gNB200a. The second cell of the DU of gNB200b receives the RA preamble. The DU of gNB200b derives a TA value that UE100 should apply to the second cell based on the received RA preamble.

[0074] In step S23, the DU of gNB200b notifies the CU of gNB200b of the derived TA value.

[0075] In step S24, the CU of gNB200b notifies the CU of gNB200a of the TA value notified in step S23.

[0076] In step S25, the CU of gNB200a notifies the DU of gNB200a of the TA value notified in step S24.

[0077] In step S26, the DU of gNB200a notifies the UE100 of the TA value notified in step S25 by a cell switching command MAC CE.

[0078] Thus, when attempting to include a TA value in the cell switch command MAC CE in inter-CU LTM, a delay in the transfer of the TA value occurs from step S23 to step S25. Therefore, there is a possibility that the transfer of the TA value will not be in time for the DU of gNB200a to decide to transmit the cell switch command MAC CE based on the L1 measurement report. Therefore, if the DU of gNB200a waits for the transfer of the TA value before transmitting the cell switch command MAC CE to UE100, there is a problem that it cannot cause UE100 to perform LTM cell switching at the appropriate time. In addition, gNB200a (first cell, serving cell) cannot perform data communication with UE100 while UE100 is performing a random access procedure with gNB200b (second cell, LTM candidate cell). The gNB 200a must receive a TA notification from the gNB 200b to know whether the random access procedure has been completed, but even during this period, it cannot resume data communication with the UE 100. In other words, there is a problem in that data communication with the UE 100 must be stopped in the section from step S22a to S24 (or S25).

[0079] In addition, conditional LTM is scheduled to be newly introduced in 3GPP Release 19.

[0080] 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. Instead of the cell switch command MAC transmitted from the gNB 200, the UE 100 performs LTM cell switching to an LTM candidate cell that satisfies the preset execution conditions (radio quality conditions). This makes it possible to eliminate the need to send and receive an L1 measurement report and a cell switch command MAC, and may enable high-speed LTM cell switching.

[0081] In this way, the conditional LTM does not require the cell switching command MAC CE. However, there is a problem in that a method for notifying the UE 100 of the TA value is unknown.

[0082] Therefore, in the embodiment, for UL early synchronization in inter-CU LTM and / or conditional LTM, it is assumed that the gNB200 (DU) that has received an RA preamble from the UE100 transmits an RAR including a TA value derived based on the RA preamble to the UE100. Note that the conditional LTM may be an intra-CU conditional LTM. The conditional LTM may also be an inter-CU conditional LTM.

[0083] 9 is a diagram showing an example of a procedure for performing UL early synchronization using CFRA in inter-CU LTM. In the illustrated example, an inter-CU LTM scenario is shown, but an inter-CU conditional LTM scenario may also be used. Alternatively, an intra-CU conditional LTM scenario may also be used. In this case, the gNB200a and gNB200b in FIG. 9 may be replaced with the same gNB200. UL early synchronization using CBRA may also be used. When CBRA is used, the RA preamble assignment (step S31) does not need to be transmitted.

[0084] In step S31, the DU (first cell, Serving cell) of gNB200a transmits a PDCCH order (RA Preamble assignment) to the UE 100, instructing the UE 100 to transmit a CFRA RA preamble to the second cell (LTM candidate cell, Candidate cell) of gNB200b. The UE 100 receives the PDCCH order from the first cell of the DU of gNB200a.

[0085] In step S32, UE100 transmits an RA preamble to the second cell of the DU of gNB200b in response to receiving the PDCCH order from the first cell of the DU of gNB200a. The second cell of the DU of gNB200b receives the RA preamble. The DU of gNB200b derives a TA value that UE100 should apply to the second cell based on the received RA preamble.

[0086] In step S33, the DU of the gNB 200b notifies the UE 100 of the derived TA value by RAR. The UE 100 associates the TA value acquired by the RAR with the cell ID of the second cell and stores it. When performing LTM cell switching to the second cell, the UE 100 performs uplink transmission to the second cell using the stored TA value.

[0087] Such an operation can solve the above-mentioned problems in inter-CU LTM and / or conditional LTM.

[0088] (3.1) First Embodiment of Inter-CU LTM and / or Conditional LTM A first embodiment of the inter-CU LTM and / or conditional LTM will be described. The first embodiment is based on the operation shown in FIG.

[0089] In the conventional LTM, since it is not assumed that an RAR is returned in response to the RA preamble transmission, there is a possibility that the UE 100 does not wait for the RAR. Therefore, even if the RAR is transmitted from the second cell to the UE 100, there is a concern that the UE 100 will not be able to receive the RAR because it does not monitor the RAR. In the first embodiment, with regard to UL early synchronization in the inter-CU LTM and / or the conditional LTM, an operation for enabling the UE 100 to properly receive the RAR from the second cell will be described.

[0090] FIG. 10 is a diagram showing the operation of the UE 100 according to the first embodiment.

[0091] In step S51, UE 100 in an RRC connected state in the first cell receives downlink signaling related to LTM from the first cell. In the first embodiment, the downlink signaling includes a notification indicating that the second cell (LTM candidate cell) will transmit (reply) an RAR in response to the RA preamble transmission. The notification may be information that configures or requests UE 100 to perform a reception process (monitoring) of the RAR from the second cell after transmitting the RA preamble. The notification may be information that configures or requests UE 100 to perform CBRA (Collision Based Random Access) with the second cell (LTM candidate cell). In the case of a CBRA procedure, UE 100 waits for an RAR after transmitting the RA preamble, as in the conventional case.

[0092] In step S52, the UE 100 transmits an RA preamble for UL early synchronization with the second cell, which is a candidate for cell switching in LTM, to the second cell based on the downlink signaling in step S51. Here, in response to receiving the notification in step S51, the UE 100 performs a receiving process (monitoring) of the RAR from the second cell after transmitting the RA preamble.

[0093] In step S53, the UE 100 receives an RAR from the second cell based on the downlink signaling (notification) in step S51, the RAR including information on a TA value to be applied to uplink transmission to the second cell. The UE 100 associates the TA value acquired by the RAR with the cell ID of the second cell and stores the associated TA value. When performing LTM cell switching to the second cell, the UE 100 performs uplink transmission to the second cell using the stored TA value.

[0094] According to this type of operation, UE100 can determine whether or not an RAR will be returned from the second cell in response to the RA preamble transmission based on the downlink signaling received from the first cell, so UE100 can wait for (monitor) the RAR from the second cell, and UE100 can receive the RAR more reliably.

[0095] The UE 100 that performs such an operation includes a receiver 110 that receives downlink signaling from the first cell when the UE 100 is in an RRC connected state in the first cell, the receiver 110 including a notification indicating that the second cell will transmit (reply) an RAR in response to the RA preamble transmission, and a transmitter 120 that transmits an RA preamble to the second cell for uplink early synchronization with the second cell that is a candidate for LTM cell switching based on the downlink signaling (see FIG. 2). The receiver 110 receives an RAR from the second cell that includes information on the TA value to be applied to uplink transmission to the second cell. Meanwhile, the gNB 200 includes a transmitter 210 that transmits downlink signaling to the UE 100 including a notification indicating that the second cell will transmit (reply) an RAR in response to the RA preamble transmission (see FIG. 3).

[0096] In the first embodiment, the first cell and the second cell may be managed by different gNBs 200. That is, the operation of FIG. 10 may be applied to an inter-CU LTM scenario.

[0097] In the first embodiment, the LTM to which the operation of FIG. 10 is applied may be a conditional LTM.

[0098] In the first operation pattern of the first embodiment, the downlink signaling in step S51 is a PDCCH order (RA Preamble assignment) instructing the second cell to transmit an RA preamble of CFRA. In response to the PDCCH order from the first cell including a notification (a notification indicating that the second cell will transmit (reply to) the RAR in response to the RA preamble transmission), the UE 100 may receive the RAR from the second cell (step S53).

[0099] 11 is a diagram showing an example of information included in a PDCCH order (DCI Format 1_0) used in the first operation pattern of the first embodiment. The UE 100 receives DCI Format 1_0 having a CRC scrambled with the C-RNTI from the first cell.

[0100] The PDCCH order used in the first operation pattern of the first embodiment includes a notification indicating that the second cell (LTM candidate cell) will transmit (reply) an RAR in response to the RA preamble transmission. The notification may indicate that the LTM candidate cell indicated by the following "Cell indicator" will transmit an RAR. The notification may be 1-bit flag information that is set to "True (1)" when an RAR is to be transmitted (reply) in response to the RA preamble transmission, and that is set to "False (0)" when an RAR is not to be transmitted (reply) in response to the RA preamble transmission.

[0101] The PDCCH order used in the first operation pattern of the first embodiment may further include at least one of the following information (fields):

[0102] Frequency domain resource assignment If the CRC of DCI Format 1_0 is scrambled by the C-RNTI and the "Frequency domain resource assignment" field is all ones, the DCI Format 1_0 is used for a random access procedure initiated by a PDCCH order.

[0103] Random access preamble index This is the preamble index of the RA preamble.

[0104] SSB index: This is the index of the SSB used to determine the RACH occasion in RA preamble transmission (PRACH transmission).

[0105] PRACH Mask index This is an index indicating the RACH occasion associated with the SSB indicated by the "SSB index" for PRACH transmission.

[0106] Cell indicator: This is an indicator indicating the cell of the corresponding PRACH transmission when the higher layer parameter EarlyUlSyncConfig is set in the UE 100. When "Cell indicator" is "0", it is mapped to the serving cell, and other indexes are mapped to the LTM candidate cells set in EarlyUlSyncConfig in ascending order of the LTM candidate cell identifiers set in RRC (ltm-CandidateId).

[0107] PRACH retransmission indicator This is an indicator that shows whether the PRACH transmission is an initial transmission or a retransmission.

[0108] In the second operation pattern of the first embodiment, the downlink signaling in step S51 is an RRC message including a candidate cell setting (LTM candidate cell setting) for the second cell. The RRC message may be the RRC Reconfiguration message in step S4 of Fig. 6. In response to the candidate cell setting from the first cell including a notification (a notification indicating that the second cell will transmit (reply) an RAR in response to the RA preamble transmission), the UE 100 may receive an RAR from the second cell (step S53).

[0109] FIG. 12 is a diagram illustrating an example of information included in an RRC message (RRC Reconfiguration message) used in the second operation pattern of the first embodiment.

[0110] In the illustrated example, the RRC Reconfiguration message sent by the first cell (gNB 200) to the UE 100 includes an LTM candidate cell setting list (ltm-CandidateToAddModList). The LTM candidate cell setting list (ltm-CandidateToAddModList) includes one or more LTM candidate cell settings (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 (Channel State Information-Reference Signal) resource configuration (NZP-CSI-RS-Resource), a TCI state configuration (TCI-State), a UE-based TA measurement configuration (ltm-UE-MeasuredTA), etc. At least one of the parameters other than the RRC configuration (RRCReconfiguration) may be included in the RRC configuration (RRCReconfiguration).

[0111] In the second operation pattern of the first embodiment, the LTM candidate cell configuration (LTM-Candidate) includes a notification indicating that an RAR will be transmitted (reply) in response to the RA preamble transmission when the corresponding LTM candidate cell transmits an RAR in response to the RA preamble transmission (for example, when the corresponding LTM candidate cell is an inter-CU cell and / or when the corresponding LTM candidate cell is a conditional LTM LTM candidate cell). The notification may be included in the early UL synchronization configuration (EarlyUL-SyncConfig) or the RRC configuration (RRCReconfiguration). The notification may be 1-bit flag information that is set to "True (1)" when an RAR is transmitted (reply) in response to the RA preamble transmission, and that is set to "False (0)" when an RAR is not transmitted (reply) in response to the RA preamble transmission. In addition, the UE 100 may perform CBRA on the LTM candidate cell in response to the notification (setting) (S52 in FIG. 10). The UE 100 may perform RA preamble transmission in the CBRA procedure when the UE 100 receives a PDCCH order (S51 in FIG. 10).

[0112] In the case of inter-CU LTM, at least some of the information in the LTM candidate cell setting (LTM-Candidate) is generated by the gNB200b that manages the LTM candidate cell (second cell) and is notified to the UE100 from the gNB200b via the gNB200a that manages the serving cell (first cell). Therefore, the gNB200a (first network node) that manages the first cell may receive a notification from the gNB200b (second network node) that manages the second cell indicating that an RAR will be transmitted (reply) in response to the RA preamble transmission. The gNB200a may transmit downlink signaling (RRC message) including the notification received from the gNB200b to the UE100.

[0113] In the third operation pattern of the first embodiment, the downlink signaling in step S51 may be an RRC message including a setting for inter-CU LTM cell switching or a setting for conditional LTM cell switching. The RRC message may be the RRC Reconfiguration message in step S4 of Fig. 6. The UE 100 may regard the setting as a notification indicating that an RAR will be transmitted (a reply) in response to the RA preamble transmission, and may receive the RAR from the second cell (step S53).

[0114] In the third operation pattern of the first embodiment, in the RRC message (RRC Reconfiguration message) used in the second operation pattern of the first embodiment, the LTM candidate cell setting (LTM-Candidate) includes information indicating inter-CU LTM when the corresponding LTM candidate cell is an inter-CU cell. The LTM candidate cell setting (LTM-Candidate) includes information indicating conditional LTM when the corresponding LTM candidate cell is an LTM candidate cell for conditional LTM. The UE 100 may regard this information as a notification indicating that an RAR will be transmitted (reply) in response to the RA preamble transmission, and receive an RAR from the corresponding LTM candidate cell. In this way, in the third operation pattern of the first embodiment, the LTM candidate cell implicitly notifies the UE 100 that it will transmit (reply) an RAR in response to the RA preamble transmission.

[0115] In the first embodiment, the gNB 200 (gNB 200a) that manages the first cell may transmit to the UE 100 information indicating the maximum waiting time (also referred to as the "RAR reception window") for waiting to receive an RAR after transmitting an RA preamble. The UE 100 may receive information indicating the maximum waiting time (RAR reception window) from the first cell and perform reception processing (monitoring) of the RAR within the RAR reception window. The information indicating the maximum waiting time (RAR reception window) may be included in the RRC Reconfiguration message (for example, LTM candidate cell setting (LTM-Candidate)) of step S4 of FIG. 6. The UE 100 may regard the information indicating the maximum waiting time (RAR reception window) as a notification indicating that an RAR will be transmitted (reply) in response to the RA preamble transmission, and may receive an RAR from the corresponding LTM candidate cell.

[0116] When the UE 100 does not receive the RAR from the second cell (LTM candidate cell) within the RAR reception window, the UE 100 may transmit the RA preamble to the second cell again. This allows the UE 100 to try to receive the RAR from the second cell again.

[0117] Alternatively, when the UE 100 does not receive an RAR from the second cell (LTM candidate cell) within the RAR reception window, the UE 100 may perform a random access procedure for the second cell at the time of LTM cell switching to the second cell. That is, when the UE 100 does not receive an RAR from the second cell (LTM candidate cell) within the RAR reception window, the UE 100 may consider that UL early synchronization has failed and perform RACH-based LTM cell switching.

[0118] Alternatively, if UE100 does not receive an RAR from the second cell (LTM candidate cell) within the RAR reception window, UE100 may transmit failure information to the first cell. The failure information may include information (e.g., cell ID or configuration ID) of the LTM candidate cell from which the RAR was not received. The gNB200 (gNB200a) managing the first cell can determine the failure of RAR reception at UE100 based on the failure information. In this case, gNB200 (gNB200a) may transmit a PDCCH order to UE100 again. Alternatively, if UE100 does not receive an RAR from the second cell (LTM candidate cell) within the RAR reception window, UE100 may start PDCCH standby in the first cell. In other words, UE100 terminates the random access procedure with the second cell and resumes communication in the first cell.

[0119] (3.2) Second Embodiment of Inter-CU LTM and / or Conditional LTM The second embodiment of the inter-CU LTM and / or conditional LTM will be described, focusing mainly on the differences from the first embodiment described above. The first embodiment is an embodiment based on the operation shown in FIG. 9 . The second embodiment can be implemented in combination with the operation of the first embodiment described above. However, the second embodiment may be based on the first embodiment described above. The second embodiment does not have to be based on the operation of the first embodiment described above.

[0120] In the operation of FIG. 9 , UE 100 receives a PDCCH order (RA Preamble assignment) from a first cell (Serving cell) in step S31, transmits an RA preamble to a second cell (Candidate cell) in step S32, receives an RAR including a TA value from the second cell (Candidate cell) in step S33, saves the TA value, and resumes communication with the first cell (Serving cell).

[0121] Here, in the period from step S32 to step S33, the UE 100 is synchronized with the second cell (Candidate cell) and is not synchronized with the first cell (Serving cell), and therefore, there is a possibility that the UE 100 cannot communicate with the first cell (Serving cell). In particular, it is difficult for the UE 100 to perform downlink (DL) reception from the first cell (Serving cell) in the period from step S32 to step S33. Therefore, in the second embodiment, when the first cell (Serving cell) transmits the RA Preamble assignment, the first cell (Serving cell) does not perform (i.e., suspends) DL transmission to the UE 100.

[0122] When step S33 is completed, UE 100 synchronizes with the first cell (serving cell) and becomes able to communicate with the first cell (serving cell), but the first cell (serving cell) cannot grasp the timing when step S33 is completed (i.e., the timing when UE 100 receives the RAR). Therefore, there is a problem that it is not possible to determine the optimal timing when DL transmission from the first cell (serving cell) to UE 100 should be resumed.

[0123] In the following first to third operation patterns of the second embodiment, operations for solving such problems will be described.

[0124] (3.2.1) First Operation Pattern of the Second Embodiment Figure 13 is a diagram showing an example of operation according to the first operation pattern of the second embodiment. In the illustrated example, an inter-CU LTM scenario is shown, but it may also be an inter-CU conditional LTM scenario. Alternatively, it may also be an intra-CU conditional LTM scenario. In this case, the gNB200a and gNB200b in Figure 9 can be read as the same gNB200.

[0125] First, an overview of the operation shown in FIG. 13 will be described.

[0126] UE100 in the RRC connected state in a first cell (Serving cell) suspends communication with the first cell and transmits an RA preamble for uplink early synchronization with a second cell (Candidate cell) that is a candidate for cell switching in LTM to the second cell (step S63). UE100 receives an RAR from the second cell that includes information on a TA value to be applied to uplink transmission to the second cell (step S64). In response to receiving the RAR, UE100 transmits uplink signaling to the first cell to resume communication with the first cell (step S65).

[0127] According to such an operation, the gNB 200a managing the first cell can determine, based on the uplink signaling of step S65, the optimal timing for resuming DL transmission to the UE 100. As a result, the UE 100 can quickly resume communication with the first cell.

[0128] The uplink signaling of step S65 may be a request to resume communication with the first cell. The uplink signaling may be a notification (completion notification) indicating that an RAR has been received from the second cell. Here, the UE 100 may transmit (notify) the uplink signaling by including the TA value included in the RAR received from the second cell in step S64. The uplink signaling of step S65 may be a medium access control element (MAC CE), an RRC message, or uplink control information (UCI). The RRC message may be, for example, a UE Assistance Information message. The uplink signaling of step S65 may include the cell ID (PhysCellId) or configuration ID (LTM-CandidateId) of the second cell.

[0129] Next, the operation shown in FIG. 13 will be described in detail.

[0130] In step S61, the DU (first cell, Serving cell) of the gNB200a transmits a PDCCH order (RA Preamble assignment) to the UE100 to instruct (trigger) the transmission of a CFRA RA preamble to the second cell (LTM candidate cell, Candidate cell) of the gNB200b. The DU of the gNB200a may notify the CU of the gNB200a of the transmission. The UE100 receives the PDCCH order from the first cell of the DU of the gNB200a.

[0131] In step S62, the DU (first cell, Serving cell) of gNB200a suspends communication (particularly DL communication) with UE100 in response to the transmission of the PDCCH order in step S61. The DU of gNB200a may notify the CU of gNB200a of the suspension.

[0132] In step S63, in response to receiving the PDCCH order in step S61, UE100 synchronizes (DL synchronizes) with the second cell of the DU of gNB200b and transmits an RA preamble to the second cell of the DU of gNB200b. Also, UE100 suspends communication with the first cell of the DU of gNB200a. Note that suspending communication may mean stopping PDCCH monitoring of the first cell, or may mean abandoning synchronization with the first cell. The second cell of the DU of gNB200b receives the RA preamble. The DU of gNB200b derives a TA value that UE100 should apply to the second cell based on the received RA preamble. The DU of gNB200a may notify the CU of gNB200a of the reception of the RA preamble.

[0133] In step S64, the DU of the gNB 200b notifies the UE 100 of the derived TA value by RAR. The UE 100 associates the TA value acquired by the RAR with the cell ID of the second cell and stores it. Note that when performing LTM cell switching to the second cell, the UE 100 performs uplink (UL) transmission to the second cell using the stored TA value.

[0134] In step S65, in response to receiving the RAR in step S64, the UE 100 synchronizes (DL synchronizes) the DU of the gNB 200a with the first cell and / or starts PDCCH monitoring of the first cell, and transmits UL signaling to the first cell to resume communication with the first cell. The DU of the gNB 200a receives the UL signaling. The DU of the gNB 200a may notify the CU of the gNB 200a of the reception of the UL signaling.

[0135] Here, the UE 100 may transmit the UL signaling (UCI) on the PUCCH using a PUCCH resource previously allocated in the first cell. The UE 100 may transmit a scheduling request (SR) to the first cell, transmit a buffer status report (BSR) to the first cell using a PUSCH resource allocated from the first cell in accordance with the SR, and transmit the UL signaling (MAC CE or RRC message) to the first cell using the PUSCH resource allocated from the first cell in accordance with the BSR. Alternatively, the UE 100 may transmit the UL signaling (MAC CE or RRC message) to the first cell using a (periodic) PUSCH resource allocated by a configured grant (CG) from the first cell. The UE 100 may receive a PDCCH (UL Grant) from the first cell and transmit the UL signaling using the resource. In this case, the first cell may blindly transmit a UL Grant (Dynamic Grant) around the timing when it is expected that communication with UE 100 can be resumed (i.e., the timing when it is expected that UE 100 will receive RAR in S64).

[0136] In step S66, the DU of gNB200a resumes communication (DL communication) with UE100 in the first cell. For example, the DU of gNB200a (first cell) allocates PDSCH resources to UE100 via PDCCH and transmits DL data to UE100 on the PDSCH. In response to the transmission of the UL signaling in step S65, UE100 resumes monitoring the PDCCH of the first cell and receives DL data on the PDSCH of the first cell.

[0137] (3.2.2) Second Operation Pattern of Second Embodiment Fig. 14 is a diagram showing an example of operation according to the second operation pattern of the second embodiment. In the illustrated example, an inter-CU LTM scenario is shown, but an inter-CU conditional LTM scenario may also be used.

[0138] First, an overview of the operation shown in FIG. 14 will be described.

[0139] The gNB 200a managing the first cell instructs the UE 100 in the RRC connected state in the first cell to transmit an RA preamble for uplink early synchronization with the second cell, which is a candidate for LTM cell switching, to the second cell, and suspends communication with the UE 100 (steps S71 and S72). The gNB 200a receives a message from the gNB 200b managing the second cell indicating that the second cell has received the RA preamble and transmitted an RAR to the UE 100 (step S75). In response to receiving the message, the gNB 200a resumes communication with the UE 100 (step S76).

[0140] According to such an operation, the gNB 200a managing the first cell can determine, based on the message of step S75, the optimal timing for resuming DL transmission to the UE 100. As a result, the UE 100 can quickly resume communication with the first cell.

[0141] The message of step S75 may be a newly introduced Xn message. The message of step S75 may include an identifier of the UE 100 and / or an identifier of the second cell. The gNB 200b manages the CFRA resource used for the RA preamble transmission of step S73 in association with the UE 100, and may identify the UE 100 based on the RA preamble of step S73 and include the identifier of the identified UE 100 in the message of step S75.

[0142] Next, the operation shown in FIG. 14 will be described in detail.

[0143] The operations of steps S71 to S74 are the same as the operations shown in steps S61 to S64 of Figure 13. In step S74, the DU of gNB200b may notify the CU of gNB200b that it has transmitted the RAR to UE100. The DU of gNB200b may transmit the notification to the CU in response to receiving an ACK corresponding to the RAR from UE100.

[0144] In step S75, the CU of gNB200b transmits a message to the CU of gNB200a indicating that the RAR has been transmitted to the UE 100. The CU of gNB200a receives the message.

[0145] In step S76, in response to receiving the message of step S75, the CU of the gNB200a instructs the DU of the gNB200a to resume communication (DL communication) with the UE100 in the first cell. In response to the instruction, the DU of the gNB200a resumes communication (DL communication) with the UE100 in the first cell. For example, the DU (first cell) of the gNB200a assigns PDSCH resources to the UE100 via the PDCCH and transmits DL data to the UE100 on the PDSCH. The UE100 resumes monitoring the PDCCH of the first cell and receives DL data on the PDSCH of the first cell.

[0146] (3.2.3) Third Operation Pattern of Second Embodiment Fig. 15 is a diagram showing an example of operation according to the third operation pattern of the second embodiment. In the illustrated example, an inter-CU LTM scenario is shown, but an inter-CU conditional LTM scenario may also be used.

[0147] First, an overview of the operation shown in FIG. 15 will be described.

[0148] The gNB 200a managing the first cell performs gNB-to-gNB communication (inter-network node communication) with the gNB 200b managing the second cell, which is a candidate for LTM cell switching (step S80). The gNB 200a instructs the UE 100 in the RRC connected state in the first cell to transmit an RA preamble for uplink early synchronization with the second cell to the second cell, and suspends communication with the UE 100 (steps S81 and S82). After suspending communication with the UE 100, the gNB 200a resumes communication with the UE 100 in response to the elapse of the time determined by the inter-gNB communication in step S80 (also referred to as the "communication suspension time").

[0149] Thus, in the third operation pattern of the second embodiment, the communication interruption time between the first cell and the UE 100 due to the RA procedure to the second cell is determined between the gNBs through inter-gNB communication. As a result, the gNB 200a managing the first cell can determine the optimal timing for resuming DL transmission to the UE 100 based on the inter-gNB communication. As a result, the UE 100 can quickly resume communication with the first cell.

[0150] The communication interruption time may be the time from when the first cell transmits an RA Preamble assignment to the UE 100 until the second cell transmits (returns) an RAR to the UE 100. The communication interruption time may be the time during which DL data transmission of the first cell is stopped. The communication interruption time may be the time of the RAR reception window described above.

[0151] In step S80, the gNB 200a may receive a message from the gNB 200b including information indicating the communication interruption time. In this case, the communication interruption time may be determined by the gNB 200b. The message may further include an identifier of the UE 100 and / or an identifier of the second cell.

[0152] Alternatively, in step S80, the gNB 200a may transmit a message including information indicating the communication interruption time to the gNB 200b. In this case, the communication interruption time may be determined by the gNB 200a. The message may further include an identifier of the UE 100 and / or an identifier of the first cell and / or an identifier of the second cell.

[0153] The message in step S80 may be an Xn message defined in the Xn Application Protocol (AP). The message may be an Xn SETUP REQUEST message, an Xn SETUP RESPONSE message, a gNB Configuration Update message, a HANDOVER REQUEST message, a HANDOVER REQUEST ACKNOWLEDGE message, or a new message for LTM.

[0154] Next, the operation shown in FIG. 15 will be described in detail.

[0155] In step S80, the CU of gNB200a and the CU of gNB200b perform gNB-to-gNB communication (network node-to-network node communication) to exchange information about communication interruption time.

[0156] The CU of gNB200b may transmit a message including information indicating the communication interruption time to the CU of gNB200a. For example, the DU of gNB200b may notify the CU of gNB200b of the communication interruption time determined according to the performance and / or load status of the DU, and the CU of gNB200b may transmit the message to the CU of gNB200a based on the notification.

[0157] Here, the signaling from the DU of gNB200b to the CU of gNB200b may be an F1 message defined by the F1 AP. The message may be an F1 SETUP REQUEST message, an F1 SETUP RESPONSE message, a gNB-DU CONFIGURATION UPDATE message, or a new message for LTM.

[0158] Alternatively, the CU of the gNB 200a may transmit a message including information indicating the communication interruption time to the CU of the gNB 200b. For example, the CU of the gNB 200a may notify the CU of the gNB 200b of the requested communication interruption time.

[0159] The operations of steps S81 to S84 are the same as those shown in Fig. 14. In step S81, the DU of the gNB 200a transmits a PDCCH order (RA Preamble assignment) to the UE 100. The DU of the gNB 200a may notify the CU of the gNB 200a of the transmission. The DU or CU of the gNB 200a may start a timer in which a communication interruption time is set when transmitting the PDCCH order in step S81.

[0160] In step S85, the DU or CU of the gNB200a recognizes that the communication interruption time has elapsed (for example, expiration of the timer) and resumes communication (DL communication) with the UE100 in the first cell. When the CU of the gNB200a makes this recognition, the CU of the gNB200a may instruct the DU of the gNB200a to resume communication (DL communication) with the UE100 in the first cell. The DU (first cell) of the gNB200a assigns PDSCH resources to the UE100 via the PDCCH and transmits DL data to the UE100 on the PDSCH. The UE100 resumes monitoring the PDCCH of the first cell and receives DL data on the PDSCH of the first cell.

[0161] (3.3) Third Embodiment of Inter-CU LTM and / or Conditional LTM A third embodiment of inter-CU LTM and / or conditional LTM will be described, focusing on differences from the first and second embodiments. The third embodiment is based on the first operation pattern of the second embodiment. Note that the third embodiment may be applied to conditional LTM, specifically, intra-CU conditional LTM or inter-CU conditional LTM.

[0162] In the first operation pattern of the second embodiment described above, as shown in Fig. 13, the UE 100 in the RRC connected state in the first cell (Serving cell) suspends communication with the first cell and transmits an RA preamble for uplink early synchronization with the second cell (Candidate cell), which is a candidate for cell switching in LTM, to the second cell (Step S63). The UE 100 receives an RAR from the second cell, which includes information on the TA value to be applied to uplink transmission to the second cell (Step S64). In response to receiving the RAR, the UE 100 transmits UL signaling to the first cell to resume communication with the first cell (Step S65).

[0163] In the third embodiment, details of UL radio resources for transmitting UL signaling to the first cell for resuming communication with the first cell will be described.

[0164] In the third embodiment, in response to receiving an RAR from a second cell (Candidate cell), the UE 100 transmits UL signaling for resuming communication with the first cell (Serving cell) to the first cell using a physical uplink shared channel (PUSCH) resource, a physical uplink control channel (PUCCH) resource, or a physical random access channel (PRACH) resource. This allows the UE 100 to appropriately transmit UL signaling for resuming communication with the first cell. In addition, the gNB 200 managing the first cell can determine the optimal timing for resuming DL transmission to the UE 100 based on the UL signaling, allowing the UE 100 to quickly resume communication with the first cell.

[0165] UE 100 that performs such operation has a control unit 130 that, when in an RRC connected state in a first cell, suspends communication with the first cell and transmits an RA preamble to the second cell for uplink early synchronization with a second cell that is a candidate for LTM cell switching, a receiving unit 110 that receives an RAR from the second cell including information on the TA value to be applied to uplink transmission to the second cell, and a transmitting unit 120 that, in response to receiving the RAR, transmits UL signaling to the first cell using a PUSCH resource, a PUCCH resource, or a PRACH resource to resume communication with the first cell.

[0166] On the other hand, gNB200 has a control unit 230 that suspends communication with UE100 when UE100 in an RRC connected state in a first cell transmits an RA preamble for uplink early synchronization with a second cell that is a candidate for LTM cell switching to the second cell, and a receiving unit 220 that receives UL signaling for resuming communication with the first cell from UE100 that has received an RAR from the second cell that includes information about the TA value, using a PUSCH resource, a PUCCH resource, or a PRACH resource.

[0167] In a first operation pattern of the third embodiment, the UE 100 receives an RRC reconfiguration message from a first cell, the RRC reconfiguration message including a configured grant (CG) setting for allocating a PUSCH resource. The UE 100 transmits UL signaling for resuming communication to the first cell by using the PUSCH resource allocated by the CG.

[0168] In the second operation pattern of the third embodiment, upon receiving the RAR from the second cell, the UE 100 uses the PUCCH resource to transmit a scheduling request (SR) to the first cell as UL signaling to resume communication.

[0169] In the third operation pattern of the third embodiment, in response to receiving an RAR from the second cell, UE 100 uses a PUCCH resource to transmit uplink control information (UCI) to the first cell, the uplink control information including information for resuming communication with the first cell.

[0170] In a fourth operation pattern of the third embodiment, the UE 100 receives configuration information for configuring a PRACH resource for contention-free random access (CFRA) from the first cell. The UE 100 transmits an RA preamble of the CFRA to the first cell as UL signaling for resuming communication, using the configured PRACH resource.

[0171] (3.3.1) First Operation Pattern of the Third Embodiment Figure 16 is a diagram showing an example of operation according to the first operation pattern of the third embodiment. In the illustrated example, an inter-CU LTM scenario is shown, but an intra-CU LTM scenario may also be used. In this case, gNB200a and gNB200b can be read as the same gNB200.

[0172] First, an overview of the operation shown in FIG. 16 will be described.

[0173] The UE 100 receives an RRC reconfiguration message including a CG setting for allocating a PUSCH resource from the first cell (step S100). The UE 100 transmits UL signaling for resuming communication to the first cell by using the PUSCH resource allocated by the CG (step S106).

[0174] The UE 100 receives a PDCCH order for triggering transmission of an RA preamble from the first cell (step S101). The UE 100 may activate a CG configuration in response to the reception of the PDCCH order (step S102).

[0175] The UE 100 may discard the CG setting in response to transmitting the UL signaling for resuming communication to the first cell (step S107).

[0176] Next, the details of the operation shown in Fig. 16 will be described, but redundant explanations of operations similar to those in the first operation pattern of the second embodiment will be omitted.

[0177] In step S100, the gNB 200a (first cell, serving cell) transmits an RRC reconfiguration message including a CG setting for allocating a PUSCH resource to the UE 100. The UE 100 receives the RRC reconfiguration message and stores the CG setting. At this stage, the CG setting is in an inactive state, and the UE 100 may not be allocated a periodic PUSCH resource. The CG setting may include any of the following information: that the CG setting is for the UL signaling (notification regarding resynchronization in step 106), or that the CG setting is in an inactive state, that it becomes active when it receives an RA Preamble Assignment (step 101), that it becomes active when it transmits a Random Access Preamble (step S104), that it becomes active when it receives a Random Access Response (step S105), or that it becomes active when it is synchronized with the Serving Cell (gNB 200a). Becoming active may mean that the UE 100 recognizes that PUSCH resources are assigned according to the CG setting.

[0178] In step S101, gNB200a (first cell, Serving cell) transmits a PDCCH order (RA Preamble assignment) to UE100, which instructs (triggers) the transmission of a CFRA RA preamble to a second cell (LTM candidate cell, Candidate cell) of gNB200b. UE100 receives the PDCCH order from the first cell of gNB200a. The PDCCH order may include information for activating the CG configuration.

[0179] In step S102, the UE 100 may activate the CG setting in response to receiving a PDCCH order from the first cell. Alternatively, the UE 100 may activate the CG setting in response to receiving an RAR from the second cell (step S105). Alternatively, the UE 100 may activate the CG setting in response to transmitting a Random Access Preamble to the second cell (step S104). Alternatively, the UE 100 may activate the CG setting in response to re-synchronization with the first cell. Note that, if the UE 100 has already received an Uplink grant (PUSCH resource allocation) from the first cell, the UE 100 may discard the Uplink grant. Alternatively, the UE 100 may perform the operation of step S104 after completing transmission in response to the UL grant.

[0180] In step S103, gNB200a (first cell, serving cell) suspends communication (particularly DL communication) with UE100 in response to the transmission of the PDCCH order in step S101.

[0181] In step S104, in response to receiving the PDCCH order in step S101, the UE 100 synchronizes with the second cell of the gNB 200b (DL synchronization) and transmits an RA preamble to the second cell of the gNB 200b. The UE 100 also suspends communication with the first cell of the gNB 200a. The second cell of the gNB 200b receives the RA preamble. The gNB 200b derives a TA value that the UE 100 should apply to the second cell based on the received RA preamble.

[0182] In step S105, the gNB 200b notifies the UE 100 of the derived TA value by RAR. The UE 100 associates the TA value acquired by the RAR with the cell ID of the second cell and stores it. Note that when the UE 100 performs LTM cell switching to the second cell, it performs uplink (UL) transmission to the second cell using the stored TA value.

[0183] In step S106, in response to receiving the RAR in step S105, the UE 100 synchronizes with the first cell of the gNB 200a (DL synchronization) and / or starts PDCCH monitoring of the first cell, and transmits UL signaling to the first cell to resume communication with the first cell. The gNB 200a receives the UL signaling.

[0184] In step S107, the UE 100 may discard (or deactivate) the CG configuration in response to transmitting UL signaling to the first cell to resume communication.

[0185] In step S108, gNB200a resumes communication (DL communication) with UE100 in the first cell. For example, gNB200a (first cell) allocates PDSCH resources to UE100 by PDCCH and transmits DL data to UE100 on the PDSCH. In response to the transmission of the UL signaling in step S106, UE100 resumes monitoring the PDCCH of the first cell and receives DL data on the PDSCH of the first cell.

[0186] (3.3.2) Second Operation Pattern of the Third Embodiment Figure 17 is a diagram showing an example of operation according to the second operation pattern of the third embodiment. In the illustrated example, an inter-CU LTM scenario is shown, but an intra-CU LTM scenario may also be used. In this case, gNB200a and gNB200b can be read as the same gNB200.

[0187] First, an overview of the operation shown in FIG. 17 will be described.

[0188] In response to receiving the RAR from the second cell (step S114), the UE 100 uses the PUCCH resource to transmit a scheduling request (SR) to the first cell as UL signaling for resuming communication (step S115). The gNB 200a (first cell) regards the first SR after transmitting the PDCCH order to the UE 100 as UL signaling for resuming communication, and resumes communication with the UE 100 (step S116).

[0189] Next, the details of the operation shown in Fig. 17 will be explained, but duplicate explanations of operations similar to those described above will be omitted.

[0190] In step S111, gNB200a (first cell, serving cell) transmits to UE100 a PDCCH order (RA Preamble assignment) that instructs (trigger) the transmission of a CFRA RA preamble to gNB200b's second cell (LTM candidate cell, candidate cell).

[0191] In step S112, gNB200a (first cell, serving cell) suspends communication (particularly DL communication) with UE100 in response to the transmission of the PDCCH order in step S111.

[0192] In step S113, in response to receiving the PDCCH order in step S111, the UE 100 synchronizes with the second cell of the gNB 200b (DL synchronization) and transmits an RA preamble to the second cell of the gNB 200b. In addition, the UE 100 suspends communication with the first cell of the gNB 200a.

[0193] In step S114, gNB200b notifies UE100 of the derived TA value by RAR.

[0194] In step S115, in response to receiving the RAR in step S114, UE100 synchronizes with the first cell of gNB200a (DL synchronization) and / or starts PDCCH monitoring of the first cell, and transmits an SR to the first cell on the PUCCH as UL signaling to resume communication with the first cell. Here, when the MAC layer of UE100 receives the RAR, it may notify the PHY layer of UE100 of this and cause the PHY layer to trigger the transmission of an SR. The MAC layer of UE100 may trigger the SR in response to re-establishing DL synchronization with the first cell (in response to receiving this information from the PHY layer).

[0195] In step S116, gNB200a regards the SR of step S115 as UL signaling for resuming communication and resumes communication (DL communication) with UE100 in the first cell. For example, gNB200a (first cell) allocates PDSCH resources to UE100 by PDCCH and transmits DL data to UE100 on the PDSCH. In response to the transmission of the SR of step S115, UE100 resumes monitoring the PDCCH of the first cell and receives DL data on the PDSCH of the first cell.

[0196] (3.3.3) Third Operation Pattern of the Third Embodiment Figure 18 is a diagram showing an example of operation according to the third operation pattern of the third embodiment. In the example shown, an inter-CU LTM scenario is shown, but an intra-CU LTM scenario may also be used. In this case, gNB200a and gNB200b can be read as the same gNB200.

[0197] First, an overview of the operation shown in FIG. 18 will be described.

[0198] In response to receiving the RAR from the second cell (step S124), the UE 100 uses the PUCCH resource to transmit uplink control information (UCI) including information for resuming communication with the first cell to the first cell (step S125a). Thus, in this operation pattern, instead of the SR in the second operation pattern of the above-mentioned third embodiment, the UCI including information for resuming communication with the first cell is used. The UE 100 may include information indicating whether or not the reception of the RAR was successful in the UCI.

[0199] Next, the details of the operation shown in Fig. 18 will be described. Here, a duplicated description of the operation similar to that of the second operation pattern of the third embodiment will be omitted.

[0200] In step S125a, in response to receiving the RAR in step S124, UE100 synchronizes with the first cell of gNB200a (DL synchronization) and / or starts PDCCH monitoring of the first cell, and transmits UCI including information for resuming communication with the first cell to the first cell on the PUCCH. Here, when the MAC layer of UE100 receives the RAR, it may notify the PHY layer of UE100 of this fact (and whether or not the RAR was successfully received) and cause the PHY layer to trigger the transmission of the UCI. The MAC layer of UE100 may trigger the transmission of the UCI in response to re-establishing DL synchronization with the first cell (in response to receiving this fact from the PHY layer).

[0201] (3.3.4) Fourth Operation Pattern of the Third Embodiment Figure 19 is a diagram showing an example of operation according to the fourth operation pattern of the third embodiment. In the example shown, an inter-CU LTM scenario is shown, but an intra-CU LTM scenario may also be used. In this case, gNB200a and gNB200b can be read as the same gNB200.

[0202] First, an overview of the operation shown in FIG. 19 will be described.

[0203] The UE 100 receives configuration information for configuring a PRACH resource for contention-free random access (CFRA) from the first cell (step S130). The UE 100 transmits a CFRA RA preamble to the first cell using the configured PRACH resource as UL signaling for resuming communication (step S135). The gNB 200a (first cell) regards the first CFRA RA preamble after transmitting the PDCCH order to the UE 100 as UL signaling for resuming communication, and resumes communication with the UE 100 (step S137).

[0204] Next, the details of the operation shown in Fig. 19 will be explained, but duplicate explanations of operations similar to those described above will be omitted.

[0205] In step S130, the gNB 200a (first cell, serving cell) transmits an RRC reconfiguration message including configuration information (RACH config dedicated) for configuring a PRACH resource for CFRA to the UE 100. The UE 100 receives the RRC reconfiguration message and stores the configuration information (RACH config dedicated).

[0206] In step S131, gNB200a (first cell, serving cell) transmits to UE100 a PDCCH order (RA Preamble assignment) that instructs (trigger) the transmission of a CFRA RA preamble to gNB200b's second cell (LTM candidate cell, candidate cell).

[0207] In step S132, gNB200a (first cell, serving cell) suspends communication (particularly, DL communication) with UE100 in response to the transmission of the PDCCH order in step S131.

[0208] In step S133, in response to receiving the PDCCH order in step S131, the UE 100 synchronizes with the second cell of the gNB 200b (DL synchronization) and transmits an RA preamble to the second cell of the gNB 200b. In addition, the UE 100 suspends communication with the first cell of the gNB 200a.

[0209] In step S134, gNB200b notifies UE100 of the derived TA value by RAR. UE100 associates the TA value acquired by the RAR with the cell ID of the second cell and stores it. Note that when performing LTM cell switching to the second cell, UE100 performs uplink (UL) transmission to the second cell using the stored TA value.

[0210] In step S135, in response to receiving the RAR in step S135, the UE 100 synchronizes with the first cell of the gNB 200a (DL synchronization) and / or starts PDCCH monitoring of the first cell, and uses the configuration information (RACH config dedicated) set in step S130 to transmit a CFRA RA preamble to the first cell as UL signaling for resuming communication with the first cell. The gNB 200a receives the CFRA RA preamble.

[0211] In step S136, the UE 100 may discard (or deactivate) the configuration information (RACH config dedicated) set in step S130 in response to the PRACH transmission in step S135.

[0212] In step S137, gNB200a resumes communication (DL communication) with UE100 in the first cell. For example, gNB200a (first cell) allocates PDSCH resources to UE100 by PDCCH and transmits DL data to UE100 on the PDSCH. In response to the transmission of the PRACH transmission in step S135, UE100 resumes monitoring the PDCCH of the first cell and receives DL data on the PDSCH of the first cell.

[0213] (4) Example of Conditional LTM Operation An example of conditional LTM (also referred to as "C-LTM") operation will be described, assuming the operation of the mobile communication system 1 according to the first to third embodiments described above.

[0214] In this operation example, the gNB 200 sets in advance in the UE 100 an execution condition (cell switching trigger condition, CondEvent), which is a radio quality condition for performing cell switching, in the RRC Reconfiguration message of step S4 of FIG. 6. The UE 100 performs cell switching by LTM (also referred to as "LTM cell switching") when the radio quality condition is satisfied. This makes it possible to skip the operations from the L1 Measurement Report of step S7 of FIG. 6 to the cell switching command (MAC CE) of step S9. As a result, it is possible to speed up cell switching from the first cell to the second cell and minimize interruptions in data communication.

[0215] Here, in conventional LTM, the TA value to be applied to the second cell (LTM candidate cell, target cell) is notified from the gNB 200 to the UE 100 in the cell switching command of step S9. On the other hand, in C-LTM, the cell switching command of step S9 is skipped, so there is a problem that the UE 100 cannot obtain the TA value to be applied to the second cell from the gNB 200. As a result, the UE 100 may need to perform a random access procedure for the second cell when the set radio quality condition is satisfied. Therefore, it is difficult to speed up cell switching from the first cell to the second cell.

[0216] Therefore, in this operation example, the UE 100 in which C-LTM is set transmits a preamble to the LTM candidate cell (specifically, transmits an RA preamble) before the set radio quality condition is satisfied, and acquires the TA value by receiving a random access response (RAR) including the TA value of the LTM candidate cell. As a result, when the radio quality condition for the second cell is satisfied, the UE 100 already has the TA value of the second cell, so that a random access procedure for the second cell is not required, and cell switching from the first cell to the second cell can be speeded up.

[0217] The following description will discuss an example in which the UE 100 acquires the TA value of the second cell by RAR. However, the UE 100 may acquire the TA value by an RAR in a format different from that of the conventional RAR. For example, a MAC protocol data unit (PDU) or MAC CE different from that of the conventional RAR may be used, or an RRC message may be used. When the MAC CE is used, the UE 100 may acquire the TA value by a new MAC CE (e.g., Early Sync TA notification MAC CE) dedicated to early synchronization.

[0218] Also, normally, when the UE 100 acquires a TA value in the RAR, the UE 100 immediately applies the TA value. However, in the case of C-LTM, when the UE 100 acquires the TA value in the RAR and the radio quality condition for the second cell is not satisfied, the UE 100 should not immediately apply the TA value. In such a case, if the UE 100 immediately applies the TA value, there is a concern that the UE 100 may not be able to properly communicate with the first cell, which is the current serving cell.

[0219] Therefore, in this operation example, even if UE 100 acquires a TA value in RAR, it retains the TA value without applying it until the set radio quality condition is satisfied (that is, it continues to apply the TA value of the first cell, which is the current serving cell, until then). Then, when the radio quality condition is satisfied, UE 100 applies the TA value and performs LTM cell switching.

[0220] 20 is a diagram showing the operation of the UE 100 according to this operation example. Description of the contents that overlap with the operation according to the above-described embodiment (including the conventional LTM cell switching procedure in FIG. 6) will be omitted.

[0221] In step S201, the UE 100 receives, from the first cell, information indicating a radio quality condition that must be satisfied in order to perform an LTM cell switch, which switches the serving cell of the UE 100 from the first cell to the second cell by LTM, and instruction information instructing transmission of a CFRA preamble to the second cell. The UE 100 receives the information indicating the radio quality condition from the first cell in an RRC Reconfiguration message (see step S4 in FIG. 6 ). The RRC Reconfiguration message may include a RACH setting, for example, a CFRA setting, used for transmitting an RA preamble to the second cell. The RRC Reconfiguration message may include a notification indicating that the second cell will transmit (reply) an RAR in response to the RA preamble transmission, and / or information indicating an RAR reception window.

[0222] The RRC Reconfiguration message may include an LTM candidate cell configuration list, which is a list including configuration information (LTM candidate cell configuration) of each of a plurality of second cells. Each entry of the LTM candidate cell configuration list, the LTM candidate cell configuration, includes a cell ID of the corresponding second cell and information indicating a radio quality condition to be applied to the corresponding second cell. The LTM candidate cell configuration may include a RACH configuration (CFRA configuration) of the corresponding second cell.

[0223] The radio quality may be at least one measurement value of a reference signal received power (RSRP), a reference signal received quality (RSRQ), a signal-to-interference-and-noise ratio (SINR), and a received signal strength indicator (RSSI). The reference signal may be an SSB (SS / PBCH block) or a CSI (Channel State Information) reference signal. The information indicating the radio quality condition may be information indicating a radio quality condition that should be satisfied for the first cell (current serving cell) and / or the second cell (LTM candidate cell) to trigger an LTM cell switch. For example, the information indicating the radio quality condition may be an event (condition) such as "Neighbor becomes offset better than SpCell," i.e., a trigger condition indicating that the radio quality of the second cell has improved by an offset compared to the radio quality of the first cell. In this case, the information indicating the radio quality condition may include an offset value. Alternatively, the information indicating the wireless quality condition may be information indicating an event (condition) such as “Nightbour / SpCell becomes better / worse than threshold.” In this case, the information indicating the wireless quality condition may include a threshold.

[0224] The UE 100 receives, in a PDCCH order or an RRC Reconfiguration message, instruction information instructing transmission of a CFRA preamble to the second cell.

[0225] Specifically, the UE 100 receives a PDCCH order including instruction information from the first cell. The PDCCH order may include a notification indicating that the second cell will transmit (reply) an RAR in response to the RA preamble transmission, and / or information indicating an RAR reception window. As described above, when DCI Format 1_0 is used and all "Frequency domain resource assignment" fields in the DCI are set to "1", the DCI is treated as a PDCCH order. The PDCCH order may include a cell indicator indicating the second cell to which the RA preamble is to be transmitted. The cell indicator may indicate multiple second cells as destinations of the RA preamble. However, a new DCI format may be used instead of the conventional PDCCH order.

[0226] In step S202, the UE 100 may evaluate whether the set radio quality condition is satisfied. For example, the UE 100 measures the radio quality of the first cell and / or the radio quality of the second cell, and evaluates (determines) whether the measured value satisfies the set radio quality condition. The UE 100, in which a plurality of second cells are set as LTM candidate cells, performs the evaluation in step S202 for each second cell.

[0227] In step S203, the UE 100 transmits an RA preamble to the second cell in accordance with the instruction information (specifically, transmits an RA preamble of CFRA), and then receives an RAR from the second cell, the RAR including a TA value derived based on the RA preamble transmission. When the UE 100 is instructed to transmit preambles to a plurality of second cells, the UE 100 may transmit preambles to each of the second cells and acquire the TA value of each of the second cells.

[0228] In step S204, the UE 100 holds the TA value until it is evaluated that the radio quality condition is satisfied for any of the second cells. Specifically, even if the UE 100 acquires the TA value by the RAR, the UE 100 holds the TA value without applying it until the radio quality condition is satisfied for any of the second cells (that is, the TA value of the first cell continues to be applied until that time).

[0229] In step S205, in response to the evaluation that the radio quality condition is satisfied for any second cell, the UE 100 performs the LTM cell switch by applying the TA value of the second cell for which the radio quality condition is satisfied. Here, the UE 100 performs the LTM cell switch by applying the TA value that it holds without performing a random access procedure for the second cell. When performing the LTM cell switch, the UE 100 transmits an RRC Reconfiguration Complete message to the second cell. The UE 100 may consider that the execution of the LTM cell switch has been completed successfully when it is determined that the network (second cell) has successfully received the first uplink data.

[0230] According to such an operation, the UE 100 skips the operations from the L1 Measurement Report in step S7 to the cell switching command (MAC CE) in step S9, and can quickly perform LTM cell switching when the radio quality condition set by the gNB 200 is satisfied. In addition, when the radio quality condition is satisfied, the UE 100 does not need to perform a random access procedure for the second cell, and cell switching can be accelerated.

[0231] However, since the second cell is a non-serving cell, it is also assumed that the conventional UE 100 does not have the capability to receive the RAR from the second cell. Therefore, the UE 100 having this capability may transmit capability information indicating that it has the capability to receive the RAR from the second cell to the network (gNB 200). Such capability information may be transmitted in a UE Capability message, which is a type of RRC message. For example, when connected to the gNB 200, the UE 100 may transmit a UE Capability message to the gNB 200 in response to receiving a capability inquiry from the gNB 200.

[0232] 21 is a diagram showing an example of operation of the mobile communication system 1 according to this operation example. Here, in an intra-gNB (intra-CU) scenario, an example in which the UE 100 receives a PDCCH order while performing evaluation of the second cell will be mainly described, focusing on differences from the above-described operation.

[0233] The operations of steps S301 to S305 are the same as those of the conventional LTM cell switching procedure of FIG. 6. However, in step S304, the gNB 200 transmits an RRC Reconfiguration message including an LTM candidate cell configuration for C-LTM (C-LTM Candidate Configuration) to the UE 100. The LTM candidate cell configuration for C-LTM may include an LTM candidate cell configuration list, which is a list including configuration information (LTM candidate cell configuration) for each of a plurality of second cells. The LTM candidate cell configuration, which is each entry in the LTM candidate cell configuration list, includes the cell ID of the corresponding second cell and information indicating the radio quality conditions to be applied to the corresponding second cell. The LTM candidate cell configuration may include the RACH configuration (CFRA configuration) of the corresponding second cell. The LTM candidate cell configuration may include a timer value of a timer that determines an upper limit of the RAR waiting period (RAR reception window). This timer may be a timer separate from a conventional RACH / RAR related timer. The timer value may be an offset value with respect to the conventional timer value, or the like. The UE 100 may start the timer when transmitting an RA preamble to the corresponding second cell, and if the timer expires without receiving an RAR, may transmit an RA preamble to the corresponding second cell again. The UE 100 may determine that a random access procedure is necessary for the corresponding second cell at the time of cell switching.

[0234] In step S306, the UE 100 performs downlink synchronization processing (DL synchronization processing) for the second cell.

[0235] In step S307, the UE 100 starts evaluating whether the radio quality condition set in step S304 is satisfied. For example, the UE 100 measures the radio quality of the first cell and / or the radio quality of the second cell, and evaluates (determines) whether the measured value satisfies the set radio quality condition. The UE 100, in which a plurality of second cells are set as LTM candidate cells, starts evaluation for each second cell.

[0236] In step S308, gNB200 (first cell) transmits a PDCCH order to UE100. UE100 receives the PDCCH order (instruction information) from the first cell.

[0237] In step S309, UE100 transmits a CFRA RA preamble to the second cell specified in the PDCCH order (instruction information). Here, UE100 may transmit a preamble based on the CFRA setting set in step S304. gNB200 (second cell) receives the RA preamble and derives a TA value based on the received RA preamble. When preamble transmission is instructed for multiple second cells, UE100 may transmit a preamble to each second cell. In this case, gNB200 may derive a TA value for each second cell.

[0238] In step S310, the gNB200 (the first cell or the second cell) transmits an RAR including the derived TA value to the UE100. The UE100 receives the RAR from the first cell or the second cell. The RAR may include the TA values ​​of each of a plurality of second cells. For example, the RAR may include a plurality of sets of cell IDs and TA values ​​of the second cells. Upon receiving the RAR from the second cell, the UE100 may transmit uplink signaling to the first cell to resume communication with the first cell.

[0239] In step S311, the UE 100 holds the TA value acquired in step S310 without applying it. The UE 100 may hold the TA value for each of the plurality of second cells.

[0240] In step S312, the UE 100 determines whether or not the radio quality condition is satisfied for any of the second cells. If it is determined that the radio quality condition is satisfied for any of the second cells (step S312: YES), in step S313, the UE 100 determines LTM cell switching to the second cell for which the radio quality condition is satisfied.

[0241] In step S314, the UE 100 detaches from the first cell which is the source cell and applies the configuration of the second cell which is the target cell. Here, the UE 100 applies the TA value held for the second cell determined in step S312.

[0242] In step S315, the UE 100 applies the TA value to the second cell (target cell) determined in step S312 and transmits an RRC Reconfiguration Complete message. The gNB 200 receives the RRC Reconfiguration Complete message. When the UE 100 determines that the second cell has successfully received the first uplink data, it may consider that the execution of the LTM cell switch from the first cell to the second cell has been successfully completed.

[0243] (5) Other Embodiments The above-described operational flows are not limited to being implemented independently, but can be implemented by combining two or more operational flows. For example, some steps of one operational flow may be added to another operational flow, or some steps of one operational flow may be replaced with some steps of another operational flow. In each flow, it is not necessary to execute all steps, and only some steps may be executed. Furthermore, the order of steps in each flow may be changed as appropriate.

[0244] 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.

[0245] 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.

[0246] 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).

[0247] 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.

[0248] 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.

[0249] 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.

[0250] This application claims priority from Japanese Patent Application No. 2024-128112 (filed August 2, 2024), the entire contents of which are incorporated herein by reference.

[0251] (6) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0252] Supplementary Note 1: A communication method used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a user equipment in a Radio Resource Control (RRC) Connected state in a first cell suspends communication with the first cell and transmits, to the second cell, a random access (RA) preamble for uplink early synchronization with a second cell that is a candidate for cell switching of the LTM; the user equipment receiving, from the second cell, a random access response (RAR) including information on a timing advance (TA) value to be applied to uplink transmissions to the second cell; and, in response to receiving the RAR, the user equipment transmitting, to the first cell, uplink signaling for resuming communication with the first cell, using a physical uplink shared channel (PUSCH) resource, a physical uplink control channel (PUCCH) resource, or a physical random access channel (PRACH) resource.

[0253] Supplementary Note 2: The communication method according to Supplementary Note 1, further comprising receiving, from the first cell, an RRC reconfiguration message including a setting of a configured grant for allocating the PUSCH resource, and wherein the user equipment transmits the uplink signaling to the first cell using the PUSCH resource allocated in the configured grant.

[0254] Supplementary Note 3: The communication method according to Supplementary Note 2, further comprising receiving, from the first cell, a PDCCH order for triggering transmission of the RA preamble, and wherein the user equipment activates setting of the configured grant in response to receiving the PDCCH order.

[0255] Supplementary Note 4: The communication method according to Supplementary Note 2 or 3, wherein the user equipment discards the setting of the configured grant in response to transmitting the uplink signaling to the first cell.

[0256] Supplementary Note 5: The communication method according to Supplementary Note 1, wherein the user equipment transmits a scheduling request (SR) as the uplink signaling to the first cell using the PUCCH resource in response to receiving the RAR from the second cell.

[0257] Supplementary Note 6: The communication method according to Supplementary Note 1, wherein the user equipment transmits uplink control information (UCI) including information for resuming communication with the first cell to the first cell using the PUCCH resource in response to receiving the RAR from the second cell.

[0258] Supplementary Note 7: The communication method of Supplementary Note 1, further comprising: the user equipment receiving, from the first cell, configuration information for configuring the PRACH resources for contention-free random access (CFRA); and the user equipment transmitting, to the first cell, an RA preamble for the CFRA as the uplink signaling, using the configured PRACH resources.

[0259] Supplementary Note 8: The communication method according to any one of Supplementary Notes 1 to 7, wherein the LTM is a conditional LTM.

[0260] Supplementary Note 9: A user equipment used in a mobile communication system supporting L1 / L2 Triggered Mobility (LTM), comprising: a controller configured, when in a Radio Resource Control (RRC) Connected state in a first cell, to suspend communication with the first cell and transmit, to the second cell, a random access (RA) preamble for uplink early synchronization with a second cell that is a candidate for cell switching of the LTM; a receiver configured to receive from the second cell a random access response (RAR) including information on a timing advance (TA) value to be applied to uplink transmission to the second cell; and a transmitter configured, in response to reception of the RAR, to transmit uplink signaling to the first cell for resuming communication with the first cell, using a physical uplink shared channel (PUSCH) resource, a physical uplink control channel (PUCCH) resource, or a physical random access channel (PRACH) resource.

[0261] Supplementary Note 10: A network node managing a first cell in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a controller configured to suspend communication with a first cell when a user equipment in a Radio Resource Control (RRC) Connected state in the first cell transmits, to the second cell, a random access (RA) preamble for uplink early synchronization with a second cell that is a candidate for cell switching of the LTM; and a receiver configured to receive, from the user equipment that has received from the second cell a random access response (RAR) including information of a timing advance (TA) value, uplink signaling for resuming communication with the first cell, using a physical uplink shared channel (PUSCH) resource, a physical uplink control channel (PUCCH) resource, or a physical random access channel (PRACH) resource.

[0262] 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 used in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: a user equipment in a Radio Resource Control (RRC) Connected state in a first cell suspends communication with the first cell and transmits a random access (RA) preamble for uplink early synchronization with a second cell that is a candidate for cell switching of the LTM to the second cell; and, in response to communication with the first cell becoming possible, the user equipment transmits uplink signaling for resuming communication with the first cell to the first cell using a Physical Uplink Shared Channel (PUSCH) resource, a Physical Uplink Control Channel (PUCCH) resource, or a Physical Random Access Channel (PRACH) resource.

2. The communication method according to claim 1, wherein communication with the first cell is enabled when the user equipment achieves synchronization with the first cell.

3. The communication method according to claim 1, wherein communication with the first cell is enabled when the user equipment receives a random access response (RAR) from the second cell, the RAR including information on a timing advance (TA) value to be applied to uplink transmission to the second cell.

4. The communication method according to claim 1, further comprising: the user equipment receiving, from the first cell, an RRC reconfiguration message including a configured grant setting for allocating the PUSCH resource; and the user equipment transmitting the uplink signaling to the first cell using the PUSCH resource allocated in the configured grant.

5. The communication method according to claim 2, further comprising the user equipment receiving a PDCCH order from the first cell for triggering transmission of the RA preamble, and the user equipment activating the setting of the configured grant in response to receiving the PDCCH order.

6. The communication method according to claim 2 or 3, wherein the user equipment discards the setting of the configured grant in response to transmitting the uplink signaling to the first cell.

7. The communication method according to claim 1, wherein the user equipment transmits a scheduling request (SR) as the uplink signaling to the first cell using the PUCCH resource.

8. The communication method according to claim 1, wherein the user equipment transmits uplink control information (UCI) including information for resuming communication with the first cell to the first cell using the PUCCH resource.

9. The communication method according to claim 1, further comprising the user equipment receiving, from the first cell, configuration information for configuring the PRACH resource for contention-free random access (CFRA), and the user equipment transmitting an RA preamble for the CFRA to the first cell as the uplink signaling using the configured PRACH resource.

10. The communication method of claim 1, wherein the LTM is a conditional LTM.

11. A user equipment used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a control unit that, when in a Radio Resource Control (RRC) Connected state in a first cell, suspends communication with the first cell and transmits a random access (RA) preamble for uplink early synchronization with a second cell that is a candidate for cell switching of the LTM to the second cell; and a transmission unit that, when communication with the first cell becomes possible, transmits uplink signaling to the first cell for resuming communication with the first cell using a physical uplink shared channel (PUSCH) resource, a physical uplink control channel (PUCCH) resource, or a physical random access channel (PRACH) resource.

12. A network node that manages a first cell in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: a control unit that suspends communication with a user equipment when a user equipment in a radio resource control (RRC) connected state in the first cell transmits a random access (RA) preamble for uplink early synchronization with a second cell that is a candidate for cell switching of the LTM to the second cell; and a receiving unit that receives uplink signaling from the user equipment for resuming communication with the first cell, using a physical uplink shared channel (PUSCH) resource, a physical uplink control channel (PUCCH) resource, or a physical random access channel (PRACH) resource, in response to the user equipment being able to communicate with the first cell.

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