Communication method and user device

Abstract

This communication method is used in a mobile communication system supporting LTM and includes: user equipment communicating with a master node and a secondary node; the user equipment storing a first LTM configuration including information used for performing a first LTM cell switching, which is an LTM cell switching from the master node to another master node; the user equipment storing a second LTM configuration including information used for performing a second LTM cell switching, which is an LTM cell switching within the secondary node; and the user equipment disabling the use of the second LTM configuration on the basis of performing the first LTM cell switching using the first LTM configuration.

Description

Communication method and user device

[0001] The present disclosure relates to a communication method and user equipment 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), causing the network node to change the serving cell of the user equipment.

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

[0005] This disclosure provides techniques for improving LTM.

[0006] A communication method according to a first aspect of the present disclosure is a method used in a mobile communication system supporting L1 / L2 Triggered Mobility (LTM). The communication method includes: a user equipment (UE) communicating with a master node and a secondary node; storing, by the UE, a first LTM configuration including information used for a first LTM cell switch, which is an LTM cell switch from the master node to another master node; storing, by the UE, a second LTM configuration including information used for a second LTM cell switch, which is an LTM cell switch within the secondary node; and disabling, by the UE, the second LTM configuration in response to an instruction received from the master node or a network node that is the other master node, based on the UE performing the first LTM cell switch using the first LTM configuration.

[0007] A user equipment according to a second aspect of the present disclosure is an equipment used in a mobile communication system supporting L1 / L2 Triggered Mobility (LTM). The user equipment includes: a communication unit that communicates with a master node and a secondary node; and a control unit that stores a first LTM configuration including information used for a first LTM cell switch, which is an LTM cell switch from the master node to another master node, and a second LTM configuration including information used for a second LTM cell switch, which is an LTM cell switch within the secondary node. The control unit disables the second LTM configuration in response to an instruction received from the master node or a network node that is the other master node, based on performing the first LTM cell switch using the first LTM configuration.

[0008] 1 is a diagram illustrating an example of the configuration of a mobile communication system according to an embodiment. FIG. 2 is a diagram illustrating an example of the configuration of a UE (user equipment) according to an embodiment. FIG. 3 is a diagram illustrating an example of the configuration of a gNB (network node) according to an embodiment. FIG. 4 is a diagram illustrating a protocol stack configuration of a radio interface of a user plane that handles data. FIG. 5 is a diagram illustrating a protocol stack configuration of a radio interface of a control plane that handles signaling (control signals). FIG. 6 is a diagram for explaining dual connectivity (DC) according to an embodiment. FIG. 7 is a diagram illustrating an example of a cell switching procedure by LTM in an intra-CU (i.e., within the same gNB) according to an embodiment. FIG. 8 is a diagram illustrating an example of the operation of inter-CU LTM according to an embodiment. FIG. 9 is a diagram illustrating an example of LTM setting (LTM Candidate Configuration) set to a UE from a network (e.g., MN) according to an embodiment. FIG. 10 is a diagram illustrating a first case of inter-CU MCG LTM according to an embodiment. FIG. 11 is a diagram illustrating a second case of inter-CU MCG LTM according to an embodiment. FIG. 12 is a diagram illustrating an example of a first operation pattern according to an embodiment. FIG. 13 is a diagram illustrating an example of a second operation pattern according to an embodiment.

[0009] 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.

[0010] (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.

[0011] 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.

[0012] 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).

[0013] 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").

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data.

[0026] 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.

[0027] 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 the successfully decoded DCI as DCI addressed to the UE. The DCI transmitted from gNB200 has a CRC parity bit scrambled by the RNTI added.

[0028] 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.

[0029] 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.

[0030] The PDCP layer performs header compression / decompression, encryption / decryption, and the like.

[0031] 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.

[0032] 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).

[0033] 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.

[0034] 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.

[0035] 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").

[0036] (2) Overview of Dual Connectivity The mobile communication system 1 supports dual connectivity (DC). Fig. 6 is a diagram for explaining DC according to the embodiment.

[0037] A DC may be set for the UE 100 in the RRC connected state. In the DC, the UE 100 performs radio communication with a master cell group (MCG) managed by a master node (MN) 200M and a secondary cell group (SCG) managed by a secondary node (SN). The MN 200M and the SN 200S are connected to each other via an inter-node interface.

[0038] MN200M is also referred to as a master gNB (MgNB) when it is a 5G / NR node. SN200M is also referred to as a secondary gNB (SgNB) when it is a 5G / NR node. The inter-node interface may be an Xn interface in 5G / NR. In the following embodiments, communication between gNBs 200 is assumed to be performed over the Xn interface, but the inter-node interface may be named differently in 6G.

[0039] For example, MN200M transmits a predetermined message (for example, an SN Addition Request message) to SN200S, and MN200M transmits an RRC Reconfiguration message to UE100, whereby SCG is set in UE100 and DC is started. In DC, UE100 in the RRC connected state is assigned radio resources from the respective schedulers of MN200M and SN200S, and performs radio communication using the radio resources of MN200M and the radio resources of SN200S.

[0040] The MN 200M may have a control plane connection with the CN 20. The MN 200M provides the primary radio resources of the UE 100. The MN 200M manages an MCG, which is a group of serving cells associated with the MN 200M. The MCG has a primary cell (PCell) and optionally has one or more secondary cells (SCells).

[0041] On the other hand, the SN 200S may not have a control plane connection with the CN 20. The SN 200S provides additional radio resources to the UE 100. The SN 200S manages an SCG, which is a group of serving cells associated with the SN 200S. The SCG has a primary / secondary cell (PSCell) and optionally has one or more SCells. Note that the PCell of the MCG and the PSCell of the SCG are sometimes referred to as special cells (SpCells).

[0042] (3) Overview of LTM The mobile communication system 1 supports LTM (L1 / L2-triggered mobility).

[0043] 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, which is an RRC message (specifically, an RRC Reconfiguration message), from the gNB 200 to the UE 100.

[0044] On the other hand, LTM is a technology for shortening mobility delay (specifically, serving cell switching delay) compared to a general handover procedure by triggering serving cell switching by signaling of lower layers, 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 by MAC CE to instruct the serving cell switching by a cell switching command.

[0045] Specifically, in LTM, first, gNB200 prepares LTM settings for candidate cells to be switched to, and provides the LTM settings to UE100 via RRC signaling.

[0046] Secondly, the UE 100 performs a synchronization process with the LTM candidate cell by early synchronization (Early sync).

[0047] Third, the gNB 200 receives an L1 measurement report from the UE 100, determines a serving cell switch to the target cell based on the L1 measurement report, and transmits a cell switch command (Cell Switch Command) indicating the target cell (LTM setting) to the UE 100 via a MAC control element (CE). The serving cell switch trigger is conveyed in a MAC CE including at least a candidate setting index (ID) together with a beam indicator.

[0048] Fourth, the UE 100 switches the serving cell in response to a cell switch command MAC CE from the gNB 200 (source cell).

[0049] In this way, a serving cell switch is triggered by selecting the LTM configuration as the target configuration by gNB200. The LTM configuration can be added, changed, and released by gNB200 via RRC signaling.

[0050] The following principles apply to LTM:

[0051] Each LTM setting can be provided as a delta setting relative to a reference setting that is used to form the complete LTM setting.

[0052] If a full LTM configuration is applied, the current UE configuration is replaced upon a serving cell switch. The reconfiguration procedure does this but does not necessarily reset the MAC, RLC or PDCP layers.

[0053] The user plane continues without a reset if configured in RRC signaling to avoid additional delays in data recovery.

[0054] - Security is not updated in LTM.

[0055] LTM between subsequent LTM configurations can be performed without RRC reconfiguration, i.e., the UE 100 does not release other LTM configurations after an LTM is triggered.

[0056] (3.1) LTM for Intra-CU Figure 7 is a diagram showing an example of a cell switching procedure using LTM in intra-CU (i.e., within the same gNB 200). In the illustrated example, UE 100 performs a serving cell switch from a first cell of gNB 200 to a second cell.

[0057] Here, the first cell and the second cell may be formed by different TRPs (Transmission and Reception Points). In the following description of the embodiment, the second cell is also referred to as an "LTM candidate cell (or LTM candidate cell)" until a serving cell switch by LTM is determined, and after a serving cell switch by LTM is determined, the second cell is also referred to as a "target cell". The first cell is also referred to as a "source cell" or a "(current) serving cell".

[0058] In step S1, UE100 is in an RRC connected state in the cell (first cell) of gNB200.

[0059] 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.

[0060] In step S3, gNB200 decides to use LTM based on the Measurement Report message and starts preparing an LTM candidate cell.

[0061] In step S4, the gNB 200 (first cell) transmits an RRC message, specifically an RRC Reconfiguration message, to the UE 100, including LTM configurations (LTM Candidate Configurations) of one or more LTM candidate cells. The LTM configurations may include a random access channel (RACH) configuration, such as a contention-free random access (CFRA) configuration, used to transmit RA preambles to the corresponding LTM candidate cells. 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.

[0062] In step S5, UE100 saves the LTM setting and sends an RRC Reconfiguration Complete message to gNB200 (first cell).

[0063] 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 (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).

[0064] 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.

[0065] 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).

[0066] In step S8, gNB200 decides to switch the serving cell to the target cell (second cell).

[0067] 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 determined by UL early synchronization (i.e., a TA value derived based on the RA preamble).

[0068] 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).

[0069] 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).

[0070] 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.

[0071] (3.2) Inter-CU LTM The LTM introduced in 3GPP Release 18 only supports intra-CU and does not support inter-CU (i.e., between different gNBs 200) LTM. 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).

[0072] 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.

[0073] An example of the operation of the Inter CU LTM will be described based on the operation of Fig. 7. Fig. 8 is a diagram showing an example of the operation of the Inter CU LTM.

[0074] In step S101, the UE 100 transmits an L3 (RRC) Measurement Report to the gNB 200a. The gNB 200a receives the L3 (RRC) Measurement Report.

[0075] In step S102, gNB200a determines to use inter-gNB (inter-CU) LTM based on the L3 (RRC) Measurement Report in step S101 and starts preparing an LTM candidate cell. Here, it is assumed that the second cell of gNB200b is determined as the LTM candidate cell.

[0076] In step S103, the gNB 200a transmits a request message (LTM HO Request) indicating a serving cell change by LTM to the gNB 200b. The gNB 200b receives the request message (LTM HO Request). The request message (LTM HO Request) includes an LTM indicator and may be a Handover Request message used in a general handover. Alternatively, the request message (LTM HO Request) may be a new message different from the Handover Request message, for example, an LTM Handover Request message. Note that the request message (LTM HO Request) may include, similarly to a general handover, the RRC setting information of the UE 100, a cell identifier indicating the second cell, and the like.

[0077] In step S104, the gNB200b determines whether or not to accept the request of step S103 (Admission control). Here, the description will proceed assuming that the request of step S103 is accepted. In this case, the gNB200b may configure a CFRA resource for early synchronization in the second cell. Note that, if the request of step S103 is rejected, the gNB200b may send a rejection message to the gNB200a. The rejection message may include information indicating that inter-gNB LTM cannot be used.

[0078] In step S105, the gNB200b transmits an acknowledgement message (LTM HO Request Ack) indicating acceptance of the request of step S103 to the gNB200a. The gNB200a receives the acknowledgement message (LTM HO Request Ack). The acknowledgement message (LTM HO Request Ack) may include an LTM indicator and may be a Handover Request Ack message used in general handover. Alternatively, the acknowledgement message (LTM HO Request Ack) may be a new message different from the Handover Request Ack message, for example, an LTM Handover Request Ack message. The acknowledgement message (LTM HO Request Ack) may include information indicating an early synchronization CFRA resource (e.g., an RA preamble and / or a PRACH (Physical Random Access Channel) resource) configured by the gNB200b for the second cell. Note that the acknowledgement message (LTM HO Request Ack) may include RRC reconfiguration information (RRC Reconfiguration) of the UE100 to be applied in the second cell, as in a general handover. The acknowledgement message (LTM HO Request Ack) 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.

[0079] In step S106, the gNB 200a transmits an RRC Reconfiguration message including the LTM configuration (LTM Candidate Configuration, LTM-Candidate) of the second cell to the UE 100. The UE 100 receives the RRC Reconfiguration message. The RRC Reconfiguration message may include information indicating the CFRA resource for early synchronization configured by the gNB 200b for 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.

[0080] In step S107, the UE 100 saves the LTM setting and transmits an RRC Reconfiguration Complete message to the gNB 200a. The gNB 200a receives the RRC Reconfiguration Complete message.

[0081] In step S108, the UE 100 may transmit an L1 measurement report (or an L3 measurement report) to the gNB 200a for the gNB 200a to determine early synchronization. The gNB 200a may receive the L1 measurement report (or the L3 measurement report).

[0082] In step S109, gNB200a may make a decision to perform early synchronization.

[0083] In step S110, the gNB 200a may transmit an Early sync CFRA Request message to the gNB 200b, which is a request message requesting preparation of a CFRA resource for early synchronization, specifically, configuration and / or activation (validation) of a CFRA resource for early synchronization. The gNB 200b may receive the request message (Early sync CFRA Request message). The request message (Early sync CFRA Request message) may include an identifier (Xn-AP UE ID) for identifying the UE 100 and / or an identifier (cell ID) for identifying the second cell.

[0084] In step S111, gNB200b may prepare CFRA resources for early synchronization.

[0085] In step S112, the gNB 200b may transmit a notification message indicating that the CFRA resource for early synchronization has been prepared, for example, an Early sync CFRA Request Ack message, to the gNB 200a. The gNB 200a may receive the notification message (Early sync CFRA Request Ack message).

[0086] In step S113, the gNB 200a may transmit a PDCCH order to the UE 100 and instruct the UE 100 to perform CFRA for early synchronization. The UE 100 receives the PDCCH order. The PDCCH order may include information (Target cell indicator) for identifying the second cell as a target of the CFRA. 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.

[0087] In step S114, the UE 100 may perform early synchronization of the downlink (DL) with the second cell. For example, the UE 100 performs timing synchronization using the SSB (PSS / SSS) of the second cell. Note that the UE 100 may have performed DL synchronization before this point.

[0088] In step S115, the UE 100 transmits a CFRA, specifically, an RA preamble on a PRACH, to the second cell specified in the PDCCH order in order to perform early synchronization of the uplink (UL) with the second cell. The gNB 200b receives the RA preamble. The UE 100 identifies the CFRA resource (for example, an RA preamble and / or a PRACH resource) based on information set in the SIB or the like and information such as a "Random Access Preamble Index" and a "PRACH Mask Index" in the PDCCH order.

[0089] In step S116, the gNB 200b may transmit an RAR including a TA value derived based on the RA preamble to the UE 100. The UE 100 may receive the RAR. Step S116 may be an optional step that is executed only if there is a setting from the gNB 200a (e.g., the setting in step S106).

[0090] The UE 100 may transmit a notification (Early Sync Complete) indicating that the UL early synchronization with the second cell has been completed to the gNB 200a (step S117). The notification (Early Sync Complete) may include the TA value notified by the RAR.

[0091] In step S118, the gNB 200b may transmit to the gNB 200a a notification message (Early Sync Complete) indicating that UL early synchronization with the UE 100 has been completed. The gNB 200a may receive the notification message (Early Sync Complete). The notification message (Early Sync Complete) may include the TA value derived based on the RA preamble of step S115.

[0092] In step S119, the UE 100 transmits an L1 measurement report to the gNB 200a. The gNB 200a receives the L1 measurement report.

[0093] In step S120, when gNB200a determines that the possibility of LTM execution has increased, for example, based on the L1 measurement report of step S119, it may transmit a UL resource request message to gNB200b. gNB200b may receive the request message. The UL resource request may be a request for preparation or activation of CFRA resources. The UL resource request may be a request for preparation or execution of UL grant transmission to UE100. The UL resource request may be a request for preparation or activation of UL configured grant (CG) resources. Note that the transmission of the request message in step S120 may be simultaneous with the LTM execution decision in step S121. The transmission may be after the LTM execution decision in step S121.

[0094] In step S121, gNB200a decides to perform LTM based on the L1 measurement report of step S119.

[0095] In step S122, in response to the LTM execution decision, the gNB 200a transmits a cell switching command (MAC CE) to the UE 100. The UE 100 receives the cell switching command. The cell switching command may include the TA value notified to the gNB 200a in step S117 or S118.

[0096] In step S123, in response to the reception of the cell switching command, the UE 100 detaches from the first cell (source cell) and applies the LTM setting of the second cell (target cell).

[0097] In step S124, if the cell switch command does not include a TA value (a valid TA value), the UE 100 may perform a random access procedure for the second cell.

[0098] In step S125, the UE 100 transmits an RRC Reconfiguration Complete message to the second cell. The gNB 200b receives the RRC Reconfiguration Complete message.

[0099] In step S126, the gNB 200b transmits DCI including a CRC (Cyclic Redundancy Code) scrambled with the C-RNTI assigned to the UE 100 to the UE 100 on the PDCCH, and may transmit a Contention Resolution MAC CE to the UE 100 on the PDSCH assigned by the DCI. The UE 100 may receive the DCI and the Contention Resolution MAC CE.

[0100] In step S127, gNB200b may transmit a notification message (LTM HO Success) to gNB200a indicating that the inter-network node LTM to the second cell has been completed. gNB200a may receive the notification message (LTM HO Success).

[0101] (4) Operation of Mobile Communication System Based on the above-described configuration and operation, the operation of the mobile communication system 1 according to the embodiment will be described. In the following embodiment, in a DC scenario such as that shown in Fig. 6, an operation of switching the MN 200M by the Inter-CU LTM (also referred to as "Inter-CU MCG LTM") as shown in Fig. 8 is assumed.

[0102] (4.1) LTM Configuration Fig. 9 is a diagram showing an example of an LTM configuration (LTM Candidate Configuration) set in the UE 100 from the network 5 (for example, the MN 200M). In the following embodiment, it is assumed that the UE 100 receives the LTM configuration as shown in Fig. 9 in, for example, an RRC Reconfiguration message from the MN 200M, and stores the received LTM configuration.

[0103] According to the current 3GPP agreement, LTM candidates for inter-CU LTM cannot be configured simultaneously for MCG and SCG, but there are no restrictions on intra-CU LTM. Based on this agreement, intra-CU LTM in Release 18 and inter-CU LTM in Release 19 can be configured as shown in Figure 9.

[0104] The UE 100 receives an MCG LTM Config (also referred to as the "first LTM configuration"), which is an LTM configuration for the MCG (MN 200M), and stores the received MCG LTM Config as an MCG VarLTM-Config (first LTM configuration). The first LTM configuration is a configuration for performing LTM cell switching for the MN 200M (PCell). The first LTM configuration can be configured with a mixture of intra-CU cells and inter-CU cells. When performing LTM cell switching for the MN 200M (PCell), it is necessary to release the SN 200S. Therefore, the first LTM configuration may also include a configuration of the SN 200S for performing SN addition after completing the LTM cell switching.

[0105] In the illustrated example, the first LTM configuration includes a plurality of LTM candidate configurations (LTM-Candidate). Specifically, the first LTM configuration includes an LTM candidate configuration (LTM-Candidate) with a candidate configuration index (ID) = 1, an LTM candidate configuration (LTM-Candidate) with a candidate configuration index (ID) = 2, ... Each LTM candidate configuration (LTM-Candidate) includes an MCG configuration used for LTM cell switching of the MCG (MN200M), and an SCG configuration used to add an SCG (SN200S) after LTM cell switching of the MCG (MN200M). Note that, when the UE 100 receives a cell switching command MAC CE from the MN 200M, the UE 100 determines to switch the LTM cell of the MCG (MN200M) (i.e., to use the first LTM configuration). On the other hand, when the UE 100 receives the cell switching command MAC CE from the SN 200S, the UE 100 determines that the LTM cell switching of the SCG (SN 200S) is to be performed (that is, the second LTM setting is used).

[0106] Furthermore, UE100 receives SCG LTM Config (also referred to as "second LTM setting"), which is an LTM setting for SCG (SN200S), and stores the received SCG LTM Config as SCG VarLTM-Config (second LTM setting). The second LTM setting is a setting for performing LTM cell switching for SN200S (PSCell). The second LTM setting can only be set for intra-CU cells. When performing LTM cell switching for SN200S (PSCell), the LTM cell switching is performed while UE100 is connected to MN200M, so the first LTM setting does not include the setting for MN200M, and only includes the setting for SN200S.

[0107] In the illustrated example, the second LTM configuration includes a plurality of LTM candidate configurations (LTM-Candidate). Specifically, the second LTM configuration includes an LTM candidate configuration (LTM-Candidate) with a candidate configuration index (ID) = 1, an LTM candidate configuration (LTM-Candidate) with a candidate configuration index (ID) = 2, .... Each LTM candidate configuration (LTM-Candidate) includes an SCG configuration used for LTM cell switching within the SN200S.

[0108] In the current LTM technical specifications, when the UE 100 performs an LTM cell switch, the UE 100 retains the stored LTM configuration without releasing it in order to perform a subsequent LTM cell switch (also referred to as "Subsequent LTM") without RRC reconfiguration. Specifically, the UE 100 can repeat the procedures of initial synchronization, LTM cell switch execution, and LTM cell switch completion without releasing other LTM candidate configurations after each LTM cell switch is completed. Therefore, the UE 100 retains the LTM configuration until an instruction is received from the network 5 side. In addition, in the current LTM technical specifications, the UE 100 performs L1 measurement for each configured candidate cell and transmits an L1 measurement report to the gNB 200.

[0109] (4.2) Inter-CU MCG LTM For the Inter-CU MCG LTM, there are a first case in which SN200S is released (i.e., a case in which SN200S is changed) and a second case in which SN200S is not changed.

[0110] FIG. 10 is a diagram for explaining a first case of inter-CU MCG LTM according to the embodiment.

[0111] In step S11, UE100 in the RRC connected state is performing DC communication with MN200M#1 (MCG) and SN200S#1 (SCG). UE100 has the LTM setting as shown in FIG. 9 configured by, for example, an RRC Reconfiguration message from MN200M#1. UE100 stores the first LTM setting and the second LTM setting as shown in FIG. 9. The first LTM setting includes information (MCG setting) used for the first LTM cell switching (i.e., inter-CU MCG LTM), which is the LTM cell switching from MN200M#1 to another MN200M#2. The second LTM configuration includes information (SCG configuration) used for the second LTM cell switch (i.e., intra-CU SCG LTM), which is an LTM cell switch within SN200S#1.

[0112] In step S12, UE100 performs LTM cell switching by inter-CU MCG LTM using the first LTM configuration. Specifically, UE100 performs LTM cell switching from MN200M#1 to MN200M#2. Here, UE100 applies the target MCG configuration in the stored first LTM configuration and establishes a connection with MN200M#2. UE100 also releases the connection with SN200S#1.

[0113] In step S13a, UE100 applies the SCG setting associated with the target MCG in the stored first LTM setting, and establishes a connection with SN200S#2 (SN Addition). In the first case, SN200S is changed from SN200S#1 to SN200S#2 in association with the inter-CU MCG LTM. Note that in the first case, SN200S#2 does not necessarily have to be added in association with the inter-CU MCG LTM. In other words, changing SN200S is not limited to cases where SN200S is changed before and after inter-CU MCG LTM, and may also include cases where SN200S is not added after inter-CU MCG LTM.

[0114] In step S14a, UE100 performs DC communication with MN200M#2 (MCG) and SN200S#2 (SCG).

[0115] FIG. 11 is a diagram for explaining a second case of the inter-CU MCG LTM according to the embodiment.

[0116] Steps S11 and S12 are the same as in the first case.

[0117] In step S13b, UE100 applies the SCG setting associated with the target MCG in the stored first LTM setting and establishes a connection with SN200S#1 (SN Addition). In the second case, SN200S#1 is not changed due to the inter-CU MCG LTM.

[0118] In step S14b, UE100 performs DC communication with MN200M#2 (MCG) and SN200S#1 (SCG).

[0119] As described above, there are a first case and a second case for the inter-CU MCG LTM. In the second case, since the SN200S#1 is not changed in association with the inter-CU MCG LTM, the second LTM setting shown in FIG. 9 can be applied after the inter-CU MCG LTM. Therefore, the UE100 can apply the second LTM setting to LTM cell switching (intra-CU LTM) within the SN200S#1 by retaining the second LTM setting even after the inter-CU MCG LTM.

[0120] On the other hand, in the first case, since the SN200S is changed due to the inter-CU MCG LTM, the second LTM configuration shown in FIG. 9 cannot be applied after the inter-CU MCG LTM. However, since the UE100 maintains the second LTM configuration even after the inter-CU MCG LTM, the UE100 may need to perform L1 measurement (and report) for each candidate cell in the second LTM configuration. It is not desirable for such unnecessary processing to increase the power consumption of the UE100. Furthermore, in the first case, maintaining the second LTM configuration by the UE100 even after the inter-CU MCG LTM may result in unnecessary consumption of the memory capacity of the UE100. Therefore, various problems may occur if the UE100 maintains the second LTM configuration even after the inter-CU MCG LTM.

[0121] Therefore, in the embodiment, the UE 100 that stores the first LTM setting and the second LTM setting disables the second LTM setting based on performing the first LTM cell switch using the first LTM setting. This makes it possible to solve problems that may occur when the UE 100 retains the second LTM setting even after the inter-CU MCG LTM. Note that disabling the second LTM setting means discarding the second LTM setting. Note that "discard" can be replaced with (synonymous with) "delete" or "release."

[0122] Alternatively, disabling the second LTM setting may be suspending the second LTM setting. Suspension may be a state in which use of the setting is stopped (interrupted). After suspending the second LTM setting, the UE 100 may resume the second LTM setting when adding the SN 200S #1. Suspension may also be a state in which the setting is used. Note that suspension and resume are interchangeable (synonymous) with deactivation and activation, respectively.

[0123] The UE 100 according to the embodiment has a communication unit (wireless communication unit 140) that communicates with MN 200M #1 and SN 200S #1, and a control unit 130 that stores a first LTM setting including information used for a first LTM cell switch, which is an LTM cell switch from MN 200M #1 to another MN 200M #2, and a second LTM setting including information used for a second LTM cell switch, which is an LTM cell switch within SN 200S #1. The control unit 130 disables the second LTM setting based on the first LTM cell switch being performed using the first LTM setting.

[0124] In a first operation pattern of the embodiment, when the UE 100 performs a first LTM cell switch using the first LTM setting and SN200S#1 is changed with the first LTM cell switch (i.e., the first case), the UE 100 discards the second LTM setting. On the other hand, when the UE 100 performs a first LTM cell switch using the first LTM setting and SN200S#1 is not changed with the first LTM cell switch (i.e., the second case), the UE 100 retains the second LTM setting. This allows the UE 100 to perform an appropriate operation for each case, since the second LTM setting is retained while the second LTM setting is discarded in the first case.

[0125] In a second operation pattern of the embodiment, when UE100 performs a first LTM cell switch using the first LTM configuration, UE100 discards the second LTM configuration regardless of whether SN200S#1 is changed due to the first LTM cell switch. Thus, in the second operation pattern, UE100 discards the second LTM configuration in both the first case and the second case. This simplifies the operation of UE100 compared to the first operation pattern. Note that in the second operation pattern, after the first LTM cell switch, the second LTM configuration may be reconfigured in UE100 by an RRC Reconfiguration message from network 5 (e.g., MN200M#2).

[0126] (4.3) Example of First Operation Pattern FIG. 12 is a diagram illustrating an example of a first operation pattern according to the embodiment.

[0127] In step S201, UE100 performs DC communication with MN200M#1 and SN200S#1 shown in FIG. 10 or 11. UE100 also stores a first LTM setting (MCG VarLTM-Config) including information used for a first LTM cell switch (inter-CU MCG LTM), which is an LTM cell switch from MN200M#1 to another MN200M#2. UE100 also stores a second LTM setting (SCG VarLTM-Config) including information used for a second LTM cell switch (intra-CU SCG LTM), which is an LTM cell switch within SN200S#1.

[0128] In step S202, the network 5 (for example, MN 200M#1) determines whether to perform MCG LTM of an inter-CU or intra-CU. If it is determined that MCG LTM is to be performed (step S202: YES), the process proceeds to step S205.

[0129] If it is determined that MCG LTM is not to be performed (step S202: NO), in step S203, the network 5 (e.g., SN200S#1) determines whether to perform SCG LTM of the intra-CU. If it is determined that SCG LTM of the intra-CU is to be performed (step S203: YES), in step S204, the network 5 (e.g., SN200S#1) controls the UE 100 to perform SCG LTM of the intra-CU. In this case, the UE 100 performs SCG LTM of the intra-CU using the second LTM setting (SCG VarLTM-Config). At that time, the UE 100 maintains its connection with the MN 200M#1. On the other hand, if it is determined that SCG LTM of the intra-CU is not to be performed (step S203: NO), the process returns to step S202.

[0130] If it is determined that MCG LTM of the intra-CU is to be performed (step S205: NO), in step S207, the network 5 (e.g., MN200M#1) controls the UE100 to perform MCG LTM of the intra-CU. In this case, the UE100 performs MCG LTM of the intra-CU using the first LTM setting (MCGVarLTM-Config). At that time, the UE100 maintains the connection with the SN200S#1.

[0131] On the other hand, if it is determined that an MCG LTM of an inter-CU is to be performed (step S205: YES), in step S206, the network 5 (e.g., MN200M#1) releases SN200S#1 and controls UE100 to perform an MCG LTM of an inter-CU. In this case, UE100 releases SN200S#1 and performs an MCG LTM of an inter-CU using the first LTM setting (VarLTM-Config for MCG). Specifically, UE100 performs an MCG LTM of an inter-CU from MN200M#1 to MN200M#2, as shown in FIG. 10 or 11 .

[0132] In step S208, the UE 100 or the network 5 (for example, the MN 200M#2) determines whether the SN 200S#1 is changed in accordance with the MCG LTM of the inter-CU.

[0133] When UE100 makes the determination in step S208, UE100 determines whether SN200S#1 will be changed in conjunction with the MCG LTM of the inter-CU by comparing the node identifier (gNB Identity) contained in the system information of SN200S#1 before the MCG LTM of the inter-CU with the node identifier (gNB Identity) contained in the system information of SN200S (SN200S#1 or SN200S#2) with which UE100 communicates after the MCG LTM of the inter-CU.

[0134] Specifically, since the MCG VarLTM-Config includes the settings of the SCG (SN200S) to be connected after executing the InterCU MCG LTM, the UE100 applies the settings to acquire the system information block type 1 (SIB1) broadcast by the SN200S, and acquires the gNB identity in the NCI (NR cell identity) included in the SIB1. The NCI includes the gNB identity and cell identifier (Cell Identity), and the gNB identity can be 22 bits or 32 bits long. The bit length of the gNB identity can be determined from the gNB-ID-Length in SIB1. In a similar manner, UE100 knows the gNB identity of SN200S #1 before the MCG LTM of the inter-CU. Then, UE100 can determine whether SN200S is changed before and after the MCG LTM of the inter-CU by comparing the gNB identity of SN200S before and after the MCG LTM of the inter-CU.

[0135] When the determination in step S208 is made by the network 5, the MN 200M#2 may make the determination in step S208 in the following manner: The MN 200M#2 may notify the MN 200M#1 of the determination result.

[0136] Specifically, first, MN200M # 2 exchanges cell information (List of Served Cells NR) in Xn Setup Request and Xn Setup Response messages to establish an Xn connection with SN200S (which is simply a neighboring gNB at this point). The cell information (Served Cell Information NR) in the List of Served Cells NR includes NR PCI (Physical Cell ID) and DL Frequency Info (ARFCN: Absolute radio-frequency channel number). This allows MN200M#2 to obtain information about the cells under the control of SN200S with which the Xn connection is established.

[0137] Secondly, MN200M#2 receives a request message (LTM HO Request) from MN200M#1 and receives the UE context in the HandoverPreparationInformation included in the message. The UE context includes CellGroupConfig for SN200S#1 (SCG) as information set in UE100. MN200M#2 acquires two pieces of information, physCellId and FrequencyInfoDL, in CellGroupConfig of SN200S#1 (SCG) set in UE100.

[0138] Third, MN200M#2 identifies the information (physCellId and FrequencyInfoDL) of SN200S (MCG) to be added after inter-CU MCG LTM from the cell information at the time of establishment of Xn connection, and compares the identified information with the information (physCellId and FrequencyInfoDL) in the UE context. That is, MN200M#2 can determine whether SN200S is changed before and after inter-CU MCG LTM by comparing the information (physCellId and FrequencyInfoDL) of SN200S before and after inter-CU MCG LTM.

[0139] Alternatively, when the network 5 makes the determination in step S208, the MN 200M#2 may make the determination in step S208 in the following manner: The MN 200M#2 may notify the MN 200M#1 of the determination result.

[0140] First, MN200M#2 receives a request message (LTM HO Request) from MN200M#1, and the message includes the gNB identity of SN200S that currently has a DC connection. MN200M#2 identifies the gNB identity of SN200S that currently has a DC connection.

[0141] Secondly, MN200M#2 obtains information about SN200S that is set in advance in UE100, specifically, the settings of the SCG (SN200S) to be connected after executing InterCU MCG LTM in VarLTM-Config for MCG, and from the settings, identifies the gNB Identity after executing InterCU MCG LTM.

[0142] Third, MN200M#2 can determine whether SN200S is changed before and after interCU MCG LTM by comparing the gNB identity of SN200S that currently has a DC connection with the gNB identity after interCU MCG LTM is executed.

[0143] When the network 5 makes the determination in step S208, the network node (MN200M#1 or MN200M#2) may send notification information to UE100 to discard the second LTM setting (VarLTM-Config for SCG) during the MCG LTM of the interCU, depending on the determination result that SN200S will be changed before and after the MCG LTM of the interCU.

[0144] For example, before executing the MCG LTM of the inter-CU, MN200M#1 may include the notification information in a cell switching command MAC CE that instructs the execution of the MCG LTM of the inter-CU, and transmit the cell switching command MAC CE including the notification information to UE100. In this case, UE100 may discard the second LTM setting (VarLTM-Config for SCG) in response to receiving the cell switching command MAC CE including the notification information. After receiving the cell switching command MAC CE including the notification information, UE100 may discard the second LTM setting (VarLTM-Config for SCG) when accessing or connecting to SN200S#2.

[0145] Alternatively, MN200M#2 may include the notification information in the LTM candidate configuration in the acknowledgement message (LTM HO Request Ack) before executing the MCG LTM of the inter-CU. In this case, MN200M#1 transmits an RRC Reconfiguration message including the notification information to UE100. For example, MN200M#1 may include the notification information in an LTM-Candidate whose SN200S is changed before and after the MCG LTM of the inter-CU, among multiple LTM-Candidates in the first LTM setting shown in Fig. 9 . In this case, the UE 100 may discard the second LTM setting (SCG VarLTM-Config) in response to applying the LTM-Candidate including the notification information and executing (applying) the MCG LTM. The UE 100 may discard the second LTM setting (SCG VarLTM-Config) when accessing or connecting to the SN 200S #2.

[0146] Alternatively, MN200M#2 may include the notification information in an RRC Reconfiguration message that is sent from MN200M#2 to UE 100 after completion of InterCU MCG LTM. In this case, UE 100 may discard the second LTM setting (VarLTM-Config for SCG) in response to receiving the RRC Reconfiguration message from MN200M#2.

[0147] If it is determined that SN200S#1 is changed in accordance with the MCG LTM of the inter-CU (step S208: YES), in step S209, UE100 discards the second LTM setting (VarLTM-Config for SCG). On the other hand, if it is determined that SN200S#1 is not changed in accordance with the MCG LTM of the inter-CU (step S208: NO), in step S210, UE100 retains the second LTM setting (VarLTM-Config for SCG).

[0148] After receiving the cell switching command MAC CE, the UE 100 may discard the second LTM setting when it determines that the SN 200S # 1 will be changed due to the MCG LTM of the inter-CU. That is, the UE 100 may discard the second LTM setting (VarLTM-Config for SCG) when it receives the cell switching command MAC CE from the MN 200M # 1 and determines that the SN 200S to be connected after the LTM execution is different from the SN 200S before the LTM execution. Alternatively, the UE 100 may discard the second LTM setting (VarLTM-Config for SCG) when it accesses the MN 200M # 2 or when it establishes a connection with the MN 200M # 2. Alternatively, after accessing MN200M#2, UE100 may discard the second LTM setting (VarLTM-Config for SCG) when an SCG is set from MN200M#2. In other words, UE100 may discard the second LTM setting (VarLTM-Config for SCG) at the time when DC is set from MN200M#2.

[0149] When the second LTM setting (VarLTM-Config for SCG) is discarded, MN200M#1 or MN200M#2 may transmit to SN200S#1 a notification indicating that UE100 has discarded the second LTM setting (VarLTM-Config for SCG). This allows SN200S#1 to understand that the second LTM setting (VarLTM-Config for SCG) has been discarded.

[0150] In step S211, UE100 adds SN200S (SN200S#1 or SN200S#2) and performs DC communication between the added SN200S and MN200M#2.

[0151] (4.4) Example of Second Operation Pattern FIG. 13 is a diagram illustrating an example of a second operation pattern according to the embodiment.

[0152] In step S301, UE100 performs DC communication with MN200M#1 and SN200S#1 shown in FIG. 10 or 11. UE100 also stores a first LTM setting (MCG VarLTM-Config) including information used for a first LTM cell switch (inter-CU MCG LTM), which is an LTM cell switch from MN200M#1 to another MN200M#2. UE100 also stores a second LTM setting (SCG VarLTM-Config) including information used for a second LTM cell switch (intra-CU SCG LTM), which is an LTM cell switch within SN200S#1.

[0153] In step S302, the network 5 (for example, MN 200M#1) determines whether to perform MCG LTM of an inter-CU or intra-CU. If it is determined that MCG LTM is to be performed (step S302: YES), the process proceeds to step S305.

[0154] If it is determined that MCG LTM is not to be performed (step S302: NO), in step S303, the network 5 (e.g., SN200S#1) determines whether to perform SCG LTM of the intra-CU. If it is determined that SCG LTM of the intra-CU is to be performed (step S303: YES), in step S304, the network 5 (e.g., SN200S#1) controls the UE 100 to perform SCG LTM of the intra-CU. In this case, the UE 100 performs SCG LTM of the intra-CU using the second LTM setting (SCG VarLTM-Config). At that time, the UE 100 maintains its connection with the MN 200M#1. On the other hand, if it is determined that SCG LTM of the intra-CU is not to be performed (step S303: NO), the process returns to step S302.

[0155] If it is determined that MCG LTM of the intra-CU is to be performed (step S305: NO), in step S308, the network 5 (e.g., MN200M#1) controls UE100 to perform MCG LTM of the intra-CU. In this case, UE100 performs MCG LTM of the intra-CU using the first LTM setting (VarLTM-Config for MCG). At that time, UE100 maintains the connection with SN200S#1 and holds the second LTM setting (VarLTM-Config for SCG).

[0156] On the other hand, if it is determined that an MCG LTM of an inter-CU is to be performed (step S305: YES), in step S306, the network 5 (e.g., MN200M#1) releases SN200S#1 and controls UE100 to perform an MCG LTM of an inter-CU. In this case, UE100 releases SN200S#1 and performs an MCG LTM of an inter-CU using the first LTM setting (VarLTM-Config for MCG). Specifically, UE100 performs an MCG LTM of an inter-CU from MN200M#1 to MN200M#2, as shown in FIG. 10 or 11 .

[0157] In the second operation pattern, when UE100 performs cell switching of MCG LTM of inter-CU using the first LTM setting, UE100 discards the second LTM setting (VarLTM-Config for SCG) regardless of whether SN200S#1 is changed due to cell switching of MCG LTM of inter-CU. This determination may be made by UE100. This determination may also be made by the network 5 (for example, MN200M#1 or MN200M#2).

[0158] When the network 5 makes the determination in step S305, the network node (MN200M#1 or MN200M#2) may, depending on the determination result that MCG LTM of the inter-CU is to be performed, send notification information to UE100 to discard the second LTM setting (VarLTM-Config for SCG) during MCG LTM of the inter-CU.

[0159] For example, before executing the MCG LTM of the inter-CU, MN200M#1 may include the notification information in a cell switching command MAC CE that instructs the execution of the MCG LTM of the inter-CU, and transmit the cell switching command MAC CE including the notification information to UE100. In this case, UE100 may discard the second LTM setting (VarLTM-Config for SCG) in response to receiving the cell switching command MAC CE including the notification information. After receiving the cell switching command MAC CE including the notification information, UE100 may discard the second LTM setting (VarLTM-Config for SCG) when accessing or connecting to SN200S#2.

[0160] Alternatively, MN200M#2 may include the notification information in the LTM candidate configuration in the acknowledgement message (LTM HO Request Ack) before executing the MCG LTM of the inter-CU. In this case, MN200M#1 transmits an RRC Reconfiguration message including the notification information to UE100. For example, MN200M#1 may include the notification information in an LTM-Candidate whose SN200S is changed before and after the MCG LTM of the inter-CU, among multiple LTM-Candidates in the first LTM setting shown in Fig. 9 . In this case, the UE 100 may discard the second LTM setting (SCG VarLTM-Config) in response to applying the LTM-Candidate including the notification information and executing (applying) the MCG LTM. The UE 100 may discard the second LTM setting (SCG VarLTM-Config) when accessing or connecting to the SN 200S #2.

[0161] Alternatively, MN200M#2 may include the notification information in an RRC Reconfiguration message that is sent from MN200M#2 to UE 100 after completion of InterCU MCG LTM. In this case, UE 100 may discard the second LTM setting (VarLTM-Config for SCG) in response to receiving the RRC Reconfiguration message from MN200M#2.

[0162] When the UE 100 makes the determination in step S305, the UE 100 may determine whether to perform the MCG LTM of the inter-CU based on whether or not the UE 100 has received an instruction from a network node (e.g., MN200M#1) to re-establish PDCP and / or update the security key. If it is determined that the MCG LTM of the inter-CU is to be performed, the UE 100 may discard the second LTM setting (VarLTM-Config for SCG) at the timing of the instruction, the timing of receiving the cell switching command MAC CE, the timing of performing the MCG LTM of the inter-CU (the timing of applying the first LTM setting), the timing of accessing the MN 200M#2, or the timing of establishing a connection with the MN 200M#2. Alternatively, after accessing MN200M#2, UE100 may discard the second LTM setting (VarLTM-Config for SCG) when an SCG is set from MN200M#2. In other words, UE100 may discard the second LTM setting (VarLTM-Config for SCG) at the time when DC is set from MN200M#2.

[0163] When the second LTM setting (VarLTM-Config for SCG) is discarded, MN200M#1 or MN200M#2 may transmit to SN200S#1 a notification indicating that UE100 has discarded the second LTM setting (VarLTM-Config for SCG). This allows SN200S#1 to understand that the second LTM setting (VarLTM-Config for SCG) has been discarded.

[0164] In step S307, UE100 adds SN200S (SN200S#1 or SN200S#2) and performs DC communication between the added SN200S and MN200M#2.

[0165] (5) Other Embodiments The LTM in the above-described embodiments may be read as a conditional LTM. For example, the above-described MCG LTM of an inter-CU or intra-CU may be a conditional LTM of an MCG of an inter-CU or intra-CU. Furthermore, the above-described SCG LTM of an intra-CU may be a conditional LTM of an SCG of an intra-CU. In conditional LTM, for example, the RRC Reconfiguration message in step S4 of FIG. 6 includes information indicating the execution conditions (e.g., radio quality conditions) of LTM cell switching for each LTM candidate cell. Instead of the cell switching 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 eliminates the need to send and receive L1 measurement reports and cell switching commands MAC, enabling faster LTM cell switching.

[0166] The DC in the above-described embodiment may be read as multi-connectivity. In the DC in the above-described embodiment, one SN 200S is added. However, the operation according to the above-described embodiment may be applied to multi-connectivity in which multiple SNs 200S are added. In multi-connectivity, the UE 100 can simultaneously communicate with one MN 200M and multiple SNs 200S.

[0167] 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.

[0168] 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.

[0169] 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.

[0170] 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).

[0171] 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.

[0172] As used in this disclosure, the terms "based on" and "depending on / in response to" do not mean "based only on" or "depending only on," unless expressly stated otherwise. The term "based on" means both "based only on" and "based at least in part on." Similarly, the term "depending on" means both "depending only on" and "depending at least in part on." The terms "include," "comprise," and variations thereof do not mean including only the listed items, but may mean including only the listed items or may include additional items in addition to the listed items. Additionally, as used in this disclosure, the term "or" is not intended to mean an exclusive or. Furthermore, as used in this disclosure, any reference to an element using a designation such as "first," "second," etc. does not generally limit the quantity or order of those elements. These designations may be used herein as a convenient method of distinguishing between two or more elements. Thus, a reference to a first and a second element does not imply that only two elements may be employed therein, or that the first element must precede the second element in some way. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles are intended to include the plural unless the context clearly indicates otherwise.

[0173] 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.

[0174] This application claims priority from Japanese Patent Application No. 2024-115588 (filed July 19, 2024), the entire contents of which are incorporated herein by reference.

[0175] (6) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0176] Supplementary Note 1: A communication method used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a user equipment communicating with a master node and a secondary node; the user equipment storing a first LTM configuration including information used for a first LTM cell switch, which is an LTM cell switch from the master node to another master node; the user equipment storing a second LTM configuration including information used for a second LTM cell switch, which is an LTM cell switch within the secondary node; and the user equipment disabling the second LTM configuration based on the first LTM cell switch being performed using the first LTM configuration.

[0177] Supplementary Note 2: The communication method according to Supplementary Note 1, wherein disabling the second LTM setting is discarding the second LTM setting.

[0178] Supplementary Note 3: When the user equipment performs the first LTM cell switch using the first LTM configuration and the secondary node is changed due to the first LTM cell switch, the user equipment discards the second LTM configuration. The communication method described in Supplementary Note 2.

[0179] Supplementary Note 4: The communication method according to Supplementary Note 3, further comprising: when the user equipment performs the first LTM cell switch using the first LTM configuration and the secondary node is not changed due to the first LTM cell switch, retaining the second LTM configuration.

[0180] Supplementary Note 5: The communication method according to Supplementary Note 3 or 4, further comprising the user equipment determining whether the secondary node is changed in association with the first LTM cell switch.

[0181] Supplementary Note 6: The user equipment determines whether the secondary node is changed due to the first LTM cell switching by comparing a node identifier included in system information of the secondary node before the first LTM cell switching with a node identifier included in system information of a secondary node with which the user equipment communicates after the first LTM cell switching.

[0182] Supplementary Note 7: The communication method according to Supplementary Note 5, further comprising: the user equipment receiving a cell switch command MAC CE from the master node; and after receiving the cell switch command MAC CE, the user equipment discards the second LTM configuration when it determines that the secondary node will be changed due to the first LTM cell switch.

[0183] Supplementary Note 8: The communication method according to Supplementary Note 3, further comprising the master node or the network node being the other master node determining whether the secondary node is changed due to the first LTM cell switch.

[0184] Supplementary Note 9: The communication method according to Supplementary Note 8, further comprising: the network node transmitting notification information to the user equipment to cause the second LTM configuration to be discarded in response to a determination result that the secondary node is changed.

[0185] Supplementary Note 10: When the user equipment performs the first LTM cell switch using the first LTM configuration, the user equipment discards the second LTM configuration regardless of whether the secondary node is changed due to the first LTM cell switch. The communication method described in Supplementary Note 2.

[0186] Supplementary Note 11. The communication method according to Supplementary Note 10, further comprising the network node being the master node or the other master node determining whether the user equipment performs the first LTM cell switch.

[0187] Supplementary Note 12: The communication method according to Supplementary Note 11, further comprising the network node transmitting, to the user equipment, notification information for causing the second LTM configuration to be discarded in response to a determination result that the user equipment performs the first LTM cell switch.

[0188] Supplementary Note 13: The communication method of Supplementary Note 10, further comprising: determining whether to perform the first LTM cell switch based on whether the user equipment is instructed by a network node to perform PDCP re-establishment and / or security key update.

[0189] Supplementary Note 14: The communication method according to Supplementary Note 13, wherein the user equipment discards the second LTM configuration when the PDCP re-establishment and / or the security key update is instructed by the network node.

[0190] Supplementary Note 15: The communication method according to Supplementary Note 9 or 12, wherein the master node transmits an RRC message including the notification information to the user equipment before the first LTM cell switch is performed.

[0191] Supplementary Note 16: The communication method according to Supplementary Note 9 or 12, wherein the master node transmits a cell switch command MAC CE including the notification information to the user equipment before the first LTM cell switch is executed.

[0192] Supplementary Note 17: The communication method according to Supplementary Note 9 or 12, wherein the another master node transmits an RRC message including the notification information to the user equipment after the first LTM cell switch is performed.

[0193] Supplementary Note 18: The communication method according to Supplementary Note 9 or 12, wherein the notification information is included in an RRC message or a cell switch command (MAC CE) transmitted by the master node to the user equipment, and the user equipment discards the second LTM configuration upon receiving the cell switch command (MAC CE).

[0194] Supplementary Note 19: The communication method according to Supplementary Note 9 or 12, wherein the notification information is included in an RRC message or a cell switch command (MAC CE) transmitted by the master node to the user equipment, and the user equipment, after receiving the cell switch command (MAC CE), discards the second LTM configuration when accessing a target specified in the cell switch command (MAC CE).

[0195] Supplementary Note 20: The communication method according to Supplementary Note 2, wherein the user device, after accessing the other master node, discards the second LTM setting when establishing a connection with the other master node.

[0196] Supplementary Note 21: The communication method according to Supplementary Note 2, wherein the user equipment discards the second LTM configuration when a secondary cell group is configured from the other master node after accessing the other master node.

[0197] Supplementary Note 22: The communication method of Supplementary Note 2, further comprising the network node being the master node or the other master node sending a notification to the secondary node indicating that the user equipment has abandoned the second LTM configuration.

[0198] Supplementary Note 23: A user equipment used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a communication unit that communicates with a master node and a secondary node; and a control unit that stores a first LTM configuration including information used for a first LTM cell switch, which is an LTM cell switch from the master node to another master node, and a second LTM configuration including information used for a second LTM cell switch, which is an LTM cell switch within the secondary node, wherein the control unit disables the second LTM configuration based on the first LTM cell switch being performed using the first LTM configuration.

[0199] 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 200M: MN 200S: SN 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 communicating with a master node and a secondary node; the user equipment storing a first LTM configuration including information used for a first LTM cell switch, which is an LTM cell switch from the master node to another master node; the user equipment storing a second LTM configuration including information used for a second LTM cell switch, which is an LTM cell switch within the secondary node; and the user equipment disabling the second LTM configuration in response to an instruction received from the master node or a network node that is the other master node, based on the user equipment performing the first LTM cell switch using the first LTM configuration.

2. The communication method according to claim 1, wherein disabling the second LTM setting comprises discarding the second LTM setting.

3. The communication method according to claim 2, wherein the user equipment discards the second LTM configuration when the first LTM cell switch is performed using the first LTM configuration and when the secondary node is changed in association with the first LTM cell switch.

4. The communication method according to claim 3, further comprising: when the user equipment performs the first LTM cell switch using the first LTM configuration and the secondary node is not changed in association with the first LTM cell switch, retaining the second LTM configuration.

5. The communication method according to claim 3 or 4, further comprising the user equipment determining whether the secondary node is changed in association with the first LTM cell switch.

6. The communication method according to claim 5, wherein the user equipment determines whether the secondary node will be changed in association with the first LTM cell switching by comparing a node identifier included in system information of the secondary node before the first LTM cell switching with a node identifier included in system information of a secondary node with which the user equipment communicates after the first LTM cell switching.

7. The communication method according to claim 5, further comprising: the user equipment receiving a cell switch command MAC CE from the master node; and after receiving the cell switch command MAC CE, the user equipment discarding the second LTM configuration when it determines that the secondary node will be changed in association with the first LTM cell switch.

8. The communication method according to claim 3, further comprising the network node being the master node or the other master node determining whether the secondary node is changed in association with the first LTM cell switch.

9. The communication method according to claim 8, further comprising the network node transmitting notification information to the user equipment to cause the second LTM configuration to be discarded in response to a determination result that the secondary node is to be changed.

10. The communication method according to claim 2, wherein when the user equipment performs the first LTM cell switch using the first LTM configuration, the user equipment discards the second LTM configuration regardless of whether the secondary node is changed as a result of the first LTM cell switch.

11. The communication method according to claim 10, further comprising the network node being the master node or the other master node determining whether the user equipment performs the first LTM cell switch.

12. The communication method according to claim 11, further comprising the network node transmitting notification information to the user equipment to cause the second LTM configuration to be discarded in response to a determination result that the user equipment will perform the first LTM cell switch.

13. The communication method according to claim 10, further comprising: determining whether the user equipment performs the first LTM cell switch based on whether the user equipment receives an instruction from a network node to re-establish PDCP and / or update a security key.

14. The communication method according to claim 13, wherein the user equipment discards the second LTM configuration when the network node instructs the user equipment to re-establish the PDCP and / or update the security key.

15. The communication method according to claim 9 or 12, wherein the master node transmits an RRC message including the notification information to the user equipment before the first LTM cell switch is performed.

16. The communication method according to claim 9 or 12, wherein the master node transmits a cell switch command MAC CE including the notification information to the user equipment before the first LTM cell switch is executed.

17. The communication method according to claim 9 or 12, wherein the other master node transmits an RRC message including the notification information to the user equipment after the first LTM cell switch is executed.

18. The communication method according to claim 9 or 12, wherein the notification information is included in an RRC message or a cell switch command (MAC CE) transmitted by the master node to the user equipment, and the user equipment discards the second LTM configuration upon receiving the cell switch command (MAC CE).

19. The communication method according to claim 9 or 12, wherein the notification information is included in an RRC message or a cell switch command (MAC CE) transmitted by the master node to the user equipment, and the user equipment, after receiving the cell switch command (MAC CE), discards the second LTM configuration when accessing a target specified in the cell switch command (MAC CE).

20. The communication method according to claim 2, wherein the user device, after accessing the other master node, discards the second LTM setting when establishing a connection with the other master node.

21. The communication method according to claim 2, wherein the user equipment discards the second LTM configuration when a secondary cell group is configured from the other master node after accessing the other master node.

22. The communication method of claim 2, further comprising the network node being the master node or the other master node sending a notification to the secondary node indicating that the user equipment has abandoned the second LTM configuration.

23. A user equipment used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a communication unit that communicates with a master node and a secondary node; and a control unit that stores a first LTM configuration including information used for a first LTM cell switch, which is an LTM cell switch from the master node to another master node, and a second LTM configuration including information used for a second LTM cell switch, which is an LTM cell switch within the secondary node, wherein the control unit disables the second LTM configuration in response to an instruction received from the master node or a network node that is the other master node, based on performing the first LTM cell switch using the first LTM configuration.

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