Communication method and user device

Abstract

This communication method executed by a user device includes: receiving, from a serving cell of the user device, a first identifier associated with the serving cell and a second identifier associated with an LTM (L1 / L2 Triggered Mobility) candidate cell configured in the user device; storing an SCG LTM configuration used for LTM cell-switching in a secondary cell group (SCG) configured in the user device; triggering LTM cell-switching from the serving cell to the LTM candidate cell; and discarding the SCG LTM configuration in response to the LTM cell-switching being triggered and the first identifier and the second identifier being different.

Description

Communication Method and User Equipment

[0001] The present disclosure relates to a communication method and a user equipment used in a mobile communication system.

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

[0003] On the other hand, in 3GPP Release 18 (3GPP Release 18), the technical specifications of LTM (L1 / L2-Triggered Mobility), which is a new procedure for serving cell switching, are formulated. In LTM, a network node receives a layer 1 (L1) measurement report from a user equipment, and based on this, the network node changes the serving cell of the user equipment by a cell switching command signaled to the user equipment by a media access control (MAC) control element (CE).

[0004] 3GPP Technical Specification "3GPP TS 38.300 V18.2.0"

[0005] The present disclosure provides a technology for improving LTM.

[0006] The first aspect of the communication method is a communication method performed by a user device, comprising: receiving from the serving cell a first identifier associated with the serving cell of the user device and a second identifier associated with an LTM (L1 / L2 Triggered Mobility) candidate cell set up in the user device; storing an SCG LTM setting used for switching LTM cells within a secondary cell group (SCG) set up in the user device; triggering an LTM cell switch from the serving cell to the LTM candidate cell; and discarding the SCG LTM setting in accordance with the triggering of the LTM cell switch and the fact that the first identifier and the second identifier are different.

[0007] A user device according to the second embodiment includes a receiving unit that receives from the serving cell a first identifier associated with the serving cell of the user device and a second identifier associated with an LTM (L1 / L2 Triggered Mobility) candidate cell set in the user device, and a control unit that stores an SCG LTM setting used for switching LTM cells within a secondary cell group (SCG) set in the user device. The control unit triggers an LTM cell switch from the serving cell to the LTM candidate cell and discards the SCG LTM setting if the first identifier and the second identifier are different.

[0008] This figure shows an example configuration of a mobile communication system according to an embodiment. This figure shows an example configuration of a UE (User Equipment) according to an embodiment. This figure shows an example configuration of a gNB (Network Node) according to an embodiment. This figure shows the configuration of the protocol stack of the wireless interface of the user plane that handles data. This figure shows the configuration of the protocol stack of the wireless interface of the control plane that handles signaling (control signals). This figure is for explaining dual connectivity (DC) according to the first embodiment. This figure shows an example of a cell switching procedure by LTM in an intraCU (i.e., within the same gNB) according to the first embodiment. This figure shows an example of operation of an interCU LTM according to the first embodiment. This figure shows an example of an LTM setting (LTM Candidate Configuration) set on the UE from a network (e.g., MN) according to the first embodiment. This figure is for explaining the first case of an interCU MCG LTM according to the first embodiment. This figure is for explaining the second case of an interCU MCG LTM according to the first embodiment. This figure shows an example of the first operation pattern of an LTM according to the first embodiment. This figure shows an example of the second operation pattern of an LTM according to the first embodiment. This figure shows an example of LTM settings (LTM-Config) set in the UE according to the first embodiment. This figure shows an example of the operation of the UE according to the first embodiment. This figure is for explaining the overview of the operation according to the second embodiment. This figure shows an example of the operation of the mobile communication system according to the second embodiment.

[0009] A mobile communication system according to an embodiment will be described with reference to the drawings. In the drawings, identical or similar parts are denoted by the same or similar reference numerals.

[0010] (1) First Embodiment The first embodiment will be described below.

[0011] (1.1) Diagram 1 of the mobile communication system configuration shows an example of the configuration of the mobile communication system 1 according to this embodiment. The mobile communication system 1 conforms to the 5th generation system (5GS) of the 3GPP standard. In the following description, 5GS will be used as an example, but the mobile communication system may also have an LTE (Long Term Evolution) system applied to it at least partially. The mobile communication system may also have a 6th generation (6G) system applied to it at least partially.

[0012] The mobile communication system 1 comprises 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, NG-RAN 10 may be simply referred to as RAN 10, and 5GC 20 may be simply referred to as the core network (CN) 20. RAN 10 and CN 20 constitute the network 5 of the mobile communication system 1.

[0013] UE100 is a mobile wireless communication device. UE100 can be any device used by a user. For example, UE100 can 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 attached to a sensor, a vehicle or a device attached to a vehicle (Vehicle UE), or an aircraft or a device attached to an aircraft (Aerial UE). The link from UE100 to network 5 in the transmission direction is called the uplink (UL), and the link from network 5 to UE100 in the transmission direction is called the downlink (DL).

[0014] NG-RAN10 includes a base station (referred to as "gNB" in the 5G system) 200, which is a type of network node. The gNBs 200 are interconnected via the Xn interface, which is an inter-base station interface. The gNBs 200 manage one or more cells. The gNBs 200 perform wireless communication with UEs 100 that have established a connection with their own cell. The gNBs 200 have radio resource management (RRM) functions, user data (hereinafter simply referred to as "data") routing functions, measurement and control functions for mobility control and scheduling, etc. "Cell" is used as a term to indicate the smallest unit of a wireless communication area. "Cell" is also used as a term to indicate a function or resource that performs wireless communication with a UE 100. "Cell" is identified by a cell identifier (cell ID). One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

[0015] Furthermore, gNBs can also connect to the EPC (Evolved Packet Core), which is the core network of LTE. LTE base stations can also connect to 5GC. LTE base stations and gNBs can also be connected via an inter-base station interface.

[0016] 5GC20 includes AMF (Access and Mobility Management Function) and UPF (User Plane Function) 300. The AMF performs various mobility controls for UE100. The AMF manages the mobility of UE100 by communicating with UE100 using NAS (Non-Access Stratum) signaling. The UPF controls data transfer. The AMF and UPF are connected to gNB200 via the NG interface, which is the base station-core network interface.

[0017] Figure 2 shows an example configuration of UE100 (user device) according to this embodiment. UE100 includes 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 gNB200.

[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 the radio signal received by the antenna into a baseband signal (received signal) and outputs it to the control unit 130.

[0019] The transmitting unit 120 performs various types of transmissions under the control of the control unit 130. The transmitting unit 120 includes an antenna and a transmitter. The transmitter converts the baseband signal (transmission signal) output by the control unit 130 into a wireless signal and transmits it from the antenna.

[0020] The control unit 130 performs various control and processing operations in the UE 100. Such processing includes processing in each layer described later. The operation of the UE 100 described above and later may also be controlled by the control unit 130. 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 for processing by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation, demodulation, encoding, and decoding of baseband signals. The CPU executes programs stored in memory and performs various processing operations.

[0021] Figure 3 shows an example configuration of a gNB200 (network node) according to this embodiment. The gNB200 includes a transmitting unit 210, a receiving unit 220, a control unit 230, and a network communication unit 240. The transmitting unit 210 and the receiving unit 220 constitute a wireless communication unit 250 that performs wireless communication with the UE100. The network communication unit 240 includes a transmitting unit 241 that performs transmission and a receiving unit 242 that performs reception.

[0022] The transmitting unit 210 performs various types of transmissions under the control of the control unit 230. The transmitting unit 210 includes an antenna and a transmitter. The transmitter converts the baseband signal (transmission signal) output by the control unit 230 into a wireless 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 the radio signal received by the antenna into a baseband signal (received signal) and outputs it to the control unit 230.

[0024] The control unit 230 performs various control and processing operations in the gNB 200. Such processing includes processing in each layer described later. The operation of the gNB 200 described above and later may also be controlled by 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 for processing by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation, demodulation, encoding, decoding, etc. of the baseband signal. The CPU executes programs stored in memory and performs various processing operations.

[0025] The network communication unit 240 is connected to an adjacent base station via the Xn interface, which is an inter-base station interface. The network communication unit 240 is connected to the AMF / UPF 300 via the NG interface, which is an inter-base station-core network interface. The gNB 200 may consist of a central unit (CU) and a distributed unit (DU) (i.e., functionally divided), and the two units may be connected by the F1 interface, which is a front-haul interface. In this case, each of the gNB-DU and gNB-CU may have a functional block configuration similar to the one shown in Figure 3. However, the gNB-CU is assumed not to have a wireless communication unit 250.

[0026] Figure 4 shows the configuration of the protocol stack for the user plane's wireless interface that handles data.

[0027] The user plane radio interface protocol comprises a physical (PHY) layer, a MAC (Medium Access Control) layer, an RLC (Radio Link Control) layer, a PDCP (Packet Data Convergence Protocol) layer, and an SDAP (Service Data Adaptation Protocol) 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 the 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 the DCI addressed to its own UE. The DCI transmitted from gNB200 has a CRC parity bit added, which is scrambled by the RNTI.

[0029] The MAC layer performs data priority control, retransmission processing using Hybrid ARQ (HARQ: Hybrid Automatic Repeat reQuest), and random access procedures. Data and control information are transmitted between the MAC layer of UE100 and the MAC layer of gNB200 via the transport channel. The MAC layer of gNB200 includes a scheduler. The scheduler determines the transport format for the up and down links (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be allocated to UE100.

[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 UE100 and the RLC layer of gNB200 via a logical channel.

[0031] The PDCP layer performs header compression / decompression, encryption / decryption, etc.

[0032] The SDAP layer maps IP flows, which are the units under which the core network performs QoS (Quality of Service) control, to wireless bearers, which are the units under which the AS (Access Stratum) performs QoS control. Note that if the RAN is connected to the EPC, the SDAP is not required.

[0033] Figure 5 shows the configuration of the protocol stack of the wireless interface of the control plane that handles signaling (control signals).

[0034] The protocol stack of the control plane's wireless interface includes an RRC (Radio Resource Control) layer and a NAS (Non-Access Stratum) layer, instead of the SDAP layer shown in Figure 4.

[0035] RRC signaling for various settings is transmitted between the RRC layer of the UE100 and the RRC layer of the gNB200. The RRC layer controls the logical channel, transport channel, and physical channel in response to the establishment, re-establishment, and release of the radio bearer. If there is a connection (RRC connection) between the RRC of the UE100 and the RRC of the gNB200, the UE100 is in the RRC connected state. If there is no connection (RRC connection) between the RRC of the UE100 and the RRC of the gNB200, the UE100 is in the RRC idle state. If the connection between the RRC of the UE100 and the RRC of the gNB200 is suspended, the UE100 is in the RRC inactive state.

[0036] The NAS layer (also simply referred to as "NAS"), located above the RRC layer, handles session management and mobility management, among other things. NAS signaling is transmitted between the NAS layer of the UE100 and the NAS layer of the AMF300. The UE100 also has an application layer in addition to the wireless interface protocol. Furthermore, the layer below the NAS layer is called the AS layer (also simply referred to as "AS").

[0037] (1.2) Overview of Dual Connectivity The mobile communication system 1 supports dual connectivity (DC). Figure 6 is a diagram illustrating DC according to this embodiment.

[0038] A UE100 in RRC connected state can be configured as a DC. In the DC, the UE100 communicates wirelessly with the master cell group (MCG) managed by the master node (MN) 200M and the secondary cell group (SCG) managed by the secondary node (SN). The MN200M and SN200S are connected to each other via an inter-node interface.

[0039] MN200M is also called the master gNB (MgNB) when it is a 5G / NR node. SN200S is also called the secondary gNB (SgNB) when it is a 5G / NR node. The inter-node interface may be a 5G / NR Xn interface. In the following embodiments, communication between gNB200s is assumed to take place over the Xn interface, but the inter-node interface may have a different name in 6G.

[0040] For example, when MN200M sends a predetermined message (e.g., an SN Addition Request message) to SN200S, and MN200M sends an RRC Reconfiguration message to UE100, the SCG is set to UE100 and DC starts. In DC, UE100, in an RRC connected state, is allocated radio resources from the schedulers of MN200M and SN200S, and performs wireless communication using the radio resources of MN200M and SN200S.

[0041] The MN200M may have a control plane connection to the CN20. The MN200M provides the primary radio resources for the UE100. The MN200M manages the MCG, which is a group of serving cells associated with the MN200M. The MCG has a primary cell (PCell) and optionally one or more secondary cells (SCell).

[0042] On the other hand, the SN200S may not have a control plane connection with the CN20. The SN200S provides additional radio resources to the UE100. The SN200S manages an SCG which is a group of serving cells associated with the SN200S. The SCG has a primary secondary cell (PSCell) and optionally one or more SCells. Note that the PCell of the MCG and the PSCell of the SCG may be referred to as a special cell (SpCell).

[0043] (1.3) Overview of LTM The mobile communication system 1 according to the present embodiment supports LTM (L1 / L2 Triggered Mobility).

[0044] In a general handover procedure, a serving cell switch is triggered by signaling at a higher layer, specifically, at the RRC layer of the L3 layer. Such a general handover is also referred to as an L3 handover. In an L3 handover, an L3 measurement result (Measurement Report) message, which is an RRC message, is transmitted from the UE100 to the gNB200. Based on the measurement report message, the gNB200 determines the handover of the UE100, and a handover command (specifically, an RRC Reconfiguration message), which is an RRC message, is transmitted from the gNB200 to the UE100 to instruct cell switching.

[0045] On the other hand, LTM is a technology for shortening the mobility delay (specifically, the delay of serving cell switching) compared to the general handover procedure by triggering a serving cell switch by signaling at layer 1 (L1) and / or layer 2 (L2), which are lower layers. In LTM, the gNB200 receives an L1 measurement report from the UE100, and based on this, the gNB200 instructs a serving cell switch by a cell switch command signaled to the UE100 by means of a MAC CE.

[0046] Specifically, in LTM, first, the gNB 200 prepares an LTM candidate cell setting regarding the candidate of the target cell and provides the LTM candidate cell setting to the UE 100 by RRC signaling.

[0047] Second, the UE 100 performs synchronization processing with the LTM candidate cell by early sync.

[0048] Third, the gNB 200 receives an L1 measurement report from the UE 100, determines the serving cell switch to the target cell based on the L1 measurement report, and transmits a cell switch command (Cell Switch Command) indicating the target cell (LTM candidate cell setting) to the UE 100 by a MAC control element (CE). The serving cell switch trigger is transmitted by a MAC CE including at least a candidate setting index (setting ID) together with a beam indicator.

[0049] Fourth, the UE 100 switches the serving cell according to the cell switch command MAC CE from the gNB 200 (source cell).

[0050] In this way, by selecting the LTM candidate cell setting as the target setting by the gNB 200, the serving cell switch is triggered. The LTM candidate cell setting can be added, changed, and released by the gNB 200 via RRC signaling.

[0051] The following principles apply to LTM.

[0052] - Each LTM candidate cell setting can be provided as a difference setting (delta setting) with respect to the reference setting used to form a complete LTM candidate cell setting.

[0053] - When a complete LTM candidate cell setting is applied, the current UE setting is replaced at the time of serving cell switch. The replacement is performed in the reconfiguration procedure, but the MAC, RLC, or PDCP layer is not necessarily reset.

[0054] - The user plane continues without reset if it is set by RRC signaling for the purpose of avoiding additional delay in data recovery.

[0055] - Security updates are not provided in LTM versions.

[0056] - Subsequent LTMs (Subsequent LTMs) can be performed between subsequent LTM candidate cell settings without RRC reconfiguration. In other words, UE100 does not release other LTM candidate cell settings after an LTM is triggered.

[0057] (1.4) IntraCU LTM Figure 7 shows an example of a cell switching procedure using LTM in an intraCU (i.e., within the same gNB200). In the illustrated example, UE100 performs an LTM-based serving cell switch (i.e., LTM cell switch) from the first cell to the second cell of the gNB200.

[0058] Here, the first cell and the second cell may be formed by different TRPs (Transmission and Reception Points). In the following description of the embodiments, the second cell will also be referred to as the "LTM candidate cell (or candidate cell)" until the LTM decides to switch the serving cell, and the second cell will also be referred to as the "target cell" after the LTM decides to switch the serving cell. The first cell will also be referred to as the "source cell" or the "(current) serving cell".

[0059] In step S1, UE100 is in an RRC connected state in the cell (first cell) of gNB200.

[0060] In step S2, UE100 sends a Measurement Report message, which is an RRC message, to gNB200 (first cell). The measurement report, which includes the L3 measurement result, is also called the L3 Measurement Report (L3 MR).

[0061] In step S3, gNB200 decides to use LTM based on the L3 measurement report and begins preparing LTM candidate cells.

[0062] In step S4, gNB200 (first cell) sends an LTM configuration, including the LTM Candidate Configuration for one or more LTM candidate cells, to UE100 in an RRC message, specifically an RRC Reconfiguration message. The LTM candidate cell configuration may include a random access channel (RACH) configuration used for sending RA preambles to the corresponding LTM candidate cells, such as a conflict-free random access (CFRA) configuration. Such a RACH configuration may be referred to as an Early Ul Sync Configuration. CFRA is a random access procedure in which UE100 is allocated dedicated RACH resources (e.g., dedicated preamble sequences and / or dedicated time and frequency resources) and no RACH conflicts occur between UE100s.

[0063] In step S5, UE100 saves the LTM settings (LTM candidate cell settings) and sends an RRC Reconfiguration Complete message to gNB200 (first cell).

[0064] In step S6, UE100 may perform synchronization with the LTM candidate cell (second cell) before receiving the cell switching command MAC CE from the first cell. Such synchronization may be called Early Sync. Here, UE100 may perform Downlink Synchronization (DL Synchronization) with the LTM candidate cell and then perform Early Timing Advance (TA) acquisition (i.e., UL Early Sync) with the LTM candidate cell requested by gNB200 (serving cell). This is performed by a CFRA triggered by a PDCCH order from the first cell. Note that if 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. Furthermore, if Early Ul Sync Config is set on UE100, the PDCCH order may include a cell indicator that indicates the corresponding RACH transmission cell, i.e., which LTM candidate cell UE100 should send a random access preamble (RA preamble) to.

[0065] UE100 transmits an RA preamble to the designated LTM candidate cell (second cell). To minimize the interruption of serving cell communication by CFRA to the LTM candidate cell, during early synchronization, UE100 does not receive a Random Access Response (RAR) from the LTM candidate cell for the purpose of obtaining the TA value. The TA value of the LTM candidate cell (target cell) is indicated by the cell switching command MAC CE in step S9. The TA value is used to adjust the uplink transmission timing of UE100.

[0066] In step S7, UE100 performs a Layer 1 (L1) measurement on the configured LTM candidate cell and transmits a measurement report (also referred to as "L1 measurement report (L1 MR)") including the L1 measurement result to gNB200 (first cell). The L1 measurement result may be, for example, L1-RSRP and / or L1-SINR.

[0067] In step S8, the gNB200 decides to switch the serving cell to the target cell (second cell).

[0068] In step S9, the gNB200 (first cell) sends a cell switching command MAC CE to the UE100, which includes the candidate setting index (setting ID) of the target cell. The cell switching command MAC CE may include the TA value obtained by UL early synchronization (i.e., the TA value derived based on the RA preamble).

[0069] In step S10, UE100 switches to the settings for the target cell (second cell). Specifically, UE100 detaches from the first cell and applies the settings for the target cell (second cell).

[0070] In step S11, if the serving cell switch requires the execution of a random access procedure (for example, if the cell switch command MAC CE does not contain a valid TA value), UE 100 executes a random access procedure on the target cell (RACH-based LTM cell switch). However, if UE 100 does not need to obtain the TA of the target cell during the serving cell switch (for example, if the cell switch command MAC CE contains a valid TA value), the random access procedure can be skipped (RACH-less LTM cell switch).

[0071] In step S12, UE100 indicates that the serving cell switch to the target cell has been successfully completed by sending, for example, an RRC Reconfiguration Complete message to the target cell (second cell). Subsequently, UE100 may perform steps S6 to S12 multiple times for subsequent LTM serving cell switches based on the settings provided in step S4.

[0072] (1.5) Inter-CU LTM The LTM introduced in 3GPP Release 18 only supports intra-CUs and does not support inter-CU (i.e., between different gNB200s) LTM. In other words, with conventional LTM, it is possible to switch LTM cells between cells under the same CU (same gNB200), but it is not possible to switch LTM cells between cells under different CUs (different gNB200s).

[0073] On the other hand, 3GPP Release 19 will introduce InterCU LTM. In InterCU LTM, the UE100 will perform LTM cell switching from the first cell of one gNB200 to the second cell of another gNB200.

[0074] Based on the operation shown in Figure 7, an example of the operation of the InterCU LTM will be described. Figure 8 is a diagram showing an example of the operation of the InterCU LTM.

[0075] In step S101, UE100 sends an L3 (RRC) Measurement Report to gNB200a. gNB200a receives the L3 (RRC) Measurement Report.

[0076] In step S102, gNB200a decides to use inter-gNB (interCU) LTM based on the L3 (RRC) Measurement Report from step S101 and begins preparing the LTM candidate cell. Here, it is assumed that the second cell of gNB200b has been selected as the LTM candidate cell.

[0077] In step S103, gNB200a sends a request message (LTM HO Request) to gNB200b indicating that the serving cell change is due to LTM. gNB200b receives the request message (LTM HO Request). The request message (LTM HO Request) may include 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. The request message (LTM HO Request) may include RRC configuration information for UE100 and a cell identifier indicating the second cell, similar to a typical handover.

[0078] In step S104, gNB200b decides whether to accept the request in step S103 (Admission control). Here, we will proceed assuming that the request in step S103 is accepted. In this case, gNB200b may set up a CFRA resource for early synchronization in the second cell. If the request in step S103 is rejected, gNB200b may send a rejection message to gNB200a. The rejection message may include information indicating that inter-gNB LTM is unavailable.

[0079] In step S105, gNB200b sends an acknowledgment message (LTM HO Request Ack) to gNB200a indicating acceptance of the request in step S103. gNB200a receives the acknowledgment message (LTM HO Request Ack). The acknowledgment message (LTM HO Request Ack) may include an LTM Indicator and may be a Handover Request Ack message used in general handovers. Alternatively, the acknowledgment 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 acknowledgment message (LTM HO Request Ack) may include information indicating the early synchronization CFRA resources (e.g., RA preamble and / or PRACH (Physical Random Access Channel) resources) set up by gNB200b for the second cell. The acknowledgment message (LTM HO Request Ack) may also include RRC reconfiguration information for UE100 applied in the second cell, similar to a general handover. The acknowledgment message (LTM HO Request Ack) may also include a notification indicating that the second cell will send (reply to) an RAR in response to the RA preamble transmission, and / or information indicating the RAR reception window.

[0080] In step S106, gNB200a sends an RRC Reconfiguration message to UE100 that includes the LTM configuration (LTM Candidate Configuration, LTM-Candidate) for the second cell. UE100 receives the RRC Reconfiguration message. The RRC Reconfiguration message may include information indicating the early synchronization CFRA resource configured by gNB200b for the second cell. The RRC Reconfiguration message may also include a notification that the second cell will send (reply to) an RAR in response to an RA preamble transmission, and / or information indicating the RAR reception window.

[0081] In step S107, UE100 saves the LTM settings and sends an RRC Reconfiguration Complete message to gNB200a. gNB200a receives the RRC Reconfiguration Complete message.

[0082] In step S108, UE100 may send an L1 measurement report (or L3 measurement report) to gNB200a for gNB200a to determine about early synchronization. gNB200a may receive an L1 measurement report (or L3 measurement report).

[0083] In step S109, gNB200a may perform an early synchronization decision.

[0084] In step S110, gNB200a may send an Early sync CFRA Request message to gNB200b, which is a request message requesting the preparation of a CFRA resource for early synchronization, specifically, the configuration and / or activation of a CFRA resource for early synchronization. gNB200b may receive the request message (Early sync CFRA Request message). The request message (Early sync CFRA Request message) may include an identifier for identifying UE100 (Xn-AP UE ID) and / or an identifier for identifying the second cell (cell ID).

[0085] In step S111, gNB200b may prepare CFRA resources for early synchronization.

[0086] In step S112, gNB200b may send a notification message to gNB200a indicating that the CFRA resources for early synchronization are ready, for example, an Early sync CFRA Request Ack message. gNB200a may receive the notification message (Early sync CFRA Request Ack message).

[0087] In step S113, gNB200a may send a PDCCH order to UE100, instructing UE100 to perform a CFRA for early synchronization. UE100 receives the PDCCH order. The PDCCH order may include information for identifying the second cell as the target of the CFRA (Target cell indicator). The PDCCH order may also include a notification indicating that the second cell will send (reply with) a RAR in response to the RA preamble transmission, and / or information indicating the RAR reception window.

[0088] In step S114, UE100 may perform early synchronization of the downlink (DL) with the second cell. For example, UE100 may perform timing synchronization using the second cell's SSB (Synchronization Signal Block) (PSS / SSS: Primary Synchronization Signal / Secondary Synchronization Signal). Note that UE100 may perform DL synchronization before this point.

[0089] In step S115, UE100 transmits a CFRA, specifically an RA preamble on PRACH, to the second cell specified in the PDCCH order in order to perform early synchronization of the uplink (UL) with the second cell. gNB200b receives the RA preamble. UE100 identifies the CFRA resources (e.g., the RA preamble and / or PRACH resources) using information set in the SIB, etc., and information such as the "Random Access Preamble index" and "PRACH Mask Index" in the PDCCH order.

[0090] In step S116, gNB200b may send a RAR containing the TA value derived based on the RA preamble to UE100. UE100 may receive the RAR. Step S116 may be an optional step that is performed only if there is a setting from gNB200a (for example, the setting in step S106).

[0091] UE100 may send an Early Sync Complete notification to gNB200a indicating that UL early synchronization with the second cell is complete (step S117). The Early Sync Complete notification may include the TA value notified in RAR.

[0092] In step S118, gNB200b may send a notification message (Early Sync Complete) to gNB200a indicating that UL early synchronization with UE100 has been completed. gNB200a 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 in step S115.

[0093] In step S119, UE100 sends the L1 measurement report to gNB200a. gNB200a receives the L1 measurement report.

[0094] In step S120, if gNB200a determines, for example, that the likelihood of LTM execution has increased based on the L1 measurement report in step S119, it may send a UL resource request message to gNB200b. gNB200b may receive the request message. The UL resource request may be a request for the preparation or activation of a CFRA resource. The UL resource request may be a request for the preparation or execution of a UL grant transmission to UE100. The UL resource request may be a request for the preparation or activation of a UL configured grant (CG) resource. 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 also be after the LTM execution decision in step S121.

[0095] In step S121, gNB200a decides to perform LTM based on the L1 measurement report from step S119.

[0096] In step S122, gNB200a sends a cell switching command (MAC CE) to UE100 in response to the LTM execution decision. UE100 receives the cell switching command. The cell switching command may include the TA value notified to gNB200a in step S117 or S118.

[0097] In step S123, upon receiving a cell switching command, UE100 detaches from the first cell (source cell) and applies the LTM settings of the second cell (target cell).

[0098] In step S124, if the cell switching command does not contain a TA value (a valid TA value), UE100 may execute a random access procedure on the second cell.

[0099] In step S125, UE100 sends an RRC Reconfiguration Complete message to the second cell. gNB200b receives the RRC Reconfiguration Complete message.

[0100] In step S126, gNB200b may transmit a DCI including a CRC (Cyclic Redundancy Code) scrambled with the C-RNTI assigned to UE100 to UE100 on the PDCCH, and transmit a Contention Resolution MAC CE to UE100 on the PDSCH assigned with the DCI. UE100 may receive the DCI and the Contention Resolution MAC CE.

[0101] In step S127, gNB200b may send 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).

[0102] (1.6) LTM in DC Scenario Based on the above configuration and operation, the LTM in the DC scenario will be described. In this embodiment, in the DC scenario shown in Figure 6, we assume the operation of switching the MN200M by the interCU LTM shown in Figure 8 (also referred to as "interCU MCG LTM").

[0103] (1.6.1) LTM Configuration Diagram 9 is a diagram showing an example of an LTM configuration (LTM Candidate Configuration) set on UE100 from network 5 (for example, MN200M). In this embodiment, UE100 receives the LTM configuration shown in Figure 9, for example, in an RRC Reconfiguration message from MN200M, and stores the received LTM configuration.

[0104] According to the current 3GPP agreement, it is not possible to simultaneously set LTM candidates for InterCU LTM for MCG and SCG, but there are no restrictions regarding IntraCU LTM. Based on this agreement, the IntraCU LTM for Release 18 and the InterCU LTM for Release 19 can be set as shown in Figure 9.

[0105] UE100 receives the MCG LTM Config (also referred to as "MCG LTM Config"), which is the LTM configuration for MCG (MN200M), and stores the received MCG LTM Config as MCG VarLTM-Config (MCG LTM Config). The MCG LTM Config is a setting for performing LTM cell switching for MN200M (PCell). The MCG LTM Config can be configured with a mix of intraCU cells and interCU cells. Since it is necessary to release SN200S when performing LTM cell switching for MN200M (PCell), the MCG LTM Config may also include a setting for SN200S to perform SN addition after the LTM cell switching is completed.

[0106] In the illustrated example, the MCG LTM setting includes multiple LTM candidate settings (LTM-Candidate). Specifically, the MCG LTM setting includes an LTM candidate setting (LTM-Candidate) with candidate setting index (ID) = 1, an LTM candidate setting (LTM-Candidate) with candidate setting index (ID) = 2, and so on. Each LTM candidate setting (LTM-Candidate) includes an MCG setting used for LTM cell switching of the MCG (MN200M) and an SCG setting used to add an SCG (SN200S) after the LTM cell switching of the MCG (MN200M). Note that when UE100 receives the cell switching command MAC CE from MN200M, it determines that it will perform LTM cell switching of the MCG (MN200M) (i.e., use the MCG LTM setting). On the other hand, if UE100 receives the cell switching command MAC CE from SN200S, it determines that it is an LTM cell switching operation by SCG (SN200S).

[0107] Furthermore, UE100 receives the SCG LTM Config (also referred to as "SCG LTM Config"), which is the LTM configuration for SCG (SN200S), and stores the received SCG LTM Config as SCG VarLTM-Config (SCG LTM Config). The SCG LTM Config is a setting for performing LTM cell switching for SN200S (PSCell). The SCG LTM Config can only configure cells in the intraCU. When performing LTM cell switching for SN200S (PSCell), the UE100 remains connected to MN200M during the LTM cell switching; therefore, the MCG LTM Config does not include settings for MN200M, and only settings for SN200S.

[0108] In the illustrated example, the SCG LTM setting includes multiple LTM candidate settings (LTM-Candidate). Specifically, the SCG LTM setting includes an LTM candidate setting (LTM-Candidate) with candidate setting index (ID) = 1, an LTM candidate setting (LTM-Candidate) with candidate setting index (ID) = 2, and so on. Each LTM candidate setting (LTM-Candidate) includes an SCG setting used for LTM cell switching within the SN200S.

[0109] Furthermore, according to the current LTM technical specifications, when UE100 performs an LTM cell switchover, it retains the stored LTM settings without releasing them in order to perform subsequent LTM cell switches (also referred to as "Subsequent LTM") without RRC reconfiguration. Specifically, after each LTM cell switchover is completed, UE100 can repeat the procedure of initial synchronization, LTM cell switchover execution, and LTM cell switchover completion without releasing other LTM candidate configurations. Therefore, UE100 retains the LTM settings until instructed to do so from the network 5 side. In addition, according to the current LTM technical specifications, UE100 performs L1 measurements on each configured candidate cell and sends the L1 measurement report to gNB200.

[0110] (1.6.2) InterCU MCG LTM Regarding the InterCU MCG LTM, there is 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.

[0111] Figure 10 is a diagram illustrating the first case of the InterCU MCG LTM according to this embodiment.

[0112] In step S11, UE100, in the RRC connected state, is performing DC communication with MN200M#1 (MCG) and SN200S#1 (SCG). UE100 has LTM settings as shown in Figure 9, for example, set by an RRC Reconfiguration message from MN200M#1. UE100 stores the LTM settings for MCG and SCG as shown in Figure 9. The LTM settings for MCG include information (MCG settings) used for interCU MCG LTM, which is LTM cell switching from MN200M#1 to another MN200M#2. The LTM settings for SCG include information (SCG settings) used for intraCU SCG LTM, which is LTM cell switching within SN200S#1.

[0113] In step S12, UE100 performs LTM cell switching using the InterCU MCG LTM with the MCG LTM settings. Specifically, UE100 performs LTM cell switching from MN200M#1 to MN200M#2. Here, UE100 applies the target MCG setting in the stored MCG LTM settings and establishes a connection with MN200M#2. UE100 also releases the connection with SN200S#1.

[0114] In step S13a, UE100 applies the SCG setting associated with the target MCG in the stored MCG LTM settings and establishes a connection with SN200S#2 (SN Addition). In the first case, SN200S is changed from SN200S#1 to SN200S#2 in conjunction with the InterCU MCG LTM. However, in the first case, SN200S#2 is not necessarily added in conjunction with the InterCU MCG LTM. That is, the change of SN200S is not limited to cases where SN200S is changed before and after the InterCU MCG LTM, but may also include cases where SN200S is not added after the InterCU MCG LTM.

[0115] In step S14a, UE100 performs DC communication with MN200M#2 (MCG) and SN200S#2 (SCG).

[0116] Figure 11 is a diagram illustrating a second case of the InterCU MCG LTM according to this embodiment.

[0117] Steps S11 and S12 are the same as in the first case.

[0118] In step S13b, UE100 applies the SCG setting associated with the target MCG in the stored MCG LTM settings and establishes a connection with SN200S#1 (SN Addition). In the second case, SN200S#1 is not changed in conjunction with the interCU MCG LTM.

[0119] In step S14b, UE100 performs DC communication with MN200M#2 (MCG) and SN200S#1 (SCG).

[0120] Thus, there are two cases for InterCU MCG LTM. In the second case, since SN200S#1 is not changed in conjunction with InterCU MCG LTM, the SCG LTM settings shown in Figure 9 can be applied after InterCU MCG LTM. Therefore, UE100 can apply the SCG LTM settings to LTM cell switching (IntraCU LTM) within SN200S#1 by retaining the SCG LTM settings even after InterCU MCG LTM.

[0121] On the other hand, in the first case, since SN200S is changed in conjunction with InterCU MCG LTM, the SCG LTM settings shown in Figure 9 can no longer be applied after InterCU MCG LTM. However, since UE100 retains the SCG LTM settings even after InterCU MCG LTM, it may be necessary to perform L1 measurements (and reports) for each candidate cell during the SCG LTM settings. It is undesirable for the power consumption of UE100 to increase due to such unnecessary processing. Also, in the first case, UE100 retaining the SCG LTM settings even after InterCU MCG LTM may result in the unnecessary consumption of UE100's memory capacity. Therefore, various problems may arise if UE100 retains the SCG LTM settings even after InterCU MCG LTM.

[0122] Therefore, in this embodiment, the UE 100, which stores the MCG LTM settings and the SCG LTM settings, disables the SCG LTM settings based on the fact that InterCU MCG LTM is performed using the MCG LTM settings. This solves the problem that may arise if the UE 100 retains the SCG LTM settings after InterCU MCG LTM. Disabling the SCG LTM settings means discarding the SCG LTM settings. Note that "discard" can be replaced with (synonymous with) "delete" or "release".

[0123] Alternatively, disabling the SCG LTM settings may be done by suspending the SCG LTM settings. Suspension means that the use of the settings is stopped (interrupted). UE100 may resume the SCG LTM settings when adding SN200S#1 after suspending them. Suspension means that the settings are in use. Note that suspend and resume are interchangeable (synonymous) with deactivation and activation, respectively.

[0124] The UE100 according to this embodiment includes a communication unit (wireless communication unit 140) that communicates with MN200M#1 and SN200S#1, and a control unit 130 that stores an MCG LTM setting that includes information used for interCU MCG LTM, which is LTM cell switching from MN200M#1 to another MN200M#2, and an SCG LTM setting that includes information used for intraCU SCG LTM, which is LTM cell switching within SN200S#1. The control unit 130 disables the SCG LTM setting based on the fact that interCU MCG LTM is performed using the MCG LTM setting.

[0125] In the first operation pattern of this embodiment, when UE100 performs interCU MCG LTM using the MCG LTM setting and SN200S#1 is changed in conjunction with the interCU MCG LTM (i.e., first case), it discards the SCG LTM setting. On the other hand, when performing interCU MCG LTM using the MCG LTM setting and SN200S#1 is not changed in conjunction with the interCU MCG LTM (i.e., second case), UE100 retains the SCG LTM setting. As a result, the SCG LTM setting is discarded in the first case, while the SCG LTM setting is retained in the second case, making it possible to perform appropriate operations for each case.

[0126] In the second operation pattern of this embodiment, when UE100 performs interCU MCG LTM using the MCG LTM setting, it discards the SCG LTM setting regardless of whether SN200S#1 is changed as a result of interCU MCG LTM. Thus, in the second operation pattern, UE100 discards the SCG LTM setting in both the first and second cases. This simplifies the operation of UE100 compared to the first operation pattern. In the second operation pattern, after interCU MCG LTM, the SCG LTM setting may be set again on UE100 by an RRC Reconfiguration message from network 5 (e.g., MN200M#2).

[0127] (1.6.3) Example of the first operation pattern Figure 12 shows an example of the first operation pattern of this embodiment.

[0128] In step S201, UE100 performs DC communication with MN200M#1 and SN200S#1 as shown in Figure 10 or Figure 11. UE100 also stores an MCG LTM configuration (MCG VarLTM-Config) containing information used for inter-CU MCG LTM, which is LTM cell switching from MN200M#1 to another MN200M#2. Furthermore, UE100 stores an SCG LTM configuration (SCG VarLTM-Config) containing information used for intra-CU SCG LTM, which is LTM cell switching within SN200S#1.

[0129] In step S202, network 5 (for example, MN200M#1) determines whether or not to perform MCG LTM on the interCU or intraCU. If it is determined that MCG LTM should be performed (step S202: YES), the process proceeds to step S205.

[0130] If it is determined that MCG LTM will not be performed (step S202: NO), in step S203, network 5 (e.g., SN200S#1) determines whether or not to perform SCG LTM on the intra CU. If it is determined that SCG LTM on the intra CU will be performed (step S203: YES), in step S204, network 5 (e.g., SN200S#1) controls UE100 to perform SCG LTM on the intra CU. In this case, UE100 performs SCG LTM on the intra CU using the SCG LTM setting (SCG VarLTM-Config). At that time, UE100 maintains the connection with MN200M#1. On the other hand, if it is determined that SCG LTM on the intra CU will not be performed (step S203: NO), the process returns to step S202.

[0131] If it is determined that MCG LTM should be performed on the intra CU (step S205: NO), in step S207, network 5 (for example, MN200M#1) controls UE100 to perform MCG LTM on the intra CU. In this case, UE100 performs MCG LTM on the intra CU using the MCG LTM setting (MCG VarLTM-Config). At that time, UE100 maintains its connection with SN200S#1.

[0132] On the other hand, if it is determined that an MCG LTM should be performed on the interCU (step S205: YES), in step S206, network 5 (for example, MN200M#1) releases SN200S#1 and controls UE100 to perform an MCG LTM on the interCU. In this case, UE100 releases SN200S#1 and performs an MCG LTM on the interCU using the MCG LTM setting (MCG VarLTM-Config). Specifically, UE100 performs an MCG LTM on the interCU from MN200M#1 to MN200M#2, as shown in Figure 10 or Figure 11.

[0133] In step S208, UE100 or network 5 (for example, MN200M#2) determines whether SN200S#1 is changed in accordance with the MCG LTM of the interCU.

[0134] If UE100 performs the determination in step S208, UE100 compares the node identifier (gNB Identity) contained in the system information of SN200S#1 before the interCU's MCG LTM with the node identifier (gNB Identity) contained in the system information of the SN200S (SN200S#1 or SN200S#2) with which UE100 communicates after the interCU's MCG LTM to determine whether SN200S#1 is changed in conjunction with the interCU's MCG LTM.

[0135] Specifically, the MCG-VarLTM-Config includes settings for the SCG (SN200S) to be connected after the InterCU MCG LTM is executed. Therefore, by applying these settings, the UE100 can obtain the System Information Block Type 1 (SIB1) broadcast by the SN200S and obtain the gNB Identity in the NCI (NR cell identity) contained in the SIB1. The NCI includes the gNB Identity and the 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 the SIB1. Similarly, UE100 can determine the gNB Identity of SN200S#1 before the interCU's MCG LTM. Then, by comparing the gNB Identity of SN200S before and after the interCU's MCG LTM, UE100 can determine whether or not SN200S is changed before and after the interCU's MCG LTM.

[0136] If the network 5 performs the determination in step S208, MN200M#2 may perform the determination in step S208 in the following manner. MN200M#2 may also notify MN200M#1 of the determination result.

[0137] Specifically, firstly, MN200M#2 exchanges cell information (List of Served Cells NR) with SN200S (which at this point is simply an adjacent gNB) using Xn Setup Request and Xn Setup Response messages to establish an Xn connection. 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 SN200S that establish an Xn connection.

[0138] Secondly, MN200M#2 receives a request message (LTM HO Request) from MN200M#1 and receives the UE context in HandoverPreparationInformation contained in the message. The UE context includes CellGroupConfig for SN200S#1 (SCG) as information set in UE100. MN200M#2 retrieves two pieces of information from CellGroupConfig for SN200S#1 (SCG) set in UE100: physCellId and FrequencyInfoDL.

[0139] Thirdly, MN200M#2 identifies the SN200S (MCG) information (physisCellId and FrequencyInfoDL) to be added after the interCU MCG LTM from the cell information at the time of Xn connection establishment, and compares this identified information with the information (physisCellId and FrequencyInfoDL) in the UE context. In other words, by comparing the SN200S information (physisCellId and FrequencyInfoDL) before and after the interCU MCG LTM, MN200M#2 can determine whether or not the SN200S is changed before and after the interCU MCG LTM.

[0140] Alternatively, if the network 5 performs the determination in step S208, MN200M#2 may perform the determination in step S208 in the following manner. MN200M#2 may notify MN200M#1 of the determination result.

[0141] Firstly, MN200M#2 receives a request message (LTM HO Request) from MN200M#1, which 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.

[0142] Secondly, MN200M#2 obtains the information of SN200S that is set in UE100 in advance, specifically the settings of the SCG (SN200S) to be connected after the execution of InterCU MCG LTM in MCG VarLTM-Config, and identifies the gNB Identity after the execution of InterCU MCG LTM from these settings.

[0143] Thirdly, MN200M#2 can determine whether the SN200S is modified before and after the InterCU MCG LTM by comparing the gNB Identity of the SN200S currently connected to the DC with the gNB Identity after the InterCU MCG LTM is executed.

[0144] If the network 5 makes the determination in step S208, the network node (MN200M#1 or MN200M#2) may send notification information to the UE100 to discard the SCG LTM setting (SCG VarLTM-Config) during the interCU's MCG LTM, depending on the determination result that the SN200S is changed before and after the interCU's MCG LTM.

[0145] For example, MN200M#1 may include the notification information in the cell switching command MAC CE that instructs the execution of the InterCU's MCG LTM before executing the InterCU's MCG LTM, and send the cell switching command MAC CE containing the notification information to UE100. In this case, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) upon receiving the cell switching command MAC CE containing the notification information. After receiving the cell switching command MAC CE containing the notification information, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) when accessing or connecting to SN200S#2.

[0146] Alternatively, MN200M#2 may include notification information in the LTM candidate configuration within the acknowledgment message (LTM HO Request Ack) before executing the interCU's MCG LTM. In this case, MN200M#1 sends an RRC Reconfiguration message containing the notification information to UE100. For example, MN200M#1 may include the notification information in the LTM candidate among the multiple LTM candidates in the MCG LTM configuration shown in Figure 9, where SN200S is changed before and after the interCU's MCG LTM. In this case, UE100 may discard the SCG LTM configuration (SCG VarLTM-Config) when executing (applying) MCG LTM by applying the LTM-Candidate containing the notification information. UE100 may also discard the SCG LTM configuration (SCG VarLTM-Config) when accessing or connecting to SN200S#2.

[0147] Alternatively, MN200M#2 may include the notification information in the RRC Reconfiguration message it sends to UE100 after the InterCU MCG LTM is completed. In this case, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) upon receiving the RRC Reconfiguration message from MN200M#2.

[0148] If it is determined that SN200S#1 will be changed in conjunction with the InterCU's MCG LTM (step S208: YES), in step S209, UE100 discards the SCG LTM setting (SCG VarLTM-Config). On the other hand, if it is determined that SN200S#1 will not be changed in conjunction with the InterCU's MCG LTM (step S208: NO), in step S210, UE100 retains the SCG LTM setting (SCG VarLTM-Config).

[0149] UE100 may discard the SCG LTM setting after receiving the cell switching command MAC CE and determining that SN200S#1 will be changed in accordance with the InterCU's MCG LTM. In other words, UE100 may discard the SCG LTM setting (SCG VarLTM-Config) at the time UE100 receives the cell switching command MAC CE from MN200M#1 and determines that the SN200S to be connected after LTM execution is different from the SN200S before LTM execution. Alternatively, UE100 may discard the SCG LTM setting (SCG VarLTM-Config) when accessing MN200M#2 or when establishing a connection with MN200M#2. Alternatively, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) after accessing MN200M#2 and when SCG is configured from MN200M#2. In other words, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) at the same time that DC is configured from MN200M#2.

[0150] If the SCG LTM configuration (SCG VarLTM-Config) is discarded, MN200M#1 or MN200M#2 may send a notification to SN200S#1 indicating that UE100 has discarded the SCG LTM configuration (SCG VarLTM-Config). This allows SN200S#1 to understand that the SCG LTM configuration (SCG VarLTM-Config) has been discarded.

[0151] In step S211, UE100 adds an SN200S (SN200S#1 or SN200S#2) and performs DC communication between the added SN200S and MN200M#2.

[0152] (1.6.4) Example of a second operation pattern Figure 13 shows an example of a second operation pattern of this embodiment.

[0153] In step S301, UE100 performs DC communication with MN200M#1 and SN200S#1 as shown in Figure 10 or Figure 11. UE100 also stores an MCG LTM configuration (MCG VarLTM-Config) containing information used for inter-CU MCG LTM, which is LTM cell switching from MN200M#1 to another MN200M#2. Furthermore, UE100 stores an SCG LTM configuration (SCG VarLTM-Config) containing information used for intra-CU SCG LTM, which is LTM cell switching within SN200S#1.

[0154] In step S302, network 5 (for example, MN200M#1) determines whether or not to perform MCG LTM on the interCU or intraCU. If it is determined that MCG LTM should be performed (step S302: YES), the process proceeds to step S305.

[0155] If it is determined that MCG LTM will not be performed (step S302: NO), in step S303, network 5 (e.g., SN200S#1) determines whether or not to perform SCG LTM on the intra CU. If it is determined that SCG LTM on the intra CU will be performed (step S303: YES), in step S304, network 5 (e.g., SN200S#1) controls UE100 to perform SCG LTM on the intra CU. In this case, UE100 performs SCG LTM on the intra CU using the SCG LTM setting (SCG VarLTM-Config). At that time, UE100 maintains the connection with MN200M#1. On the other hand, if it is determined that SCG LTM on the intra CU will not be performed (step S303: NO), the process returns to step S302.

[0156] If it is determined that MCG LTM should be performed on the intraCU (step S305: NO), in step S308, network 5 (for example, MN200M#1) controls UE100 to perform MCG LTM on the intraCU. In this case, UE100 performs MCG LTM on the intraCU using the MCG LTM setting (MCG VarLTM-Config). At that time, UE100 maintains the connection with SN200S#1 and retains the SCG LTM setting (SCG VarLTM-Config).

[0157] On the other hand, if it is determined that an MCG LTM should be performed on the interCU (step S305: YES), in step S306, network 5 (for example, MN200M#1) releases SN200S#1 and controls UE100 to perform an MCG LTM on the interCU. In this case, UE100 releases SN200S#1 and performs an MCG LTM on the interCU using the MCG LTM setting (MCG VarLTM-Config). Specifically, UE100 performs an MCG LTM on the interCU from MN200M#1 to MN200M#2, as shown in Figure 10 or Figure 11.

[0158] In the second operation pattern, when UE100 performs cell switching of the InterCU's MCG LTM using the MCG LTM settings, it discards the SCG LTM settings (SCG VarLTM-Config) regardless of whether SN200S#1 is changed as a result of the InterCU's MCG LTM cell switching. This determination may be made by UE100. This determination may also be made by network 5 (for example, MN200M#1 or MN200M#2).

[0159] If the network 5 makes the determination in step S305, the network node (MN200M#1 or MN200M#2) may send notification information to the UE100 to discard the SCG LTM setting (SCG VarLTM-Config) during the interCU's MCG LTM, depending on the determination result that the interCU will perform MCG LTM.

[0160] For example, MN200M#1 may include the notification information in the cell switching command MAC CE that instructs the execution of the InterCU's MCG LTM before executing the InterCU's MCG LTM, and send the cell switching command MAC CE containing the notification information to UE100. In this case, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) upon receiving the cell switching command MAC CE containing the notification information. After receiving the cell switching command MAC CE containing the notification information, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) when accessing or connecting to SN200S#2.

[0161] Alternatively, MN200M#2 may include notification information in the LTM candidate configuration within the acknowledgment message (LTM HO Request Ack) before executing the interCU's MCG LTM. In this case, MN200M#1 sends an RRC Reconfiguration message containing the notification information to UE100. For example, MN200M#1 may include the notification information in the LTM candidate among the multiple LTM candidates in the MCG LTM configuration shown in Figure 9, where SN200S is changed before and after the interCU's MCG LTM. In this case, UE100 may discard the SCG LTM configuration (SCG VarLTM-Config) when executing (applying) MCG LTM by applying the LTM-Candidate containing the notification information. UE100 may also discard the SCG LTM configuration (SCG VarLTM-Config) when accessing or connecting to SN200S#2.

[0162] Alternatively, MN200M#2 may include the notification information in the RRC Reconfiguration message it sends to UE100 after the InterCU MCG LTM is completed. In this case, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) upon receiving the RRC Reconfiguration message from MN200M#2.

[0163] If UE100 performs the determination in step S305, UE100 may determine whether to perform MCG LTM on the interCU based on whether or not it has been instructed by the network node (e.g., MN200M#1) to re-establish PDCP and / or update the security key. If it determines to perform MCG LTM on the interCU, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) at the timing of the instruction, the timing of receiving the cell switching command MAC CE, the timing of performing MCG LTM on the interCU (the timing of applying the MCG LTM settings), the timing of accessing MN200M#2, or the timing of establishing a connection with MN200M#2. Alternatively, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) after accessing MN200M#2 and when SCG is configured from MN200M#2. In other words, UE100 may discard the SCG LTM settings (SCG VarLTM-Config) at the same time that DC is configured from MN200M#2.

[0164] If the SCG LTM configuration (SCG VarLTM-Config) is discarded, MN200M#1 or MN200M#2 may send a notification to SN200S#1 indicating that UE100 has discarded the SCG LTM configuration (SCG VarLTM-Config). This allows SN200S#1 to understand that the SCG LTM configuration (SCG VarLTM-Config) has been discarded.

[0165] In step S307, UE100 adds an SN200S (SN200S#1 or SN200S#2) and performs DC communication between the added SN200S and MN200M#2.

[0166] (1.6.5) Identifier for identifying InterCU LTM The above operation does not specifically describe how UE100 determines whether to perform MCG LTM on an InterCU or an IntraCU.

[0167] When performing an InterCU MCG LTM, a security key update is required in UE100. Furthermore, in the first operation pattern described above, UE100 discards the SCG LTM setting when performing an InterCU MCG LTM using the MCG LTM setting, and when SN200S#1 is changed as a result of the InterCU MCG LTM (i.e., the first case). In the second operation pattern described above, when UE100 performs an InterCU MCG LTM using the MCG LTM setting, it discards the SCG LTM setting regardless of whether SN200S#1 is changed as a result of the InterCU MCG LTM.

[0168] On the other hand, when performing MCG LTM on an intraCU, security key updates are not required on the UE100. Specifically, when performing MCG LTM on an intraCU interDU, the UE100 performs an L2 reset (i.e., MAC reset and RLC re-establishment), but does not perform PDCP re-establishment. In contrast, when performing MCG LTM on an intraCU intraDU, the UE100 does not perform RLC re-establishment or PDCP re-establishment. Furthermore, when performing MCG LTM on an intraCU, the UE100 retains the SCG LTM settings without discarding them.

[0169] In the following embodiment, it is assumed that a new identifier is introduced to enable UE100 to determine whether to perform MCG LTM on the interCU or the intraCU. This new identifier is set in UE100 in association with a cell during LTM configuration (LTM-Config).

[0170] The new identifier is a temporary identifier (index) that indicates the aggregate unit (gNB-CU) to which the cell belongs, and may be referred to as the "CU index" below. The new identifier may also be an identifier for determining whether or not a security key update (PDCP re-establishment) is required during LTM cell switching, and may be referred to as "PdcpNoResetID" or "NoSecurityKeyUpdateID" below.

[0171] Figure 14 shows an example of an LTM setting (LTM-Config) set in the UE100 according to this embodiment. In Figure 14, "-r18" indicates an information element introduced in Release 18 of the 3GPP standard, and "-r19" indicates an information element newly introduced in Release 19 of the 3GPP standard.

[0172] The LTM configuration (LTM-Config) E1 includes the LTM reference configuration (ltm-ReferenceConfiguration-r18), the LTM candidate release list (ltm-CandidateToReleaseList-r18), the LTM candidate addition / modification list (ltm-CandidateToAddModList-r18) E2, the serving cell DU index (ltm-ServingCellNoResetID-r18) E4, and the serving cell CU index (ltm-ServingCellPdcpNoResetID-r19) E5.

[0173] The LTM candidate addition / modification list (ltm-CandidateToAddModList-r18) E2 is a list of LTM candidate cell settings (LTM-Candidate-r18) to be added or modified.

[0174] The entry in the list, LTM Candidate Cell Settings (LTM-Candidate-r18), includes LTM Candidate ID (ltm-CandidateId-r18), LTM Candidate Cell ID (ltm-CandidatePCI-r18), SSB Settings (ltm-SSB-Config-r18), LTM Candidate RRC Settings (ltm-CandidateConfig-r18), LTM Candidate Cell DU Index (ltm-NoResetID-r18) E6, and LTM Candidate Cell CU Index (ltm-PdcpNoResetID-r19) E7.

[0175] The Serving Cell DU Index (ltm-ServingCellNoResetID-r18) E4 is a temporary identifier (index) of the DU to which the serving cell belongs, and is used by UE100 to determine whether an L2 reset is necessary during LTM cell switching. Similarly, the LTM Candidate Cell DU Index (ltm-NoResetID-r18) E6 is a temporary identifier (index) of the gNB-DU to which the corresponding LTM candidate cell belongs, and is used by UE100 to determine whether an L2 reset is necessary during LTM cell switching to that LTM candidate cell.

[0176] Specifically, when UE100 switches an intraCU to an LTM cell (target cell), if the serving cell DU index (ltm-ServingCellNoResetID-r18) E4 and the LTM candidate cell DU index (ltm-NoResetID-r18) E6 of the target cell are different, i.e., if it is an LTM cell switch of an intraCU interDU, UE100 performs an L2 reset including RLC re-establishment.

[0177] On the other hand, in the case of an intraCU switching to an LTM cell (target cell), if the serving cell DU index (ltm-ServingCellNoResetID-r18) E4 and the LTM candidate cell DU index (ltm-NoResetID-r18) E6 of the target cell are the same, that is, in the case of an intraCU intraDU switching to an LTM cell, UE100 does not perform RLC re-establishment.

[0178] The Serving Cell CU Index (ltm-ServingCellPdcpNoResetID-r19) E5 is a temporary identifier (index) of the gNB-CU to which the serving cell belongs, and is used by UE100 to determine whether a PDCP reset (PDCP re-establishment) and / or security key update are necessary during LTM cell switching. Similarly, the LTM Candidate Cell CU Index (ltm-PdcpNoResetID-r19) E7 is a temporary identifier (index) of the gNB-CU to which the corresponding LTM candidate cell belongs, and is used by UE100 to determine whether a PDCP reset (PDCP re-establishment) and / or security key update are necessary during LTM cell switching to that LTM candidate cell.

[0179] Specifically, when switching an LTM cell to an LTM candidate cell (target cell), UE100 performs a PDCP reset (PDCP re-establishment) and / or security key update and L2 reset if the serving cell CU index (ltm-ServingCellPdcpNoResetID-r19) E5 and the LTM candidate cell CU index (ltm-PdcpNoResetID-r19) E7 of the target cell are different, i.e., it is an LTM cell switch for an interCU. In this embodiment, when switching an MCG LTM cell for an interCU, UE100 performs a PDCP reset (PDCP re-establishment) and L2 reset, and discards the stored SCG LTM settings (SCG-VarLTM-Config).

[0180] On the other hand, when switching to an LTM cell candidate cell (target cell), if the serving cell CU index (ltm-ServingCellPdcpNoResetID-r19) E5 and the LTM candidate cell CU index (ltm-PdcpNoResetID-r19) E7 of the target cell are the same, i.e., if it is an LTM cell switch of an intra CU, UE100 does not perform a PDCP reset (PDCP re-establishment) and / or security key update. In this embodiment, when it is an MCG LTM cell switch of an intra CU, UE100 retains the stored SCG LTM settings (SCG VarLTM-Config) without discarding them.

[0181] Figure 15 shows an example of the operation of UE100 according to this embodiment. First, an overview of the operation will be explained.

[0182] Firstly, UE100 receives from the serving cell a first identifier associated with UE100's serving cell (serving cell CU index (ltm-ServingCellPdcpNoResetID-r19) E5) and a second identifier associated with the LTM candidate cell set in UE100 (LTM candidate cell CU index (ltm-PdcpNoResetID-r19) E7) (step 401). Also, UE100 stores the SCG LTM settings used for switching LTM cells within the SCG that are set in UE100 (step 401). Secondly, UE100 triggers an LTM cell switch from a serving cell to an LTM candidate cell (step S402: YES), and discards the stored SCG LTM settings in response to the fact that the first identifier and the second identifier are different (step S403: YES) (step S405). Specifically, UE100 triggers an LTM cell switch (step S402: YES), discards the SCG LTM settings in response to the fact that the first identifier and the second identifier are different (step S403: YES), and the triggered LTM cell switch is an MCG LTM cell switch (step S404: YES).

[0183] This makes it possible to solve the above-mentioned problem that occurs when UE100 retains the SCG LTM settings even after interCU MCG LTM. In particular, UE100 can appropriately determine whether or not to discard the SCG LTM settings based on the first identifier (serving cell CU index (ltm-ServingCellPdcpNoResetID-r19) E5) and the second identifier (LTM candidate cell CU index (ltm-PdcpNoResetID-r19) E7).

[0184] UE100 may, upon triggering an LTM cell switch (step S402: YES) and confirming that the first identifier and the second identifier are the same (step S403: NO), maintain the stored SCG LTM settings without discarding them (steps S409, S412).

[0185] Next, we will describe the details of the operation of the UE100 according to this embodiment.

[0186] As shown in Figure 15, in step S401, UE100 performs DC communication with MN200M#1 and SN200S#1 as shown in Figure 10 or Figure 11. UE100 also stores an MCG LTM configuration (MCG VarLTM-Config) containing information used for inter-CU MCG LTM, which is LTM cell switching from MN200M#1 to another MN200M#2. Furthermore, UE100 stores an SCG LTM configuration (SCG VarLTM-Config) containing information used for intra-CU SCG LTM, which is LTM cell switching within SN200S#1.

[0187] Specifically, UE100 receives the LTM settings for MCG and SCG from MN200M#1 (serving cell) via RRC signaling (RRC Reconfiguration message) and stores each received setting. Note that for the SCG LTM settings, UE100 may also receive them from SN200S#1 via RRC signaling (RRC Reconfiguration message). The MCG LTM settings may be the LTM settings (LTM-Config) shown in Figure 14. The LTM settings (LTM-Config) include first and second configuration information. The second configuration information is for LTM candidate cells, and the first configuration information is different from the second configuration information. The first identifier associated with the serving cell of UE100 (serving cell CU index (ltm-ServingCellPdcpNoResetID-r19) E5) is included in the first configuration information. The second identifier associated with the LTM candidate cell set in UE100 (LTM candidate cell CU index (ltm-PdcpNoResetID-r19) E7) is included in the second configuration information.

[0188] In step S402, UE100 determines whether or not to trigger an LTM cell switchover. UE100 triggers an LTM cell switchover in response to receiving a cell switchover command (MAC CE) from the serving cell. Alternatively, in the case of conditional LTM described later, UE100 may trigger an LTM cell switchover in response to the fulfillment of the radio quality conditions set by the serving cell.

[0189] If an LTM cell switch is triggered (step S402: YES), in step S403, UE100 compares a first identifier associated with the serving cell (source cell) (serving cell CU index (ltm-ServingCellPdcpNoResetID-r19) E5) with a second identifier associated with the LTM candidate cell, which is the target cell for the LTM cell switch (LTM candidate cell CU index (ltm-PdcpNoResetID-r19) E7). That is, UE100 compares the CU index of the source cell with the CU index of the target cell.

[0190] If the first identifier and the second identifier are different (step S403: YES), that is, if it is an LTM cell switchover of the interCU, in step S404, UE100 determines whether the triggered LTM cell switchover is an LTM cell switchover of the MCG. For example, UE100 may determine that it is an LTM cell switchover of the MCG when it receives a cell switchover command (MAC CE) from the serving cell of MN200M#1.

[0191] If the triggered LTM cell switch is an MCG LTM cell switch (step S404: YES), in step S405, UE100 performs an interCU MCG LTM cell switch. Specifically, UE100 applies the target cell settings and starts accessing the target cell. UE100 also performs a PDCP reset (PDCP re-establishment). UE100 may also perform an L2 reset in addition to the PDCP reset (PDCP re-establishment). Furthermore, UE100 discards the stored SCG LTM settings (SCG VarLTM-Config). If the triggered LTM cell switch is an SCG LTM cell switch (step S404: NO), in step S406, UE100 performs an SCG LTM cell switch. Furthermore, during the switching of the SCG's LTM cell, UE100 may perform a PDCP reset (PDCP re-establishment) and / or an L2 reset. In addition, UE100 retains (does not discard) the stored SCG LTM settings (SCG VarLTM-Config).

[0192] If the first identifier and the second identifier are the same (step S403: NO), that is, if it is an LTM cell switch of an intraCU, in step S407, UE100 compares the serving cell DU index (ltm-ServingCellNoResetID-r18) E4 with the LTM candidate cell DU index (ltm-NoResetID-r18) E6 associated with the LTM candidate cell which is the target cell for the LTM cell switch. In other words, UE100 compares the DU index of the source cell with the DU index of the target cell.

[0193] If the serving cell DU index (ltm-ServingCellNoResetID-r18) E4 and the LTM candidate cell DU index (ltm-NoResetID-r18) E6 are different (step S407: YES), that is, if it is an LTM cell switchover of an intraCU interDU, in step S408, UE100 determines whether the triggered LTM cell switchover is an LTM cell switchover of an MCG.

[0194] If the triggered LTM cell switch is an MCG LTM cell switch (step S408: YES), in step S409, UE100 performs an MCG LTM cell switch for the intraCU interDU. UE100 also performs an L2 reset, including RLC re-establishment. Furthermore, UE100 retains the stored SCG LTM settings (SCG VarLTM-Config) without discarding them. If the triggered LTM cell switch is an SCG LTM cell switch (step S408: NO), in step S410, UE100 performs an SCG LTM cell switch.

[0195] If the serving cell DU index (ltm-ServingCellNoResetID-r18) E4 and the LTM candidate cell DU index (ltm-NoResetID-r18) E6 are the same (step S407: NO), that is, if it is an LTM cell switchover of an intraCU intraDU, in step S411, UE100 determines whether the triggered LTM cell switchover is an LTM cell switchover of an MCG.

[0196] If the triggered LTM cell switch is an MCG LTM cell switch (step S411: YES), in step S412, UE100 performs an MCG LTM cell switch for the intraCU intraDU. In this case, UE100 does not need to perform an L2 reset including RLC re-establishment. Furthermore, UE100 retains the stored SCG LTM settings (SCG VarLTM-Config) without discarding them. If the triggered LTM cell switch is an SCG LTM cell switch (step S411: NO), in step S413, UE100 performs an SCG LTM cell switch.

[0197] In this example, it is assumed that UE100 receives and stores an LTM setting that includes the first identifier and the second identifier. However, there may also be cases where UE100 receives and stores an LTM setting that does not include the first identifier and the second identifier. If UE100 does not have the second identifier of the target cell (i.e., does not include the ltm-PdcpNoResetID field of LTM-Candidate IE) and UE100 does not have the first identifier (i.e., does not store the value of ltm-ServingCellPdcpNoResetID in VarLTM-ServingCellPdcpNoResetID), then UE100 may perform the same operation as YES in step S403. In other words, if UE100 does not store the first identifier and the second identifier, and it is an LTM cell switchover for the MCG, UE100 may perform PDCP re-establishment and L2 reset, and discard the LTM settings for the SCG (step S405).

[0198] (2) Second Embodiment The second embodiment will be described mainly in terms of the differences from the first embodiment described above. This embodiment may be implemented in combination with the first embodiment described above. In this embodiment, the switching of LTM cells of the MCG is mainly assumed, but the switching of LTM cells of the SCG may also be assumed.

[0199] In this embodiment, the operation for appropriately managing the new identifiers (CU index, PdcpNoResetID) on the network 5 side will be described. The CU index may be set to a different value for each UE100 for the same gNB-CU. Furthermore, in Subsequent LTM, LTM cell switching is performed without RRC signaling, making it difficult to update the CU index set in UE100 using RRC signaling. Therefore, the question arises as to how to manage the CU index.

[0200] In this embodiment, the new identifiers described above (CU index, PdcpNoResetID) may be referred to as "Rel-19 ID". Also, gNB-CU may be simply referred to as "CU", and gNB-DU may be simply referred to as "DU". Communication between CUs shall be performed over the Xn interface. Communication between CUs and DUs shall be performed over the F1 interface.

[0201] The following embodiment describes the operation for properly managing the CU index (Rel-19 ID). Figure 16 is a diagram illustrating the overview of the operation according to this embodiment.

[0202] In this embodiment, the CU of the first network node gNB (also referred to as "serving gNB" or "source gNB") 200a, which manages the serving cells of the UE 100, manages each CU index set in the UE 100.

[0203] Specifically, firstly, the CU of gNB200a assigns a CU index to the CU of the adjacent gNB (also referred to as the "candidate gNB" or "target gNB") 200b, which is the second network node that manages the LTM candidate cell set up in UE100, during the preparation phase for LTM cell switching. In the illustrated example, the CU of gNB200a assigns non-overlapping CU indexes to each of the CUs of gNB200b1 to gNB200b3. The CU of gNB200a maintains a list containing the assigned CU indexes (also referred to as the "CU index list" or "Rel-19 ID list").

[0204] Secondly, the CU of gNB200a notifies the gNB200b managing the target cell (the new serving cell of UE100) of the CU index list during the execution or completion phase of the LTM cell switchover. In the illustrated example, the CU of gNB200a notifies the CU index list to any of the gNB200b from gNB200b1 to gNB200b3 that manages the target cell on which the LTM cell switchover occurred. In this way, the CU of gNB200a takes over the CU index list to the gNB200b managing the new serving cell of UE100 in response to the execution of the LTM cell switchover. As a result, the serving gNB always has the CU index list. gNB200b, which has inherited the CU index list, may add a new CU index to the CU index list when adding an LTM candidate cell setting for UE100, or remove a CU index from the CU index list when deleting an LTM candidate cell setting.

[0205] In this embodiment, gNB200a transmits to the CU of gNB200b a temporary identifier (CU index) that is assigned to the CU of gNB200b to which the LTM candidate cell set in UE100 belongs. As described in the first embodiment, this temporary identifier (CU index) is an identifier set in UE100 as part of the setting information regarding the LTM candidate cell.

[0206] In this embodiment, the CU of gNB200a determines the CU index (Rel-19 ID) to be assigned to the CU of gNB200b during the preparation phase for LTM cell switching, and sends a request message containing the determined CU index (Rel-19 ID) to the CU of gNB200b. In the illustrated example, gNB200a assigns Rel-19 ID:1 to gNB200b1, Rel-19 ID:2 to gNB200b2, and Rel-19 ID:3 to gNB200b3. The request message used to notify gNB200b2 of the CU index (Rel-19 ID) may be a HANDOVER REQUEST message requesting preparation for LTM cell switching, or a new message for notifying the CU index (Rel-19 ID).

[0207] The CU of gNB200b sends a response message to the CU of gNB200a, which is a response message to a request message from the CU of gNB200a, and includes LTM candidate cell settings (LTM candidate), which are configuration information for LTM candidate cells. The LTM candidate cell settings (LTM candidate) in the response message include the CU index notified by the CU of gNB200a. The LTM candidate cell settings (LTM candidate) are contained in the RRC container in the response message and are set in UE100 via gNB200a.

[0208] The CU of gNB200a maintains a list of CU indices assigned to each gNB200b CU (i.e., CU indices for each LTM candidate cell set in UE100). The CU index list may be managed in association with identifiers of UE100. Each entry in the CU index list includes a CU index and a unique identifier of the gNB200b corresponding to that CU index. The unique identifier may be a Global gNB ID.

[0209] Thus, the CU of gNB200a holds a CU index list that associates a temporary identifier, the CU index, with a unique identifier, the Global gNB ID. The CU of gNB200a may hold a different CU index list for each UE100.

[0210] Each entry in the CU index list may include the following information:

[0211] CU index.

[0212] - ID for identifying the gNB The ID may be a Source Global gNB ID (an ID for identifying gNB200a) or a Candidate (Target) Global gNB ID (an ID for identifying gNB200b).

[0213] - ID for identifying UE100 The ID may be Source NG-RAN node UE XnAP ID (ID for identifying UE100 in gNB200a), Target NG-RAN node UE XnAP ID (ID for identifying UE100 in gNB200b), or 5G-S-TMSI (ID for identifying UE in AMF).

[0214] Then, during the execution or completion phase of the LTM cell switchover, the CU of gNB200a sends a message containing the CU index list to the CU of gNB200b, which manages the new serving cell of UE100.

[0215] For example, gNB200a may send a message to gNB200b indicating that it has sent a cell switch command (MAC CE) to UE100 instructing a cell switch to an LTM candidate cell, including a CU index list. Specifically, after sending an LTM cell switch command MAC CE to UE100, gNB200a's CU may send a CELL SWITCH NOTIFICATION message to gNB200b notifying it that a cell switch has been performed, including the CU index list it holds.

[0216] Alternatively, the CU of gNB200a may send a message containing the CU index list to gNB200b in response to a request from the CU of gNB200b. The CU of gNB200a may send the CU index list to the CU of gNB200b at the time it is requested by the CU of gNB200b.

[0217] For example, the CU of gNB200b may receive a notification from the DU of gNB200b indicating that it has detected access from UE100, and in response to receiving the notification, it may send a send request to gNB200a requesting the transmission of a message containing the CU index list. That is, the CU of gNB200b may request the CU of gNB200a to transmit the CU index list when it receives an ACCESS SUCCESS message from the DU of gNB200b.

[0218] Alternatively, the CU of gNB200b may receive a transfer of the RRC message received by the DU of gNB200b from the UE100, and in response to receiving the transfer, send a transmission request to gNB200a requesting the transmission of a message including the CU index list. That is, the CU of gNB200b may request the CU of gNB200a to send the CU index list when it receives the UL RRC MESSAGE TRANSFER message from the DU of gNB200b.

[0219] As another example, gNB200a may receive a release message from gNB200b to release the context information (UE context) of UE100, and in response to receiving the release message, send a message containing the CU index list to gNB200b. That is, the CU of gNB200a may send the CU index list to the CU of gNB200b at the time it receives the UE CONTEXT RELEASE message from the CU of gNB200b.

[0220] Furthermore, the CU of gNB200a may send a CU index list to each gNB200b during the preparation phase for LTM cell switching. For example, the CU of gNB200a may include the CU index list in the HANDOVER REQUEST message requesting preparation for LTM cell switching and send it to gNB200b.

[0221] Figure 17 shows an example of the operation of the mobile communication system 1 according to this embodiment.

[0222] 1: After setting up the measurement parameters for UE100, gNB200a receives the L3 measurement report from UE100.

[0223] 2: Based on the L3 measurement report from UE100, the CU of gNB200a determines one or more gNB200b as candidates for LTM cell switching.

[0224] 3. The CU of gNB200a assigns a CU index to each CU of gNB200b and notifies each CU of gNB200b with a HANDOVER REQUEST message. The CU of gNB200b receives a HANDOVER REQUEST message containing the CU index assigned to it.

[0225] 4. The CU of the gNB200b sends a UE CONTEXT SETUP REQUEST message to the DU of the gNB200b.

[0226] 5: The DU of the gNB200b sends the UE CONTEXT SETUP RESPONSE message to the CU of the gNB200b.

[0227] 6: The CU of gNB200b sends a HANDOVER REQUEST ACKNOWLEDGE message to the CU of gNB200a, setting the CU index notified by the CU of gNB200a into the LTM candidate in the RRC container.

[0228] 7: The CU of gNB200a sends a UE CONTEXT MODIFICATION REQUEST message to the DU of gNB200a. Here, the CU of gNB200a sends a UE CONTEXT MODIFICATION REQUEST message to the DU of gNB200a that includes information about early synchronization and a list of LTM configuration ID mappings for accepted LTM candidate cells.

[0229] 8: The DU of gNB200a sends a UE CONTEXT MODIFICATION RESPONSE message to the CU of gNB200a. Specifically, the DU of gNB200a responds with a UE CONTEXT MODIFICATION RESPONSE message containing the updated lower layer configuration.

[0230] 9: The CU of gNB200a sends a UE CONTEXT UPDATE message to the CU of gNB200b. The UE CONTEXT UPDATE message is a new signaling message, and is used by the CU of gNB200a to request a UE CONTEXT UPDATE from the CU of gNB200b.

[0231] 10: The CU of gNB200b sends a UE CONTEXT MODIFICATION REQUEST message to the DU of gNB200b. Specifically, the CU of gNB200b sends a UE CONTEXT MODIFICATION REQUEST message to the DU of gNB200b containing information for Subsequent LTM and / or information for updating the settings of LTM candidate cells.

[0232] 11: The DU of gNB200b sends a UE CONTEXT MODIFIATION RESPONSE message to the CU of gNB200b. Specifically, the DU of gNB200b responds with a UE CONTEXT MODIFIATION RESPONSE message containing the updated lower layer configuration.

[0233] 12: The CU of gNB200b sends the LTM UE CONTEXT UPDATE ACKNOWLEDGE message to the CU of gNB200a.

[0234] 13: The CU of gNB200a sends a DL RRC MESSAGE TRANSFER message containing an RRC Reconfiguration message to the DU of gNB200a.

[0235] 14: The DU of gNB200a sends the RRC Reconfiguration message received from the CU of gNB200a to UE100. At this point, the CU index of each gNB200b is set in UE100.

[0236] 15: UE100 sends the RRC Reconfiguration Complete message to gNB200a.

[0237] 16: The DU of gNB200a sends a UL RRC MESSAGE TRANSFER message containing the RRC Reconfiguration Complete message received from UE100 to the CU of gNB200a.

[0238] 17: The DU of gNB200a sends a PDCCH order to UE100 for early synchronization. Upon receiving the PDCCH order, UE100 sends a random access preamble to the DU of gNB200b. However, in the case of UE-based TA measurement, UE100 performs early synchronization itself.

[0239] 18: The DU of gNB200b sends the TA value derived from the random access preamble from UE100 to the CU of gNB200b in the DU-CU TA INFORMATION TRANSFER message.

[0240] 19: The CU of gNB200b sends a CU-CU TA INFORMATION TRANSFER message containing the TA value to the CU of gNB200a.

[0241] 20: The CU of gNB200a sends a CU-DU TA INFORMATION TRANSFER message containing the TA value to the DU of gNB200a.

[0242] 21: UE100 sends the L1 measurement report to the DU of gNB200a.

[0243] 22: For gNB200a, the DU is determined by switching the LTM cell based on the L1 measurement report.

[0244] 23: The DU of gNB200a sends LTM cell switch command MAC CE to UE100.

[0245] 24: The DU of gNB200a sends a DU-CU CELL SWITCH NOTIFICATION message to the CU of gNB200a to notify that it has sent an LTM Cell Switch Command MAC CE to UE100.

[0246] 25: The CU of gNB200a sends a CU-CU CELL SWITCH NOTIFICATION message to the CU of gNB200b to notify that it has sent an LTM Cell Switch Command MAC CE to UE100. The CU-CU CELL SWITCH NOTIFICATION message includes the target cell ID and the TCI State ID. The CU-CU CELL SWITCH NOTIFICATION message may also include a CU index list (Rel-19 ID list).

[0247] 26: The CU of gNB200b sends a CU-DU CELL SWITCH NOTIFICATION message to the DU of gNB200b to notify that it has sent an LTM Cell Switch Command MAC CE to UE100.

[0248] 27: UE100 accesses the target cell (DU of gNB200b).

[0249] 28: When the DU of gNB200b detects access to UE100, it sends an ACCESS SUCCESS message to the CU of gNB200b.

[0250] 29: The CU of gNB200b may send an ID LIST REQUEST message to the CU of gNB200a requesting the submission of a CU index list (Rel-19 ID list).

[0251] 30: When the CU of gNB200a receives an ID LIST REQUEST message, it sends an ID LIST REQUEST ACKNOWLEDGE message containing the CU index list to the CU of gNB200b.

[0252] 31: UE100 sends the RRC Reconfiguration Complete message to the DU of gNB200b.

[0253] 32: The DU of gNB200b sends a UL RRC MESSAGE TRANSFER message containing the RRC Reconfiguration Complete message received from UE100 to the CU of gNB200b.

[0254] 33: The CU of gNB200b may send an ID LIST REQUEST message to the CU of gNB200a requesting the submission of a CU index list (Rel-19 ID list).

[0255] 34: When the CU of gNB200a receives an ID LIST REQUEST message, it sends an ID LIST REQUEST ACKNOWLEDGE message containing the CU index list to the CU of gNB200b.

[0256] 35: The CU of gNB200b sends a UE CONTEXT RELEASE message to the CU of gNB200a.

[0257] 36: The CU of gNB200a may send an ID LIST NOTIFICATION message containing a CU index list to the CU of gNB200b.

[0258] 37: The CU of gNB200a sends the UE CONTEXT RELEASE COMMAND message to the DU of gNB200a.

[0259] 38: The DU of gNB200a sends the UE CONTEXT RELEASE COMPLETE message to the CU of gNB200a.

[0260] (3) Other Embodiments The LTM in the above embodiments may be read as a conditional LTM. For example, the MCG LTM of the interCU or intraCU described above may be a conditional LTM of the MCG of the interCU or intraCU. Also, the SCG LTM of the intraCU described above may be a conditional LTM of the SCG of the intraCU. In a conditional LTM, for example, the RRC Reconfiguration message in step S4 of Figure 6 includes information indicating the execution conditions for LTM cell switching (e.g., wireless quality conditions) for each LTM candidate cell. Instead of the cell switching command MAC transmitted from the gNB200, the UE100 performs LTM cell switching for LTM candidate cells that meet the set execution conditions (wireless quality conditions). This eliminates the need to send and receive L1 measurement reports and cell switching command MACs, enabling faster LTM cell switching.

[0261] The DC in the above-described embodiment may be read as multi-connectivity. In the DC of the above-described embodiment, there is one additional SN200S. However, the operation according to the above-described embodiment may be applied to multi-connectivity in which multiple SN200S are added. In multi-connectivity, UE100 can perform simultaneous communication between one MN200M and multiple SN200S.

[0262] The above-described operation flows can be performed not only independently, but also in combination of two or more operation flows. For example, some steps of one operation flow may be added to another operation flow, or some steps of one operation flow may be replaced with some steps of another operation flow. It is not necessary to execute all steps in each flow; only some steps may be executed. Furthermore, the order of steps in each flow may be changed as appropriate.

[0263] In the embodiments and examples described above, an example in which the base station is an NR base station (gNB) was described, but the base station may also be an LTE base station (eNB) or a 6G base station. Furthermore, 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 an IAB node. Furthermore, UE100 may be an MT (Mobile Termination) of an IAB node. That is, UE100 may be a terminal function unit (a type of communication module) for the base station to control a relay device that performs signal relay. Such a terminal function unit is referred to as an MT. Examples of multi-transmission architectures (MTs) include IAB-MT, NCR (Network Controlled Repeater)-MT, and RIS (Reconfigurable Intelligent Surface)-MT.

[0264] Furthermore, 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). Additionally, a network node may consist of a combination of at least a part of the core network device and at least a part of a base station.

[0265] A program may be provided that causes a computer to execute each process performed by the UE100 or gNB200. The program may be recorded on a computer-readable medium. Using a computer-readable medium, it is possible to install the program on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transient recording medium. The non-transient recording medium is not particularly limited, but may be a recording medium such as a CD-ROM and / or DVD-ROM. Alternatively, the circuits that execute each process performed by the UE100 or gNB200 may be integrated, and at least a part of the UE100 or gNB200 may be configured as a semiconductor integrated circuit (chipset, SoC: System on a chip).

[0266] The functions realized by UE100 or gNB200 may be implemented in a circuit or processing circuit, including a general-purpose processor, application processor, integrated circuit, ASICs (Application Specific Integrated Circuits), CPU (a Central Processing Unit), conventional circuitry, and / or a combination thereof, programmed to realize the described functions. A processor, including transistors and / or other circuitry, is considered a circuit or processing circuit. A processor may be a programmed processor that executes a program stored in memory. In this specification, circuitry, unit, and means are hardware programmed to realize or execute the described functions. The hardware may be any hardware disclosed herein, or any hardware known to be programmed to perform or execute the functions described herein. If the hardware is a processor that is considered to be of the type of circuit, the circuit, means, or unit is a combination of hardware and software used to constitute the hardware and / or processor.

[0267] The phrases “based on” and “depending on / in response to” as used in this disclosure do not mean “based solely on” or “in response solely” unless otherwise specified. “Based on” means both “based solely on” and “at least partially on.” Similarly, “depending” means both “at least partially on” and “in at least partially on.” The terms “include,” “comprise,” and variations thereof do not mean that only the listed items are included, but that they may include only the listed items or may include additional items in addition to the listed items. Furthermore, the term “or” as used in this disclosure is not intended to mean exclusive OR. Moreover, 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 way to distinguish between two or more elements. Therefore, references to the first and second elements do not imply that only two elements may be adopted therein, or that the first element must precede the second element in any way. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles shall be plural unless it is clearly indicated from the context that they are not.

[0268] Although the embodiments have been described in detail above with reference to the drawings, the specific configuration is not limited to those described above, and various design changes can be made without departing from the gist of the invention.

[0269] This application claims priority to Japanese Patent Application No. 2024-194726 (filed November 6, 2024), the entirety of which is incorporated into the specification of this application.

[0270] (4) Additional notes: Features of the above-described embodiments are noted below.

[0271] - Appendix 1 A communication method performed by a user device, comprising: receiving from the serving cell a first identifier associated with the serving cell of the user device and a second identifier associated with an LTM (L1 / L2 Triggered Mobility) candidate cell set up in the user device; storing an SCG LTM setting used for switching LTM cells within a secondary cell group (SCG) set up in the user device; triggering an LTM cell switch from the serving cell to the LTM candidate cell; and discarding the SCG LTM setting in accordance with the triggering of the LTM cell switch and the fact that the first identifier and the second identifier are different.

[0272] - Appendix 2 The communication method according to Appendix 1, wherein the user device triggers the LTM cell switching and retains the SCG LTM settings without discarding them, depending on whether the first identifier and the second identifier are the same.

[0273] - Appendix 3 The communication method according to Appendix 1 or 2, wherein the user device triggers the LTM cell switching, and discards the SCG LTM setting in accordance with the fact that the first identifier and the second identifier are different and the triggered LTM cell switching is an LTM cell switching of a master cell group (MCG).

[0274] - Appendix 4 The communication method according to any one of the appendices 1 to 3, wherein the first identifier is an index indicating the aggregation unit (CU) to which the serving cell belongs, and the second identifier is an index indicating the CU to which the LTM candidate cell belongs.

[0275] - Appendix 5 The user device receives LTM settings including first setting information and second setting information from the serving cell, the second setting information is setting information for the LTM candidate cell, the first setting information is setting information different from the second setting information, the first identifier is included in the first setting information, and the second identifier is included in the second setting information. A communication method according to any one of the appendices 1 to 4.

[0276] - Appendix 6 The communication method according to any one of Appendix 1 to 5, wherein the user device triggers the LTM cell switching in response to receiving a cell switching command from the serving cell, and discards the SCG LTM setting in response to receiving the cell switching command and the first identifier and the second identifier being different.

[0277] - Appendix 7 The communication method according to any one of Appendix 1 to 5, wherein the user device triggers the LTM cell switching in response to the fulfillment of the wireless quality conditions set by the serving cell, and discards the SCG LTM setting in response to the fulfillment of the wireless quality conditions and the first identifier and the second identifier being different.

[0278] - Appendix 8 A user device comprising: a receiving unit that receives from the serving cell a first identifier associated with the serving cell of the user device and a second identifier associated with an LTM (L1 / L2 Triggered Mobility) candidate cell set in the user device; and a control unit that stores an SCG LTM setting used for switching LTM cells within a secondary cell group (SCG) set in the user device, wherein the control unit triggers an LTM cell switch from the serving cell to the LTM candidate cell, and discards the SCG LTM setting if the first identifier and the second identifier are different.

[0279] 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 performed by a user device, comprising: receiving from the serving cell a first identifier associated with the serving cell of the user device and a second identifier associated with an LTM (L1 / L2 Triggered Mobility) candidate cell set up in the user device; storing an SCG LTM setting used for switching LTM cells within a secondary cell group (SCG) set up in the user device; triggering an LTM cell switch from the serving cell to the LTM candidate cell; and discarding the SCG LTM setting in accordance with the triggering of the LTM cell switch and the fact that the first identifier and the second identifier are different.

2. The communication method according to claim 1, wherein the user device triggers the LTM cell switching and retains the SCG LTM settings without discarding them, depending on whether the first identifier and the second identifier are the same.

3. The communication method according to claim 1 or 2, wherein the user device triggers the LTM cell switching, and discards the SCG LTM setting in accordance with the fact that the first identifier and the second identifier are different and the triggered LTM cell switching is an LTM cell switching of a master cell group (MCG).

4. The communication method according to claim 1 or 2, wherein the first identifier is an index indicating the aggregation unit (CU) to which the serving cell belongs, and the second identifier is an index indicating the CU to which the LTM candidate cell belongs.

5. The communication method according to claim 1 or 2, wherein the user device receives LTM settings including first setting information and second setting information from the serving cell, the second setting information being setting information for the LTM candidate cell, the first setting information being setting information different from the second setting information, the first identifier being included in the first setting information, and the second identifier being included in the second setting information.

6. The communication method according to claim 1 or 2, wherein the user device triggers the LTM cell switching in response to receiving a cell switching command from the serving cell, and discards the SCG LTM setting in response to receiving the cell switching command and the first identifier and the second identifier being different.

7. The communication method according to claim 1 or 2, wherein the user device triggers the LTM cell switching in response to the fulfillment of the wireless quality conditions set by the serving cell, and discards the SCG LTM setting in response to the fulfillment of the wireless quality conditions and the first identifier and the second identifier being different.

8. A user device comprising: a receiving unit that receives from a serving cell a first identifier associated with the serving cell of the user device and a second identifier associated with an LTM (L1 / L2 Triggered Mobility) candidate cell set in the user device; and a control unit that stores an SCG LTM setting used for switching LTM cells within a secondary cell group (SCG) set in the user device, wherein the control unit triggers an LTM cell switch from the serving cell to the LTM candidate cell, and discards the SCG LTM setting if the first identifier and the second identifier are different.

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