Mobile communication method and network node

Abstract

This communication method for switching, by using L1 / L2 triggered mobility (LTM), a serving cell of a user device from a first cell of a first network node to a second cell of a second network node, comprises: receiving, by the first network node from the second network node, a handover request approval message including a dedicated random access channel (RACH) configuration for contention free random access (CFRA) used for early uplink synchronization performed on the second cell by the user device; and transmitting, by the first network node, a radio resource control (RRC) reconfiguration message including the dedicated RACH configuration to the user device. The handover request approval message includes notification information indicating the existence of the configuration of the CFRA.

Description

Mobile communication method and network node

[0001] The present disclosure relates to a communication method for use in a mobile communication system.

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

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

[0004] 3GPP Technical Specification "3GPP TS 38.300 V18.2.0 (2024-06)"

[0005] The present disclosure provides techniques for enabling LTM between network nodes, specifically, inter-CU (Central Unit) LTM.

[0006] A communication method according to a first aspect of the present disclosure is a communication method for switching a serving cell of a user equipment from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), the communication method comprising: the first network node transmitting, to the second network node, a handover request message including information requesting configuration for early uplink synchronization to be performed by the user equipment to the second cell; and the first network node receiving, from the second network node, a handover request acknowledgement message including the configuration for the early uplink synchronization.

[0007] In a communication method according to a second aspect of the present disclosure, the first network node receives the handover request acknowledgement message from the second network node, the handover request acknowledgement message including information indicating that LTM configuration will be performed.

[0008] A communication method according to a third aspect of the present disclosure includes a network node that operates as a first network node when switching a serving cell of a user equipment from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), the network node including: a transmitter that transmits, to the second network node, a handover request message including information requesting configuration for early uplink synchronization to be performed by the user equipment with the second cell; and a receiver that receives, from the second network node, a handover request acknowledgement message including the configuration for the early uplink synchronization.

[0009] A communication method according to a fourth aspect of the present disclosure is a communication method for switching a serving cell of a user equipment (UE) from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), the communication method including: receiving, from the first cell, a radio resource control (RRC) reconfiguration message including a dedicated random access channel (RACH) configuration for contention-free random access (CFRA) used for early uplink synchronization to the second cell; and triggering, based on the reception of the RRC reconfiguration message, the CFRA to the second cell, even if the user equipment does not receive, from the first cell, a physical downlink control channel (PDCCH) order triggering the CFRA to the second cell.

[0010] 1 is a diagram illustrating an example of the configuration of a mobile communication system according to an embodiment. FIG. 2 is a diagram illustrating an example of the configuration of a UE (user equipment) according to an embodiment. FIG. 3 is a diagram illustrating an example of the configuration of a gNB (network node) according to an embodiment. FIG. 4 is a diagram illustrating a protocol stack configuration of a radio interface of a user plane that handles data. FIG. 5 is a diagram illustrating a protocol stack configuration of a radio interface of a control plane that handles signaling (control signals). FIG. 6 is a diagram illustrating an example of a procedure for intra-CU LTM. FIG. 7 is a diagram for explaining the operating environment of a mobile communication system according to an embodiment. FIG. 8 is a diagram illustrating the operation of a source gNB in ​​a first operation pattern for inter-CU LTM according to the first embodiment. FIG. 9 is a diagram illustrating the operation of a candidate gNB in ​​a second operation pattern for inter-CU LTM according to the first embodiment. FIG. 10 is a diagram illustrating the operation of a source gNB in ​​a third operation pattern for inter-CU LTM according to the first embodiment. FIG. 11 is a diagram illustrating an example of an operation sequence of a mobile communication system according to the first embodiment. FIG. 12 is a diagram illustrating the operation of a UE according to a modified example of the first embodiment. FIG. 13 is a diagram for explaining a problem according to the second embodiment. FIG. 14 is a diagram illustrating an operation example according to the first operation pattern of the second embodiment. FIG. 15 is a diagram illustrating an operation example according to the second operation pattern of the second embodiment. FIG. 16 is a diagram illustrating an operation example according to the third operation pattern of the second embodiment. FIG. 17 is a diagram illustrating an operation example according to the fourth operation pattern of the second embodiment. 1 illustrates the problem of early RACH due to PDCCH order in Inter CU LTM, Inter CU MCG LTM with SN release, and LTM Config stored in the UE before Inter CU MCG LTM with SN release.

[0011] A mobile communication system according to an embodiment will be described with reference to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0012] (1) Configuration of a Mobile Communication System FIG. 1 is a diagram showing an example of the configuration of a mobile communication system 1 according to this embodiment. The mobile communication system 1 conforms to the 3GPP standard 5th Generation System (5GS). While the following description uses 5GS as an example, the mobile communication system may also be at least partially based on an LTE (Long Term Evolution) system. The mobile communication system may also be at least partially based on a 6th Generation (6G) system.

[0013] The mobile communication system 1 includes a user equipment (UE) 100, a 5G radio access network (NG-RAN: Next Generation Radio Access Network) 10, and a 5G core network (5GC: 5G Core Network) 20. Hereinafter, the NG-RAN 10 may be simply referred to as the RAN 10. Furthermore, the 5GC 20 may be simply referred to as the core network (CN) 20. The RAN 10 and the CN 20 constitute a network 5 of the mobile communication system 1.

[0014] The UE 100 is a mobile wireless communication device. The UE 100 may be any device used by a user. For example, the UE 100 may be a mobile phone terminal (including a smartphone) and / or a tablet terminal, a notebook PC, a communication module (including a communication card or chipset), a sensor or a device provided in a sensor, a vehicle or a device provided in a vehicle (Vehicle UE), or an aircraft or a device provided in an aircraft (Aerial UE). A link in the transmission direction from the UE 100 to the network 5 is referred to as an uplink (UL), and a link in the transmission direction from the network 5 to the UE 100 is referred to as a downlink (DL).

[0015] The NG-RAN 10 includes a base station (referred to as "gNB" in the 5G system) 200, which is a type of network node. The gNBs 200 are connected to each other via an Xn interface, which is an interface between base stations. The gNB 200 manages one or more cells. The gNB 200 performs wireless communication with the UE 100 that has established a connection with its own cell. The gNB 200 has a radio resource management (RRM) function, a routing function for user data (hereinafter simply referred to as "data"), a measurement control function for mobility control and scheduling, etc. The term "cell" is used to indicate the smallest unit of a wireless communication area. The term "cell" is also used to indicate a function or resource that performs wireless communication with the UE 100. One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

[0016] In addition, gNBs can also be connected to the Evolved Packet Core (EPC), which is the core network of LTE. LTE base stations can also be connected to 5GC. LTE base stations and gNBs can also be connected via an inter-base station interface.

[0017] The 5GC20 includes an AMF (Access and Mobility Management Function) and a UPF (User Plane Function) 300. The AMF performs various mobility controls for the UE 100. The AMF manages the mobility of the UE 100 by communicating with the UE 100 using NAS (Non-Access Stratum) signaling. The UPF controls data forwarding. The AMF and the UPF are connected to the gNB 200 via an NG interface, which is an interface between a base station and a core network.

[0018] 2 is a diagram showing an example of the configuration of a UE 100 (user equipment) according to this embodiment. The UE 100 has a receiving unit 110, a transmitting unit 120, and a control unit 130. The receiving unit 110 and the transmitting unit 120 constitute a wireless communication unit 140 that performs wireless communication with the gNB 200.

[0019] The receiving unit 110 performs various types of reception under the control of the control unit 130. The receiving unit 110 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 130.

[0020] The transmitting unit 120 performs various transmissions under the control of the control unit 130. The transmitting unit 120 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 130 into a radio signal and transmits it from the antenna.

[0021] The control unit 130 performs various controls and processes in the UE 100. Such processes include processes of each layer described below. The operations of the UE 100 described above and below may be operations controlled by the control unit 230. The control unit 130 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processing by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0022] 3 is a diagram showing an example configuration of a gNB 200 (network node) according to this embodiment. The gNB 200 has a transmitter 210, a receiver 220, a controller 230, and a network communication unit 240. The transmitter 210 and the receiver 220 constitute a wireless communication unit 250 that performs wireless communication with the UE 100. The network communication unit 240 has a transmitter 241 that transmits and a receiver 242 that receives.

[0023] The transmitting unit 210 performs various transmissions under the control of the control unit 230. The transmitting unit 210 includes an antenna and a transmitter. The transmitter converts a baseband signal (transmission signal) output by the control unit 230 into a radio signal and transmits it from the antenna.

[0024] The receiving unit 220 performs various types of reception under the control of the control unit 230. The receiving unit 220 includes an antenna and a receiver. The receiver converts a radio signal received by the antenna into a baseband signal (received signal) and outputs the baseband signal to the control unit 230.

[0025] The control unit 230 performs various controls and processes in the gNB 200. Such processes include processes for each layer described below. The operations of the gNB 200 described above and below may be operations under the control of the control unit 230. The control unit 230 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processing by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0026] The network communication unit 240 is connected to adjacent base stations via an Xn interface, which is an interface between base stations. The network communication unit 240 is connected to the AMF / UPF 300 via an NG interface, which is an interface between a base station and a core network. The gNB 200 is composed of a CU (Central Unit) and a DU (Distributed Unit) (i.e., functionally divided), and the two units may be connected by an F1 interface, which is a fronthaul interface.

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

[0028] The user plane radio interface protocol includes a physical (PHY) layer, a medium access control (MAC) layer, a radio link control (RLC) layer, a packet data convergence protocol (PDCP) layer, and a service data adaptation protocol (SDAP) layer.

[0029] The PHY layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layer of UE100 and the PHY layer of gNB200 via a physical channel. The PHY layer of UE100 receives downlink control information (DCI) transmitted from gNB200 on a physical downlink control channel (PDCCH). Specifically, UE100 performs blind decoding of the PDCCH using a radio network temporary identifier (RNTI) and acquires successfully decoded DCI as DCI addressed to the UE. The DCI transmitted from gNB200 has a CRC (Cyclic Redundancy Code) parity bit scrambled by the RNTI added.

[0030] The MAC layer performs data priority control, retransmission processing using Hybrid Automatic Repeat reQuest (HARQ), random access procedures, etc. Data and control information are transmitted between the MAC layer of the UE 100 and the MAC layer of the gNB 200 via a transport channel. The MAC layer of the gNB 200 includes a scheduler. The scheduler determines the uplink and downlink transport format (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be allocated to the UE 100.

[0031] The RLC layer transmits data to the receiving RLC layer using the functions of the MAC layer and PHY layer. Data and control information are transmitted between the RLC layer of the UE 100 and the RLC layer of the gNB 200 via a logical channel.

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

[0033] The SDAP layer maps IP flows, which are units for Quality of Service (QoS) control by the core network, to radio bearers, which are units for QoS control by the Access Stratum (AS). Note that if the RAN is connected to the EPC, SDAP may not be required.

[0034] FIG. 5 is a diagram showing the configuration of a protocol stack of a radio interface of a control plane that handles signaling (control signals).

[0035] The protocol stack of the radio interface of the control plane has an RRC (Radio Resource Control) layer and an NAS (Non-Access Stratum) layer instead of the SDAP layer shown in FIG.

[0036] RRC signaling for various settings is transmitted between the RRC layer of UE100 and the RRC layer of gNB200. The RRC layer controls logical channels, transport channels, and physical channels according to the establishment, re-establishment, and release of radio bearers. When there is a connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC connected state. When there is no connection (RRC connection) between the RRC of UE100 and the RRC of gNB200, UE100 is in an RRC idle state. When the connection between the RRC of UE100 and the RRC of gNB200 is suspended, UE100 is in an RRC inactive state.

[0037] The NAS layer (also simply referred to as "NAS") located above the RRC layer performs session management, mobility management, etc. NAS signaling is transmitted between the NAS layer of the UE 100 and the NAS layer of the AMF 300A. Note that the UE 100 has an application layer and the like in addition to the radio interface protocol. Also, a layer lower than the NAS layer is referred to as the AS layer (also simply referred to as "AS").

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

[0039] In a typical handover procedure, a serving cell switch is triggered by signaling in the upper layer L3, specifically, the RRC layer. Such a typical handover is also referred to as an L3 handover. In an L3 handover, an L3 measurement report message, which is an RRC message, is transmitted from the UE 100 to the gNB 200. The gNB 200 determines the handover of the UE 100 based on the Measurement Report message, and instructs the cell switch by transmitting a handover command (specifically, an RRC Reconfiguration message) which is an RRC message from the gNB 200 to the UE 100.

[0040] On the other hand, LTM is a technology for shortening mobility delay (specifically, serving cell switching delay) compared to a typical handover procedure by triggering a serving cell switch by signaling of a lower layer, Layer 1 (L1) and / or Layer 2 (L2). In LTM, the gNB 200 receives an L1 measurement report from the UE 100, and based on the L1 measurement report, the gNB 200 signals the UE 100 via a MAC CE to instruct the serving cell switch by a cell switch command.

[0041] Specifically, in LTM, first, gNB200 prepares an LTM candidate cell configuration for a candidate cell to be switched to, and provides the LTM candidate cell configuration to UE100 via RRC signaling.

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

[0043] Third, the gNB 200 receives an L1 measurement report from the UE 100, determines a serving cell switch to the target cell based on the L1 measurement report, and transmits a cell switch command (Cell Switch Command) indicating the target cell (LTM candidate cell setting) to the UE 100 via a MAC control element (CE). The serving cell switch trigger is conveyed in a MAC CE including at least a candidate setting index together with a beam indicator.

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

[0045] In this way, a serving cell switch is triggered by selecting an LTM candidate cell setting as a target setting by gNB200. An LTM candidate cell setting can be added, changed, and released by gNB200 via RRC signaling.

[0046] The following principles apply to LTM:

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

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

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

[0050] - Security is not updated in LTM.

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

[0052] 6 is a diagram showing an example of a cell switching procedure using LTM. In the illustrated example, it is assumed that UE 100 performs serving cell switching from a first cell of gNB 200 to a second cell.

[0053] Here, the first cell and the second cell may be formed by different TRPs (Transmission and Reception Points). In the following description of the embodiment, the second cell is also referred to as an "LTM candidate cell" until a serving cell switch by LTM is determined, and after a serving cell switch by LTM is determined, the second cell is also referred to as a "target cell". The first cell is also referred to as a "source cell".

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

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

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

[0057] In step S4, the gNB 200 transmits to the UE 100 an RRC Reconfiguration message including an LTM candidate cell configuration (LTM Candidate Configuration) of one or more LTM candidate cells. The LTM candidate cell configuration may include a random access channel (RACH) configuration used for RA preamble transmission to the corresponding LTM candidate cell, for example, a contention-free random access (CFRA) configuration. Such a RACH configuration may be referred to as an early UL synchronization configuration (EarlyUlSyncConfig). Early UL synchronization may also be referred to as early TA or early TA acquisition. CFRA is a random access procedure in which a dedicated RACH resource (e.g., a dedicated preamble sequence and / or a dedicated time-frequency resource) is assigned to the UE 100, and no RACH contention occurs between the UEs 100.

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

[0059] In step S6, the UE 100 may perform synchronization processing with the LTM candidate cell before receiving the cell switch command MAC CE. Such synchronization processing may be referred to as early synchronization (Early sync). Here, the UE 100 may perform downlink synchronization processing (DL synchronization processing) for the LTM candidate cell, and then perform early timing advance (TA) acquisition in the LTM candidate cell requested by the gNB 200 (source cell). This is performed by CFRA triggered by a PDCCH order (PDCCH order) from the source cell. Note that when DCI Format 1_0 is used and all "Frequency domain resource assignment" fields in the DCI are set to "1", the DCI is treated as a PDCCH order. In addition, when early UL synchronization setting (EarlyUlSyncConfig) is configured in UE100, the PDCCH order may include a cell indicator indicating the corresponding RACH transmission cell, i.e., to which LTM candidate cell UE100 should transmit a random access preamble (RA preamble).

[0060] The UE 100 transmits an RA preamble to the designated LTM candidate cell. In order to minimize communication interruption of the source cell due to CFRA for the LTM candidate cell, in early synchronization, the UE 100 does not receive a random access response (RAR) for the purpose of acquiring a TA value from the LTM candidate cell. The TA value of the LTM candidate cell (target cell) is indicated in the cell switching command MAC CE in step S9. Note that the TA value is a value for adjusting the uplink transmission timing of the UE 100.

[0061] In step S7, the UE 100 performs layer 1 (L1) measurement in the configured LTM candidate cell and transmits a physical layer measurement report (L1 measurement report) to the gNB 200. The L1 measurement report is transmitted and received in the PHY layer L1 or L2 (e.g., MAC layer). For example, the UE 100 transmits L1-RSRP and / or L1-SINR to the gNB 200 via a PUCCH (Physical Uplink Control Channel) and / or a PUSCH (Physical Uplink Shared Channel).

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

[0063] In step S9, the gNB 200 transmits a cell switch command MAC CE including a candidate configuration index of the target cell to the UE 100. The cell switch command MAC CE may include a TA value obtained by early synchronization.

[0064] In step S10, the UE 100 switches to the configuration of the target cell. Specifically, the UE 100 detaches from the source cell (first cell) and applies the configuration of the target cell.

[0065] In step S11, if the serving cell switch needs to include execution of a random access procedure (for example, if the cell switch command MAC CE does not include a valid TA value), the UE 100 executes the random access procedure for the target cell (RACH-based LTM cell switch). Note that, if the UE 100 does not need to acquire the TA of the target cell at the time of serving cell switch (for example, if the cell switch command MAC CE includes a valid TA value), it can skip the random access procedure (RACH-less LTM cell switch).

[0066] In step S12, the UE 100 indicates that the serving cell switch to the target cell has been successfully completed. Thereafter, the UE 100 may perform steps S6 to S12 multiple times for subsequent LTM serving cell switches based on the configuration provided in step S4.

[0067] (3) First embodiment regarding inter-CU LTM The LTM specified in Release 18 of the 3GPP standard is an intra-CU (Central Unit) LTM and does not support inter-CU LTM. That is, in conventional LTM, it is possible to perform LTM cell switching between cells under the same CU (same gNB 200), but it is not possible to perform LTM cell switching between cells under different CUs (different gNB 200). On the other hand, in Release 19 of the 3GPP standard, support for inter-CU LTM is being considered.

[0068] FIG. 7 is a diagram for explaining the operating environment of the mobile communication system 1 according to the first embodiment.

[0069] The UE100 is in an RRC connected state in the cell of the gNB200S. The gNB200S is an example of a first network node. The cell of the gNB200S is an example of a first cell. The cell of the gNB200S is also referred to as a source cell or a current serving cell. The gNB200S is also referred to as a source gNB200S or a serving gNB200S.

[0070] The gNB200C manages the cells adjacent to the cell of gNB200S. In the illustrated example, the cells adjacent to the cell of gNB200S are managed by gNB200C1 and gNB200C2. gNB200C1 is an example of a second network node, and gNB200C2 is an example of a third network node. The cell of gNB200C1 is also referred to as LTM candidate cell #1, and the cell of gNB200C2 is also referred to as LTM candidate cell #2. Each of LTM candidate cell #1 and LTM candidate cell #2 is an example of a second cell.

[0071] When gNB200C1 and gNB200C2 are not particularly distinguished, they are referred to as candidate gNB200C, and when LTM candidate cell #1 and LTM candidate cell #2 are not distinguished, they are simply referred to as LTM candidate cells. However, after a cell switch by LTM is determined, the determined LTM candidate cell is also referred to as a target cell.

[0072] In the illustrated example, there are two candidate gNB200C and two LTM candidate cells, but there may be one or three or more candidate gNB200C, and there may be one or three or more LTM candidate cells. Also, one candidate gNB200C may manage multiple LTM candidate cells.

[0073] 7 are connected to each other by an Xn interface, which is an interface between network nodes (or, from another perspective, an interface between CUs). Communication between the gNBs 200 is assumed to be performed over the Xn interface.

[0074] Each candidate gNB200C provides RRC Reconfiguration (with sync) including configuration information of its own LTM candidate cell (LTM candidate cell configuration) to UE100 via gNB200S. In the illustrated example, gNB200C1 provides RRC Reconfiguration (with sync) including LTM candidate cell configuration of LTM candidate cell #1 to UE100 via gNB200S. Also, gNB200C2 provides RRC Reconfiguration (with sync) including LTM candidate cell configuration of LTM candidate cell #2 to UE100 via gNB200S. The LTM candidate cell configuration may include early UL synchronization configuration (CFRA configuration), as described above. The LTM candidate cell configuration may include configuration of a PUSCH resource to be used when performing the first UL transmission (for example, transmission of an RRC Reconfiguration Complete message to the target cell) with the corresponding LTM candidate cell as the target cell.

[0075] After storing the LTM candidate cell setting of each LTM candidate cell, the UE 100 performs early UL synchronization with one or more LTM candidate cells, and performs LTM cell switching with the LTM candidate cell specified in the cell switching command MAC CE from the source cell as the target cell. However, if the TA value of the LTM candidate cell is the same as the TA value of the source cell (TA = same as source), or if the TA value of the LTM candidate cell is zero (TA = 0), the UE 100 may not perform early UL synchronization with the LTM candidate cell.

[0076] (3.1) First Operation Pattern of First Embodiment First, the first operation pattern related to inter-CU LTM will be described.

[0077] As described above, the early UL synchronization is configured in the RRC Reconfiguration (with sync) generated by the candidate gNB200C. The RRC message is stored in an RRC container (transparent container) in a Handover Request Acknowledge message on the Xn interface and notified to the UE100 via the source gNB200S. Therefore, the source gNB200S extracts the RRC message from the RRC container and transmits it to the UE100 without deciphering the contents.

[0078] Therefore, there is a problem that the source gNB200S does not know whether early UL synchronization is set in the UE100. Therefore, the source gNB200S does not know whether to transmit, for example, a PDCCH order that triggers a CFRA to the LTM candidate cell to the UE100.

[0079] In addition, there is also a problem in that it is not clear whether the source gNB200S or the candidate gNB200C makes the decision that the TA value of the LTM candidate cell is the same as the TA value of the source cell (TA = same as source) or that the TA value of the LTM candidate cell is zero (TA = 0). In the first operation pattern, it is assumed that the source gNB200S can make this decision.

[0080] In the first operation pattern, an operation is described that makes it easy for the source gNB200S to determine whether early UL synchronization is set in the UE100.

[0081] Figure 8 is a diagram showing the operation of the source gNB200S in the first operation pattern for inter-CU LTM in the first embodiment.

[0082] In step S15, the source gNB200S that manages the source cell sends a request message to the candidate gNB200C that manages the LTM candidate cell to prepare for LTM cell switching.

[0083] In step S16, the source gNB200S receives a response message to the request message of step S15 from the candidate gNB200C.

[0084] In the first operation pattern, the request message of step S15 includes an information element for indicating to the candidate gNB200C whether or not to provide in the response message an early UL synchronization setting to be used for early UL synchronization performed by the UE100 to the LTM candidate cell.

[0085] This allows the candidate gNB200C that receives the request message to determine whether to provide early UL synchronization setting in the response message based on the information element from the source gNB200S. In addition, the source gNB200S can understand whether the candidate gNB200C provides early UL synchronization in the response message.

[0086] If the source gNB200S decides to set a predetermined TA value to the UE100 as the TA value to be applied to the LTM candidate cell, in step S15, the source gNB200S may send to the candidate gNB200C a request message including an information element indicating that the predetermined TA value is to be set to the UE100. The predetermined TA value is the same TA value as the TA value of the source cell (TA = same as source) or zero (TA = 0). In this case, the candidate gNB200C decides not to provide early UL synchronization setting in the response message, and in step S16, the source gNB200S receives a response message from the candidate gNB200C that does not include the early UL synchronization setting.

[0087] On the other hand, if the source gNB200S determines that the candidate gNB200C provides early UL synchronization, in step S15, the source gNB200S may send a request message including an information element for requesting the provision of an early UL synchronization setting to the candidate gNB200C. In this case, the candidate gNB200C determines to provide the early UL synchronization setting in a response message, and in step S16, the source gNB200S receives a response message including the early UL synchronization setting from the candidate gNB200C. The early UL synchronization setting may include information indicating the CFRA to be used for early UL synchronization. Alternatively, the early UL synchronization setting may include setting information regarding UE-based TA measurement, in which the UE100 itself measures the TA value to be applied to the LTM candidate cell.

[0088] The request message of step S15 may be a Handover Request message transmitted over the Xn interface and including information about LTM. That is, the request message of step S15 may be a message common to the Handover Request message used in L3 handover and including additional information indicating that it is a request for LTM setup. The response message of step S16 may be a Handover Request Acknowledge message transmitted over the Xn interface.

[0089] Alternatively, the request message in step S15 may be transmitted on the Xn interface and may be a new message for the LTM (e.g., an LTM Configuration Request message) different from the Handover Request message. The response message in step S16 may be transmitted on the Xn interface and may be a new message for the LTM (e.g., an LTM Configuration Request Acknowledge message) different from the Handover Request Acknowledge message.

[0090] (3.2) Second operation pattern of the first embodiment The second operation pattern, like the first operation pattern, relates to an operation for facilitating the source gNB200S to determine whether early UL synchronization is set in the UE100. In the second operation pattern, the candidate gNB200C explicitly notifies the source gNB200S that the candidate gNB200C has provided early UL synchronization setting.

[0091] Figure 9 is a diagram showing the operation of candidate gNB200C in the second operation pattern for inter-CU LTM in the first embodiment.

[0092] In step S21, the candidate gNB200C that manages the LTM candidate cell receives a request message for preparing an LTM cell switch from the source gNB200S that manages the source cell.

[0093] In step S22, the candidate gNB200C transmits a response message to the request message of step S21 to the source gNB200S. In the second operation pattern, the response message of step S22 includes 1) a transparent container (RRC container) including an early UL synchronization setting and provided to the UE100 via the source gNB200S, and 2) notification information for notifying the source gNB200S that the early UL synchronization setting is included in the transparent container in the response message.

[0094] As a result, the source gNB200S that receives the response message can understand that the candidate gNB200C has provided early UL synchronization setting based on the notification information in the response message without having to decipher the contents of the transparent container (RRC container).

[0095] The early UL synchronization configuration included in the response message of step S22 may include information indicating that a predetermined TA value is set in the UE 100 as the TA value to be applied to the LTM candidate cell. The predetermined TA value may be the same as the TA value of the source cell, or may be zero. Alternatively, the early UL synchronization configuration may include information indicating a CFRA resource to be used for early UL synchronization. Alternatively, the early UL synchronization configuration may include configuration information related to UE-based TA measurement in which the UE 100 itself measures the TA value to be applied to the LTM candidate cell.

[0096] The notification information included in the response message of step S22 may be configured to be able to identify, as types of early UL synchronization configuration, the following: A) information indicating that a predetermined TA value is set in UE 100 (TA=0 or TA=same as source configuration), B) information indicating a CFRA resource (CFRA configuration), C) configuration information regarding UE-based TA measurement (UE-based TA measurement configuration). That is, the notification information may be information indicating the type of early UL synchronization configuration included in a transparent container in the response message.

[0097] For example, if candidate gNB200C provides CFRA configuration as early UL synchronization configuration, source gNB200S can transmit a PDCCH order that triggers CFRA to the LTM candidate cell to UE100. If candidate gNB200C provides early UL synchronization configuration other than CFRA configuration, source gNB200S does not need to transmit the PDCCH order to UE100.

[0098] The request message of step S21 may be a Handover Request message transmitted over the Xn interface and including information about LTM. That is, the request message of step S21 may be a message common to the Handover Request message used in L3 handover and including additional information indicating that it is a request for LTM setup. The response message of step S22 may be a Handover Request Acknowledge message transmitted over the Xn interface.

[0099] Alternatively, the request message in step S21 may be transmitted on the Xn interface and may be a new message for the LTM (e.g., an LTM Configuration Request message) different from the Handover Request message. The response message in step S22 may be transmitted on the Xn interface and may be a new message for the LTM (e.g., an LTM Configuration Request Acknowledge message) different from the Handover Request Acknowledge message.

[0100] (3.3) Third Operation Pattern of the First Embodiment In LTM, LTM candidate cell settings (RRC Reconfiguration) of multiple LTM candidate cells are configured in advance in the UE 100. Therefore, each LTM candidate cell needs to reserve UL resources for the UE 100 so as not to allocate them to other UEs. The UL resources may be CFRA resources used for early UL synchronization and / or PUSCH resources used by the UE 100 for the first PUSCH transmission to the target cell (e.g., transmission of an RRC Reconfiguration Complete message). Therefore, in LTM, there is a problem that resource utilization efficiency in multiple LTM candidate cells may decrease. In addition, since the candidate gNB 200C needs to continue reception processing for the UL resources and continue waiting for transmission from the UE 100, there is also a problem that power consumption of the candidate gNB 200C may increase.

[0101] In particular, the longer the time from when the candidate gNB200C determines the LTM candidate cell setting to when the LTM cell switching is performed, the more UL resources are unused in each LTM candidate cell, resulting in a decrease in resource utilization efficiency. Such a problem can be significant in inter-CU LTM involving network side signaling. In the third operation pattern, even in the case of inter-CU LTM, an operation that can suppress a decrease in resource utilization efficiency of each LTM candidate cell will be described.

[0102] Figure 10 is a diagram showing the operation of the source gNB200S in the third operation pattern for inter-CU LTM in the first embodiment.

[0103] In step S31, the source gNB200S transmits a request message to the candidate gNB200C to prepare for LTM cell switching. In response to the request message, the candidate gNB200C determines UL resources for the UE100 in the LTM candidate cell, but deactivates the UL resources. The deactivation state may be a state in which the UL resources can be allocated to other UEs. The deactivation state may be a state in which the candidate gNB200C does not perform reception processing for the UL resources.

[0104] In step S32, the source gNB200S receives a response message from the candidate gNB200C in response to the request message of step S31. The response message may include information on the UL resources determined by the candidate gNB200C.

[0105] In step S33, the source gNB200S sends an indication regarding the LTM candidate cell to the UE100.

[0106] In step S34, based on the transmission of the instruction in step S33, the source gNB200S transmits a message to the candidate gNB200C including information for making UL resources for UE100 available to UE100 in the LTM candidate cell (e.g., activating the UL resources). Here, "based on the transmission of the instruction" may mean based on having transmitted the instruction, or may mean based on the instruction being transmitted in the future.

[0107] As a result, the candidate gNB200C can deactivate the UL resources at the time of transmitting the response message in step S32. Thereafter, when the candidate gNB200C receives a message from the source gNB200S based on the transmission of an instruction to the UE100, the candidate gNB200C activates the UL resources and makes the UL resources available to the UE100 in the LTM candidate cell. Here, while the UL resources are in an inactive state, it is possible to suppress a decrease in resource utilization efficiency in the LTM candidate cell and reduce power consumption in the candidate gNB200C.

[0108] The UL resources determined by the candidate gNB200C may include CFRA resources used for early UL synchronization. The instruction in step S33 may be a PDCCH order instructing the execution of CFRA for the LTM candidate cell. In this case, the candidate gNB200C can deactivate the CFRA resources at the time of transmitting the response message in step S32. Thereafter, when the candidate gNB200C receives a message based on the transmission of the PDCCH order to the UE100 from the source gNB200S, the candidate gNB200C activates the CFRA resources and makes the CFRA resources available to the UE100 in the LTM candidate cell.

[0109] Alternatively, the instruction in step S33 may be an RRC Reconfiguration message including configuration information of the LTM candidate cell (LTM candidate cell configuration). In response to receiving the RRC Reconfiguration Complete message from the UE100, the source gNB200S may transmit a message indicating completion of the LTM configuration (or a request to activate CFRA resources) to the candidate gNB200C.

[0110] The UL resources determined by the candidate gNB200C may include PUSCH resources. The instruction in step S33 may be a cell switch command MAC CE instructing execution of LTM cell switch for the LTM candidate cell. In this case, the candidate gNB200C can deactivate the PUSCH resources at the time of transmitting the response message in step S32. Thereafter, when the candidate gNB200C receives a message based on the transmission of the cell switch command MAC CE to the UE100 from the source gNB200S, the candidate gNB200C activates the PUSCH resources and makes the PUSCH resources available to the UE100 in the LTM candidate cell.

[0111] (3.4) Example of operation sequence of the first embodiment Figure 11 is a diagram showing an example of the operation sequence of the mobile communication system 1 according to the first embodiment. This sequence corresponds to a specific example of the first to third operation patterns described above, but is merely an example. In the example shown, it is assumed that the source gNB 200S and the candidate gNB 200C are each separated into a CU and a DU. Communication between the CU and the DU is performed over the F1 interface, which is a fronthaul interface.

[0112] Steps S101 to S113 are the preparation phase in InterCU LTM.

[0113] In step S101, UE100 may send an L3 measurement report message to the CU of source gNB200S.

[0114] In step S102, the CU of the source gNB200S decides to configure inter-CU LTM (LTM configuration) for the UE100.

[0115] In step S103, the CU of the source gNB200S sends an Xn Handover Request message to the CU of the candidate gNB200C. The message includes one or more of the following information:

[0116] Information indicating an LTM configuration request (LTM config. Request): Alternatively, a new Xn message (for example, LTM Configuration Request) may be used.

[0117] Information on UL synchronization configuration (information element related to the above-described first operation pattern) (UL sync config.): This may be information to set TA=0 or TA=same as source. Alternatively, it may be information to request the setting of early UL synchronization configuration (CFRA or UE-based TA measurement).

[0118] In step S104, the CU of candidate gNB200C decides to accept the LTM setting (accept inter-CU LTM for UE100) (Admission control).

[0119] In step S105, the CU of candidate gNB200C sends a UE Context Setup Request message to the DU of candidate gNB200C.

[0120] In step S106, in response to receiving the UE Context Setup Request message, the DU of the candidate gNB200C sends a UE Context Setup Response message to the CU of the candidate gNB200C.

[0121] In step S107, the CU of the candidate gNB200C sends an Xn Handover Request Acknowledge message including an RRC container (transparent container) to the CU of the source gNB200S. In addition to the RRC container, the message includes one or more of the following information:

[0122] Information indicating that LTM configuration will be performed (LTM config. Indication): Alternatively, a new Xn message (for example, an LTM Configuration Request Acknowledge message) may be used.

[0123] Information indicating UL synchronization setting in an RRC container (information element related to the above-described second operation pattern) (UL sync config.): This may be information indicating TA=0 or TA=same as source. It may also be information indicating early UL synchronization setting (CFRA or UE-based TA measurement).

[0124] Information (UL resource event request) on whether or not a notification is required from the source gNB200S (whether or not a notification operation described later is required): This may be information on whether or not it is necessary to provide information on PDCCH order transmission. It may also be information on whether or not it is necessary to provide information on cell switching command MAC CE transmission.

[0125] In step S108, the CU of the source gNB200S sends a UE Context Modification Request message to the DU of the source gNB200S.

[0126] In step S109, the DU of the source gNB200S sends a UE Context Modification Response message to the CU of the source gNB200S.

[0127] In step S110, after the UE Context Modification is successful, the CU of the source gNB200S transmits an RRC Reconfiguration message to the UE100. Specifically, the CU of the source gNB200S extracts the RRC Reconfiguration message from the RRC container included in the message of step S107, and transmits an RRC Reconfiguration message to the UE100 to add it as the LTM setting (LTM candidate cell setting) of the UE100.

[0128] In step S111, UE100 sends an RRC Reconfiguration Complete message to the CU of source gNB200S.

[0129] In step S112, the CU of the source gNB200S may transmit a message (LTM Configuration Complete) indicating completion of the LTM configuration (LTM candidate cell configuration) to the CU of the candidate gNB200C. In this case, in step S113, in response to receiving the message, the CU of the candidate gNB200C may transmit a message (CFRA resource activation) for activating a CFRA resource for the UE100 to the DU of the candidate gNB200C, and activate the CFRA resource.

[0130] Steps S114 to S123 are the early synchronization phase in inter-CU LTM.

[0131] In step S114, UE100 may send an L1 measurement report to the DU of source gNB200S.

[0132] In step S115, the DU of the source gNB200S determines to transmit a PDCCH order in early synchronization (early TA) based on the L1 measurement report of step S114. Note that this determination may be made when early TA is re-executed (PDCCH order is re-transmitted). For example, this determination may be made when it is necessary to update (re-acquire) the TA value for a UE moving at high speed. The DU of the source gNB200S transmits to the UE100 a PDCCH order that triggers a CFRA for the LTM candidate cell.

[0133] In step S116, the DU of the source gNB200S sends a notification (PDCCH order indication) to the CU of the source gNB200S indicating that a PDCCH order is about to be transmitted or has been transmitted.

[0134] In step S117, in response to receiving the notification in step S116, the CU of the source gNB200S transmits a notification message (PDCCH order indication) indicating that a PDCCH order is about to be transmitted or has been transmitted to the CU of the candidate gNB200C. The notification message may be a message requesting activation of a CFRA resource for early UL synchronization. The notification message may include the cell ID of the LTM candidate cell to which a PRACH (Physical Random Access Channel) is to be transmitted.

[0135] In step S118, in response to receiving the message of step S117, the CU of candidate gNB200C may send a message (CFRA resource activation) to the DU of candidate gNB200C to activate the CFRA resource for UE100, and activate the CFRA resource (step S119).

[0136] In step S120, UE100 transmits a CFRA RA preamble to the DU of candidate gNB200C using the CFRA resource (PRACH resource) set in step S110 in accordance with the PDCCH order of step S115.

[0137] In step S121, the DU of candidate gNB200C sends a message (DU-CU TA information) including the TA value derived based on the RA preamble of the CFRA in step S120 to the CU of candidate gNB200C.

[0138] In step S122, the CU of candidate gNB200C sends a message (CU-CU TA information) including the TA value to the CU of source gNB200S.

[0139] In step S123, the CU of the source gNB200S transmits a message (CU-DU TA information) including the TA value to the DU of the source gNB200S. The TA value is notified to the UE100 by the cell switching command MAC CE in step S125.

[0140] Steps S124 to S131 are the execution phase in the InterCU LTM.

[0141] In step S124, UE100 may send an L1 measurement report to the DU of source gNB200S.

[0142] In step S125, the DU of the source gNB200S sends a cell switching command MAC CE to the UE100.

[0143] In step S126, in response to transmitting or having transmitted the cell switch command MAC CE, the DU of the source gNB200S transmits a notification (Cell switch command indication) indicating that the cell switch command MAC CE is to be transmitted or has been transmitted to the CU of the source gNB200S.

[0144] In step S127, the CU of the source gNB200S transmits a notification message (Cell switch command indication) indicating that a cell switch command MAC CE will be transmitted or has been transmitted to the CU of the candidate gNB200C. The notification message may include a cell ID of the target LTM candidate cell. The notification message may be a notification requesting activation of a PUSCH resource. The notification may request activation of a PUSCH resource (configured grant resource) determined for the LTM candidate cell. The notification may request dynamic PUSCH resource allocation (dynamic grant transmission) to the UE100.

[0145] In step S128, in response to receiving the message in step S127, the CU of the candidate gNB200C may send a message (PUSCH resource activation) for activating a PUSCH resource for the UE100 to the DU of the candidate gNB200C, and activate the PUSCH resource (step S129).

[0146] In step S130, UE100 uses the PUSH resource to send an RRC Reconfiguration Complete message to the CU of candidate gNB200C.

[0147] In step S131, in response to receiving the message of step S130, the CU of candidate gNB200C sends a UE Context Release message to the CU of source gNB200S, and the CU of source gNB200S releases the UE Context.

[0148] (3.5) Modification of the First Embodiment A modification of the first embodiment will now be described.

[0149] In the third operation pattern of the first embodiment described above, the source gNB200S transmits a message based on the transmission of an instruction (PDCCH order or cell switching command MAC CE) to the candidate gNB200C, and the candidate gNB200C activates UL resources for the UE100 in the LTM candidate cell. Here, due to transmission delays on the Xn interface (and the F1 interface), there is a risk that the UE100 will transmit to the LTM candidate cell using the UL resources before the LTM candidate cell activates the UL resources. In this case, since the UL transmission cannot be received by the LTM candidate cell, the UE100 will waste power by retransmission or generate unnecessary UL interference.

[0150] One method for solving this problem is for the source gNB200S to send a message to the candidate gNB200C before sending the instruction, wait a predetermined time (after suspending transmission of the instruction to the UE100), and then send the instruction to the UE100. However, particularly in the case of the cell switching command MAC CE, if the radio environment of the UE100 deteriorates while transmission is suspended on the source gNB200S side, there is a problem that the cell switching command MAC CE will not reach the UE100.

[0151] In this modification, an operation in which the third operation pattern of the above embodiment is improved in order to solve such a problem will be described.

[0152] FIG. 12 is a diagram showing the operation of UE 100 according to this modification.

[0153] In step S41, the UE 100 receives from the source gNB 200S (source cell) an instruction regarding the LTM candidate cell and information specifying a waiting time from receiving the instruction until transmitting to the LTM candidate cell. The information specifying the waiting time may be information included in the instruction. The information may be information set in the UE 100 by an RRC message (RRC Reconfiguration) prior to the instruction.

[0154] The instruction may be a PDCCH order instructing the LTM candidate cell to perform CFRA. In this case, the waiting time may be a waiting time from receiving the PDCCH order from the source cell to transmitting a PRACH to the LTM candidate cell (specifically, transmitting a CFRA preamble).

[0155] Alternatively, the instruction may be a cell switch command MAC CE instructing the LTM candidate cell to perform LTM cell switch. In this case, the waiting time may be a waiting time from receiving the cell switch command MAC CE from the source cell to transmitting RRC Reconfiguration Complete to the LTM candidate cell (target cell).

[0156] In addition, the source gNB200S or the candidate gNB200C may determine the waiting time from the amount of delay in communication between the source gNB200S and the candidate gNB200C. When the candidate gNB200C determines the waiting time, the candidate gNB200C may notify the UE100 via the source gNB200S of an RRC container (RRC Reconfiguration) including information indicating the determined waiting time.

[0157] The waiting time may be specified (set) to the UE 100 individually for each LTM candidate cell.

[0158] The waiting time may be a timer value.

[0159] In step S42, the UE 100 waits until a designated waiting time has elapsed since receiving the instruction in step S41.

[0160] In step S43, when the waiting time has elapsed, the UE 100 performs transmission (PRACH transmission or RRC Reconfiguration Complete transmission) to the LTM candidate cell specified in the instruction in step S41.

[0161] The operation when this modification is applied to the sequence of FIG. 11 will be described.

[0162] As shown in FIG. 11 , in step S115, the source gNB 200S transmits a PDCCH order to the UE 100 to instruct (trigger) a CFRA to the LTM candidate cell. The PDCCH order may include information (e.g., a timer value) indicating the waiting time until the CFRA starts. Alternatively, the RRC Reconfiguration message in step S110 may include information (e.g., a timer value) indicating the waiting time until the CFRA starts. The waiting time may be specified (set) individually to the UE 100 for each LTM candidate cell. The UE 100 receives the PDCCH order, measures the waiting time, and suspends PRACH transmission during the waiting time. The UE 100 may set a timer value that specifies the waiting time and start the timer. In step S120, the UE 100 transmits a PRACH to the LTM candidate cell after the waiting time has elapsed (e.g., when the timer expires).

[0163] As shown in FIG. 11, in step S125, the source gNB 200S transmits a cell switch command MAC CE to the UE 100 to instruct the UE 100 to perform LTM cell switch to the LTM candidate cell. The cell switch command MAC CE may include information indicating a waiting time until the start of PDSCH (RRC Reconfiguration Complete) transmission to the LTM candidate cell (target cell). Alternatively, the RRC Reconfiguration message of step S110 may include information (for example, a timer value) indicating a waiting time until the start of PDSCH (RRC Reconfiguration Complete) transmission. The waiting time may be specified (set) to the UE 100 individually for each LTM candidate cell. The UE 100 receives the cell switching command MAC CE, measures the waiting time, and reserves the PDSCH (RRC Reconfiguration Complete) transmission during the waiting time. The UE 100 may set a timer value that specifies the waiting time and start the timer. In step S130, the UE 100 transmits the PDSCH (RRC Reconfiguration Complete) to the LTM candidate cell (target cell) after the waiting time has elapsed (for example, when the timer expires).

[0164] (4) Second Embodiment Regarding LTM of Inter-CUs, the second embodiment regarding LTM of inter-CUs will be described, focusing mainly on the differences from the first embodiment. The second embodiment may be implemented in combination with the first embodiment. Note that in the following description of the second embodiment, steps similar to those in the operation of FIG. 11 are denoted by the same reference numerals.

[0165] Fig. 13 is a diagram for explaining the problem associated with the second embodiment. In Fig. 13, steps similar to those in the operation of the first embodiment (the operation of Fig. 11) are denoted by the same reference numerals (however, the reference numerals have an "a" suffix).

[0166] In the second operation pattern of the first embodiment described above, an example was described in which the candidate gNB200C notifies the source gNB200S that the candidate gNB200C has provided an early UL synchronization setting, in order to solve the problem of the source gNB200S knowing whether early UL synchronization is set in the UE100. For example, the candidate gNB200C included in a response message (Handover Request Acknowledge message) to the source gNB200S: 1) a transparent container (RRC container) including the early UL synchronization setting and provided to the UE100 via the source gNB200S, and 2) notification information for notifying the source gNB200S that the early UL synchronization setting is included in the transparent container in the response message.

[0167] Here, the early UL synchronization setting (i.e., CFRA setting) includes a general RACH setting (RACH-Config Generic) and a dedicated RACH setting (RACH-Config Dedicated), but in the above-described first embodiment, these are not particularly distinguished. The general RACH setting (RACH-Config Generic) may be information for specifying random access parameters for both normal random access and beam failure recovery. It may be a RACH setting common to a plurality of UEs 100, or may be a cell-specific RACH setting. On the other hand, the dedicated RACH setting (RACH-Config Dedicated) is a RACH setting specific to a UE. The dedicated RACH setting (RACH-Config Dedicated) may be information for specifying random access parameters dedicated to a UE. It should be noted that the general RACH configuration (RACH-Config Generic) may be included in the dedicated RACH configuration (RACH-Config Dedicated).

[0168] In addition, in the conventional intra-CU LTM, a general RACH configuration (RACH-Config Generic) is provided to the UE 100 in an RRC Reconfiguration message, and content corresponding to a dedicated RACH configuration (RACH-Config Dedicated) is included in a PDCCH order from a first cell (source cell) to the UE 100. For example, the PDCCH order includes a "Random Access Preamble index," an "SS / PBCH index," and a "PRACH Mask index" as information in the dedicated RACH configuration (RACH-Config Dedicated). In addition, the PDCCH order includes a "UL / SUL indicator" and a cell indicator. At the time of receiving the PDCCH order, the CFRA setting is completed in the UE 100. However, in the inter-CU LTM, there is a problem that the details regarding the PDCCH order have not yet been determined.

[0169] In order to solve the above-mentioned problems, in the first operation pattern of the second embodiment, the candidate gNB200C includes notification information indicating the presence of CFRA configuration in a handover request acknowledgement message (Handover Request Acknowledge message), similar to the second operation pattern of the first embodiment described above. This allows the serving gNB200S to understand that it should transmit a PDCCH order to the UE100 based on the notification information.

[0170] In order to solve the above-mentioned problem, in the second operation pattern of the second embodiment, the UE 100 notifies the first cell (serving gNB 200S) that the early TA of the CFRA has been set by using an RRC reconfiguration complete (RRC Reconfiguration Complete) message indicating that the setting process using the RRC Reconfiguration message has been completed. Specifically, the UE 100 includes in the RRC Reconfiguration Complete message notification information indicating that a dedicated RACH setting was included in the RRC Reconfiguration message. This allows the serving gNB 200S to understand that the early TA of the CFRA has been set for the UE 100 in response to the notification from the UE 100.

[0171] In the first and second operation patterns of the second embodiment, the RRC Reconfiguration message provided to the UE 100 includes a dedicated RACH setting (RACH-Config Dedicated) and a general RACH setting (RACH-Config Generic). Therefore, the serving gNB 200S does not need to include information in the dedicated RACH setting (RACH-Config Dedicated) in the PDCCH order. Therefore, in the first and second operation patterns of the second embodiment, the serving gNB 200S does not include information in the dedicated RACH setting in the PDCCH order, but includes a cell indicator indicating the second cell in the PDCCH order. This reduces the size (amount of information) of the PDCCH order.

[0172] In order to solve the above-mentioned problem, in the third operation pattern of the second embodiment, assuming the operation of the third operation pattern of the first embodiment described above, the candidate gNB 200C provides a dedicated RACH setting (RACH-Config Dedicated) to the serving gNB 200S in a response message to a request message requesting reservation or activation of radio resources for CFRA. In this case, since the dedicated RACH setting (RACH-Config Dedicated) is not included in a transparent container, the serving gNB 200S can easily obtain the dedicated RACH setting (RACH-Config Dedicated) and can understand that a PDCCH order should be sent to the UE 100.

[0173] In order to solve the above-mentioned problem, in the fourth operation pattern of the second embodiment, an early TA CFRA is performed without a PDCCH order. Specifically, when the UE 100 receives an RRC Reconfiguration message including a dedicated RACH setting (RACH-Config Dedicated) from the first cell, the UE 100 triggers a CFRA to the second cell even if it does not receive a PDCCH order from the first cell that triggers a CFRA to the second cell. This eliminates the need to transmit a PDCCH order from the serving gNB 200S to the UE 100, making it possible to realize efficient inter-CU LTM.

[0174] (4.1) First Operation Pattern of the Second Embodiment In the first operation pattern of the second embodiment, the candidate gNB 200C includes notification information indicating the presence of a CFRA setting in a handover request acknowledgement message (Handover Request Acknowledge message), similar to the second operation pattern of the first embodiment described above. Also, in the first operation pattern of the second embodiment, the serving gNB 200S does not include information in the dedicated RACH setting (RACH-Config Dedicated) in the PDCCH order, but includes a cell indicator indicating the second cell in the PDCCH order.

[0175] Fig. 14 is a diagram showing an example of operation according to the first operation pattern of the second embodiment. In Fig. 14, steps similar to those in Fig. 11 are denoted by the same reference numerals (however, the reference numerals have a "b" suffix).

[0176] In step S101b, UE100 transmits an L3 measurement report to the serving gNB200S (first cell).

[0177] In step S103b, the serving gNB200S sends a HANDOVER REQUEST message (Xn Handover Request message) to the candidate gNB200C.

[0178] In step S107b, the candidate gNB200C transmits a HANDOVER REQUEST ACKNOWLEDGE message (Xn Handover Request Acknowledge message) to the serving gNB200S. Here, in the HANDOVER REQUEST ACKNOWLEDGE message, the candidate gNB200C configures a dedicated RACH setting (RACH-Config Dedicated) and a general RACH setting (RACH-Config Generic) in the handover command (RRC container) to the UE100. However, as described in the third operation pattern of the first embodiment, the candidate gNB200C does not need to reserve resources for CFRA of the early TA at this point. In addition, in the HANDOVER REQUEST ACKNOWLEDGE message, the serving gNB200S is notified that an early TA for CFRA has been set.

[0179] In step S110b, the serving gNB200S transmits an RRC Reconfiguration message including a handover command (RRC container) from the candidate gNB200C to the UE100. The RRC Reconfiguration message includes a dedicated RACH setting (RACH-Config Dedicated) and a general RACH setting (RACH-Config Generic). Therefore, when the UE100 receives the RRC Reconfiguration message from the serving gNB200S, the CFRA setting, i.e., the dedicated RACH setting (RACH-Config Dedicated) and the general RACH setting (RACH-Config Generic), are completed.

[0180] In step S111b, UE100 sends an RRC Reconfiguration Complete message to serving gNB200S.

[0181] In step S112b, the serving gNB200S sends a message (CFRA Resource Reserve Request message) to the candidate gNB200C requesting that a CFRA resource for early TA be reserved or activated, similar to the third operation pattern of the first embodiment.

[0182] In step S201b, when the candidate gNB200C receives the message of step S112b, it reserves or activates resources for the CFRA of the early TA and sends a response message (CFRA Resource Reserve Request Acknowledge message) to the serving gNB200S.

[0183] Here, if the candidate gNB200C is able to reserve or activate the CFRA resources configured in the dedicated RACH setting (RACH-Config Dedicated) in step S107b, it may notify the serving gNB200S of this. If the candidate gNB200C is unable to reserve or activate the CFRA resources configured in the dedicated RACH setting (RACH-Config Dedicated) in step S107b, it may notify the serving gNB200S of this. If the serving gNB200S receives a notification that it was unable to reserve or activate the CFRA resources, it does not send (cancels) the PDCCH order. Alternatively, if the candidate gNB200C is unable to reserve or activate the CFRA resource set in the dedicated RACH setting (RACH-Config Dedicated) in step S107b, the candidate gNB200C may notify the serving gNB200S of a new dedicated RACH setting (RACH-Config Dedicated). In this case, the serving gNB200S may transmit the new dedicated RACH setting (RACH-Config Dedicated) to the UE100 in a PDCCH order.

[0184] In step S115b, the serving gNB 200S transmits a PDCCH order that triggers a CFRA to the second cell to the UE 100. Here, the serving gNB 200S does not include information in the dedicated RACH configuration (RACH-Config Dedicated) in the PDCCH order, but includes a cell indicator (Cell indicator) indicating the second cell in the PDCCH order.

[0185] The serving gNB 200S may transmit the cell indicator by setting a specific value (e.g., 0000, 1111, NULL, etc.) in the same field as the dedicated RACH setting (RACH-Config Dedicated) in the PDCCH order. In this case, the UE 100 transmits the RA preamble on the PRACH using the dedicated RACH setting (RACH-Config Dedicated) set in step S110b. Alternatively, the PDCCH order in step S115b may be a newly defined DCI format. Alternatively, the PDCCH order may include information (e.g., a flag) indicating that the dedicated RACH setting (RACH-Config Dedicated) is not included. The UE 100 recognizes from the information that the received PDCCH order does not include a dedicated RACH setting (RACH-Config Dedicated) and includes only a cell indicator (Cell indicator) indicating the second cell, and can decode the PDCCH with a reduced amount of information without error.

[0186] Alternatively, the serving gNB 200S may transmit a new dedicated RACH setting (RACH-Config Dedicated) in a PDCCH order. In this case, the UE 100 does not use the dedicated RACH setting (RACH-Config Dedicated) set in step S110b, and follows the setting of the new dedicated RACH setting (RACH-Config Dedicated) in the PDCCH order.

[0187] In step S120b, UE100 transmits a PRACH to candidate gNB200C.

[0188] The subsequent operations are the same as those in the first embodiment.

[0189] Thus, in the first operation pattern of the second embodiment, the candidate gNB200C notifies the serving gNB200S that it has set an early TA of the CFRA using a HANDOVER REQUEST ACKNOWLEDGE message. The candidate gNB200C also sets a dedicated RACH setting (RACH-Config Dedicated) in the handover command.

[0190] In the first operation pattern of the second embodiment, the serving gNB200S transmits a request message to the candidate gNB200C to reserve resources for CFRA for early TA before transmitting a PDCCH order. The serving gNB200S also transmits a cell indicator to the UE100 in the PDCCH order.

[0191] (4.2) Second Operation Pattern of Second Embodiment In the second operation pattern of the second embodiment, the UE 100 notifies the first cell (serving gNB 200S) that an early TA of the CFRA has been set by an RRC Reconfiguration Complete message. Specifically, the UE 100 includes, in the RRC Reconfiguration Complete message, notification information indicating that a dedicated RACH setting was included in the RRC Reconfiguration message.

[0192] In the second operation pattern of the second embodiment, a dedicated RACH setting (RACH-Config Dedicated) and a general RACH setting (RACH-Config Generic) are included in the RRC Reconfiguration message provided to the UE 100. Therefore, the serving gNB 200S does not include information in the dedicated RACH setting in the PDCCH order, but includes a cell indicator indicating the second cell in the PDCCH order.

[0193] Fig. 15 is a diagram showing an example of operation according to the second operation pattern of the second embodiment. In Fig. 15, steps similar to those in Fig. 11 are given the same reference numerals (except that the reference numerals have a "c" suffix). Here, differences from the first operation pattern of the second embodiment will be mainly described.

[0194] In step S107c, the candidate gNB200C sends a HANDOVER REQUEST ACKNOWLEDGE message to the serving gNB200S. The candidate gNB200C sets a dedicated RACH setting (RACH-Config Dedicated) in the handover command. The candidate gNB200C does not need to reserve resources for the CFRA of the early TA at this point.

[0195] In step S110c, the serving gNB200S transmits an RRC Reconfiguration message to the UE100. When the UE100 receives the RRC Reconfiguration message from the serving gNB200S, the CFRA configuration (dedicated RACH configuration (RACH-Config Dedicated) and general RACH configuration (RACH-Config Generic)) is completed. Also, the UE100 understands that the CFRA has been configured at the early TA at this point.

[0196] In step S111c, the UE 100 transmits an RRC Reconfiguration Complete message to the serving gNB 200S. Here, the UE 100 notifies the serving gNB 200S that a dedicated RACH setting (RACH-Config Dedicated) (early TA CFRA) has been configured. The notification may include an ID (index) for identifying the dedicated RACH setting (RACH-Config Dedicated). The ID may be an LTM candidate ID, an LTM candidate PCI, or a Target cell ID. Note that the notification may be transmitted using a UE Assistance Information message instead of an RRC Reconfiguration Complete message. Alternatively, the notification may be notified using a MAC CE. When an RRC message (RRC Reconfiguration Complete message or UE Assistance Information message) is used, the notification may be further notified from the CU of the serving gNB200S to the DU. On the other hand, when MAC CE is used, the DU of the serving gNB200S can receive the notification directly from the UE100, so notification from the CU to the DU is not necessary.

[0197] The subsequent operations are the same as those in the above-described embodiment.

[0198] Thus, in the second operation pattern of the second embodiment, UE100 notifies the serving gNB200S that an early TA for CFRA has been set using an RRC Reconfiguration Complete message.

[0199] (4.3) Third operation pattern of the second embodiment In the third operation pattern of the second embodiment, the candidate gNB200C provides a dedicated RACH configuration (RACH-Config Dedicated) to the serving gNB200S in a response message to a request message requesting reservation or activation of radio resources for CFRA.

[0200] Fig. 16 is a diagram showing an example of operation according to the third operation pattern of the second embodiment. In Fig. 16, steps similar to those in Fig. 11 are given the same reference numerals (except that the reference numerals have a "d" suffix). Here, differences from the above-mentioned operation patterns will be mainly described.

[0201] In step S107d, the candidate gNB200C transmits a HANDOVER REQUEST ACKNOWLEDGE message to the serving gNB200S. Here, the candidate gNB200C notifies the serving gNB200S that it has configured an early TA for CFRA. The HANDOVER REQUEST ACKNOWLEDGE message may include a generic RACH configuration (RACH-Config Generic) without including a dedicated RACH configuration (RACH-Config Dedicated). Alternatively, the candidate gNB200C may include a dedicated RACH configuration (RACH-Config Dedicated) in the HANDOVER REQUEST ACKNOWLEDGE message. Candidate gNB200C does not need to reserve or activate resources for early TA CFRA at this point.

[0202] In step S110d, the serving gNB 200S transmits an RRC Reconfiguration message including a generic RACH configuration (RACH-Config Generic) to the UE 100. Upon receiving the RRC Reconfiguration message from the serving gNB 200S, the UE 100 completes only a portion of the CFRA configuration, i.e., the generic RACH configuration (RACH-Config Generic).

[0203] In step S111d, UE100 sends an RRC Reconfiguration Complete message to serving gNB200S.

[0204] In step S112d, the serving gNB200S sends a message to the candidate gNB200C requesting that resources for the early TA CFRA be reserved or activated.

[0205] In step S201d, the candidate gNB200C sends a dedicated RACH setting (RACH-Config Dedicated) to the serving gNB200S in a response message to the resource reservation request.

[0206] In step S115d, the serving gNB 200S transmits a PDCCH order to the UE 100. The PDCCH order includes a setting equivalent to a dedicated RACH setting (RACH-Config Dedicated) and a cell indicator (Cell indicator). When the UE 100 receives the PDCCH order, the CFRA setting, that is, the dedicated RACH setting (RACH-Config Dedicated) and the general RACH setting (RACH-Config Generic) are completed.

[0207] In step S120d, UE100 transmits a PRACH to candidate gNB200C.

[0208] The subsequent operations are the same as those in the above-described embodiment.

[0209] Thus, in the third operation pattern of the second embodiment, the candidate gNB200C transmits the settings necessary for the serving gNB200S to transmit a PDCCH order to the serving gNB200S. Specifically, the candidate gNB200C transmits a dedicated RACH setting (RACH-Config Dedicated) to the serving gNB200S in a CFRA resource reservation (or activation) response message. However, the candidate gNB200C may also transmit the dedicated RACH setting (RACH-Config Dedicated) to the serving gNB200S in a HANDOVER REQUEST ACKNOWLEDGE message.

[0210] (4.4) Fourth Operation Pattern of Second Embodiment In the fourth operation pattern of the second embodiment, when the UE 100 receives an RRC Reconfiguration message including a dedicated RACH setting (RACH-Config Dedicated) from the first cell, the UE 100 triggers a CFRA to the second cell even if the UE 100 does not receive a PDCCH order that triggers a CFRA to the second cell from the first cell.

[0211] Fig. 17 is a diagram showing an example of operation according to the fourth operation pattern of the second embodiment. In Fig. 17, steps similar to those in Fig. 11 are given the same reference numerals (except that the reference numerals have an "e" suffix). Here, differences from the above-mentioned operation patterns will be mainly described.

[0212] In step S101e, the UE 100 transmits an L3 measurement report to the serving gNB 200S (first cell). The serving gNB 200S makes an LTM decision based on the L3 measurement report. In this operation pattern, the serving gNB 200S decides to immediately perform an LTM cell switch and proceeds with the procedure without a PDCCH order. However, if the serving gNB 200S does not immediately perform an LTM cell switch, it may trigger a CFRA with a PDCCH order as in the conventional case.

[0213] In step S103e, the serving gNB200S transmits a HANDOVER REQUEST message to the candidate gNB200C to notify the candidate gNB200C of the LTM decision. The HANDOVER REQUEST message may include the measurement results in the L3 measurement report of step S101e. In this case, the candidate gNB200C may check the measurement results transferred in the HANDOVER REQUEST message and determine whether to perform early TA without a PDCCH order.

[0214] In step S107e, the candidate gNB200C transmits a HANDOVER REQUEST ACKNOWLEDGE message to the serving gNB200S. If early TA without a PDCCH order is to be performed according to the above determination, the candidate gNB200C sets a dedicated RACH setting (RACH-Config Dedicated) in the handover command in the HANDOVER REQUEST ACKNOWLEDGE message. In this operation pattern, the candidate gNB200C reserves or activates CFRA resources for early TA at this point. The candidate gNB200C may include a notification that CFRA without a PDCCH order will be performed in the HANDOVER REQUEST ACKNOWLEDGE message (handover command).

[0215] In step S110e, the serving gNB 200S transmits an RRC Reconfiguration message to the UE 100. Upon receiving the RRC Reconfiguration message from the serving gNB 200S, the UE 100 completes the CFRA configuration, i.e., the dedicated RACH configuration (RACH-Config Dedicated) and the general RACH configuration (RACH-Config Generic). At this point, the UE 100 understands that the CFRA has been configured with the early TA.

[0216] When the UE 100 receives notification information of a dedicated RACH setting (RACH-Config Dedicated) and an early TA instruction without a PDCCH order in an RRC Reconfiguration message, the UE 100 may use this as a trigger to transmit a PRACH to the second cell. In this case, the UE 100 may transmit an RRC Reconfiguration Complete message to the serving gNB 200S after transmitting the PRACH. In this case, since the early TA has ended without the serving gNB 200S knowing, the serving gNB 200S does not need to be concerned about the early TA.

[0217] In step S111e, the UE 100 transmits an RRC Reconfiguration Complete message to the serving gNB 200S. Here, the UE 100 notifies the serving gNB 200S that it will start early TA. Specifically, the UE 100 includes in the RRC Reconfiguration Complete message notification information indicating that a dedicated RACH setting (RACH-Config Dedicated) is included in the RRC Reconfiguration message or an early TA instruction without a PDCCH order. Note that, if there are multiple cells in which CFRA is configured, the UE 100 may arbitrarily determine which LTM candidate cell to perform CFRA on. When notifying that the early TA will be started, the UE 100 may include the LTM candidate ID in the RRC Reconfiguration Complete message.

[0218] In step S120e, UE100 transmits a PRACH to candidate gNB200C.

[0219] The subsequent operations are the same as those in the above-described embodiment.

[0220] As described above, in the fourth operation pattern of the second embodiment, the UE 100 performs early TA without a PDCCH order. The UE 100 may notify the serving gNB 200S of the start of early TA by an RRC Reconfiguration Complete message.

[0221] (5) Other embodiments The LTM in the above-described embodiments may be read as conditional LTM. For example, the above-described LTM of an inter-CU or intra-CU may be conditional LTM of an inter-CU or intra-CU. In conditional LTM, for example, the RRC Reconfiguration message in step S4 of FIG. 6 includes information indicating the execution conditions (e.g., radio quality conditions) of LTM cell switching for each LTM candidate cell. The UE 100 performs LTM cell switching to an LTM candidate cell that satisfies the preset execution conditions (radio quality conditions) instead of the cell switching command MAC CE transmitted from the gNB 200. This enables faster LTM cell switching.

[0222] The above-described operational flows are not limited to being implemented independently, but can be implemented by combining two or more operational flows. For example, some steps of one operational flow may be added to another operational flow, or some steps of one operational flow may be replaced with some steps of another operational flow. In each flow, it is not necessary to execute all steps, and only some steps may be executed. Furthermore, the order of steps in each flow may be changed as appropriate.

[0223] In the above-described embodiments and examples, an example in which the base station is an NR base station (gNB) has been described, but the base station may be an LTE base station (eNB) or a 6G base station. The base station may also be a relay node such as an IAB (Integrated Access and Backhaul) node. The base station may also be a DU of the IAB node. The UE 100 may also be an MT (Mobile Termination) of the IAB node. That is, the UE 100 may be a terminal function unit (a type of communication module) for the base station to control a relay that relays signals. Such a terminal function unit is referred to as an MT. Examples of MTs include, in addition to IAB-MT, NCR (Network Controlled Repeater)-MT and RIS (Reconfigurable Intelligent Surface)-MT.

[0224] The term "network node" primarily refers to a base station, but may also refer to a core network device or a part of a base station (CU, DU, or RU). A network node may also be configured by a combination of at least a part of a core network device and at least a part of a base station.

[0225] A program that causes a computer to execute each process performed by the UE 100 or the gNB 200 may be provided. The program may be recorded on a computer-readable medium. Using a computer-readable medium, the program can be installed on a computer. Here, the computer-readable medium on which the program is recorded may be a non-transitory recording medium. The non-transitory recording medium is not particularly limited, and may be, for example, a recording medium such as a CD-ROM and / or a DVD-ROM. Furthermore, circuits that execute each process performed by the UE 100 or the gNB 200 may be integrated, and at least a portion of the UE 100 or the gNB 200 may be configured as a semiconductor integrated circuit (chip set, SoC: System on a chip).

[0226] The functions performed by UE100 or gNB200 may be implemented in circuitry or processing circuitry, including general-purpose processors, application-specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), CPUs (Central Processing Units), conventional circuits, and / or combinations thereof, programmed to perform the described functions. A processor includes transistors and / or other circuits and is considered to be circuitry or processing circuitry. A processor may also be a programmed processor that executes a program stored in memory. In this specification, circuitry, unit, or means refers to hardware that is programmed to perform the described functions or hardware that executes them. The hardware may be any hardware disclosed herein or any hardware known to be programmed or capable of performing the described functions. If the hardware is a processor, the circuitry, means, or unit is a combination of hardware and software used to configure the hardware and / or processor.

[0227] As used in this disclosure, the terms "based on" and "depending on / in response to" do not mean "based only on" or "depending only on," unless expressly stated otherwise. The term "based on" means both "based only on" and "based at least in part on." Similarly, the term "depending on" means both "depending only on" and "depending at least in part on." The terms "include," "comprise," and variations thereof do not mean including only the listed items, but may mean including only the listed items or may include additional items in addition to the listed items. Additionally, the term "or," as used in this disclosure, is not intended to mean an exclusive or. Furthermore, any reference to elements using designations such as "first," "second," etc., as used in this disclosure does not generally limit the quantity or order of those elements. These designations may be used herein as a convenient method of distinguishing between two or more elements. Thus, a reference to a first and a second element does not imply that only two elements may be employed therein or that the first element must precede the second element in some way. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles shall include the plural unless the context clearly indicates otherwise.

[0228] The above describes the embodiments in detail with reference to the drawings, but the specific configuration is not limited to that described above, and various design changes can be made within the scope that does not deviate from the gist of the invention.

[0229] This application claims priority to U.S. Provisional Application No. 63 / 679,736, filed August 6, 2024, the entire contents of which are incorporated herein by reference.

[0230] (6) First Supplementary Note The following is a supplementary note regarding the features of the above-described embodiment.

[0231] Supplementary Note 1: A communication method for switching a serving cell of a user equipment from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), the communication method comprising: receiving, by the first network node, a handover request acknowledgement message from the second network node, the handover request acknowledgement message including a dedicated random access channel (RACH) configuration for contention-free random access (CFRA) to be used for early uplink synchronization of the user equipment to the second cell; and transmitting, by the first network node, a radio resource control (RRC) reconfiguration message to the user equipment, the dedicated RACH configuration, the handover request acknowledgement message including notification information indicating that the CFRA configuration exists.

[0232] Supplementary Note 2. The communication method of Supplementary Note 1, further comprising the first network node transmitting to the user equipment a Physical Downlink Control Channel (PDCCH) order triggering the CFRA to the second cell, wherein the PDCCH order does not include information on the dedicated RACH configuration and includes a cell indicator indicating the second cell.

[0233] Supplementary Note 3: A communication method for switching a serving cell of a user equipment from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), the communication method comprising: receiving, from the first cell, a Radio Resource Control (RRC) reconfiguration message, the RRC reconfiguration message including a dedicated Random Access Channel (RACH) configuration for Contention Free Random Access (CFRA) used for early uplink synchronization to the second cell; and transmitting, from the user equipment, an RRC reconfiguration complete message to the first cell, the RRC reconfiguration complete message indicating that a configuration process based on the RRC reconfiguration message has been completed, the RRC reconfiguration complete message including notification information indicating that the dedicated RACH configuration was included in the RRC reconfiguration message.

[0234] Supplementary Note 4: A communication method for switching a serving cell of a user equipment from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), the communication method comprising: the first network node transmitting, to the second network node, a request message requesting reservation or activation of radio resources for Contention Free Random Access (CFRA) to be used for early uplink synchronization of the user equipment to the second cell; and the first network node receiving, from the second network node, a response message in response to the request message, wherein the response message includes configuration of a Dedicated Random Access Channel (RACH) for the CFRA.

[0235] Supplementary Note 5. The communication method of Supplementary Note 4, wherein the first network node transmits the request message to the second network node before transmitting a Physical Downlink Control Channel (PDCCH) order to the user equipment, the PDCCH order triggering the CFRA to the second cell.

[0236] Supplementary Note 6: A communication method for switching a serving cell of a user equipment from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), the communication method comprising: receiving, by the user equipment, a Radio Resource Control (RRC) reconfiguration message from the first cell, the RRC reconfiguration message including a Dedicated Random Access Channel (RACH) configuration for Contention Free Random Access (CFRA) used for early uplink synchronization to the second cell; and triggering, by the user equipment, the CFRA to the second cell based on the reception of the RRC reconfiguration message, even if the user equipment does not receive, from the first cell, a Physical Downlink Control Channel (PDCCH) order for triggering the CFRA to the second cell.

[0237] Supplementary Note 7: The communication method according to Supplementary Note 6, further comprising the user equipment transmitting an RRC reconfiguration complete message to the first cell indicating that a configuration process using the RRC reconfiguration message has been completed, wherein the RRC reconfiguration complete message includes notification information indicating that the CFRA to the second cell is triggered.

[0238] (7) Appendix 2 1. Introduction In RAN #102, a new work item for NR mobility extension phase 4 was approved. The purpose of inter-CU LTM support is described in the WID as follows:

[0239] -Specify support for inter-CU Layer 2 mobility (LTM).

[0240] The case where the CU acts as the MN when the DC is not configured is given priority.

[0241] As a secondary priority, NR-DC is set to support the case where the CU acts as an SN and the MCG is not changed.

[0242] As a secondary priority, NR-DC is configured to support the case where the CU acts as the MN and the SCG is not changed or the SCG is released.

[0243] Note: Cases where LTM is set in both MCG and SCG are excluded.

[0244] According to Rel-18 LTM, it specifies support for subsequent LTM mobility procedures aimed at avoiding RRC setup during cell switching.

[0245] Coordination with SA3 is required regarding security key handling.

[0246] Note: The Rel-18 inter-CU LTM procedure is considered the baseline for adding inter-CU support.

[0247] This appendix discusses potential challenges to supporting InterCU LTM.

[0248] 2. Discussion 2.1. Early RACH via PDCCH Order In RAN2#125-bis, it was agreed that early RACH via PDCCH order is also supported for inter-CU LTM.

[0249] Agreements regarding the early synchronization phase: 1. Early DL and UL synchronization will also be supported in Inter CU LTM. This will be communicated to RAN3. Early DL synchronization using CSI-RS should be considered, subject to approval by RAN1. 2. Early RACH via PDCCH order will be supported in Inter CU LTM. 3. For early TA acquisition, the Rel-18 option will be the baseline. RAR-based options require further study.

[0250] In Rel-18 intra-CU LTM, CFRA without network response (i.e., early RACH with PDCCH order) is supported in the early TA process. During the early TA process, the serving cell sends a PDCCH order to the UE, and the LTM candidate cell receives a PRACH from the UE. Since the serving cell and the LTM candidate cell belong to the same CU, close coordination between the two cells can be easily achieved.

[0251] In the case of Rel-19 inter-CU LTM, since the LTM candidate cell and the serving cell belong to different CUs, some interaction between the CUs is considered to be necessary. Regarding early RACH by PDCCH order in inter-CU LTM, the following issues are considered, which are also shown in Figure 18.

[0252] - Currently, the serving gNB does not know whether the LTM candidate gNB has configured the UE with an early RACH due to a PDCCH order, and therefore does not know whether it needs to send a PDCCH order to the UE.

[0253] -In addition, when the serving gNB needs to send a PDCCH order, the serving gNB does not know the configuration of the dedicated preamble that should be notified to the UE via the PDCCH order.

[0254] Regarding these issues, the following three approaches can be considered: Approach 1: Whether the UE is configured with early TA via PDCCH order and the dedicated preamble configuration are notified from the LTM candidate gNB to the serving gNB via the Xn interface.

[0255] The LTM candidate gNB notifies the serving gNB of the settings required to send a PDCCH order to the UE.

[0256] The LTM candidate gNB must explicitly inform the serving gNB (e.g., in a HANDOVER REQUEST ACKNOWLEDGE message) that it has configured the UE with an early RACH via PDCCH order, because RAN2 assumes that the serving gNB does not interpret the RRC container.

[0257] - Approach 2: The UE notifies the serving gNB via the Uu interface whether or not the UE is configured for early TA via a PDCCH order.

[0258] The LTM candidate gNB configures the UE in advance with RRC Reconfiguration, including the settings (i.e., dedicated preamble) required to perform early RACH via a PDCCH order. In this case, the serving gNB needs to know whether the UE has been configured with a dedicated preamble via RRC Reconfiguration in order to determine whether it needs to indicate a dedicated preamble in the PDCCH order.

[0259] If the UE is configured with a dedicated preamble, the serving gNB only needs to send a PDCCH order to the UE at the optimal timing, because the UE has already been configured with RACH-Config Generic and RACH-Config Dedicated (i.e., early RACH configuration by PDCCH order) by RRC Reconfiguration.

[0260] In this case, the serving gNB does not need to include the Random Access Preamble index, SS / PBCH index, and PRACH Mask index (i.e., equivalent to RACH-Config Dedicated) in the PDCCH order. Note that in Rel-18 intra-CU LTM, since the LTM candidate cell does not configure RACH-Config Dedicated for early TA in RRC Reconfiguration (i.e., LTM Config), the PDCCH order uses DCI format 1_0 with the following dedicated preamble configuration:

[0261] If the CRC of DCI format 1_0 is scrambled with the C-RNTI and the "Frequency domain resource assignment" field is all ones, then DCI format 1_0 is for a random access procedure initiated by a PDCCH order, and all remaining fields are set as follows:

[0262] - Random Access Preamble index - 6 bits according to ra-PreambleIndex in section 5.1.2 of [8, TS38.321]

[0263] - UL / SUL indicator - 1 bit. If the value of "Random Access Preamble index" is not all zeros and the UE has supplementaryUplink configured in the ServingCellConfig in the cell, this field indicates the UL carrier for transmitting the PRACH in the cell. Otherwise, this field is reserved.

[0264] - SS / PBCH index - 6 bits. If the value of "Random Access Preamble index" is not all zeros, this field indicates the SS / PBCH to be used to determine the RACH occasion for PRACH transmission. Otherwise, this field is reserved.

[0265] - PRACH Mask index - 4 bits. If the value of "Random Access Preamble index" is not all zeros, this field indicates the RACH occasion associated with the SS / PBCH indicated by "SS / PBCH index" for PRACH transmission according to clause 5.1.1 of [8, TS38.321]. Otherwise, this field is reserved.

[0266] - Cell indicator - the number of bits corresponding to the log_2(C+1) ceiling function indicating the cell for the corresponding PRACH transmission if the UE is configured with the higher layer parameter EarlyUlSyncConfig, where C is the number of candidate cells configured with the higher layer parameter EarlyUlSyncConfig, and 0 bits otherwise. Bit field index 0 of the Cell indicator field is mapped to the serving cell, the other bit field indices are mapped to the candidate cells configured with the higher layer parameter EarlyUlSyncConfig according to the ascending order of the candidate identities configured by ltm-CandidateId, and bit field index 1 is mapped to the candidate cell with the smallest candidate identity.

[0267] Approach 3: Early TA is performed automatically by the UE when configured by the LTM candidate gNB without a PDCCH order from the serving gNB (i.e., PDCCH orderless early TA).

[0268] Similar to Approach 2, the LTM candidate gNB pre-configures the UE with RRC Reconfiguration, which includes the settings necessary to perform early RACH via a PDCCH order.

[0269] Approach 3 differs from the previous two approaches in that it does not require a PDCCH order. Similar to conventional L3 handover, once the UE is configured with RRC Reconfiguration, the UE immediately sends a Random Access Preamble to the LTM candidate gNB. In other words, the RACH-Config Dedicated that the UE receives from the LTM candidate gNB acts as an implicit PDCCH order.

[0270] Approach 3 may be particularly useful when an LTM cell switch is likely to occur immediately. Additionally, from the serving gNB's perspective, early TA is performed transparently (i.e., the serving gNB does not need to send a PDCCH order for early TA).

[0271] Approaches 1 and 2 assume that the serving gNB sends the PDCCH order to the UE as agreed in RAN2. Approach 1 is the conventional method but has implications for the RAN3 specification. Approach 2 has the advantage of reducing the signaling overhead associated with the PDCCH order.

[0272] Approach 3 is not discussed in RAN2, but is worth discussing because it does not require any coordination between gNBs regarding PDCCH orders for early TA.

[0273] Proposal 1: RAN2 should discuss how to allocate dedicated preambles to UEs, for example, using PDCCH order as Rel-18 LTM did (approach 1), using RRC Reconfiguration (in LTM config) as Rel-15 L3 handover did (approach 2), or without PDCCH order (approach 3).

[0274] 2.2. Inter-CU MCG LTM with SN Release RAN2#127 agreed to clarify the DC scenario for Inter-CU LTM.

[0275] Agreements on Inter-CU LTM: DC Clarifications 1. LTM configuration with Inter-CU LTM candidate cells can be configured by either MCG or SCG (but not both simultaneously) and it is up to the network how to handle this (further details, if any, are up to RAN3). No restrictions on Intra-CU LTM candidate cells.

[0276] RAN#104 approved a revised WID that includes three DC scenarios as follows:

[0277] If DC is configured, InterCU LTM can be configured either in MN or SN, but not both at the same time. In such cases:

[0278] As a secondary priority, it supports the case where the CU acts as the SN and the MN does not change.

[0279] As a secondary priority, it supports the case where the CU acts as the MN and the SN is not changed or the SN is released.

[0280] According to WID, there are three scenarios for InterCU LTM with DC. Among them, "InterCU MCG LTM with SN release" has an issue with handling the SCG LTM Config. The scenario for InterCU MCG LTM with SN release is shown in Figure 19.

[0281] According to the following TS 38.331, the UE maintains the LTM Config of the MCG and SCG in separate VarLTM-Configs, and each LTM Config is set from the MN and SN, respectively. That is, the problem is that the MN cannot delete the SCG LTM Config.

[0282] The network configures the UE with one or more LTM candidate configurations in the LTM-Config IE.

[0283] In NR-DC, the UE may receive two independent ltm-Configs:

[0284] - the ltm-Config associated with the MCG contained in the RRCReconfiguration message received via SRB1, and

[0285] - The ltm-Config associated with the SCG embedded in the RRCReconfiguration message received via SRB3 or alternatively via SRB1.

[0286] According to the agreement of RAN2#126, before "Inter-CU MCG LTM with SN Release", the UE will be configured with two LTM Configs as follows:

[0287] MCG LTM Config: A mix of intra-CU cells and inter-CU cells can be set.

[0288] SCG LTM Config: Only intra-CU (= intra-SN) cells can be set.

[0289] According to TS38.300 below, for subsequent LTMs, the UE maintains the LTM Config until the NW explicitly instructs deletion. Therefore, the UE maintains the SCG LTM Config even after the execution of Inter-CU MCG LTM. Note that in this scenario, the SCG is released during Inter-CU MCG LTM.

[0290] However, it is unclear whether the target MCG will use a new DC with the same SN (i.e., the source SN for the intra-CU LTM config stored by the UE). That is, the target MCG may not use a DC, or may configure the UE with a new DC with a target SN different from the source SN. Since the SCG is released in the "Inter-CU MCG LTM with SN Release" scenario, the stored SCG LTM Config (for intra-SN) is no longer necessary from the target MCG's point of view. Note that maintaining a DC with the same SN after the execution of Inter-CU MCG LTM is considered in the "Inter-CU MCG LTM with SN unchanged" scenario.

[0291] Subsequent LTMs are performed by repeating the steps of early synchronization, LTM cell switch execution, and LTM cell switch completion without releasing other LTM candidate settings after each LTM cell switch completion.

[0292] According to 38.300 below, the UE should perform L1 measurements as long as the LTM Config in the LTM preparation phase is applicable. Therefore, maintaining unnecessary LTM Config increases the power consumption of the UE.

[0293] 5. The UE performs L1 measurements on the configured candidate cells and sends L1 measurement reports to the gNB. L1 measurements should be performed as long as RRC reconfiguration (step 2) is applicable.

[0294] To avoid this issue, unnecessary SCG LTM Config should be deleted when "Inter CU MCG LTM with SN Release" is executed, which can only be done by the SN according to the current specification. However, this SN is already released by the target MN when Inter CU MCG LTM is executed. In the current specification, neither the UE nor the MN can delete the SCG LTM Config. Therefore, RAN2 should discuss how to delete unnecessary SCG LTM Config when the SN is not configured in the UE.

[0295] Proposal 2: RAN2 should discuss how to delete the SCG LTM Config after performing inter-CU MCG LTM with SN release.

[0296] 1: Mobile communication system 5: Network 10: RAN 20: CN 100: UE 110: Receiving unit 120: Transmitting unit 130: Control unit 140: Wireless communication unit 200: gNB 210: Transmitting unit 220: Receiving unit 230: Control unit 240: Network communication unit 241: Transmitting unit 242: Receiving unit 250: Wireless communication unit 300: AMF / UPF

Claims

1. A communication method for switching a serving cell of a user equipment from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), the communication method comprising: the first network node transmitting, to the second network node, a handover request message including information requesting configuration for early uplink synchronization to be performed by the user equipment to the second cell; and the first network node receiving, from the second network node, a handover request acknowledgement message including the configuration for the early uplink synchronization.

2. The communication method according to claim 1, wherein the first network node transmits the handover request message to the second network node, the handover request message further including information indicating that the message is an LTM setting request.

3. The communication method according to claim 2, wherein the first network node receives the handover request acknowledgement message from the second network node, the handover request acknowledgement message including information indicating that LTM configuration is to be performed.

4. The method of claim 1, further comprising the first network node receiving a message from the second network node including a TA value for the early uplink synchronization.

5. The communication method according to claim 1, further comprising the first network node transmitting, in response to transmitting a cell switch command to the user equipment, a message to the second network node notifying the second network node of the transmission of the cell switch command.

6. A network node that operates as a first network node when switching a serving cell of a user equipment from a first cell of a first network node to a second cell of a second network node by LTM (L1 / L2 Triggered Mobility), the network node having: a transmitter that transmits a handover request message to the second network node, the handover request message including information requesting configuration for early uplink synchronization to be performed by the user equipment with the second cell; and a receiver that receives a handover request acknowledgement message from the second network node, the handover request acknowledgement message including the configuration for the early uplink synchronization.

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