Communication method and user device

Abstract

This communication method for switching the serving cell of a user device from a first cell to a second cell by L1 / L2 Triggered Mobility (LTM) comprises: the user device receiving, from the first cell, a cell radio network temporary identifier (C-RNTI) assigned to the user device for the second cell; and the user device transmitting, to the second cell, a contention-based random access (CBRA) preamble to be used for uplink early synchronization in the LTM, and the C-RNTI, by means of a message A (MSGA) in a two-step RA procedure.

Description

Communication method and user device

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

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

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

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

[0005] This disclosure provides techniques for improving LTM.

[0006] A 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 second network node transmitting, to the first network node, a HANDOVER REQUEST ACKNOWLEDGE message including an LTM candidate cell configuration for the second cell and a configuration for uplink early synchronization; and the second network node transmitting, to the first network node, a message including a TA value obtained by the uplink early synchronization.

[0007] A communication method according to a second aspect of the present disclosure is a communication method for switching a serving cell of a user equipment from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the communication method including: receiving, by the user equipment, a Cell Radio Network Temporary Identifier (C-RNTI) assigned to the user equipment for the second cell from the first cell; and transmitting, by the user equipment, a contention-based random access (CBRA) preamble used for uplink early synchronization in the LTM and the C-RNTI to the second cell by a message A (MSGA) in a two-step RA procedure.

[0008] A user equipment according to a third aspect of the present disclosure is a user equipment that switches a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), and includes: a receiving unit that receives, from the first cell, a Cell Radio Network Temporary Identifier (C-RNTI) assigned to the user equipment for the second cell; and a transmitting unit that transmits, to the second cell, a contention-based random access (CBRA) preamble used for uplink early synchronization in the LTM and the C-RNTI by a message A (MSGA) in a two-step RA procedure.

[0009] FIG. 1 is a diagram showing an example of the configuration of a mobile communication system according to an embodiment. FIG. 2 is a diagram showing an example of the configuration of a UE (user equipment) according to an embodiment. FIG. 3 is a diagram showing an example of the configuration of a gNB (network node) according to an embodiment. FIG. 4 is a diagram showing the configuration of a protocol stack of the radio interface of the user plane that handles data. FIG. 5 is a diagram showing the configuration of a protocol stack of the radio interface of the control plane that handles signaling (control signals). FIG. 6 is a diagram showing an example of a cell switching procedure by LTM in intra-CU (i.e., within the same gNB). FIG. 7 is a diagram showing an example of operation when CFRA-based early TA is used as early synchronization (early TA) in inter-CU LTM. FIG. 8 is a diagram for explaining an overview of a general two-step RA procedure. FIG. 9 is a diagram showing an example of operation of a mobile communication system according to an embodiment.

[0010] A mobile communication system according to an embodiment will be described with reference to the drawings. In the description of the drawings, the same or similar parts are denoted by the same or similar reference numerals.

[0011] (1) Configuration of a Mobile Communication System FIG. 1 is a diagram showing an example of the configuration of a mobile communication system 1 according to this embodiment. The mobile communication system 1 conforms to the 3GPP standard 5th Generation System (5GS). While the following description uses 5GS as an example, the mobile communication system may also be at least partially based on an LTE (Long Term Evolution) system. The mobile communication system may also be at least partially based on a 6th Generation (6G) system.

[0012] The mobile communication system 1 includes a user equipment (UE) 100, a 5G radio access network (NG-RAN: Next Generation Radio Access Network) 10, and a 5G core network (5GC: 5G Core Network) 20. Hereinafter, the NG-RAN 10 may be simply referred to as the RAN 10. Furthermore, the 5GC 20 may be simply referred to as the core network (CN) 20. The RAN 10 and the CN 20 constitute a network 5 of the mobile communication system 1.

[0013] The UE 100 is a mobile wireless communication device. The UE 100 may be any device used by a user. For example, the UE 100 may be a mobile phone terminal (including a smartphone) and / or a tablet terminal, a notebook PC, a communication module (including a communication card or chipset), a sensor or a device provided in a sensor, a vehicle or a device provided in a vehicle (Vehicle UE), or an aircraft or a device provided in an aircraft (Aerial UE). A link in the transmission direction from the UE 100 to the network 5 is referred to as an uplink (UL), and a link in the transmission direction from the network 5 to the UE 100 is referred to as a downlink (DL).

[0014] The NG-RAN 10 includes a base station (referred to as "gNB" in the 5G system) 200, which is a type of network node. The gNBs 200 are connected to each other via an Xn interface, which is an interface between base stations. The gNB 200 manages one or more cells. The gNB 200 performs wireless communication with the UE 100 that has established a connection with its own cell. The gNB 200 has a radio resource management (RRM) function, a routing function for user data (hereinafter simply referred to as "data"), a measurement control function for mobility control and scheduling, etc. The term "cell" is used to indicate the smallest unit of a wireless communication area. The term "cell" is also used to indicate a function or resource that performs wireless communication with the UE 100. One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

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

[0016] The 5GC20 includes an AMF (Access and Mobility Management Function) and a UPF (User Plane Function) 300. The AMF performs various mobility controls for the UE 100. The AMF manages the mobility of the UE 100 by communicating with the UE 100 using NAS (Non-Access Stratum) signaling. The UPF controls data forwarding. The AMF and the UPF are connected to the gNB 200 via an NG interface, which is an interface between a base station and a core network.

[0017] 2 is a diagram showing an example of the configuration of a UE 100 (user equipment) according to this embodiment. The UE 100 has a receiving unit 110, a transmitting unit 120, and a control unit 130. The receiving unit 110 and the transmitting unit 120 constitute a wireless communication unit 140 that performs wireless communication with the gNB 200.

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

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

[0020] The control unit 130 performs various controls and processes in the UE 100. Such processes include processes of each layer described below. The operations of the UE 100 described above and below may be operations controlled by the control unit 230. The control unit 130 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processing by the processor. The processor may include a baseband processor and a CPU (Central Processing Unit). The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0021] 3 is a diagram showing an example configuration of a gNB 200 (network node) according to this embodiment. The gNB 200 has a transmitter 210, a receiver 220, a controller 230, and a network communication unit 240. The transmitter 210 and the receiver 220 constitute a wireless communication unit 250 that performs wireless communication with the UE 100. The network communication unit 240 has a transmitter 241 that transmits and a receiver 242 that receives.

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

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

[0024] The control unit 230 performs various controls and processes in the gNB 200. Such processes include processes for each layer described below. The operations of the gNB 200 described above and below may be operations under the control of the control unit 230. The control unit 230 includes at least one processor and at least one memory. The memory stores programs executed by the processor and information used in the processing by the processor. The processor may include a baseband processor and a CPU. The baseband processor performs modulation / demodulation and encoding / decoding of baseband signals. The CPU executes programs stored in the memory to perform various processes.

[0025] The network communication unit 240 is connected to adjacent base stations via an Xn interface, which is an interface between base stations. The network communication unit 240 is connected to the AMF / UPF 300 via an NG interface, which is an interface between a base station and a core network. The gNB 200 is composed of a CU (Central Unit) and a DU (Distributed Unit) (i.e., functionally divided), and the two units may be connected by an F1 interface, which is a fronthaul interface.

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

[0027] The user plane radio interface protocol includes a physical (PHY) layer, a medium access control (MAC) layer, a radio link control (RLC) layer, a packet data convergence protocol (PDCP) layer, and a service data adaptation protocol (SDAP) layer.

[0028] The PHY layer performs encoding / decoding, modulation / demodulation, antenna mapping / demapping, and resource mapping / demapping. Data and control information are transmitted between the PHY layer of UE100 and the PHY layer of gNB200 via a physical channel. The PHY layer of UE100 receives downlink control information (DCI) transmitted from gNB200 on a physical downlink control channel (PDCCH). Specifically, UE100 performs blind decoding of the PDCCH using a radio network temporary identifier (RNTI) and acquires successfully decoded DCI as DCI addressed to the UE. The DCI transmitted from gNB200 has a CRC (Cyclic Redundancy Code) parity bit scrambled by the RNTI added.

[0029] The MAC layer performs data priority control, retransmission processing using Hybrid Automatic Repeat reQuest (HARQ), random access procedures, etc. Data and control information are transmitted between the MAC layer of the UE 100 and the MAC layer of the gNB 200 via a transport channel. The MAC layer of the gNB 200 includes a scheduler. The scheduler determines the uplink and downlink transport format (transport block size, modulation and coding scheme (MCS)) and the resource blocks to be allocated to the UE 100.

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

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

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

[0033] FIG. 5 is a diagram showing the configuration of a protocol stack of a radio interface of a control plane that handles signaling (control signals).

[0034] The protocol stack of the radio interface of the control plane has an RRC (Radio Resource Control) layer and an NAS (Non-Access Stratum) layer instead of the SDAP layer shown in FIG.

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

[0036] The NAS layer (also simply referred to as "NAS") located above the RRC layer performs session management, mobility management, etc. NAS signaling is transmitted between the NAS layer of the UE 100 and the NAS layer of the AMF 300A. Note that the UE 100 has an application layer and the like in addition to the radio interface protocol. Also, a layer lower than the NAS layer is referred to as the AS layer (also simply referred to as "AS").

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

[0038] In a typical handover procedure, a serving cell switch is triggered by signaling in the upper layer L3, specifically, the RRC layer. Such a typical handover is also referred to as an L3 handover. In an L3 handover, an L3 measurement report message, which is an RRC message, is transmitted from the UE 100 to the gNB 200. The gNB 200 determines the handover of the UE 100 based on the Measurement Report message, and instructs the cell switch by transmitting a handover command (specifically, an RRC Reconfiguration message) which is an RRC message from the gNB 200 to the UE 100.

[0039] On the other hand, LTM is a technology for shortening mobility delay (specifically, serving cell switching delay) compared to a typical handover procedure by triggering a serving cell switch by signaling of a lower layer, Layer 1 (L1) and / or Layer 2 (L2). In LTM, the gNB 200 receives an L1 measurement report from the UE 100, and based on the L1 measurement report, the gNB 200 signals the UE 100 via a MAC CE to instruct the serving cell switch by a cell switch command.

[0040] Specifically, in LTM, first, gNB200 prepares LTM settings for candidate cells to be switched to, and provides the LTM settings to UE100 via RRC signaling.

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

[0042] Third, the gNB 200 receives an L1 measurement report from the UE 100, determines a serving cell switch to the target cell based on the L1 measurement report, and transmits a cell switch command (Cell Switch Command) indicating the target cell (LTM setting) to the UE 100 via a MAC control element (CE). The serving cell switch trigger is conveyed in a MAC CE including at least a candidate setting index (ID) together with a beam indicator.

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

[0044] In this way, a serving cell switch is triggered by selecting the LTM configuration as the target configuration by gNB200. The LTM configuration can be added, changed, and released by gNB200 via RRC signaling.

[0045] The following principles apply to LTM:

[0046] Each LTM setting can be provided as a delta setting relative to a reference setting that is used to form the complete LTM setting.

[0047] If a full LTM configuration is applied, the current UE configuration is replaced upon a serving cell switch. The reconfiguration procedure does this but does not necessarily reset the MAC, RLC or PDCP layers.

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

[0049] - Security is not updated in LTM.

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

[0051] 6 is a diagram showing an example of a cell switching procedure by LTM in intra-CU (i.e., within the same gNB 200). In the illustrated example, it is assumed that UE 100 performs serving cell switching from a first cell of gNB 200 to a second cell.

[0052] Here, the first cell and the second cell may be formed by different TRPs (Transmission and Reception Points). In the following description of the embodiment, the second cell is also referred to as an "LTM candidate cell" until a serving cell switch by LTM is determined, and the second cell is also referred to as a "target cell" after a serving cell switch by LTM is determined. The first cell is also referred to as a "source cell" or a "(current) serving cell".

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

[0054] In step S2, the UE 100 transmits a Measurement Report message, which is an RRC message, to the gNB 200 (first cell). The measurement report transmitted by the RRC message is also referred to as an "L3 measurement report."

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

[0056] In step S4, the gNB 200 (first cell) transmits an RRC message, specifically an RRC Reconfiguration message, to the UE 100, including LTM configurations (LTM Candidate Configurations) of one or more LTM candidate cells. The LTM configurations may include a random access channel (RACH) configuration, such as a contention-free random access (CFRA) configuration, used to transmit an RA preamble to the corresponding LTM candidate cell. Such a RACH configuration may be referred to as an early UL synchronization configuration (EarlyUlSyncConfig). CFRA is a random access procedure in which a dedicated RACH resource (e.g., a dedicated preamble sequence and / or a dedicated time-frequency resource) is assigned to the UE 100, and no RACH contention occurs between the UEs 100.

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

[0058] In step S6, the UE 100 may perform synchronization processing with the LTM candidate cell (second cell) before receiving the cell switch command MAC CE from the first cell. Such synchronization processing may be referred to as early synchronization (Early sync) or early timing advance (Early TA). Here, the UE 100 may perform downlink synchronization processing (DL synchronization processing) for the LTM candidate cell, and then perform early timing advance (TA) acquisition (i.e., UL early synchronization) in the LTM candidate cell requested by the gNB 200 (serving cell). This is performed by a CFRA triggered by a PDCCH order (PDCCH order) from the first cell. Note that when DCI Format 1_0 is used and all "Frequency domain resource assignment" fields in the DCI are set to "1", the DCI is treated as a PDCCH order. Note that, when early UL synchronization configuration (EarlyUlSyncConfig) is configured in the UE 100, the PDCCH order may include a corresponding RACH transmission cell, i.e., a cell indicator indicating to which LTM candidate cell the UE 100 should transmit a random access preamble (RA preamble).

[0059] The UE 100 transmits an RA preamble to the designated LTM candidate cell (second cell). In order to minimize communication interruption of the serving cell due to CFRA for the LTM candidate cell, in early synchronization, the UE 100 does not receive a random access response (RAR) for the purpose of acquiring a TA value from the LTM candidate cell. The TA value of the LTM candidate cell (target cell) is indicated in the cell switching command MAC CE in step S9. Note that the TA value is a value for adjusting the uplink transmission timing of the UE 100.

[0060] In step S7, the UE 100 performs layer 1 (L1) measurement in the configured LTM candidate cell and transmits a physical layer measurement report (also referred to as an "L1 measurement report") to the gNB 200 (first cell). The L1 measurement report is transmitted and received at L1, which is the PHY layer. For example, the UE 100 transmits L1-RSRP and / or L1-SINR to the gNB 200 via a PUCCH (Physical Uplink Control Channel) and / or a PUSCH (Physical Uplink Shared Channel).

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

[0062] In step S9, the gNB 200 (first cell) transmits a cell switch command MAC CE including a candidate configuration index of the target cell to the UE 100. The cell switch command MAC CE may include a TA value obtained by UL early synchronization (i.e., a TA value derived based on the RA preamble).

[0063] In step S10, the UE 100 switches to the configuration of the target cell (second cell). Specifically, the UE 100 detaches from the first cell and applies the configuration of the target cell (second cell).

[0064] In step S11, if the serving cell switch needs to include execution of a random access procedure (for example, if the cell switch command MAC CE does not include a valid TA value), the UE 100 executes the random access procedure for the target cell (RACH-based LTM cell switch). Note that, if the UE 100 does not need to acquire the TA of the target cell at the time of serving cell switch (for example, if the cell switch command MAC CE includes a valid TA value), it can skip the random access procedure (RACH-less LTM cell switch).

[0065] In step S12, the UE 100 indicates that the serving cell switch to the target cell has been successfully completed, for example, by sending an RRC Reconfiguration Complete message to the target cell (second cell). Thereafter, the UE 100 may perform steps S6 to S12 multiple times for subsequent LTM serving cell switches based on the configuration provided in step S4.

[0066] (3) Inter-CU LTM The LTM introduced in 3GPP Release 18 only supports intra-CU and does not support inter-CU (i.e., between different gNBs 200) LTM. That is, in conventional LTM, it is possible to perform LTM cell switching between cells under the same CU (same gNB 200), but it is not possible to perform LTM cell switching between cells under different CUs (different gNBs 200).

[0067] Meanwhile, Inter-CU LTM is scheduled to be newly introduced in 3GPP Release 19. In Inter-CU LTM, UE100 performs LTM cell switching from a first cell of one gNB200 (also referred to as "serving gNB200S") to a second cell of another gNB200 (also referred to as "LTM candidate gNB200C"). The serving gNB200S manages the serving cell of UE100, and the LTM candidate gNB200C manages the LTM candidate cell of UE100.

[0068] 7 is a diagram showing an example of operation when CFRA-based early TA is used as early synchronization (early TA) in inter-CU LTM. In the following embodiment, communication between the serving gNB200S and the LTM candidate gNB200C is performed over the Xn interface, which is an interface between gNBs.

[0069] In step S51, UE100 in an RRC connected state in the serving cell of the serving gNB200S transmits an L3 measurement report to the serving gNB200S. The serving gNB200S that receives the L3 measurement report decides to use LTM based on the L3 measurement report.

[0070] In step S52, the serving gNB200S transmits a HANDOVER REQUEST message to the LTM candidate gNB200C that manages the LTM candidate cell to request preparation for LTM cell switching. The LTM candidate gNB200C that receives the HANDOVER REQUEST message allocates (reserves) dedicated CFRA resources in the LTM candidate cell for the UE100.

[0071] In step S53, the LTM candidate gNB200C sends a HANDOVER REQUEST ACKNOWLEDGE message to the serving gNB200S, including an LTM candidate cell setting including a CFRA setting.

[0072] In step S54, the serving gNB200S sends an RRC Reconfiguration message to the UE100, which includes the LTM candidate cell setting from the LTM candidate gNB200C as the LTM setting.

[0073] In step S55, UE100, which has received the RRC Reconfiguration message, saves the LTM setting and sends an RRC Reconfiguration Complete message to the serving gNB200S.

[0074] In step S56, the serving gNB200S decides, for example based on an L1 measurement report from UE100, to have UE100 perform early synchronization to the LTM candidate cell, and sends a PDCCH order to UE100 instructing it to transmit a CFRA RA preamble to the LTM candidate cell.

[0075] In step S57, the UE 100 that has received the PDCCH order transmits a CFRA RA preamble to the LTM candidate cell on the PRACH (Physical Random Access Channel). The LTM candidate gNB 200C that has received the CFRA RA preamble derives a TA value based on the RA preamble, and uniquely identifies the UE 100 based on the RA preamble (CFRA resource), and can provide the TA value together with the identification information of the UE 100 to the serving gNB 200S. As a result, the serving gNB 200S can transmit a cell switching command including the TA value for the UE 100 to the UE 100.

[0076] In such an operation, the LTM candidate gNB200C does not know the timing when the serving gNB200S transmits a PDCCH order to the UE100. Therefore, the LTM candidate gNB200C needs to reserve CFRA resources for the UE100 from the time it transmits a HANDOVER REQUEST ACKNOWLEDGE message to the serving gNB200S until it receives an RA preamble from the UE100, which may result in a waste of radio resources (specifically, CFRA resources). Also, although multiple LTM candidate cells are typically configured for the UE100, there may be LTM candidate cells among them that the UE100 does not access, which may result in a further waste of radio resources.

[0077] One possible solution to this further waste of radio resources (CFRA resources) is to introduce contention-based random access (CBRA) for early synchronization (early TA). When CBRA is applied to early TA acquisition, although RA preamble contention may occur, each LTM candidate cell does not need to reserve CFRA resources.

[0078] When the CBRA procedure is applied to the early TA of LTM, it is assumed that UE100 transmits a CBRA RA preamble in step S57. However, since the CBRA RA preamble is randomly selected by UE100, the LTM candidate gNB200C that receives the CBRA RA preamble cannot uniquely identify UE100 based on the received RA preamble. Therefore, the LTM candidate gNB200C cannot provide the TA value together with the identification information of UE100 to the serving gNB200S, and cannot include the TA value in the cell switch command.

[0079] To solve this problem, in the inter-CU LTM according to the embodiment, first, the receiver 110 of the UE 100 receives a cell radio network temporary identifier (C-RNTI) assigned to the UE 100 for the LTM candidate cell from the serving cell (serving gNB 200S). Specifically, the UE 100 receives the C-RNTI (i.e., the C-RNTI for the candidate cell) included in the LTM candidate cell configuration (Var LTM-Config) corresponding to the LTM candidate cell. Second, the transmitter 120 of the UE 100 transmits a CBRA preamble (RA preamble) used for UL early synchronization in LTM and the C-RNTI to the LTM candidate cell (LTM candidate gNB 200C) by message A (MSGA) in a two-step RA procedure (also referred to as "two-step RACH").

[0080] Thus, in the embodiment, UE100 uses MSGA in the two-step RA procedure to transmit the RA preamble of CBRA to the LTM candidate cell and transmits the C-RNTI for the LTM candidate cell. As a result, the LTM candidate gNB200C (LTM candidate cell) can derive a TA value using the RA preamble and can uniquely identify UE100 using the C-RNTI. Specifically, the LTM candidate gNB200C receives an MSGA including the RA preamble and C-RNTI from UE100, derives a TA value based on the RA preamble, and associates the derived TA value with the C-RNTI.

[0081] Then, the LTM candidate gNB200C can provide the serving gNB200S with the TA value together with the identification information of the UE100. As a result, the serving gNB200S can include the TA value for the UE100 in the cell switching command to be sent to the UE100. The UE100 receives a cell switching command from the serving gNB200S (serving cell) that includes the TA value and instructs LTM cell switching to the LTM candidate cell.

[0082] An overview of a general two-step RA procedure will now be described with reference to Figure 8. As shown in Figure 8, the general two-step RA procedure consists of two steps: transmission of a message A (MSGA) from the UE 100 to the gNB 200, and transmission of a message B (MSGB) from the gNB 200 to the UE 100. The two-step RA procedure supports contention-based random access (CBRA) and contention-free random access (CFRA), but in the embodiment, CBRA is used.

[0083] MSGA combines the transmission of message 1 (MSG1) and message 3 (MSG3) in the four-step RA procedure into one step. MSG1 is the transmission of an RA preamble (Random Access Preamble) on the PRACH. MSG3 is the transmission of a PUSCH payload (PUSCH payload), specifically, an RRC message. The RRC message is, for example, an RRC Setup Request message. However, in the embodiment, the MSGA transmitted by the UE 100 to the LTM candidate gNB 200C does not necessarily include an RRC message as a PUSCH payload. In the embodiment, the MSGA transmitted by the UE 100 to the LTM candidate gNB 200C only needs to include at least a C-RNTI as a PUSCH payload. For example, the C-RNTI may be included in the MAC CE, and the UE 100 may transmit the MAC CE including the C-RNTI on the PUSCH of MsgA.

[0084] The MSGB is a response to the MSGA. The MSGB may include a contention resolution response and / or a fallback instruction. In a typical two-step RA procedure, the UE 100 monitors a response from the gNB 200 within a set window after transmitting the MSGA. In the case of CBRA, if contention resolution is successful upon receiving the network response (MSGB), the UE 100 terminates the random access procedure. On the other hand, if a fallback instruction is received in the MSGB, the UE 100 transmits MSG3 using the UL grant scheduled in the fallback instruction and monitors contention resolution. If contention resolution is not successful after retransmission of MSG3, the UE 100 returns to MSGA transmission.

[0085] In an embodiment, the MSGA is used for early TA of the LTM. Therefore, the UE 100 that transmitted the MSGA to the LTM candidate gNB 200C does not need to monitor the MSGB (contention resolution) from the LTM candidate gNB 200C. If the LTM candidate gNB 200C can acquire the C-RNTI in the PUSCH payload, it can determine that it has been received without contention, so contention resolution is not required. After transmitting the MSGA to the LTM candidate gNB 200C (LTM candidate cell), the UE 100 resumes communication with the serving gNB 200S (serving cell) without waiting for the reception of a response message (MSGB) from the LTM candidate cell. When the LTM candidate gNB200C (LTM candidate cell) of the embodiment receives MSGA from UE100 and is able to obtain the C-RNTI in the PUSH payload, it may determine that the CBRA is access for early TA and may not need to send a response message (MSGB) to UE100.

[0086] When performing CBRA, UE100 generally needs to monitor contention resolution messages from LTM candidate cells, which lengthens the time during which communication is interrupted between UE100 and the serving gNB200S. In an embodiment, UE100 does not need to monitor contention resolution messages from LTM candidate cells, so the time during which communication is interrupted between UE100 and the serving gNB200S can be shortened.

[0087] In addition, in an embodiment, the UE 100 may receive a PDCCH order instructing CBRA preamble transmission from the serving gNB 200S (serving cell). In response to receiving the PDCCH order, the UE 100 may transmit MSGA to the LTM candidate cell.

[0088] FIG. 9 is a diagram showing an example of the operation of the mobile communication system 1 according to the embodiment.

[0089] In step S61, UE100 in an RRC connected state in the serving cell of the serving gNB200S transmits an L3 measurement report to the serving gNB200S. The serving gNB200S that receives the L3 measurement report decides to use LTM based on the L3 measurement report.

[0090] In step S62, the serving gNB200S transmits a HANDOVER REQUEST message to the LTM candidate gNB200C that manages the LTM candidate cell to request preparation for LTM cell switching. In an embodiment, the LTM candidate gNB200C that receives the HANDOVER REQUEST message may not allocate dedicated CFRA resources in the LTM candidate cell for the UE100.

[0091] In step S63, the LTM candidate gNB200C transmits a HANDOVER REQUEST ACKNOWLEDGE message including an LTM candidate cell configuration to the serving gNB200S. The LTM candidate cell configuration includes resource configuration for CBRA and a C-RNTI that the UE100 should use for communication with the LTM candidate cell. The LTM candidate gNB200C may set a radio quality condition (e.g., a radio quality threshold) under which the UE100 starts CBRA in the LTM candidate cell configuration. The LTM candidate gNB200C may also include information (e.g., a flag) that allows early TA by CBRA (or two-step RA) to be performed in the LTM candidate cell configuration.

[0092] In step S64, the serving gNB200S sends an RRC Reconfiguration message to the UE100, which includes the LTM candidate cell setting from the LTM candidate gNB200C as the LTM setting.

[0093] In step S65, the UE 100 that has received the RRC Reconfiguration message saves the LTM setting and transmits an RRC Reconfiguration Complete message to the serving gNB 200S. Using this message, the UE 100 may notify the serving gNB 200S that early TA by CBRA has been configured.

[0094] In step S66, the serving gNB200S may decide to have the UE100 perform early synchronization to the LTM candidate cell, for example, based on an L1 measurement report from the UE100, and may transmit a PDCCH order to the UE100 instructing the UE100 to transmit a CBRA RA preamble to the LTM candidate cell. The PDCCH order may indicate that it is a CBRA PDCCH order in a reserved bit in the DCI (for example, "1" is set if it is a CBRA PDCCH order, and "0" is set otherwise).

[0095] Alternatively, the UE 100 may trigger the CBRA to the LTM candidate cell in response to the radio quality condition set in the RRC Reconfiguration message being satisfied. For example, the UE 100 may measure the radio quality of the reference signal of the LTM candidate cell, and when the radio quality exceeds a threshold, may trigger the CBRA to the LTM candidate cell.

[0096] Alternatively, the UE 100 may trigger the CBRA at its own discretion. For example, based on the results of radio quality measurements, the UE 100 may estimate an appropriate timing for early TA before the timing of LTM cell switching execution, and trigger the CBRA at the estimated timing. The estimation may be inference using a machine learning model. Note that the UE 100 may notify the serving gNB 200S that it will transmit the CBRA to the LTM candidate gNB 200C when the radio quality condition is satisfied or when the UE 100 triggers the CBRA at its own discretion. The notification may be performed by an RRC message or by a MAC CE. This notification allows the serving gNB 200S to appropriately make a decision to temporarily suspend communication with the UE 100.

[0097] In step S67, the UE 100 transmits the RA preamble and PUSCH payload of the CBRA on the PRACH to the LTM candidate gNB 200C. Here, the UE 100 notifies the LTM candidate gNB 200C of the C-RNTI by including the C-RNTI in the LTM candidate cell setting (Var LTM-Config) corresponding to the LTM candidate cell in the PUSCH payload. Note that in the early TA CBRA, it may be assumed that the PRACH and PUSCH are not repeatedly transmitted. That is, the UE 100 may transmit the PRACH and PUSCH payload only once. The LTM candidate gNB 200C receives the PRACH and PUSCH payload. After UE100 sends MSGA to LTM candidate gNB200C (LTM candidate cell), it resumes communication with serving gNB200S (serving cell) without waiting to receive a response message (MSGB) from the LTM candidate cell.

[0098] If the LTM candidate gNB200C can receive the PRACH and PUSCH payload without collision, it can decode and decrypt the C-RNTI in the PUSCH payload. After deriving the TA value based on the PRACH (RA preamble), the LTM candidate gNB200C associates the C-RNTI with the TA value and stores it. As described above, the LTM candidate gNB200C does not send a contention resolution message to the UE100 (contention resolution is not required).

[0099] On the other hand, if the LTM candidate gNB200C is unable to receive the PRACH and PUSCH payload due to collision, it cannot decode the C-RNTI in the PUSCH payload. In this case, in step S68, the LTM candidate gNB200C may transmit a dedicated RACH setting (RACH-Config Dedicated) for CFRA to the serving gNB200S. In step S69, the serving gNB200S may transmit a PDCCH order including information in the dedicated RACH setting (RACH-Config Dedicated) to the UE100. As a result, if a collision occurs in CBRA, it is possible to switch to CFRA. Alternatively, in step S68, the LTM candidate gNB200C may notify (request) the serving gNB200S that the UE100 should perform CBRA again. In this case, in step S69, the serving gNB200S re-triggers CBRA using the reserved bit of the PDCCH order. This operation is particularly effective when CFRA resources are depleted when a collision occurs in CBRA.

[0100] In step S70, the LTM candidate gNB200C that has acquired the C-RNTI and the TA value transmits a message including the identification information of the UE100 and the TA value to the serving gNB200S. The message may be a notification message (Early Sync Complete) indicating that UL early synchronization has been completed. The identification information in the message may be the identifier of the UE100 on the Xn interface (e.g., UE XnAP ID). The serving gNB200S stores the TA value notified in the message in association with the identification information of the UE100.

[0101] In step S72, the serving gNB200S determines an LTM cell switch to the LTM candidate cell, for example, based on the L1 measurement report from the UE100 (step S71), and transmits a cell switch command MAC CE instructing the UE100 to switch the LTM cell to the LTM candidate cell. The cell switch command MAC CE includes an index (candidate setting index) corresponding to the LTM candidate cell and a TA value obtained by UL early synchronization (early TA). Since the cell switch command MAC CE includes a valid TA value, the UE100 determines to skip the RA procedure to the LTM candidate cell (RACH-less LTM cell switch).

[0102] In step S73, the UE 100 applies the TA value to the LTM candidate cell (LTM candidate gNB 200C) and transmits an RRC Reconfiguration Complete message. This completes the LTM cell switching successfully.

[0103] (4) Other embodiments The operation of the inter-CU LTM according to the above-described embodiments may be applied to intra-CU LTM. In this case, the serving gNB200S and the LTM candidate gNB200C in FIG. 9 become one gNB200, and the one gNB200 manages the first cell and the second cell.

[0104] The LTM in the above-described embodiments may be interpreted as a conditional LTM. For example, the above-described MCG LTM of an inter-CU or intra-CU may be a conditional LTM of the MCG of an inter-CU or intra-CU. Furthermore, the above-described SCG LTM of an intra-CU may be a conditional LTM of the SCG of an intra-CU. In conditional LTM, for example, the RRC Reconfiguration message in step S4 of FIG. 6 includes information indicating the execution conditions (e.g., radio quality conditions) of LTM cell switching for each LTM candidate cell. The UE 100 performs LTM cell switching to an LTM candidate cell that satisfies the preset execution conditions (radio quality conditions) instead of the cell switching command MAC transmitted from the gNB 200. This eliminates the need to transmit and receive an L1 measurement report and a cell switching command MAC CE, enabling faster LTM cell switching.

[0105] 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.

[0106] 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.

[0107] 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.

[0108] 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).

[0109] The functions performed by the above-described communication device (such as the UE 100 or the gNB 200) may be implemented in circuitry or processing circuitry, including general-purpose processors, application-specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), a CPU (Central Processing Unit), conventional circuits, and / or combinations thereof, programmed to perform the described functions. A processor includes transistors and / or other circuits and is considered to be circuitry or processing circuitry. A processor may be a programmed processor that executes a program stored in a memory. In this specification, a circuitry, unit, or means is hardware that is programmed to realize or executes a described function. The hardware may be any hardware disclosed herein or any hardware known to be programmed to realize or execute the described function. If the hardware is a processor, which is considered to be a type of circuitry, the circuitry, means, or unit is a combination of hardware and software used to configure the hardware and / or processor.

[0110] As used in this disclosure, the terms "based on" and "depending on / in response to" do not mean "based only on" or "depending only on," unless expressly stated otherwise. The term "based on" means both "based only on" and "based at least in part on." Similarly, the term "depending on" means both "depending only on" and "depending at least in part on." The terms "include," "comprise," and variations thereof do not mean including only the listed items, but may mean including only the listed items or may include additional items in addition to the listed items. Additionally, as used in this disclosure, the term "or" is not intended to mean an exclusive or. Furthermore, as used in this disclosure, any reference to an element using a designation such as "first," "second," etc. does not generally limit the quantity or order of those elements. These designations may be used herein as a convenient method of distinguishing between two or more elements. Thus, a reference to a first and a second element does not imply that only two elements may be employed therein, or that the first element must precede the second element in some way. In this disclosure, where articles are added by translation, such as a, an, and the in English, these articles are intended to include the plural unless the context clearly indicates otherwise.

[0111] 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.

[0112] This application claims priority from Japanese Patent Application No. 2024-132772 (filed August 8, 2024), the entire contents of which are incorporated herein by reference.

[0113] (5) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0114] Supplementary Note 1: A communication method for switching a serving cell of a user equipment from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the communication method comprising: the user equipment receiving, from the first cell, a Cell Radio Network Temporary Identifier (C-RNTI) assigned to the user equipment for the second cell; and the user equipment transmitting, to the second cell, a contention-based random access (CBRA) preamble used for uplink early synchronization in the LTM and the C-RNTI by a message A (MSGA) in a two-step RA procedure.

[0115] Supplementary Note 2: The communication method according to Supplementary Note 1, further comprising: a network node managing the second cell receiving the MSGA from the user equipment; the network node deriving a Timing Advance (TA) value based on the preamble; and the network node associating the TA value with the C-RNTI.

[0116] Supplementary Note 3: The communication method of Supplementary Note 1 or 2, further comprising the user equipment receiving, from the first cell, a cell switch command including a timing advance (TA) value derived based on the preamble, the cell switch command indicating an LTM cell switch to the second cell.

[0117] Supplementary Note 4: The communication method according to any one of Supplementary Notes 1 to 3, further comprising receiving a Physical Downlink Control Channel (PDCCH) order from the first cell instructing transmission of a preamble of the CBRA, wherein the user equipment transmits the MSGA to the second cell in response to receiving the PDCCH order.

[0118] Supplementary Note 5: The communication method according to any one of Supplementary Notes 1 to 4, further comprising the user equipment resuming communication with the first cell after transmitting the MSGA without waiting for receipt of a response message from the second cell.

[0119] Supplementary Note 6: A user equipment that switches a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), comprising: a receiver that receives, from the first cell, a Cell Radio Network Temporary Identifier (C-RNTI) assigned to the user equipment for the second cell; and a transmitter that transmits, to the second cell, a preamble of a Contention Based Random Access (CBRA) used for uplink early synchronization in the LTM and the C-RNTI by a Message A (MSGA) in a two-step RA procedure.

[0120] 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 200S: Serving 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 second network node transmitting, to the first network node, a HANDOVER REQUEST ACKNOWLEDGE message including an LTM candidate cell configuration for the second cell and a configuration for uplink early synchronization; and the second network node transmitting, to the first network node, a message including a TA value obtained by the uplink early synchronization.

2. A communication method for switching a serving cell of a user equipment from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), the communication method comprising: the user equipment receiving, from the first cell, a Cell Radio Network Temporary Identifier (C-RNTI) assigned to the user equipment for the second cell; and the user equipment transmitting, to the second cell, a contention-based random access (CBRA) preamble used for uplink early synchronization in the LTM and the C-RNTI by a message A (MSGA) in a two-step RA procedure.

3. The communication method according to claim 2, further comprising: a network node managing the second cell receiving the MSGA from the user equipment; the network node deriving a timing advance (TA) value based on the preamble; and the network node associating the TA value with the C-RNTI.

4. The communication method of claim 2, further comprising the user equipment receiving from the first cell a cell switch command including a timing advance (TA) value derived based on the preamble and instructing an LTM cell switch to the second cell.

5. The communication method of claim 2, further comprising receiving a Physical Downlink Control Channel (PDCCH) order from the first cell instructing transmission of a preamble of the CBRA, wherein the user equipment transmits the MSGA to the second cell in response to receiving the PDCCH order.

6. The communication method of claim 2, further comprising the user equipment resuming communication with the first cell after transmitting the MSGA without waiting for receipt of a response message from the second cell.

7. A user equipment that switches a serving cell from a first cell to a second cell by LTM (L1 / L2 Triggered Mobility), comprising: a receiving unit that receives, from the first cell, a Cell Radio Network Temporary Identifier (C-RNTI) assigned to the user equipment for the second cell; and a transmitting unit that transmits, to the second cell, a contention-based random access (CBRA) preamble used for uplink early synchronization in the LTM and the C-RNTI by a message A (MSGA) in a two-step RA procedure.

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