Communication method and network node

Abstract

According to the present invention, a communication method for performing serving cell switching for switching a serving cell of a user device from a first cell to a second cell includes: receiving, by a second network node managing the second cell, an uplink reference signal transmitted by the user device to a first network node managing the first cell; generating, by the second network node, timing error information on an error of the reception timing of the uplink reference signal with respect to the frame timing of the second cell on the basis of the uplink reference signal; and transmitting the timing error information to the first network node by the second network node.

Description

Communication method and network node

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

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

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

[0004] In 3GPP Release 18, LTM is limited to serving cell switching between cells belonging to the same network node and does not support serving cell switching between cells belonging to different network nodes (i.e., inter-network node LTM).

[0005] 3GPP contribution: R2-2309335

[0006] The present disclosure relates to a communication method and a network node for realizing inter-network node LTM using UE-based timing advance (TA) measurements that can obtain TA values ​​without a random access (RA) procedure.

[0007] A communication method according to a first aspect is a communication method for performing serving cell switching, which switches a serving cell of a user equipment from a first cell to a second cell, and includes: a second network node managing a second cell receiving an uplink reference signal transmitted by the user equipment to a first network node managing the first cell; the second network node generating timing error information relating to an error in reception timing of the uplink reference signal relative to frame timing of the second cell based on the uplink reference signal; and the second network node transmitting the timing error information to the first network node.

[0008] A network node according to a second aspect is a network node that manages a second cell in a mobile communication system that performs serving cell switching to switch the serving cell of a user equipment from a first cell to a second cell, and includes: a receiving unit that receives an uplink reference signal transmitted by the user equipment to another network node that manages the first cell; a control unit that generates timing error information relating to an error in the reception timing of the uplink reference signal relative to the frame timing of the second cell based on the uplink reference signal; and a transmitting unit that transmits the timing error information to the other network node.

[0009] A network node according to a third aspect is a network node that manages a first cell in a mobile communication system that performs serving cell switching to switch the serving cell of a user equipment from a first cell to a second cell, and has a receiving unit that, in response to the uplink reference signal transmitted by the user equipment to the network node being received by another network node that manages the second cell, receives timing error information relating to an error in the reception timing of the uplink reference signal relative to the frame timing of the second cell from the other network node that manages 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 an LTM procedure whose specifications are being developed in 3GPP Release 18. FIG. 7 is a diagram for explaining an operation scenario of a mobile communication system according to an embodiment. FIG. 8 is a diagram illustrating an example of a basic operation of LTM between network nodes according to an embodiment. FIG. 9 is a diagram for explaining an overview of UE-based TA measurement according to an embodiment. FIG. 10 is a diagram illustrating the operation of a gNB according to an embodiment. FIG. 11 is a diagram illustrating an example of a first operation pattern according to an embodiment. FIG. 12 is a diagram illustrating an example of a second operation pattern according to an embodiment.

[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 an 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 that is 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 a 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).

[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 illustrating an example configuration of a UE 100 (user equipment) according to an embodiment. The UE 100 includes a receiving unit 110, a transmitting unit 120, and a control unit 130. The receiving unit 110 and the transmitting unit 120 configure 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 an 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 the successfully decoded DCI as DCI addressed to the UE. The DCI transmitted from gNB200 has a CRC parity bit scrambled by the RNTI added.

[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 according to the embodiment supports LTM (L1 / L2-triggered mobility).

[0039] LTM is a technology for shortening mobility delay (specifically, serving cell switching delay) compared to a general handover procedure by triggering serving cell switching by signaling of a lower layer, Layer 1 (L1) and / or Layer 2 (L2). In a general handover procedure, a measurement report (Measurement Report) message, which is an RRC message, is transmitted from the UE 100 to the gNB 200, and the gNB 200 determines handover of the UE 100 based on the Measurement Report message. The handover is instructed by transmitting a handover command (specifically, an RRC Reconfiguration message) which is an RRC message from the gNB 200 to the UE 100.

[0040] In contrast, in LTM, first, the gNB 200 prepares an LTM candidate cell configuration for a candidate cell to be switched to and provides the LTM candidate cell configuration to the UE 100 by RRC signaling. Second, the UE 100 performs synchronization processing with the candidate cell by early synchronization (Early sync). Third, the gNB 200 receives an L1 measurement report from the UE 100, determines a serving cell switch to a target cell based on the L1 measurement report, and transmits a cell switch command (Cell Switch Command) indicating the target cell (LTM candidate cell configuration) to the UE 100 by MAC CE. The serving cell switch trigger is conveyed in a MAC CE that includes at least a candidate configuration index together with a beam indicator. Fourth, the UE 100 changes the serving cell in response to the cell switch command from the gNB 200 (source cell). 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.

[0041] The following principles apply to LTM:

[0042] Each LTM candidate cell configuration can be provided as a differential configuration (delta configuration) relative to the reference configuration used to form the complete candidate cell configuration.

[0043] If a full candidate cell configuration is applied, it replaces the current UE configuration upon a serving cell switch. The reconfiguration procedure does the replacement but does not necessarily reset the MAC, RLC or PDCP layers.

[0044] The user plane continues without a reset if configured in RRC signaling to avoid additional delays in data recovery.

[0045] - Security is not updated in LTM.

[0046] 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.

[0047] 6 is a diagram showing an example of an LTM procedure being specified in 3GPP Release 18. In the illustrated example, UE 100 performs serving cell switching from a first cell of gNB 200 to a second cell. 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 a "candidate cell (or LTM candidate cell)" until the serving cell switching by LTM is determined, and after the serving cell switching 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".

[0048] In step S1, UE100 is in an RRC connected state in the cell (first cell, source cell) of gNB200.

[0049] In step S2, UE100 transmits a Measurement Report message, which is an RRC message, to gNB200.

[0050] In step S3, gNB200 decides to use LTM based on the Measurement Report message and starts preparing the candidate cell.

[0051] In step S4, gNB200 sends an RRC Reconfiguration message to UE100, which includes LTM candidate cell configurations (LTM Candidate Configurations) for one or more candidate cells.

[0052] In step S5, UE100 saves the LTM candidate cell setting and sends an RRC Reconfiguration Complete message to gNB200.

[0053] In step S6, the UE 100 may perform synchronization processing with the candidate cell before receiving the cell switch command. Such synchronization processing is called early synchronization (Early sync). Here, the UE 100 may perform early timing advance (TA) acquisition in the candidate cell requested by the gNB 200 (source cell) before receiving the cell switch command of step S9. This is performed by contention-free random access (CFRA) triggered by a PDCCH command (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. The UE 100 transmits a random access preamble (RA preamble) to the specified candidate cell. In order to minimize communication interruption of the source cell due to CFRA for the 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 candidate cell. The TA value of the candidate cell (target cell) is indicated in the cell switching command in step S9. The TA value is a value for adjusting the uplink transmission timing of the UE 100.

[0054] In step S7, the UE 100 performs layer 1 (L1) measurement in the configured candidate cell and transmits a physical layer measurement report (L1 Measurement Report) to the gNB 200. 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).

[0055] In step S8, gNB200 decides to switch the serving cell to the target cell (second cell).

[0056] In step S9, gNB200 transmits a Cell Switch Command (MAC CE) including a candidate configuration index of the target cell to UE100. The Cell Switch Command may include a TA value obtained by early synchronization.

[0057] 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.

[0058] In step S11, if the serving cell switch needs to include the execution of a random access procedure (for example, if the Cell Switch Command does not include a valid TA value), the UE 100 executes the random access procedure for the target cell. Note that, if the UE 100 does not need to acquire the TA of the target cell at the time of the serving cell switch (for example, if the Cell Switch Command includes a valid TA value), the random access procedure can be skipped.

[0059] 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.

[0060] (3) Basic Operation Example of Inter-Network Node LTM The LTM of 3GPP Release 18 as described above is limited to serving cell switching between cells belonging to the same gNB 200 (same CU). Therefore, serving cell switching between cells belonging to different gNBs 200 (different CUs) cannot be realized by LTM. Note that LTM between cells belonging to different gNBs 200 (different CUs) may be referred to as inter-network node LTM, specifically, inter-gNB LTM (inter-gNB LTM) or inter-CU LTM (inter-CU LTM). Here, the operation for realizing inter-network node LTM will be described.

[0061] 7 is a diagram for explaining an operation scenario of the mobile communication system 1 according to the embodiment. In the following description of the embodiment, the operation scenario shown in FIG.

[0062] UE100 performs serving cell switching from a first cell (source cell) of gNB200a, which is a source gNB, to a second cell of gNB200b. gNB200a is an example of a first network node, and gNB200b is an example of a second network node. The first cell is formed by a first TRP of gNB200a, and the second cell is formed by a second TRP of gNB200b. Until a serving cell switch by LTM is determined, the second cell is also referred to as a "candidate cell (or LTM candidate cell)." After a serving cell switch by LTM is determined, the second cell is also referred to as a "target cell." An Xn interface is established between gNB200a and gNB200b. Communication between gNB200a and gNB200b is assumed to be performed over the Xn interface.

[0063] In an embodiment, the gNB 200a performs wireless communication with the UE 100 in an RRC connected state in the first cell of the gNB 200a. The gNB 200a transmits to the gNB 200b a request message to request a change of the serving cell of the UE 100 from the first cell of the gNB 200a to the second cell of the gNB 200b. Here, when the serving cell change is performed by LTM, the gNB 200a transmits to the gNB 200b a request message indicating that the serving cell change is by LTM. This allows the gNB 200b to understand that a serving cell change by LTM is requested rather than a general handover, based on the request message from the gNB 200a.

[0064] The request message may be a handover request (HO Request) message that can be used in a handover procedure that instructs a handover from the gNB 200a to the UE 100 by an RRC message. The request message indicating that the serving cell is switched by LTM may be an HO Request message including an LTM indicator. This makes it possible to use the HO Request message used in general handovers in LTM between network nodes, making it easier to minimize changes to technical specifications.

[0065] Alternatively, the request message indicating a serving cell switch by LTM may be a request message for LTM that is different from the HO Request message. The request message for LTM may be a request message that is used exclusively for LTM.

[0066] The request message indicating the serving cell switch by LTM may include information indicating whether the gNB 200b needs to configure a contention-free random access (CFRA) resource. The CFRA resource is used in early synchronization performed by the UE 100 with respect to the second cell before the serving cell switch instruction by the MAC CE. This allows the gNB 200b to determine whether to configure the CFRA resource based on the request message.

[0067] The gNB 200a may send another request message to the gNB 200b requesting that the gNB 200b configure or activate CFRA resources for early synchronization, thereby allowing the gNB 200b to appropriately configure or activate CFRA resources for early synchronization.

[0068] The gNB 200a may transmit a PDCCH command (PDCCH order) instructing the UE 100 to perform CFRA (i.e., transmit an RA preamble), to the UE 100. The PDCCH order may include information for identifying the second cell. This makes it easy for the UE 100 to identify whether the target of the RA preamble transmission is the first cell or the second cell.

[0069] The gNB 200a may receive from the gNB 200b a notification indicating that the early synchronization performed by the UE 100 with respect to the second cell has been successful. This allows the gNB 200a to understand whether the early synchronization performed by the UE 100 with respect to the second cell has been successful.

[0070] Alternatively, gNB200a may receive a notification from UE100 indicating that UE100 has successfully performed early synchronization with the second cell.

[0071] Fig. 8 is a diagram showing an example of a basic operation of the LTM between network nodes according to the embodiment. In Fig. 8, steps that can be omitted are indicated by dashed lines. Note that, although a duplicated explanation of the operation explained in Fig. 6 will be omitted, the operation explained in Fig. 6 may be applied as appropriate.

[0072] In step S101, the UE 100 transmits an L3 (RRC) Measurement Report to the gNB 200a. The gNB 200a receives the L3 (RRC) Measurement Report.

[0073] In step S102, gNB200a determines to use inter-gNB LTM based on the L3 (RRC) Measurement Report in step S101 and starts preparing a candidate cell. Here, it is assumed that the second cell of gNB200b is determined as the candidate cell.

[0074] In step S103, the gNB 200a transmits a request message (LTM HO Request) indicating that the serving cell is switched by LTM to the gNB 200b. The gNB 200b receives the request message (LTM HO Request). The request message (LTM HO Request) includes an LTM indicator and may be a Handover Request message used in a general handover. Alternatively, the request message (LTM HO Request) may be a new message different from the Handover Request message, for example, an LTM Handover Request message. The request message (LTM HO Request) may include information indicating whether or not early synchronization configuration, i.e., configuration of CFRA resources for early synchronization is necessary (or may be information proposing configuration of early synchronization). Note that the request message (LTM HO Request) may include RRC configuration information of the UE 100 and a cell identifier indicating the second cell, as in a general handover.

[0075] In step S104, the gNB200b determines whether or not to accept the request of step S103 (Admission control). Here, the description will proceed assuming that the request of step S103 is accepted. In this case, the gNB200b may configure a CFRA resource for early synchronization in the second cell. Note that, if the request of step S103 is rejected, the gNB200b may send a rejection message to the gNB200a. The rejection message may include information indicating that inter-gNB LTM cannot be used.

[0076] In step S105, the gNB200b transmits an acknowledgement message (LTM HO Request Ack) indicating acceptance of the request of step S103 to the gNB200a. The gNB200a receives the acknowledgement message (LTM HO Request Ack). The acknowledgement message (LTM HO Request Ack) includes an LTM indicator and may be a Handover Request Ack message used in a general handover. Alternatively, the acknowledgement message (LTM HO Request Ack) may be a new message different from the Handover Request Ack message, for example, an LTM Handover Request Ack message. The acknowledgement message (LTM HO Request Ack) may include information indicating an early synchronization CFRA resource (e.g., an RA preamble and / or a PRACH (Physical Random Access Channel) resource) configured by the gNB200b for the second cell. Note that the acknowledgement message (LTM HO Request Ack) may include RRC reconfiguration information (RRC Reconfiguration) of the UE100 to be applied in the second cell, as in a general handover.

[0077] In step S106, the gNB 200a transmits an RRC Reconfiguration message including an LTM candidate cell configuration (LTM Candidate Configuration) of the second cell to the UE 100. The UE 100 receives the RRC Reconfiguration message. The RRC Reconfiguration message may include information indicating the early synchronization CFRA resource configured by the gNB 200b for the second cell.

[0078] In step S107, the UE 100 saves the LTM candidate cell setting and transmits an RRC Reconfiguration Complete message to the gNB 200a. The gNB 200a receives the RRC Reconfiguration Complete message.

[0079] In step S108, the UE 100 may transmit an L1 measurement report (or an L3 measurement report) to the gNB 200a for the gNB 200a to determine early synchronization. The gNB 200a may receive the L1 measurement report (or the L3 measurement report).

[0080] In step S109, gNB200a may make a decision to perform early synchronization.

[0081] In step S110, the gNB 200a may transmit an Early sync CFRA Request message to the gNB 200b, which is a request message requesting preparation of a CFRA resource for early synchronization, specifically, configuration and / or activation (validation) of a CFRA resource for early synchronization. The gNB 200b may receive the request message (Early sync CFRA Request message). The request message (Early sync CFRA Request message) may include an identifier (Xn-AP UE ID) for identifying the UE 100 and / or an identifier (cell ID) for identifying the second cell.

[0082] In step S111, gNB200b may prepare CFRA resources for early synchronization.

[0083] In step S112, the gNB 200b may transmit a notification message indicating that the CFRA resource for early synchronization has been prepared, for example, an Early sync CFRA Request Ack message, to the gNB 200a. The gNB 200a may receive the notification message (Early sync CFRA Request Ack message).

[0084] In step S113, the gNB 200a transmits a PDCCH order to the UE 100 and instructs the UE 100 to perform CFRA for early synchronization. The UE 100 receives the PDCCH order. The PDCCH order may include information (Target cell indicator) for identifying the second cell as a target of the CFRA. The information may be the cell ID (or cell index) of the second cell. The information may be an index in the list of LTM candidate cell settings in step S106. The information may be information (index) that specifies the TRP corresponding to the second cell.

[0085] In step S114, the UE 100 may perform early synchronization of the downlink (DL) with the second cell. For example, the UE 100 performs timing synchronization using the SSB (PSS / SSS) of the second cell. Note that the UE 100 may have performed DL synchronization before this point.

[0086] In step S115, the UE 100 transmits a CFRA, specifically, an RA preamble on a PRACH, to the second cell specified in the PDCCH order in order to perform early synchronization of the uplink (UL) with the second cell. The gNB 200b receives the RA preamble. The UE 100 identifies the CFRA resource (for example, an RA preamble and / or a PRACH resource) based on information set in the SIB or the like and information such as a "Random Access Preamble index" and a "PRACH Mask Index" in the PDCCH order.

[0087] In step S116, the gNB 200b may transmit an RAR including a TA value derived based on the RA preamble to the UE 100. The UE 100 may receive the RAR. Step S116 may be an optional step that is executed only if there is a setting from the gNB 200a (e.g., the setting in step S106). The UE 100 may transmit a notification (Early Sync Complete) indicating that UL early synchronization with the second cell has been completed to the gNB 200a (step S117). The notification (Early Sync Complete) may include the TA value notified in the RAR.

[0088] In step S118, the gNB 200b may transmit to the gNB 200a a notification message (Early Sync Complete) indicating that UL early synchronization with the UE 100 has been completed. The gNB 200a may receive the notification message (Early Sync Complete). The notification message (Early Sync Complete) may include the TA value derived based on the RA preamble of step S115.

[0089] In step S119, the UE 100 transmits an L1 measurement report to the gNB 200a. The gNB 200a receives the L1 measurement report.

[0090] In step S120, when gNB200a determines that the possibility of LTM execution has increased, for example, based on the L1 measurement report of step S119, it may transmit a UL resource request message to gNB200b. gNB200b may receive the request message. The UL resource request may be a request for preparation or activation of CFRA resources. The UL resource request may be a request for preparation or execution of UL grant transmission to UE100. The UL resource request may be a request for preparation or activation of UL configured grant (CG) resources. Note that the transmission of the request message in step S120 may be simultaneous with the LTM execution decision in step S121. The transmission may be after the LTM execution decision in step S121.

[0091] In step S121, gNB200a decides to perform LTM based on the L1 measurement report of step S119.

[0092] In step S122, in response to the LTM execution decision, the gNB 200a transmits a Cell switch command (MAC CE) to the UE 100. The UE 100 receives the Cell switch command. The Cell switch command may include the TA value notified to the gNB 200a in step S117 or S118.

[0093] In step S123, in response to the reception of the Cell switch command, the UE 100 detaches from the first cell (source cell) and applies the LTM candidate cell setting of the second cell (target cell).

[0094] In step S124, if the Cell switch command does not include a TA value (a valid TA value), the UE 100 may perform a random access procedure for the second cell.

[0095] In step S125, the UE 100 transmits an RRC Reconfiguration Complete message to the second cell. The gNB 200b receives the RRC Reconfiguration Complete message.

[0096] In step S126, the gNB 200b transmits DCI including a CRC (Cyclic Redundancy Code) scrambled with the C-RNTI assigned to the UE 100 to the UE 100 on the PDCCH, and may transmit a Contention Resolution MAC CE to the UE 100 on the PDSCH assigned by the DCI. The UE 100 may receive the DCI and the Contention Resolution MAC CE.

[0097] In step S127, gNB200b may transmit a notification message (LTM HO Success) to gNB200a indicating that the inter-network node LTM to the second cell has been completed. gNB200a may receive the notification message (LTM HO Success).

[0098] (4) Inter-Network Node LTM Using UE-Based TA Measurement In the above-described basic operation example of the inter-network node LTM, the UE 100 has realized UL early synchronization with the second cell by using an RA procedure (specifically, CFRA). In the following embodiment, an operation for realizing UL early synchronization by using UE-based TA measurement that can acquire a TA value without an RA procedure in the inter-network node LTM will be described.

[0099] (4.1) Overview of UE-based TA Measurement FIG. 9 is a diagram for explaining an overview of UE-based TA measurement according to an embodiment. In the illustrated example, it is assumed that the frame timing between a first cell (source cell) and a second cell (candidate cell, target cell) is asynchronous, and the frame timing difference between the first cell and the second cell is also referred to as "Tdiff_s-t_nw." In the illustrated example, "Tdiff_s-t_nw" is the time from time t1 to time t3. Note that if the first cell and the second cell are perfectly synchronized, the frame timing difference "Tdiff_s-t_nw" is zero. As a synchronization method, for example, synchronization is achieved using GNSS (Global Navigation Satellite System) and / or IEEE 1588.

[0100] The UE-based TA measurement includes, for example, the following procedures.

[0101] STEP 1: UE100 is in an RRC connected state in the first cell and knows the TA value being applied in the first cell. The TA value being applied to UE100 in the first cell is also referred to as "TA_s." "TA_s" is the time by which the UL frame timing precedes the DL frame timing in UE100. In the illustrated example, the TA value "TA_s" being applied in the first cell is the time from time t6 to time t7. Note that the TA value is used to control the UL transmission timing of each UE100 so that UL transmissions from all UE100 are synchronized when received by the serving cell (gNB200). UE100 closer to the TRP of the cell has a short propagation delay, so the TA value is small. On the other hand, UE100 farther from the TRP of the cell has a long propagation delay, so the TA value is large. Generally, the TA value "TA_s" is set to the UE 100 by the serving cell (gNB 200) in an RA response during the RA procedure, and then adjusted by a TA command (MAC CE) transmitted from the serving cell (gNB 200) to the UE 100. Therefore, the serving cell (gNB 200) also knows the TA value "TA_s".

[0102] STEP 2: The UE 100 performs RSTD (Reference Signal Timing Difference) measurement and generates reference signal time difference information "Tdiff_s-t_ue" relating to the reception timing difference between the DL reference signal of the first cell and the DL reference signal of the second cell. The RSTD measurement measures the reception timing difference between the DL reference signal of the first cell and the DL reference signal of the second cell, and determines the timing difference "Tdiff_s-t_ue" between the DL radio frames of the first cell and the second cell at the receiving end of the UE 100. In the illustrated example, the timing difference "Tdiff_s-t_ue" between the DL radio frames of the first cell and the second cell is the time from time t3 to time t4.

[0103] STEP 3: The second cell (gNB200) receives an UL reference signal (e.g., SRS (Sounding Reference Signal)) transmitted from UE100 to the first cell, and generates timing error information "TA_temp_t" regarding the error in the reception timing of the UL reference signal relative to the frame timing of the second cell. Here, "TA_s" is applied to the UL reference signal transmitted from UE100 to the first cell. The second cell (gNB200) grasps the reception error "TA_temp_t" between the second cell's own UL radio frame and the UL reference signal from UE100. In the illustrated example, the reception timing error "TA_temp_t" of the UL reference signal relative to the frame timing of the second cell is the time from time t1 to time t5. Note that STEP 3 may be performed before STEP 2 or may be performed simultaneously with STEP 2.

[0104] STEP 4: From "TA_s", "Tdiff_s-t_ue", and "TA_temp_t", a TA value "TA_t" to be applied by UE 100 in the second cell is calculated by the following equation (1): TA_t = (TA_temp_t + TA_s) - Tdiff_s-t_ue (1) However, the TA value "TA_t" to be applied by UE 100 in the second cell may also be calculated by the following equation (2) that further takes into account the inter-cell synchronization error "Tdiff_s-t_nw": TA_t = (TA_temp_t + TA_s) - (Tdiff_s-t_ue + Tdiif_s-t_nw) (2) Alternatively, "Tdiff_s-t_nw" may be used only for the second cell to receive the UL reference signal from the UE 100, i.e., to calculate "TA_temp_t".

[0105] Therefore, by aggregating parameters (variables) such as "TA_s", "Tdiff_s-t_ue", and "TA_temp_t" in UE100 or gNB200 and calculating equation (1), the TA value "TA_t" to be applied by UE100 in the second cell can be calculated without UE100 performing an RA procedure to the second cell. "TA_s", "Tdiff_s-t_ue", "TA_temp_t", and "Tdiff_s-t_nw" can also be aggregated in UE100 or gNB200 and calculated according to equation (2) to calculate "TA_t".

[0106] (4.2) Overview of Inter-Network Node LTM Using UE-Based TA Measurement In the case of inter-gNB LTM as shown in Figures 7 and 8, signaling is required to aggregate "TA_s", "Tdiff_s-t_ue", and "TA_temp_t" as shown in Figure 9, but there is a problem that such signaling is pending. The following two operation patterns can be considered as a method of aggregating these parameters.

[0107] First operation pattern: Aggregation to UE 100 This is rational because it is UE 100 that ultimately applies the TA value.

[0108] - Second operation pattern: Aggregate to the first cell (gNB200) This is reasonable because it is the first cell that ultimately sends the Cell switch command to UE100.

[0109] In the case of inter-gNB LTM, in both the first operation pattern and the second operation pattern, it is considered necessary for the second cell (gNB200b) to measure "TA_temp_t" and for the second cell (gNB200b) to notify "TA_temp_t" to the first cell (gNB200a).

[0110] Figure 10 is a diagram showing the operation of gNB200b according to the embodiment.

[0111] In step S11, the gNB200b managing the second cell (candidate cell, target cell) may receive reference signal configuration information indicating the UL reference signal configuration set in the UE100 from the gNB200a managing the first cell (source cell, current serving cell). Here, the gNB200b may receive the reference signal configuration information from the gNB200a over the Xn interface. The gNB200b may receive the UL reference signal from the UE100 based on the reference signal configuration information.

[0112] In step S12, gNB200b receives the UL reference signal transmitted by UE100 to the first cell (gNB200a). That is, gNB200b intercepts the UL reference signal transmitted by UE100 for the first cell.

[0113] In step S13, gNB200b generates timing error information "TA_temp_t" regarding the error in the reception timing of the UL reference signal relative to the frame timing of the second cell based on the UL reference signal received in step S12.

[0114] In step S14, gNB200b transmits the timing error information "TA_temp_t" generated in step S13 to gNB200a. Here, gNB200b may transmit the timing error information "TA_temp_t" to gNB200a over the Xn interface.

[0115] According to this operation, it is possible to realize LTM between network nodes using UE-based TA measurement that can acquire a TA value without an RA procedure. The gNB200b that performs this operation has a receiver 220 that receives an UL reference signal transmitted by the UE100 to another network node (gNB200a) that manages the first cell, a controller 230 that generates timing error information "TA_temp_t" regarding an error in the reception timing of the UL reference signal relative to the frame timing of the second cell based on the UL reference signal, and a transmitter 241 that transmits the timing error information "TA_temp_t" to the other network node (gNB200a). On the other hand, gNB200a has a receiving unit 242 that receives timing error information "TA_temp_t" regarding the error in the reception timing of the UL reference signal relative to the frame timing of the second cell from another network node (gNB200b) that manages the second cell in response to the other network node (gNB200b) receiving the UL reference signal transmitted by UE100 to gNB200a.

[0116] The receiver 242 of the gNB 200b may receive frame timing information "Tdiff_s-t_nw" regarding the frame timing difference between the first cell and the second cell from the gNB 200a. The receiver 220 of the gNB 200b may receive the UL reference signal from the UE 100 based on the frame timing information "Tdiff_s-t_nw". This makes it easier to receive the UL reference signal from the UE 100.

[0117] In the first operation pattern, the receiving unit 242 of gNB200a receives timing error information "TA_temp_t" from gNB200b. The transmitting unit 210 of gNB200a transmits the timing error information "TA_temp_t" to UE100. This allows UE100 to appropriately calculate the TA value "TA_t" that UE100 should apply in the second cell using the timing error information "TA_temp_t".

[0118] In the first operation pattern, the transmitter 210 of the gNB 200a may transmit frame timing information "Tdiff_s-t_nw" regarding the frame timing difference between the first cell and the second cell to the UE 100. This allows the UE 100 to calculate the TA value "TA_t" using equation (2).

[0119] In the second operation pattern, the receiver 242 of the gNB 200a receives timing error information "TA_temp_t" from the gNB 200b, and the receiver 220 of the gNB 200a receives reference signal time difference information "Tdiff_s-t_ue" (RSTD) relating to the reception timing difference between the DL reference signal of the first cell and the DL reference signal of the second cell from the UE 100. The control unit 230 of the gNB 200a determines the TA value "TA_t" that the UE 100 should apply to the second cell based on the TA value "TA_s" currently applied to the first cell that it grasps (manages), the timing error information "TA_temp_t", and the reference signal time difference information "Tdiff_s-t_ue" (RSTD) (for example, calculated using equation (1)). The control unit 230 of the gNB 200a may determine the TA value "TA_t" using equation (2) further based on "Tdiff_s-t_nw". The transmission unit 210 of the gNB 200a transmits the determined TA value "TA_t" to the UE 100. This allows the parameters to be aggregated in the gNB 200a, and the TA value "TA_t" to be notified to the UE 100 under the initiative of the gNB 200a.

[0120] Here, the transmitter 210 of the gNB 200a may transmit a medium access control element (MAC CE) including the determined TA value "TA_t" to the UE 100. The MAC CE may be a cell switching command MAC CE that instructs the serving cell to be switched from the first cell to the second cell. This allows the TA value "TA_t" determined by the gNB 200a to be efficiently notified to the UE 100.

[0121] In both the first operation pattern and the second operation pattern, the receiver 220 of the gNB 200b may receive the TA value "TA_t" applied by the UE 100 from the UE 100 that has accessed the second cell. This allows the gNB 200b to appropriately communicate with the UE 100 based on the TA value "TA_t" in the second cell after cell switching.

[0122] (4.3) Specific example of inter-network node LTM using UE-based TA measurement As a specific example of inter-network node LTM using UE-based TA measurement, a first operation pattern and a second operation pattern will be described. In the following first operation pattern and second operation pattern, duplicated explanations of the operations described in Figure 6 and the operations described in Figure 8 will be omitted, but the operations described in Figure 6 and the operations described in Figure 8 may be applied as appropriate. In the following first operation pattern and second operation pattern, it is assumed that inter-node communication between the first cell (gNB 200a) and the second cell (gNB 200b) is performed on the Xn interface.

[0123] (4.3.1) Example of the first operation pattern Figure 11 is a diagram showing an example of the first operation pattern according to the embodiment. The first operation pattern is a pattern in which information (parameters) is collected in the UE 100 and the UE 100 calculates the TA value "TA_t" of the second cell (gNB 200b). In the illustrated example, it is assumed that the first cell (gNB 200a) and the second cell (gNB 200b) know the DL frame timing error "Tdiff_s-t_nw" between the cells. However, when using equation (1) and / or when the first cell (gNB 200a) and the second cell (gNB 200b) are synchronized, it is not necessary to handle "Tdiff_s-t_nw".

[0124] In step S200, the first cell (gNB200a) may transmit a message including the DL frame timing error "Tdiff_s-t_nw" (Radio frame timing diff.) to the second cell (gNB200b). Alternatively, the second cell (gNB200b) may transmit a message including the DL frame timing error "Tdiff_s-t_nw" to the first cell (gNB200a). The message may be an Xn Handover Request message requesting handover of the UE100, or a gNB configuration update message notifying a gNB setting update. In addition, the first cell (gNB200a) and / or the second cell (gNB200b) may transmit RRC signaling including the DL frame timing error "Tdiff_s-t_nw" to the UE 100. The RRC signaling may be an RRC Reconfiguration message or a system information block (SIB).

[0125] In step S201, the UE 100 manages (updates) the TA value "TA_s" of the first cell (gNB 200a) based on signaling from the first cell (gNB 200a). For example, the TA value "TA_s" is set to the UE 100 from the first cell (gNB 200a) in an RA response during the RA procedure, and then adjusted by a TA command (MAC CE) transmitted from the first cell (gNB 200a) to the UE 100.

[0126] In step S202, the first cell (gNB200a) transmits (configures) an UL reference signal configuration (RS configuration) to the UE 100 so as to configure transmission of an UL reference signal to the UE 100. For example, the UL reference signal configuration (RS configuration) may be an SRS configuration (SRS config.).

[0127] In step S203, the first cell (gNB 200a) may transmit to the second cell (gNB 200b) a message including reference signal configuration information (RS config. info) indicating the UL reference signal configuration configured in the UE 100. The message may be an Xn Handover Request message or a gNB configuration update message.

[0128] Here, the reference signal configuration information (RS config. info) may include at least one of the parameters of SRS-config., which is an RRC information element (IE) configured in the UE 100. For example, the reference signal configuration information (RS config. info) includes at least one of the number of SRS ports, comb pattern information, time / frequency starting point (reference resource) information / repetition (periodic) information, RS transmission trigger type (periodic, semi-persistent, periodic), and sequence ID.

[0129] In step S204, the UE 100 transmits a UL reference signal (e.g., SRS) to the first cell (gNB 200a) based on the setting received in step S202. Here, the TA value "TA_s" of the first cell (gNB 200a) is applied to the transmission of the UL reference signal.

[0130] The second cell (gNB200b) receives (intercepts) the UL reference signal from the UE100 using reference signal configuration information (RS config. info) from the first cell (gNB200a). Alternatively, the second cell (gNB200b) may specify the UL reference signal configuration (RS configuration) to be set for the UE100 to the first cell (gNB200a), and may receive the UL reference signal from the UE100 using the specified UL reference signal configuration (RS configuration).

[0131] In step S205, the second cell (gNB200b) measures the UL reception timing error (error from the UL radio frame; "TA_temp_t") using the UL reference signal from UE100.

[0132] In step S206, the second cell (gNB200b) transmits a message including "TA_temp_t" (Temp TA value) as an RRC container (RRC Reconfiguration) to the first cell (gNB200a). The message may be an Xn Handover Request Ack message or a gNB configuration update message. The message (RRC container) may include at least one of "Tdiff_s-t_nw" (Radio frame timing diff.), RSTD measurement configuration (RSTD meas. config.), and UE-based TA measurement (UE-based TA meas. config.).

[0133] In step S207, the first cell (gNB 200a) transmits a message including the information received from the second cell (gNB 200b) in step S206 to the UE 100. The message may be an RRC Reconfiguration message. The RRC Reconfiguration message may be an RRC Reconfiguration with sync message including the RRC container. The RRC Reconfiguration message may include Conditional reconfiguration and / or LTM configuration including the information received from the second cell (gNB 200b) in step S206 as information elements.

[0134] The message of step S207 includes at least one of "TA_temp_t" (Temp TA value), "Tdiff_s-t_nw" (Radio frame timing diff.), RSTD measurement configuration (RSTD meas. config.), and UE-based TA measurement (UE-based TA meas. config.). This information may be linked to the cell ID of the second cell (gNB200b). For example, the information may be notified in each entry of the candidate cell configuration list in the LTM configuration. In addition, when the first cell and the second cell are synchronized, "Tdiff_s-t_nw" (Radio frame timing diff.) may notify "Tdiff_s-t_nw=0", or may not include the IE (NULL), or may be information indicating synchronization.

[0135] In step S208, the UE 100 performs RSTD measurement and generates "Tdiff_s-t_ue".

[0136] In step S209, the UE 100 calculates the TA value "TA_t" for the second cell (gNB 200b) using equation (1) or equation (2). When the UE 100 calculates the TA value "TA_t", it may start a TAT (Time Alignment Timer) that determines the expiration date of the TA value "TA_t". The UE 100 may store information (which may be a label) indicating that the calculated TA value "TA_t" is a TA value calculated by UE-based TA measurement.

[0137] In step S210, the first cell (gNB 200a) transmits a Cell switch command MAC CE to the UE 100, instructing cell switching to the second cell (gNB 200b). The MAC CE may not include the TA value "TA_t". The MAC CE may include an instruction to apply the UE-based TA measurement value (TA_t). In addition, instead of the first cell (gNB200a) instructing UE100 to switch cells to the second cell (gNB200b), the first cell (gNB200a) may set execution conditions for cell switching to the second cell (gNB200b) in UE100, and UE100 may voluntarily trigger cell switching to the second cell (gNB200b) when the execution conditions are met.

[0138] In step S211, UE100 applies the TA value (TA_t) calculated for the second cell (gNB200b) and performs UL transmission (PUSCH transmission: transmission of an RRC Reconfiguration Complete message) for the second cell (gNB200b).

[0139] The UE 100 may include the TA value (TA_t) in the message of step S211 or another message (for example, a UE Assistance Information message) and notify the second cell (gNB 200b). In the case where the TA value measured / calculated by the UE-based TA measurement is applied, and the TA value is applied, the TA value may be included in the message and notified to the second cell (gNB 200b). Alternatively, the UE 100 may notify the second cell (gNB 200b) of the currently applied TA value (not limited to the TA value of the UE-based TA measurement) in the message, including the TA value. By notifying the TA value, the second cell (gNB 200b) can know the TA value currently applied by the UE 100. As a result, the second cell (gNB200b) starts TA management of UE100.

[0140] In addition, if UE100 is unable to calculate the TA value (TA_t), it may perform an RA procedure for the second cell (gNB200b) and then perform UL transmission (PUSCH transmission: transmission of an RRC Reconfiguration Complete message) for the second cell (gNB200b).

[0141] (4.3.2) Example of Second Operation Pattern An example of the second operation pattern according to the embodiment will be described, focusing on the differences from the above-described first operation pattern (FIG. 11).

[0142] 12 is a diagram showing an example of a second operation pattern according to the embodiment. The second operation pattern is a pattern in which information (parameters) is collected in the first cell (gNB200a), and the first cell (gNB200a) calculates the TA value (TA_t) of the second cell (gNB200b). In the illustrated example, it is assumed that the first cell (gNB200a) and the second cell (gNB200b) know the DL frame timing error "Tdiff_s-t_nw" between the cells. However, when using equation (1), and / or when the first cell (gNB200a) and the second cell (gNB200b) are synchronized, it is not necessary to handle "Tdiff_s-t_nw".

[0143] Steps S301 to S305 are similar to steps S200 to S205 in FIG.

[0144] In step S306, the second cell (gNB200b) transmits a message including "TA_temp_t" (Temp TA value) to the first cell (gNB200a). The message may be an Xn Handover Request Ack message or a gNB configuration update message.

[0145] In step S307, the first cell (gNB 200a) transmits a message including an RSTD measurement configuration to the UE 100. The message may be an RRC Reconfiguration message. The RRC Reconfiguration message may include an LTM configuration, and the LTM configuration may include the RSTD measurement configuration.

[0146] In step S308, the UE 100 performs RSTD measurement and generates "Tdiff_s-t_ue".

[0147] In step S309, the UE 100 transmits a message including the RSTD measurement result "Tdiff_s-t_ue" to the first cell (gNB 200a). The message may be a measurement report (meas. report) message, which is an RRC message.

[0148] In step S310, the first cell (gNB200a) calculates the TA value "TA_t" for the second cell (gNB200b) using equation (1) or equation (2).

[0149] In step S311, the first cell (gNB200a) transmits a Cell switch command MAC CE including the calculated TA value "TA_t" to the UE100.

[0150] In step S312, the UE 100 applies the TA value (TA_t) calculated for the second cell (gNB 200b) and performs UL transmission (PUSCH transmission: transmission of an RRC Reconfiguration Complete message) to the second cell (gNB 200b). As in the first operation pattern, the UE 100 includes the TA value (TA_t) in the message of step S312 or another message (for example, a UE Assistance Information message) and notifies the second cell (gNB 200b).

[0151] (5) Other Embodiments In the above embodiment, an example has been described in which various parameters are signaled over the Xn interface by inter-node communication between the first cell (gNB 200a) and the second cell (gNB 200b). However, various parameters may be signaled over the F1 interface between the DU and CU.

[0152] 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.

[0153] 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.

[0154] That is, the UE 100 may be a terminal function unit (a type of communication module) for a base station to control a repeater that relays signals. Such a terminal function unit is referred to as an MT. Examples of the MT include, in addition to the IAB-MT, an NCR (Network Controlled Repeater)-MT and a RIS (Reconfigurable Intelligent Surface)-MT.

[0155] 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.

[0156] 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).

[0157] The functions performed by the UE 100 or the gNB 200 (network node) 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 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.

[0158] 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.

[0159] 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.

[0160] This application claims priority from Japanese Patent Application No. 2023-202058 (filed November 29, 2023), the entire contents of which are incorporated herein by reference.

[0161] (6) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0162] (Supplementary Note 1) A communication method for performing serving cell switching, which switches a serving cell of a user equipment from a first cell to a second cell, comprising: a second network node managing a second cell receiving an uplink reference signal transmitted by the user equipment to a first network node managing the first cell; the second network node generating timing error information relating to an error in reception timing of the uplink reference signal relative to frame timing of the second cell, based on the uplink reference signal; and the second network node transmitting the timing error information to the first network node.

[0163] (Supplementary Note 2) The communication method according to Supplementary Note 1, further comprising: the second network node receiving, from the first network node, reference signal configuration information indicating an uplink reference signal configuration configured in the user equipment; and the second network node receiving the uplink reference signal from the user equipment based on the reference signal configuration information.

[0164] (Supplementary Note 3) The communication method according to Supplementary Note 2, further comprising the second network node receiving frame timing information from the first network node relating to a frame timing difference between the first cell and the second cell, and the second network node receiving the uplink reference signal from the user equipment further based on the frame timing information.

[0165] (Supplementary Note 4) The communication method according to any one of Supplementary Notes 1 to 3, further comprising: the first network node receiving the timing error information from the second network node; and the first network node transmitting the timing error information to the user equipment.

[0166] (Supplementary Note 5) The communication method according to any one of Supplementary Notes 1 to 4, further comprising the first network node transmitting to the user equipment frame timing information relating to a frame timing difference between the first cell and the second cell.

[0167] (Supplementary Note 6) The communication method according to any one of Supplementary Notes 1 to 5, further comprising receiving, by the second network node, from the user equipment that has accessed the second cell, a timing advance value applied by the user equipment.

[0168] (Supplementary Note 7) The communication method according to any one of Supplementary Notes 1 to 3, further comprising: the first network node receiving the timing error information from the second network node; the first network node receiving, from the user equipment, reference signal time difference information relating to a reception timing difference between a downlink reference signal of the first cell and a downlink reference signal of the second cell; the first network node determining, based on the timing error information and the reference signal time difference information, a timing advance value to be applied by the user equipment for the second cell; and the first network node transmitting the timing advance value to the user equipment.

[0169] (Supplementary Note 8) The communication method according to Supplementary Note 7, wherein the first network node transmits to the user equipment a Medium Access Control Element (MAC CE) including the timing advance value, the MAC CE being a cell switch command MAC CE indicating a serving cell switch from the first cell to the second cell.

[0170] (Supplementary Note 9) A network node that manages a second cell in a mobile communication system that performs serving cell switching to switch a serving cell of a user equipment from a first cell to a second cell, the network node comprising: a receiver that receives an uplink reference signal transmitted by the user equipment to another network node that manages the first cell; a controller that generates timing error information relating to an error in reception timing of the uplink reference signal relative to frame timing of the second cell, based on the uplink reference signal; and a transmitter that transmits the timing error information to the other network node.

[0171] (Supplementary Note 10) A network node that manages a first cell in a mobile communication system that performs serving cell switching to switch a serving cell of a user equipment from a first cell to a second cell, the network node comprising: a receiving unit that, in response to reception by another network node that manages a second cell of an uplink reference signal transmitted by the user equipment to the network node, receives timing error information regarding an error in reception timing of the uplink reference signal relative to frame timing of the second cell from the other network node that manages the second cell.

[0172] 1: Mobile communication system 5: Network 10: CN 20: RAN 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 communications method for performing serving cell switching, which switches a serving cell of a user equipment from a first cell to a second cell, comprising: a second network node managing a second cell receiving an uplink reference signal transmitted by the user equipment to a first network node managing the first cell; the second network node generating timing error information regarding an error in the reception timing of the uplink reference signal relative to the frame timing of the second cell based on the uplink reference signal; and the second network node transmitting the timing error information to the first network node.

2. The communication method according to claim 1, further comprising the second network node receiving, from the first network node, reference signal configuration information indicating an uplink reference signal configuration configured in the user equipment, and the second network node receiving the uplink reference signal from the user equipment based on the reference signal configuration information.

3. The communications method according to claim 2, further comprising the second network node receiving from the first network node frame timing information relating to a frame timing difference between the first cell and the second cell, and the second network node receiving the uplink reference signal from the user equipment further based on the frame timing information.

4. The method of claim 1, further comprising: the first network node receiving the timing error information from the second network node; and the first network node transmitting the timing error information to the user equipment.

5. The method of claim 1, further comprising the first network node transmitting frame timing information to the user equipment relating to a frame timing difference between the first cell and the second cell.

6. The communication method according to claim 1, further comprising: the second network node receiving, from the user equipment that has accessed the second cell, a timing advance value that the user equipment is applying.

7. The communication method according to claim 1, further comprising: the first network node receiving the timing error information from the second network node; the first network node receiving from the user equipment reference signal time difference information relating to a reception timing difference between a downlink reference signal of the first cell and a downlink reference signal of the second cell; the first network node determining a timing advance value to be applied by the user equipment for the second cell based on the timing error information and the reference signal time difference information; and the first network node transmitting the timing advance value to the user equipment.

8. The method of claim 7, wherein the first network node transmits a Medium Access Control and Control Element (MAC CE) to the user equipment, the MAC CE including the timing advance value, the MAC CE being a cell switch command MAC CE instructing a serving cell switch from the first cell to the second cell.

9. A network node that manages a second cell in a mobile communication system that performs serving cell switching to switch a serving cell of a user equipment from a first cell to a second cell, the network node having: a receiving unit that receives an uplink reference signal transmitted by the user equipment to another network node that manages the first cell; a control unit that generates timing error information regarding an error in the reception timing of the uplink reference signal relative to the frame timing of the second cell based on the uplink reference signal; and a transmitting unit that transmits the timing error information to the other network node.

10. A network node managing a first cell in a mobile communication system that performs serving cell switching to switch a serving cell of a user equipment from a first cell to a second cell, the network node having a receiving unit that receives timing error information regarding an error in the reception timing of the uplink reference signal relative to the frame timing of the second cell from another network node managing the second cell in response to the uplink reference signal transmitted by the user equipment to the network node being received by the other network node managing the second cell.

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