WO2025115878A1 - Communication method and network node - Google Patents

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

This disclosure relates to a communication method and a network node for use in a mobile communication system.

3GPP (3rd Generation Partnership Project) (registered trademark; the same applies below) defines the 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) for a user device in a radio resource control (RRC) connected state is instructed by sending an RRC layer message (so-called handover command), which corresponds to Layer 3 (L3), from a network node to the user device.

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. In LTM, a network node receives a Layer 1 (L1) measurement report from a user equipment, and based on that, the network node signals a cell switching command to the user equipment via the medium access control (MAC) control element (CE), causing the network node to change the serving cell of the user equipment.

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).

3GPP contribution: R2-2309335

The present disclosure relates to a communication method and a network node for realizing LTM between network nodes using UE-based timing advance (TA) measurements that can obtain a TA value without a random access (RA) procedure.

The communication method according to the first aspect is a communication method for performing serving cell switching to switch a serving cell of a user equipment from a first cell to a second cell, and includes: a second network node managing the 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 related 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 the second network node transmitting the timing error information to the first network node.

The network node according to the second aspect is 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, and includes 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 related 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 transmitter that transmits the timing error information to the other network node.

The network node according to the third aspect is 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, and has a receiving unit that receives timing error information related to an error in the reception timing of the uplink reference signal relative to the frame timing of the second cell from another network node that manages 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 that manages the second cell.

FIG. 1 is a diagram illustrating a configuration example of a mobile communication system according to an embodiment. FIG. 2 is a diagram illustrating a configuration example of a UE (user equipment) according to an embodiment. A diagram showing an example configuration of a gNB (network node) according to an embodiment. A diagram showing the configuration of a protocol stack of a wireless interface of a user plane that handles data. A diagram showing the configuration of a protocol stack of the wireless interface of the control plane that handles signaling (control signals). A diagram showing an example of an LTM procedure being specified in 3GPP Release 18. FIG. 2 is a diagram for explaining an operation scenario of the mobile communication system according to the embodiment. FIG. 1 is a diagram illustrating an example of a basic operation of LTM between network nodes according to an embodiment. FIG. 2 is a diagram for explaining an overview of UE-based TA measurement according to an embodiment. A diagram showing the operation of a gNB according to an embodiment. FIG. 4 is a diagram illustrating an example of a first operation pattern according to the embodiment. FIG. 11 is a diagram illustrating an example of a second operation pattern according to the embodiment.

The mobile communication system according to the embodiment will be described with reference to the drawings. In the drawings, the same or similar parts are denoted by the same or similar reference numerals.

(1) Configuration of a Mobile Communication System FIG. 1 is a diagram showing a configuration example of a mobile communication system 1 according to an embodiment. The mobile communication system 1 complies with the 3GPP standard 5th generation system (5GS: 5th Generation System). In the following description, 5GS is taken as an example, but an LTE (Long Term Evolution) system may be applied at least in part to the mobile communication system. A sixth generation (6G) system may be applied at least in part to the mobile communication system.

The mobile communication system 1 has 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. In the following, the NG-RAN 10 may be simply referred to as the RAN 10. Also, the 5GC 20 may be simply referred to as the core network (CN) 20. The RAN 10 and the CN 20 constitute the network 5 of the mobile communication system 1.

UE100 is a mobile wireless communication device. UE100 may be any device that is used by a user. For example, UE100 is 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).

NG-RAN10 includes base station 200 (referred to as "gNB" in the 5G system), which is a type of network node. gNB200 is connected to each other via an Xn interface, which is an interface between base stations. gNB200 manages one or more cells. gNB200 performs wireless communication with UE100 that has established a connection with its own cell. gNB200 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. "Cell" is used as a term indicating the smallest unit of a wireless communication area. "Cell" is also used as a term indicating a function or resource for performing wireless communication with UE100. One cell belongs to one carrier frequency (hereinafter simply referred to as "frequency").

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 a base station-to-base station interface.

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

FIG. 2 is a diagram showing an example of the configuration of a UE 100 (user equipment) according to an 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.

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

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 the baseband signal (transmission signal) output by the control unit 130 into a radio signal and transmits it from the antenna.

The control unit 130 performs various controls and processes in the UE 100. Such processes include the processes of each layer described below. The operations of the UE 100 described above and below may be operations under the control of 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.

FIG. 3 is a diagram showing an example of the configuration of a gNB 200 (network node) according to an embodiment. The gNB 200 has a transmitting unit 210, a receiving unit 220, a control unit 230, and a network communication unit 240. The transmitting unit 210 and the receiving unit 220 constitute a wireless communication unit 250 that performs wireless communication with the UE 100. The network communication unit 240 has a transmitting unit 241 that performs transmission and a receiving unit 242 that performs reception.

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 the baseband signal (transmission signal) output by the control unit 230 into a radio signal and transmits it from the antenna.

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

The control unit 230 performs various controls and processes in the gNB 200. Such processes include the processes of 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.

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. Note that the gNB 200 may be composed of a CU (Central Unit) and a DU (Distributed Unit) (i.e., functionally divided), and the two units may be connected via an F1 interface, which is a fronthaul interface.

Figure 4 shows the protocol stack configuration of the wireless interface of the user plane that handles data.

The user plane radio interface protocol has 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.

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 CRC parity bits scrambled by the RNTI added.

The MAC layer performs data priority control, retransmission processing using Hybrid Automatic Repeat reQuest (HARQ), and random access procedures. Data and control information are transmitted between the MAC layer of UE100 and the MAC layer of gNB200 via a transport channel. The MAC layer of gNB200 includes a scheduler. The scheduler determines the transport format (transport block size, modulation and coding scheme (MCS)) of the uplink and downlink and the resource blocks to be assigned to UE100.

The RLC layer uses the functions of the MAC layer and PHY layer to transmit data to the RLC layer on the receiving side. Data and control information are transmitted between the RLC layer of UE100 and the RLC layer of gNB200 via logical channels.

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

The SDAP layer maps IP flows, which are the units for which the core network controls QoS (Quality of Service), to radio bearers, which are the units for which the AS (Access Stratum) controls QoS. Note that if the RAN is connected to the EPC, SDAP is not necessary.

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

The protocol stack of the radio interface of the control plane has an RRC (Radio Resource Control) layer and a NAS (Non-Access Stratum) layer instead of the SDAP layer shown in Figure 4.

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 in response 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.

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 UE100 and the NAS layer of AMF300A. In addition to the radio interface protocol, UE100 also has an application layer, etc. Also, the layer below the NAS layer is called the AS layer (also simply referred to as "AS").

(2) Overview of LTM The mobile communication system according to the embodiment supports LTM (L1/L2-triggered mobility).

LTM is a technology for shortening mobility delays (specifically, delays in serving cell switching) compared to general handover procedures by triggering serving cell switching through signaling in the lower layers, Layer 1 (L1) and/or Layer 2 (L2). In a general handover procedure, a Measurement Report message, which is an RRC message, is sent from UE100 to gNB200, and gNB200 decides to handover UE100 based on the Measurement Report message, and the handover is instructed by sending a handover command, which is an RRC message (specifically, an RRC Reconfiguration message), from gNB200 to UE100.

In contrast, in LTM, first, gNB200 prepares an LTM candidate cell setting for a candidate cell to be switched to, and provides the LTM candidate cell setting to UE100 by RRC signaling. Second, UE100 performs synchronization processing with the candidate cell by early synchronization (Early sync). Third, gNB200 receives an L1 measurement report from UE100, determines serving cell switching 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 setting) to UE100 by MAC CE. The serving cell switch trigger is transmitted by MAC CE including at least a candidate setting index together with a beam indicator. Fourth, UE100 changes the serving cell in response to the cell switch command from gNB200 (source cell). In this manner, the gNB 200 triggers a serving cell switch by selecting the LTM candidate cell setting as the target setting. The LTM candidate cell setting can be added, changed, and released by the gNB 200 via RRC signaling.

The following principles apply to LTM:

- Each LTM candidate cell configuration can be provided as a differential configuration (delta configuration) relative to a reference configuration that is used to form the complete candidate cell configuration.

- If a full candidate cell configuration is applied, it replaces the current UE configuration during a serving cell switch. A reconfiguration procedure does this but does not necessarily reset the MAC, RLC or PDCP layers.

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

- Security is not updated in LTM.

- LTM between subsequent LTM candidate cell configurations can be performed without RRC reconfiguration. That is, UE100 does not release other LTM candidate cell configurations after LTM is triggered.

FIG. 6 is a diagram showing an example of the LTM procedure, the specifications of which are being formulated in 3GPP Release 18. In the illustrated example, UE 100 performs serving cell switching from the first cell of gNB 200 to the 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 the second cell is also referred to as a "target cell" after the serving cell switching by LTM is determined. The first cell is also referred to as a "source cell".

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

In step S2, UE100 transmits a measurement report message, which is an RRC message, to gNB200.

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

In step S4, gNB200 transmits an RRC Reconfiguration message to UE100, the RRC Reconfiguration message including LTM candidate cell configuration (LTM Candidate Configuration) for one or more candidate cells.

In step S5, UE100 stores the LTM candidate cell setting and transmits an RRC Reconfiguration Complete message to gNB200.

In step S6, UE100 may perform synchronization processing with the candidate cell before receiving the cell switching command. Such synchronization processing is called early synchronization. Here, UE100 may perform early timing advance (TA) acquisition with the candidate cell requested by gNB200 (source cell) before receiving the cell switching 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. UE100 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 a candidate cell, in early synchronization, UE100 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 UE100.

In step S7, UE100 performs layer 1 (L1) measurements on the configured candidate cells and transmits a physical layer measurement report (L1 Measurement Report) to gNB200. The L1 Measurement Report is transmitted and received at L1, which is the PHY layer. For example, UE100 transmits L1-RSRP and/or L1-SINR to gNB200 via PUCCH (Physical Uplink Control Channel) and/or PUSCH (Physical Uplink Shared Channel).

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

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 the TA value obtained by early synchronization.

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

In step S11, if the serving cell switching needs to include the execution of a random access procedure (for example, if the Cell Switch Command does not contain a valid TA value), UE100 executes a random access procedure for the target cell. Note that, if UE100 does not need to acquire the TA of the target cell when switching the serving cell (for example, if the Cell Switch Command contains a valid TA value), it can skip the random access procedure.

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

(3) Basic operation example of LTM between network nodes 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, an operation for realizing inter-network node LTM will be described.

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

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 the first TRP of gNB200a, and the second cell is formed by the second TRP of gNB200b. Until the serving cell switching by LTM is determined, the second cell is also called a "candidate cell (or LTM candidate cell)", and after the serving cell switching by LTM is determined, the second cell is also called a "target cell". An Xn interface is established between gNB200a and gNB200b. Communication between gNB200a and gNB200b is assumed to be performed on the Xn interface.

In the embodiment, gNB200a performs wireless communication with UE100 in an RRC connected state in the first cell of gNB200a. gNB200a transmits a request message to gNB200b to request a change of the serving cell of UE100 from the first cell of gNB200a to the second cell of gNB200b. Here, when gNB200a changes the serving cell by LTM, it transmits a request message to gNB200b indicating that the serving cell is switched by LTM. This allows gNB200b to understand that a serving cell switch by LTM is requested, rather than a general handover, based on the request message from gNB200a.

The request message may be a handover request (HO Request) message that can be used in a handover procedure that instructs a handover from gNB200a to UE100 via an RRC message. The request message indicating that the serving cell is being switched by LTM may be a HO Request message that includes an LTM indicator. This makes it possible to reuse the HO Request message used in general handovers in LTM between network nodes, making it easier to minimize changes to technical specifications.

Alternatively, the request message indicating that the serving cell is being switched 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 used exclusively for LTM.

The request message indicating that the serving cell is to be switched by LTM may include information indicating whether or not gNB200b needs to configure contention-free random access (CFRA) resources. The CFRA resources are used in early synchronization that UE100 performs with the second cell before the serving cell switching instruction is issued by MAC CE. This allows gNB200b to determine whether or not to configure CFRA resources based on the request message.

gNB200a may send another request message to gNB200b requesting that gNB200b configure or activate CFRA resources for early synchronization. This allows gNB200b to appropriately configure or activate CFRA resources for early synchronization.

The gNB200a may transmit a PDCCH command (PDCCH order) to the UE100 to instruct the execution of CFRA (i.e., RA preamble transmission). The PDCCH order may include information for identifying the second cell. This makes it easier for the UE100 to identify whether the target of the RA preamble transmission is the first cell or the second cell.

The gNB200a may receive a notification from the gNB200b indicating that the early synchronization performed by the UE100 with the second cell has been successful. This allows the gNB200a to know whether or not the early synchronization performed by the UE100 with the second cell has been successful.

Alternatively, gNB200a may receive a notification from UE100 indicating that early synchronization performed by UE100 with the second cell has been successful.

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

In step S101, UE100 transmits an L3 (RRC) Measurement Report to gNB200a. gNB200a receives the L3 (RRC) Measurement Report.

In step S102, gNB200a decides 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 has been determined as the candidate cell.

In step S103, gNB200a transmits a request message (LTM HO Request) indicating that the serving cell is switched by LTM to gNB200b. gNB200b 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 general handovers. 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 is required, i.e., whether or not CFRA resources for early synchronization need to be set (or may include information proposing early synchronization). Note that the request message (LTM HO Request) may include RRC setting information of UE 100 and a cell identifier indicating the second cell, as in a general handover.

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

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

In step S106, gNB200a transmits an RRC Reconfiguration message including an LTM candidate cell configuration (LTM Candidate Configuration) for the second cell to UE100. UE100 receives the RRC Reconfiguration message. The RRC Reconfiguration message may include information indicating the CFRA resource for early synchronization configured by gNB200b for the second cell.

In step S107, UE100 saves the LTM candidate cell setting and transmits an RRC Reconfiguration Complete message to gNB200a. gNB200a receives the RRC Reconfiguration Complete message.

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).

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

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

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

In step S112, gNB200b may send a notification message, for example, an Early sync CFRA Request Ack message, to gNB200a indicating that preparation of CFRA resources for early synchronization has been completed. gNB200a may receive the notification message (Early sync CFRA Request Ack message).

In step S113, gNB200a transmits a PDCCH order to UE100 and instructs UE100 to perform CFRA for early synchronization. UE100 receives the PDCCH order. The PDCCH order may include information (Target cell indicator) for identifying the second cell as the target of the CFRA. The information may be the cell ID (or cell index) of the second cell. The information may be an index of 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.

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 prior to this point.

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

In step S116, gNB200b may transmit an RAR including a TA value derived based on the RA preamble to UE100. UE100 may receive the RAR. Step S116 may be an optional step that is executed only if there is a configuration from gNB200a (e.g., the configuration of step S106). UE100 may transmit a notification (Early Sync Complete) to gNB200a indicating that UL early synchronization with the second cell has been completed (step S117). The notification (Early Sync Complete) may include the TA value notified in the RAR.

In step S118, gNB200b may transmit a notification message (Early Sync Complete) to gNB200a indicating that UL early synchronization with UE100 has been completed. gNB200a may receive the notification message (Early Sync Complete). The notification message (Early Sync Complete) may include the TA value derived based on the RA preamble of step S115.

In step S119, UE100 transmits the L1 measurement report to gNB200a. gNB200a receives the L1 measurement report.

In step S120, when gNB200a determines that the possibility of LTM execution has increased based on, for example, the L1 measurement report in 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 decision to execute LTM in step S121. The transmission may be after the decision to execute LTM in step S121.

In step S121, gNB200a decides to execute LTM based on the L1 measurement report of step S119.

In step S122, gNB200a transmits a Cell switch command (MAC CE) to UE100 in response to the decision to execute LTM. UE100 receives the Cell switch command. The Cell switch command may include the TA value notified to gNB200a in step S117 or S118.

In step S123, in response to receiving the Cell switch command, UE100 detaches from the first cell (source cell) and applies the LTM candidate cell setting of the second cell (target cell).

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.

In step S125, UE100 transmits an RRC Reconfiguration Complete message to the second cell. gNB200b receives the RRC Reconfiguration Complete message.

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

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

(4) LTM between network nodes using UE-based TA measurements
In the above-described basic operation example of the inter-network node LTM, the UE 100 realizes UL early synchronization with the second cell by using the RA procedure (specifically, CFRA). In the following embodiment, in the inter-network node LTM, an operation for realizing UL early synchronization by using UE-based TA measurement capable of acquiring a TA value without the RA procedure will be described.

(4.1) Overview of UE-based TA Measurement FIG. 9 is a diagram for explaining an overview of UE-based TA measurement according to the embodiment. In the illustrated example, the frame timing between the first cell (source cell) and the second cell (candidate cell, target cell) is assumed to be 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 completely 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 IEEE1588.

UE-based TA measurements have the following procedures, for example:

STEP 1: UE100 is in an RRC connected state in the first cell and is aware of the TA value being applied in the first cell. The TA value being applied in the first cell by UE100 is also referred to as "TA_s". "TA_s" is the time during 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. The TA value is used to control the UL transmission timing of each UE100 so that the UL transmissions from all UE100 are synchronized when received by the serving cell (gNB200). UE100 closer to the TRP of the cell has a shorter propagation delay, and therefore a smaller TA value. On the other hand, UE100 farther from the TRP of the cell has a longer propagation delay, and therefore a larger TA value. 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) sent from the serving cell (gNB 200) to the UE 100. Therefore, the serving cell (gNB 200) also knows the TA value "TA_s".

STEP 2: UE100 performs RSTD (Reference Signal Timing Difference) measurement and generates reference signal time difference information "Tdiff_s-t_ue" regarding 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 UE100. 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.

STEP 3: The second cell (gNB200) receives a 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. STEP 3 may be performed before STEP 2 or simultaneously with STEP 2.

STEP 4: From "TA_s", "Tdiff_s-t_ue", and "TA_temp_t", a TA value "TA_t" to be applied by the UE 100 in the second cell is calculated by the following formula (1):
TA_t = (TA_temp_t + TA_s) - Tdiff_s-t_ue (1)
However, the TA value “TA_t” to be applied by the UE 100 in the second cell may be calculated by the following equation (2) further taking into account the synchronization error “Tdiff_s−t_nw” between the cells:
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 a UL reference signal from the UE 100, i.e., to calculate "TA_temp_t".

Therefore, by aggregating parameters (variables) such as "TA_s", "Tdiff_s-t_ue", and "TA_temp_t" in UE100 or gNB200 and calculating formula (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" may be aggregated in UE100 or gNB200 and "TA_t" may be calculated by calculating formula (2).

(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 are considered as a method of aggregating these parameters.

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

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

In the case of inter-gNB LTM, in both the first and second operation patterns, 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).

FIG. 10 is a diagram showing the operation of gNB200b according to the embodiment.

In step S11, the gNB 200b managing the second cell (candidate cell, target cell) may receive reference signal setting information indicating the UL reference signal setting set in the UE 100 from the gNB 200a managing the first cell (source cell, current serving cell). Here, the gNB 200b may receive the reference signal setting information from the gNB 200a on the Xn interface. The gNB 200b may receive the UL reference signal from the UE 100 based on the reference signal setting information.

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 to the first cell.

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.

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.

According to this operation, it becomes possible to realize LTM between network nodes using UE-based TA measurement that can obtain 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 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, 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 UL reference signal transmitted by UE100 to gNB200a being received by the other network node (gNB200b).

The receiving unit 242 of gNB200b may receive frame timing information "Tdiff_s-t_nw" regarding the frame timing difference between the first cell and the second cell from gNB200a. The receiving unit 220 of gNB200b may receive a UL reference signal from UE100 based on the frame timing information "Tdiff_s-t_nw". This makes it easier to receive the UL reference signal from UE100.

In the first operation pattern, the receiver 242 of gNB200a receives timing error information "TA_temp_t" from gNB200b. The transmitter 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".

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).

In the second operation pattern, the receiver 242 of gNB200a receives timing error information "TA_temp_t" from gNB200b, and the receiver 220 of gNB200a 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 UE100. The control unit 230 of gNB200a determines the TA value "TA_t" that UE100 should apply to the second cell based on the TA value "TA_s" 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 formula (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.

Here, the transmitter 210 of the gNB 200a may transmit a medium access control and 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 switching of the serving cell 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.

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.

(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, the operation described in FIG. 6 and the operation described in FIG. 8 will not be described repeatedly, but the operation described in FIG. 6 and the operation described in FIG. 8 may be applied as appropriate. In the following first operation pattern and second operation pattern, the inter-node communication between the first cell (gNB 200a) and the second cell (gNB 200b) is assumed to be performed on the Xn interface.

(4.3.1) Example of the first operation pattern FIG. 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 formula (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".

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 UE100, or a gNB configuration update message notifying a gNB setting update. In addition, the first cell (gNB 200a) and/or the second cell (gNB 200b) 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).

In step S201, UE100 manages (updates) the TA value "TA_s" of the first cell (gNB200a) based on signaling from the first cell (gNB200a). For example, the TA value "TA_s" is set in UE100 from the first cell (gNB200a) in the RA response during the RA procedure, and then adjusted by a TA command (MAC CE) sent from the first cell (gNB200a) to UE100.

In step S202, the first cell (gNB200a) transmits (configures) an UL reference signal configuration (RS configuration) to the UE100 to configure the UE100 to transmit an UL reference signal. For example, the UL reference signal configuration (RS configuration) may be an SRS configuration (SRS config.).

In step S203, the first cell (gNB200a) may transmit a message including reference signal setting information (RS config. info) indicating the UL reference signal setting set in the UE100 to the second cell (gNB200b). The message may be an Xn Handover Request message or a gNB configuration update message.

Here, the reference signal setting information (RS config. info) may include at least one of the parameters of the SRS-config., which is an RRC information element (IE) set in the UE 100. For example, the reference signal setting 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 (cycle) information, RS transmission trigger type (aperiodic, semi-persistent, periodic), and sequence ID.

In step S204, the UE 100 transmits a UL reference signal (e.g., SRS) to the first cell (gNB 200a) based on the settings 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.

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

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 the UE100.

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.).

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 above-mentioned RRC container. The RRC Reconfiguration message may include, as information elements, Conditional reconfiguration and/or LTM configuration including the information received from the second cell (gNB 200b) in step S206.

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.). These pieces of 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 list of candidate cell settings 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", may not include the IE (NULL), or may be information indicating synchronization.

In step S208, UE100 performs RSTD measurement and generates "Tdiff_s-t_ue".

In step S209, UE100 calculates the TA value "TA_t" for the second cell (gNB200b) using formula (1) or formula (2). When UE100 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". UE100 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 together with the calculated TA value "TA_t".

In step S210, the first cell (gNB200a) transmits a Cell switch command MAC CE to the UE100 to instruct cell switching to the second cell (gNB200b). 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 the 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 the UE100, and the UE100 may voluntarily trigger cell switching to the second cell (gNB200b) when the execution conditions are satisfied, instead of the first cell (gNB200a) instructing the UE100 to switch cells to the second cell (gNB200b).

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

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

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).

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

FIG. 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 formula (1) and/or when the first cell (gNB200a) and the second cell (gNB200b) are synchronized, it is not necessarily necessary to handle "Tdiff_s-t_nw".

Steps S301 to S305 are similar to steps S200 to S205 in FIG. 11.

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.

In step S307, the first cell (gNB 200a) transmits a message including the 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.

In step S308, UE100 performs RSTD measurement and generates "Tdiff_s-t_ue".

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.

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

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

In step S312, the UE 100 applies the calculated TA value (TA_t) to 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 may notify the second cell (gNB 200b) of the TA value (TA_t) by including it in the message of step S312 or another message (e.g., a UE Assistance Information message).

(5) Other embodiments In the above embodiment, an example has been described in which various parameters are signaled on 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 between the DU and the CU on the F1 interface.

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

In the above-mentioned embodiment and example, 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 be a DU of an IAB node. The UE 100 may also be an MT (Mobile Termination) of an IAB node.

In other words, UE100 may be a terminal function unit (a type of communication module) that allows a base station to control a repeater that relays signals. Such a terminal function unit is called an MT. Examples of MT include, in addition to IAB-MT, NCR (Network Controlled Repeater)-MT and RIS (Reconfigurable Intelligent Surface)-MT.

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

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

The functions realized by UE100 or gNB200 (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 realize the described functions. A processor includes transistors and 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, circuitry, unit, and means are hardware that is programmed to realize 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 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.

As used in this disclosure, the terms "based on" and "depending on/in response to" do not mean "based only on" or "only in response to," unless otherwise specified. The term "based on" means both "based only on" and "based at least in part on." Similarly, the term "in response to" means both "based only on" and "at least in part on." The terms "include," "comprise," and variations thereof do not mean including only the items listed, but may include only the items listed, or may include additional items in addition to the items listed. In addition, 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 is not intended to generally limit the quantity or order of those elements. These designations may be used herein as a convenient way to distinguish between two or more elements. Thus, a reference to a first and 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 manner. In this disclosure, where articles are added by translation, such as, for example, a, an, and the in English, these articles are intended to include the plural unless the context clearly indicates otherwise.

The above describes the embodiments in detail with reference to the drawings, but the specific configuration is not limited to the above, and various design changes can be made without departing from the spirit of the invention.

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.

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

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

(Appendix 2)
The second network node further comprises receiving, from the first network node, reference signal configuration information indicating an uplink reference signal configuration configured in the user equipment;
The communication method according to claim 1, wherein the second network node receives the uplink reference signal from the user equipment based on the reference signal configuration information.

(Appendix 3)
The method further comprises 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;
The communication method of claim 2, wherein the second network node receives the uplink reference signal from the user equipment further based on the frame timing information.

(Appendix 4)
said first network node receiving said timing error information from said second network node;
4. The method of any of claims 1 to 3, further comprising the first network node transmitting the timing error information to the user equipment.

(Appendix 5)
5. The method of any of claims 1 to 4, 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.

(Appendix 6)
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 accessing the second cell, a timing advance value applied by the user equipment.

(Appendix 7)
said first network node receiving said timing error information from said second network node;
The first network node receives, 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;
determining, by the first network node, 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;
4. The method of any of claims 1 to 3, further comprising the first network node transmitting the timing advance value to the user equipment.

(Appendix 8)
the first network node transmitting a Medium Access Control and Control Element (MAC CE) including the timing advance value to the user equipment;
The communication method according to Supplementary Note 7, wherein the MAC CE is a cell switch command MAC CE instructing a serving cell switch from the first cell to the second cell.

(Appendix 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, comprising:
A receiver 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 of a reception timing of the uplink reference signal with respect to a frame timing of the second cell based on the uplink reference signal;
a transmitter for transmitting the timing error information to the other network node.

(Appendix 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, comprising:
A network node comprising: 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 managing the second cell, 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 the other network node.

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: Transmitter 220: Receiver 230: Controller 240: Network communication unit 241: Transmitter 242: Receiver 250: Wireless communication unit 300: AMF/UPF

Claims (10)

A communication method for performing serving cell switching for switching a serving cell of a user equipment from a first cell to a second cell, comprising:
receiving, by a second network node managing the second cell, an uplink reference signal transmitted by the user equipment to a first network node managing the first cell;
generating, based on the uplink reference signal, timing error information relating to an error of a reception timing of the uplink reference signal relative to a frame timing of the second cell;
said second network node transmitting said timing error information to said first network node. The second network node further comprises receiving, from the first network node, reference signal configuration information indicating an uplink reference signal configuration configured in the user equipment;
The communication method according to claim 1 , wherein the second network node receives the uplink reference signal from the user equipment based on the reference signal configuration information. The method further comprises 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;
The method of claim 2 , wherein the second network node receives the uplink reference signal from the user equipment further based on the frame timing information. said first network node receiving said timing error information from said second network node;
The method of claim 1 , further comprising the first network node transmitting the timing error information to the user equipment. 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. The method of claim 1 , further comprising: the second network node receiving, from the user equipment accessing the second cell, a timing advance value applied by the user equipment. said first network node receiving said timing error information from said second network node;
The first network node receives, 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;
determining, by the first network node, 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;
The method of claim 1 , further comprising: the first network node transmitting the timing advance value to the user equipment. the first network node transmitting a Medium Access Control and Control Element (MAC CE) including the timing advance value to the user equipment;
The communication method according to claim 7 , wherein the MAC CE is a cell switch command MAC CE instructing a serving cell switch from the first cell to the second cell. 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, comprising:
A receiver 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 of a reception timing of the uplink reference signal with respect to a frame timing of the second cell based on the uplink reference signal;
a transmitter for transmitting the timing error information to the other network node. 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, comprising:
A network node comprising: 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 managing the second cell, 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 the other network node.

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