Communication method, user device, and network node

Abstract

This communication method is used in a mobile communication system that supports L1 / L2 Triggered Mobility (LTM) and includes: a user device receiving, from a first cell, a setting for performing LTM cell switching for switching a serving cell of the user device from a first cell to a second cell based on LTM; transmitting, to the first cell, notification information for requesting the first cell to transmit instruction information instructing a contention-free random access (CFRA) preamble transmission to the second cell; and performing the preamble transmission to the second cell in response to receiving the instruction information from the first cell.

Description

COMMUNICATION METHOD, USER EQUIPMENT, AND NETWORK NODE

[0001] The present disclosure relates to a communication method, a user equipment, and a network node.

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

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

[0004] 3GPP Technical Specification "3GPP TS 38.300 V18.0.0 (2023-12)"

[0005] The technology of the present disclosure aims to provide a communication method, a user equipment, and a network node that can improve LTM.

[0006] A communication method according to a first aspect of the present disclosure is a communication method used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), and includes the steps of: receiving, in a user equipment, from a first cell, a configuration for performing LTM cell switching, which switches a serving cell of the user equipment from a first cell to a second cell by the LTM; transmitting, to the first cell, notification information for requesting the first cell to transmit instruction information instructing the first cell to transmit a preamble for Contention Free Random Access (CFRA) to the second cell; and transmitting the preamble to the second cell in response to receiving the instruction information from the first cell.

[0007] A user equipment according to a second aspect of the present disclosure is a user equipment used in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), and includes: a receiving unit that receives from the first cell a setting for performing LTM cell switching, which switches the serving cell of the user equipment from a first cell to a second cell using the LTM; a transmitting unit that transmits to the first cell notification information requesting the first cell to transmit instruction information instructing the first cell to transmit a preamble for Contention Free Random Access (CFRA) to the second cell; and a control unit that transmits the preamble to the second cell in response to receiving the instruction information from the first cell.

[0008] A network node according to a third aspect of the present disclosure is a network node that manages a first cell in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), and includes: a transmitter that transmits to the user equipment a configuration for performing LTM cell switching, which switches a serving cell of a user equipment from the first cell to a second cell by the LTM; and a receiver that receives, from the user equipment, notification information for requesting the first cell to transmit instruction information instructing the first cell to transmit a preamble of Contention Free Random Access (CFRA) to the second cell, and the transmitter transmits the instruction information to the user equipment in response to the notification information.

[0009] 1 is a diagram showing an example of the configuration of a mobile communication system according to an embodiment. FIG. 2 is a diagram showing an example of the configuration of a UE (user equipment) according to an embodiment. FIG. 3 is a diagram showing an example of the configuration of a gNB (network node) according to an embodiment. FIG. 4 is a diagram showing a protocol stack configuration of a radio interface of a user plane that handles data. FIG. 5 is a diagram showing a protocol stack configuration of a radio interface of a control plane that handles signaling (control signals). FIG. 6 is a diagram showing an example of an LTM procedure. FIG. 7 is a diagram showing the operation of a UE according to the first embodiment. FIG. 8 is a diagram showing a first operation pattern of the mobile communication system according to the first embodiment. FIG. 9 is a diagram showing an example of the configuration of an RRC Reconfiguration message according to the first operation pattern of the first embodiment. FIG. 10 is a diagram showing a second operation pattern of the mobile communication system according to the first embodiment. FIG. 11 is a diagram showing a third operation pattern of the mobile communication system according to the first embodiment. FIG. 12 is a diagram for explaining an example of UE-based TA measurement according to a second embodiment. FIG. 13 is a diagram showing the operation of a UE 100 according to the second embodiment.

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

[0011] (1) First Embodiment The first embodiment will be described.

[0012] (1.1) Configuration of a Mobile Communication System FIG. 1 is a diagram showing an example of the configuration of a mobile communication system 1 according to this embodiment. The mobile communication system 1 conforms to the 3GPP standard 5th Generation System (5GS). While the following description uses 5GS as an example, the mobile communication system may also be at least partially based on an LTE (Long Term Evolution) system. The mobile communication system may also be at least partially based on a 6th Generation (6G) system.

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

[0014] The UE 100 is a mobile wireless communication device. The UE 100 may be any device 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 showing an example of the configuration of a UE 100 (user equipment) according to this embodiment. The UE 100 has a receiving unit 110, a transmitting unit 120, and a control unit 130. The receiving unit 110 and the transmitting unit 120 constitute a wireless communication unit 140 that performs wireless communication with the gNB 200.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0038] (1.2) Overview of LTM The mobile communication system 1 according to this embodiment supports LTM (L1 / L2-triggered mobility).

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

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

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

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

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

[0044] Fourth, UE100 switches the serving cell in response to a cell switching command from gNB200 (source cell).

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

[0046] The following principles apply to LTM:

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

[0048] If a full candidate cell configuration is applied, it replaces the current UE configuration upon a serving cell switch. A reconfiguration procedure does this but does not necessarily reset the MAC, RLC or PDCP layers.

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

[0050] - Security is not updated in LTM.

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

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

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

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

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

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

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

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

[0059] In step S6, the UE 100 may perform synchronization processing with the candidate cell before receiving the cell switching command. Such synchronization processing may be referred to as early synchronization (Early sync). Here, the UE 100 may perform downlink synchronization processing (DL synchronization processing) for the candidate cell, and then perform early timing advance (TA) acquisition in the candidate cell requested by the gNB 200 (source cell). This is performed by a 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. In addition, when early UL synchronization setting (EarlyUlSyncConfig) is configured in UE 100, the PDCCH order may include a cell indicator indicating a corresponding RACH transmission cell, i.e., a candidate cell to which UE 100 should transmit a random access preamble (RA preamble).

[0060] The UE 100 transmits an RA preamble to the designated candidate cell. In order to minimize communication interruption of the source cell due to the 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. Note that the TA value is a value for adjusting the uplink transmission timing of the UE 100.

[0061] In step S7, the UE 100 performs layer 1 (L1) measurement in the configured 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).

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

[0063] 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.

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

[0065] In step S11, if the serving cell switch needs to include 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.

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

[0067] (1.3) Overview of Operation According to First Embodiment Introduction of a conditional LTM (C-LTM) procedure is planned for consideration in 3GPP Release 19. The cell switching procedure using LTM as shown in Fig. 6 can be applied to the C-LTM procedure, similar to a conditional L3 handover (CHO).

[0068] In this embodiment, the gNB 200 sets in advance in the UE 100 a switching condition (also referred to as a "cell switching trigger condition" or "execution condition"), which is a radio quality condition for performing cell switching, in the RRC Reconfiguration message of step S4. The UE 100 performs cell switching by LTM (also referred to as "LTM cell switching") when the switching condition is satisfied. This makes it possible to skip the operations from the L1 Measurement Report of step S7 to the cell switching command (MAC CE) of step S9. As a result, it is possible to speed up cell switching from the first cell to the second cell and minimize interruptions in data communication.

[0069] Here, in the conventional LTM, the TA value to be applied to the second cell (candidate cell, target cell) is notified from the gNB 200 to the UE 100 in the cell switching command of step S9. On the other hand, in C-LTM, the cell switching command of step S9 is skipped, so there is a problem that the UE 100 cannot obtain the TA value to be applied to the second cell from the gNB 200. As a result, the UE 100 may need to perform a random access procedure for the second cell when the set switching conditions are met. Therefore, it is difficult to speed up cell switching from the first cell to the second cell.

[0070] Therefore, in this embodiment, the UE 100 in which C-LTM is set receives a PDCCH order including instruction information instructing the transmission of a CFRA preamble to the second cell from the first cell (gNB 200), transmits a preamble to the candidate cell (specifically, transmits an RA preamble), and receives a random access response (RAR) including the TA value of the candidate cell to acquire the TA value. As a result, for example, when the switching condition for the second cell is satisfied, the UE 100 already has the TA value of the second cell, so a random access procedure for the second cell is not required, and cell switching from the first cell to the second cell can be accelerated.

[0071] As described above, when DCI Format 1_0 is used and the "Frequency domain resource assignment" field in the DCI is set to all "1", the DCI is treated as a PDCCH order. The PDCCH order may include a cell indicator indicating a second cell to which the RA preamble is to be transmitted. The cell indicator may indicate multiple second cells as destinations of the RA preamble. However, a new DCI format may be used instead of the conventional PDCCH order.

[0072] In addition, in this embodiment, an example will be described in which the UE 100 acquires the TA value of the second cell by RAR. However, the UE 100 may acquire the TA value by other signaling instead of RAR. For example, a MAC protocol data unit (PDU) or MAC CE different from the RAR may be used, or an RRC message may be used. When the MAC CE is used, the UE 100 may acquire the TA value by a new MAC CE (e.g., Early Sync TA notification MAC CE) dedicated to early synchronization.

[0073] However, there is a problem in determining the timing at which the gNB200 (first cell) transmits a PDCCH order to the UE100. For example, if the timing at which the gNB200 transmits a PDCCH order to the UE100 is earlier than the timing at which the switching condition is satisfied in the UE100, the timing at which the UE100 acquires the TA value of the second cell will be too early. As a result, a situation may occur in which the TA value becomes an inappropriate value when the switching condition is satisfied, and / or the validity period of the TA value expires (i.e., the time alignment timer expires). On the other hand, if the timing at which the gNB200 transmits a PDCCH order to the UE100 is later than the timing at which the switching condition is satisfied in the UE100, it will be impossible to speed up cell switching.

[0074] Therefore, in this embodiment, the UE 100 transmits notification information for requesting the first cell to transmit a PDCCH order (instruction information) to the first cell. This enables the gNB 200 (first cell) to transmit the PDCCH order to the UE 100 at an appropriate timing.

[0075] Also, normally, when the UE 100 acquires a TA value in the RAR, the TA value is immediately applied. However, in the case of C-LTM, when the UE 100 acquires a TA value in the RAR, if the switching condition for the second cell is not satisfied, the TA value should not be immediately applied. In such a case, if the UE 100 immediately applies the TA value, there is a concern that communication with the first cell, which is the current serving cell, may not be performed appropriately.

[0076] Therefore, in this embodiment, when the UE 100 acquires the TA value by the RAR before the switching condition is satisfied, the UE 100 holds the TA value without applying it until the switching condition is satisfied (that is, the TA value of the first cell, which is the current serving cell, continues to be applied until then). Then, when the switching condition is satisfied, the UE 100 applies the TA value and performs LTM cell switching.

[0077] Fig. 7 is a diagram showing the operation of the UE 100 according to this embodiment. Description of the contents that overlap with the conventional LTM cell switching procedure in Fig. 6 will be omitted.

[0078] In step S11, UE 100 receives, from the first cell, notification condition information, which is information indicating a notification condition to be satisfied in order to transmit notification information to the first cell, and switching condition information, which indicates a switching condition to be satisfied in order to perform LTM cell switching, which switches the serving cell of UE 100 from the first cell to the second cell by LTM. UE 100 receives the notification condition information and the switching condition information from the first cell in an RRC Reconfiguration message (see step S4 in FIG. 6 ). The RRC Reconfiguration message may include a RACH configuration, for example, a CFRA configuration, used for RA preamble transmission to the second cell.

[0079] The RRC Reconfiguration message may include a candidate cell configuration list, which is a list including configuration information (candidate cell configuration) of each of a plurality of second cells. Each candidate cell configuration, which is an entry in the candidate cell configuration list, includes a cell ID of the corresponding second cell, information indicating a notification condition to be applied to the corresponding second cell, and information indicating a switching condition to be applied to the corresponding second cell. The candidate cell configuration may include a RACH configuration (CFRA configuration) of the corresponding second cell.

[0080] The radio quality may be at least one measurement value of a reference signal received power (RSRP), a reference signal radio quality (RSRQ), a signal-to-interference-and-noise ratio (SINR), and a received signal strength indicator (RSSI). The reference signal may be an SS / PBCH block (SSB) or a channel state information (CSI) reference signal. The information indicating the switching condition may be information indicating radio quality that should be satisfied for the first cell (current serving cell) and / or the second cell (candidate cell) to trigger an LTM cell switch. For example, the information indicating the switching condition may be an event (condition) such as "Neighbor becomes offset better than SpCell," i.e., a trigger condition indicating that the radio quality of the second cell has improved by an offset compared to the radio quality of the first cell. In this case, the information indicating the switching condition may include an offset value. Alternatively, the information indicating the switching condition may be information indicating an event (condition) such as “Nightbour / SpCell becomes better / worse than threshold.” In this case, the information indicating the switching condition may include a threshold.

[0081] In step S12, the UE 100 evaluates whether the set notification condition is satisfied. For example, the UE 100 measures the radio quality of the first cell and / or the radio quality of the second cell, and evaluates (determines) whether the measurement value satisfies the set notification condition. The UE 100, in which a plurality of second cells are set as candidate cells, performs the evaluation in step S12 for each second cell. The UE 100 transmits notification information to the first cell in response to the evaluation that the notification condition for any of the second cells is satisfied.

[0082] Here, the UE 100 may transmit the notification information to the first cell by a MAC CE, an RRC message, an UCI (Uplink Control Information), or an SR (Scheduling Request). That is, the notification information may be transmitted by being included in the MAC CE, an RRC message, an UCI, or an SR. The notification information may include a cell ID (or a configuration ID) of the second cell for which the notification condition is satisfied. This allows the gNB 200 to know for which second cell the notification condition is satisfied.

[0083] In step S13, UE100 receives the PDCCH order transmitted from the first cell (gNB200) based on the notification information.

[0084] In step S14, in response to the reception of the PDCCH order, the UE 100 performs preamble transmission (specifically, transmission of an RA preamble of CFRA) to the second cell specified in the PDCCH order.

[0085] In step S15, the UE 100 receives an RAR including a TA value derived based on the preamble transmission from the first cell or the second cell. Note that, when preamble transmission is instructed for a plurality of second cells, the UE 100 may transmit a preamble to each second cell and acquire the TA value of each second cell.

[0086] In step S16, the UE 100 holds the TA value until it is evaluated that the switching condition is satisfied for the second cell. Specifically, even if the UE 100 acquires the TA value by the RAR, the UE 100 holds the TA value without applying it until the switching condition is satisfied for any second cell (that is, the TA value of the first cell continues to be applied until then).

[0087] In step S17, in response to the evaluation that the switching condition is satisfied for any second cell, the UE 100 performs the LTM cell switching by applying the TA value of the second cell for which the switching condition is satisfied. Here, the UE 100 performs the LTM cell switching by applying the TA value that it holds without performing a random access procedure for the second cell. When performing the LTM cell switching, the UE 100 transmits an RRC Reconfiguration Complete message to the second cell. The UE 100 may consider that the execution of the LTM cell switching has been completed successfully when it is determined that the network (second cell) has successfully received the first uplink data.

[0088] According to this operation, the UE 100 skips the operations from the L1 Measurement Report in step S7 to the cell switching command (MAC CE) in step S9, and can quickly perform LTM cell switching when the switching conditions set by the gNB 200 are satisfied. In addition, when the switching conditions are satisfied, the UE 100 does not need to perform a random access procedure for the second cell, and cell switching can be accelerated.

[0089] Note that whether the UE 100 receives the RAR from the first cell or the second cell may be determined in the 3GPP technical specifications. Alternatively, whether the UE 100 receives the RAR from the first cell or the second cell may be specified (configured) by the gNB 200 to the UE 100 by an RRC Reconfiguration message. In this case, the specification (configuration) may be performed independently for each of the multiple second cells, or the specification (configuration) may be performed commonly for all the second cells. Alternatively, the UE 100 may receive the RAR from a candidate cell (second cell) specified in the PDCCH order.

[0090] However, since the second cell is a non-serving cell, it is also assumed that the conventional UE 100 does not have the capability to receive the RAR from the second cell. Therefore, the UE 100 having this capability may transmit capability information indicating that it has the capability to receive the RAR from the second cell to the network (gNB 200). Such capability information may be transmitted in a UE Capability message, which is a type of RRC message. For example, when connected to the gNB 200, the UE 100 may transmit a UE Capability message to the gNB 200 in response to receiving a capability inquiry from the gNB 200.

[0091] In this embodiment, an example will be mainly described in which a notification condition is set from the gNB 200 (first cell) to the UE 100 separately from the switching condition. However, the notification condition may be common to the switching condition. The UE 100 may evaluate whether the switching condition is satisfied for the second cell, and may transmit notification information to the first cell in response to the evaluation that the switching condition is satisfied. However, in this case, it takes time from when the switching condition is satisfied to when the TA value is acquired, and there is a possibility that cell switching cannot be sufficiently accelerated.

[0092] Also, in this embodiment, an example will be mainly described in which MAC CE, RRC message, UCI, or SR is used as notification information transmitted from UE 100 to gNB 200 (first cell). However, an event-triggered L1 Measurement Report may be used as notification information. In this case, the notification condition may be a trigger condition that triggers the transmission of the L1 Measurement Report. As described above, the L1 Measurement Report is transmitted and received via L1. For example, UE 100 transmits L1-RSRP and / or L1-SINR to gNB 200 via PUCCH and / or PUSCH.

[0093] (1.4) Specific Operation Examples First to third operation patterns will be described as specific examples of the operation of the mobile communication system 1 according to the embodiment. Hereinafter, a scenario in which the first cell and the second cell belong to the same gNB 200 (i.e., the same CU) is referred to as an intra-gNB scenario, and a scenario in which the first cell and the second cell belong to different gNBs 200 (i.e., different CUs) is referred to as an inter-gNB scenario.

[0094] (1.4.1) First Operation Pattern Figure 8 is a diagram showing a first operation pattern of the mobile communication system 1 according to this embodiment. The first operation pattern is an operation pattern in which, in an intra-gNB scenario, the UE 100 receives a PDCCH order while performing evaluation of the second cell.

[0095] The operations of steps S101 to S105 are the same as those of the conventional LTM cell switching procedure of FIG. 6. However, in step S104, the gNB 200 transmits an RRC Reconfiguration message including an LTM candidate cell configuration for C-LTM (C-LTM Candidate Configuration) to the UE 100. The LTM candidate cell configuration for C-LTM may include a candidate cell configuration list, which is a list including configuration information (candidate cell configuration) for each of a plurality of second cells. Each entry in the candidate cell configuration list includes the cell ID of the corresponding second cell, notification condition information indicating the notification condition to be applied to the corresponding second cell, and switching condition information indicating the switching condition to be applied to the corresponding second cell. The candidate cell configuration may include the RACH configuration (CFRA configuration) of the corresponding second cell. The candidate cell configuration may include a timer value of a timer that determines an upper limit of the RAR waiting period. This timer may be a timer separate from a conventional RACH / RAR related timer. The timer value may be an offset value with respect to the conventional timer value, or the like. The UE 100 may start the timer when transmitting an RA preamble to the corresponding second cell, and if the timer expires without receiving an RAR, may transmit an RA preamble to the corresponding second cell again. The UE 100 may determine that a random access procedure is necessary for the corresponding second cell at the time of cell switching.

[0096] The notification condition information may include at least one of the following information 1) to 3).

[0097] 1) Information indicating radio quality conditions to be satisfied for the first cell and / or the second cell to trigger transmission of notification information: The radio quality may be at least one measurement value of Reference Signal Received Power (RSRP), Reference Signal Radio Quality (RSRQ), Signal-to-Interference-and-Noise Ratio (SINR), and Received Signal Strength Indicator (RSSI). The reference signal may be an SS / PBCH block (SSB) or a Channel State Information (CSI) reference signal. The information indicating the switching condition may be information indicating radio quality to be satisfied for the first cell (current serving cell) and / or the second cell (candidate cell) to trigger LTM cell switching. For example, the information indicating the switching condition may be an event (condition) such as "Neighbor becomes offset better than SpCell," i.e., a trigger condition indicating that the radio quality of the second cell has improved by an offset amount relative to the radio quality of the first cell. In this case, the information indicating the switching condition may include an offset value. Alternatively, the information indicating the switching condition may be information indicating an event (condition) such as "Neighbor / SpCell becomes better / worse than threshold." In this case, the information indicating the switching condition may include a threshold.

[0098] 2) Information indicating a geographical location condition of the UE 100 that must be satisfied to trigger transmission of notification information: The geographical location condition may be a geographical location range (i.e., a geographical area) in which the UE 100 should trigger transmission of the notification information. The geographical location range may be determined by latitude and longitude. The geographical location range may be determined by latitude, longitude, and altitude. The UE 100 measures (i.e., positions) its own geographical location using, for example, a Global Navigation Satellite System (GNSS) receiver that it has, and transmits the notification information in response to the location condition specified in the notification condition information being satisfied.

[0099] 3) Information indicating an artificial intelligence or machine learning (AI (Artificial Intelligence) / ML (Machine Learning)) model used to infer whether or not a trigger condition for transmission of notification information has been satisfied: Such an AI / ML model may be a trained model that uses, for example, time-series data of wireless quality measurement values ​​as input, derives (infers) and outputs an optimal trigger timing for transmitting the notification information. UE 100 may hold such an AI / ML model in advance. UE 100 transmits the notification information in response to inferring that a trigger condition for transmission of the notification information has been satisfied using the AI / ML model.

[0100] In step S106, the UE 100 performs downlink synchronization processing (DL synchronization processing) for the second cell.

[0101] In step S107, the UE 100 starts evaluating whether the conditions (notification condition, switching condition) set in step S104 are satisfied. For example, the UE 100 measures the radio quality of the first cell and / or the radio quality of the second cell, and evaluates (determines) whether the measured value satisfies the set switching condition. The UE 100, in which multiple second cells are set as candidate cells, starts evaluation for each second cell.

[0102] If it is determined that the notification condition is satisfied (step S107: YES), in step S108, the UE 100 transmits notification information to the first cell in response to the evaluation that the notification condition is satisfied for any second cell. As described above, the notification information may be transmitted by being included in a MAC CE, an RRC message, a UCI, an SR, or an L1 Measurement Report. The notification information may include a cell ID (or a configuration ID) of the second cell for which the notification condition is satisfied.

[0103] In step S109, in response to receiving the notification information, gNB200 (first cell) transmits a PDCCH order to UE100. UE100 receives the PDCCH order (instruction information) from the first cell.

[0104] In step S110, UE100 transmits a CFRA RA preamble to the second cell specified in the PDCCH order (instruction information). Here, UE100 may transmit the preamble based on the CFRA setting set in step S104. gNB200 (second cell) receives the RA preamble and derives a TA value based on the received RA preamble. When preamble transmission is instructed for multiple second cells, UE100 may transmit a preamble to each second cell. In this case, gNB200 may derive a TA value for each second cell.

[0105] In step S111, the gNB200 (the first cell or the second cell) transmits an RAR including the derived TA value to the UE100. The UE100 receives the RAR from the first cell or the second cell. The RAR may include the TA values ​​of each of the second cells. For example, the RAR may include multiple sets of cell IDs and TA values ​​of the second cells.

[0106] In step S112, the UE 100 holds the TA value acquired in step S110 without applying it. The UE 100 may hold the TA value for each of the plurality of second cells.

[0107] In step S113, the UE 100 determines whether or not the switching condition is satisfied for any of the second cells. If it is determined that the switching condition is satisfied for any of the second cells (step S113: YES), in step S114, the UE 100 determines LTM cell switching to the second cell for which the switching condition is satisfied.

[0108] In step S115, the UE 100 detaches from the first cell, which is the source cell, and applies the configuration of the second cell, which is the target cell. Here, the UE 100 applies the TA value held for the second cell determined in step S114.

[0109] In step S116, the UE 100 applies the TA value to the second cell (target cell) determined in step S114 and transmits an RRC Reconfiguration Complete message. The gNB 200 receives the RRC Reconfiguration Complete message. When the UE 100 determines that the second cell has successfully received the first uplink data, it may consider that the execution of the LTM cell switching from the first cell to the second cell has been successfully completed.

[0110] FIG. 9 is a diagram showing an example of the configuration of an RRC Reconfiguration message (RRC message of step S104) according to the first operation pattern of this embodiment.

[0111] In the illustrated example, the RRC Reconfiguration message includes, as an information element, a conditional LTM configuration (C-LTM config) for configuring the conditional LTM cell switching in the UE 100. The conditional LTM configuration includes, as an information element, a candidate cell configuration list (C-LTM Config List) for adding the configuration of the conditional LTM cell switching for each candidate cell (second cell).

[0112] The candidate cell configuration list includes, as entries, candidate cell configurations, which are conditional LTM cell switching configurations for each candidate cell (second cell). The candidate cell configuration includes a configuration ID for identifying the configuration, a cell ID for identifying the corresponding candidate cell (second cell), notification condition information indicating notification conditions to be applied to the corresponding candidate cell (second cell), switching condition information indicating switching conditions to be applied to the corresponding candidate cell (second cell), an RRC configuration to be applied to communication with the corresponding candidate cell (second cell), and a RACH configuration (CFRA configuration) to be applied to the corresponding candidate cell (second cell). The candidate cell configuration may include a timer value (RAR timer value) of a timer that determines an upper limit of the RAR standby period for the corresponding candidate cell (second cell). The candidate cell configuration may also include information (RAR cell information) indicating whether the RAR should be received from the first cell or the second cell for the corresponding candidate cell (second cell).

[0113] In the illustrated example, a conditional LTM configuration (C-LTM Config) is newly provided instead of LTM Config, which is an information element introduced in 3GPP Release 18. However, as another configuration example, 3GPP Release 18 may be utilized, and the cell switching execution condition and the TA value may be added to each entry of the candidate cell configuration list (LTM Candidate To Add Mod List in 3GPP Release 18) in the LTM Config.

[0114] (1.4.2) Second Operation Pattern The second operation pattern will be described, focusing mainly on the differences from the first operation pattern. The second operation pattern is an operation pattern in which the UE 100 receives an RAR from the first cell in an inter-gNB scenario.

[0115] FIG. 10 is a diagram showing a second operation pattern of the mobile communication system 1 according to this embodiment.

[0116] In step S131, UE100 is in an RRC connected state in the first cell of gNB200a.

[0117] In step S132, UE100 transmits a measurement report message to gNB200a (first cell).

[0118] In step S133, gNB200a determines an LTM candidate cell (second cell) based on the Measurement Report message.

[0119] In step S134, the gNB 200a transmits a request message requesting preparation for LTM cell switching of the UE 100 to the gNB 200b managing the second cell. The request message may be a message transmitted on the Xn interface, for example, an Xn Handover Request message. The request message may include information requesting preparation (reservation) of CFRA resources. The request message may include information requesting LTM configuration. In response to receiving the request message, the gNB 200b reserves CFRA resources and determines LTM configuration.

[0120] In step S135, the gNB 200b transmits an acknowledgement message to the gNB 200a in response to the request message of step S134. The acknowledgement message may be a message transmitted on the Xn interface, for example, an Xn Handover Request Acknowledge message. The acknowledgement message includes the LTM configuration, and the LTM configuration includes CFRA information (CFRA configuration) regarding the reserved CFRA resources.

[0121] In step S136, gNB200a (first cell) transmits to UE100 an RRC Reconfiguration message including an LTM candidate cell configuration (C-LTM Candidate Configuration) including the LTM configuration received from gNB200b. The configuration of the LTM candidate cell configuration is the same as in the first operation pattern. The LTM candidate cell configuration includes a CFRA configuration based on the CFRA information from gNB200a (first cell).

[0122] In step S137, UE100 stores the LTM candidate cell setting received from gNB200a (first cell) and transmits an RRC Reconfiguration Complete message to gNB200a (first cell).

[0123] In step S138, the UE 100 performs downlink synchronization processing (DL synchronization processing) for the second cell.

[0124] In step S139, the UE 100 starts evaluating whether the condition set in step S136 is satisfied. For example, the UE 100 measures the radio quality of the first cell and / or the radio quality of the second cell, and evaluates (determines) whether the measurement value satisfies the set switching condition. The UE 100, in which a plurality of second cells are set as candidate cells, starts evaluation for each second cell.

[0125] If it is determined that the notification condition is satisfied for any of the second cells (step S139: YES), in step S140, the UE 100 transmits notification information to the first cell.

[0126] In step S141, gNB200a (first cell) may transmit a message including information requesting activation of CFRA resources to gNB200b. In response to receiving the message, gNB200b may activate the CFRA resources reserved for UE100. Note that gNB200b may allocate the CFRA resources to other UE100 until the CFRA resources are activated. This enables effective use of radio resources.

[0127] In step S142, the gNB 200a (first cell) transmits a PDCCH order specifying the second cell to the UE 100. The UE 100 receives the PDCCH order (instruction information) from the first cell.

[0128] In step S143, the UE 100 transmits a CFRA RA preamble to the second cell specified in the PDCCH order. Here, the UE 100 may transmit the preamble based on the CFRA setting set in step S136. The gNB 200b receives the RA preamble and derives a TA value based on the received RA preamble. The gNB 200b may identify the UE 100 that transmitted the preamble based on the RACH resource (CFRA resource) used for the preamble transmission. When preamble transmission is instructed for multiple second cells, the UE 100 may transmit a preamble to each second cell. In this case, the corresponding gNB 200 may derive a TA value for each second cell.

[0129] In step S144, the gNB 200b transmits a message including the TA value derived based on the RA preamble to the gNB 200a. The gNB 200a receives the message. The message may be a message transmitted on the Xn interface, for example, a new TA INFORMATION TRANSFER message. In the message, the TA value derived based on the RA preamble may be associated with the cell ID of the second cell and identification information for identifying the UE 100 specified based on the RACH resource (CFRA resource). The gNB 200a may identify the UE 100 corresponding to the TA value notified from the gNB 200b based on the identification information.

[0130] In step S145, the gNB 200a (first cell) transmits an RAR including the TA value notified from the gNB 200b to the UE 100. The UE 100 receives the RAR from the first cell. The RAR may include the TA values ​​of each of a plurality of second cells. For example, the RAR may include a plurality of sets of the cell IDs and TA values ​​of the second cells.

[0131] In step S146, the UE 100 holds the TA value acquired in step S143 without applying it. The UE 100 may hold the TA value for each of the plurality of second cells.

[0132] In step S147, the UE 100 determines whether or not the switching condition is satisfied for any of the second cells. If it is determined that the switching condition is satisfied for any of the second cells (step S147: YES), in step S148, the UE 100 determines LTM cell switching to the second cell for which the switching condition is satisfied.

[0133] In step S149, the UE 100 detaches from the first cell which is the source cell and applies the configuration of the second cell which is the target cell. Here, the UE 100 applies the TA value held for the second cell determined in step S146.

[0134] In step S150, the UE 100 applies the TA value to the second cell (target cell) determined in step S148 and transmits an RRC Reconfiguration Complete message. The gNB 200b (second cell) receives the RRC Reconfiguration Complete message. When the UE 100 determines that the second cell has successfully received the initial uplink data, the UE 100 may consider that the execution of the LTM cell switch from the first cell to the second cell has been successfully completed. The gNB 200b may transmit a notification to the gNB 200a indicating that the UE 100 has accessed the second cell.

[0135] (1.4.3) Third Operation Pattern The third operation pattern will be described, focusing on the differences from the first and second operation patterns. The third operation pattern is an operation pattern in which the UE 100 receives an RAR from the second cell in an inter-gNB scenario.

[0136] FIG. 11 is a diagram showing a third operation pattern of the mobile communication system 1 according to this embodiment.

[0137] The operations in steps S151 to S163 are the same as those in the second operation pattern.

[0138] In step S164, the gNB 200b (second cell) transmits an RAR including a TA value derived based on the RA preamble to the UE 100. The UE 100 receives the RAR from the second cell.

[0139] In step S165, the UE 100 holds the TA value acquired in step S142 without applying it. The UE 100 may hold the TA value for each of the plurality of second cells.

[0140] The operations in steps S166 to S169 are the same as those in the second operation pattern.

[0141] (2) Second Embodiment The second embodiment will be described mainly focusing on the differences from the first embodiment. The second embodiment is an embodiment that assumes a normal LTM, not a C-LTM. However, the second embodiment may also be applied to a C-LTM.

[0142] In the first embodiment, the UE 100 realizes early TA for the second cell (candidate cell, target cell) by triggering CFRA by the PDCCH order. In the second embodiment, instead of such CFRA, early TA for the second cell is realized by UE-based TA measurement. In the UE-based TA measurement, the UE 100 itself measures the TA value to be applied to the second cell.

[0143] (2.1) UE-Based TA Measurement FIG. 12 is a diagram illustrating an example of UE-based TA measurement according to this embodiment. In the illustrated example, it is assumed that the frame timing between the first cell and the second 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.

[0144] The UE-based TA measurement includes, for example, the following procedures.

[0145] 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".

[0146] 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.

[0147] 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.

[0148] 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".

[0149] Therefore, by calculating equation (1) based on parameters (variables) such as "TA_s", "Tdiff_s-t_ue", and "TA_temp_t", the TA value "TA_t" that UE100 should apply in the second cell can be derived without UE100 performing an RA procedure to the second cell.

[0150] (2.2) Operation According to the Second Embodiment The UE 100 configured for UE-based TA measurement attempts to acquire the TA value of the second cell by performing UE-based TA measurement. However, it is also assumed that the UE-based TA measurement fails, that is, the UE 100 cannot acquire a valid TA value by the UE-based TA measurement.

[0151] Therefore, in the second embodiment, the UE 100 transmits the above-described notification information to the first cell in response to a failure of the UE-based TA measurement. The notification information may be failure notification information indicating a failure of the UE-based TA measurement. This allows the UE 100 to transmit a CFRA preamble triggered by a PDCCH order from the first cell to the second cell, and to fall back to a conventional LTM procedure in which the CFRA is used to acquire the TA value of the second cell.

[0152] FIG. 13 is a diagram showing the operation of the UE 100 according to this embodiment.

[0153] In step S21, the UE 100 receives, from the first cell, a UE-based TA measurement configuration for performing LTM cell switching. The UE 100 receives the UE-based TA measurement configuration from the first cell in an RRC Reconfiguration message (see step S4 in FIG. 6 ). The UE-based TA measurement configuration may include at least one of information indicating a second cell to be the target of the UE-based TA measurement and a timer setting value of a timer for determining whether the UE-based TA measurement is successful (i.e., a timer that determines an upper limit of the time for performing the UE-based TA measurement). The RRC Reconfiguration message may include a RACH configuration, for example, a CFRA configuration, used for transmitting an RA preamble to the second cell.

[0154] For example, the RRC Reconfiguration message may include a candidate cell configuration list, which is a list including configuration information (candidate cell configuration) of each of a plurality of second cells. Each entry of the candidate cell configuration list may include a cell ID of the corresponding second cell and a UE-based TA measurement configuration to be applied to the corresponding second cell. The candidate cell configuration may include a RACH configuration (CFRA configuration) of the corresponding second cell.

[0155] In step S22, the UE 100 starts UE-based TA measurement for the second cell based on the UE-based TA measurement configuration. The UE 100 may start the UE-based TA measurement in response to receiving a cell switch command from the first cell that does not include a valid TA value. The UE 100 may perform UE-based TA measurement only for second cells for which the UE-based TA measurement is configured in the candidate cell configuration, among multiple second cells.

[0156] In step S23, UE 100 detects a failure of UE-based TA measurement. For example, UE 100 starts a timer at the timing of receiving the RRC Reconfiguration message in step S21, the timing of transmitting an RRC Reconfiguration Complete message in response to the RRC Reconfiguration message, the timing of starting UE-based TA measurement in step S22, or the timing of receiving a cell switching command. If UE 100 succeeds in measuring the TA value of the second cell while the timer is running, UE 100 stops the timer. If UE 100 does not succeed in measuring the TA value of the second cell and the timer expires, UE 100 determines that UE-based TA measurement has failed.

[0157] In step S24, the UE 100 transmits notification information to the first cell (gNB 200) in response to a failure in the UE-based TA measurement for any of the second cells. As described above, the notification information may be transmitted by being included in a MAC CE, an RRC message, a UCI, an SR, or an L1 Measurement Report. The notification information may include the cell ID (or configuration ID) of the second cell for which the UE-based TA measurement failed. This allows the gNB 200 to know for which second cell the UE-based TA measurement failed. Note that the UE 100 may transmit the notification information to the first cell (gNB 200) only when the UE-based TA measurement failed for a second cell for which the RACH setting (CFRA setting) is configured in the candidate cell setting among the plurality of second cells.

[0158] In step S25, UE100 receives the PDCCH order transmitted from the first cell (gNB200) based on the notification information.

[0159] In step S26, in response to the reception of the PDCCH order, the UE 100 performs preamble transmission (specifically, transmission of an RA preamble of CFRA) to the second cell specified in the PDCCH order.

[0160] In step S27, UE100 receives a cell switching command from the first cell (gNB200) including a TA value derived based on the preamble transmission.

[0161] In step S28, the UE 100 performs LTM cell switching by applying the TA value of the second cell to the second cell specified in the cell switching command. Here, the UE 100 performs LTM cell switching by applying the TA value notified in the cell switching command without performing a random access procedure for the second cell. When performing LTM cell switching, the UE 100 transmits an RRC Reconfiguration Complete message to the second cell. The UE 100 may consider that the execution of the LTM cell switching has been completed successfully when it is determined that the network (second cell) has successfully received the first uplink data.

[0162] (3) Other Embodiments 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.

[0163] In the above-described embodiments and examples, an example in which the base station is an NR base station (gNB) has been described, but the base station may be an LTE base station (eNB) or a 6G base station. The base station may also be a relay node such as an IAB (Integrated Access and Backhaul) node. The base station may also be a DU of the IAB node. The UE 100 may also be an MT (Mobile Termination) of the IAB node. That is, the UE 100 may be a terminal function unit (a type of communication module) for the base station to control a relay that relays signals. Such a terminal function unit is referred to as an MT. Examples of MTs include, in addition to IAB-MT, NCR (Network Controlled Repeater)-MT and RIS (Reconfigurable Intelligent Surface)-MT.

[0164] 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.

[0165] 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).

[0166] The functions performed by UE100 or gNB200 may be implemented in circuitry or processing circuitry, including general-purpose processors, application-specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), CPUs (Central Processing Units), conventional circuits, and / or combinations thereof, programmed to perform the described functions. A processor includes transistors and / or other circuits and is considered to be circuitry or processing circuitry. A processor may also be a programmed processor that executes a program stored in memory. In this specification, circuitry, unit, or means refers to hardware that is programmed to perform the described functions or hardware that executes them. The hardware may be any hardware disclosed herein or any hardware known to be programmed or capable of performing the described functions. If the hardware is a processor, the circuitry, means, or unit is a combination of hardware and software used to configure the hardware and / or processor.

[0167] 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.

[0168] 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.

[0169] This application claims priority from Japanese Patent Application No. 2024-017583 (filed February 8, 2024), the entire contents of which are incorporated herein by reference.

[0170] (4) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0171] Supplementary Note 1: A communication method used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), the communication method comprising: in a user equipment, receiving, from the first cell, a configuration for performing LTM cell switching, which switches a serving cell of the user equipment from a first cell to a second cell by the LTM; transmitting, to the first cell, notification information for requesting the first cell to transmit instruction information instructing the first cell to transmit a preamble for Contention Free Random Access (CFRA) to the second cell; and transmitting the preamble to the second cell in response to receiving the instruction information from the first cell.

[0172] Supplementary Note 2: The communication method according to Supplementary Note 1, wherein the user equipment receives a Physical Downlink Control Channel (PDCCH) command from the first cell, the PDCCH command including the indication information.

[0173] Supplementary Note 3: The communication method according to Supplementary Note 1 or 2, wherein the user equipment transmits a Medium Access Control Element (MAC CE) including the notification information to the first cell.

[0174] Supplementary Note 4: The communication method according to any one of Supplementary Notes 1 to 3, wherein the user equipment transmits the preamble to the second cell in response to the instruction information, receives a timing advance value derived based on the preamble transmission from the first cell or the second cell, and applies the timing advance value to perform the LTM cell switching.

[0175] Supplementary Note 5: The communication method according to any one of Supplementary Notes 1 to 4, wherein the configuration includes information indicating a switching condition that must be satisfied in order to perform the LTM cell switching.

[0176] Supplementary Note 6: The communication method according to any one of Supplementary Notes 1 to 5, wherein the configuration further includes information indicating a notification condition that must be satisfied in order to transmit the notification information to the first cell, and the user equipment evaluates whether the notification condition is satisfied, and transmits the notification information to the first cell in response to the evaluation that the notification condition is satisfied.

[0177] Supplementary Note 7: The communication method according to any one of Supplementary Notes 1 to 5, wherein the user equipment evaluates whether the switching condition is satisfied for the second cell, and transmits the notification information to the first cell when the evaluation indicates that the switching condition is satisfied.

[0178] Supplementary Note 8: The communication method according to any one of Supplementary Notes 1 to 7, wherein a first network node managing the first cell transmits, to a second network node managing the second cell, request information requesting reservation or activation of CFRA resources for the second cell in response to receiving the notification information from the user equipment.

[0179] Supplementary Note 9: The communication method according to any one of Supplementary Notes 1 to 8, wherein the configuration includes user equipment measurement configuration information for the user equipment itself to measure a timing advance value to be applied to the second cell, and the user equipment starts the user equipment measurement based on the configuration information, and transmits the notification information to the first cell in response to a failure of the user equipment measurement.

[0180] Supplementary Note 10: The communication method according to Supplementary Note 9, wherein the notification information is failure notification information indicating a failure of the user equipment measurement.

[0181] Supplementary Note 11: A user equipment used in a mobile communication system supporting LTM (L1 / L2 Triggered Mobility), comprising: a receiver that receives, from the first cell, a setting for performing LTM cell switching, which switches a serving cell of the user equipment from a first cell to a second cell by the LTM; a transmitter that transmits, to the first cell, notification information for requesting the first cell to transmit instruction information instructing the first cell to transmit a preamble of Contention Free Random Access (CFRA) to the second cell; and a controller that transmits the preamble to the second cell in response to receiving the instruction information from the first cell.

[0182] Supplementary Note 12: A network node that manages a first cell in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: a transmitter that transmits to the user equipment a configuration for performing LTM cell switching, which switches a serving cell of a user equipment from the first cell to a second cell by the LTM; and a receiver that receives, from the user equipment, notification information for requesting the first cell to transmit instruction information instructing the first cell to transmit a preamble of Contention Free Random Access (CFRA) to the second cell, wherein the transmitter transmits the instruction information to the user equipment in response to the notification information.

[0183] 1: Mobile communication system 5: Network 10: RAN 20: CN 100: UE 110: Receiving unit 120: Transmitting unit 130: Control unit 140: Wireless communication unit 200: gNB 210: Transmitting unit 220: Receiving unit 230: Control unit 240: Network communication unit 241: Transmitting unit 242: Receiving unit 250: Wireless communication unit 300: AMF / UPF

Claims

1. A communication method used in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: in a user equipment, receiving from the first cell a configuration for performing LTM cell switching, which switches the serving cell of the user equipment from a first cell to a second cell using LTM; transmitting to the first cell notification information requesting the first cell to transmit instruction information instructing the first cell to transmit a preamble for Contention Free Random Access (CFRA) to the second cell; and transmitting the preamble to the second cell in response to receiving the instruction information from the first cell.

2. The communication method according to claim 1, wherein the user equipment receives a Physical Downlink Control Channel (PDCCH) command including the indication information from the first cell.

3. The communication method according to claim 1, wherein the user equipment transmits a medium access control element (MAC CE) including the notification information to the first cell.

4. The communication method according to claim 1, wherein the user equipment transmits the preamble to the second cell in accordance with the instruction information, receives a timing advance value derived based on the preamble transmission from the first cell or the second cell, and applies the timing advance value to perform the LTM cell switching.

5. The communication method according to claim 1, wherein the configuration includes information indicating a switching condition that must be satisfied in order to perform the LTM cell switch.

6. The communication method according to claim 5, wherein the configuration further includes information indicating a notification condition that must be satisfied in order to transmit the notification information to the first cell, and the user equipment evaluates whether the notification condition is satisfied, and transmits the notification information to the first cell in response to the evaluation that the notification condition is satisfied.

7. The communication method according to claim 5, wherein the user equipment evaluates whether the switching condition is satisfied for the second cell, and transmits the notification information to the first cell in response to the evaluation that the switching condition is satisfied.

8. The communication method according to claim 1, wherein a first network node managing the first cell, in response to receiving the notification information from the user equipment, transmits request information to a second network node managing the second cell, requesting reservation or activation of CFRA resources for the second cell.

9. The communication method according to claim 1, wherein the configuration includes user equipment measurement configuration information for the user equipment itself to measure a timing advance value to be applied to the second cell, and the user equipment starts the user equipment measurement based on the configuration information, and transmits the notification information to the first cell in response to a failure of the user equipment measurement.

10. The communication method according to claim 9, wherein the notification information is failure notification information indicating a failure of the user equipment measurement.

11. A user equipment used in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: a receiving unit that receives from the first cell a setting for performing LTM cell switching, which switches the serving cell of the user equipment from a first cell to a second cell using the LTM; a transmitting unit that transmits to the first cell notification information requesting the first cell to transmit instruction information instructing the first cell to transmit a preamble for Contention Free Random Access (CFRA) to the second cell; and a control unit that transmits the preamble to the second cell in response to receiving the instruction information from the first cell.

12. A network node that manages a first cell in a mobile communication system that supports LTM (L1 / L2 Triggered Mobility), comprising: a transmitter that transmits to the user equipment a setting for performing LTM cell switching, which switches the serving cell of the user equipment from the first cell to a second cell using the LTM; and a receiver that receives, from the user equipment, notification information that requests the first cell to transmit instruction information instructing the first cell to transmit a contention-free random access (CFRA) preamble to the second cell, and the transmitter transmits the instruction information to the user equipment in response to the notification information.

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