Communication method, network node, and user equipment

Abstract

A communication method for performing cell switching that switches a serving cell of user equipment from a first cell to a second cell includes: a first condition for executing early uplink synchronization with the second cell and / or a second condition for executing the cell switching being set as a condition for executing an operation related to the cell switching for the user equipment by a first network node that manages the first cell; and a notification indicating the execution of the operation being received by the first network node, on the basis of the condition being satisfied in the user equipment, from the user equipment or from a second network node that manages the second cell.

Description

COMMUNICATION METHOD, NETWORK NODE, AND USER EQUIPMENT

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

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

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

[0004] 3GPP contribution: R2-2309335

[0005] The present disclosure relates to a communication method, a network node, and a user equipment for efficiently implementing conditional long-term mobile (C-LTM) and / or RACH-less conditional handover (RL-CHO).

[0006] A communication method according to a first aspect is a communication method for performing cell switching, which switches a serving cell of a user equipment from a first cell to a second cell, and includes: a first network node managing the first cell setting, in the user equipment, a first condition for performing uplink early synchronization with the second cell and / or a second condition for performing the cell switch as conditions for performing an operation related to the cell switch; and the first network node receiving, based on the conditions being satisfied in the user equipment, a notification indicating the execution of the operation from the user equipment or a second network node managing the second cell.

[0007] A network node according to a second aspect is a network node that manages a first cell in a mobile communication system that performs cell switching to switch a serving cell of a user equipment from a first cell to a second cell, and includes: a control unit that sets, in the user equipment, a first condition for performing uplink early synchronization with the second cell and / or a second condition for performing the cell switching as conditions for performing an operation related to the cell switching; and a receiving unit that receives, based on the conditions being satisfied in the user equipment, a notification indicating the execution of the operation from the user equipment or a second network node that manages the second cell.

[0008] A user equipment according to a third aspect is a user equipment in a mobile communication system that performs cell switching to switch the serving cell of the user equipment from a first cell to a second cell, and has: a receiving unit that receives, from a first network node that manages the first cell, information that sets a first condition for performing uplink early synchronization with the second cell and / or a second condition for performing the cell switching as conditions for performing an operation related to the cell switching; and a transmitting unit that transmits a notification to the first network node indicating that the operation is to be performed based on the conditions being satisfied.

[0009] A network node according to a fourth aspect is a network node that manages a second cell in a mobile communication system that performs cell switching to switch a serving cell of a user equipment from a first cell to a second cell, and includes: a receiving unit that receives a random access signal for uplink early synchronization from the user equipment, in response to the condition being set for performing uplink early synchronization with the second cell, and the condition being satisfied in the user equipment; and a transmitting unit that transmits a notification indicating the execution of the uplink early synchronization to the first network node based on the reception of the random access signal.

[0010] 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 for explaining an operation scenario of a mobile communication system according to an embodiment. FIG. 7 is a diagram showing an example of an LTM procedure that is being specified in 3GPP Release 18. FIG. 8 is a diagram showing an overview of UE operation related to C-LTM / RL-CHO according to an embodiment. FIG. 9 is a diagram showing a specific example of C-LTM / RL-CHO operation of a mobile communication system according to an embodiment. FIG. 10 is a diagram for explaining another operation scenario of a mobile communication system according to an embodiment. FIG. 11 is a diagram showing gNB operation related to improvement of C-LTM / RL-CHO according to an embodiment. FIG. 12 is a diagram showing an example of a first operation pattern according to an embodiment. FIG. 13 is a diagram showing an example of a second operation pattern according to an embodiment.

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

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

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

[0014] The UE 100 is a mobile wireless communication device. The UE 100 may be any device that is used by a user. For example, the UE 100 may be a mobile phone terminal (including a smartphone) and / or a tablet terminal, a notebook PC, a communication module (including a communication card or a chipset), a sensor or a device provided in a sensor, a vehicle or a device provided in a vehicle (Vehicle UE), or an aircraft or a device provided in an aircraft (Aerial UE).

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

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

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

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

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

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

[0021] The control unit 130 performs various controls and processes in the UE 100. Such processes include processes of each layer described below. The operations of the UE 100 described above and below may be operations 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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0038] (2) Overview of LTM LTM (L1 / L2-triggered mobility) is a technology for shortening mobility delay (specifically, delay in serving cell switching) compared to general handover procedures by triggering serving cell switching (cell switching) through signaling of lower layers, Layer 1 (L1) and / or Layer 2 (L2).

[0039] In a typical 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 (specifically, an RRC Reconfiguration message), which is an RRC message, from gNB200 to UE100.

[0040] A common handover application procedure is a conditional handover (CHO). In a conditional handover, a measurement report message, which is an RRC message, is transmitted from the UE 100 to the gNB 200. Based on the measurement report message, the gNB 200 determines a handover candidate cell (CHO candidate cell) for the UE 100, and the gNB 200 sets a handover execution condition (CHO trigger condition) in the UE 100 using an RRC message, an RRC Reconfiguration message. The UE 100 evaluates whether the handover execution condition is met and accesses the target cell when the handover execution condition is met (i.e., the UE 100 spontaneously triggers a handover).

[0041] In contrast, in LTM, first, the gNB 200 prepares an LTM candidate cell configuration for a candidate cell to be switched to and provides the LTM candidate cell configuration to the UE 100 by RRC signaling. Second, the UE 100 performs synchronization processing with the candidate cell by early synchronization (Early sync). Third, the gNB 200 receives an L1 measurement report from the UE 100, determines cell switching 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 configuration) to the UE 100 by MAC CE. The cell switch trigger is conveyed in a MAC CE that includes at least a candidate configuration index together with a beam indicator. Fourth, the UE 100 changes the serving cell in response to the cell switch command from the gNB 200 (source cell). In this way, cell switching is triggered by gNB200 by selecting the LTM candidate cell setting as the target setting. The LTM candidate cell setting can be added, changed, and released by gNB200 via RRC signaling.

[0042] The following principles apply to LTM:

[0043] 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.

[0044] If a full candidate cell configuration is applied, it replaces the current UE configuration at cell switch. A reconfiguration procedure does the replacement but does not necessarily reset the MAC, RLC or PDCP layers.

[0045] The user plane continues without a reset if configured in RRC signaling to avoid additional delays in data recovery.

[0046] - Security is not updated in LTM.

[0047] 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.

[0048] 6 is a diagram for explaining an operation scenario of the mobile communication system 1 according to the embodiment. The UE 100 performs serving cell switching (cell switching) from a first cell #a (source cell, current serving cell) of the gNB 200 to a second cell #b of the same gNB 200. Note that the cell switching may be synonymous with handover. The first cell #a and the second cell #b may be configured with different TRPs (Transmission and Reception Points). Until the cell switching by LTM is determined, the second cell #b is also referred to as a "candidate cell (or LTM candidate cell)", and after the cell switching by LTM is determined, the second cell #b is also referred to as a "target cell".

[0049] FIG. 7 is a diagram showing an example of an LTM procedure that is currently being developed as a specification in 3GPP Release 18.

[0050] In step S1, UE100 is in an RRC connected state in the cell (first cell) of gNB200.

[0051] In step S2, UE100 transmits a Measurement Report message, which is an RRC message, to gNB200.

[0052] In step S3, gNB200 decides to use LTM based on the Measurement Report message and starts preparing the candidate cell.

[0053] In step S4, gNB200 sends an RRC Reconfiguration message to UE100, which includes LTM candidate cell configurations (LTM Candidate Configurations) for one or more candidate cells.

[0054] In step S5, UE100 saves the LTM candidate cell setting and sends an RRC Reconfiguration Complete message to gNB200.

[0055] In step S6, the UE 100 may perform synchronization processing with the candidate cell before receiving the cell switch command. Such synchronization processing is called early synchronization (Early sync). Here, the UE 100 may perform early timing advance (TA) acquisition in the candidate cell requested by the gNB 200 (source cell) before receiving the cell switch command of step S9. This is performed by contention-free random access (CFRA) triggered by a PDCCH command (PDCCH order) from the source cell. Note that when DCI Format 1_0 is used and all "Frequency domain resource assignment" fields in the DCI are set to "1", the DCI is treated as a PDCCH order. The UE 100 transmits a random access preamble (RA preamble) to the specified candidate cell. In order to minimize communication interruption of the source cell due to 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 of step S9. The TA value is a value for adjusting the uplink transmission timing of the UE 100. The TA value may be an offset value representing the difference between the frame start timing of the downlink and the frame start timing of the uplink.

[0056] 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).

[0057] In step S8, gNB200 decides to perform cell switching to the target cell (second cell).

[0058] In step S9, the gNB 200 transmits a Cell Switch Command (MAC CE) including a candidate configuration index of the target cell to the UE 100. The Cell Switch Command may include a TA value determined by early synchronization.

[0059] 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 #a) and applies the configuration of the target cell.

[0060] In step S11, if the 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 cell switch (for example, if the Cell Switch Command includes a valid TA value), the random access procedure can be skipped.

[0061] In step S12, the UE 100 indicates that the 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 cell switches based on the configuration provided in step S4.

[0062] (3) C-LTM / RL-CHO In the cell switching procedure by LTM as shown in Fig. 7, similar to the conditional handover, by setting the execution condition (trigger condition) of cell switching in advance in UE 100 by the RRC Reconfiguration message of step S4, it is possible to skip the operations from the L1 Measurement Report of step S7 to the Cell Switch Command (MAC CE) of step S9, thereby speeding up cell switching and minimizing momentary interruptions in data communication. Such a conditional cell switching procedure by LTM is also called conditional LTM (C-LTM: Conditional LTM).

[0063] Also, one of the features of LTM is early synchronization (Early sync). Early synchronization makes it possible to skip the random access procedure when performing a cell switch. Such early synchronization may also be applicable to conditional handover. Conditional handover that uses early synchronization to skip the random access procedure when performing a cell switch (handover) is also called RACH-less CHO (RL-CHO). Note that although RL-CHO skips the random access procedure when performing a cell switch, it performs the random access procedure when performing early synchronization, and does not mean that it is a procedure that does not perform the random access procedure at all.

[0064] Here, in conventional LTM early synchronization, the UE 100 triggers CFRA by receiving a PDCCH order from the source cell before receiving a Cell Switch Command (MAC CE) from the current serving cell (source cell). However, in C-LTM and / or RL-CHO (hereinafter referred to as "C-LTM / RL-CHO"), the UE 100 voluntarily triggers cell switching, so the gNB 200 cannot determine when to cause the UE 100 to perform early synchronization. Therefore, there is a concern that the PDCCH order cannot be used and CFRA cannot be used. Although it is possible to reserve resources for CFRA for the UE 100 in the candidate cell, the gNB 200 cannot determine when the UE 100 will trigger CFRA, which may result in wasted CFRA resources. In addition, in C-LTM / RL-CHO, the gNB200 does not transmit a Cell Switch Command (MAC CE) to the UE100 as in conventional LTM, so the TA value cannot be notified to the UE100 by the Cell Switch Command (MAC CE).

[0065] In the embodiment, in addition to setting the cell switching with a trigger condition in the UE 100, by setting the early synchronization with a trigger condition in the UE 100, the UE 100 can spontaneously trigger each of the early synchronization and the cell switching. Here, in the early synchronization, contention-free random access (CBRA) is used instead of CFRA.

[0066] 8 is a diagram showing an outline of the operation of UE 100 regarding C-LTM / RL-CHO according to the embodiment. The operation of FIG. 8 is an operation for performing cell switching to change the serving cell of UE 100 in the RRC connected state from the first cell #a to the second cell #b. Specifically, the operation of FIG. 8 is an operation for realizing C-LTM / RL-CHO.

[0067] In step S11, the UE 100 receives configuration information from the first cell #a (gNB 200) including first information for specifying a first condition for triggering early synchronization related to cell switching and second information for specifying a second condition for triggering cell switching. As a result, the trigger conditions for early synchronization and the trigger conditions for cell switching are each set in the UE 100. Hereinafter, such configuration information may be referred to as "C-LTM / RL-CHO configuration".

[0068] In step S12, the UE 100 performs early synchronization including a random access procedure for the second cell #b in response to the first condition set in step S11 being satisfied. Here, the early synchronization includes downlink early synchronization and uplink early synchronization. The UE 100 performs the random access procedure for uplink early synchronization (i.e., acquisition of a TA value).

[0069] In step S13, after the early synchronization is completed, the UE 100 performs cell switching without a random access procedure in response to the second condition set in step S11 being satisfied.

[0070] According to this operation, the UE 100 can spontaneously trigger early synchronization when the first condition is set by the gNB 200 and the first condition is met. Therefore, the UE 100 can trigger early synchronization even without receiving a PDCCH order from the first cell #a (source cell). Then, after the early synchronization is completed, the UE 100 performs cell switching to the second cell #b (target cell) without a random access procedure when the set second condition is met. This enables C-LTM / RL-CHO to be realized, and the time from when the second condition is met to when the cell switching is completed can be shortened.

[0071] The UE 100 performing such an operation has a receiver 110 that receives from the first cell #a configuration information including first information for specifying a first condition for triggering early synchronization regarding cell switching from the first cell #a to the second cell #b and second information for specifying a second condition for triggering cell switching, and a control unit 130 that performs early synchronization including a random access procedure for the second cell #b in response to the first condition being satisfied. After the early synchronization is completed, the control unit 130 performs cell switching without a random access procedure in response to the second condition being satisfied. Meanwhile, the gNB 200 has a transmitter 210 that transmits to the UE 100 in the first cell #a configuration information including the first information for specifying a first condition for triggering early synchronization regarding cell switching from the first cell #a to the second cell #b and second information for specifying a second condition for triggering cell switching.

[0072] The first information in step S11 may include at least one of the following information 1) to 4).

[0073] 1) Information indicating reception quality conditions to be satisfied for the first cell #a (serving cell) and / or the second cell #b (candidate cell) to trigger early synchronization: The reception quality may be at least one measurement value among reference signal received power (RSRP), reference signal received quality (RSRQ), signal-to-interference-and-noise ratio (SINR), received signal strength indicator (RSSI), bit error rate (BER), and block error rate (BLER). The reception quality condition may be a threshold value to be compared with these measurement values. The information indicating the reception quality condition may include a threshold value for the first cell #a (serving cell) and a threshold value for the second cell #b (candidate cell). In step S12, the UE 100 performs early synchronization in response to the reception quality condition specified in the first information being satisfied (i.e., the reception quality measurement value satisfying the threshold condition).

[0074] 2) Information indicating a geographical location condition of the UE 100 that must be satisfied to trigger early synchronization: The geographical location condition may be a geographical location range (i.e., a geographical area) in which the UE 100 should trigger early synchronization. The UE 100 measures (i.e., positions) its own geographical location using, for example, a Global Navigation Satellite System (GNSS) receiver that the UE 100 has, and performs early synchronization when the location condition specified in the first information is satisfied.

[0075] 3) Information indicating an artificial intelligence or machine learning (AI / ML) model used to infer whether the first condition is satisfied: Such an AI / ML model may be a trained model that uses, for example, time-series data of reception quality measurements as input, derives (infers) and outputs an optimal trigger timing for early synchronization. UE 100 may store such an AI / ML model in advance. In step S12, UE 100 performs early synchronization in response to the inference that the first condition is satisfied using the AI / ML model.

[0076] 4) Information indicating reception of the first information or the setting information of step S11 as the first condition: In this case, in step S12, the UE 100 may immediately perform early synchronization in response to reception of the first information or the setting information of step S11. That is, the UE 100 may perform early synchronization in response to the setting of C-LTM / RL-CHO. The information of 4) may be flag information included in the setting information of step S11. The information may be character string information indicating the name of the first information or the setting information of step S11.

[0077] The configuration information of step S11 may include a TA value for the second cell #b. In step S12, the UE 100 may perform early synchronization (specifically, uplink early synchronization) only when the TA value for the second cell #b is not included in the configuration information. When the TA value for the second cell #b is included in the configuration information, the UE 100 may skip the early synchronization of step S12 (specifically, uplink early synchronization).

[0078] The random access procedure in the early synchronization of step S12 may be a contention-based random access (CBRA) procedure. This allows the UE 100 to perform the random access procedure even if it does not receive a PDCCH order from the first cell #a (serving cell). Information indicating candidates for PRACH (Physical Random Access Channel) resources to be applied to CBRA (specifically, candidates for random access preambles and / or candidates for PRACH time / frequency resources) may be included in system information (SIB: System Information Block) broadcast from the second cell #b (candidate cell). This information may be included in the configuration information of step S11. The configuration information of step S11 may explicitly or implicitly specify the CBRA procedure to the UE 100 as the random access procedure in the early synchronization. Unlike the CFRA in which a dedicated random access preamble is assigned to the UE 100, the CBRA allows the UE 100 to select a random access preamble by itself from random access preamble candidates (a group of random access preambles). Therefore, while the CFRA does not cause contention of random access preambles between UEs, the CBRA may cause contention of random access preambles between UEs.

[0079] The CBRA procedure in early synchronization in step S12 includes transmitting a random access preamble (RA preamble) from UE 100 to the second cell #b, and UE 100 receiving a random access response (RAR) from the second cell #b.

[0080] In step S12, the UE 100 identifies a PRACH resource to be used exclusively for transmitting a random access preamble for early synchronization. For example, some of the candidates for the PRACH resource for CBRA are prepared (designated) as PRACH resources dedicated to early synchronization. The UE 100 transmits the random access preamble using the identified PRACH resource (the random access preamble dedicated to early synchronization and / or the PRACH time-frequency resource dedicated to early synchronization).

[0081] The RAR includes a TA value for the second cell #b. In step S13, when the second condition is satisfied, the UE 100 checks whether the TA value notified in the RAR is valid. If the TA value is valid, the UE 100 performs cell switching without a random access procedure. On the other hand, if the TA value is invalid when the second condition is satisfied, the UE 100 performs cell switching with a random access procedure in step S13.

[0082] The setting information in step S11 may include at least one of the following information 1) to 3).

[0083] 1) Timer value indicating the validity period of the TA value: In this case, when the UE 100 receives an RAR including the TA value, it starts a timer (TAT: Time Alignment Timer) to which the timer value is set. The UE 100 considers the TA value to be valid while the TAT is running, and considers the TA value to be invalid when the TAT expires.

[0084] 2) Information indicating a geographical range in which the TA value is valid: In this case, the UE 100 checks whether the geographical location of the UE 100 is within a specified geographical range to confirm whether the TA value is valid. If the UE 100's geographical location is within the specified geographical range, the UE 100 considers the TA value to be valid. On the other hand, if the UE 100's geographical location is outside the specified geographical range, the UE 100 considers the TA value to be invalid.

[0085] 3) Information indicating the reception quality range of the second cell #b for which the TA value is valid: In this case, in order to confirm whether the TA value is valid, the UE 100 checks whether the reception quality of the second cell #b in the UE 100 is within a specified reception quality range. If the reception quality of the second cell #b is within the specified reception quality range, the UE 100 considers the TA value to be valid. On the other hand, if the reception quality of the second cell #b is outside the specified reception quality range, the UE 100 considers the TA value to be invalid.

[0086] The RAR may include a temporary identifier of the UE 100 in the second cell #b, for example, a Cell Radio Network Temporary Identifier (C-RNTI). Such a C-RNTI is also referred to as a Temporary C-RNTI. In step S13, the UE 100 may transmit an RRC message (for example, an RRC Reconfiguration Complete message) including the Temporary C-RNTI to the second cell #b as the first transmission to the second cell #b.

[0087] FIG. 9 is a diagram showing a specific example of the C-LTM / RL-CHO operation of the mobile communication system 1 according to the embodiment.

[0088] In step S101, UE100 is in an RRC connected state (RRC Connected) in the first cell #a (serving cell) of gNB200. UE100 may transmit capability information (UE capability) indicating that it supports C-LTM / RL-CHO to gNB200. gNB200 may receive the capability information (UE capability).

[0089] In step S102, the UE 100 transmits a Measurement Report message, which is an RRC message, to the gNB 200 (serving cell). The gNB 200 receives the Measurement Report message. The Measurement Report message includes at least one measurement value of the reception quality of the first cell #a (serving cell) and the reception quality of the second cell #b (candidate cell).

[0090] In step S103, gNB200 decides to configure C-LTM / RL-CHO for UE100 and identifies a candidate cell.

[0091] In step S104, gNB200 transmits the C-LTM / RL-CHO configuration to UE100. gNB200 may transmit an RRC message (specifically, an RRC Reconfiguration message) including the C-LTM / RL-CHO configuration to UE100. UE100 receives the C-LTM / RL-CHO configuration from gNB200 (serving cell).

[0092] The C-LTM / RL-CHO configuration includes a) configuration information related to conditional early synchronization and b) other mobility-related configuration information. The configuration information related to conditional early synchronization in a) may include at least one of the following information a1) to a4).

[0093] a1) First Condition: Such configuration information (Early sync trigger config.) explicitly indicates the first condition. The first condition may be a wireless quality threshold such as RSRP / RSRQ / SINR. The first condition may be a data reception quality threshold such as BLER. The first condition may be location information indicating a geographical range determined by latitude, longitude, altitude, etc. The first condition may be an event (e.g., a measurement event or a conditional event) defined in the RRC protocol. The first condition may be information indicating the use of model inference using an AI (Artificial Intelligence) / ML (Machine Learning) model, such as a model ID or function ID of the AI / ML model.

[0094] a2) Information specifying execution of conditional early synchronization: Such configuration information (Early sync trigger config.) implicitly indicates the first condition. When this information is included in the C-LTM / RL-CHO configuration, the UE 100 determines to execute conditional early synchronization. In this case, the UE 100 may determine the trigger timing of early synchronization by referring to the C-LTM / RL-CHO execution condition (second condition). For example, the UE 100 adds an offset to the reception quality or its threshold used in the conditional event (for example, adds +3 dB to the RSRP value of the serving cell). The UE 100 triggers early synchronization when such a first condition is satisfied. Alternatively, the UE 100 may immediately trigger early synchronization when it receives an RRC Reconfiguration message including configuration information for conditional early synchronization.

[0095] a3) Configuration information for PRACH resources dedicated to early synchronization: By defining a PRACH resource dedicated to early synchronization, the gNB 200 can distinguish between PRACH transmission for initial access that is not a cell switch and PRACH transmission for early synchronization. Based on the configuration information in a3), the UE 100 selects a PRACH resource to be used for early synchronization from among PRACH resource candidates dedicated to early synchronization among PRACH resource candidates for CBRA. This division of PRACH resources is also referred to as PRACH partitioning. In PRACH partitioning, a portion of the PRACH time-frequency resources and / or a portion of the random access preamble (preamble sequence) is designated (prepared) for a specific use. For example, the UE 100 selects a PRACH resource to be used for early synchronization from among PRACH resource candidates dedicated to early synchronization among random access preamble candidates for CBRA. By using such a PRACH resource dedicated to early synchronization, the gNB 200 that received the random access preamble from the UE 100 can identify that the RA transmission (access) is due to early synchronization of C-LTM / RL-CHO based on the PRACH resource applied to the received random access preamble. Note that the setting information for the PRACH resource dedicated to early synchronization is not limited to the case where it is signaled individually to the UE in the C-LTM / RL-CHO setting, but the first cell #a or the second cell #b may be signaled commonly to multiple UEs in the SIB.

[0096] a4) Configuration information regarding the validity period of the TA value: The TA value for the second cell #b, which is a candidate cell, can change its optimal value depending on the positional relationship (propagation distance) between the UE 100 and the second cell #b, so the validity period of the TA value is determined. The configuration information of a4) may indicate the validity period of the TA value as any of the following.

[0097] Time length: In this case, the configuration information of a4) indicates the time length of the validity period of the TA value acquired by the UE 100. For example, the configuration information may be a timer value set in a timer that measures the validity period of the TA value.

[0098] UE movement distance: In this case, the setting information of a4) is the movement distance of the UE 100 after the UE 100 acquires the TA value, and indicates the upper limit of the movement distance of the UE 100 for which the TA value is valid. The setting information may be a movement distance threshold. In other words, the setting information may be information on the area range for which the TA value is valid.

[0099] UE location: In this case, the configuration information of a4) indicates the geographical location (for example, latitude, longitude, and altitude) of the UE 100 for which the TA value is valid.

[0100] Reception quality of second cell #b: In this case, the configuration information of a4) may be a reception quality threshold (RSRP threshold, etc.) of second cell #b, which is a candidate cell. UE 100 considers that the TA value is valid if the reception quality of second cell #b is within a reception quality range specified by the configuration information. When multiple candidate cells exist, the configuration information of a4) may be specified separately for each candidate cell. That is, the configuration information may be associated with the cell ID of the candidate cell.

[0101] Meanwhile, the other mobility-related configuration in b) may include at least one of the following information in b1) to b4). Note that the configuration information in b) may be similar to configuration information for general CHO (conditional reconfiguration) that does not use conditional early synchronization. When multiple candidate cells exist, the other mobility-related configuration in b) may be specified separately for each candidate cell.

[0102] b1) Candidate Cell ID: The cell ID of the candidate cell.

[0103] b2) Second Condition (CondEvent): Information indicating a reception quality condition that must be met for the serving cell and / or candidate cell to trigger a cell switch.

[0104] b3) RRC configuration: The RRC configuration applied in the candidate cell.

[0105] b4) Configured Grant (CG) Configuration: This is configuration information for configuring uplink radio resources (e.g., PUSCH resources) available for initial uplink transmission in a candidate cell.

[0106] In step S105, the UE 100 transmits an RRC Reconfiguration Complete message indicating that the configuration by the RRC Reconfiguration message of step S104 has been completed to the gNB 200 (serving cell). The gNB 200 receives the RRC Reconfiguration Complete message.

[0107] In step S106, the UE 100 performs reception quality measurement based on the setting in step S104 (particularly, setting information related to conditional early synchronization), evaluates whether or not a first condition (Early sync trigger condition) is satisfied, detects that the condition is satisfied, and triggers early synchronization.

[0108] In step S107, the UE 100 performs downlink early synchronization (DL synchronization) with the second cell #b. The UE 100 may establish downlink synchronization with the second cell #b by using synchronization signals (primary synchronization signal and secondary synchronization signal) included in a synchronization signal (SSB) received from the second cell #b, which is a candidate cell.

[0109] In step S108, the UE 100 performs early synchronization (UL synchronization) of the uplink with the second cell #b.

[0110] Specifically, in step S108a, UE100 performs PRACH transmission (random access preamble transmission) using a PRACH resource dedicated to early synchronization among CBRA resources. gNB200 receives the random access preamble from UE100. gNB200 recognizes that PRACH transmission has been performed using a PRACH resource dedicated to early synchronization and determines that it is a C-LTM / RL-CHO RACH procedure. gNB200 may recognize that transmitting the RAR in step S108b and receiving Msg3 (RRC Reconfiguration Complete message) in step S111 will be delayed.

[0111] In step S108b, gNB200 transmits an RAR (Random Access Response) to UE100. UE100 receives the RAR. As with a normal CBRA procedure, the RAR may include at least one of a TA value, a Temporary C-RNTI, and an UL grant indicating an uplink radio resource (e.g., a PUSCH resource) used in step S111.

[0112] In step S109, the UE 100 starts a TA validity check based on the setting in step S104 (particularly, setting information related to conditional early synchronization). For example, the UE 100 may start a timer (TAT).

[0113] In step S110, the UE 100 evaluates whether or not a trigger condition (Execution condition) for cell switching by C-LTM / RL-CHO is satisfied based on the settings in step S104 (especially other mobility-related settings), and detects that the condition is satisfied. For example, the UE 100 detects that CondEvent A3 is satisfied.

[0114] In step S111, if the TA value is valid, the UE 100 transmits an RRC Reconfiguration Complete message (Msg3) to the second cell #b (target cell). The UE 100 may determine that the TA value is valid if the timer (TAT) is running. The gNB 200 receives the RRC Reconfiguration Complete message.

[0115] Here, the UE 100 may transmit the RRC Reconfiguration Complete (Msg3) using uplink resources (PUSCH transmission) allocated by either a CG (configured grant) or a UL grant. For example, if the CG setting has been performed in step S104, the UE 100 performs PUSCH transmission using the CG resource. If the CG setting has not been performed in step S104, the UE 100 monitors the PDCCH of the second cell #b (target cell), and when receiving a PDCCH (UL grant, also referred to as "Dynamic grant") scrambled with the Temporary C-RNTI, the UE 100 may perform PUSCH transmission according to the UL grant. When the UE 100 receives the UL grant in step S108b, the UE 100 may perform PUSCH transmission in accordance with the UL grant.

[0116] On the other hand, if the TA value is not valid, the UE 100 performs a new RACH procedure (CBRA) for the second cell #b and transmits an RRC Reconfiguration Complete message to the second cell #b. For example, if the timer (TAT) has expired, the UE 100 determines that the TA value is invalid. Alternatively, if the TA value is not valid, the UE 100 may perform early synchronization again. Whether to perform early synchronization when the TA expires may be configured in the UE 100 by the gNB 200.

[0117] In step S108, UE 100 may perform CFRA using the PDCCH order, similar to step S6 in Fig. 6. Furthermore, when the timer (TAT) expires (i.e., before step S111), UE 100 may discard the currently acquired TA value and may perform the UL synchronization process (CBRA) of step S108 again. Furthermore, UE 100 may stop (or discard) the timer (TAT) when cell switching is completed in step S113.

[0118] In step S112, gNB200 transmits Msg4 to UE100. UE100 receives Msg4 from gNB200 (target cell). UE100 may determine that C-LTM / RL-CHO is completed when it receives PDCCH (DCI including CRC (Cyclic Redundancy Code) scrambled with Temporary C-RNTI) from the target cell (step S113). The DL transmission from gNB200 may be accompanied by PDSCH (Contention Resolution MAC CE).

[0119] If a contention occurs and contention resolution fails, the UE 100 may perform the RACH procedure (CBRA) again. The UE 100 may consider such a failure as a handover failure (HOF). The UE 100 may initiate access (CBRA) to another candidate cell.

[0120] In the above-described operation scenario, cell switching (C-LTM / RL-CHO) between cells within the same gNB (within the same CU), i.e., intra-gNB C-LTM / RL-CHO was assumed. However, as shown in FIG. 10, the operation according to the above-described embodiment may be applied to cell switching (C-LTM / RL-CHO) between cells of different gNBs (different CUs), i.e., inter-gNB C-LTM / RL-CHO. In the illustrated example, the first cell #a may be managed by the gNB 200a, and the second cell #a may be managed by the gNB 200b. In this case, between steps S103 and S104 of Figure 9, a C-LTM / RL-CHO request message may be sent from gNB200a to gNB200b over the Xn interface, and a C-LTM / RL-CHO response message may be sent from gNB200b to gNB200a over the Xn interface.

[0121] Here, the request message may be a Handover Request message including information indicating that the C-LTM / RL-CHO is a C-LTM / RL-CHO or that the C-LTM / RL-CHO is desired. The request message may be a request message used exclusively for the C-LTM / RL-CHO. The Handover Request message is a request message used in general handover and conditional handover.

[0122] The response message may be a Handover Request Acknowledge message. This response message may be a response message used exclusively for C-LTM / RL-CHO. The Handover Request Acknowledge message is a response message used for general handover and conditional handover. The response message may include information indicating that the C-LTM / RL-CHO has admitted the HO and / or settings related to early synchronization (such as trigger conditions).

[0123] (4) Improvement of C-LTM / RL-CHO In the case of general LTM, the first cell #a (gNB200) explicitly transmits a Cell Switch Command MAC CE to the UE 100, so the first cell #a (gNB200) knows when the UE 100 performs cell switching. However, in the case of C-LTM / RL-CHO, since cell switching is performed at the UE origin, the first cell #a (gNB200) does not know when the UE 100 performs cell switching.

[0124] Therefore, the first cell #a (gNB200) has a problem in that it does not know when to stop DL transmission to the UE 100. After the UE 100 performs cell switching, the DL packet transmitted by the first cell #a (gNB200) cannot be received by the UE 100, resulting in a waste of radio resources.

[0125] In addition, in the case of inter-gNB C-LTM / RL-CHO, the source gNB (gNB200a) does not know when to perform data forwarding of DL data addressed to the UE. For example, if the UE 100 forwards DL data from the source gNB (gNB200a) to the target gNB (gNB200b) before performing cell switching, it may result in a waste of backhaul communication resources.

[0126] In the following embodiment, the operation is described to enable the first cell #a (gNB200a) to grasp the timing when UE100 performs UL early synchronization and / or cell switching in C-LTM / RL-CHO, and to efficiently implement C-LTM / RL-CHO.

[0127] 11 is a diagram showing the operation of the gNB200 regarding the improvement of C-LTM / RL-CHO according to the embodiment. In the case of intra-gNB C-LTM / RL-CHO, the gNB200 that performs this operation is the gNB200 that manages the first cell #a and the second cell #b (see FIG. 6). On the other hand, in the case of inter-gNB C-LTM / RL-CHO, the gNB200 that performs this operation is the gNB200a that manages the first cell #a (see FIG. 10).

[0128] In step S21, gNB200 (gNB200a) managing the first cell #a sets a first condition for performing UL early synchronization with the second cell #b and / or a second condition for performing cell switching in UE100 as conditions for performing operations related to cell switching.

[0129] In step S22, the gNB 200 (gNB 200a) managing the first cell #a receives a notification indicating the execution of an operation related to cell switching from the UE 100 or the gNB 200b managing the second cell #b based on the condition being satisfied in the UE 100. For example, the notification may be a notification indicating the execution of UL early synchronization.

[0130] In step S23, in response to receiving the notification in step S22, the gNB200 (gNB200a) managing the first cell #a stops DL transmission to the UE100. In addition, in response to receiving the notification, the gNB200 (gNB200a) managing the first cell #a may start data forwarding to transfer DL data addressed to the UE100 to the gNB200b.

[0131] According to such an operation, C-LTM / RL-CHO can be efficiently implemented. Note that such an operation can be applied to both intra-gNB C-LTM / RL-CHO and inter-gNB C-LTM / RL-CHO, but in the case of intra-gNB C-LTM / RL-CHO, data forwarding is not required.

[0132] A gNB200 (gNB200a) that performs such operations has a control unit 230 that sets a first condition for performing UL early synchronization and / or a second condition for performing cell switching in UE100 as conditions for performing operations related to cell switching, and a receiving unit 220 or 242 that receives a notification indicating the execution of the operation from UE100 or gNB200b that manages second cell #b based on the conditions being satisfied in UE100.

[0133] In a first operation pattern of the embodiment, the gNB200 (gNB200a) receives the notification of step S22 from the UE100. The gNB200 (gNB200a) may receive a notification indicating the execution of UL early synchronization from the UE100 based on the first condition being satisfied in the UE100. The gNB200 (gNB200a) may receive a notification indicating the execution of cell switching from the UE100 based on the second condition being satisfied in the UE100.

[0134] In the second operation pattern of the embodiment, assuming inter-gNB C-LTM / RL-CHO, the gNB 200a receives the notification of step S22 from the gNB 200b. Specifically, the gNB 200a receives a notification from the gNB 200b indicating the execution of UL early synchronization based on the first condition being satisfied in the UE 100.

[0135] (4.1) An example of a first operation pattern A UE 100 relating to the first operation pattern has a receiver 110 that receives information from a gNB 200 that manages a first cell #a, setting a first condition for performing UL early synchronization with a second cell #b and / or a second condition for performing cell switching, as conditions for performing an operation related to cell switching, and a transmitter 120 that transmits a notification to a gNB 200a indicating that the operation is to be performed based on the conditions being satisfied.

[0136] 12 is a diagram showing an example of a first operation pattern according to an embodiment. In the illustrated example, an inter-gNB C-LTM / RL-CHO scenario is assumed. However, the first operation pattern according to the embodiment may be applied to an intra-gNB C-LTM / RL-CHO scenario. In addition, in the following description of the embodiment, redundant explanations of the operations described with reference to FIGS. 7 and 9 will be omitted, but the operations described with reference to FIGS. 7 and 9 may be applied as appropriate.

[0137] In step S201, UE100 is in an RRC connected state with the first cell #a (gNB200a) as the serving cell.

[0138] In step S202, the UE 100 may transmit a measurement report message to the first cell #a (gNB 200a). The first cell #a (gNB 200a) may receive the measurement report message.

[0139] In step S203, the first cell #a (gNB200a) prepares inter-gNB C-LTM / RL-CHO with the second cell #b (gNB200b).

[0140] In step S204, the first cell #a (gNB 200a) transmits an RRC Reconfiguration message including an LTM setting to the UE 100. The LTM setting includes first information, which is a setting of a first condition (Early sync trigger condition), and second information, which is a setting of a second condition (Execution condition). The UE 100 receives an RRC Reconfiguration message including the LTM setting.

[0141] In step S205, the UE 100 transmits an RRC Reconfiguration Complete message to the first cell #a (gNB 200a). The first cell #a (gNB 200a) receives the RRC Reconfiguration Complete message.

[0142] In step S206, the UE 100 confirms that the set first condition (Early sync trigger condition) is satisfied.

[0143] In step S207, the UE 100 performs DL early synchronization processing with the second cell #b (gNB 200b) in response to the first condition (Early sync trigger condition) being satisfied.

[0144] In step S208, the UE 100 performs UL early synchronization processing with the second cell #b (gNB 200b) in response to the first condition (Early sync trigger condition) being satisfied. Here, the UE 100 may acquire the TA value of the second cell #b (gNB 200b) on a random access (RA) basis. The UE 100 may acquire the TA value of the second cell #b (gNB 200b) by UE-based measurement.

[0145] In step S209, the UE 100 notifies the first cell #a (gNB 200a) that the early synchronization process has been completed (successfully). The UE 100 may transmit the notification to the first cell #a (gNB 200) using a MAC CE, an RRC message (e.g., a UE assistance information message), or UCI (Uplink Control Information). The notification may include an identifier (cell ID, etc.) of the corresponding second cell #b (gNB 200b). The notification may include information indicating that the C-LTM / RL-CHO early synchronization trigger condition has been met. The first cell #a (gNB 200a) receives the notification.

[0146] The UE 100 may transmit the notification when the first condition is satisfied. That is, the UE 100 may transmit the notification before performing the early synchronization process. In this case, the notification may notify that the early synchronization process will be performed thereafter.

[0147] In step S210, the UE 100 confirms that the set second condition (Execution condition) is satisfied.

[0148] In step S211, the UE 100 notifies the first cell #a (gNB 200a) of the execution of cell switching by C-LTM / RL-CHO in response to the second condition (Execution condition) being satisfied. The UE 100 may transmit the notification to the first cell #a (gNB 200) in a MAC CE, an RRC message (for example, a UE assistance information message), or UCI (Uplink Control Information). The notification may include an identifier (cell ID, etc.) of the corresponding second cell #b (gNB 200b). The notification may include information indicating that the cell switching trigger condition of C-LTM / RL-CHO has been satisfied. That is, the UE 100 notifies that the second condition (Execution condition) of C-LTM / RL-CHO, which is different from the trigger condition of the existing CHO, is satisfied. This information may be a configuration index (LTM configuration index) included in the LTM setting of step S204. The first cell #a (gNB 200a) receives the notification.

[0149] In addition, UE100 may transmit both the notification of step S209 and the notification of step S211. UE100 may transmit only one of the notification of step S209 and the notification of step S211. The first cell #a (gNB200) may set which notification to transmit to UE100 in step S204.

[0150] In step S212, the first cell #a (gNB200a) stops DL transmission to the UE 100 in response to receiving the notification of either step S209 or step S211. For example, the first cell #a (gNB200a) may stop allocating DL radio resources to the UE 100.

[0151] In step S213, in response to receiving the notification of either step S209 or step S211, the first cell #a (gNB 200a) starts data forwarding to forward DL data addressed to the UE 100 to the second cell #b (gNB 200b). For example, the first cell #a (gNB 200a) may send an Early Status Transfer to the second cell #b (gNB 200b) to start data forwarding.

[0152] In addition, the first cell #a (gNB200a) may perform the processes of steps S212 and S213 after a certain period of time has elapsed since receiving the notification of either step S209 or step S211.

[0153] In step S214, the UE 100 performs cell switching. Specifically, the UE 100 detaches from the first cell #a (gNB 200a), and in step S215, transmits an RRC Reconfiguration Complete message to the second cell #b (gNB 200b). Note that the UE 100 may transmit the notification of step S211 immediately before detaching from the first cell #a (gNB 200a).

[0154] (4.2) Example of Second Operation Pattern The second operation pattern will be described, focusing mainly on the differences from the first operation pattern.

[0155] The gNB200b according to the second operation pattern has a receiver 220 that receives a random access signal (RA preamble) for UL early synchronization from the UE100 in which a condition for performing UL early synchronization with the second cell #b is set, in response to the condition being satisfied in the UE100, and a transmitter 241 that transmits a notification indicating the execution of UL early synchronization to the gNB200a based on the reception of the random access signal. That is, when the second cell #b (gNB200b) performs UL early synchronization with the UE100, it notifies the first cell #a (gNB200a) that UL synchronization with the UE100 has been performed.

[0156] FIG. 13 is a diagram illustrating an example of a second operation pattern according to the embodiment.

[0157] Steps S301 to S308 are the same as steps S201 to S208 in the first operation pattern (FIG. 12).

[0158] In step S309, the second cell #b (gNB200b) notifies the first cell #a (gNB200a) that the early synchronization process (UL early synchronization) has been completed (successfully) following the completion (success) of the UL early synchronization process. The second cell #b (gNB200b) may transmit the notification to the first cell #a (gNB200a) using an Xn Handover Request Ack message or a gNB configuration update message notifying of a gNB configuration update.

[0159] The notification in step S309 may include an identifier of the UE 100 on the Xn interface (such as Xn-AP-UE-ID).

[0160] The notification of step S309 may include the cell ID (target cell ID, or a list of target cell IDs to which the TA is applicable) of the cell from which the UE 100 obtained the TA value. The cell ID may be used by the first cell #a (gNB 200a) to identify the C-LTM / RL-CHO cell switching execution trigger condition and to estimate the execution timing of the cell switching in the UE 100.

[0161] The notification of step S309 may include information on UL reception power (e.g., PRACH reception power, SRS reception power, etc.) from the UE 100. The reception power value may be used by the first cell #a (gNB 200a) to grasp the link state (e.g., path loss) between the UE 100 and the second cell #b (gNB 200b) and to estimate the execution timing of cell switching in the UE 100. In C-LTM / RL-CHO, since the first cell #a (gNB 200a) cannot acquire the L1 measurement report as shown in FIG. 7, the reception power value can be used to estimate the link state between the UE 100 and the second cell #b (gNB 200b). In addition, power ramping that increases the UL transmission power each time an RA preamble is transmitted may be applied to the PRACH, but in the UL early synchronization of C-LTM / RL-CHO, the transmission power of the PRACH may be limited to be fixed (no ramping).

[0162] The notification in step S309 may be a request to start early status transfer (data forwarding). That is, upon completion of the early synchronization process of the UE 100, the second cell #b (gNB 200b) requests data forwarding from the first cell #a (gNB 200a) (that is, notifies that preparations for receiving data forwarding are complete).

[0163] In step S310, in response to receiving the notification in step S309, the first cell #a (gNB 200a) starts data forwarding to transfer DL data addressed to the UE 100 to the second cell #b (gNB 200b). For example, the first cell #a (gNB 200a) may send an Early Status Transfer to the second cell #b (gNB 200b) to start data forwarding.

[0164] In step S311, in response to receiving the notification in step S309, the first cell #a (gNB200a) stops DL transmission to the UE100. For example, the first cell #a (gNB200a) may stop allocating DL radio resources to the UE100.

[0165] In addition, the first cell #a (gNB200a) may perform the processes of steps S310 and S311 after a certain period of time has elapsed since receiving the notification of step S309.

[0166] In step S312, the UE 100 confirms that the set second condition (Execution condition) is satisfied.

[0167] In step S313, the UE 100 executes cell switching in response to the second condition (Execution condition) being satisfied. Specifically, the UE 100 detaches from the first cell #a (gNB 200a), and in step S314, transmits RRC Reconfiguration Complete to the second cell #b (gNB 200b).

[0168] (5) Other Embodiments In the above embodiment, an example of signaling on the Xn interface by inter-node communication between the first cell #a (gNB 200a) and the second cell #b (gNB 200b) has been described. However, it may also be applied to signaling performed on the F1 interface between the DU and CU.

[0169] 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.

[0170] 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.

[0171] That is, the UE 100 may be a terminal function unit (a type of communication module) for a base station to control a repeater that relays signals. Such a terminal function unit is referred to as an MT. Examples of the MT include, in addition to the IAB-MT, an NCR (Network Controlled Repeater)-MT and a RIS (Reconfigurable Intelligent Surface)-MT.

[0172] 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.

[0173] 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).

[0174] The functions performed by the UE 100 or the gNB 200 (network node) may be implemented in circuitry or processing circuitry, including general-purpose processors, application-specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), CPUs (Central Processing Units), conventional circuits, and / or combinations thereof, programmed to perform the described functions. A processor includes transistors and other circuits and is considered to be circuitry or processing circuitry. A processor may also be a programmed processor that executes a program stored in memory. In this specification, circuitry, unit, or means refers to hardware that is programmed to perform the described functions or hardware that executes them. The hardware may be any hardware disclosed herein or any hardware known to be programmed or capable of performing the described functions. If the hardware is a processor, the circuitry, means, or unit is a combination of hardware and software used to configure the hardware and / or processor.

[0175] 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.

[0176] 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.

[0177] This application claims priority from Japanese Patent Application No. 2023-203218 (filed November 30, 2023), the entire contents of which are incorporated herein by reference.

[0178] (6) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0179] (Supplementary Note 1) A communication method for performing cell switching to switch a serving cell of a user equipment from a first cell to a second cell, the communication method comprising: a first network node managing the first cell setting, in the user equipment, a first condition for performing uplink early synchronization with the second cell and / or a second condition for performing the cell switch as conditions for performing an operation related to the cell switching; and the first network node receiving, from the user equipment or a second network node managing the second cell, a notification indicating the execution of the operation, based on the conditions being satisfied in the user equipment.

[0180] (Supplementary Note 2) The communication method according to Supplementary Note 1, further comprising the first network node stopping downlink transmissions to the user equipment in response to receiving the notification.

[0181] (Supplementary Note 3) The communication method according to Supplementary Note 1 or 2, further comprising the first network node initiating data forwarding to forward downlink data addressed to the user equipment to the second network node in response to receiving the notification.

[0182] (Supplementary Note 4) The communication method according to any one of Supplementary Notes 1 to 3, wherein the first network node receives, from the user equipment, the notification indicating the execution of the uplink early synchronization based on the first condition being satisfied in the user equipment.

[0183] (Supplementary Note 5) The communication method according to any one of Supplementary Notes 1 to 3, wherein the first network node receives, from the user equipment, the notification indicating the execution of the cell switch based on the second condition being satisfied in the user equipment.

[0184] (Supplementary Note 6) The communication method according to any one of Supplementary Notes 1 to 3, wherein the first network node receives the notification indicating the execution of the uplink early synchronization from the second network node based on the first condition being satisfied in the user equipment.

[0185] (Supplementary Note 7) A network node that manages a first cell in a mobile communication system that performs cell switching to switch a serving cell of a user equipment from a first cell to a second cell, the network node comprising: a control unit that sets, in the user equipment, a first condition for performing uplink early synchronization with the second cell and / or a second condition for performing the cell switching, as conditions for performing an operation related to the cell switching; and a receiving unit that receives, from the user equipment or a second network node that manages the second cell, a notification indicating the execution of the operation, based on the conditions being satisfied in the user equipment.

[0186] (Supplementary Note 8) A user equipment in a mobile communication system that performs cell switching to switch a serving cell of the user equipment from a first cell to a second cell, the user equipment comprising: a receiver that receives, from a first network node that manages the first cell, information that sets a first condition for performing uplink early synchronization with the second cell and / or a second condition for performing the cell switching, as conditions for performing an operation related to the cell switching; and a transmitter that transmits a notification to the first network node indicating that the operation is to be performed, based on the conditions being satisfied.

[0187] (Supplementary Note 9) A network node that manages a second cell in a mobile communication system that performs cell switching to switch a serving cell of a user equipment from a first cell to a second cell, the network node comprising: a receiver that receives, from the user equipment, a random access signal for uplink early synchronization in response to a condition for performing uplink early synchronization with the second cell being set, the condition being set in the user equipment; and a transmitter that transmits, based on the reception of the random access signal, a notification to the first network node indicating the execution of the uplink early synchronization.

[0188] 1: Mobile communication system 5: Network 10: CN 20: RAN 100: UE 110: Receiving unit 120: Transmitting unit 130: Control unit 140: Wireless communication unit 200: gNB 210: Transmitting unit 220: Receiving unit 230: Control unit 240: Network communication unit 241: Transmitting unit 242: Receiving unit 250: Wireless communication unit 300: AMF / UPF

Claims

1. A communications method for performing cell switching to switch a serving cell of a user equipment from a first cell to a second cell, comprising: a first network node managing the first cell setting in the user equipment a first condition for performing uplink early synchronization with the second cell and / or a second condition for performing the cell switching as a condition for performing an operation related to the cell switching; and the first network node receiving a notification indicating the execution of the operation from the user equipment or a second network node managing the second cell based on the condition being satisfied in the user equipment.

2. The method of claim 1, further comprising the first network node ceasing downlink transmissions to the user equipment in response to receiving the notification.

3. The communication method according to claim 1, further comprising the first network node initiating data forwarding in response to receiving the notification, the data forwarding comprising forwarding downlink data addressed to the user equipment to the second network node.

4. A communication method according to any one of claims 1 to 3, wherein the first network node receives the notification indicating execution of the uplink early synchronization from the user equipment based on the first condition being satisfied in the user equipment.

5. The method of any one of claims 1 to 3, wherein the first network node receives the notification indicating execution of the cell switch from the user equipment based on the second condition being satisfied in the user equipment.

6. A communication method according to any one of claims 1 to 3, wherein the first network node receives the notification indicating execution of the uplink early synchronization from the second network node based on the first condition being satisfied in the user equipment.

7. A network node managing a first cell in a mobile communication system that performs cell switching to switch a serving cell of a user equipment from a first cell to a second cell, the network node comprising: a control unit that sets, in the user equipment, a first condition for performing uplink early synchronization with the second cell and / or a second condition for performing the cell switching as a condition for performing an operation related to the cell switching; and a receiving unit that receives a notification indicating the execution of the operation from the user equipment or a second network node that manages the second cell based on the condition being satisfied in the user equipment.

8. A user equipment in a mobile communication system that performs cell switching to switch a serving cell of the user equipment from a first cell to a second cell, the user equipment having: a receiving unit that receives, from a first network node that manages the first cell, information that sets a first condition for performing uplink early synchronization with the second cell and / or a second condition for performing the cell switching, as a condition for performing an operation related to the cell switching; and a transmitting unit that transmits a notification to the first network node indicating the execution of the operation based on the condition being satisfied.

9. A network node that manages a second cell in a mobile communication system that performs cell switching to switch a serving cell of a user equipment from a first cell to a second cell, the network node having: a receiving unit that receives a random access signal for uplink early synchronization from the user equipment, in response to the condition being set for performing uplink early synchronization with the second cell, and a transmitting unit that transmits a notification indicating the execution of the uplink early synchronization to the first network node based on the reception of the random access signal.