Communication method and network node

Abstract

In the present invention, a communication method for switching a serving cell of a user device between network nodes by L1 / L2 Triggered Mobility (LTM) involves: a first network node that manages a source cell transmitting, to each of a plurality of network nodes corresponding to a plurality of LTM candidate cells that are candidates for a target cell, a request message for preparing for LTM cell switching; the first network node receiving a response message, which includes an LTM candidate cell setting, from each of the plurality of network nodes; and the first network node transmitting a first notification message, which includes the LTM candidate cell setting of another network node, to each of the plurality of network nodes.

Description

Communication method and network node

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

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

[0003] Meanwhile, 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 present disclosure provides techniques for enabling LTM between network nodes with subsequent LTM cell switching, specifically, inter-CU (Central Unit) LTM with subsequent LTM cell switching.

[0006] A communication method according to a first aspect is a communication method for switching a serving cell of a user equipment between network nodes by LTM (L1 / L2 Triggered Mobility), the method comprising: a first network node managing a source cell transmitting a request message for preparing an LTM cell switch to each of a plurality of network nodes corresponding to a plurality of LTM candidate cells that are candidates for a target cell; the first network node receiving a response message including an LTM candidate cell configuration from each of the plurality of network nodes; and the first network node transmitting a first notification message to each of the plurality of network nodes including the LTM candidate cell configuration of another network node.

[0007] A network node according to a second aspect is a network node that manages a source cell in a mobile communication system that is capable of switching a serving cell of a user equipment between network nodes by LTM (L1 / L2 Triggered Mobility), and includes: a transmitter that transmits a request message for preparing for LTM cell switching to each of a plurality of network nodes corresponding to a plurality of LTM candidate cells that are candidates for a target cell; and a receiver that receives, from each of the plurality of network nodes, a response message including an LTM candidate cell configuration. The transmitter transmits, to each of the plurality of network nodes, a first notification message including the LTM candidate cell configuration of another network node.

[0008] 1 is a diagram showing an example of the configuration of a mobile communication system according to an embodiment; FIG. 2 is a diagram showing an example of the configuration of a UE (user equipment) according to an embodiment; FIG. 3 is a diagram showing an example of the configuration of a gNB (network node) according to an embodiment; FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data; 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); FIG. 6 is a diagram showing an example of an LTM procedure for intra-CU; FIG. 7 is a diagram for explaining the operating environment of a mobile communication system according to an embodiment; FIG. 8 is a flow diagram showing an overview of the operation of a source gNB according to an embodiment; and FIG. 9 is a sequence diagram showing a specific example of the operation of a mobile communication system according to an embodiment.

[0009] 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.

[0010] (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.

[0011] 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.

[0012] The UE 100 is a mobile wireless communication device. The UE 100 may be any device used by a user. For example, the UE 100 may be a mobile phone terminal (including a smartphone) and / or a tablet terminal, a notebook PC, a communication module (including a communication card or chipset), a sensor or a device provided in a sensor, a vehicle or a device provided in a vehicle (Vehicle UE), or an aircraft or a device provided in an aircraft (Aerial UE). A link in the transmission direction from the UE 100 to the network 5 is referred to as an uplink (UL), and a link in the transmission direction from the network 5 to the UE 100 is referred to as a downlink (DL).

[0013] 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").

[0014] 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.

[0015] 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.

[0016] 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.

[0017] The receiving unit 110 performs various reception operations 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] FIG. 4 is a diagram showing the configuration of a protocol stack of a radio interface of a user plane that handles data.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] 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.

[0030] The PDCP layer performs header compression / decompression, encryption / decryption, and the like.

[0031] 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.

[0032] 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).

[0033] 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.

[0034] 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.

[0035] 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").

[0036] (2) Overview of LTM The mobile communication system 1 according to this embodiment supports LTM (L1 / L2-triggered mobility).

[0037] 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.

[0038] 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.

[0039] 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.

[0040] Secondly, the UE 100 performs a synchronization process with the LTM candidate cell by early synchronization (Early sync).

[0041] 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 via a MAC control element (CE). The serving cell switch trigger is conveyed in a MAC CE including at least a candidate setting index together with a beam indicator.

[0042] Fourth, the UE 100 switches the serving cell in response to a cell switch command MAC CE from the gNB 200 (source cell).

[0043] 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.

[0044] The following principles apply to LTM:

[0045] Each LTM candidate cell configuration can be provided as a differential configuration (delta configuration) relative to a reference configuration used to form the complete LTM candidate cell configuration.

[0046] If a full LTM candidate cell configuration is applied, the current UE configuration is replaced upon a serving cell switch. The reconfiguration procedure does this but does not necessarily reset the MAC, RLC or PDCP layers.

[0047] The user plane continues without a reset if configured in RRC signaling to avoid additional delays in data recovery.

[0048] - Security is not updated in LTM.

[0049] LTM between subsequent LTM candidate cell configurations (subsequent LTM cell switching) can be performed without RRC reconfiguration, i.e., the UE 100 does not release other LTM candidate cell configurations after LTM is triggered.

[0050] 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.

[0051] Here, the first cell and the second cell may be formed by different TRPs (Transmission and Reception Points). In the following description of the embodiment, the second cell is also referred to as an "LTM candidate cell (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".

[0052] In step S1, UE100 is in an RRC connected state in the cell (first cell, source cell) of gNB200.

[0053] In step S2, UE100 transmits a measurement report (L3 Measurement Report) message, which is an RRC message, to gNB200.

[0054] In step S3, gNB200 decides to use LTM based on the Measurement Report message and starts preparing an LTM candidate cell.

[0055] In step S4, the gNB 200 transmits to the UE 100 an RRC Reconfiguration message including an LTM candidate cell configuration (LTM Candidate Configuration) of one or more LTM candidate cells. The LTM candidate cell configuration may include a random access channel (RACH) configuration used for RA preamble transmission to the corresponding LTM candidate cell, for example, 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.

[0056] In step S5, UE100 saves the LTM candidate cell setting and sends an RRC Reconfiguration Complete message to gNB200.

[0057] In step S6, the UE 100 may perform synchronization processing with the LTM candidate cell before receiving the cell switch command MAC CE. 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 LTM candidate cell, and then perform early timing advance (TA) acquisition in the LTM candidate cell requested by the gNB 200 (source cell). This is performed by CFRA triggered by a PDCCH order (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 UE100, the PDCCH order may include a cell indicator indicating the corresponding RACH transmission cell, i.e., to which LTM candidate cell UE100 should transmit a random access preamble (RA preamble).

[0058] The UE 100 transmits an RA preamble to the designated LTM candidate cell. In order to minimize communication interruption of the source cell due to CFRA for the LTM candidate cell, in early synchronization, the UE 100 does not receive a random access response (RAR) for the purpose of acquiring a TA value from the LTM candidate cell. The TA value of the LTM candidate cell (target cell) is indicated in the cell switching command MAC CE in step S9. Note that the TA value is a value for adjusting the uplink transmission timing of the UE 100.

[0059] In step S7, the UE 100 performs layer 1 (L1) measurement in the configured LTM 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).

[0060] In step S8, gNB200 decides to switch the serving cell to the target cell (second cell).

[0061] 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 MAC CE may include a TA value obtained by early synchronization.

[0062] 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.

[0063] In step S11, if the serving cell switch needs to include execution of a random access procedure (for example, if the cell switch command MAC CE does not include a valid TA value), the UE 100 executes a random access procedure for the target cell (RACH (Random Access Channel)-based LTM cell switch). Note that, if the UE 100 does not need to acquire the TA of the target cell at the time of serving cell switch (for example, if the cell switch command MAC CE includes a valid TA value), it can skip the random access procedure (RACH-less LTM cell switch).

[0064] 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.

[0065] (3) Operation related to LTM of inter-CU The LTM specified in Release 18 of the 3GPP standard is an LTM of an intra-CU (Central Unit) and does not support LTM of an inter-CU. That is, in conventional LTM, it is possible to perform LTM cell switching between cells under the same CU (same gNB 200), but it is not possible to perform LTM cell switching between cells under different CUs (different gNB 200). On the other hand, in Release 19 of the 3GPP standard, support for LTM of inter-CU is being considered.

[0066] FIG. 7 is a diagram for explaining the operating environment of the mobile communication system 1 according to the embodiment.

[0067] The UE100 is in an RRC connected state in the cell of the gNB200S. The gNB200S is an example of a first network node. The cell of the gNB200S is an example of a first cell. The cell of the gNB200S is also referred to as a source cell or a current serving cell. The gNB200S is also referred to as a source gNB200S or a serving gNB200S.

[0068] The gNB200C manages the cells adjacent to the cell of gNB200S. In the illustrated example, the cells adjacent to the cell of gNB200S are managed by gNB200C1 and gNB200C2. gNB200C1 is an example of a second network node, and gNB200C2 is an example of a third network node. The cell of gNB200C1 is also referred to as LTM candidate cell #1, and the cell of gNB200C2 is also referred to as LTM candidate cell #2. Each of LTM candidate cell #1 and LTM candidate cell #2 is an example of a second cell.

[0069] When gNB200C1 and gNB200C2 are not particularly distinguished, they are referred to as candidate gNB200C, and when LTM candidate cell #1 and LTM candidate cell #2 are not distinguished, they are simply referred to as LTM candidate cells. However, after a cell switch by LTM is determined, the determined LTM candidate cell is also referred to as a target cell.

[0070] In the illustrated example, there are two candidate gNB200C and two LTM candidate cells, but there may be one candidate gNB200C or three or more. There may also be one LTM candidate cell or three or more. Furthermore, one candidate gNB200C may manage multiple LTM candidate cells.

[0071] 7 are connected to each other by an Xn interface, which is an interface between network nodes (or, from another perspective, an interface between CUs). Communication between the gNBs 200 is assumed to be performed over the Xn interface.

[0072] Each candidate gNB200C provides RRC Reconfiguration (with sync) including configuration information of its own LTM candidate cell (LTM candidate cell configuration) to UE100 via gNB200S. In the illustrated example, gNB200C1 provides RRC Reconfiguration (with sync) including LTM candidate cell configuration of LTM candidate cell #1 to UE100 via gNB200S. Also, gNB200C2 provides RRC Reconfiguration (with sync) including LTM candidate cell configuration of LTM candidate cell #2 to UE100 via gNB200S. The LTM candidate cell configuration may include early UL synchronization configuration (CFRA configuration), as described above. The LTM candidate cell configuration may include configuration of a PUSCH resource to be used when performing the first UL transmission (for example, transmission of an RRC Reconfiguration Complete message to the target cell) with the corresponding LTM candidate cell as the target cell.

[0073] After storing the LTM candidate cell setting of each LTM candidate cell, the UE 100 performs early UL synchronization with one or more LTM candidate cells, and performs LTM cell switching with the LTM candidate cell specified in the cell switching command MAC CE from the source cell as the target cell. However, if the TA value of the LTM candidate cell is the same as the TA value of the source cell (TA = same as source), or if the TA value of the LTM candidate cell is zero (TA = 0), the UE 100 may not perform early UL synchronization with the LTM candidate cell.

[0074] (4) Operation for Inter-CU LTM with Subsequent LTM Cell Switching The LTM defined in Release 18 of the 3GPP standard supports subsequent LTM cell switching.

[0075] In the subsequent LTM cell switching, the UE 100, which has set the LTM candidate cell settings of a plurality of LTM candidate cells by RRC signaling, performs the LTM cell switching to one LTM candidate cell, and then reuses the LTM candidate cell setting without releasing it, and performs the LTM cell switching to another LTM candidate cell. Therefore, the subsequent LTM cell switching requires only one RRC signaling (RRC setting) even when the UE 100 performs the LTM cell switching multiple times, so that the signaling can be reduced and the operation can be accelerated.

[0076] However, since the subsequent LTM cell switching in Release 18 of the 3GPP standard assumes only intra-CU (intra-gNB), it is assumed that one CU (one gNB) centrally manages the LTM candidate cell settings of multiple LTM candidate cells. However, if subsequent LTM cell switching between inter-CUs (different gNBs) is enabled, each candidate gNB must understand the LTM candidate cell settings of other candidate gNBs.

[0077] For example, in the operating environment shown in Figure 7, assume that UE100 performs LTM cell switching from the source cell of source gNB200S to LTM candidate cell #1 of candidate gNB200C1, and then performs LTM cell switching to LTM candidate cell #2 of candidate gNB200C2 by subsequent LTM cell switching. In this case, candidate gNB200C1 needs to transmit, for example, a cell switching command MAC CE (and PDCCH order) to UE100 in order to perform LTM cell switching of UE100 to LTM candidate cell #2 of candidate gNB200C2.

[0078] This cell switching command MAC CE (and PDCCH order) must include parameters related to the LTM candidate cell setting of the LTM candidate cell #2 of the candidate gNB200C2. Therefore, if the candidate gNB200C1 does not understand the LTM candidate cell setting of the LTM candidate cell #2 of the candidate gNB200C2, it cannot perform LTM cell switching to the LTM candidate cell #2. Also, if the setting (LTM setting) of the LTM candidate cell #2 of the candidate gNB200C2 has not been performed in the UE100, an error (cell switching is not possible) will occur even if the cell switching command MAC CE is sent, so it is necessary to understand the setting status in the UE100.

[0079] Therefore, the conventional mechanism for switching the subsequent LTM cell has a problem in that it is not possible to realize switching the subsequent LTM cell in an inter-CU. In the following embodiment, a technique for realizing switching the subsequent LTM cell in an inter-CU will be described.

[0080] Figure 8 is a flow chart showing an overview of the operation of the source gNB200S in the embodiment.

[0081] In step S11, the source gNB200S that manages the source cell sends a request message for preparing for LTM cell switching to each of multiple candidate gNB200C corresponding to multiple LTM candidate cells that are candidates for the target cell.

[0082] In step S12, the source gNB200S receives a response message including an LTM candidate cell setting from each of the multiple candidate gNB200C.

[0083] In step S13, the source gNB200S sends a first notification message to each of the multiple candidate gNB200C, including the LTM candidate cell configuration of the other candidate gNB200C.

[0084] This allows each candidate gNB to understand the LTM candidate cell settings of other candidate gNBs, making it possible to realize subsequent LTM cell switching in inter-CU.

[0085] The multiple candidate gNB200C may include candidate gNB200C1 and candidate gNB200C2 (see FIG. 7). In response to configuring the LTM candidate cell setting of candidate gNB200C1 (LTM candidate cell setting of LTM candidate cell #1) in UE100, the source gNB200S may send a first notification message including the LTM candidate cell setting of candidate gNB200C1 (LTM candidate cell setting of LTM candidate cell #1) to candidate gNB200C2. Conversely, in response to configuring the LTM candidate cell setting of candidate gNB200C2 (LTM candidate cell setting of LTM candidate cell #2) in UE100, the source gNB200S may send a first notification message including the LTM candidate cell setting of candidate gNB200C2 (LTM candidate cell setting of LTM candidate cell #2) to candidate gNB200C1.

[0086] Alternatively, the source gNB200S may transmit a first notification message including the LTM candidate cell setting of the candidate gNB200C1 (LTM candidate cell setting of LTM candidate cell #1) to the candidate gNB200C2 in response to configuring a modification of the LTM candidate cell setting of the candidate gNB200C1 (LTM candidate cell setting of LTM candidate cell #1) in the UE100. Conversely, the source gNB200S may transmit a first notification message including the LTM candidate cell setting of the candidate gNB200C2 (LTM candidate cell setting of LTM candidate cell #2) to the candidate gNB200C1 in response to configuring a modification of the LTM candidate cell setting of the candidate gNB200C2 (LTM candidate cell setting of LTM candidate cell #2) in the UE100.

[0087] Alternatively, the source gNB200S may send to the candidate gNB200C2 a first notification message that does not include the LTM candidate cell setting of the candidate gNB200C1 (the LTM candidate cell setting of the LTM candidate cell #1) in response to configuring the deletion (release) of the LTM candidate cell setting of the candidate gNB200C1 (the LTM candidate cell setting of the LTM candidate cell #1) in the UE100. Conversely, the source gNB200S may send to the candidate gNB200C1 a first notification message that does not include the LTM candidate cell setting of the candidate gNB200C2 (the LTM candidate cell setting of the LTM candidate cell #2) in response to configuring the deletion (release) of the LTM candidate cell setting of the candidate gNB200C2 (the LTM candidate cell setting of the LTM candidate cell #2) in the UE100.

[0088] The source gNB200S may transmit a second notification message to the candidate gNB200C2 in response to the execution of the LTM cell switch with the LTM candidate cell #1 of the candidate gNB200C1 as the target cell. In response to receiving the second notification message, the candidate gNB200C2 may deactivate the UL resources of the LTM candidate cell configuration of the candidate gNB200C2 (i.e., the UL resources determined for the UE100 for the LTM candidate cell #2).

[0089] Conversely, the source gNB200S may send a second notification message to the candidate gNB200C1 in response to an LTM cell switch being performed with the LTM candidate cell #2 of the candidate gNB200C2 as the target cell. In response to receiving the second notification message, the candidate gNB200C1 may deactivate the UL resources of the LTM candidate cell configuration of the candidate gNB200C1 (i.e., the UL resources determined for the UE100 for the LTM candidate cell #1).

[0090] Note that deactivating the UL resource may mean making the UL resource temporarily assignable to another UE 100. Here, the UL resource may be a CFRA resource used in early UL synchronization and / or a PUSCH resource used when performing a first UL transmission (e.g., transmission of an RRC Reconfiguration Complete message).

[0091] This allows for effective utilization of resources of the LTM candidate cell. Deactivating the UL resource may mean that the candidate gNB 200C does not perform reception processing for the UL resource. This reduces the power consumption of the candidate gNB 200C.

[0092] After an LTM cell switch is performed with the LTM candidate cell #1 of candidate gNB200C1 as the target cell, candidate gNB200C1 may send a message to candidate gNB200C2 to activate the UL resources of candidate gNB200C2 (i.e., the UL resources determined for UE100 for LTM candidate cell #2) before performing a subsequent LTM cell switch to the LTM candidate cell #2 of candidate gNB200C2.

[0093] Conversely, after an LTM cell switch is performed with the LTM candidate cell #2 of candidate gNB200C2 as the target cell, candidate gNB200C2 may send a message to candidate gNB200C1 to activate the UL resources of candidate gNB200C1 (i.e., the UL resources determined for UE100 for LTM candidate cell #1) before performing a subsequent LTM cell switch to the LTM candidate cell #1 of candidate gNB200C1.

[0094] The request message of step S11 may be a Handover Request message transmitted on the Xn interface and including information on inter-CU LTM cell switching corresponding to subsequent LTM cell switching. That is, the request message of step S11 may be a message common to the Handover Request message used in L3 handover and including additional information indicating that it is a request for inter-CU LTM setup involving subsequent LTM cell switching. The response message of step S12 may be a Handover Request Acknowledge message transmitted on the Xn interface.

[0095] Alternatively, the request message in step S11 may be a new LTM-directed message (e.g., an LTM Configuration Request message) transmitted over the Xn interface and different from the Handover Request message. The new LTM-directed message may be a message used exclusively for inter-CU LTM cell switching corresponding to subsequent LTM cell switching. The response message in step S12 may be a new LTM-directed message (e.g., an LTM Configuration Request Acknowledge message) transmitted over the Xn interface and different from the Handover Request Acknowledge message.

[0096] 9 is a sequence diagram showing a specific example of the operation of the mobile communication system 1 according to the embodiment. Here, differences from the LTM procedure in FIG. 6 will be mainly described, and overlapping descriptions will be omitted.

[0097] In step S101, the UE 100 is in an RRC connected state in a cell (source cell) of the source gNB 200S. The UE 100 transmits a measurement report (L3 Measurement Report) message to the source gNB 200S.

[0098] In step S102, based on the measurement report message, the source gNB200S decides to use inter-CU LTM corresponding to subsequent LTM cell switching and starts preparing the LTM candidate cell.

[0099] In step S103, the source gNB200S transmits an Xn Handover Request message to the candidate gNB200C1. The message may include information indicating that subsequent-LTM is to be performed, or information indicating that subsequent LTM is not to be performed. Alternatively, the message may be a new Xn message such as an "S-LTM Request."

[0100] In step S104, the source gNB200S transmits an Xn Handover Request message to the candidate gNB200C2. The message may include information indicating that subsequent-LTM is to be performed, or information indicating that subsequent LTM is not to be performed. Alternatively, the message may be a new Xn message such as an "S-LTM Request."

[0101] In step S105, the candidate gNB200C1 transmits a Handover Request Acknowledge message, which is a response message to the request message of step S103, to the source gNB200S. Alternatively, the message may be a new Xn message such as "S-LTM Request Acknowledge".

[0102] In step S106, the candidate gNB200C2 transmits a Handover Request Acknowledge message, which is a response message to the request message of step S104, to the source gNB200S. Alternatively, the message may be a new Xn message such as "S-LTM Request Acknowledge".

[0103] In step S107, the source gNB200S, which has received a Handover Request Acknowledge message from each LTM candidate gNB200C, notifies the UE100 of the RRC Reconfiguration message contained in the RRC container in these messages as an LTM setting (LTM candidate cell setting). The LTM configuration (LTM candidate cell configuration) is provided as a list of one or more LTM candidate cell configurations, and the entries of the list may include a configuration ID (LTM-CandidateId), a cell ID (PhysCellId), an early UL synchronization configuration (EarlyUL-SyncConfig), an SSB configuration (ltm-SSB-Config), a CSI-RS resource configuration (NZP-CSI-RS-Resource), and a TCI state configuration (TCI-State), etc. In this operation example, the LTM candidate cell configuration of the LTM candidate cell #1 of the candidate gNB200C1 and the LTM candidate cell configuration of the LTM candidate cell #2 of the candidate gNB200C2 are provided to the UE100.

[0104] In step S108, UE100 sends an RRC Reconfiguration Complete message to source gNB200S.

[0105] In step S109, the UE 100 stores a list of LTM candidate cell settings.

[0106] In step S110, the source gNB200S, which has received the RRC Reconfiguration Complete message from UE100, transmits a first notification message (S-LTM config. update) to the candidate gNB200C1 to notify the update (addition, change, and / or deletion) of the LTM candidate cell setting in UE100. Also, in step S111, the source gNB200S transmits a first notification message (S-LTM config. update) to the candidate gNB200C2 to notify the update (addition, change, and / or deletion) of the LTM candidate cell setting in UE100. In the illustrated example, the first notification message (S-LTM config. update) includes a list of LTM candidate cell configurations configured in the UE 100 in step S107.

[0107] In step S112, in response to receiving the first notification message (S-LTM config. update) of step S110, candidate gNB200C1 stores the LTM candidate cell configuration set for UE100, in particular the LTM candidate cell configuration of LTM candidate cell #2 of candidate gNB200C2, and activates UL resources (e.g., CFRA resources and / or PUSCH resources) for UE100 in LTM candidate cell #1.

[0108] In step S113, in response to receiving the first notification message (S-LTM config. update) in step S111, candidate gNB200C2 stores the LTM candidate cell setting configured for UE100, in particular the LTM candidate cell setting for LTM candidate cell #1 of candidate gNB200C1, and activates UL resources (e.g., CFRA resources and / or PUSCH resources) for UE100 in LTM candidate cell #2.

[0109] In step S114, the source gNB200S sends a PDCCH order to the UE100 that triggers a CFRA to the candidate gNB200C1 (LTM candidate cell #1).

[0110] In step S115, UE100 that has received the PDCCH order performs early UL synchronization by transmitting a CFRA RA preamble to candidate gNB200C1 (LTM candidate cell #1). Candidate gNB200C1 may derive a TA value based on the RA preamble and notify the source gNB200S of the derived TA value.

[0111] In step S116, candidate gNB200C1 deactivates the CFRA resource for UE100 in LTM candidate cell #1.

[0112] In step S117, UE100 transmits an L1 measurement report (L1 Measurement Report) to source gNB200S.

[0113] In step S118, the source gNB200S determines LTM cell switching to LTM candidate cell #1 based on the L1 measurement report message, and transmits a cell switching command MAC CE to the UE100 instructing LTM cell switching to the LTM candidate cell #1 (target cell).

[0114] In step S119, the UE 100 transmits an RRC Reconfiguration Complete message to the candidate gNB 200C1 using the PUSCH resource of the LTM candidate cell #1. This completes the LTM cell switch from the source cell to the LTM candidate cell #1.

[0115] In step S120, candidate gNB200C1 sends a UE Context Release message to source gNB200S.

[0116] In step S121, the source gNB200S, which has received the UE Context Release message from the candidate gNB200C1, determines that the LTM cell switch to the LTM candidate cell #1 has been completed, and sends a second notification message (Subsequent LTM in-progress) to the candidate gNB200C2 indicating that a subsequent LTM cell switch procedure is pending.

[0117] The second notification message (Subsequent LTM in-progress) of step S121 may include at least one of the following information: - Information indicating the candidate gNB200C1 to which UE100 has completed access (cell switching) (e.g., gNB ID and / or cell ID); - Information indicating UE100 (e.g., XnAP (Application Protocol) UE ID); - TNL (Transport Network Layer) address information of gNB200C1 (e.g., IP (Internet Protocol) address and / or GTP (GPRS Tunneling Protocol) tunnel endpoint ID).

[0118] In step S122, candidate gNB200C2, which receives a second notification message (Subsequent LTM in-progress) from source gNB200S, deactivates UL resources (e.g., CFRA resources and / or PUSCH resources) for UE100 in LTM candidate cell #2.

[0119] In step S123, the source gNB200S sends a UE Context Release Ack message to the candidate gNB200C1 in response to the UE Context Release message of step S120.

[0120] In step S124, the UE 100 is in an RRC connected state with the LTM candidate cell #1 of the candidate gNB 200C1 as the current serving cell. The UE 100 transmits an L1 Measurement Report message to the candidate gNB 200C1.

[0121] In step S125, when the possibility of a subsequent LTM cell switch to, for example, LTM candidate cell #2 increases based on the L1 measurement report message from UE100, candidate gNB200C1 sends a message (CFRA / PUSCH resource activation request) to candidate gNB200C2 to request activation of UL resources (e.g., CFRA resources and / or PUSCH resources) for UE100 in LTM candidate cell #2.

[0122] The message sent to gNB200C2 in step S125 may be a Handover Request (or S-LTM Request). The message may include information about candidate gNB200C1 and / or UE100 as included in the second notification message of step S121. By including such information, candidate gNB200C2 can recognize that the message is a message for a subsequent LTM. Alternatively, candidate gNB200C2 can recognize that candidate gNB200C1 is the current source gNB200.

[0123] In step S126, in response to receiving a message (CFRA / PUSCH resource activation request) from the candidate gNB200C1, the candidate gNB200C2 activates UL resources (e.g., CFRA resources and / or PUSCH resources) for the UE100 in the LTM candidate cell #2. Thereafter, a subsequent LTM cell switch from the LTM candidate cell #1 to the LTM candidate cell #2 may be performed in a procedure similar to steps S114 to S119.

[0124] In step S118, the UE 100 that has received the LTM cell switching command MAC CE may perform error processing if there is no configuration of the target cell specified in the command (that is, if the target cell does not exist in the list of LTM candidate cell configurations). The error processing may be to notify the source gNB 200S of information about the failure of the LTM cell switching. The notification may be performed by MAC CE or an RRC message. The error processing may be to execute an RRC re-establishment procedure. The error processing may be to record (log) the error information. The UE 100 may report the record to the gNB 200 when requested by the gNB 200.

[0125] (5) 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.

[0126] 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.

[0127] 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.

[0128] 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).

[0129] 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.

[0130] 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.

[0131] 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.

[0132] This application claims priority from Japanese Patent Application No. 2024-058529 (filed April 1, 2024), the entire contents of which are incorporated herein by reference.

[0133] (6) Supplementary Notes The following are additional notes regarding the features of the above-described embodiment.

[0134] Supplementary Note 1: A communication method for switching a serving cell of a user equipment between network nodes by LTM (L1 / L2 Triggered Mobility), the communication method comprising: a first network node managing a source cell transmitting a request message for preparing an LTM cell switch to each of a plurality of network nodes corresponding to a plurality of LTM candidate cells that are candidates for a target cell; the first network node receiving, from each of the plurality of network nodes, a response message including an LTM candidate cell configuration; and the first network node transmitting, to each of the plurality of network nodes, a first notification message including the LTM candidate cell configuration of another network node.

[0135] Supplementary Note 2: The communication method according to Supplementary Note 1, wherein the plurality of network nodes include a second network node and a third network node, and the first network node transmits the first notification message including the LTM candidate cell configuration of the second network node to the third network node in response to configuring the LTM candidate cell configuration of the second network node in the user equipment.

[0136] Supplementary Note 3: The communication method according to Supplementary Note 1 or 2, wherein the plurality of network nodes include a second network node and a third network node, and the first network node transmits a second notification message to the third network node in response to the LTM cell switch being performed with an LTM candidate cell of the second network node as the target cell.

[0137] Supplementary Note 4: The communication method according to Supplementary Note 3, wherein the third network node deactivates uplink resources of an LTM candidate cell configuration of the third network node in response to receiving the second notification message.

[0138] Supplementary Note 5: The communication method according to Supplementary Note 4, wherein the second network node transmits a message to the third network node to activate the uplink resources of the third network node before performing a subsequent LTM cell switch following the LTM cell switch to an LTM candidate cell of the third network node.

[0139] Supplementary Note 6: The communication method according to any one of Supplementary Notes 1 to 5, wherein the request message is a Handover Request message transmitted on an interface between network nodes and including information regarding an inter-network node LTM cell switch corresponding to a subsequent LTM cell switch.

[0140] Supplementary Note 7: The communication method according to any one of Supplementary Notes 1 to 5, wherein the request message is a message transmitted on an interface between network nodes and is a message directed to the LTM for an LTM cell switch between network nodes corresponding to a subsequent LTM cell switch.

[0141] Supplementary Note 8: A network node managing a source cell in a mobile communication system capable of switching a serving cell of a user equipment between network nodes by LTM (L1 / L2 Triggered Mobility), comprising: a transmitter that transmits a request message for preparation of LTM cell switching to each of a plurality of network nodes corresponding to a plurality of LTM candidate cells that are candidates for a target cell; and a receiver that receives, from each of the plurality of network nodes, a response message including an LTM candidate cell configuration, wherein the transmitter transmits, to each of the plurality of network nodes, a first notification message including the LTM candidate cell configuration of another network node.

[0142] 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 for switching a serving cell of a user equipment between network nodes by LTM (L1 / L2 Triggered Mobility), the communication method comprising: a first network node managing a source cell transmitting a request message for preparing an LTM cell switch to each of a plurality of network nodes corresponding to a plurality of LTM candidate cells that are candidates for a target cell; the first network node receiving a response message including an LTM candidate cell configuration from each of the plurality of network nodes; and the first network node transmitting a first notification message to each of the plurality of network nodes including the LTM candidate cell configuration of another network node.

2. The communication method according to claim 1, wherein the plurality of network nodes include a second network node and a third network node, and the first network node transmits the first notification message including the LTM candidate cell configuration of the second network node to the third network node in response to configuring the LTM candidate cell configuration of the second network node in the user equipment.

3. The communication method according to claim 1, wherein the plurality of network nodes include a second network node and a third network node, and the first network node transmits a second notification message to the third network node in response to the LTM cell switch being performed with an LTM candidate cell of the second network node as the target cell.

4. The communication method according to claim 3, wherein the third network node deactivates uplink resources of the LTM candidate cell configuration of the third network node in response to receiving the second notification message.

5. The communication method according to claim 4, wherein the second network node transmits a message to the third network node to activate the uplink resources of the third network node before performing a subsequent LTM cell switch following the LTM cell switch to an LTM candidate cell of the third network node.

6. The communication method according to any one of claims 1 to 5, wherein the request message is a Handover Request message transmitted on an interface between network nodes and containing information regarding an inter-network node LTM cell switch corresponding to a subsequent LTM cell switch.

7. A communication method according to any one of claims 1 to 5, wherein the request message is a message transmitted on an interface between network nodes and is a message directed to the LTM for an LTM cell switch between network nodes corresponding to a subsequent LTM cell switch.

8. A network node that manages a source cell in a mobile communication system that can switch a serving cell of a user equipment between network nodes by LTM (L1 / L2 Triggered Mobility), comprising: a transmitter that transmits a request message for preparing for LTM cell switching to each of a plurality of network nodes corresponding to a plurality of LTM candidate cells that are candidates for a target cell; and a receiver that receives a response message including an LTM candidate cell configuration from each of the plurality of network nodes, wherein the transmitter transmits a first notification message to each of the plurality of network nodes that includes the LTM candidate cell configuration of another network node.

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