Terminal device, base station device, and wireless communication system

Abstract

The present invention is provided with: a reception unit that receives a first signal including change destination information specifying a first cell and a second cell as cell-change destination candidates; and a processing unit that, when first processing for switching a serving cell from a third cell to the first cell is initiated, uses a first setting applied in the first cell to perform the first processing. When the first processing fails, the processing unit uses a third setting applied in the third cell to perform an RRC re-establishment procedure. In the RRC re-establishment procedure, when a prescribed condition is satisfied, the processing unit performs second processing while, in the RRC re-establishment procedure, when the prescribed condition is not satisfied, the processing unit performs third processing.

Description

Terminal equipment, base station equipment, wireless communication systems

[0001] This invention relates to terminal equipment, base station equipment, and wireless communication systems.

[0002] Currently, mobile device traffic (smartphones and feature phones) accounts for the majority of network resources. Furthermore, the amount of traffic used by mobile devices is expected to continue to increase.

[0003] Furthermore, in addition to traffic used by mobile devices, IoT (Internet of Things) services (e.g., traffic systems, smart meters, monitoring systems for devices, etc.) are also being deployed. Therefore, networks are required to support services with diverse requirements. To support such diverse services, for example, the communication standards for fifth-generation mobile communication (5G or NR (New Radio)) (e.g., Non-Patent Documents 1 to 14) have been formulated with support for many use cases classified as eMBB (Enhanced Mobile Broadband), Massive MTC (Machine Type Communications), and URLLC (Ultra-Reliable and Low Latency Communication) in mind. Technologies related to NR or 5G systems are described, for example, in Non-Patent Documents 1 to 10 below.

[0004] Furthermore, in 3GPP (3rd Generation Partnership Project (registered trademark)), extension technologies for the above communication standards are continuously being studied and standardized. Currently, technologies related to mobility performance improvement are being studied in 3GPP. As one of the study items, a specification for a technology called LTM (L1 / L2-triggered mobility), which aims to achieve effects such as reduction of mobility delay by having a base station determine to cause a terminal to perform serving cell switching using a layer 1 measurement signal transmitted from the terminal and instruct the terminal to perform serving cell switching using a layer 2 signal, has been established in Rel-18 (Non-Patent Document 11). Furthermore, it is planned that in Rel-19, specifications for LTM that further extends the LTM in Rel-18, which only supports LTM within a CU (Centralized Unit) (intra-CU), to also support LTM between CUs (inter-CU), and specifications for Conditional LTM (CLTM), in which instead of the base station instructing the terminal to perform serving cell switching using a layer 2 signal, the terminal performs cell switching to a corresponding candidate cell when the terminal satisfies a condition, will be established (Non-Patent Document 12).

[0005] 3GPP TS38.300 v18.5.0 NR Overview Specification 3GPP TS38.211 v18.5.0 NR PHY Channel and Modulation Specification 3GPP TS38.321 v18.5.0 NR MAC Specification 3GPP TS38.322 v18.2.0 NR RLC Specification 3GPP TS38.323 v18.5.0 NR PDCP Specification 3GPP TS37.324 v18.0.0 NR SDAP Specification 3GPP TS38.331 v18.5.0 NR RRC Specification 3GPP TS37.340 v18.5.0 NR MR-DC Specification 3GPP TS38.304 v18.4.0 NR Idle Mode and Inactive Mode Specification 3GPP TS38.306 v18.5.0 NR UE Capability Specification 3GPP RP-223520 "Revised WID on Further NR mobility enhancements" 3GPP RP-242356 "Revised Work Item: NR mobility enhancements Phase 4"

[0006] By the way, the specific methods for Rel-19's LTM, namely inter-CU compatible LTM and conditional LTM, have not been determined. Furthermore, the specific methods for recovering RRC (Radio Resource Control) connections in the event of failure of inter-CU compatible LTM or conditional LTM have not been determined. If these are not determined, for example, in inter-CU compatible LTM or conditional LTM, if some of the terminal's data or information is lost, or if the RRC connection recovers after an LTM cell switchover fails, communication may not be properly maintained.

[0007] The disclosed technology, made in view of the above, provides a method for enabling the correct continuation of communication in the recovery process of an RRC connection after inter-CU compatible LTM cell switching and / or conditional LTM cell switching.

[0008] In one aspect, the terminal device includes a receiving unit that receives a first signal containing change destination information that designates a first cell and a second cell as candidate cell change destinations, and a processing unit that, when a first process of switching a serving cell from a third cell to the first cell is activated, performs the first process using a first setting applied to the first cell, wherein if the first process fails, the processing unit performs an RRC re-establishment procedure using a third setting applied to the third cell, the processing unit performs a second process if predetermined conditions are met in the RRC re-establishment procedure, and the processing unit performs a third process if the predetermined conditions are not met in the RRC re-establishment procedure.

[0009] Furthermore, in one aspect, the base station device is provided, comprising: a transmitting unit that transmits a first signal containing change destination information that designates a first cell and a second cell as candidate cell change destinations; and a processing unit that, when a first process of switching the serving cell from a third cell to the first cell is activated in a terminal device, causes the terminal device to perform the first process using a first setting applied to the first cell; the processing unit, when the first process fails, causes the terminal device to perform an RRC re-establishment procedure using a third setting applied to the third cell; the processing unit, if predetermined conditions are met in the RRC re-establishment procedure, causes the processing unit to perform a second process; and the processing unit, if the predetermined conditions are not met in the RRC re-establishment procedure, causes the processing unit to perform a third process.

[0010] Furthermore, in one aspect, the present invention provides a wireless communication system comprising: a base station device that transmits a first signal containing change destination information that designates a first cell and a second cell as candidate cell change destinations; and a terminal device that receives the first signal, wherein the terminal device, when a first process for switching the serving cell from a third cell to the first cell is initiated, performs the first process using a first setting applied to the first cell; when the first process fails, performs an RRC re-establishment procedure using a third setting applied to the third cell; in the RRC re-establishment procedure, performs a second process if predetermined conditions are met; and in the RRC re-establishment procedure, performs a third process if the predetermined conditions are not met.

[0011] In the terminal, this enables the recovery process of RRC connections after inter-CU compatible LTM cell switching and / or conditional LTM cell switching, allowing communication to continue correctly.

[0012] Figure 1 shows an example of the network configuration of Embodiment 1. Figure 2 shows an example of a functional configuration block diagram of a base station in the wireless communication system of Embodiment 1. Figure 3 shows an example of a functional configuration block diagram of a terminal in the wireless communication system of Embodiment 1. Figure 4 shows an example of the LTM cell switching sequence that ensures communication is properly continued in the fast LTM recovery process in Embodiment 1. Figure 5 shows an example of the LTM cell switching failure process flow when the MCG LTM cell switching fails in Embodiment 1. Figure 6 shows an example of the hardware configuration of a base station. Figure 7 shows an example of the hardware configuration of a terminal.

[0013] Hereinafter, this embodiment will be described in detail with reference to the drawings. The problems and embodiments described herein are examples only and do not limit the scope of the rights of this application. In particular, even if the wording of the description is different, if it is technically equivalent, the technology of this application can be applied even with different wording and does not limit the scope of the rights. Furthermore, each embodiment can be appropriately combined as long as the processing content is not contradictory.

[0014] Furthermore, the terminology and technical content used in this specification may be appropriately adapted from the terminology and technical content described in specifications and contributions of communication standards such as 3GPP. Examples of such specifications are those described in Non-Patent Documents 1 to 14.

[0015] The following describes in detail, with reference to the drawings, embodiments of the base station, terminal, wireless communication system, and communication method disclosed in this application. The following embodiments are not intended to limit the disclosed technology. Embodiment 1

[0016] <Example Configuration of Wireless Communication System 1> Figure 1 is a diagram showing an example of wireless communication system 1 in Embodiment 1. Wireless communication system 1 has base stations 100A and 100B, and terminals 200A, 200B, and 200C. Base station 100A forms cell C10A. Base station 100B forms cell C10B. Terminal 200A is located within cells C10A and C10B. Terminal 200B is located within cell C10A. Terminal 200C is located within cell C10B. When base stations 100A and 100B are not distinguished, they are simply referred to as base station 100. When terminals 200A, 200B, and 200C are not distinguished, they are simply referred to as terminal 200.

[0017] Wireless communication system 1 is a wireless communication system in which, for example, terminal 200 communicates with base station 100A or base station 100B. Alternatively, wireless communication system 1 is a wireless communication system in which, for example, terminal 200 communicates with base stations 100A and 100B using MR-DC (Multi Radio Dual Connectivity). When communicating using MR-DC, for example, base station 100A is the master base station, and base station 100B is the secondary base station. Hereafter, the master base station may be referred to as MN (Master Node), and the secondary base station as SN (Secondary Node). Note that MR-DC may also be simply referred to as DC.

[0018] Furthermore, the base station 100 may be a small wireless base station (including micro wireless base stations, femto wireless base stations, etc.) such as a macro wireless base station or pico wireless base station, or a wireless base station of various sizes, and may be described as a wireless communication device, communication device, transmitting device, gNB (Node B), etc. Also, the terminal 200 may be a wireless terminal such as a mobile phone, smartphone, PDA (Personal Digital Assistant), personal computer, vehicle, airplane, drone, or other devices with wireless communication capabilities, or a device (sensor device, etc.) mounted on a robot, AV equipment, home appliances, office equipment, vending machine, other household equipment, industrial equipment, etc., and may be described as a wireless communication device, communication device, receiving device, mobile station, UE (User Equipment), user equipment, etc.

[0019] Base station 100 is connected to the network via wired connections to the core network (not shown in the diagram) and network devices (higher-level devices and other base stations). Alternatively, base station 100 may be connected to the network devices wirelessly instead of via wired connections.

[0020] The base station 100 may separate its wireless communication function with the terminal 200 from its digital signal processing and control functions into separate devices. In this case, the device with wireless communication functionality can be called an RRH (Remote Radio Head), and the device with digital signal processing and control functions can be called a BBU (Base Band Unit). The RRH may be installed extending from the BBU, and they may be connected by a wired connection such as an optical fiber. Alternatively, they may be connected wirelessly. Furthermore, instead of separating into RRH and BBU as described above, the base station 100 may be separated into, for example, a CU (Central Unit), a DU (Distributed Unit), and a RU (Radio Unit). The CU may include, for example, the functions of the RRC (Radio Resource Control) layer. The CU may also include, for example, the functions of the PDCP (Packet Data Convergence Protocol) layer. The CU may also include, for example, the functions of the SDAP (Service Data Adaptation Protocol) layer. The DU may include, for example, the functions of the MAC (Media Access Control) layer. Furthermore, the DU includes, for example, the functionality of the RLC (Radio Link Control) layer. The RU includes at least an RF radio circuit. The DU and RU may be integrated into a single unit.

[0021] Meanwhile, terminal 200 communicates with base station 100 via wireless communication. The RAT (Radio Access Technology) that provides the wireless connection is, for example, E-UTRA (Evolved Universal Terrestrial Radio Access), which is a fourth-generation RAT, NR (New Radio), which is a fifth-generation RAT, or a sixth-generation, seventh-generation, or later RAT.

[0022] <Example of Base Station 100 Configuration> Next, the base station 100 will be described. Figure 2 is a diagram showing an example of the functional configuration of the base station 100. The base station 100 has a wireless communication unit 110, a control unit 120, a storage unit 130, and a communication unit 140.

[0023] The wireless communication unit 110 consists of a transmitting unit 111 and a receiving unit 112, and communicates wirelessly with the terminal 200. Specifically, the transmitting unit 111 transmits downlink signals to the terminal 200, such as measurement signals (e.g., SSB, reference signals), random access procedure signals, RRC layer signals, downlink data signals, and downlink control signals.

[0024] The receiving unit 112 can receive uplink signals transmitted from the terminal 200, such as random access procedure signals, RRC layer signals, uplink data signals, and uplink control signals.

[0025] The control unit 120 controls the base station 100. Specifically, the control unit 120 can control the RRC connection with the terminal 200, process signals received by the receiving unit 112, create transport blocks (TBs), and map transport blocks to radio resources. The control unit 120 also performs processing related to radio protocols, such as the Radio Resource Control (RRC) layer, Service Data Adaptation Protocol (SDAP) layer, Packet Data Convergence Protocol (PDCP) layer, Radio Link Control (RLC) layer, and Medium Access Control (MAC) layer.

[0026] The memory unit 130 can store, for example, downlink data signals.

[0027] The communication unit 140 connects to network devices (e.g., host devices, other base stations) via wired or wireless connections and performs communication. Data signals received by the communication unit 140 for the terminal 200 can be stored in the storage unit 130. Note that the wireless communication unit 110 and the communication unit 140 may be collectively referred to as the communication unit.

[0028] <Example Configuration of Terminal 200> Next, terminal 200 will be described. Figure 3 is a diagram showing an example of the functional configuration of terminal 200. As shown in Figure 3, terminal 200 comprises a communication unit 210, a control unit 220, and a storage unit 230. Each of these components is connected in such a way that signals and data can be input and output in one direction or bidirectionally. Note that the communication unit 210 can be described separately as a transmitting unit 211 and a receiving unit 212.

[0029] The transmitting unit 211 transmits data signals and control signals wirelessly via an antenna. The antenna may be the same for both transmission and reception. The transmitting unit 211 transmits, for example, uplink signals such as random access procedure signals, RRC layer signals, uplink data signals, and uplink control signals.

[0030] The receiving unit 212 receives downlink signals transmitted from the base station 100, such as random access procedure signals, downlink data signals, and downlink control signals. The received signals may also include reference signals used for channel estimation and demodulation. The receiving unit 212 can also receive and measure measurement signals transmitted from the base station 100.

[0031] The control unit 220 controls the terminal 200. Specifically, the control unit 220 can control the RRC connection with the base station 100, signal processing of signals received by the receiving unit 312, creation of transport blocks (TBs), and mapping of transport blocks to radio resources. The control unit 220 can also control the measurement of measurement signals in the receiving unit 212. Furthermore, the control unit 220 executes processing related to radio protocols, such as the Radio Resource Control (RRC) layer, Service Data Adaptation Protocol (SDAP) layer, Packet Data Convergence Protocol (PDCP) layer, Radio Link Control (RLC) layer, and Medium Access Control (MAC) layer.

[0032] The storage unit 230 can store, for example, uplink data signals. The storage unit 230 can also store configuration information (or setting information) related to wireless communication transmitted from the base station 100.

[0033] <RRC Status> The RRC status of terminal 200 will be explained below.

[0034] The RRC status of terminal 200 refers to the state of terminal 200's RRC connection. The state in which an RRC connection with base station 100 has not been established is called RRC idle mode (RRC_IDLE). The state in which an RRC connection with base station 100 has been established is called RRC connected mode (RRC_CONNECTED). The state in which the RRC connection with base station 100 is suspended is called RRC inactive mode (RRC_INACTIVE).

[0035] <Wireless Bearer> An example of a wireless bearer used in wireless communication system 1 will be described.

[0036] A signaling radio bearer (SRB) is a bearer primarily used for transmitting and receiving RRC signals. RRC signals are also called RRC messages. There are several types of SRBs, such as SRB0, which is used for transmitting and receiving RRC messages using CCCH (Common Control Channel), and SRB1, SRB2, and SRB3, which are used for transmitting RRC messages using DCCH (Dedicated Control Channel).

[0037] A Data Radio Bearer (DRB) is a bearer primarily used for sending and receiving user data. User data includes, for example, IP (Internet Protocol) packets.

[0038] SRB1 is established when terminal 200 transitions from RRC idle mode to RRC connection mode. SRB2 and DRB are established after the AS (Access Stratum) security of wireless communication system 1 is activated. One or more DRBs are set depending on the QoS (Quality of Service) characteristics of the user data being transmitted and received. SRB3 may be established on the SN side when MR-DC is configured on terminal 200.

[0039] Hereafter, when the term "wireless bearer" is used without distinguishing between SRB and DRB, the wireless bearer may be an SRB, a DRB, or both an SRB and a DRB.

[0040] Each wireless bearer other than SRB0 is configured with a PDCP (Packet Data Convergence Protocol) entity and one or more RLC (Radio Link Control) bearers. An RLC bearer consists of an RLC entity and a logical channel. SRB0 does not have a PDCP entity and consists only of an RLC bearer. Wireless bearers are connected to MAC (Medium Access Control) entities through logical channels. There is one MAC entity for each cell group (CG).

[0041] <Cell Groups> This section describes the cell groups configured in terminal 200.

[0042] A cell group may consist of only one Special Cell (SpCell), or it may consist of one SpCell and one or more Secondary Cells (SCells). When an MR-DC is configured, two cell groups are formed: a Master Cell Group (MCG) composed of a Master Node (MN), and a Secondary Cell Group (SCG) composed of a Secondary Node (SN). An SpCell in an MCG is sometimes called a Primary Cell (PCell). An SpCell in an SCG is sometimes called a Primary SCG Cell (PSCell).

[0043] A PCell is a cell on the primary frequency of the MCG and is used for establishing and re-establishing RRC connections. When base station 100 requests a handover from terminal 200, a new PCell specified by base station 100 is used for random access.

[0044] PSCell is a cell on the SCG side of the primary frequency. PSCell is specified by the base station 100, and is used for random access when performing addition or change of PSCell in SCG.

[0045] SCell is a cell that provides additional radio resources in addition to SpCell when carrier aggregation (CA: Carrier Aggregation) is configured for the terminal 200.

[0046] <Bearer Types in MR-DC> The bearer types in MR-DC will be described below.

[0047] There are six types of bearer in MR-DC, depending on whether the PDCP termination point is on the MN side or the SN side, whether the RLC bearer is associated with the MCG, the SCG, or both the MCG and the SCG.

[0048] A radio bearer whose PDCP termination point is on the MN side may be referred to as an MN-terminated bearer, and a radio bearer whose PDCP termination point is on the SN side may be referred to as an SN-terminated bearer. Further, a radio bearer whose RLC bearer is associated with the MCG may be referred to as an MCG bearer, a radio bearer whose RLC bearer is associated with the SCG may be referred to as an SCG bearer, and a bearer associated with both the MCG and the SCG may be referred to as a split bearer.

[0049] SRB1 and SRB2 are each configured with either a bearer type of MN-terminated MCG bearer or MN-terminated split bearer. SRB3 is configured only as an SN-terminated SCG bearer.

[0050] DRB is configured with any one of the six types of bearer. That is, it is configured with any one of MN-terminated MCG bearer, MN-terminated SCG bearer, MN-terminated split bearer, SN-terminated MCG bearer, SN-terminated SCG bearer, and SN-terminated split bearer.

[0051] Terminal 200 can determine whether the PDCP termination point is on the MN side or the SN side based on whether each DRB is associated with the security key on the MN side or the security key on the SN side. Specifically, if a DRB is associated with the master key (the security key on the MN side), terminal 200 determines that this DRB is an MN-terminated bearer. If a DRB is associated with the secondary key (the security key on the SN side), terminal 200 determines that this DRB is an SN-terminated bearer.

[0052] <Reconfiguration with Sync> This section explains reconfiguration with sync.

[0053] Synchronized reconfiguration refers to a procedure performed by the terminal 200 by including a parameter (reconfigurationWithSync: hereafter sometimes referred to as the synchronized reconfiguration parameter) that instructs the base station 100 to perform synchronized reconfiguration in the RRC reconfiguration message (RRCReconfiguration) sent to the terminal 200.

[0054] Synchronized reconfiguration is a procedure for changing the SpCell that terminal 200 will use as its serving cell. When terminal 200 is instructed to perform synchronized reconfiguration, it applies the target settings received from base station 100 and performs actions such as random access to the target SpCell, MAC reset, and PDCP data recovery. If synchronized reconfiguration involves updating security keys, it also performs actions such as PDCP entity re-establishment and RLC entity re-establishment. Note that the destination is sometimes called the target, while the source is sometimes called the source of the change.

[0055] Furthermore, the process when synchronized reset parameters are included under MCG setting parameters, i.e., the synchronized reset process on the MCG side, is sometimes called a handover. Also, the process when synchronized reset parameters are included under SCG setting parameters, i.e., the synchronized reset process on the SCG side, is sometimes called a PSCell addition and / or PSCell change.

[0056] <Handover Failure Handling> This section explains the handling of synchronous reset (handover) failures on the MCG side.

[0057] If terminal 200, which has received an RRC reset message containing synchronized reset parameters, is unable to successfully complete the synchronized reset process within a certain period of time, the synchronized reset will fail. Failure to successfully complete the synchronized reset process within a certain period of time means, for example, that the timer for detecting synchronized reset failures, which starts when an RRC reset message containing synchronized reset parameters is received, expires before the synchronized reset process is successfully completed.

[0058] If the MCG's synchronized reconfiguration fails, terminal 200 reverts its settings to those used by the source PCell and performs the RRC re-establishment procedure. When reverting terminal 200's settings to those used by the source PCell, the values ​​of state variables and buffer states in each entity of each wireless bearer are also reverted to the values ​​used by the source (values ​​immediately before the handover process).

[0059] In the RRC re-establishment procedure, terminal 200 selects a cell and performs processing according to the selected cell. For example, if the selected cell is an NR cell, it sends an RRC re-establishment request message (RRCReestablishmentRequest) to base station 100. If a cell of a different RAT, such as E-UTRA, is selected, the terminal transitions to the RRC idle state. The RRC re-establishment procedure is also performed when terminal 200 detects a radio link failure (RLF).

[0060] <Rel-18 LTM> This section provides an overview of intra-CU (L1 / L2-triggered mobility) as specified in Rel-18.

[0061] In the CU-based LTM specified in Rel-18, the base station 100 sends an RRC reset message to the terminal 200 that includes settings for up to eight candidate cells that will be the target of synchronized reset, thereby causing the terminal 200 to retain the settings for the candidate cells. The settings for the candidate cells include synchronized reset parameters, target settings, and an LTM candidate identifier (ltm-CandidateId), which is an index for uniquely identifying the candidate cell. Subsequently, the base station 100 determines the target for synchronized reset from the Layer 1 (L1) measurement information sent from the terminal 200, and instructs the terminal 200 to perform synchronized reset to the target using a MAC control signal (MAC CE: MAC Control Element) that includes the target's LTM candidate identifier. The synchronized reset process in LTM is sometimes called LTM cell switch, cell switch, or LTM. After terminal 200 successfully switches to the target LTM cell, the candidate cell settings held by terminal 200 are not released and are used for subsequent LTM cell switches.

[0062] In Rel-18, intra-CU LTM cell switching includes intra-DU LTM cell switching and inter-DU LTM cell switching. When terminal 200 performs inter-DU LTM cell switching, it performs a Layer 2 (L2) reset on the DRB. An L2 reset is, for example, the re-establishment of RLC entities. Furthermore, an L2 reset includes, for example, the process by which terminal 200 performs data recovery of PDCP entities on an AM (Acknowledged Mode) DRB that has re-established RLC entities, and recovers data that was erased by the re-establishment of RLC entities. In order for terminal 200 to distinguish between intra-DU LTM cell switching and inter-DU LTM cell switching and to determine whether or not an L2 reset is necessary, each candidate cell setting includes an identifier called an LTM reset-free identifier (ltm-NoResetID). Candidate cells under the same DU (Digital Unit) have the same LTM (Long-Term Reset) no-reset identifier, while candidate cells under different DUs have different LTM no-reset identifiers. If the LTM no-reset identifier is not included in the settings of a candidate cell, terminal 200 considers all candidate cells to belong to different DUs.

[0063] In addition, during LTM cell switching within the CU in Rel-18, security key updates and re-establishment of PDCP and RLC entities associated with LTM cell switching are not performed.

[0064] Furthermore, if the MCG's LTM cell switching fails, or if a wireless link failure is detected, and the cell selected in the RRC re-establishment procedure is one of the candidate cells, and the terminal 200 is set to a parameter (attemptLTM-Switch) that allows recovery of the RRC connection, the terminal 200 will perform an LTM cell switch to the selected cell. This process of recovering the RRC connection may be called fast LTM recovery. Note that if the MCG's LTM cell switching fails, the terminal 200 will revert the settings to those used by the source PCell and perform the RRC re-establishment procedure, similar to when the normal synchronized reconfiguration on the MCG side fails. However, in this case, the state variables of the SRB's PDCP entity associated with the MCG will not be reverted to those used by the source PCell but will be retained.

[0065] <Rel-19LTM> This section provides a partial overview of the currently defined outlines for the inter-CU compatible LTM and conditional LTM, which are currently under discussion as Rel-19LTM.

[0066] In inter-CU (Central Unit) LTM (Long-Term Memory) systems, similar to intra-CU LTMs, the base station 100 allows the terminal 200 to maintain up to eight candidate cell settings. However, candidate cells can include not only cells within the same CU but also cells under different CUs. When switching LTM cells to cells under different CUs, updating the security key is mandatory, and this requires the re-establishment of PDCP and RLC entities. Furthermore, to distinguish between intra-CU LTM cell switching and inter-CU LTM cell switching, and to determine whether processing related to security key updates is necessary, each candidate cell setting includes an identifier called an LTM security change-free identifier (ltm-NoSecurityChangeID). Candidate cells under the same CU have the same LTM security change-free identifier, while candidate cells under different CUs have different LTM security change-free identifiers. Note that the LTM security change-free identifier may be referred to by a different name.

[0067] Furthermore, in LTMs that support inter-CUs, discussions are underway to allow changes in the DRB bearer type during LTM cell switching when MR-DC is configured on terminal 200. When an LTM cell switch causes a change in the PDCP termination point of a DRB, the security key changes from a master key to a secondary key, or from a secondary key to a master key, resulting in the re-establishment of PDCP entities and RLC entities. In addition, when an LTM cell switch causes no change in the PDCP termination point, but the bearer type of the RLC bearer changes, it may result in the release of RLC entities.

[0068] Conditional LTM is a method of LTM that is initiated by the terminal 200 without receiving an LTM cell switching signal from the base station 100, when the execution conditions for LTM cell switching are met. When the base station 100 has the terminal 200 maintain settings for up to eight candidate cells, it also has the terminal 200 maintain the execution conditions for LTM cell switching for each candidate cell. The execution conditions are, for example, measurement results. For example, if the measurement result of a certain candidate cell exceeds the measurement result of the serving cell by a threshold, the LTM cell switching execution condition for this candidate cell is met. Note that LTM cell switching using conditional LTM is limited to within the same CU, similar to intra-CU LTM in Rel-18, and includes intra-DU LTM cell switching and inter-DU LTM cell switching. In the case of inter-DU conditional LTM cell switching, the terminal 200 performs an L2 reset process.

[0069] Furthermore, it has been agreed to support fast LTM recovery in CU-compatible LTM and conditional LTM.

[0070] However, with inter-CU compatible LTM and conditional LTM, issues arise where fast LTM recovery is not performed correctly.

[0071] For example, if the RLC entity is re-established or released during processing prior to the LTM cell switchover failure, the data held in the RLC entity will be deleted. It is virtually impossible to recover this deleted data when reverting the terminal 200 settings to the settings used by the source PCell after the LTM cell switchover failure. Therefore, when performing a fast LTM recovery afterward, it is necessary to restore the data by performing data recovery on the PDCP entity as appropriate.

[0072] Furthermore, if, for example, the PDCP entity is re-established due to processing before the LTM cell switchover failure, the header compression information will be initialized and data in the SRB will be deleted. This initialized information and deleted data cannot be recovered when the terminal 200 settings are restored to the settings used by the source PCell after the LTM cell switchover failure. If a fast LTM recovery is then performed, it may affect proper data transmission and reception. Therefore, it is necessary to avoid performing a fast LTM recovery.

[0073] Furthermore, for example, in conditional LTM, the base station 100 cannot distinguish between normal conditional LTM and fast LTM recovery. Therefore, it may not be able to perform processing specific to fast LTM recovery. Processing specific to fast LTM recovery includes, for example, processing that takes into account that the SRB state variable is retained when the terminal 200 returns to the source PCell settings after an LTM cell switching failure. For this reason, it is necessary for the base station 100 to be able to distinguish between normal conditional LTM and fast LTM recovery.

[0074] <Example of a method for correctly continuing communication during fast LTM recovery processing> Figure 4 shows an example of an LTM cell switching sequence that correctly continues communication during fast LTM recovery processing in Embodiment 1.

[0075] Terminal 200 performs a UE capability transfer procedure with base station 100 (step S401). When terminal 200 receives a UE capability inquiry message from base station 100, it sends a UE capability information message to base station 100, which includes terminal 200's capability information. As UE capability information, terminal 200 includes information indicating support for Rel-19 LTM in the UE capability information message. Information indicating support for Rel-19 LTM includes, for example, information indicating support for inter-CU compatible LTM. Information indicating support for Rel-19 LTM also includes, for example, information indicating support for conditional LTM. Information indicating support for Rel-19 LTM also includes, for example, information indicating support for fast LTM recovery.

[0076] Next, the base station 100 sends an RRC reset message to the terminal 200, including up to eight candidate cell settings. The terminal 200 receives the RRC reset message (step S402). The terminal 200 stores the LTM setting information (including the candidate cell settings) included in the received RRC reset message. The terminal 200 also stores the execution conditions for performing an LTM cell switch for each candidate cell if the LTM settings include such conditions. The terminal 200 may also store the value of the parameter (ltm-ServingCellNoResetID) included in the LTM settings, which indicates the LTM reset-free identifier of the current serving cell, as a variable. The terminal 200 may also store the value of the parameter (ltm-ServingCellNoSecurityChangeID) included in the LTM settings, which indicates the LTM security change-free identifier of the current serving cell, as a variable. The storage location is, for example, the storage unit 230. The information corresponding to the candidate cell settings is an example of change destination information. Furthermore, the RRC reset message is an example of the first signal. Note that, for example, when multiple cells are set as candidate cells, the RRC reset message includes information corresponding to the setting of candidate cells, such as target cell change information that designates at least the first and second cells as candidate cell change targets.

[0077] Terminal 200 initiates an LTM cell switch to one of the candidate cells (let's call it cell X) in the current serving cell (let's call it cell Z) (step S403). The LTM cell switch to cell X is initiated, for example, by receiving an LTM cell switch command MAC CE from base station 100. Alternatively, the LTM cell switch to cell X is initiated when the conditions for executing the LTM cell switch to cell X are met. Furthermore, the LTM cell switch to cell X is initiated by fast LTM recovery. Note that the LTM cell switch process is an example of the first process.

[0078] When the LTM cell switch to cell X is initiated, terminal 200 starts a timer (T304) for detecting synchronization-enabled reset failures (step S404).

[0079] Terminal 200 applies the settings for cell X and performs an LTM cell switching process from cell Z to cell X (step S405). Note that the LTM cell switching process may be a collective term for steps S403 to S405, or a collective term for steps S403 and S405, or a collective term for S405.

[0080] The process of applying the settings of cell X may include process A. Process A is, for example, the process by which terminal 200 determines whether or not it is an inter-CU LTM cell switchover based on the LTM security change-free identifier included in the settings of the candidate cell of cell X. If the LTM security change-free identifier included in the settings of the candidate cell of cell X is different from the LTM security change-free identifier of the current serving cell stored in step S402, it is determined that it is an inter-CU LTM cell switchover. Furthermore, terminal 200 performs, for example, processing related to the inter-CU LTM cell switchover. Processing related to the inter-CU LTM cell switchover includes, for example, updating the security key. Processing related to the inter-CU LTM cell switchover also includes, for example, the process of re-establishing the PDCP entity for the wireless bearer. Processing related to the inter-CU LTM cell switchover also includes, for example, the process of re-establishing the RLC entity for the wireless bearer.

[0081] Furthermore, the process of applying the settings of cell X may include process B. Process B is, for example, the process by which terminal 200 determines whether there is a DRB whose PDCP termination point has been changed, based on the security key associated with each DRB. If the security key associated with the DRB differs between the setting in the source and the candidate cell setting in cell X, it is determined that the PDCP termination point for this DRB has been changed. Furthermore, terminal 200 performs, for example, processing related to the change in the PDCP termination point. Processing related to the change in the PDCP termination point includes, for example, the process of re-establishing the PDCP entity for this DRB. Processing related to the change in the PDCP termination point also includes, for example, the process of re-establishing the RLC entity for this DRB. The security key associated with the DRB is either the master key or the secondary key. The security key associated with the DRB can be determined from the keyToUse parameter. Process B does not need to be performed if, for example, process A has been performed.

[0082] Furthermore, the process of applying the settings of cell X may include process C. Process C is, for example, the process by which terminal 200 determines from the cell group to which the logical channel is associated whether there is a DRB in which RLC entities have been released due to a change in the RLC bearer type. Furthermore, if there is a DRB in which RLC entities have been released, and this DRB is an AM DRB, terminal 200 performs data recovery of the PDCP entities (PDCP data recovery). Note that process C does not need to be performed if, for example, process A has been performed. Also, process C does not need to be performed for a DRB in which, for example, process B has been performed.

[0083] The process of applying the settings of cell X also includes, for example, process D. Process D is, for example, the process by which terminal 200 determines whether an L2 reset is necessary for some or all of the configured wireless bearers, based on the LTM reset no-reset identifier (ltm-NoResetID) included in the settings of the candidate cells of cell X. If the settings of the candidate cells of cell X do not include the LTM reset no-reset identifier, or if the LTM reset no-reset identifier included in the settings of the candidate cells of cell X is different from the LTM reset no-reset identifier of the current serving cell stored in step S402, it is determined that an L2 reset is necessary, and an L2 reset is performed. Process D does not need to be performed if, for example, process A has been performed. Also, process D does not need to be performed for DRBs that have undergone, for example, process B.

[0084] Furthermore, if, for example, the LTM security change-free identifier included in the candidate cell settings of cell X is different from the LTM security change-free identifier of the current serving cell stored in step S402, the terminal 200 stores the LTM security change-free identifier included in the candidate cell settings of cell X as the LTM security change-free identifier of the current serving cell.

[0085] Furthermore, if, for example, the terminal 200 finds that the LTM reset-free identifier included in the candidate cell settings for cell X is different from the LTM reset-free identifier of the current serving cell stored in step S402, it stores the LTM reset-free identifier included in the candidate cell settings for cell X as the LTM reset-free identifier of the current serving cell.

[0086] If the timer started in step S404 expires before the LTM cell switching of terminal 200 is completed, an LTM cell switching failure is detected (step S406). When terminal 200 detects an LTM cell switching failure, it performs an LTM cell switching failure process (step S407).

[0087] Figure 5 shows an example of the flow of the LTM cell switching failure process in the case where the MCG LTM cell switching fails in Embodiment 1.

[0088] Terminal 200 restores its settings to those used by the source PCell (step S501). At this time, if a parameter indicating permission for fast LTM recovery is set, terminal 200 retains some or all of the state variables of some or all of the wireless bearers without restoring them to the settings used by the source PCell. Some wireless bearers are, for example, SRBs associated with MCG. Some state variables are, for example, the state variables of PDCP entities. The parameter indicating permission for fast LTM recovery is, for example, the attemptLTM-Switch parameter.

[0089] Terminal 200 performs the RRC re-establishment procedure (step S502).

[0090] In the RRC connection re-establishment procedure, terminal 200 determines whether a predetermined condition is met and performs processing based on that determination (step S503). The predetermined condition may include, for example, condition A. Condition A is, for example, that a parameter meaning that fast LTM recovery is permitted is set on terminal 200. The parameter meaning that fast LTM recovery is permitted is, for example, the attemptLTM-Switch parameter.

[0091] Furthermore, the specified conditions include, for example, condition B. Condition B is, for example, that the cell selected in the RRC re-establishment procedure is one of the candidate cells, or that the cell selected in the RRC re-establishment procedure is one of the candidate cells within the same CU. A cell selected in the RRC re-establishment procedure being one of the candidate cells within the same CU means, for example, that the source cell before the LTM cell switchover and one of the selected candidate cells are cells within the same CU. The determination that a cell selected in the RRC re-establishment procedure is one of the candidate cells within the same CU is made, for example, by checking whether the LTM security change-free identifier included in the settings of the selected candidate cell is the same as the LTM reset-free identifier of the current serving cell. Let cell Y be an example of a cell that meets condition B.

[0092] Furthermore, the specified conditions may include, for example, condition C. Condition C is, for example, that in the LTM cell switching process before the failure, the PDCP entities of some or all wireless bearers were not re-established. Not having the PDCP entities of some or all wireless bearers re-established includes, for example, that it is not an LTM cell switching between CUs. The determination that it is not an LTM cell switching between CUs is made, for example, by checking that in the LTM cell switching process before the failure, the LTM security change-free identifier included in the candidate cell setting of the target (cell X) is the same as the LTM reset-free identifier of the current serving cell. Also, not having the PDCP entities of some or all wireless bearers re-established means, for example, that there are no DRBs whose PDCP termination points have been changed. Not having a DRB whose PDCP termination point has been changed means, for example, that there are no DRBs whose associated security key (keyToUse) has been changed.

[0093] Furthermore, the specified conditions may include, for example, condition D. Condition D is, for example, that the RLC entities of some or all wireless bearers have not been re-established or released.

[0094] Furthermore, the specified conditions may include, for example, condition E. Condition E may be, for example, that MR-DC is not set on terminal 200. Not having MR-DC set on terminal 200 means, for example, that MR-DC is not set in the source settings of terminal 200 before the LTM cell switching failure, and / or that MR-DC is not set in the target settings of terminal 200 before the LTM cell switching failure. Also, condition E may be, for example, that MR-DC in the inter-CU compatible LTM is not set on terminal 200. Not having MR-DC in the inter-CU compatible LTM on terminal 200 means, for example, that MR-DC in the inter-CU compatible LTM is not set in the source settings of terminal 200 before the LTM cell switching failure, and / or that MR-DC in the inter-CU compatible LTM is not set in the target settings of terminal 200 before the LTM cell switching failure. The determination that a system is an inter-CU compatible LTM may be made, for example, by setting an LTM security change-free identifier on terminal 200, or by setting other parameters. Alternatively, the determination that a system is an inter-CU compatible LTM may be made by other methods, such as terminal 200 transmitting a UE capability to base station 100 indicating that it supports Rel-19 LTM.

[0095] In other words, the specified conditions are those that include some or all of conditions A, B, C, D, and E.

[0096] Note that the LTM cell switching process before failure is, for example, the LTM cell switching process from cell Z to cell X. Also, the LTM cell switching process before failure is, for example, the process in step S405 in Figure 4.

[0097] If terminal 200 determines that a predetermined condition is met (Yes in step S503), it performs process X. Process X is, for example, the process of performing an LTM cell switch for one of the selected candidate cells (cell Y). If terminal 200 determines that a predetermined condition is not met (No in step S503), it performs process Y. Process Y is, for example, the process of sending an RRC re-establishment request message (RRCReestablishmentRequest) to base station 100 if the selected cell is an NR cell. Process Y is, for example, the process of transitioning to an RRC idle state if the selected cell is a cell of a different RAT such as E-UTRA. Process X is an example of the second process. Process Y is an example of the third process.

[0098] Process X may include process Z in addition to the processes in steps S403 and S404 of Figure 4. Process Z may include, for example, the process of re-establishing the RLC entities of some or all of the wireless bearers. Some of the wireless bearers whose RLC entities are re-established may be, for example, DRBs. Some of the wireless bearers whose RLC entities are re-established may be, for example, DRBs whose RLC entities were re-established or released in the LTM cell switching process before the failure. Process Z may include, for example, the process of performing data recovery of PDCP entities for AM DRBs whose RLC entities were re-established or released. Process Z is an example of the fourth process.

[0099] Process Z is performed, for example, after an inter-CU LTM cell switching failure. The determination that an inter-CU LTM cell switching failure has occurred may be made, for example, by whether an LTM security change-free identifier is set on terminal 200, or by whether other parameters are set. The determination that an inter-CU LTM cell switching failure has occurred may also be made by other methods, such as terminal 200 transmitting a UE capability to base station 100 indicating that it supports Rel-19 LTM.

[0100] Furthermore, process Z is performed, for example, after a conditional LTM cell switchover has failed. The determination that a conditional LTM cell switchover has failed may be made, for example, by determining that the terminal 200 has been configured for conditional LTM. Alternatively, the determination that a conditional LTM cell switchover has failed may be made by other methods, such as when the terminal 200 transmits a UE capability to the base station 100 indicating that it supports Rel-19 LTM.

[0101] Furthermore, in process X, terminal 200 may include parameter A in the RRC Reconfiguration Complete message (RRCReconfigurationComplete) that it sends to base station 100 when performing an LTM cell switch to cell Y. Parameter A is, for example, a parameter indicating that it is a fast LTM recovery. Parameter A is also, for example, a parameter indicating that it is a fast LTM recovery after an LTM cell switch failure. For example, if it is a fast LTM recovery after an LTM cell switch failure, terminal 200 includes parameter A in the RRC Reconfiguration Complete message that it sends to base station 100 when performing an LTM cell switch to cell Y. An LTM cell switch failure includes, for example, some or all of the following: an LTM cell switch failure within a Rel-18 CU, an LTM cell switch failure between CUs, and a conditional LTM failure. Furthermore, if, for example, a conditional LTM setting has been configured, terminal 200 includes parameter A in the RRC reconfiguration completion message it sends to base station 100 when performing an LTM cell switch to cell Y. Note that the parameter indicating that it is a fast LTM recovery can be rephrased as the parameter indicating that a fast LTM recovery has been performed. Including parameter A in the RRC reconfiguration completion message (RRCReconfigurationComplete) is an example of the fifth process.

[0102] Furthermore, after an LTM cell switchover fails due to fast LTM recovery, fast LTM recovery does not need to be supported. In other words, after an LTM cell switchover fails due to fast LTM recovery, for example, process X is not performed, and process Y is performed instead.

[0103] Fast LTM recovery may also be performed during the RRC re-establishment procedure after terminal 200 detects a wireless link failure, provided that certain conditions are met.

[0104] This embodiment makes it possible to correctly continue communication during the RRC connection recovery process (fast LTM recovery process) after an LTM cell switching failure corresponding to inter-CU cell switching, and after a conditional LTM cell switching failure. Hardware configuration of each device in each embodiment

[0105] The hardware configuration of each device in the wireless communication system of each embodiment will be described based on Figures 6 to 7.

[0106] Figure 6 shows an example of the hardware configuration of base station 100. As shown in Figure 6, the base station 100 has, as hardware components, an RF (Radio Frequency) circuit 320 equipped with an antenna 410, a CPU (Central Processing Unit) 330, a DSP (Digital Signal Processor) 340, a memory 350, and a network IF (Interface) 360. The CPU is connected via a bus to enable input and output of various signals and data signals. The memory 350 includes at least one of RAM (Random Access Memory), such as SDRAM (Synchronous Dynamic Random Access Memory), ROM (Read Only Memory), and flash memory, and stores programs, control information, and data signals.

[0107] The correspondence between the functional configuration of the base station 100 shown in Figure 2 and the hardware configuration of the base station 100 shown in Figure 6 will be explained. The transmitting unit 111 and the receiving unit 112 (or wireless communication unit 110) are realized by, for example, an RF circuit 320, or an antenna 310 and an RF circuit 320. The control unit 120 is realized by, for example, a CPU 330, a DSP 340, a memory 350, and a digital electronic circuit (not shown). Examples of digital electronic circuits include ASIC (Application Specific Integrated Circuit), FPGA (Field Programmable Gate Array), and LSI (Large Scale Integration). The storage unit 130 is realized by, for example, a memory 350. The communication unit 140 is realized by, for example, a network IF 360.

[0108] Furthermore, while the base station 100 can generate multiple data signals transmitted in multiple subbands, the filters that generate these signals may be configured independently for each subband.

[0109] Figure 7 shows an example of the hardware configuration of terminal 200. As shown in Figure 7, terminal 200 has, as hardware components, an RF circuit 420 equipped with an antenna 410, a CPU 430, a DSP 440, and a memory 440. The memory 440 includes at least one of RAM such as SDRAM, ROM, and flash memory, and stores programs, control information, and data signals.

[0110] The correspondence between the functional configuration of terminal 200 shown in Figure 3 and the hardware configuration of terminal 200 shown in Figure 7 will be explained. The transmitting unit 211 and the receiving unit 212 (or the communication unit 210) are realized by, for example, an RF circuit 420, or an antenna 410 and an RF circuit 420. The control unit 220 is realized by, for example, a CPU 430, a DSP 440, a memory 450, and a digital electronic circuit (not shown). Examples of digital electronic circuits include ASICs, FPGAs, and LSIs. The storage unit 230 is realized by, for example, a memory 450.

[0111] <Other> Furthermore, each embodiment may be combined as appropriate, provided it is not inconsistent.

[0112] In addition, the message sending, receiving, and processing in sequences and flows described in each embodiment may not be performed in order, or the order may be changed. Furthermore, some message sending, receiving, and processing in sequences and flows may not be performed at all.

[0113] Furthermore, the functions and processes described for terminal 200 may also be functions and processes of base station 100. Similarly, the functions and processes described for base station 100 may also be functions and processes of terminal 200.

[0114] Furthermore, if condition "A" and condition "B" are contradictory, condition "B" may be expressed as an "other" condition of condition "A".

[0115] Each embodiment can be summarized, for example, as shown in (1) to (12) below.

[0116] (1) A terminal device comprising: a receiving unit that receives a first signal including change destination information that designates a first cell and a second cell as candidate cell change destinations; and a processing unit that, when a first process of switching a serving cell from a third cell to the first cell is activated, performs the first process using a first setting applied to the first cell, wherein if the first process fails, the processing unit performs an RRC re-establishment procedure using a third setting applied to the third cell; the processing unit performs a second process if predetermined conditions are met in the RRC re-establishment procedure; and the processing unit performs a third process if the predetermined conditions are not met in the RRC re-establishment procedure.

[0117] (2) The terminal device according to (1), wherein the first process is activated when the receiving unit receives a second signal containing information about cell switching, and in accordance with the information about cell switching contained in the second signal.

[0118] (3) The terminal device according to (1), wherein the first process is activated when the measurement result for the first cell satisfies the conditions for performing the first process.

[0119] (4) The terminal device according to (1), (2), or (3), wherein the predetermined conditions include some or all of the following: the first parameter is set; the second cell is selected and the second cell is a cell belonging to the same base station as the first cell; some or all of the PDCP entities of some or all radio bearers have not been re-established in the switching process before the failure; and MR-DC is not set.

[0120] (5) The terminal device according to (1), (2), (3), or (4), wherein the second process is a process of switching the serving cell to the second cell using the second setting applied in the second cell.

[0121] (6) The terminal device according to (1), or (2), or (3), or (4), or (5), wherein the second process includes the fourth process.

[0122] (7) The terminal device according to (1), (2), (3), (4), or (5), wherein the second process includes a process that performs the fourth process when the first condition is met, the first condition being that a parameter indicating that it is an inter-CU compatible LTM or a parameter indicating that it is a conditional LTM is set.

[0123] (8) The terminal device according to (6) or (7), wherein the fourth process is to re-establish the RLC entity of the first wireless bearer, and if the RLC entity of the first wireless bearer is in acknowledged mode, to perform data recovery of the PDCP entity for the wireless bearer.

[0124] (9) The terminal device according to (1), (2), (3), (4), (5), (6), or (7), wherein the second process includes a process of performing a fifth process if the first condition is met, and the fifth process includes a parameter in the RRC reset completion message indicating that the second process has been performed.

[0125] (10) The terminal device according to (1), (2), (3), or (4), wherein the third process is the process of sending an RRC re-establishment request message.

[0126] (11) A base station device comprising: a transmitting unit that transmits a first signal including change destination information that designates a first cell and a second cell as candidate cell change destinations; and a processing unit that, when a first process of switching a serving cell from a third cell to the first cell is activated in a terminal device, causes the terminal device to perform the first process using a first setting applied in the first cell, wherein if the first process fails, the processing unit causes the terminal device to perform an RRC re-establishment procedure using a third setting applied in the third cell; the processing unit causes the terminal device to perform a second process if predetermined conditions are met in the RRC re-establishment procedure; and the processing unit causes the terminal device to perform a third process if the predetermined conditions are not met in the RRC re-establishment procedure.

[0127] (12) A wireless communication system comprising: a base station device that transmits a first signal including destination information that designates a first cell and a second cell as candidates for cell change destinations; and a terminal device that receives the first signal, wherein the terminal device, when a first process for switching a serving cell from a third cell to the first cell is activated, performs the first process using a first setting applied to the first cell; when the first process fails, performs an RRC re-establishment procedure using a third setting applied to the third cell; in the RRC re-establishment procedure, performs a second process if predetermined conditions are met; and in the RRC re-establishment procedure, performs a third process if the predetermined conditions are not met.

[0128] Although each embodiment describes an example of a base station and a terminal, the disclosed technology is not limited to these examples and can be applied to various devices such as electronic equipment mounted on automobiles, trains, airplanes, satellites, electronic equipment transported by drones, robots, AV equipment, home appliances, office equipment, vending machines, and other everyday devices.

[0129] Furthermore, although each embodiment was explained using fifth-generation mobile communication as an example, the disclosed technology is not limited to these. For example, the disclosed technology may be applied to mobile communication of different generations, such as sixth-generation or seventh-generation.

[0130] 1 Wireless Communication System 100 100A 100B Base Station C10A C10B Cell 110 Wireless Communication Unit 111 Transmitter Unit 112 Receiver Unit 120 Control Unit 130 Memory Unit 140 Communication Unit 200 200A 200B 200C Terminal 210 Communication Unit 211 Transmitter Unit 212 Receiver Unit 220 Control Unit 230 Memory Unit 310 Antenna 320 RF Circuit 330 CPU 340 DSP 350 Memory 360 Network IF 410 Antenna 420 RF Circuit 430 CPU 440 DSP 450 Memory

Claims

1. A terminal device comprising: a receiving unit that receives a first signal containing change destination information that designates a first cell and a second cell as candidate cell change destinations; and a processing unit that, when a first process for switching a serving cell from a third cell to the first cell is activated, performs the first process using a first setting applied to the first cell, wherein if the first process fails, the processing unit performs an RRC re-establishment procedure using a third setting applied to the third cell; the processing unit performs a second process if predetermined conditions are met in the RRC re-establishment procedure; and the processing unit performs a third process if the predetermined conditions are not met in the RRC re-establishment procedure.

2. The terminal device according to claim 1, wherein the first process is activated when the receiving unit receives a second signal containing information regarding cell switching, and in accordance with the information regarding cell switching contained in the second signal.

3. The terminal device according to claim 1, wherein the first process is activated when the measurement result for the first cell satisfies the conditions for performing the first process.

4. The terminal device according to any one of claims 1 to 3, wherein the predetermined conditions include some or all of the following: the first parameter is set; the second cell is selected and the second cell is a cell belonging to the same base station as the first cell; in the switching process before failure, some or all of the PDCP entities of some or all radio bearers have not been re-established; and MR-DC is not set.

5. The terminal device according to any one of claims 1 to 3, wherein the second process is a process of switching the serving cell to the second cell using a second setting applied in the second cell.

6. The terminal device according to any one of claims 1 to 3, wherein the second process includes a fourth process.

7. The terminal device according to any one of claims 1 to 3, wherein the second process includes a process of performing a fourth process if the first condition is met, and the first condition is that a parameter indicating that it is an inter-CU corresponding LTM or a parameter indicating that it is a conditional LTM is set.

8. The terminal device according to claim 6 or claim 7, wherein the fourth process is to re-establish the RLC entity of the first wireless bearer, and if the RLC entity of the first wireless bearer is in acknowledged mode, to perform data recovery of the PDCP entity for the first wireless bearer.

9. The terminal device according to any one of claims 1 to 3, wherein the second process includes a process of performing a fifth process if the first condition is met, and the fifth process includes a parameter in the RRC reset completion message indicating that the second process has been performed.

10. The terminal device according to any one of claims 1 to 3, wherein the third process is the process of sending an RRC re-establishment request message.

11. A base station device comprising: a transmitting unit that transmits a first signal including change destination information that designates a first cell and a second cell as candidate cell change destinations; and a processing unit that, when a first process of switching a serving cell from a third cell to the first cell is activated in a terminal device, causes the terminal device to perform the first process using a first setting applied in the first cell, wherein if the first process fails, the processing unit causes the terminal device to perform an RRC re-establishment procedure using a third setting applied in the third cell; the processing unit causes the terminal device to perform a second process if predetermined conditions are met in the RRC re-establishment procedure; and the processing unit causes the terminal device to perform a third process if the predetermined conditions are not met in the RRC re-establishment procedure.

12. A wireless communication system comprising: a base station device that transmits a first signal including change destination information that designates a first cell and a second cell as candidate cell change destinations; and a terminal device that receives the first signal, wherein the terminal device performs the first process using a first setting applied to the first cell when a first process for switching the serving cell from a third cell to the first cell is initiated; performs an RRC re-establishment procedure using a third setting applied to the third cell when the first process fails; performs a second process if predetermined conditions are met in the RRC re-establishment procedure; and performs a third process if the predetermined conditions are not met in the RRC re-establishment procedure.

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