Terminal device and wireless communication system

Abstract

The present disclosure suggests that a terminal device, which communicates with a plurality of base station devices while switching a connection destination, acquires a communication parameter associated with at least one handover-destination-candidate base station device and information on an LTM implementation condition, determines whether the LTM implementation condition associated with the handover-destination-candidate base station device is satisfied before receiving, from a handover source base station device, a Cell Switch Command instructing a change of a connection destination cell, and transmits an RA preamble to the handover-destination-candidate base station device associated with the LTM implementation condition when the LTM implementation condition is satisfied.

Description

Terminal device and wireless communication system

[0001] The present disclosure relates to a terminal device and a wireless communication system.

[0002] In a mobile communication system, as a terminal device moves, a handover is performed to switch the cell (base station device) to which the terminal device is connected. Conventionally, after receiving a handover instruction from a base station device, the terminal device performs a process of establishing a connection by establishing synchronization and setting up in the cell to which the connection is to be switched. This process of switching the connection takes a certain amount of time, and the terminal device cannot communicate user data while this process is being performed.

[0003] In contrast, in the technique described in Non-Patent Document 1, a handover source base station provides a terminal device with communication parameters in the radio resource control (RRC) layer for connection with another handover destination base station before the actual handover is performed. Then, the handover source base station transmits a Layer 1 or Layer 2 command to the terminal device at the timing when the handover is to be performed, instructing the terminal device to switch cells. Upon receiving the command, the terminal device executes a random access procedure with the handover destination base station, thereby completing the handover without subsequent configuration processing in the RRC layer. This cell switching technology is called LTM (L1L2 Triggered Mobility) and is intended to suppress degradation of communication efficiency related to handover.

[0004] 3GPP (registered trademark) Contribution, R2-2209255

[0005] In the handover process using LTM disclosed in Non-Patent Document 1, the terminal device needs to receive a Cell Switch command from the base station device.

[0006] However, if the terminal device cannot receive the Cell Switch Command, the terminal device cannot initiate handover processing on its own, and must execute cell search processing and reconnection processing after detecting an RLF (Radio Link Failure). For this reason, the LTM technology of Non-Patent Document 1 has a problem in that it cannot shorten the communication interruption time and cannot speed up the handover processing.

[0007] In view of such circumstances, the present disclosure proposes a technology that enables a terminal device to perform LTM before receiving a Cell Switch command, thereby speeding up handover processing and shortening the interruption time.

[0008] In order to solve the above problem, the present disclosure proposes, as an example, a terminal device that communicates with a plurality of base station devices while switching connection destinations, the terminal device including: a storage device that stores a program related to handover; and a processor that reads the program from the storage device and executes handover processing, wherein the processor executes the following processes: acquiring, from the connected handover source base station device, communication parameters and information on conditions for implementing LTM associated with at least one handover destination candidate base station device; determining whether the conditions for implementing LTM associated with the handover destination candidate base station device are satisfied before receiving, from the handover source base station device, a Cell Switch Command that instructs a change of connection destination cell; and, if the conditions for implementing LTM are satisfied, transmitting an RA preamble to the handover destination candidate base station device associated with the conditions for implementing LTM.

[0009] According to the technology of the present disclosure, handover processing can be performed even when a terminal device cannot receive a Cell Switch command, thereby realizing faster processing and shorter interruption times.

[0010] Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which the same or similar elements are designated by the same reference numerals.

[0011] The accompanying drawings are included in and form a part of the specification, illustrate embodiments of the present invention, and are used to explain the principles of the present invention together with the description. Fig. 1 is a diagram showing an example of a schematic configuration of a wireless communication system according to this embodiment. Fig. 2 is a diagram showing an example of a schematic configuration of a base station device 100 according to this embodiment. Fig. 3 is a diagram showing an example of a schematic configuration of a terminal device 200 according to this embodiment. Fig. 4 is a diagram for explaining sequence example 1 (in the case of contention-free RA) of handover processing according to this embodiment. Fig. 5 is a diagram for explaining sequence example 2 (in the case of contention-based RA) of handover processing according to this embodiment.

[0012] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be combined in any desired manner. Furthermore, the same reference numerals are used to designate identical or similar components, and redundant descriptions will be omitted.

[0013] This embodiment discloses a technique for performing LTM to a candidate cell before a terminal device receives a Cell Switch Command from a handover source base station device. The technique according to this embodiment is not limited to this, but can be applied to, for example, wireless communication in the frequency band FR1 (410 MHz to 7125 MHz).

[0014] <System Schematic Configuration Example> Fig. 1 is a diagram showing a schematic configuration example of a wireless communication system according to this embodiment. The wireless communication system is a cellular communication system configured in accordance with a cellular communication standard such as the fifth generation (5G) of the Third Generation Partnership Project (3GPP (registered trademark)) or its successor standard. The wireless communication system includes a plurality of base station devices (base station device 100_1, base station device 100_2, ...) and a terminal device 200.

[0015] In Fig. 1 , a terminal device 200 is about to move from within the range of a cell formed by a base station device (handover source base station device) 100_1 into the range of a cell formed by a base station device (handover destination base station device) 100_2. At this time, the terminal device 200 performs handover to switch the connection destination base station device from the handover source base station device 100_1 to the handover destination base station device 100_2. Note that, although Fig. 1 shows only the handover destination base station device 100_2 as a handover destination candidate, in reality, there may be multiple handover destination candidates (handover destination base station devices 100_3, 100_4, ..., 100_k, ...).

[0016] <Regarding Conditions (Events) for Implementing LTM> In the LTM according to the present embodiment, the terminal device 200 starts connection processing and cell switching processing with the handover destination base station device 100_2 or other handover destination candidate base station devices (handover destination base station device 100_k: k = 3, 4, ...) without receiving a command (cell switch command) instructing cell switching from the handover source base station device 100_1. As an example, the terminal device 200 acquires, from the handover source base station device 100_1, information on conditions (events) for starting the cell switching processing and on cell switching processing to be performed when the condition is determined to be met, and then determines whether the condition is met and performs cell switching processing according to the condition. For example, it is assumed that condition 1 is associated with cell switching processing to the handover destination base station device 100_2, and condition 2 is associated with cell switching processing to another handover destination candidate base station device (handover destination base station device 100_3). In this case, when it is determined (confirmed) that condition 1 is met, the terminal device 200 executes a cell switching process to the base station device 100_2. On the other hand, when it is determined (confirmed) that condition 2 is met, the terminal device 200 starts a cell switching process to another handover destination candidate base station device (handover destination base station device 100_3). A plurality of pieces of information regarding the conditions and the cell switching process are included in an RRC (Radio Resource Control) layer message RRC Reconfiguration, and a condition set for each candidate cell is included in condExecutionCond. In addition, communication parameters used for communication with the handover destination candidate base station device are included in condLTMReconfig. These pieces of information are provided from the handover source base station device 100_1 to the terminal device 200.

[0017] <Specific Examples of the Conditions (Events)> As conditions (events) for implementing LTM, it is conceivable to set conditions related to the radio quality of the communicating cell or candidate cell (CondEventA3, CondEventA4, CondEventA5, CondEventD1, CondEventT1), for example. In this case, for example, RRC Reconfiguration includes information such as condReconfigId (condition setting identifier) ​​= 1, condExecutionCond = condEventA4 (threshold = threshold 1), and condLTMReconfig = communication parameters for candidate cell 1, and condReconfigId = 2, condExecutionCond = condEventA4 (threshold = threshold 2), and condLTMReconfig = communication parameters for candidate cell 2, and this information is provided to the terminal device 200.

[0018] In addition, it has been considered that the base station device 100 sets information (MeasId) for setting a report of an event occurrence to the terminal device 200 in condExecutionCond. Here, MeasId is associated with an identifier measObjectId used by the terminal device to identify a measurement target of wireless quality and an identifier reportConfigId used by the terminal device to identify the timing of reporting to the base station device. The identifier measObjectId is associated with the NR or LTE frequency that the terminal device 200 should measure. The identifier reportConfigId is associated with the timing of reporting to the base station device 100, for example, the condEventA4 event and a threshold value related to that event. In this way, the terminal device 200 can recognize the wireless quality event to be determined from the MeasId set in condExecutionCond.

[0019] <Example of Internal Configuration of Base Station Device> Figure 2 is a diagram showing an example of the schematic configuration of the base station device 100. The base station device 100 includes, for example, a processor 101, a storage device 102, a ROM (Read Only Memory) 103, a RAM (Random Access Memory) 104, and a communication device 105. The processor 101 is a computer including one or more processing circuits, such as a general-purpose CPU (Central Processing Unit) or an ASIC (Application Specific Integrated Circuit), and performs overall processing of the device and each of the above-mentioned processes by reading and executing programs stored in the storage device 102 or ROM 103. The ROM 103 is a read-only memory that stores information such as programs related to the processes executed by the base station device 100 and various parameters. The RAM 104 functions as a workspace when the processor 101 executes programs and is also a random access memory that stores temporary information. The storage device 102 may be, for example, a removable external storage device. The communication device 105 may be, for example, a circuit for wireless communication of 5G or its successor standards. While FIG. 2 shows one communication device 105, the base station device 100 may include multiple communication devices. For example, the base station device 100 may have a wireless communication device for 5G and a successor standard, and a common antenna for these devices. The base station device 100 may also have separate antennas suitable for each standard. The base station device 100 may also include a wired communication device used when communicating with other base station devices or nodes in the core network. The base station device 100 may have a separate communication device 105 for each of multiple available frequency bands, or may have a common communication device 105 for at least some of these frequency bands.

[0020] The processor 101 reads a handover-related program stored in the storage device 102 or the ROM 103 and expands it into an internal memory (not shown), thereby generating an RRC setting unit 1011, an RA instruction unit 1012, a handover instruction unit 1013, an inter-base station communication unit 1014, a TA maintenance and management unit 1015, and a PUCCH / PDCCH / RA communication unit 1016.

[0021] The processing units generated within the processor 101 in FIG. 2 mainly correspond to the functions of the handover source base station device 100_1 to which the terminal device 200 is connected. The handover destination (candidate) base station device 100_2 is similar to the base station device 100_1 except that it does not transmit some signals in the RA procedure and may transmit some signals to the base station device 100_2, and is therefore not shown here. Furthermore, FIG. 2 only illustrates functions particularly related to this embodiment, and does not illustrate various other functions that the base station device 100 may have. For example, the base station device 100 naturally has other functions that base station devices compliant with 5G and its successor standards generally have. Furthermore, the functional blocks in FIG. 2 are illustrated schematically, and the respective functional blocks may be realized as an integrated unit or may be further subdivided. Furthermore, the functions in FIG. 2 may be realized, for example, by the processor 101 executing a program stored in the ROM 103 or the storage device 102, or by a processor within the communication device 105 executing predetermined software. The details of the processes executed by each functional unit will not be described here, but only the general functions thereof will be outlined.

[0022] The RRC setting unit 1011 of the handover source base station device 100_1, for example, configures the RRC layer for the connected terminal device 200. The RRC setting unit 1011 transmits a handover request to other candidate base station devices 100 (such as the handover destination base station device 100_2 or the handover destination base station device 100_3) that provide neighboring cells, acquires communication setting information (communication parameters) therefrom, and notifies the terminal device 200 of the communication setting information and the above-mentioned LTM implementation conditions (communication parameters of each candidate cell and LTM implementation conditions) via an RRC layer message. In this case, the above-mentioned communication setting information (communication parameters) also includes information required when the terminal device 200 executes a random access procedure (such as a sequence for generating an RA preamble and information on radio resources used to transmit the RA preamble). In this embodiment, the RRC layer message is an RRC Configuration message. Then, the RRC setup section 1011 receives the RRC Configuration Complete message from the terminal device 200, and thereby performs connection processing setup between the terminal device 200 and another base station device 100 (handover destination base station device 100_2).

[0023] The RA instruction unit 1012 instructs the terminal device 200, for which RRC layer configuration has been completed, to transmit an RA preamble to another base station device 100 (handover destination base station device 100_2) that is a candidate for the handover destination. Here, the instruction to transmit the RA preamble includes a sequence for generating an RA preamble that is unique to the terminal device 200 and a group of sequences that can also be used by other terminal devices 200 that are attempting to connect to other base station devices 100, and also includes information on radio resources used to transmit the RA preamble. The RA instruction unit 1012 can instruct the currently connected terminal device 200 to transmit an RA preamble to the handover destination base station device 100_2 (candidate 1) by, for example, a physical downlink control channel (PDCCH) order. In addition, the RA instruction unit 1012 can, if necessary, instruct the terminal device 200 to retransmit the RA preamble, or, if the RA preamble is configured to be transmitted repeatedly, can transmit an instruction to the terminal device 200 to stop transmitting the RA preamble.

[0024] The handover instruction unit 1013 of the handover source base station device 100_1 instructs the terminal device 200 to execute handover. The handover instruction unit 1013 transmits, for example, a Cell switch command including information such as a cell identifier capable of identifying the handover destination base station device 100_2 to the terminal device 200. Note that when a TA (Timing Advance) value to be used for uplink communication has been determined by the terminal device transmitting an RA preamble or the like, the TA value can be included in the command transmitted to the terminal device 200. This enables the terminal device 200 to transmit an uplink user data signal using the TA value immediately after handover.

[0025] The inter-base station communication unit 1014 of the base station device 100_1 communicates with each handover destination candidate base station device (such as the handover destination base station device 100_2 or 100_3) (for example, communication via a dedicated wired line). The inter-base station communication unit 1014 can, for example, notify the handover destination base station device 100_2, which is the destination of the RA preamble, to cause the connected terminal device 200 to transmit the RA preamble. The inter-base station communication unit 1014 is also used, for example, to receive information for the RA procedure in the handover destination candidate base station device (the handover destination base station device 100_2). The inter-base station communication unit 1014 can also receive, for example, a message (message 2 or message 4) to be transmitted from the handover destination base station device 100_2 to the terminal device 200 in the RA procedure. Furthermore, the handover source base station 100_1 can acquire the TA value calculated in the handover destination base station device 100_2 via the inter-base station communication unit 1014. Note that the signal received in the inter-base station communication unit 1014 is transferred to the terminal device 200 via, for example, the RA instruction unit 1012 or the like.

[0026] The TA maintenance management unit 1015 of the handover source base station 100_1 manages the TA value to be used when the terminal device 200 transmits an uplink signal to the handover destination base station device 100_2. The TA maintenance management unit 1015 can set, for example, an expiration date and an update period for the TA value, and can instruct the terminal device 200 to retransmit an RA preamble so that the TA value is updated when the expiration date expires or the update period arrives. The TA maintenance management unit 1015 can also notify the terminal device 200 of the SRS setting in the handover destination base station device 100_2 and have the terminal device 200 transmit an SRS using the SRS setting, thereby updating the TA value. The terminal device 200 can also acquire the TA value via the TA maintenance management unit 1015 of the handover destination base station device 100_2. In this case, the base station device 100_2 can transmit an instruction to transmit an RA preamble and an instruction to transmit an SRS to the terminal device 200.

[0027] The PUCCH / PDCCH / RA communication unit 1016 operates, for example, when the base station device 100 becomes the handover destination base station device 100_2. As described above, when the handover destination base station device 100_2 notifies the handover source base station device 100_1 of communication setting information (communication parameters), it can start receiving a PUCCH, transmitting a PDCCH, and receiving an RA preamble between the handover destination base station device 100_2 and the terminal device 200. For example, as described above, when performing (continuing) a conditional handover, the PUCCH / PDCCH / RA communication unit 1016 can receive an RA preamble from the terminal device 200 and transmit an RA response to the terminal device 200 in order to perform an RA (random access) procedure.

[0028] <Example of Internal Configuration of Terminal Device> Fig. 3 is a diagram showing an example of the schematic configuration of the terminal device 200. The terminal device 200 includes, for example, a processor 201, a storage device 202, a ROM (Read Only Memory) 203, a RAM (Random Access Memory) 204, a communication device 205, an input device 206, and an output device 207. The processor 201 is a computer including one or more processing circuits such as a general-purpose CPU (Central Processing Unit) or an ASIC (Application Specific Integrated Circuit), and performs the overall processing of the device and each of the above-mentioned processes by reading and executing programs stored in the storage device 202 or ROM 203. The ROM 203 is a read-only memory that stores information such as programs related to the processing executed by the terminal device 200 and various parameters. The RAM 204 functions as a workspace when the processor 201 executes a program and is a random access memory that stores temporary information. The storage device 202 is, for example, a removable external storage device. The communication device 205 is, for example, a circuit for wireless communication of 5G or its successor standard. While FIG. 3 illustrates one communication device 205, the terminal device 200 may include multiple communication devices. For example, the terminal device 200 may have a wireless communication device for 5G and a successor standard, and a common antenna for these devices. The terminal device 200 may also include a communication device compliant with a wireless communication standard other than the cellular communication standard, such as a wireless local area network (LAN) or Bluetooth (registered trademark). Furthermore, the terminal device 200 may have separate antennas suitable for each standard. The terminal device 200 may have separate communication devices 205 for each of multiple available frequency bands, or a common communication device 205 for at least some of these frequency bands.

[0029] The input device 206 can be configured with, for example, a touch panel or a numeric keypad, and is used when a user performs a predetermined operation or inputs desired information. The output device 207 can be configured with, for example, a display device, and in this embodiment, displays information indicating the communication status, information indicating the success or failure of handover, etc.

[0030] The processor 201 reads a handover-related program stored in the storage device 202 or the ROM 23 and deploys it in an internal memory (not shown), thereby generating an RRC setting unit 2011, an RA (random access) execution unit 2012, an LTM implementation condition determination unit 2014, a handover processing unit 2013, and a timer control unit 2015. Note that FIG. 3 only shows functions particularly related to this embodiment, and various other functions that the terminal device 200 may have are not shown. For example, the terminal device 200 naturally has other functions that terminal devices compliant with 5G or its successor standards generally have. Furthermore, the functional blocks in FIG. 3 are shown schematically, and the respective functional blocks may be realized as an integrated unit or may be further subdivided.

[0031] The RRC setup unit 2011 performs connection setup processing with the handover destination base station device (candidate 1) 100_2 by receiving an RRC layer message from the connected handover source base station device 100_1. Note that the RRC setup unit 2011 holds the setting of the connected handover source base station device 100_1 and the settings of multiple handover destination candidate base station devices (handover destination base station device 100_k: k=2, 3, 4, ..., n) in parallel.

[0032] The RA execution unit 2012 receives an instruction to transmit an RA preamble from the handover source base station device 100_1. The instruction to transmit the RA preamble is issued by a PDCCH order. Note that the RA execution unit 2012 can also receive, for example, an instruction to retransmit the RA preamble or an instruction to stop transmitting the RA preamble. Furthermore, in response to receiving the instruction to transmit the RA preamble, the RA execution unit 2012 transmits the RA preamble to the handover destination (candidate 1) base station device 100_2. Here, information on a sequence for generating the RA preamble and radio resources for receiving the RA preamble in the handover destination base station device (candidate 1) 100_2 is acquired from the currently connected handover source base station device 100_1. Note that the information on the sequence and radio resources may be acquired from system information transmitted by the handover destination base station device 100_2.

[0033] In response to receiving a Cell Switch command from the currently connected handover source base station device 100_1, the handover processing unit 2013 changes the connection destination to the handover destination base station device (candidate 1) 100_2. When the received Cell Switch command notifies the terminal device 200 of allocation of uplink radio resources in the handover destination base station device 100_2, the terminal device 200 can immediately transmit an uplink signal using the radio resources.

[0034] Meanwhile, the handover processing unit 2013 instructs the LTM implementation condition determination unit 2014 to determine whether the state matches the above-mentioned LTM implementation condition. That is, the LTM implementation condition determination unit 2014 determines whether the LTM implementation condition associated with the plurality of candidate cells notified from the handover source base station device 100_1 is satisfied. If the condition is met, the handover destination to be executed is uniquely determined. For example, the LTM implementation condition is determined such that a threshold included in the communication parameters is set low for a candidate cell in a good (uncongested) communication situation, and a threshold is set high for a candidate cell in a bad (congested) communication situation.

[0035] When the LTM implementation condition determination unit 2014 determines (confirms) that the LTM implementation conditions are met, the handover processing unit 2013 performs handover processing including RA execution processing for a candidate cell (handover destination base station device 100_k) that satisfies the conditions (details will be described later). For example, when it is determined that the LTM implementation conditions associated with the handover destination base station device 100_2 of candidate 1 are met, the handover processing unit 2013 can autonomously execute handover processing in the same way as when the Cell Switch Command is received, even if the Cell Switch Command cannot be received. This eliminates the need to execute reconnection processing starting from cell search, making it possible to quickly switch cells.

[0036] The timer control unit 2015 controls the start and stop of various timers (such as T304) described below. For example, the timer (T304) is started when the conditions for performing the LTM are confirmed to be met and the LTM execution is decided, and is stopped when the UE Contention Resolution Identity MAC CE or the C-RNTI addressed DCCH is received from the destination base station device 100.

[0037] <Operation when RLF is detected> As described above, the terminal device 200 starts the cell switching process when it determines (confirms) that the conditions for performing LTM specified by the handover source base station device 100_1 are met. On the other hand, when the terminal device 200 detects an RLF (radio link failure), it may start the cell switching process even if the conditions are not met.

[0038] When providing the terminal device 200 with information (communication parameters) regarding the conditions for performing LTM and the cell switching process associated with the conditions for each candidate cell, the handover source base station device 100_1 also provides information indicating attemptCondReconfig=True / False. When detecting a radio link failure (RLF), the terminal device 200 searches for and selects a cell to be used for communication. When attemptCondReconfig, which is notified together with the information regarding the cell switching process, is True, the terminal device 200 determines whether the cell selected after the search is a handover destination candidate base station device (handover destination base station device 100_k) designated in the cell switching process. When determining that the selected cell is not the handover destination base station device designated in the cell switching process, the terminal device 200 performs reconnection processing as before. If the base station device is the handover destination designated in the cell switching process (one of the notified candidate cells), the cell switching process is initiated even if the conditions are not met, thereby shortening the interruption time.

[0039] <Example of Handover Processing Sequence (In the Case of Contention-Free RA)> Fig. 4 is a diagram for explaining an example of a sequence (in the case of contention-free RA) of handover processing according to this embodiment, which is executed when a Cell Switch Command is not received and the conditions for LTM execution are met. The sequence shown in Fig. 4 is an example, and modifications such as those described above can be made in each procedure. For example, Fig. 4 shows an example in which a unique sequence is notified to the terminal device 200 as a sequence for generating an RA preamble to be transmitted to the handover destination base station device 100_2, but this may be replaced with the above-mentioned procedure in which a group of sequences usable by multiple terminal devices 200 is notified. Note that in the example of Fig. 4, it is assumed that a connection is established between the handover source base station device 100_1 and the terminal device 200. In each step, the operation is performed by each processing unit (such as the RRC setting unit 1011), but since each processing unit is a function realized by the processor 101 or 201 of the base station device 100 or the terminal device 200, the operation may be performed by the processor 101 or 201. In the following description, the processing units of the handover source base station device 100_1 are referred to as an RRC setting unit 1011_1, an RA instruction unit 1012_1, a handover instruction unit 1013_1, an inter-base station communication unit 1014_1, a TA maintenance management unit 1015_1, and a PUCCH / PDCCH / RA communication unit 1016_1. Further, the respective processing units of the handover destination base station device 100_2 are denoted as an RRC setting unit 1011_2, an RA instruction unit 1012_2, a handover instruction unit 1013_2, an inter-base station communication unit 1014_2, a TA maintenance management unit 1015_2, and a PUCCH / PDCCH / RA communication unit 1016_2. Furthermore, in the following respective steps, various communications are actually performed by the respective processing units of the base station device and the terminal device controlling the communication device 105 and the communication device 205.

[0040] (i) Step S401: The inter-base station communication unit 1014_1 of the handover source base station device 100_1 transmits a HANDOVER REQUEST message to the inter-base station communication unit 1014_2 of the handover destination base station device 100_2 forming an adjacent cell that is a candidate for the handover destination of the terminal device 200, or to another base station device (another candidate cell: the inter-base station communication unit 1014_k (k=3, 4, . . ) of the handover destination base station device 100_k).

[0041] (ii) Step S402: Upon receiving the HANDOVER REQUEST message, the inter-base station communication unit 1014_2 of the handover destination (candidate 1) base station device 100_2 returns a HANDOVER REQUEST ACK message including information on communication parameters to be used when the terminal device 200 connects to the handover source base station device 100_1. The communication parameters include CG (Configured Grant) setting information including information on beam settings, parameters for executing an RA (Random Access) procedure, radio resources (frequency and time resources) to be used, etc. in the handover destination base station device 100_2. Other candidate cell base station devices similarly provide communication parameters to the handover source base station device 100_1.

[0042] (iii) Step S403 The RRC configuration unit 1011_1 of the handover source base station device 100_1 notifies the terminal device 200 of an LTM Configuration message including the above-mentioned communication parameters and the like acquired from the handover destination base station device 100_2. At this time, the RRC configuration unit 1011_1 provides the terminal device 200 with a condition (event: LTM implementation condition) that enables the implementation of LTM before the terminal device 200 receives a Cell Switch Command handover. This process eliminates the need for the terminal device 200 to perform RRC layer configuration processing after receiving a handover to the handover destination base station device 100_2 from the RA command unit 1012_1. Note that while FIG. 4 illustrates a case where there is only one handover destination base station device, when there are multiple handover destination (candidate) base station devices, similar configuration processing is performed for each of them.

[0043] The RRC setting unit 2011 of the terminal device 200 receives, via the LTM Configuration message, for example, a measurement instruction for a synchronization signal (SS) / physical broadcast channel (PBCH) block (SSB: Synchronization Signal Block) transmitted from the handover destination base station device 100_2, an SSB index, CG (Configured Grant) setting information related to uplink resources, and information associating uplink beam information. If the SSB index is 1, the terminal device 200 uses the first CG setting information and beam information, and if the SSB index is 2, the terminal device 200 uses the second CG setting information and beam information.

[0044] For example, when the handover processing unit 2013 of the terminal device 200 measures a radio signal (e.g., SSB) transmitted from the handover destination base station device 100_2, it notifies the handover source base station device 100_1 of the measurement result of the SSB. Here, the inter-base station communication unit 1014_1 of the handover source base station device 100_1 may notify the inter-base station communication unit 1014_2 of the handover destination base station device 100_2 of the measurement result. In this case, the handover destination base station device 100_2 can specify a rough beam to be used for communication with the terminal device 200. Note that the handover instruction unit 1013 of the handover destination base station device 100_2 may transmit CSI-RS and cause the terminal device 200 to make measurements in order to form a finer beam. In this case, the inter-base station communication unit 1014_1 of the handover destination base station device 100_2 transmits CSI-RS configuration information to the handover source base station device 100_1. The handover instruction unit 1013_2 of the handover source base station device 100_1 can transfer the configuration information to the terminal device 200. The terminal device 200 may measure the CSI-RS based on the configuration information and report the measurement result to the connected handover source base station device 100_1. Then, the inter-base station communication unit 1014_1 of the handover source base station device 100_1 transfers the measurement result to the handover destination base station device 100_2. This allows the handover instruction unit 1013_2 of the handover destination base station device 100_2 to transmit a downlink signal using a fine beam immediately after the terminal device 200 is handed over to the handover destination base station device 100_2.

[0045] When a Radio Link Failure (RLF) occurs, the terminal device 200 measures the SSB of the candidate cell (handover destination base station device 100_2) and determines whether or not it is in a state where it can be connected. If it is determined that it is possible to be connected, the terminal device 200 can identify the radio resources and the uplink beam to be used for uplink transmission from the SSB index using the information received in the LTM Configuration, and can start communication.

[0046] (iv) Step S404: The handover instruction unit 1013_1 of the handover source base station device 100_1 transmits a Cell Switch Command including the TA value of the handover destination base station device 100_2 to the terminal device 200. The Cell Switch Command can be transmitted by using, for example, a medium access control element (MAC CE) or downlink control information (DCI).

[0047] When the handover processing unit 2013 of the terminal device 200 receives the Cell Switch Command, PUSCH resources are allocated by the handover destination base station device 100_2. Then, the terminal device 200 becomes able to transmit user data to the handover destination base station device 100_2 using the allocated PUSCH resources. Note that the terminal device 200 has already executed the RA procedure as described above. Therefore, when the terminal device 200 is able to receive the Cell Switch Command, it does not need to execute the RA procedure again, and can immediately transmit user data to the handover destination base station device 100_2 using the designated radio resources.

[0048] Furthermore, when the Cell Switch Command is received, the handover processing unit 2013 of the terminal device 200 may transmit an acknowledgement (ACK) or the like to the handover source base station device 100_1. Furthermore, when the Cell Switch Command is received, the handover processing unit 2013 may switch the connection destination to the handover destination base station device 100_2 and transmit an ACK, an RRCReconfigurationComplete, or the like to the handover destination base station device 100_2 by using the allocated uplink radio resources. Then, the inter-base station communication unit 1014 of the handover destination base station device 100_2 can transfer the acknowledgement to the handover source base station device 100_1. As a result, the handover source base station device 100_1 can recognize that the terminal device 200 has switched its connection destination to the handover destination base station device 100_2, and can discard the context information of the terminal device 200, for example.

[0049] (v) Step S405: The LTM implementation condition determination unit 2014 of the terminal device 200 determines (confirms) whether the above-mentioned LTM implementation conditions are met (satisfied). Specifically, the LTM implementation condition determination unit 3014 determines whether the LTM implementation conditions corresponding to each candidate cell notified from the handover source base station device 100_1 are satisfied.

[0050] (vi) Step S406: When the LTM implementation condition determination unit 2014 of the terminal device 200 confirms that the conditions for LTM implementation are met, the handover processing unit 2013 decides to execute cell switching processing to the handover destination base station device 100_k of the candidate cell corresponding to the condition match. Note that, when there are multiple candidate cells that meet the conditions for LTM implementation, for example, the candidate cell with the shortest distance between the terminal device 200 and each handover destination base station device 100_k may be selected as the switching destination, or the candidate cell with the lowest congestion level may be selected as the switching destination.

[0051] (vii) Step S407 If it is confirmed that the conditions for implementing LTM are met, the timer control unit 2015 of the terminal device 200 starts the timer (T304).

[0052] (viii) Step S408 The RA execution unit 2012 of the terminal device 200 transmits an RA preamble to the handover destination base station device (for example, the handover destination base station device 100_2) designated in the cell switching process associated with the condition.

[0053] (ix) Step S409: Upon receiving the RA preamble transmitted from the terminal device 200 via the PUCCH, the PUCCH / PDCCH communication unit 1016_2 of the handover destination base station device 100_2 transmits an RA response (UL grant) via the PDCCH to the terminal device 200, and notifies the terminal device 200 of the TA value of the handover destination base station device 100_2 and information on the radio resources to be used for uplink transmission.

[0054] (x) Step S410 The handover processing unit 2013 of the terminal device 200 transmits an RRCReconfigurationComplete to the handover destination base station device 100_2 at the instructed transmission timing of a PUSCH (Physical Uplink Shared Channel).

[0055] (xi) Step S411: If the Random Access procedure is successful, the timer control unit 2015 of the terminal device 200 stops the timer (T304) started in step S407. However, the timer stopping operation when the Random Access procedure is successful differs depending on whether the procedure is contention-free or contention-based.

[0056] In the case of CFRA (contention-free), the MAC that has received the Random Access Response notifies the RRC of successful completion of the Random Access procedure. When the RRC processing unit in the terminal device receives successful completion of the Random Access procedure from the MAC processing unit in the terminal device, the timer control unit 2015 determines that the cell switching process is successful and stops T304.

[0057] Note that after the cell switch, the terminal device 200 may be configured to retain information condLTMReconfig related to the cell switching process. By the terminal device 200 retaining information related to the cell switching process even after the cell switch, it is possible to eliminate the need to newly receive condLTMReconfig from the base station device after the cell switch. Also, information regarding whether to retain information condLTMReconfig related to the cell switching process may be notified to the terminal device 200 from the handover source base station device 100_1 before the cell switch. As an example, it is possible to notify the terminal device 200 to retain only information related to cell switching processes that have not been executed and to delete information related to cell switching processes that have been executed.

[0058] In this way, it is possible to shorten the period (instantaneous interruption time) during which user data communication is unavailable when the terminal device 200 performs handover from the handover source base station device 100_1 to the handover destination base station device 100_2. This makes it possible to provide, for example, communication services that require low latency and high reliability. Furthermore, shortening the period (instantaneous interruption time) during which user data communication is unavailable makes it possible to suppress a decrease in efficiency of wireless communication.

[0059] <Example of Handover Processing Sequence (for Contention-Based RA)> FIG. 5 is a diagram for explaining a sequence of handover processing (for contention-based RA) according to this embodiment that is executed when a Cell Switch Command is not received and the conditions for performing LTM are met.

[0060] (i) Steps S501 to S510 These are the same processes as steps S401 to S410 in FIG.

[0061] (ii) Step S511 In the case of CBRA (contention-based), the handover processing unit 2013 of the terminal device 200 transmits its own ID (C-RNTI or Temporary C-RNTI) to the handover destination base station device 100_2. Thereafter, when the handover processing unit 2013 receives DCI transmitted on the PDCCH addressed to its own ID (C-RNTI or Temporary C-RNTI) from the handover destination base station device 100_2, it determines that Contention Resolution has been successful. When it is determined that Contention Resolution is successful, the MAC processing unit in the terminal device 200 notifies the RRC processing unit in the terminal device 200 of successful completion of the Random Access procedure. When the RRC processing unit in the terminal device 200 receives successful completion of the Random Access procedure from the MAC processing unit in the terminal device, it is determined that the cell switching process is successful, and the timer control unit 2015 stops the timer (T304).

[0062] <Summary of the embodiment> (i) The terminal device 200 acquires communication parameters and information on conditions for performing LTM associated with handover destination base station devices (handover destination candidate base station devices) 100_k (k = 2, 3, 4, ..., n) from the handover source base station device 100_1. Then, the handover source base station device 100_1 transmits a Cell Switch Command instructing the terminal device 200 to change the connection destination cell. Before receiving the Cell Switch Command, the terminal device 200 determines whether the conditions for performing LTM associated with the handover destination base station device 100_2 are satisfied. If the conditions for performing LTM are satisfied, the terminal device 200 transmits an RA preamble to the handover destination base station device 100_2 associated with the conditions for performing LTM. In this way, the handover process is resumed without waiting for the detection of an RLF, so that the handover process can be speeded up and the interruption time can be shortened.

[0063] Information on conditions for performing LTM, which is provided from the handover source base station device 100_1 to the terminal device 200 together with communication parameters, includes CondEvent A3, CondEvent A4, CondEvent A5, CondEvent D1, and CondEvent T1, which are defined in the 3GPP standard. This makes it possible to realize LTM according to this embodiment by utilizing the current 3GPP standard. (ii) Furthermore, when the terminal device 200 detects a radio link failure (RLF), even if it cannot confirm that the conditions for performing LTM are satisfied, if it can be confirmed that the cell to which LTM is to be performed has a minimum radio quality usable for communication (for example, if the cell to which LTM is to be performed satisfies cell selection criterion S), the terminal device 200 determines to perform LTM and executes a process of transmitting an RA preamble to the corresponding handover destination base station device 100_k. On the other hand, when the terminal device 200 detects a radio link failure (RLF) and cannot confirm that the cell to which LTM is to be performed has a minimum radio quality usable for communication, it executes a reconnection process starting with a cell search. In this way, even when an RLF occurs, it is possible to avoid the execution of the reconnection process by using the LTM implementation setting, thereby realizing an expedited handover process and a shortened interruption time. (iii) The functions of this embodiment and each example can also be realized by software program code. In this case, a storage medium on which the program code is recorded is provided to a system or device, and the computer (or CPU or MPU) of that system or device reads the program code stored in the storage medium. In this case, the program code itself read from the storage medium realizes the functions of the above-mentioned embodiments, and the program code itself and the storage medium on which it is stored constitute the present disclosure. Examples of storage media for providing such program code include flexible disks, CD-ROMs, DVD-ROMs, hard disks, optical disks, magneto-optical disks, CD-Rs, magnetic tape, non-volatile memory cards, and ROMs.

[0064] Furthermore, an operating system (OS) running on a computer may perform some or all of the actual processing based on instructions in the program code, and the functions of the above-described embodiments may be realized by this processing.Furthermore, after the program code is read from a storage medium and written to memory on the computer, a CPU of the computer may perform some or all of the actual processing based on instructions in the program code, and the functions of the above-described embodiments may be realized by this processing.

[0065] Furthermore, the program code of the software that realizes the functions of the embodiments and each example may be distributed via a network and stored in a storage means such as a hard disk or memory of the system or device, or in a storage medium such as a CD-RW or CD-R, so that when used, the computer (or CPU or MPU) of the system or device reads and executes the program code stored in the storage means or storage medium.

[0066] The processes and techniques described herein are not inherently related to any specific device and can be implemented by a combination of components. Various types of general-purpose devices can also be added. A dedicated device may be constructed to perform the functions of this embodiment and each example. Various functions can also be formed by appropriately combining multiple components disclosed in this embodiment and each example. For example, some components may be omitted from all the components shown in the embodiment and each example, or components from different examples may be appropriately combined.

[0067] Although specific examples are described in this disclosure, they are in all respects for the purpose of explanation (understanding the technology of the present disclosure) and not for the purpose of limitation. Those skilled in the art will recognize that there are many combinations of hardware, software, and firmware suitable for implementing the technology of the present disclosure. For example, the software described can be implemented in a wide variety of programming or scripting languages, such as assembler, C / C++, Perl, Shell, PHP, Java (registered trademark), etc.

[0068] Furthermore, in the above-described embodiment, the control lines and information lines are those that are considered necessary for the explanation, and not all control lines and information lines in the product are necessarily shown. All components may be interconnected.

[0069] In addition, other implementations of the present disclosure will be apparent to those skilled in the art from consideration of the present embodiments and examples. The specification and examples are exemplary only, with the scope and spirit of the present disclosure being indicated by the following claims.

[0070] The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the gist of the invention.

[0071] This application claims priority based on Japanese Patent Application No. 2023-170284, filed on September 29, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. A terminal device that communicates with multiple base station devices while switching connection destinations, comprising: a storage device that stores a program related to handover; and a processor that reads the program from the storage device and executes handover processing, wherein the processor executes the following processes: a process of acquiring communication parameters and information on conditions for LTM implementation associated with at least one handover destination candidate base station device from a connected handover source base station device; a process of determining whether the conditions for LTM implementation associated with the handover destination candidate base station device are satisfied before receiving a Cell Switch Command from the handover source base station device instructing a change of connection destination cell; and a process of transmitting an RA preamble to the handover destination candidate base station device associated with the conditions for LTM implementation when the conditions for LTM implementation are satisfied.

2. The terminal device of claim 1, wherein the processor acquires information on specific events including CondEvent A3, CondEvent A4, CondEvent A5, CondEvent D1, and CondEvent T1, which are specified in the 3GPP standard, as conditions for implementing the LTM, for each of the at least one handover target candidate base station device.

3. The terminal device of claim 1, wherein the processor executes a process of determining that LTM can be implemented when it detects a radio link failure, when it cannot confirm that the conditions for implementing LTM are satisfied, and when it confirms a specified radio quality that can be used for communication for the cell switching destination by implementing LTM.

4. The terminal device according to claim 1, wherein the processor, when detecting a radio link failure, executes a reconnection process starting with a cell search when it is unable to confirm a predetermined radio quality that can be used for communication at the cell destination by implementing LTM.

5. A terminal device as described in claim 1, wherein the processor receives information for identifying a measurement target of wireless quality and a first identifier associated with the information, information for identifying a timing for reporting to a base station device and a second identifier associated with the information, information for associating a measurement target of wireless quality, a report target, and a reporting timing using the first identifier and the second identifier, and a third identifier associated with the information, and specifies conditions for implementing LTM using the third identifier.

6. The terminal device according to claim 5, wherein the first identifier is a measObjectId, the second identifier is a reportConfigId, and the third identifier is a MeasId.

7. The terminal device of claim 1, wherein, when the cell switching process is successful after transmitting an RA preamble, the processor deletes some or all of the first communication parameters and the first LTM implementation conditions of the first handover destination base station device, and the second communication parameters and the second LTM implementation conditions of the second handover destination base station device, in response to instructions from the handover source base station device.

8. A wireless communication system comprising a plurality of base station devices and at least one terminal device, and executing a handover from a handover source base station device to a handover destination candidate base station device, wherein the handover source base station device to which the terminal device is connected executes a process of transmitting communication parameters and information on LTM implementation conditions associated with at least one handover destination candidate base station device to the terminal device, and a process of transmitting a Cell Switch Command to the terminal device instructing the terminal device to change the connection destination to the handover destination candidate base station device, and the terminal device executes a process of determining whether the LTM implementation conditions associated with the handover destination candidate base station device are satisfied before receiving the Cell Switch Command from the handover source base station device, and a process of transmitting an RA preamble to the handover destination candidate base station device associated with the LTM implementation conditions if the LTM implementation conditions are satisfied.

9. The wireless communication system described in claim 8, wherein the terminal device acquires information on specific events including CondEvent A3, CondEvent A4, CondEvent A5, CondEvent D1, and CondEvent T1 specified in the 3GPP standard as conditions for implementing the LTM for each of the at least one handover destination candidate base station device from the handover source base station device.

10. The wireless communication system according to claim 8, wherein the terminal device executes a process of determining that LTM is executable when it detects a wireless link failure, cannot confirm that the conditions for implementing LTM are satisfied, and confirms a predetermined wireless quality that can be used for communication at the cell switching destination by implementing LTM.

11. The wireless communication system of claim 8, wherein when the terminal device detects a wireless link failure and is unable to confirm a specified wireless quality that can be used for communication at the cell destination by implementing LTM, the terminal device executes a reconnection process starting with a cell search.

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