Terminal device for executing handover with improved efficiency, control method, and program

Abstract

A terminal device conforming to the cellular communication standard of the 3rd Generation Partnership Project (3GPP) retains a Timing Advance value (TA value) used in a cell being connected and identification information associated with the TA value before receiving a Cell Switch Command instructing switching of a cell to which the terminal device is connected, switches the cell on the basis of having received the Cell Switch Command, and then performs uplink communication on the basis of the TA value associated with the identification information corresponding to the post-switching cell.

Description

Terminal device, control method, and program for executing handover with improved efficiency

[0001] The present invention relates to handover techniques in cellular communication systems.

[0002] In a mobile communication system, as a terminal device moves, the cell (base station device) to which the terminal device is connected is switched (e.g., handover). Conventionally, after receiving a handover instruction from a base station device, the terminal device establishes a connection by establishing synchronization and setting up a connection in the cell to which the connection is to be switched. This connection switching process takes a certain amount of time, and the terminal device is unable to communicate user data during this process. In contrast, in the technique described in Non-Patent Document 1, the handover source base station device provides the terminal device with communication parameters in the radio resource control (RRC) layer for connection with another handover destination base station device before the actual handover is performed. Then, at the timing when the handover is to be performed, the handover source base station device transmits a Layer 1 or Layer 2 command to the terminal device instructing the cell to be switched. Upon receiving the command, the terminal device executes a random access procedure with the handover destination base station device, thereby completing the handover without subsequent overlay processing in the RRC layer. This technique can suppress degradation of communication efficiency related to handover.

[0003] 3GPP (registered trademark) Contribution, R2-2209255

[0004] The present invention provides a technique for further improving communication efficiency when a handover is performed.

[0005] A terminal device according to one aspect of the present invention is a terminal device that complies with the cellular communication standard of the Third Generation Partnership Project (3GPP), and includes: a holding means that holds a Timing Advance value (TA value) used in a cell to which the terminal device is currently connected and identification information associated with the TA value before receiving a Cell Switch Command that instructs the terminal device to switch the cell to which the terminal device is currently connected; and a communication means that, after switching cells based on the reception of the Cell Switch Command, performs uplink communication based on the TA value associated with the identification information corresponding to the switched cell.

[0006] According to the present invention, it is possible to improve communication efficiency when a handover is performed.

[0007] 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.

[0008] The accompanying drawings are incorporated in and constitute a part of the specification, illustrate embodiments of the present invention, and together with the description are used to explain the principles of the present invention. Figure 1 is a diagram showing an example of the configuration of a wireless communication system. Figure 2 is a diagram showing an example of the hardware configuration of an apparatus. Figure 3 is a diagram showing an example of the functional configuration of a base station apparatus. Figure 4 is a diagram showing an example of the functional configuration of a terminal apparatus. Figure 5 is a diagram showing an example of the flow of processing executed in a wireless communication system.

[0009] 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.

[0010] (System Configuration) Fig. 1 shows an example of the configuration of a wireless communication system according to this embodiment. This 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 base station devices 101 to 104 and a terminal device 111. Note that, for simplicity of explanation, Fig. 1 shows only four base station devices and one terminal device, but it goes without saying that a large number of base station devices and terminal devices may exist.

[0011] 1, terminal device 111 is about to move from the range of the cell formed by base station device 101 into the range of the cell formed by base station device 102. At this time, terminal device 111 performs handover to switch the connected base station device from base station device 101 to base station device 102.

[0012] In this embodiment, a handover procedure is performed to shorten the period during which the terminal device 111 is unable to communicate user data due to handover. That is, the handover source base station device 101 performs configuration in the radio resource control (RRC) layer (configuration using an RRC message) before actually handing over the terminal device 111. That is, in a conventional handover, the terminal device 111 receives a handover instruction from the handover source base station device 101, and the RRC layer configuration for the handover destination base station device 102, which is performed during the handover process, is notified to the terminal device 111 prior to the timing when the handover process is actually performed. In this configuration, communication parameters used for connection and communication with the handover destination candidate base station device 102 are notified to the terminal device 111. The communication parameters include, for example, beam configuration in the handover destination candidate base station device 102, parameters for executing a random access procedure, and radio resources (frequency and time resources) to be used. Furthermore, when multiple communication parameters used for communication with the base station device 102 are notified, an identifier (for example, an RRC-TransactionIdentifier, a CondReconfigId, or both) uniquely associated with each of the communication parameters may be included in an RRC message notified to the terminal device 111 or in configuration parameters in the RRC layer. In this case, a CondReconfigId is set for each candidate base station device or combination of base station devices as a handover destination. Furthermore, when multiple communication parameters are notified for each base station device or combination of base station devices, an RRC-TransactionIdentifier may be set. As an example, CondReconfigId=0 is associated with the base station device 102, which is a candidate base station for handover, and further, when multiple parameters of the base station device 102 are notified, an RRC-TransactionIdentifier (e.g., 0, 1, ...) can be set for each of the parameters.Furthermore, the handover source base station device 101 may transmit to the terminal device 111 an instruction to measure synchronization signals (SS) / physical broadcast channel (PBCH) blocks (SSBs) transmitted from the handover destination candidate base station device 102. Note that the SSBs may be transmitted using beams directed in different directions. In this case, a measurement instruction may be transmitted so that the terminal device 111 measures each of the SSBs.

[0013] In accordance with a measurement instruction from the currently connected base station device 101, the terminal device 111 measures the SSB transmitted from the base station device 102, which is a candidate handover destination. The terminal device 111 then notifies the currently connected base station device 101 of the measurement results. Here, the base station device 101 may notify the currently connected base station device 101 of the measurement results to the currently connected base station device 102, which is a candidate handover destination and was the subject of the measurement. This allows the base station device 102 to identify the general direction of the beam to be used in communication with the terminal device 111 when the terminal device 111 actually performs a handover. Here, the base station device 102 may also notify the terminal device 111 of the transmission settings of channel state information (CSI)-reference signal (RS) in order to further identify the detailed beam to be used in the downlink (the link from the base station device to the terminal device). Note that this notification is performed by the base station device 102 notifying the base station device 101 of the CSI-RS transmission settings, and the base station device 101 transferring the transmission settings to the currently connected terminal device 111. Upon receiving the transmission setting, the terminal device 111 measures the CSI-RS transmitted from the base station device 102 according to the information and notifies the connected base station device 101 of the measurement result. The base station device 101 then transfers the measurement result to the base station device 102. In this way, the base station device 102 can acquire the measurement result of the CSI-RS in the terminal device 111. As a result, the base station device 102 can determine the detailed beam to be used for downlink communication after the terminal device 111 has handed over, before the handover actually occurs. This makes it possible to omit the process of determining the beam direction in detail after the handover has occurred, and shortens the period during which the efficiency of user data communication for the terminal device 111 decreases. As an example, the above-mentioned CSI-RS transmission settings can be instructed to the terminal device 111 using a CSI-RS measurement setting consisting of a CSI-RS radio resource setting (CSI-RS-Resource) and a reporting setting (csi-ReportConfig) for the base station device 102 that are pre-set in the terminal device 111 as communication parameters before handover.The CSI-RS-Resource includes the time position and frequency position of the radio resource block where the CSI-RS is transmitted. For example, the terminal device 111 can be instructed to measure a specific CSI-RS radio resource by specifying the CSI-RS-Resource using the CSI-RS-Resource id in the medium access control element (MAC CE) or downlink control information (DCI). Also, in the csi-ReportConfig, aperiodic or periodic reporting timing of the CSI report is set. For example, the terminal device 111 can be instructed to report the CSI-RS measurement results at a specific timing by specifying the csi-ReportConfig using the csi-ReportConfig id in the MAC CE or DCI. Furthermore, when the base station device 101 performs the above-mentioned measurement configuration on the terminal device 111, the base station device 101 may transmit to the terminal device 111, for example, a cell identifier or the like, included in a MAC CE or DCI, as information that enables the terminal device 111 to identify which base station device's CSI-RS transmission configuration this is.

[0014] Furthermore, the terminal device 111 executes a random access (RA) procedure before it is determined to perform a handover to the base station device 102. That is, the terminal device 111 may perform processing for establishing synchronization of the uplink (the link from the terminal device to the base station device) before the actual handover is performed. For example, the base station device 101 to which the terminal device 111 is connected transmits an instruction to the terminal device 111 to transmit a random access preamble (RA preamble) to the base station device 102. Then, in accordance with this instruction, the terminal device 111 transmits the RA preamble using radio resources prepared for the RA procedure in the base station device 102. Note that information about these radio resources and the sequence used to generate the RA preamble may be notified to the terminal device 111 by the base station device 101 during, for example, the above-mentioned RRC layer configuration processing. Furthermore, this information may be notified to the terminal device 111 when the base station device 101 issues an instruction to transmit the RA preamble. Through this procedure, the RA preamble transmitted from the terminal device 111 is received by the base station device 102. Then, the base station device 102 can determine a timing advance value (TA value) to be used when the terminal device 111 transmits an uplink signal to the base station device 102.

[0015] Thereafter, the base station device 101 may notify the terminal device 111 to perform handover at a predetermined trigger, such as when the reception quality of a radio signal from the base station device 101 at the terminal device 111 is deteriorated by a predetermined level compared to the reception quality of a radio signal from the base station device 102. This notification may be performed using a command that may be called a Cell switch command. This command may be transmitted using a Medium Access Control Element (MAC CE) or Downlink Control Information (DCI). Note that this command includes identification information of the cell formed by the handover destination base station device 102. Furthermore, when multiple settings are configured in the RRC layer used for communication with the base station device 102, identifiers (e.g., RRC-TransactionIdentifier, CondReconfigId, or both) uniquely associated with each of the settings in the RRC layer may be included in this command. Furthermore, information regarding the beam determined in the base station device 102 by measuring SSB and CSI-RS in advance as described above and notified to the base station device 101 may be notified from the base station device 101 to the terminal device 111. In response to receiving a command from the base station device 101 specifying the base station device 102 as the handover destination, the terminal device 111 changes its connection destination from the base station device 101 to the base station device 102. In this case, since the RA procedure has already been executed as described above, there is no need to execute a new RA procedure. Furthermore, since CSI-RS measurement has been performed before the handover as described above, there is no need to newly determine the precise beam to be used in downlink communications after the handover. Therefore, the terminal device 111 can communicate user data with the base station device 102 immediately after the handover. Furthermore, if the handover is successful after receiving the command, the terminal device 111 can notify the base station device 102 of the success in an uplink control message such as an RRC message of RRC Reconfiguration Complete or a MAC CE.In this case, the terminal device 111 may include an identifier (for example, an RRC-TransactionIdentifier, a CondReconfigId, or both) associated with the communication parameters used for communication with the base station device 102 in the RRC message.

[0016] Furthermore, as described above, the base station device 102 can determine the TA value in advance. Therefore, the base station device 102 can notify the base station device 101 of the TA value in advance, and the base station device 101 can notify the terminal device 111 of the TA value using the above-mentioned command. As a result, the terminal device 111 can establish uplink synchronization with the base station device 102 at the time when the base station device 101 instructs the terminal device 111 to perform handover to the base station device 102, and can therefore transmit uplink user data to the base station device 102 without re-executing the RA procedure.

[0017] The terminal device 111 can connect to multiple cells in parallel. For example, the terminal device 111 can connect to a cell provided by the base station device 101 and a cell provided by the base station device 103 in parallel. Here, when the terminal device 111 connects to multiple cells provided by different base station devices, a separate TA value is set for each cell. For example, two TA values ​​are set in the terminal device 111: a first TA value for the cell provided by the base station device 101 and a second TA value for the cell provided by the base station device 103. Note that when the same TA value is to be used, for example, when multiple cells are provided by the same base station device or base station devices located in approximately the same location, the same TA value can be used for the multiple cells. In this way, a group of cells using the same TA value is called a timing advance group (TAG). A TA value is set for each TAG.

[0018] In this embodiment, a case will be considered in which the above-described Cell Switch Command is used to perform handover of the terminal device 111 currently connected to multiple cells. When the terminal device 111 is currently connected to multiple cells and will remain connected to multiple cells after handover, the Cell Switch Command may include identification information of the multiple cells after the handover. For example, when the terminal device 111 transitions from a state in which it is connected to a first cell formed by the base station device 101 and a second cell formed by the base station device 102 to a state in which it is connected to the second cell formed by the base station device 102 and a third cell formed by the base station device 103, a Cell Switch Command including, for example, identification information of the second cell and identification information of the third cell may be transmitted to the terminal device 111. In this case, identification information of the third cell to be changed as the connection destination may be notified to the terminal device 111, and identification information of the second cell may not be notified to the terminal device 111. However, in this case, identification information of the first cell from which the switchover is to be performed may be notified to the terminal device 111. Note that, although an example in which only one of the two cells is changed has been shown here, two or more cells may be changed at once. Furthermore, an identifier associated with a setting in the RRC layer may be included in this command to indicate which cell the handover is to be performed to. For example, a setting for using multiple cells in parallel may be included in one setting in the RRC layer. For example, if a setting for connecting to the second cell formed by the base station device 102 and the third cell formed by the base station device 103 has been configured in advance in the terminal device 111, an identifier corresponding to the setting may be notified to the terminal device 111. For example, it is assumed that a first setting in which the first cell and the third cell are used in parallel, a second setting in which the second cell and the third cell are used in parallel, and a third setting in which the first cell and a fourth cell formed by the base station device 104 are used in parallel are notified to the terminal device 111 connected to the first cell and the second cell. It is also assumed that a first identifier is assigned to the first setting, a second identifier is assigned to the second setting, and a third identifier is assigned to the third setting.In this case, for example, a Cell Switch Command specifying a second identifier is notified to the terminal device 111 from the base station device 101 that forms the first cell from which the connection destination is changed, thereby allowing the terminal device 111 to switch the connection destination cell to a combination of the second cell and the third cell. Note that the Cell Switch Command at this time may be notified to the terminal device 111 from the second base station device 102, with which the connection is maintained from before the connection destination change to after the change. Furthermore, when configuration information for using multiple cells in parallel is notified to the terminal device 111 in advance, the configuration information may include TAG settings for each of the multiple cells. For example, the configuration information may notify that the second cell belongs to the first TAG and the third cell belongs to the second TAG.

[0019] Assuming that multiple cells can be switched at once in this way, it is useful to have this command notify the terminal device 111 of the TA value to be applied to each of the multiple cells after switching. That is, as described above, since the TA value is set for each TAG, when multiple cells after handover belong to different TAGs, information indicating the TA values ​​for each of these TAGs is notified to the terminal device 111, so that the terminal device 111 does not need to be notified again of the TA value to be used after switching the destination cell. In this embodiment, in view of such circumstances, multiple TA values ​​can be specified in the Cell Switch Command.

[0020] In one example, the Cell Switch Command includes the TA value itself to be used in each TAG. Furthermore, when a TA value already held in the terminal device 111 is to be used, the Cell Switch Command may include information specifying the TA value held in the terminal device 111, rather than the TA value itself. For example, when the terminal device 111 switches its connection destination from a state in which it is currently connected to a first cell formed by base station device 101 and a second cell formed by base station device 102 to a third cell formed by base station device 103, it can reuse the TA value of the second cell, among the TA values ​​used before the connection destination was switched. Here, even when the same cell is continuously used, it is possible that the TAG configuration will change. Therefore, information indicating that the TA value used in the TAG to which the second cell belongs before the change of the destination cell will be reused in the TAG to which the second cell belongs after the change of the destination cell can be notified in the Cell Switch Command.

[0021] It is also possible that, for a third cell to which the terminal device 111 was not connected, the base station device 103 determines its TA value by executing the above-described advance RA procedure, and notifies the terminal device 111 of the determined TA value in advance via the base station device 101 or the base station device 102. In such a case, the terminal device 111 may be notified by a Cell Switch Command that the previously notified TA value should be used for the TAG to which the third cell belongs. In addition, when a TA value previously held in the terminal device 111 is used, the TA value is specified by identification information associated with the TA value. This identification information may be, for example, information (tag-id) specifying a specific TAG before switching of the destination cell, information specifying a specific cell in which the TA value is set, or the like. When the base station device 101 and the base station device 102 connected to the terminal device 111 notify the terminal device 111 of a TA value in advance, they can attach individual identification information to the TA value and notify the terminal device 111. This allows the base station device 101 and the base station device 102 to use the identification information to specify, in a Cell Switch Command, which TA value to use in a specific TAG after the connection destination has been changed.

[0022] In addition, in the above-described embodiment, a case where one TAG is associated with one cell has been described, but the description of this embodiment can also be applied to a case where multiple cells are associated with one TAG and each TAG is associated with one TA value, a case where different TAGs are associated with multiple TRPs (Transmission and Reception Points) belonging to the same cell, or a case where one cell is composed of multiple TRPs. When one cell is composed of multiple TRPs, it is also possible that a TA value is associated with each TRP. In this case, as identification information associated with the TA value for identifying the TA value to be used, in addition to information specifying the TAG (tag-id), information for specifying the TRP may be notified in advance, or may be notified by a Cell Switch Command.

[0023] Furthermore, when the TA value that has been preset and held in the terminal device 111 is to be updated, the base station device 101 and the base station device 102 may include in the Cell Switch Command a difference value Ta-Tb between the updated value Ta and the value Tb before the update. In this case, a Cell Switch Command including identification information indicating which of the preset TA values ​​is to be used and the difference value can be notified to the terminal device 111. By adding the difference value to the TA value corresponding to the identification information for a specific TAG, the terminal device 111 can specify the TA value to be used after changing the cell to which it is connected.

[0024] Note that the above-mentioned methods of notifying information indicating the TA value may be used in combination. That is, for some of the multiple TAGs set after the connection destination is changed, the TA value itself may be notified, and for the remaining part, identification information previously associated with a specific TA value (TA identification information, TA value identification information, cell identification information, etc.) may be notified. Also, a difference value for updating may be notified only for some of the TA values. That is, the above-mentioned information indicating the TA value may be arbitrarily combined and notified to the terminal device 111.

[0025] As described above, when the terminal device 111 receives a Cell Switch Command, it can identify a TA value corresponding to each TAG based on information indicating the TA value included in the command. The terminal device 111 then changes the connected cell in accordance with the command, and after changing the connected cell, adjusts the transmission timing of uplink signals in cells belonging to each TAG using the identified TA value. Note that, if the terminal device 111 has previously acquired a TA value for a specific cell as described above, it can associate and store the TA value with specific identification information (identification information for the cell or identification information prepared for the TA value). If the Cell Switch Command includes identification information as information indicating the TA value for a specific TAG, the terminal device 111 can identify the TA value corresponding to the identification information as the TA value for the specific TAG. Furthermore, if the Cell Switch Command includes the TA value itself as information indicating the TA value for a specific TAG, the terminal device 111 may decide to use the value specified in the command as the TA value for that specific TAG. When the terminal device 111 receives the TA value itself, it may start a timer (timeAlignmentTimer) related to the expiration date of that TA value. This timer measures the period during which the terminal device 111 considers that timing synchronization of uplink signal transmission is achieved. The terminal device 111 acquires a value (timeAlignmentTimerDedicated) specific to the terminal device 111 or a value (timeAlignmentTimerCommon) common to terminal devices in the cell as the expiration date of the timer, and manages the TA value based on this value. The value of the timer (timeAlignmentTimer) relating to the expiration date of the TA value may be notified together with the TAG as part of the settings in the RRC layer that are set in advance in the terminal device 111 before the notification of the Cell Switch Command, or the timer value may be set together with the TAG in the Cell Switch Command.In addition, when the terminal device 111 performs an RA procedure in advance before changing the destination cell and thereby notifies the terminal device 111 of the TA value of a candidate cell after the destination change, the base station device to which the terminal device 111 is connected may notify the terminal device 111 of the expiration dates of those timers along with the notification. In addition, when the terminal device 111 receives a new TA value, it restarts the timer. When the timer related to the expiration date managed for each TAG expires, the terminal device 111 retains the TA value associated with and stored in the TAG of the expired timer, and stops transmitting uplink signals using that TA value, such as PUCCH and SRS. Thereafter, when the terminal device 111 receives a TA value or a difference value from the retained TA value from the base station device, it updates the TA value. Furthermore, when the terminal device 111 is instructed to delete the TA value from the base station device, it deletes the TA value.

[0026] In the above embodiment, the Cell Switch Command has been described as notifying the TA value of the cell to which the connection is switched, but this command may also notify the TA value of a cell to which the terminal device 111 is not connected. For example, a Cell Switch Command instructing the terminal device 111 to change the cell to which the terminal device 111 is connected from a state in which the terminal device 111 is currently connected to the first cell formed by the base station device 101 and the second cell formed by the base station device 102 to a state in which the terminal device 111 is connected to the second cell and the third cell formed by the base station device 103 may include a TA value for the fourth cell formed by the base station device 104. In this case, it is necessary that the terminal device 111 has previously executed the RA procedure for the fourth cell, the TA value determined by the base station device 104 has been notified to at least one of the base station device 101 and the base station device 102, and information on a cell managed as a cell to which the terminal device 111 will not connect after handover and a TAG to which that cell belongs have been notified to the terminal device 111 in advance in the RRC layer before the Cell Switch Command is notified. In addition, at this time, instead of the advance configuration in the RRC layer, information for identifying a cell managed as a cell to which the terminal device 111 will not connect and a TAG to which that cell belongs may be notified to the terminal device 111. In this way, by notifying the terminal device 111 of the TA values ​​for cells that are not directly involved in the change of connection destination, when the cell to which the terminal device 111 is connected is further changed and the terminal device 111 is connected to a fourth cell, it becomes possible for the terminal device 111 to establish uplink timing synchronization with the fourth base station device without executing an RA procedure again. Although the above description has been made about the fourth cell, after the terminal device 111 changes its cell connection destination, similar processing can be performed for multiple cells, such as a fifth cell or a sixth cell to which it is not connected.

[0027] (Device Configuration) Next, the device configuration will be described. FIG. 2 shows an example of the hardware configuration of a base station device and a terminal device according to this embodiment. In one example, the base station device and the terminal device include a processor 201, a ROM 202, a RAM 203, a storage device 204, and a communication circuit 205. 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 ROM 202 or the storage device 204. The ROM 202 is a read-only memory that stores information such as programs and various parameters related to the processing performed by the base station device and the terminal device. The RAM 203 functions as a workspace when the processor 201 executes a program and is a random access memory that stores temporary information. The storage device 204 is, for example, a removable external storage device. The communication circuit 205 is, for example, a circuit for wireless communication of 5G or its successor standards. Although FIG. 2 illustrates one communication circuit 205, the base station apparatus and the terminal apparatus may have multiple communication circuits. For example, the base station apparatus and the terminal apparatus may have wireless communication circuits for 5G and its successor standard, and a common antenna for these circuits. The base station apparatus and the terminal apparatus may have separate antennas suitable for each standard. The base station apparatus may also have a wired communication circuit used when communicating with other base station apparatuses or core network nodes. The terminal apparatus may also have a communication circuit conforming to a wireless communication standard other than the cellular communication standard, such as a wireless local area network (LAN) or Bluetooth (registered trademark). The base station apparatus and the terminal apparatus may have separate communication circuits 205 for each of the multiple available frequency bands, or may have a common communication circuit 205 for at least some of these frequency bands.

[0028] FIG. 3 shows an example of the functional configuration of a base station device. The base station device includes, for example, an RRC setting unit 301, an RA instruction unit 302, a TA value acquisition unit 303, an inter-base station communication unit 304, and a cell switching instruction unit 305. Note that FIG. 3 mainly shows the functions of the base station device 101 or the base station device 102 to which the terminal device 111 is connected. The base station device 103 and the base station device 104, which form candidate cells for connection change destinations, are similar to conventional base station devices except that they may transmit information about the TA value to the base station device 101 or the base station device 102, and are therefore not shown here. Note that the base station devices 101 to 104 are distinguished only by whether or not they are the connection destination of the terminal device 111, and all of these base station devices can have the functions of FIG. 3. That is, for example, when the base station device 103 or the base station device 104 is connected to another terminal device (not shown), it can use the functions of FIG. 3 to perform the above-described processing, with the base station device 101, the base station device 102, etc., as candidates for connection change destinations. Note that FIG. 3 only illustrates functions particularly related to this embodiment, and does not illustrate various other functions that the base station device may have. For example, the base station device naturally has other functions that base station devices compliant with 5G or its successor standards generally have. Furthermore, the functional blocks in FIG. 3 are illustrated schematically, and each functional block may be realized by being integrated or further subdivided. Furthermore, each function in FIG. 3 may be realized, for example, by the processor 201 executing a program stored in the ROM 202 or the storage device 204, or by a processor within the communication circuit 205 executing predetermined software. The details of the processing performed by each functional unit will not be described here, and only their general functions will be outlined.

[0029] The RRC setting unit 301, for example, configures the RRC layer for a currently connected terminal device. For example, the RRC setting unit 301 transmits a handover request to another base station device that provides a neighboring cell that is a candidate for a connection change destination of the terminal device, acquires communication setting information from the other base station device, and notifies the terminal device of the communication setting information via an RRC layer message. Here, the RRC setting unit 301 may transmit to the terminal device, in an RRC layer message, setting information about a combination of cells that the terminal device can use in parallel, based on, for example, information on frequency bands that the terminal device can use in parallel. This setting information may include, for example, information on which TAG each of the multiple cells to be combined belongs to. Here, the candidate connection destinations when the terminal device changes its connection destination may also include the currently connected cell. That is, when a terminal device is connected to a first cell and a second cell, if a combination of a second cell and a third cell is configured as a combination of candidate cells to which the terminal device is connected, the candidate cells to which the terminal device is connected will include the currently connected second cell. Therefore, the RRC configuration unit 301 may also acquire configuration information about the second cell and notify the terminal device of this. Note that this is just one example, and with regard to the configuration information specific to the second cell, it may be possible to notify the terminal device that the current configuration information should be reused and only the configuration information required when using the second cell and the third cell in combination. Note that when there are multiple candidate combinations of destination cells, if configuration information is prepared for each of the multiple candidates, specific identification information may be assigned to each of the multiple pieces of configuration information. This identification information may be, for example, an RRC-TransactionIdentifier, a CondReconfigId, or both. The message of the RRC layer is an RRC Reconfiguration message. After that, the RRC setting unit 301 performs connection processing setting between the terminal device and another base station device by receiving an RRC Reconfiguration Complete message including an RRC-TransactionIdentifier, a CondReconfigId, or both from the terminal device.

[0030] The RA instruction unit 302 instructs a terminal device for which RRC layer configuration has been completed to transmit an RA preamble for each of the candidate cells to which the terminal device is to connect. Here, the RA preamble transmission instruction includes information for identifying the base station device or cell to which the RA preamble is to be transmitted, a sequence for generating an RA preamble specific to the terminal device, and a group of sequences that can also be used by other terminal devices attempting to connect to the candidate cell to which the terminal device is to connect, and may also include information on the radio resources used to transmit the RA preamble. For example, the RA instruction unit 302 can instruct a currently connected terminal device to transmit an RA preamble to the candidate base station device to which the terminal device is to connect, using a physical downlink control channel (PDCCH) order. Furthermore, the RA instruction unit 302 can instruct the terminal device to retransmit the RA preamble as necessary, or, if configured to repeatedly transmit an RA preamble, can transmit an instruction to the terminal device to stop transmitting the RA preamble. When the base station device or cell to which the RA preamble is to be transmitted is composed of multiple TRPs (Transmission and Reception Points), in addition to information for identifying the base station device or cell to which the RA preamble is to be transmitted, information for identifying the TRP to which the RA preamble is to be transmitted may be included in the transmission instruction.

[0031] The TA value acquisition unit 303 acquires, for each of the candidate cells to which the terminal device is to connect, a TA value determined as a result of the RA instruction unit 302 instructing the terminal device to transmit an RA preamble. Furthermore, the TA value acquisition unit 303 may acquire not only the TA values ​​for candidate cells to which the terminal device is not currently connected, but also the TA values ​​for cells currently connected to the terminal device that are included as candidate cells after the connection destination is changed, among the cells. The TA value acquisition unit 303 does not need to acquire the TA value for the cell to which the terminal device is currently connected, among the candidate cells. The inter-base station communication unit 304 communicates with a base station device that forms a candidate cell to which the terminal device is to connect. For example, the inter-base station communication unit 304 may notify another base station device to which the RA preamble is to be transmitted that the currently connected terminal device is to transmit an RA preamble. Furthermore, the inter-base station communication unit 304 is used, for example, to receive information for an RA procedure in the base station device that forms the candidate cell to which the terminal device is to connect, from that base station device. The TA value acquisition unit 303 can acquire, for example, a TA value calculated in a base station device that forms a candidate cell via the inter-base station communication unit 304 .

[0032] The cell switching instruction unit 305 instructs the terminal device to execute cell switching when the terminal device is in a state where it should execute cell switching, for example, when the reception quality of a radio signal from another base station device that forms a candidate cell to which the terminal device is to connect becomes higher by a predetermined level or more than the radio quality between the terminal device and the terminal device. The cell switching instruction unit 305 transmits to the terminal device a Cell switch command including, for example, a cell identifier that can identify the destination cell, and identification information such as an RRC-TransactionIdentifier or a CondReconfigId that corresponds to the configuration information notified by the RRC configuration unit 301. Note that when the terminal device connects to multiple cells and multiple TAGs are configured for the multiple cells, the cell switching instruction unit 305 can transmit to the terminal device a command including information indicating the TA values ​​for each of the multiple TAGs. That is, information indicating a first TA value to be applied in a first cell among the multiple cells and a second TA value to be applied in a second cell can be included in this command and transmitted to the terminal device. This enables the terminal device to transmit uplink user data signals using the TA values ​​in the multiple cells after the cell switch, respectively, immediately after the cell switch. The information indicating the TA value here may be the TA value itself, or identification information (e.g., tag-id, cell identifier, etc.) capable of identifying the TA value held in the terminal device. Furthermore, information indicating some TA values ​​among the multiple TA values ​​may be the TA values ​​themselves, and information indicating other TA values ​​may be identification information capable of identifying the TA values. Furthermore, the cell switching instruction unit 305 may notify the terminal device, in this command, of the TA value for a candidate cell after a further cell switch after the terminal device switches its connection destination. This eliminates the need to perform an RA procedure again for a candidate cell when the terminal device switches its further connection destination cell.When notifying the terminal device of the TA value itself, the cell switching instruction section 305 can notify the terminal device of the value of timeAlignmentTimerCommon or timeAlignmentTimerDedicated related to the TA value in the Cell Switch Command.

[0033] FIG. 4 shows an example of the functional configuration of a terminal device. The terminal device includes, for example, an RRC setting unit 401, an RA processing unit 402, a TA value holding unit 403, a cell switching unit 404, and a timing adjustment unit 405. Note that FIG. 4 only shows functions particularly related to this embodiment, and various other functions that the terminal device may have are omitted from the illustration. For example, the terminal device naturally has other functions that terminal devices compliant with 5G or subsequent standards generally have. The functional blocks in FIG. 4 are shown only schematically, and the respective functional blocks may be integrated or further subdivided. Furthermore, each function in FIG. 4 may be realized, for example, by the processor 201 executing a program stored in the ROM 202 or the storage device 204, or may be realized, for example, by a processor within the communication circuit 205 executing predetermined software. The details of the processes performed by each functional unit will not be described here, and only their general functions will be outlined.

[0034] The RRC setting unit 401 performs connection setting processing for a cell that is a connection candidate by receiving an RRC layer message from a connected base station device. The RRC setting unit 401 stores a setting for the currently connected cell and a setting for the connection candidate cell in parallel. When the RRC setting unit 401 stores multiple pieces of setting information, the RRC setting unit 401 may store corresponding identification information (e.g., an RRC-TransactionIdentifier, a CondReconfigId, or both) along with the setting information. The setting information for a combination of multiple cells may include, for example, information indicating to which TAG each of the multiple cells belongs. The RA processing unit 402 transmits an RA preamble for the connection candidate cell in response to receiving an RA preamble transmission instruction from the currently connected base station device. Here, the sequence for generating the RA preamble and information on the radio resources for receiving the RA preamble in the candidate cell may be received from the currently connected base station. The sequence and radio resource information may be acquired from system information transmitted by the base station forming the candidate cell. For example, after transmitting the above-mentioned RA preamble, the RA processing unit 402 may acquire the TA value determined in the destination base station via the currently connected base station. Furthermore, if there are multiple candidate cells that can use the same TA value, the RA processing unit 402 may receive information for identifying the cells that can use the same TA value from the base station and store the TA value in association with information identifying the TAG, such as a tag-id.

[0035] The TA value holding unit 403 holds, for example, a TA value for adjusting the transmission timing of an uplink signal to a connected base station device, and a TA value acquired by the RA processing unit 402 or notified in a Cell Switch Command from the connected base station device. The TA value holding unit 403 may hold the TA value in association with, for example, identification information of a TAG to which the TA value is applied or identification information of a cell to which the TA value is applied (a cell formed by the base station device that determined the TA value). Note that, when a difference value between a post-update TA value and a pre-update TA value is notified together with identification information associated with a specific TA value in order to update the specific TA value, the TA value holding unit 403 may update the TA value corresponding to the identification information using the notified difference value. Furthermore, when the TA value holding unit 403 acquires the TA value, it may start a timer (timeAlignmentTimer). The TA value is treated as valid until the timer expires.

[0036] The cell switching unit 404 changes the destination cell in response to receiving a command (cell switch command) from the connected base station device instructing cell switching. The command may include identification information specifying which of the configuration information configured in the RRC configuration unit 401 should be used. The terminal device can identify which cell to connect to by extracting the identification information from the command and identifying the configuration information corresponding to this identification information. Note that this is just an example, and the command may include identification information of the cell specifying the destination cell. The command also includes information indicating a TA value to be used. Note that when the terminal device connects to multiple cells, the command may include information indicating a first TA value for a first cell included in the multiple cells and information indicating a second TA value for a second cell. That is, the command may include information indicating multiple TA values. Note that the information indicating the TA value may be the TA value itself or identification information from which the TA value can be identified.

[0037] The timing adjustment unit 405 adjusts the transmission timing of uplink signals, for example, based on the TA value stored in the TA value storage unit 403. Furthermore, when a connection destination cell is switched by a Cell Switch Command, the timing adjustment unit 405 adjusts the transmission timing of uplink signals after the cell switch, based on information indicating the TA value notified in the command. In this way, the TA value to be used is identified based on the information included in the Cell Switch Command, and the terminal device can transmit uplink signals at appropriate timing immediately after switching the connection destination cell.

[0038] (Processing Flow) Next, an example of the processing flow executed in the wireless communication system will be described with reference to Fig. 5. Note that the processing flow in Fig. 5 is just an example, and each procedure can be modified as described above.

[0039] In this processing example, for example, it is assumed that a terminal device initially establishes a connection with a base station device providing cell A and a base station device providing cell B and is communicating in these cells (S501). Here, it is assumed that cell A belongs to TAG1, cell B belongs to TAG2, T1 is used as the TA value in TAG1, and T2 is used as the TA value in TAG2. That is, the terminal device controls the transmission timing of an uplink signal to cell A so that it is earlier by T1 than the timing at which the signal is received, and controls the transmission timing of an uplink signal to cell B so that it is earlier by T2 than the timing at which the signal is received. Thereafter, for example, the base station device forming cell A first transmits a HANDOVER REQUEST message to the base station devices forming cells C and D, which are connection destination candidates for the terminal device after cell switching (S502). When the base station device forming cell C or cell D receives this message, it returns a HANDOVER REQUEST ACK message to the base station device forming cell A, the HANDOVER REQUEST ACK message including configuration information such as information on communication parameters to be used when the terminal device connects (S503). The base station device forming cell A may, for example, previously store configuration information for cell B to which the terminal device is already connected. The base station device forming cell A then notifies the terminal device of an RRC Reconfiguration message including configuration information such as communication parameters notified from another base station device (S504). The notified configuration information may include configuration information for using multiple cells in parallel. Furthermore, identification information capable of identifying the configuration information may be associated with the configuration information. Here, it is assumed that the terminal device is notified of Configuration 1 for using cell B and cell C in parallel, and Configuration 2 for using cell A and cell D in parallel. In addition, information such as the fact that in setting 1, cell B belongs to TAG1 and cell C belongs to TAG2, and in setting 2, cell A belongs to TAG1 and cell D belongs to TAG2 is notified to the terminal device.Upon receiving this message, the terminal device transmits an RRC Reconfiguration Complete message to the base station device of cell A, thereby completing the configuration process using the RRC layer message (S505). This process eliminates the need for the terminal device to perform the RRC layer configuration process after receiving the instruction to switch cells.

[0040] Next, the base station device of cell A instructs the terminal device to transmit an RA preamble to each cell that is a candidate for connection after the cell change (S506). Here, this instruction may include information specifying the sequence that the terminal device should use to generate the RA preamble and the radio resource from which the RA preamble should be transmitted. Note that the radio resource from which the RA preamble should be transmitted may be identified based on system information of the cell that is transmitted in the candidate cell and measured by the terminal device, for example. That is, the terminal device does not need to acquire this information from the currently connected base station device. Also, the sequence to be used when generating the RA preamble may be identified in advance. In one example, information about this sequence may have been notified to the terminal device in S504. The terminal device transmits RA preambles to cells C and D that are not currently connected in accordance with the instruction (S507). At this time, the base station device of cell A may notify the base station devices of cell C and cell D that the terminal device plans to transmit an RA preamble. At this time, the base station device of cell A can notify the base station device of cell C and the base station device of cell D of information on the sequence to be used by the terminal device to generate an RA preamble to be transmitted to each cell. This enables the base station device of cell C and the base station device of cell D to detect the RA preamble transmitted by the terminal device. Note that this is just one example, and for example, the HANDOVER REQUEST of S502 may implicitly notify that the terminal device plans to transmit an RA preamble. In this case, the HANDOVER REQUEST ACK of S503 may specify the sequence to be used by the terminal device to generate an RA preamble. Note that these are just one example, and information on the sequence to be used by the terminal device to generate an RA preamble may be shared between the base station device of cell A and the base station devices of cell C and cell D by other messages. The base station device of cell C and the base station device of cell D can calculate the TA value based on the RA preamble transmitted from the terminal device, and notify the base station device of cell A of the calculated TA value (S508).

[0041] Thereafter, for example, when a predetermined condition is satisfied, such as when the radio quality of a signal from the base station device of cell C becomes higher than the radio quality of a signal from the base station device of cell A by a predetermined level, a cell switching process may be performed in the terminal device. Assume that the base station device of cell A decides to change the cell to which the terminal device is connected, for example, from a combination of cell A and cell B to a combination of cell B and cell C. In this case, the setting for using the combination of cell B and cell C was notified to the terminal device as "Setting 1" in S504. Therefore, the base station device of cell A transmits a Cell Switch Command to the terminal device instructing the terminal device to switch the connection destination cell using "Setting 1" (S509). Note that this is just an example, and any information that allows the terminal device to identify that the combination of cell B and cell C should be used may be notified to the terminal device in this command. Note that this command includes information indicating a TA value. Here, Setting 1 notified in S504 specifies that cell B belongs to TAG1 and cell C belongs to TAG2. For this reason, the command includes multiple TA values ​​to be used in each TAG. For example, the terminal device was connected to cell B before switching the destination cell, and at that time, cell B belonged to TAG2. Therefore, it is assumed that the TA value used in TAG2 before the cell switch can be used in TAG1 after the cell switch. For this reason, information specifying the TA value of TAG2 before the cell switch (e.g., tag-id) can be included in the Cell Switch Command as information indicating the TA value of TAG1 to which cell B belongs. Furthermore, the terminal device is not connected to cell C before the cell switch, and the TA value for cell C has not been notified. For this reason, "T3," which is the TA value to be used in cell C, can be included in the Cell Switch Command as information indicating the TA value of TAG2 to which cell C belongs. This TA value has been notified from the base station device of cell C to the base station device of cell A in S508, so the base station device of cell A can include this TA value (T3) in the Cell Switch Command.

[0042] When the terminal device receives the Cell Switch Command, it changes the connection destination cell to the combination of cell B and cell C in accordance with the command. Then, the terminal device can identify the TA value to be used in each cell by using the information indicating the TA value included in the command. Therefore, the terminal device can transmit an uplink signal with appropriately adjusted transmission timing immediately after switching cells, without being notified of the TA value again from the base station device of cell C or the like (S510, S511).

[0043] In this processing example, the base station device forming cell A takes the lead in controlling cell switching and the like, but the base station device forming cell B to which the terminal device is connected may also take the lead in this control. Also, the example of FIG. 5 shows an example in which the TA value for cell D, which has already been acquired in cell A, is notified to the terminal device in the Cell Switch Command. This eliminates the need for the terminal device to execute the RA procedure again when the terminal device switches its connection destination cell to cell D after changing its connection destination cell to cells B and C. However, this is just an example, and the TA value of cell D does not need to be included in the Cell Switch Command. Also, in the above example, the TA value of cell C is notified in the Cell Switch Command. However, for example, after S508, the base station device of cell A may notify the terminal device of the TA value (T3) in association with specific identification information, such as identification information of cell C. In this case, the terminal device holds the specific identification information and the notified TA value. Then, in the Cell Switch Command of S509, the specific identification information is included as information indicating the TA value of TAG2, so that the terminal device can specify that the TA value (T3) held in association with the specific identification information should be used in TAG2. Note that, although the above description has been given of the case where TAG1 and TAG2 are used, two or more TAGs, such as TAG3, TAG4, and TAGn, may also be used.

[0044] In this way, according to the present embodiment, it is possible to significantly reduce the time from when the base station device, which was the connection destination before the cell change, decides to change the connection destination of the terminal device to another cell until the terminal device starts transmitting an uplink signal containing user data in the changed cell, thereby contributing to Goal 9 of the Sustainable Development Goals (SDGs) led by the United Nations, which is to "build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0045] 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.

[0046] This application claims priority based on Japanese Patent Application No. 2023-129494, filed on August 8, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. A terminal device conforming to a cellular communication standard of the Third Generation Partnership Project (3GPP), comprising: a storage means for storing a Timing Advance value (TA value) used in a connected cell and identification information associated with the TA value before receiving a Cell Switch Command instructing the terminal device to switch a cell to which the terminal device is connected; and a communication means for performing uplink communication based on the TA value associated with the identification information corresponding to the switched cell after switching cells based on receiving the Cell Switch Command.

2. The terminal device according to claim 1, wherein, when a TA value is included in the received Cell Switch Command, said communication means performs said uplink communication using the TA value included in said Cell Switch Command.

3. A control method executed by a terminal device conforming to a cellular communication standard of the Third Generation Partnership Project (3GPP), comprising: before receiving a Cell Switch Command instructing the terminal device to switch a cell to which the terminal device is connected, retaining a Timing Advance value (TA value) used in the connected cell and identification information associated with the TA value; and, after switching cells based on receiving the Cell Switch Command, performing uplink communication based on the TA value associated with the identification information corresponding to the switched cell.

4. The control method according to claim 3, wherein, when a TA value is included in the received Cell Switch Command, the uplink communication is performed using the TA value included in the Cell Switch Command.

5. A program for causing a computer provided in a terminal device that complies with the cellular communication standard of the Third Generation Partnership Project (3GPP) to retain a Timing Advance value (TA value) used in a cell to which the terminal device is currently connected and identification information associated with the TA value before receiving a Cell Switch Command instructing the terminal device to switch the cell to which the terminal device is currently connected, and, after switching cells based on receiving the Cell Switch Command, causing the computer to perform uplink communication based on the TA value associated with the identification information corresponding to the cell after switching.

6. The program according to claim 5, wherein, when a TA value is included in the received Cell Switch Command, the uplink communication is performed using the TA value included in the Cell Switch Command.

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