Terminal device and control method for executing handover with improved communication efficiency

Abstract

To improve communication efficiency regarding handover.SOLUTION: A terminal device receives, from a base station device to which the terminal device is connected, radio resource control (RRC) layer settings for other base station devices that are candidates for the terminal device's handover destination from the base station device, receives, from the base station device, a physical downlink control channel (PDCCH) order instructing the terminal device to transmit a random access preamble to the other base station device without performing a handover to the other base station device, transmits the random access preamble to the other base station device in accordance with the instruction, and performs a handover to the other base station device when the terminal device receives a command from the base station device instructing a handover to the other base station device.SELECTED DRAWING: Figure 5

Description

[Technical field]

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

[0002] In a mobile communication system, handover is performed to switch the cell (base station) to which the terminal device is connected as the terminal device moves. 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 in the cell to which the connection is switched. This process for switching the connection takes a certain amount of time, and the terminal device cannot communicate user data during the period during which this process is being performed. In contrast, in the method 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 base station device to which the handover is to be performed before the handover is actually performed. Then, the handover source base station device transmits a layer 1 or layer 2 command to the terminal device to instruct the cell to be switched at the timing when the handover should be performed. In response to receiving the command, the terminal device executes a random access procedure with the handover destination base station device, thereby completing the handover without carrying out subsequent local processing in the RRC layer. With such a technique, it is possible to suppress deterioration of communication efficiency related to handover. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] 3GPP (registered trademark) Contribution, R2-2209255 Summary of the Invention [Problem to be solved by the invention]

[0004] The present invention provides a technique for further improving communication efficiency regarding handover. [Means for solving the problem]

[0005] A terminal device according to one embodiment of the present invention has a receiving means for receiving, from a base station device to which the terminal device is connected, radio resource control (RRC) layer settings for another base station device that is a candidate for a handover destination from the base station device, and receiving from the base station device a physical downlink control channel (PDCCH) order instructing the terminal device to transmit a random access preamble to the other base station device without executing a handover to the other base station device, a transmitting means for transmitting the random access preamble to the other base station device in accordance with the instruction, and an executing means for executing a handover to the other base station device when a command instructing a handover to the other base station device is received from the base station device. Effect of the Invention

[0006] According to the present invention, it is possible to improve communication efficiency regarding handover. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of a wireless communication system. [Diagram 2] FIG. 2 illustrates an example of a hardware configuration of the apparatus. [Diagram 3] FIG. 2 is a diagram illustrating an example of a functional configuration of a base station device. [Figure 4] FIG. 2 is a diagram illustrating an example of a functional configuration of a terminal device. [Diagram 5] FIG. 1 is a diagram illustrating an example of a flow of processing executed in a wireless communication system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more features among the multiple features described in the embodiments may be arbitrarily combined. In addition, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0009] (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 according to 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 base station device 101, a base station device 102, and a terminal device 111. Note that, for the sake of simplicity, Fig. 1 shows only two 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.

[0010] 1, terminal device 111 is about to move from within the range of a cell formed by base station device 101 into the range of a cell formed by base station device 102. At this time, terminal device 111 will perform handover to switch the connected base station device from base station device 101 to base station device 102.

[0011] In this embodiment, a handover procedure is executed to shorten the period during which the terminal device 111 cannot communicate user data due to handover. That is, the handover source base station device 101 executes a setting (setting using an RRC message) in a radio resource control (RRC) layer 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 performs a setting in the RRC layer when starting a handover process. The setting in the RRC layer is notified to the terminal device 111 prior to the timing when the handover process should actually be performed. In the setting here, communication parameters used in 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 settings in the handover destination candidate base station device 102, parameters when performing a random access procedure, and radio resources (frequency and time resources) to be used. Furthermore, the handover source base station apparatus 101 can transmit to the terminal apparatus 111 an instruction to measure a synchronization signal (SS) / physical broadcast channel (PBCH) block (SSB) transmitted from the handover destination candidate base station apparatus 102.

[0012] The terminal device 111 measures the SSB sent from the base station device 102, which is a candidate for the handover destination, according to a measurement instruction from the connected base station device 101. The terminal device 111 then notifies the connected base station device 101 of the measurement result. Here, the base station device 101 may notify the measurement result to the base station device 102, which is a candidate for the handover destination that was the measurement target. This allows the base station device 102 to specify the rough 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 notify the terminal device 111 of the transmission setting of the channel state information (CSI)-reference signal (RS) in order to further specify the detailed beam to be used in the downlink. Note that this notification is performed by the base station device 102 notifying the base station device 101 of the transmission setting of the CSI-RS, and the base station device 101 transferring the transmission setting to the connected terminal device 111. When the terminal device 111 receives the transmission setting, it 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. Then, the base station device 101 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 is actually performed. This makes it possible to omit the process of determining the beam direction in detail after the handover is performed, and the period during which the efficiency of communication of user data of the terminal device 111 decreases can be shortened. As an example, the above-mentioned CSI-RS transmission configuration can be instructed to the terminal device 111 using a CSI-RS measurement configuration consisting of a radio resource configuration (CSI-RS-Resource) and a report configuration (csi-ReportConfig) of CSI-RS related to the base station device 102 that are preset before handover as communication parameters in the terminal device 111. The CSI-RS-Resource includes the time position and frequency position of the radio resource block in which the CSI-RS is transmitted.For example, the base station device 101 may instruct the terminal device 111 to measure the radio resource of a specific CSI-RS 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 CSI report timing is set. For example, the base station device 101 may instruct the terminal device 111 to report the measurement result of the CSI-RS at a specific timing by specifying the csi-ReportConfig using the csi-ReportConfig id in the MAC CE or DCI. Also, when performing the above-mentioned measurement configuration for 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 the MAC CE or DCI, as information for the terminal device 111 to specify which base station device's CSI-RS transmission configuration is.

[0013] In this embodiment, in order to suppress deterioration of communication efficiency in further handover, the terminal device 111 executes a random access (RA) procedure before it is determined to execute a handover to the base station device 102. That is, the terminal device 111 may perform a process for establishing uplink synchronization before actually performing a handover. 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 in the radio resource prepared for the RA procedure in the base station device 102. Note that information on the radio resource and the sequence used to generate the RA preamble may be notified to the terminal device 111 from the base station device 101 during, for example, the above-mentioned RRC layer setting process. In addition, these pieces of information may be notified to the terminal device 111 when the base station device 101 instructs the terminal device 111 to transmit the RA preamble. This instruction can be made, for example, by utilizing the PDCCH order, which is a procedure for transmitting an RA preamble from the conventional base station device 101. The PDCCH is a physical downlink control channel. The conventional PDCCH order is used to transmit an RA preamble from the terminal device 111 to the base station device 101 when uplink synchronization between the terminal device 111 and the base station device 101 to which the terminal device 111 is connected is lost. In contrast, the present embodiment differs in that the PDCCH order is used to transmit an RA preamble to an unconnected base station device 102. By 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. Then, for example, the base station device 102 notifies the base station device 101 to which the terminal device 111 is connected of the TA value, and the base station device 101 transfers the TA value to the terminal device 111, thereby notifying the terminal device 111 of the TA value.As a result, the terminal device 111 can establish uplink synchronization with the base station device 102 at the time when the handover to the base station device 102 is instructed by the base station device 101, and therefore can transmit uplink user data to the base station device 102 without performing an RA procedure again. Details of the RA procedure according to the PDCCH order performed in this embodiment will be described later.

[0014] Thereafter, the base station device 101 may notify the terminal device 111 to execute handover at a predetermined opportunity, such as when the reception quality of the 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 the 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 media access control element (MAC CE) or downlink control information (DCI). Note that this command includes identification information of a cell formed by the handover destination base station device 102. In addition, when multiple settings are set in the RRC layer used for communication with the base station device 102, an identifier uniquely associated with each setting in the RRC layer, for example, an RRC-TransactionIdentifier, may be included in this command. In addition, at this time, the TA value determined in the base station device 102 by performing the RA procedure 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. Also, information on the beam determined in the base station device 102 by measuring SSB or 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. Also, the base station device 102 may reserve uplink radio resources to be used by the terminal device 111 and notify the base station device 101 of information on the reserved radio resources. Then, the base station device 101 may notify the terminal device 111 of the information on the uplink radio resources in the above-mentioned command. According to this, immediately after receiving the above-mentioned command, the terminal device 111 can transmit user data without performing control communication with the base station device 102 after handover. Note that the radio resources for uplink user data may be notified to the terminal device 111 from the base station device 102 after handover. In this case, the terminal device 111 can immediately transmit user data in the designated radio resources because the RA procedure is performed prior to receiving the command as described above, and the RA procedure after receiving the command is omitted.Note that the base station device 101 before handover can, for example, obtain a buffer status report (BSR) from the currently connected terminal device 111 and notify the contents of the report to the handover destination base station device 102. This allows the base station device 102 to allocate an appropriate amount of radio resources to the terminal device 111 without receiving a BSR from the terminal device 111. Furthermore, in response to receiving the above-mentioned command from the base station device 101, the terminal device 111 may switch the connection destination to the base station device 102 and transmit a scheduling request (SR) to the base station device 102.

[0015] In addition, when the terminal device 111 receives the above-mentioned command by the MAC CE, it may transmit an acknowledgement (Ack) or the like to the base station device 101. In addition, when the terminal device 111 receives the above-mentioned command, it may switch the connection destination to the base station device 102 and transmit an Ack or the like to the base station device 102. In this case, the terminal device 111 may transmit an Ack to the base station device 102 in the uplink radio resource allocated as described above. In addition, the base station device 102 may transfer the acknowledgement to the base station device 101. In this way, the base station device 101 can determine that the terminal device 111 has switched the connection destination to the base station device 102, and can discard, for example, the context information of the terminal device 111. In addition, when the base station device 101 does not receive an Ack from the terminal device 111, it may resend the above-mentioned command to ensure that the handover is performed. Furthermore, when the terminal device 111 receives a command by the above-mentioned DCI, the terminal device 111 may implicitly notify the handover destination base station device 102 that the command has been received by transmitting a predetermined signal such as an SR or BSR to the base station device 102. Note that the uplink radio resource in the base station device 102 for transmitting the SR or BSR may be notified to the terminal device 111 in the DCI of the command via the base station device 101. Note that, for example, the predetermined signal such as the BSR may be transmitted using a dedicated MAC CE indicating that a handover based on the above-mentioned command has been performed, or may be transmitted in the same manner as the conventional BSR. Also, a dedicated MAC CE indicating that a handover based on the above-mentioned command has been performed may be notified to the base station device 102 separately from the SR or BSR.

[0016] It is also possible to determine whether the handover of the terminal device 111 has been successful by using a timer. For example, when the terminal device 111 receives the above-mentioned command, the terminal device 111 starts counting a predetermined period by the first timer. After the start of counting by the first timer, the terminal device 111 may receive the PDCCH from the base station device 101 while receiving the PDCCH from the base station device 102 in parallel. That is, the terminal device 111 is made capable of receiving both the above-mentioned command retransmitted from the base station device 101 and a predetermined signal transmitted from the base station device 102 when the handover is successful. The terminal device 111 may continuously observe the PDCCHs from both base station devices, for example, by switching between receiving the signal from the base station device 101 and receiving the signal from the base station device 102 in a time division manner. Also, the terminal device 111 may transmit a response signal such as the above-mentioned Ack to the base station device 102 after receiving the command, and start the second timer. The second timer measures a time shorter than the time length set by the first timer. The response signal may be a signal different from Ack, such as a Cell switch command response. In one example, the terminal device 111 may determine that the handover has been successful if it does not receive a retransmission instruction for the response signal or a negative acknowledgement (Nack) indicating that the response signal could not be received from the base station device 102 until the second timer expires. When the terminal device 111 receives a retransmission instruction or a Nack, it retransmits the response signal, resets the second timer, and starts counting again. When the terminal device 111 receives a retransmission instruction or a Nack after the first timer expires, it may determine that the handover has failed.

[0017] In another example, the terminal device 111 may determine that the handover is successful when it receives a predetermined signal from the base station device 102 while the first timer is running, without using the second timer. In this case, the terminal device 111 may stop the first timer in response to receiving this predetermined signal. Here, the predetermined signal from the base station device 102 may be, for example, a signal instructing the end of reception of the PDCCH transmitted from the base station device 101. Also, the predetermined signal from the base station device 102 may be an acknowledgement (Ack) to the above-mentioned response signal. In response to receiving this predetermined signal, the terminal device 111 ends reception of the PDCCH transmitted from the base station device 101, while continuing reception of the PDCCH from the base station device 102. On the other hand, when the terminal device 111 receives a retransmission request or Nack regarding the response signal before the first timer expires, the terminal device 111 may retransmit the response signal to the base station device 102. Furthermore, the terminal device 111 may start a third timer when transmitting a response signal, regardless of the presence or absence of the first timer, and may determine that the base station device 102 has not successfully received the response signal if the above-mentioned predetermined signal is not received before the third timer expires. In this case, the terminal device 111 may retransmit the response signal and reset the third timer if the first timer has not expired, and may determine that the handover has failed if the first timer has expired. The terminal device 111 may also set a limit on the number of times the above-mentioned response signal is retransmitted. In this case, the terminal device 111 may determine that the handover has failed in response to the number of times the response signal is retransmitted reaching a predetermined number.

[0018] If the terminal device 111 determines that the handover has failed, it may end reception of the PDCCH of the base station device 102 and continue reception of the PDCCH of the base station device 101. In this case, the terminal device 111 may notify the base station device 101 that the handover has failed. If the base station device 102 receives a response signal from the terminal device 111, it may determine that the handover has been successful and notify the base station device 101 of the success of the handover. The base station device 101 may delete the context information of the terminal device 111 based on this notification.

[0019] Furthermore, in response to receiving the Cell switch command, the terminal device 111 may terminate reception of the PDCCH from the base station device 101. In this case, when the terminal device 111 determines that the handover to the base station device 102 has failed, the terminal device 111 may execute a reconnection process.

[0020] Information on the time length to be measured by the timer and the upper limit of the number of retransmissions of the response signal can be notified to the terminal device 111, for example, during the setting process of the RRC layer described above. Note that, when there are multiple candidate cells for the handover destination, information on the timer value and the upper limit of the number of retransmissions may be notified to the terminal device 111 individually for each of the multiple cells. Also, at least a part of the multiple cells may be grouped, and common setting information for each group may be notified to the terminal device 111. Furthermore, common setting information for all cells may be notified to the terminal device 111. Also, these setting information may be notified to the terminal device 111 by, for example, a Cell switch command. Note that, in order to transmit the above-mentioned response signal, uplink radio resources are allocated, and information on the upper limit of the number of retransmissions may be notified to the terminal device 111 in an uplink grant indicating the allocation.

[0021] In this manner, it is possible to shorten the period during which user data communication is not possible when the terminal device 111 performs handover from the base station device 101 to the base station device 102. This makes it possible to provide communication services that require low latency and high reliability, for example. In addition, by shortening the period during which user data communication is not possible, it is possible to suppress a decrease in the efficiency of wireless communication.

[0022] (RA procedure based on PDCCH order) <Processing example 1> The RA procedure by the PDCCH order executed in this embodiment will be described in more detail. In one example, a case where a unique sequence for generating an RA preamble is assigned to the terminal device 111 will be described. First, the base station device 101 notifies the base station device 102 that the terminal device 111 is to start the RA procedure. This allows the base station device 101 to which the terminal device 111 is connected and the base station device 102, which is a candidate for the handover destination, to commonly recognize that the RA procedure is to be executed by the terminal device 111. Then, the base station device 101 instructs the terminal device 111 to execute the RA procedure using the PDCCH as described above. At this time, the base station device 101 can notify the terminal device 111 of information for identifying radio resources (frequency and time resources) such as SS / PBCH index and PRACH Mask index that are set by the base station device 102 so that the base station device 102 receives the RA preamble, and information indicating the sequence of the RA preamble to be used (Random Access Preamble Index). When there are multiple candidate cells for the handover destination, the base station device 101 may collectively notify the terminal device 111 of information on the multiple cells by one PDCCH. Also, the base station device 101 may notify the terminal device 111 of information on at least a part of the multiple cells by an individual PDCCH. As described above, the sequence of the RA preamble here is individually assigned to the terminal device 111. Therefore, information indicating one sequence for each candidate cell for the handover destination is notified to the terminal device 111. According to this, when the candidate base station device for the handover destination receives an RA preamble of a corresponding sequence, it can uniquely identify that the source of the RA preamble is the terminal device 111. Based on the information received from the base station device 101, the terminal device 111 transmits an RA preamble for the candidate cell for the handover destination using a designated sequence in the radio resource corresponding to the cell.For example, the base station device 101 notifies the terminal device 111 of the radio resources in which the base station device 102 is permitted to transmit an RA preamble and the sequence to be used by the terminal device 111. The terminal device 111 then transmits the RA preamble to the base station device 102 using the sequence notified by the base station device 101 in the radio resources notified by the base station device 101. The base station device 102, which is a candidate handover destination, performs a detection process for the RA preamble using the sequence designated for the terminal device 111 in the radio resources for the RA procedure, and determines whether or not the RA preamble from the terminal device 111 has been received.

[0023] At this point, the TA value does not need to be notified to the terminal device 111, and contention resolution does not need to be performed because a sequence for a unique RA preamble is specified for the terminal device 111. For this reason, the base station device 102 does not need to return an RA response to the terminal device 111. However, the base station device 102 can determine the TA value to be used by the terminal device 111 based on the reception timing of the RA preamble. Then, for example, the base station device 102 notifies the base station device 101 of the TA value. The base station device 101 can notify the terminal device 111 of information on the value together with, for example, a command described later. If the RA response is not transmitted, the terminal device 111 cannot recognize whether the RA preamble has been normally received. For this reason, in this processing example, if the base station device 102 is unable to receive the RA preamble from the terminal device 111 within a predetermined period, the base station device 102 can cause the terminal device 111 to retransmit the RA preamble via the base station device 101. For example, the base station device 102 sends a predetermined notification to the base station device 101 indicating that it was not possible to receive the RA preamble from the terminal device 111. When the base station device 101 receives this predetermined notification, it transmits a PDCCH order to the terminal device 111 again to cause the terminal device 111 to retransmit the RA preamble. When the terminal device 111 receives the PDCCH order again, it retransmits the RA preamble. On the other hand, the terminal device 111 may not retransmit the RA preamble unless it receives the PDCCH order again. That is, in this processing example, the terminal device 111 does not retransmit the RA preamble unless it is specifically instructed to do so.

[0024] In the above example, the base station device 102 does not transmit an RA response. However, the base station device 102 may transmit an RA response. In this case, the terminal device 111 can acquire the TA value by the RA response. Furthermore, the terminal device 111 may spontaneously retransmit the RA preamble in response to not receiving the RA response. Furthermore, when the terminal device 111 receives an RA response, the terminal device 111 does not perform subsequent processing of the RA procedure (processing after message 3 in the RA procedure). That is, the terminal device 111 may acquire the TA value by the RA response and end the RA procedure without performing any further processing. Furthermore, the base station device 102 does not include allocation of radio resources for message 3 in the RA response. That is, in the normal RA procedure, uplink radio resources for transmitting message 3 are specified in the RA response, but since message 3 does not need to be transmitted, radio resources for that purpose may not be specified.

[0025] In the above processing example, it has been described that the terminal device 111 does not retransmit the RA preamble unless there is an instruction to retransmit the RA preamble from the base station device 101 or the base station device 102, but this is not limited to the above. For example, the terminal device 111 may retransmit the RA preamble at a predetermined period, and repeatedly transmit the RA preamble at a predetermined period unless it receives an instruction to stop transmitting the RA preamble from the base station device 101 or the base station device 102. For example, the base station device 101 or the base station device 102 may transmit an instruction to stop transmitting the RA preamble to the terminal device 111 when the terminal device 111 determines that the transmission of the RA preamble should be stopped, for example, when the RA preamble is normally detected in the base station device 102 or a predetermined period has elapsed.

[0026] <Processing example 2> In the above-mentioned processing example 1, a unique RA preamble sequence is specified for the terminal device 111, but this is not limited thereto. In another processing example, the terminal device 111 transmits an RA preamble using one sequence selected from a group of sequences used in a contention-based RA procedure in a cell that is a candidate for handover destination. The base station device 101 acquires a measurement result of the SSB of the cell that is a candidate for handover destination from the terminal device 111 as described above, and selects a cell that causes the terminal device 111 to execute the RA procedure based on the measurement result. For example, the base station device 101 may select a cell whose wireless quality is equal to or higher than a predetermined level as a cell that causes the terminal device 111 to execute the RA procedure. The base station device 101 may also select one or more predetermined number of cells in order of better wireless quality as cells that cause the RA procedure to be executed. Then, the base station device 101 notifies the terminal device 111 of the SSB index corresponding to the selected cell. The terminal device 111 can identify wireless resources (frequency and time resources) for transmitting the RA preamble based on the SSB index. Here, the base station device 102 may designate a Cell-Radio Network Temporary Identifier (C-RNTI) for communication in the RA procedure and communication after contention resolution, and may notify the terminal device 111 of the C-RNTI via the base station device 101. Also, a temporary identifier (Temporary C-RNTI (TC-RNTI)) may be notified to the terminal device 111 instead of the C-RNTI.

[0027] Thereafter, the terminal device 111 transmits an RA preamble. For example, the terminal device 111 selects one sequence from a group of sequences corresponding to the notified SSB index, generates an RA preamble, and transmits the RA preamble in the radio resource corresponding to the SSB index. Here, it is assumed that the terminal device 111 selects one sequence from a group of sequences that can be used to access the base station device 102, and transmits the RA preamble generated using the selected sequence in the radio resource for the RA procedure of the base station device 102. Here, the group of sequences that can be used to generate an RA preamble can also be used by other terminal devices, and the base station device 102 cannot recognize whether it has received an RA preamble from the terminal device 111 just by detecting the RA preamble. For this reason, when the base station device 102 detects an RA preamble, it transmits the RA response for subsequent contention resolution. After transmitting the RA preamble, the terminal device 111 starts a timer (ra-ResponseWindow) for timing a certain period for detecting an RA response. If the terminal device 111 receives an RA response within this fixed period, it stops retransmitting the RA preamble. If the terminal device 111 does not receive an RA response within this fixed period, it retransmits the RA preamble and resets and restarts the timer. The maximum number of retransmissions of the RA preamble is specified, for example, by a signal transmitted from the base station device 101 or the base station device 102. As described above, the base station device 101 can notify the terminal device 111 of identification information (C-RNTI or TC-RNTI) of the terminal device 111. Instead of this notification, the base station device 102 may notify the terminal device 111 of the TC-RNTI in the RA response. The UE can specify the resource allocation in the frequency domain and the time domain of the physical downlink shared channel (PDSCH) to which the RA response is transmitted, by using the Random Access (RA)-RNTI.That is, the resource allocation is specified by DCI format 1_0 with CRC scrambled by RA-RNTI transmitted by the base station device 102, and the terminal device 111 can acquire the information using the RA-RNTI. Note that the RA-RNTI is identified based on the arrangement of radio resources used to transmit the RA preamble (time position and frequency position identified by a symbol, slot, or frame). Then, the terminal device 111 receives an RA response corresponding to the RA preamble transmitted by its own device in accordance with the resource allocation acquired using the RA-RNTI.

[0028] When the terminal device 111 receives the RA response, it transmits a message 3 of the RACH procedure to the base station device 102 using the uplink radio resource specified in the RA response. Here, the terminal device 111 receives a TA value to be used in the RA response, and uses the TA value to control the transmission timing of the message 3. At this point, it is not determined whether this TA value is the TA value for the terminal device 111. That is, other terminal devices that have transmitted an RA preamble in the same subframe and frequency position may also specify the same RA-RNTI and receive an RA response. For this reason, it is not determined at this point whether the TA value acquired by the terminal device 111 is a value determined based on the RA preamble transmitted by the terminal device 111. When using the C-RNTI to resolve this situation (i.e., to resolve contention), the terminal device 111 transmits the identification information (C-RNTI) of its own device notified as described above by the message 3 to the base station device 102. Furthermore, the terminal device 111 starts a timer (ra-ContentionResolutionTimer) for measuring a certain period for determining whether or not contention resolution in the base station device 102 has been successful. When the terminal device 111 receives DCI addressed to its own device's identification information (C-RNTI) before this certain period expires, it can determine that the TA value has been set based on the RA preamble transmitted by its own device. When using TC-RNTI, the terminal device 111 transmits its own device's identification information (for example, InitialUE-Identity consisting of 5G-S-TMSI and a random number) to the base station device 102 by message 3. After that, the terminal device 111 starts a timer (ra-ContentionResolutionTimer) for measuring a certain period for determining whether or not contention resolution in the base station device 102 has been successful. If message 4, the transmission of which is instructed by DCI addressed to the terminal device's own device's identification information (TC-RNTI) before this certain period expires, contains the terminal device's own device's identification information (e.g., InitialUE-Identity) transmitted to the base station device 102 in message 3, the terminal device 111 can determine that the TA value has been set based on the RA preamble transmitted by the terminal device.Here, only the TA value is determined (uplink synchronization is established), and no connection is established between the terminal device 111 and the base station device 102. On the other hand, if the terminal device 111 does not receive DCI addressed to its own device's identification information (C-RNTI or TC-RNTI) within the above-mentioned certain period, the TA value acquired in the RA response was not the TA value based on the RA preamble transmitted by its own device, and so the terminal device 111 resends the RA preamble.

[0029] The above-mentioned signal transmitted from the base station device 102 to the terminal device 111 may be notified to the terminal device 111 via the base station device 101. In this case, the base station device 102 transmits, for example, the contents of message 2 (RA response) or message 4 to the base station device 101 using the inter-base station interface, and the base station device 101 notifies the terminal device 111 of information corresponding to these messages. As described above, when a radio signal is directly transmitted from the base station device 102 to the terminal device 111, the terminal device 111 needs to perform processing to receive message 2 or message 4 from the base station device 102 while continuing to receive the PDCCH of the base station device 101. On the other hand, when message 2 or message 4 is notified to the terminal device 111 via the base station device 101, the terminal device 111 does not need to receive a radio signal from the base station device 102. It is also possible to notify only one of message 2 and message 4 to the terminal device 111 via the base station device 101.

[0030] In addition, when the terminal device 111 needs to receive a signal of a candidate cell of the handover destination, the base station device 101 may notify the terminal device 111 of the timing at which to receive a downlink signal (e.g., PDCCH) from the base station device of the candidate cell. For example, when the above-mentioned message 2 or message 4 is directly transmitted from the base station device 102 to the terminal device 111, the terminal device 111 needs to receive a signal such as a PDCCH from the base station device 102. In this case, the base station device 101 may notify the terminal device 111 of information on the timing at which a signal such as a PDCCH is transmitted in the base station device 102. In addition, when there are multiple candidate cells and the timing of the PDCCHs of these cells overlap, the base station device 101 may notify the base station device of the candidate cell of a change in reception timing so that the timings of the PDCCHs of these cells do not overlap. In addition, when there are multiple candidate cells and the timings of the PDCCHs of these cells overlap, the terminal device 111 may notify the base station device 101 of information on a cell in which the reception timing overlap occurs. After transmitting the RA preamble, the terminal device 111 starts receiving the PDCCH for the candidate cell to which the RA preamble is to be transmitted, for example, with the activation of the ra-ResponseWindow as a trigger. At this time, the terminal device 111 continues receiving the PDCCH of the connected base station device 101, and may be in a state of receiving PDCCHs from multiple base station devices in parallel. Thereafter, the terminal device 111 may end the reception of the PDCCH for the candidate cell, for example, with the expiration of the ra-ResponseWindow as a trigger. Also, the terminal device 111 may end the reception of the PDCCH, for example, in response to an instruction from the base station device (for example, the base station device 102) of the candidate cell or the base station device 101. The instruction from the base station device of the candidate cell may be transmitted directly from the base station device to the terminal device 111, or may be transmitted via the base station device 101.

[0031] In the above processing example, the terminal device 111 stops transmitting the RA preamble when the timer expires without receiving an RA response, but the present invention is not limited to this. For example, the terminal device 111 may repeatedly transmit the RA preamble at a predetermined period, and stop transmitting the RA preamble when it receives an instruction to stop transmitting the RA preamble from the base station device 101 or the base station device 102. This transmission stop instruction may be a signal different from the RA response. For example, when the base station device 101 or the base station device 102 determines that the terminal device 111 should stop transmitting the RA preamble, for example, because a predetermined period has passed, the base station device 101 or the base station device 102 may transmit an instruction to stop transmitting the RA preamble to the terminal device 111. This makes it possible to instruct the terminal device 111 to stop transmitting the RA preamble by control on the network side.

[0032] (Handling of acquired TA value) By the above-mentioned processing, the TA value to be used when the terminal device 111 transmits a signal to the base station device 102 is held in at least one of the terminal device 111 and the base station device 102, but it is assumed that the TA value will no longer be suitable for the actual environment after a certain period of time has passed. For this reason, in this processing example, the handling of the acquired TA value, such as a method for maintaining the TA value at an appropriate value or for ending the maintenance of the TA value, will be described.

[0033] In the first example, the expiration date and update period of the TA value are managed using a timer. For example, the terminal device 111 starts a predetermined timer in response to acquiring the TA value of a candidate cell from either the base station device 101 or the base station device 102 that is a candidate for the handover destination. Then, in response to expiration of the predetermined timer, the terminal device 111 may transmit an RA preamble again and execute the above-mentioned procedure again to acquire an updated TA value. Also, either the base station device 101 to which the terminal device 111 is connected or the base station device 102 that is a candidate for the handover destination starts a predetermined timer in response to the TA value being determined in the base station device 102. Then, in response to expiration of the predetermined timer, the base station device that manages the timer may instruct the terminal device 111 to execute the above-mentioned RA procedure. For example, when the base station device 101 manages the timer, the base station device 101 may transmit a PDCCH order again to execute the above-mentioned process again. Furthermore, when the base station device 102 manages the timer, the base station device 102 may transmit, to the base station device 101, a message requesting transmission of a PDCCH order to the terminal device 111. Furthermore, when the terminal device 111 continues to receive a signal from the base station device 102, the base station device 102 may directly instruct the terminal device 111 to execute an RA procedure.

[0034] In addition, the terminal device 111 may transmit a predetermined signal such as a sounding reference signal (SRS) instead of executing the RA procedure again. That is, the base station device 102 may update an appropriate TA value by receiving a predetermined uplink radio signal at the base station device 102. For this reason, the terminal device 111 may transmit a predetermined uplink signal using the TA value set for the base station device 102 at a predetermined timing, such as when a predetermined timer managed within the terminal device 111 expires. In addition, the terminal device 111 may transmit an SR, for example, to receive allocation of radio resources for transmitting a predetermined signal to the base station device 102. In addition, the terminal device 111 may transmit a predetermined uplink signal in response to receiving a transmission instruction for an uplink signal for updating the TA value from the base station device 101 or the base station device 102. For example, in response to a transmission start instruction from the base station device 102, the terminal device 111 may transmit a predetermined signal such as an aperiodic or periodic SRS. In addition, a predetermined signal such as SRS may be transmitted at a timing based on the TA value before being updated. In addition, the terminal device 111 may stop the periodic transmission of a predetermined signal in response to receiving an instruction to stop transmission of the predetermined signal from the base station device 102 during the periodic transmission of the predetermined signal. In addition, in another example, the terminal device 111 may periodically transmit a predetermined signal such as SRS in response to a trigger such as receiving an RA response from the base station device 102 without an explicit instruction from the base station device 102. In addition, when it is necessary to accept an instruction from the base station device 102, the terminal device 111 may continue to receive a signal such as a PDCCH from the base station device 102 after performing the above-mentioned RA procedure and acquiring the TA value. In addition, an instruction to transmit or stop the predetermined signal and a trigger to transmit the predetermined signal may be given by the base station device 101.

[0035] For example, the terminal device 111 may continue to measure the radio quality of a cell that is a candidate for handover destination, and when the radio quality deteriorates to a predetermined level or lower, the terminal device 111 may end the maintenance of the TA value for that cell. The terminal device 111 may decide to end the maintenance of the TA value on its own, or, for example, the base station device 101 that has received a report of the radio quality may decide and instruct the terminal device 111. The base station device 102 may measure the radio quality of a predetermined signal as described above from the terminal device 111 and determine to end the maintenance of the TA value according to the result. In this case, if the terminal device 111 is maintaining a TA value, the base station device 102 may instruct the terminal device 111 to discard the TA value. If the timer for maintaining the above-mentioned TA value is activated when the maintenance of the TA value is terminated, the terminal device 111, the base station device 101, or the base station device 102 stops the timer. The terminal device 111, the base station device 101, or the base station device 102 may prevent the timer for maintaining the above-mentioned TA value from being restarted after the timer expires. Furthermore, when the terminal device 111 continues to receive a signal such as a PDCCH from the base station device 102 in order to maintain the TA value, the terminal device 111 may end the reception of the signal. Note that the base station device 101 or the base station device 102 may instruct the terminal device 111 to end the reception of the signal from the base station device 102.

[0036] As described above, the terminal device 111 may perform handover and connect to the base station device 102 in response to receiving the Cell switch command from the base station device 101. In this case, the connection between the terminal device 111 and the base station device 101 is disconnected, but after this disconnection, the base station device 101 may be set as a candidate base station device for the handover destination of the terminal device 111. For example, the terminal device 111 may maintain the TA value of the base station device 101 while connected to the base station device 102. In addition, the terminal device 111 may continue to receive a predetermined signal (such as a PDCCH) from the base station device 101 in order to maintain the TA value of the base station device 101 while connected to the base station device 102. This prevents unnecessary execution of an RA procedure when the terminal device repeatedly performs handover between multiple base station devices, and enables more efficient communication.

[0037] (Device configuration) Next, the device configuration will be described. FIG. 2 shows an example of the hardware configuration of the base station device and the terminal device of this embodiment. In one example, the base station device and the terminal device are configured to 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 configured to include one or more processing circuits such as a general-purpose CPU (Central Processing Unit) or an ASIC (Application Specific Integrated Circuit), and executes the overall processing of the device and each of the above-mentioned processes by reading and executing a program 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 executed 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 configured, for example, by a removable external storage device. The communication circuit 205 is configured, for example, by a circuit for wireless communication of 5G or its successor standard. Although one communication circuit 205 is illustrated in FIG. 2, the base station device and the terminal device may have multiple communication circuits. For example, the base station device and the terminal device may have wireless communication circuits for 5G and its successor standard, and a common antenna for those circuits. The base station device and the terminal device may have separate antennas suitable for each standard. The base station device may further have a wired communication circuit used when communicating with other base station devices or nodes of the core network. The terminal device may further 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 device and the terminal device may have separate communication circuits 205 for each of a plurality of available frequency bands, or may have a common communication circuit 205 for at least a part of those frequency bands.

[0038] FIG. 3 shows an example of the functional configuration of a base station device. The base station device has, for example, an RRC setting unit 301, a RACH instruction unit 302, an inter-base station communication unit 303, a handover instruction unit 304, and a TA maintenance management unit 305. Note that FIG. 3 mainly shows the functions of the base station device 101 to which the terminal device 111 is connected. The handover destination candidate base station device 102 is similar to a conventional base station device except that it does not transmit some signals of the RA procedure and may transmit some signals to the base station device 101, so it is not shown here. Also, FIG. 3 shows only functions that are particularly related to this embodiment, and various other functions that the base station device may have are not shown. For example, the base station device naturally has other functions that a base station device that complies with 5G and its successor standards generally has. Also, the functional blocks in FIG. 3 are shown in a schematic manner, and each functional block may be realized as an integrated unit, or may be further subdivided. 3 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 present inside the communication circuit 205 executing predetermined software. Note that the details of the processes executed by each functional unit will not be described here in detail, and only the general functions will be outlined.

[0039] The RRC setting unit 301 performs, for example, RRC layer setting for a connected terminal device. For example, the RRC setting unit 301 transmits a handover request to another base station device that is a candidate for a handover destination and provides a neighboring cell, acquires communication setting information from the other base station device, and notifies the terminal device of the communication setting information by a message of the RRC layer. The message of the RRC layer is an RRC Reconfiguration message. Then, the RRC setting unit 301 performs connection processing setting between the terminal device and the other base station device by receiving an RRC Reconfiguration Complete message from the terminal device. The RACH instruction unit 302 instructs the terminal device, for which the RRC layer setting has been completed, to transmit an RA preamble to the other base station device that is a candidate for a handover destination. Here, the instruction to transmit the RA preamble includes 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 that are trying to connect to other base stations, and may also include information on radio resources used for transmitting the RA preamble. The RACH instruction unit 302 can instruct a connected terminal device to transmit an RA preamble to a base station device that is a candidate for handover by, for example, a physical downlink control channel (PDCCH) order. In addition, the RACH instruction unit 302 can instruct the terminal device to retransmit the RA preamble as necessary, or, when configured to repeatedly transmit the RA preamble, can transmit an instruction to stop transmitting the RA preamble to the terminal device.

[0040] The inter-base station communication unit 303 communicates with the base station device that is a candidate for the handover destination. For example, the inter-base station communication unit 303 may notify another base station device that is a destination of the RA preamble to transmit the RA preamble to the connected terminal device. The inter-base station communication unit 303 is also used, for example, to receive information for the RA procedure in the base station device that is a candidate for the handover destination. The inter-base station communication unit 303 may also receive, for example, a message (message 2 or message 4) to be transmitted from the base station device that is a candidate for the handover destination to the terminal device in the RA procedure. The TA value calculated in the base station device that is a candidate for the handover destination may be acquired via the inter-base station communication unit 303. Note that the signal received in the inter-base station communication unit 303 may be transferred to the terminal device via, for example, a RACH indication unit or the like.

[0041] The handover instruction unit 304 instructs the terminal device to execute a handover in response to the terminal device being in a state in which the terminal device should execute a handover, for example, when the reception quality of a radio signal from another base station device at the handover destination in the terminal device becomes higher than the radio quality between the terminal device and the terminal device by a predetermined level or more. The handover instruction unit 304 transmits, for example, a Cell switch command including a cell identifier capable of identifying the base station device at the handover destination to the terminal device. In addition, when a TA value to be used for uplink communication is determined by the terminal device transmitting an RA preamble, the TA value can be included in the command transmitted to the terminal device. This enables the terminal device to transmit an uplink user data signal using the TA value immediately after the handover. In addition, in the present embodiment, an example has been described in which the handover instruction unit 304 transmits the above-mentioned command after causing the terminal device to execute the RA procedure, but in some cases, the above-mentioned command may be transmitted before the RA procedure is executed in response to a request for uplink communication delay in the terminal device. In other words, the RA procedure may be performed before or after the handover command is sent or received.

[0042] The TA maintenance management unit 305 manages the TA value to be used when the terminal device transmits an uplink signal to a base station device that is a candidate for handover. The TA maintenance management unit 305 may, for example, set an expiration date or an update period for the TA value, and instruct the terminal device to retransmit an RA preamble so that the TA value is updated when the expiration date or update period expires. The TA maintenance management unit 305 may also notify the terminal device of the SRS setting in the base station device that is the handover destination, and have the terminal device transmit the SRS using the SRS setting, thereby updating the TA value. The base station device that is the handover destination may have the TA maintenance management unit 305, and the base station device may transmit an instruction to transmit the RA preamble or an instruction to transmit the SRS to the terminal device.

[0043] FIG. 4 shows an example of the functional configuration of a terminal device. The terminal device has, for example, an RRC setting unit 401, a RACH instruction receiving unit 402, a RACH processing unit 403, a handover processing unit 404, and a TA maintenance management unit 405. Note that FIG. 4 shows only functions particularly related to this embodiment, and various other functions that the terminal device may have are omitted. For example, the terminal device naturally has other functions that terminal devices conforming to 5G and subsequent standards generally have. In addition, the functional blocks in FIG. 4 are shown in a schematic manner, and each functional block may be integrated and realized, or may be further subdivided. In addition, 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 present inside the communication circuit 205 executing predetermined software. Note that the details of the processing performed by each functional unit will not be described in detail here, and only the rough functions will be outlined.

[0044] The RRC setting unit 401 performs a connection setting process with a base station device that is a candidate for handover destination by receiving an RRC layer message from a base station device that is a connection destination. The RRC setting unit 401 holds the setting of the currently connected base station device and the setting of the base station device that is a candidate for handover destination in parallel. The RACH instruction receiving unit 402 receives an instruction to transmit an RA preamble from the currently connected base station device. The instruction to transmit an RA preamble may be performed by the PDCCH order. The RACH instruction receiving unit 402 may also receive, for example, an instruction to retransmit an RA preamble or an instruction to stop transmitting an RA preamble. In response to receiving an instruction by the RACH instruction receiving unit 402, the RACH processing unit 403 transmits an RA preamble to the base station device that is a candidate for handover destination. Here, a sequence for generating an RA preamble and information on radio resources for receiving an RA preamble in the base station device that is a candidate for handover destination may be received from the currently connected base station device via the RACH instruction receiving unit 402. The information on the sequence and radio resources may be obtained from system information sent by the base station device that is a candidate for handover. The handover processing unit 404 changes the base station device to which the connection is made in response to receiving a handover command (Cell switch command) from the currently connected base station device. The terminal device does not need to perform any special setting process between the base station device that is a candidate for handover and the terminal device due to the setting process by the RRC setting unit 401 and the RACH process by the RACH processing unit 403. For this reason, when the terminal device is notified of the allocation of uplink radio resources in the base station device that is a candidate for handover in the received command, the terminal device can immediately transmit an uplink signal using the radio resources.

[0045] When the TA maintenance management unit 405 acquires a TA value for a base station device that is a candidate for a handover destination, the TA maintenance management unit 405 maintains and manages the TA value. For example, the TA maintenance management unit 405 may retransmit an RA preamble and update the TA value without receiving an instruction from the currently connected base station device or the base station device that is a candidate for a handover destination in response to expiration of the validity period of the TA value. Also, the TA maintenance management unit 405 may update the TA value by transmitting a predetermined signal such as an RA preamble or an SRS for the base station device that is a candidate for a handover destination in response to an instruction from the currently connected base station device or the base station device that is a candidate for a handover destination. Also, when the TA maintenance management unit 405 receives information indicating that the maintenance of the TA value should be terminated from the currently connected base station device, the TA maintenance management unit 405 terminates the maintenance of the TA value. Here, the TA maintenance management unit 405 may immediately discard the TA value that has been terminated, or may maintain the TA value within the validity period. Also, the TA maintenance management unit 405 may maintain and manage the TA value for the base station device that is the handover source after a handover is executed. In other words, the TA maintenance management unit 405 may maintain the TA value of the currently connected base station device as the TA value of the candidate base station device as the handover destination after handover from that base station device to another base station device is performed.

[0046] (Processing flow) FIG. 5 shows an example of the flow of processing executed in the wireless communication system of this embodiment. Note that the example shown in FIG. 5 is just one example, and each procedure can be modified as described above. For example, FIG. 5 shows an example in which a sequence specific to the terminal device is notified to the terminal device as a sequence for generating an RA preamble to be transmitted to a candidate base station device of the handover destination, but this may be replaced with the above-mentioned procedure in which a group of sequences usable by multiple terminal devices is notified. Note that in the example of FIG. 5, it is assumed that a connection is established between a first base station device of the handover source and the terminal device.

[0047] The first base station device first transmits a HANDOVER REQUEST message to a second base station device that forms an adjacent cell and is a candidate for the handover destination of the terminal device (S501). When the second base station device receives this message, it returns a HANDOVER REQUEST ACK message including information on communication parameters to be used when the terminal device connects to the first base station device (S502). Then, the first base station device notifies the terminal device of an RRC Reconfiguration message including communication parameters notified from the second base station device (S503). When the terminal device receives this message, it transmits an RRC Reconfiguration Complete message to the first base station device to complete the setting process using the message of the RRC layer. With this process, the terminal device does not need to perform the setting process of the RRC layer after being instructed by the first base station device to perform a handover to the second base station device. Note that, although FIG. 5 shows an example in which there is only one candidate base station device for the handover destination, the same setting process can be performed for multiple candidate base station devices.

[0048] Thereafter, the terminal device measures a radio signal (e.g., SSB) transmitted from the second base station device, which is a candidate for the handover destination (S505), and notifies the first base station device of the measurement result (S506). Here, the first base station device may notify the second base station device of the measurement result. In this case, the second base station device can identify a rough beam to be used for communication with the terminal device. The second base station device may transmit CSI-RS to form a more fine beam and have the terminal device measure it. In this case, the second base station device may transmit setting information of CSI-RS to the first base station device and have the terminal device transfer the setting information. The terminal device measures CSI-RS based on the setting information and reports the measurement result to the first base station device to which it is connected. Then, the first base station device transfers the measurement result to the second base station device. This enables the second base station device to transmit a downlink signal using a fine beam immediately after the terminal device has handed over to the second base station device.

[0049] Next, the first base station device instructs the terminal device to transmit an RA preamble to the second base station device (S507). Here, this instruction may include information specifying the sequence that the terminal device should use to generate the RA preamble and the radio resource to which the RA preamble should be transmitted. The radio resource to which the RA preamble should be transmitted may be specified based on system information in the second base station device that is transmitted from the second base station device and measured by the terminal device, for example. That is, this information does not need to be acquired from the connected base station device. Also, the sequence to be used when generating the RA preamble may be specified in advance. In one example, information on this sequence may be notified to the terminal device in S503. The terminal device transmits the RA preamble to the second base station device in accordance with the instruction (S508). At this time, the first base station device may notify the second base station device that the terminal device plans to transmit the RA preamble. At this time, the first base station device may notify the second base station device of information on the sequence that the terminal device uses to generate the RA preamble. This enables the second base station device to detect the RA preamble transmitted by the terminal device. Note that this is one example, and for example, the HANDOVER REQUEST in S501 may implicitly notify that the terminal device plans to transmit an RA preamble. In this case, the second base station device may specify the sequence that the terminal device should use to generate the RA preamble in the HANDOVER REQUEST ACK in S502. Note that these are one example, and information on the sequence that the terminal device should use to generate the RA preamble may be shared between the first base station device and the second base station device by other messages. The second base station device calculates a TA value based on the RA preamble transmitted from the terminal device (S509). The second base station device may notify the first base station device of the calculated TA value (S510). Note that this TA value may be managed by both the first base station device and the second base station device, or may be managed by one of them.Also, this TA value may be notified from the second base station device to the first base station device, for example, in response to the first base station device deciding to execute handover of the terminal device. For example, the first base station device may notify the second base station device of the execution of handover, and in response to the notification, the second base station device may notify the first base station device of the TA value.

[0050] At this point, the terminal device has not yet handed over to the second base station device. In this state, the terminal device continuously measures the radio signal from the second base station device (S511) and notifies the connected first base station device of the measurement result (S512). The first base station device may decide to perform handover according to the measurement result (S513). For example, when a predetermined condition is satisfied, such as when the radio quality of the signal from the second base station device becomes higher than the radio quality of the signal from the first base station device by a predetermined level or more, the first base station device may decide to handover the terminal device to the second base station device. Based on this decision, the first base station device transmits a command to instruct the terminal device to perform handover (S514). This command may include, for example, the TA value acquired in S510 and information on uplink resources for signal transmission immediately after handover. The TA value may be transferred to the terminal device immediately after being notified to the first base station device in S510, and in that case, the TA value does not need to be included in this command. The first base station device may specify uplink resource information by inquiring to the second base station device in advance. The first base station device may instruct the terminal device to use a reserved predetermined radio resource available in the second base station device without making an inquiry. When the terminal device receives this command, it may change the connection destination base station device to the second base station device and transmit a response signal to the command in the resource specified in the received command to the second base station device (S515). This allows the second base station device to confirm that the terminal device has handed over to its own device, and thereafter, communication of user data may be performed between the terminal device and the second base station device (S516). The terminal device may transmit user data to the second base station device in addition to or instead of the response signal of S515.

[0051] In this way, according to the present embodiment, it is possible to significantly reduce the time from when the handover source base station device decides to actually handover the terminal device to another base station device until the terminal device starts communicating user data with the post-handover base station device. This makes it possible to contribute to Goal 9 of the United Nations-led Sustainable Development Goals (SDGs) "Build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0052] 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.

Claims

1. A terminal device, A receiving means for receiving, from a connected base station device, a radio resource control (RRC) layer configuration for another base station device that is a candidate for a handover destination from the base station device, and receiving, from the base station device, a physical downlink control channel (PDCCH) order instructing the terminal device to transmit a random access preamble to the other base station device without executing a handover to the other base station device; a transmitting means for transmitting the random access preamble to the other base station device in accordance with the instruction; an execution means for executing a handover to the other base station when a command instructing a handover to the other base station is received from the base station; A terminal device comprising:

2. 2. The terminal device according to claim 1, wherein the receiving means receives an instruction to transmit the random access preamble to a plurality of the other base station devices via one of the PDCCHs.

3. The receiving means receives, from the base station device, a sequence specific to the terminal device used when the terminal device generates the random access preamble, and information for identifying a radio resource available when transmitting the random access preamble; The transmitting means generates the random access preamble using the sequence specific to the terminal device and transmits the random access preamble in a radio resource specified by the information.

2. The terminal device according to claim 1 .

4. The receiving means receives, from the base station device or the other base station device, a timing advance value (TA value) specified in the other base station device based on the random access preamble transmitted by the terminal device.

2. The terminal device according to claim 1 .

5. A means for maintaining the TA value is further provided, 5. The terminal device according to claim 4, wherein the transmitting means transmits a predetermined uplink signal to the other base station device in response to the expiration of the TA value or the arrival of an update period.

6. The terminal device according to claim 5, characterized in that when the transmitting means is instructed by the base station device to transmit the random access preamble in response to the expiration of the TA value or the arrival of an update period, the transmitting means transmits the random access preamble to the other base station device as the specified uplink signal.

7. The terminal device according to claim 5, characterized in that the transmitting means transmits a sounding reference signal (SRS) to the other base station device at a transmission timing determined using the TA value as the specified uplink signal at a specified period or in response to an instruction to transmit the SRS from the other base station device.

8. 6. The terminal device according to claim 5, wherein the maintaining means terminates maintaining the TA value in response to a radio quality between the terminal device and the other base station device falling below a predetermined level.

9. the transmitting means repeatedly transmits the random access preamble at a predetermined period until a transmission stop instruction of the random access preamble is received from the base station device, and stops transmitting the random access preamble in response to the reception of the transmission stop instruction.

4. The terminal device according to claim 3.

10. The receiving means receives, from the base station device, a set of sequences usable when generating the random access preamble to be transmitted to the other base station device; the transmitting means generates the random access preamble using one sequence selected from the group of sequences, and transmits the random access preamble to the other base station device; 2. The terminal device according to claim 1 .

11. The receiving means receives a response to the random access preamble from the base station device or the other base station device, When the response is received, the transmitting means transmits a Transmitting a first message to the other base station device; the receiving means receives a second message including identification information of the terminal device from the base station device or the other base station device in response to the first message; The terminal device according to claim 10 .

12. The terminal device according to claim 11, characterized in that the receiving means receives from the base station device a notification of at least one of the timing when the response to the random access preamble is transmitted from the other base station device and the timing when the second message is transmitted from the other base station device.

13. The transmitting means repeatedly retransmits the random access preamble at a predetermined period in response to an instruction to transmit the random access preamble; The receiving means receives an instruction to stop transmitting the random access preamble from the base station device, The transmitting means stops transmitting the random access preamble in response to receiving the transmission stop instruction.

2. The terminal device according to claim 1 .

14. A control method executed by a terminal device, comprising: receiving, from a base station currently connected to the terminal device, a radio resource control (RRC) layer configuration for another base station device that is a candidate for a handover destination from the base station device; receiving, from the base station device, a physical downlink control channel (PDCCH) order instructing the terminal device to transmit a random access preamble to the other base station device without executing a handover to the other base station device; Transmitting the random access preamble to the other base station device in accordance with the instruction; When a command instructing a handover to the other base station device is received from the base station device, executing a handover to the other base station device; A control method comprising:

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