Terminal device and control method for performing 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 technology in a cellular communication system.

Background Art

[0002] In a mobile communication system, as the terminal device moves, a handover is performed to switch the cell (base station device) to which the terminal device is connected. Conventionally, after receiving a handover instruction from the base station device, the terminal device performs a process of establishing synchronization and setting in the cell to which the connection is to be switched and then establishes a connection. This process for switching the connection destination takes a certain amount of time, and the terminal device cannot communicate user data during this process. On the other hand, in the method described in Non-Patent Document 1, the base station device from which the handover is to be performed provides communication parameters in the radio resource control (RRC) layer for connection with another base station device as the handover destination to the terminal device before the handover is actually performed. Then, the base station device from which the handover is to be performed transmits a layer 1 or layer 2 command for instructing the cell switch to the terminal device at the timing when the handover is to be performed. The terminal device can complete the handover by executing a random access procedure with the base station device as the handover destination in response to receiving the command, without performing subsequent ground processing in the RRC layer. By such a technique, deterioration of communication efficiency related to handover can be suppressed.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] This invention provides a technology to further improve communication efficiency related to handover. [Means for solving the problem]

[0005] A terminal device according to one aspect of the present invention includes: receiving means for receiving a physical downlink control channel (PDCCH) order from a connected base station device which the terminal device receives the setting of the radio resource control (RRC) layer for another base station device that is a candidate for handover from the base station device, and which instructs the terminal device from the base station device to transmit a random access preamble to the other base station device without performing a handover to the other base station device; transmitting means for transmitting the random access preamble to the other base station device in accordance with the instruction; and executing means for performing a handover to the other base station device when the base station device receives a command from the base station device instructing a handover to the other base station device. Furthermore, the transmitting means retransmits the random access preamble when it receives a signal from the base station device requesting retransmission of the random access preamble. do. [Effects of the Invention]

[0006] According to the present invention, communication efficiency related to handover can be improved. [Brief explanation of the drawing]

[0007] [Figure 1] This is a diagram showing an example configuration of a wireless communication system. [Figure 2] This figure shows an example of the device's hardware configuration. [Figure 3] This figure shows an example of the functional configuration of a base station device. [Figure 4] This figure shows an example of the functional configuration of a terminal device. [Figure 5] This diagram shows an example of the processing flow performed in a wireless communication system. [Modes for carrying out the invention]

[0008] The embodiments will be described in detail below with reference to the attached drawings. Note that the following embodiments do not limit the invention as defined in the claims, and not all combinations of features described in the embodiments are essential to the invention. Two or more of the features described in the embodiments may be combined in any way. Furthermore, identical or similar configurations will be given the same reference numeral, and redundant descriptions will be omitted.

[0009] (System Configuration) Figure 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 cellular communication standards such as the 5th generation (5G) and its successor standards of the 3rd Generation Partnership Project (3GPP®). The wireless communication system consists of base station equipment 101 and base station equipment 102 and terminal equipment 111. Although Figure 1 shows only two base station equipment and one terminal equipment for the sake of simplicity, a large number of base station equipment and terminal equipment can naturally exist.

[0010] In Figure 1, terminal device 111 is attempting to move from the cell range formed by base station device 101 to the cell range formed by base station device 102. At this time, terminal device 111 will perform a 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 performed to shorten the period during which the terminal device 111 is unable to communicate user data due to the handover. Specifically, the base station device 101 that is the handover source performs settings at the Radio Resource Control (RRC) layer (settings using RRC messages) before actually handing over the terminal device 111. In other words, the settings at the RRC layer that are performed when the terminal device 111 receives a handover instruction from the base station device 101 that is the handover source and starts the handover process in a conventional handover are notified to the terminal device 111 prior to the timing when the handover process should actually be performed. In these settings, communication parameters used in connection and communication with the candidate base station device 102 to which the handover is to be performed are notified to the terminal device 111. The communication parameters include, for example, beam settings, parameters for executing random access procedures, and radio resources (frequency and time resources) to be used at the candidate base station device 102 to which the handover is to be performed. Furthermore, the originating base station device 101 may send a measurement instruction to the terminal device 111 for the synchronization signal (SS) / physical broadcast channel (PBCH) block (SSB) transmitted from the candidate base station device 102 to which the handover will take place.

[0012] Terminal device 111 measures the SSB transmitted from candidate base station device 102, the handover destination, in accordance with measurement instructions from the connected base station device 101. Terminal device 111 then notifies the connected base station device 101 of the measurement results. Here, base station device 101 may also notify the candidate base station device 102, the handover destination, of the measurement results. This allows base station device 102 to determine the approximate direction of the beam to be used for communication with terminal device 111 when terminal device 111 actually performs a handover. Here, base station device 102 may further notify terminal device 111 of the channel status information (CSI)-reference signal (RS) transmission settings in order to specify the detailed beam to be used in the downlink. This notification is performed by base station device 102 notifying base station device 101 of the CSI-RS transmission settings, and base station device 101 forwarding these transmission settings to the connected terminal device 111. When terminal device 111 receives the transmission settings, it measures the CSI-RS transmitted from base station device 102 according to the information and notifies the connected base station device 101 of the measurement results. Base station device 101 then forwards the measurement results to base station device 102. In this way, base station device 102 can obtain the CSI-RS measurement results from terminal device 111. As a result, base station device 102 can determine the detailed beam to be used for downlink communication after terminal device 111 has handed over, before the handover actually takes place. This makes it possible to omit the process of determining the beam direction in detail after the handover has taken place, and shortens the period during which the efficiency of user data communication from terminal device 111 is reduced. As an example, the CSI-RS transmission settings described above can be instructed to the terminal device 111 using CSI-RS measurement settings, which consist of CSI-RS radio resource settings (CSI-RS-Resource) and reporting settings (csi-ReportConfig) for the base station device 102, which are pre-configured as communication parameters in the terminal device 111 before handover. CSI-RS-Resource includes the time position and frequency position of the radio resource block from which CSI-RS is transmitted.For example, the terminal device 111 can be instructed to measure a specific CSI-RS radio resource by identifying the CSI-RS-Resource using the CSI-RS-Resource id in the Media Access Control / Control Element (MAC CE) or Downlink Control Information (DCI). Furthermore, the reporting timing for aperiodic or periodic CSI reports can be set in csi-ReportConfig. For example, the terminal device 111 can be instructed to report the CSI-RS measurement results at a specific timing by identifying the csi-ReportConfig using the csi-ReportConfig id in the MAC CE or DCI. Additionally, when the base station device 101 makes the above-mentioned measurement settings to the terminal device 111, it may include information such as a cell identifier in the MAC CE or DCI to help the terminal device 111 identify which base station device's CSI-RS transmission settings are being used.

[0013] In this embodiment, in order to suppress the degradation of communication efficiency during further handovers, the terminal device 111 is configured to perform a random access (RA) procedure before it is decided 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 before the handover actually takes place. The base station device 101, to which the terminal device 111 is connected, sends an instruction to the terminal device 111 to send a random access preamble (RA preamble) to the base station device 102. The terminal device 111 then sends the RA preamble in accordance with this instruction using the radio resources prepared for the RA procedure at the base station device 102. This radio resource and the sequence information used to generate the RA preamble may be notified to the terminal device 111 from the base station device 101, for example, during the RRC layer configuration process described above. Alternatively, this information may be notified to the terminal device 111 when the base station device 101 issues an instruction to send the RA preamble. This instruction can be carried out, for example, by using the PDCCH order, which is a procedure for causing the conventional base station device 101 to transmit an RA preamble. PDCCH stands for Physical Downlink Control Channel. Conventional PDCCH orders are used when the uplink synchronization between the terminal device 111 and the connected base station device 101 is lost, causing the terminal device 111 to transmit an RA preamble to the base station device 101. In contrast, this embodiment differs in that it is used to transmit an RA preamble to a base station device 102 that is not connected. Through this procedure, the RA preamble transmitted from the terminal device 111 is received by the base station device 102. The base station device 102 can then determine the timing advance value (TA value) that the terminal device 111 should use when transmitting an uplink signal to the base station device 102. Then, the base station device 102 notifies the terminal device 111 of the TA value, for example, by having the terminal device 111 notify the base station device 101 of the TA value, and the base station device 101 forwards that TA value to the terminal device 111, thereby notifying the terminal device 111 of the TA value.As a result, terminal device 111 can establish synchronization of the uplink with base station device 102 at the time base station device 101 instructs it to hand over to base station device 102, and can transmit uplink user data to base station device 102 without having to perform the RA procedure again. Details of the RA procedure using PDCCH order executed in this embodiment will be described later.

[0014] Subsequently, the base station device 101 may notify the terminal device 111 to perform a handover at predetermined triggers, such as when the reception quality of the radio signal from base station device 101 at the terminal device 111 deteriorates by a predetermined level compared to the reception quality of the radio signal from base station device 102. This notification may be made 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). This command includes identification information of the cell formed by the handover destination base station device 102. Furthermore, if multiple settings are configured in the RRC layer used for communication with base station device 102, an identifier uniquely associated with each setting in the RRC layer, such as an RRC-TransactionIdentifier, may be included in this command. At this time, the TA value, which has been determined in base station device 102 and notified to base station device 101 in advance by the execution of the RA procedure as described above, may also be notified from base station device 101 to terminal device 111. Furthermore, information regarding the beam determined by the base station device 102 and notified to the base station device 101 based on the SSB and CSI-RS measurements performed in advance as described above 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 the information regarding those reserved radio resources. Then, the base station device 101 may notify the terminal device 111 of the information regarding those uplink radio resources in the command described above. In this case, the terminal device 111 can transmit user data immediately after receiving the command without performing control communication with the base station device 102 after the handover. Note that the uplink radio resources for user data may be notified to the terminal device 111 from the base station device 102 after the handover. In this case as well, since the RA procedure is performed prior to receiving the command as described above, the RA procedure after receiving the command is omitted, and the terminal device 111 can immediately transmit user data on the specified radio resources.Furthermore, the base station device 101 before the handover can, for example, obtain a buffer status report (BSR) from the connected terminal device 111 and notify the handover destination base station device 102 of its contents. 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. Alternatively, the terminal device 111 may switch its connection destination to the base station device 102 and send a scheduling request (SR) to the base station device 102 in response to receiving the above command from the base station device 101.

[0015] Furthermore, terminal device 111 may send an acknowledgment (Ack) to base station device 101 when it receives the above-mentioned MAC CE command. Alternatively, terminal device 111 may switch its connection destination to base station device 102 and send an Ack to base station device 102 when it receives the above-mentioned command. In this case, terminal device 111 can send an Ack to base station device 102 using the uplink radio resources allocated as described above. Base station device 102 can then forward the acknowledgment to base station device 101. This allows base station device 101 to determine that terminal device 111 has switched its connection destination to base station device 102, and for example, discard the context information of terminal device 111. Furthermore, if base station device 101 does not receive an Ack from terminal device 111, it may resend the above-mentioned command to ensure that the handover is performed correctly. Furthermore, when terminal device 111 receives a command via DCI as described above, it may implicitly notify the handover destination base station device 102 that the command has been received by transmitting a predetermined signal such as SR or BSR. The uplink radio resources at base station device 102 for transmitting this SR or BSR may be notified to terminal device 111 via base station device 101 in the DCI of the command. For example, the predetermined signal such as BSR may be transmitted using a dedicated MAC CE indicating that a handover based on the above command has been performed, or it may be transmitted in the same way as a conventional BSR. In addition, a dedicated MAC CE indicating that a handover based on the above command has been performed may be notified to base station device 102 separately from the SR or BSR.

[0016] Furthermore, a timer may be used to determine whether the handover of terminal device 111 was successful or not. For example, when terminal device 111 receives the above command, it starts timing a predetermined period using the first timer. After the start of timing by the first timer, terminal device 111 may receive PDCCH from base station device 102 in parallel with receiving PDCCH from base station device 101. That is, terminal device 111 is made able to receive both the above command retransmitted from base station device 101 and the predetermined signal transmitted from base station device 102 when the handover is successful. For example, terminal device 111 may continuously observe PDCCH from both base station devices by switching between receiving signals from base station device 101 and receiving signals from base station device 102 in a time-division multiplexer. Also, terminal device 111 may, for example, send a response signal such as the above Ack to base station device 102 after receiving the command and start a second timer. The second timer measures a shorter time than the time duration set by the first timer. The response signal may also be a signal different from the Ack, such as a Cell switch command response. In one example, the terminal device 111 may determine that the handover was successful if it does not receive a retransmission instruction for the response signal or a negative response (Nack) indicating that it could not receive the response signal from the base station device 102 before the second timer expires. If the terminal device 111 receives a retransmission instruction or a Nack, it retransmits the response signal, resets the second timer, and starts timing again. The terminal device 111 may determine that the handover failed if it receives a retransmission instruction or a Nack after the first timer has expired.

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

[0018] Furthermore, if terminal device 111 determines that the handover has failed, it may terminate reception of PDCCH from base station device 102 and continue reception of PDCCH from base station device 101. In this case, terminal device 111 may also notify base station device 101 that the handover has failed. Also, if base station device 102 receives a response signal from terminal device 111, it may determine that the handover was successful and notify base station device 101 of the success of the handover. Based on this notification, base station device 101 may delete the context information of terminal device 111.

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

[0020] Information on the length of time to be measured by the above timer and the upper limit value 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 above RRC layer. When there are multiple candidate cells for the handover destination, information such as the timer value and the upper limit value of the number of retransmissions may be notified to the terminal device 111 individually for each of the multiple cells. Also, at least some of the multiple cells may be grouped, and common setting information may be notified to the terminal device 111 for each group. Furthermore, common setting information may be notified to the terminal device 111 for all cells. Also, these setting information may be notified to the terminal device 111 by, for example, a Cell switch command. In addition, for transmitting the above response signal, uplink radio resources are allocated, and information on the upper limit value of the number of retransmissions may be notified to the terminal device 111 in the uplink grant indicating the allocation.

[0021] As described above, it is possible to shorten the period during which the terminal device 111 cannot perform user data communication when handing over from the base station device 101 to the base station device 102. Thereby, for example, it becomes possible to provide communication services that require low latency and high reliability. Also, by shortening the period during which user data communication cannot be performed, 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 using PDCCH order executed in this embodiment will be described in more detail. As an example, we will describe the case in which a unique sequence for generating the RA preamble is assigned to the terminal device 111. First, base station device 101 notifies base station device 102 to initiate the RA procedure to terminal device 111. This makes it possible for base station device 101, to which terminal device 111 is connected, and base station device 102, a candidate for handover, to commonly recognize that the RA procedure is being executed by terminal device 111. Then, base station device 101 instructs terminal device 111 to execute the RA procedure using PDCCH as described above. At this time, base station device 101 may notify terminal device 111 of information for identifying radio resources (frequency and time resources), such as the SS / PBCH index and PRACH Mask index, which are set by base station device 102 for receiving the RA preamble, and information indicating the sequence of RA preambles to be used (Random Access Preamble Index). Furthermore, if there are multiple candidate cells for the handover destination, the base station device 101 may consolidate information about those multiple cells and notify the terminal device 111 via a single PDCCH. Alternatively, the base station device 101 may notify the terminal device 111 of information about at least some of the multiple cells via individual PDCCHs. As described above, the RA preamble sequence 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. This allows the base station device of a candidate handover destination to uniquely identify the terminal device 111 as the source of the RA preamble when it receives the corresponding sequence. Based on the information received from the base station device 101, the terminal device 111 transmits the RA preamble for the candidate cell for the handover destination using the specified sequence in the radio resource corresponding to that cell.For example, base station device 101 notifies terminal device 111 of the radio resources at base station device 102 where transmission of RA preambles is permitted and the sequence that terminal device 111 should use. Then, terminal device 111 transmits an RA preamble to base station device 102 using the sequence notified by base station device 101 on the radio resources notified by base station device 101. Base station device 102, a candidate for handover, performs RA preamble detection processing using the sequence designated for terminal device 111 on the radio resources for the RA procedure and determines whether or not it was able to receive the RA preamble from terminal device 111.

[0023] At this point, the terminal device 111 does not need to be notified of the TA value, and since a unique sequence for the RA preamble is specified for the terminal device 111, contention resolution is not necessary. Therefore, the base station device 102 does not need to send an RA response to the terminal device 111. However, the base station device 102 can determine the TA value that the terminal device 111 should use based on the timing of the RA preamble reception. This TA value is then notified, for example, from the base station device 102 to the base station device 101. The base station device 101 can then notify the terminal device 111 of this value, for example, along with a command described later. If an RA response is not sent, the terminal device 111 cannot recognize whether the RA preamble was received successfully. Therefore, in this example, if the base station device 102 is unable to receive the RA preamble from the terminal device 111 within a predetermined period, it can have the terminal device 111 resend the RA preamble via the base station device 101. For example, base station device 102 sends a predetermined notification to base station device 101 indicating that it was unable to receive the RA preamble from terminal device 111. When base station device 101 receives this predetermined notification, it sends another PDCCH order to terminal device 111, causing it to retransmit the RA preamble. Terminal device 111 retransmits the RA preamble when it receives the PDCCH order again. On the other hand, terminal device 111 may choose not to retransmit the RA preamble unless it receives the PDCCH order again. In other words, in this example, terminal device 111 will not retransmit the RA preamble unless specifically instructed otherwise.

[0024] In the example above, we described a case where the base station device 102 does not send an RA response, but the base station device 102 may also send an RA response. In this case, the terminal device 111 can obtain the TA value from the RA response. The terminal device 111 may also spontaneously resend the RA preamble if it did not receive an RA response. If the terminal device 111 receives an RA response, it will not perform the subsequent processing of the RA procedure (processing of messages 3 and later in the RA procedure). That is, the terminal device 111 may obtain the TA value from the RA response and terminate the RA procedure without performing any further processing. The base station device 102 also does not include the allocation of radio resources for message 3 in the RA response. That is, in a normal RA procedure, the uplink radio resources for sending message 3 are specified in the RA response, but since message 3 does not need to be sent, the radio resources for that purpose do not need to be specified.

[0025] In the processing example described above, it was explained that terminal device 111 does not retransmit the RA preamble unless there is a retransmission instruction from base station device 101 or base station device 102, but this is not limited to this. For example, terminal device 111 may retransmit the RA preamble at predetermined intervals, and may repeatedly transmit the RA preamble at predetermined intervals unless it receives a stop transmission instruction for the RA preamble from base station device 101 or base station device 102. For example, base station device 101 or base station device 102 may send a stop transmission instruction for the RA preamble to terminal device 111 when it determines that terminal device 111 should stop transmitting the RA preamble, such as when the RA preamble is successfully detected by base station device 102 or when a predetermined period of time has elapsed.

[0026] <Processing Example 2> In the above-described processing example 1, a unique RA preamble sequence is specified for the terminal device 111, but this is not limited to this. In another processing example, the terminal device 111 transmits an RA preamble using one sequence selected from a group of sequences used in contention-based RA procedures in a candidate cell to be handed over. The base station device 101 obtains the SSB measurement result of the candidate cell to be handed over from the terminal device 111 as described above, and based on the measurement result, selects cells in which the terminal device 111 should execute the RA procedure. For example, the base station device 101 may select cells with radio quality above a predetermined level as cells in which the RA procedure should be executed. Alternatively, the base station device 101 may select one or more predetermined numbers of cells with good radio quality as cells in which the RA procedure should be executed. The base station device 101 then notifies the terminal device 111 of the SSB index corresponding to the selected cells. Based on that SSB index, the terminal device 111 can identify the radio resources (frequency and time resources) for transmitting the RA preamble. Here, the base station device 102 may specify a Cell-Radio Network Temporary Identifier (C-RNTI) for RA procedure communication and post-conflict resolution communication, and notify the terminal device 111 of the C-RNTI via the base station device 101. Alternatively, a temporary identifier (Temporary C-RNTI (TC-RNTI)) may be notified to the terminal device 111 instead of the C-RNTI.

[0027] Subsequently, terminal device 111 transmits an RA preamble. For example, terminal device 111 selects one sequence from a group of sequences corresponding to the notified SSB index to generate an RA preamble, and transmits that RA preamble on the radio resource corresponding to that SSB index. Here, we assume that terminal device 111 selects one sequence from a group of sequences available for access to base station device 102, and transmits the RA preamble generated using the selected sequence on the base station device 102's radio resource for RA procedures. Here, the group of sequences available for generating the RA preamble can also be used by other terminal devices, and base station device 102 cannot recognize whether it has received an RA preamble from terminal device 111 simply by detecting the RA preamble. For this reason, when base station device 102 detects an RA preamble, it transmits an RA response for subsequent conflict resolution. After transmitting the RA preamble, terminal device 111 starts a timer (ra-ResponseWindow) to measure a certain period of time for detecting the RA response. If terminal device 111 receives an RA response during this specified period, it stops retransmitting the RA preamble. If terminal device 111 does not receive an RA response during this specified period, it retransmits the RA preamble, resets the timer, and restarts. The maximum number of RA preamble retransmissions is specified, for example, by a signal transmitted from base station device 101 or base station device 102. As described above, base station device 101 may notify terminal device 111 of its identification information (C-RNTI or TC-RNTI), but instead, base station device 102 may notify terminal device 111 of the TC-RNTI in the RA response. The UE can use Random Access (RA)-RNTI to determine the frequency domain and time domain resource allocation of the physical downlink shared channel (PDSCH) to which the RA response is transmitted.In other words, the resource allocation is specified by the DCI format 1_0 with CRC scrambled by RA-RNTI transmitted by the base station device 102, and the terminal device 111 can acquire this information using RA-RNTI. RA-RNTI is identified based on the arrangement of radio resources used to transmit the RA preamble (time and frequency positions identified by symbols, slots, and frames). The terminal device 111 then receives an RA response corresponding to the RA preamble transmitted by its own device, according to the resource allocation acquired using RA-RNTI.

[0028] When terminal device 111 receives an RA response, it uses the uplink radio resources specified in the RA response to send message 3 of the RACH procedure to base station device 102. Here, terminal device 111 receives the TA value to be used in the RA response and uses that TA value to control the timing of sending message 3. At this point, it is not yet determined whether this TA value is the TA value for terminal device 111. That is, other terminal devices that have sent an RA preamble at the same subframe and frequency position may also identify the same RA-RNTI and receive the RA response. Therefore, it is not yet determined at this point whether the TA value acquired by terminal device 111 is the value determined based on the RA preamble transmitted by terminal device 111. If C-RNTI is used to resolve this situation (i.e., conflict resolution), terminal device 111 sends its own device identification information (C-RNTI), which has been notified as described above, to base station device 102 via message 3. Furthermore, the terminal device 111 starts a timer (ra-ContentionResolutionTimer) to measure a certain period of time for determining whether or not conflict resolution at the base station device 102 was successful. If the terminal device 111 receives a DCI addressed to its own identification information (C-RNTI) before this 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 sends its own identification information (for example, InitialUE-Identity, which consists of 5G-S-TMSI and a random number) to the base station device 102 via message 3. After that, the terminal device 111 starts a timer (ra-ContentionResolutionTimer) to measure a certain period of time for determining whether or not conflict resolution at the base station device 102 was successful. The terminal device 111 can determine that the TA value has been set based on the RA preamble it transmitted if the message 4, which is instructed to be sent via DCI addressed to its own identification information (TC-RNTI) before this certain period expires, contains the identification information of its own device (e.g., InitialUE-Identity) that it transmitted to the base station device 102 in message 3.Note that at this stage, only the TA value is determined (synchronization of the uplink is established), and the connection between the terminal device 111 and the base station device 102 is not established. On the other hand, if the terminal device 111 does not receive a DCI addressed to its own identification information (C-RNTI or TC-RNTI) within the aforementioned period, it will retransmit the RA preamble because the TA value obtained in the RA response was not the TA value based on the RA preamble transmitted by the device.

[0029] Furthermore, the signals transmitted from the base station device 102 to the terminal device 111 may also 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) and 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 the information corresponding to those messages. As described above, when radio signals are transmitted directly from the base station device 102 to the terminal device 111, the terminal device 111 needs to process receiving messages 2 and 4 from the base station device 102 while continuing to receive the PDCCH of the base station device 101. In contrast, when messages 2 and 4 are notified to the terminal device 111 via the base station device 101, the terminal device 111 does not need to receive radio signals from the base station device 102. Furthermore, it is also possible that only one of either message 2 or message 4 is notified to the terminal device 111 via the base station device 101.

[0030] Furthermore, if the terminal device 111 needs to receive signals from a candidate cell to which it will be handed over, the base station device 101 may notify the terminal device 111 of the timing at which it should receive downlink signals (e.g., PDCCH) from the base station device of that candidate cell. For example, if messages 2 and 4 described above are transmitted directly from the base station device 102 to the terminal device 111, the terminal device 111 needs to receive signals such as PDCCH from the base station device 102. In this case, the base station device 101 may notify the terminal device 111 of the timing at which signals such as PDCCH are transmitted from the base station device 102. Furthermore, if there are multiple candidate cells and the timing of their PDCCH signals does not overlap, the base station device 101 may notify the base station device of the candidate cell of a change in the reception timing. Furthermore, if there are multiple candidate cells and the timing of their PDCCH signals overlaps, the terminal device 111 may notify the base station device 101 of the cell in which the overlapping reception timing is occurring. After transmitting the RA preamble, the terminal device 111 begins receiving PDCCH for candidate cells that are the destination of the RA preamble, for example, triggered by the activation of the ra-ResponseWindow. At this time, the terminal device 111 also continues to receive PDCCH from the connected base station device 101, and may be receiving PDCCH from multiple base station devices in parallel. Subsequently, the terminal device 111 may terminate the reception of PDCCH for candidate cells, for example, triggered by the expiration of the ra-ResponseWindow. Alternatively, the terminal device 111 may terminate the reception of PDCCH based on instructions from, for example, the base station device of the candidate cell (e.g., base station device 102) or base station device 101. The instructions from the base station device of the candidate cell may be transmitted directly from that base station device to the terminal device 111, or transmitted via base station device 101.

[0031] In the processing example described above, the terminal device 111 stops transmitting the RA preamble because the timer expires without receiving an RA response, but this is not the only example. The terminal device 111 may, for example, repeatedly transmit the RA preamble at a predetermined interval and stop transmitting the RA preamble when it receives a transmission stop instruction from the base station device 101 or base station device 102. This transmission stop instruction may be a different signal from the RA response. For example, the base station device 101 or base station device 102 may send a transmission stop instruction to the terminal device 111 when it determines that the terminal device 111 should stop transmitting the RA preamble, such as when a predetermined period of time has elapsed. This makes it possible to instruct the cessation of RA preamble transmission by network control.

[0032] (Handling of acquired TA values) Furthermore, the above process ensures that at least one of the terminal device 111 or base station device 102 retains the TA value that should be used when the terminal device 111 transmits a signal to the base station device 102. However, it is anticipated that after a certain period of time, this TA value may become unsuitable for the real-world environment. Therefore, this processing example describes how to handle the acquired TA value, including methods for maintaining the TA value at an appropriate value or for ending the maintenance of the TA value.

[0033] In the first example, a timer is used to manage the expiration date and update cycle of the TA value. For example, terminal device 111 starts a predetermined timer when it obtains the TA value of a candidate cell from either base station device 101 or candidate handover base station device 102. Then, when the predetermined timer expires, terminal device 111 may re-transmit an RA preamble and repeat the above procedure to obtain the updated TA value. Alternatively, either base station device 101 to which terminal device 111 is connected or candidate handover base station device 102 starts a predetermined timer when the TA value is determined at base station device 102. Then, when the predetermined timer expires, the base station device managing that timer may instruct terminal device 111 to execute the above RA procedure. For example, if base station device 101 manages the timer, base station device 101 may re-transmit a PDCCH order to repeat the above process. Furthermore, if the base station device 102 manages the timer, it may send a message to the base station device 101 requesting that it send a PDCCH order to the terminal device 111. Also, if the terminal device 111 continues to receive signals from the base station device 102, the base station device 102 may directly instruct the terminal device 111 to execute the RA procedure.

[0034] The terminal device 111 may, instead of repeating the RA procedure, transmit a predetermined signal such as a sounding reference signal (SRS). That is, when a predetermined uplink radio signal is received by the base station device 102, the base station device 102 can update the appropriate TA value. For this reason, the terminal device 111 may transmit a predetermined uplink signal to the base station device 102 using the TA value set for that base station device 102 at a predetermined timing, such as when a predetermined timer managed within the device itself expires. The terminal device 111 may, for example, transmit an SR to receive an allocation of radio resources to transmit the predetermined signal to the base station device 102. The terminal device 111 may also transmit the predetermined uplink signal in response to receiving a transmission instruction from the base station device 101 or base station device 102 for updating the TA value. 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. Furthermore, predetermined signals such as SRS may be transmitted at a timing based on the TA value before it is updated. Also, the terminal device 111 may stop the periodic transmission of a predetermined signal in response to receiving a stop instruction from the base station device 102 during the periodic transmission of that predetermined signal. In another example, the terminal device 111 may periodically transmit predetermined signals such as SRS in response to triggers such as receiving an RA response from the base station device 102, without explicit instructions from the base station device 102. Furthermore, if the terminal device 111 needs to accept instructions from the base station device 102, it may continue to receive signals such as PDCCH from the base station device 102 after executing the RA procedure described above and obtaining the TA value. In addition, instructions to transmit or stop predetermined signals, or triggers for transmitting predetermined signals, may be given by the base station device 101.

[0035] The terminal device 111 may, for example, continue measuring the radio quality of a candidate cell for handover, and may terminate the maintenance of the TA value for that cell if the radio quality deteriorates to a predetermined level or below. The termination of this TA value maintenance may be decided independently by the terminal device 111, or it may be decided by the base station device 101, for example, after receiving a radio quality report, and instruct the terminal device 111 to do so. Alternatively, the base station device 102 may measure the radio quality of a predetermined signal from the terminal device 111 as described above and decide to terminate the maintenance of the TA value according to the result. In this case, if the terminal device 111 is maintaining the TA value, the base station device 102 may instruct the terminal device 111 to discard that TA value. If the maintenance of the TA value is terminated and the timer for maintaining the TA value described above is running, the terminal device 111, base station device 101, or base station device 102 will stop that timer. Furthermore, the terminal device 111, base station device 101, or base station device 102 may prevent the same timer from being restarted after the timer for maintaining the TA value described above has expired. Furthermore, if the terminal device 111 is continuing to receive signals such as PDCCH from the base station device 102 in order to maintain the TA value, it may terminate the reception of those signals. Note that the termination of signal reception from the base station device 102 may also be instructed to the terminal device 111 by the base station device 101 or base station device 102.

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

[0037] (Device configuration) Next, the device configuration will be described. Figure 2 shows an example of the hardware configuration of the base station device and terminal device of this embodiment. In one example, the base station device and terminal device are configured to include a processor 201, ROM 202, RAM 203, storage device 204, and 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 ASIC (Application-Specific Integrated Circuit), and executes the overall processing of the device and the aforementioned processing by reading and executing programs stored in the ROM 202 and 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 terminal device. The RAM 203 functions as a workspace when the processor 201 executes programs and is a random access memory that stores temporary information. The storage device 204 is configured to include, for example, a removable external storage device. The communication circuit 205 is configured to include, for example, a circuit for wireless communication of 5G or its successor standards. Although Figure 2 shows one communication circuit 205, base station equipment and terminal equipment may have multiple communication circuits. For example, base station equipment and terminal equipment may have wireless communication circuits for 5G and its successor standards, respectively, and an antenna common to those circuits. Base station equipment and terminal equipment may also have separate antennas suitable for each standard. Furthermore, base station equipment may also have wired communication circuits used when communicating with other base station equipment or nodes in the core network. Furthermore, terminal equipment may also have communication circuits compliant with wireless communication standards other than cellular communication standards, such as wireless local area networks (LANs) and Bluetooth®. Base station equipment and terminal equipment may have separate communication circuits 205 for each of the multiple usable frequency bands, or they may have a common communication circuit 205 for at least a portion of those frequency bands.

[0038] Figure 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, a RACH instruction unit 302, an inter-base station communication unit 303, a handover instruction unit 304, and a TA maintenance unit 305. Note that Figure 3 mainly shows the functions of the base station device 101 to which the terminal device 111 is connected. The candidate base station device 102 for handover is the same as a conventional base station device, except that it does not transmit some signals in the RA procedure and may transmit some signals to the base station device 101, so it is not shown here. Also, Figure 3 shows only the functions particularly relevant 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 base station devices compliant with 5G and subsequent standards generally have. Furthermore, the functional blocks in Figure 3 are shown schematically, and each functional block may be implemented as an integrated unit or further subdivided. Furthermore, each function in Figure 3 may be implemented, for example, by the processor 201 executing a program stored in the ROM 202 or the storage device 204, or by a processor located inside the communication circuit 205 executing predetermined software. Note that the details of the processing performed by each functional unit will not be explained here, and only their general functions will be outlined.

[0039] The RRC configuration unit 301, for example, configures the RRC layer for the connected terminal device. For example, the RRC configuration unit 301 sends a handover request to another base station device that is a candidate for handover and provides an adjacent cell, obtains communication configuration information from the other base station device, and notifies the terminal device of this communication configuration information using an RRC layer message. The RRC layer message is an RRC Reconfiguration message. The RRC configuration unit 301 then configures the connection process between the terminal device and the other base station device upon receiving an RRC Reconfiguration Complete message from the terminal device. The RACH instruction unit 302 instructs the terminal device, whose RRC layer configuration is complete, to send an RA preamble to the other base station device that is a candidate for handover. Here, the instruction to send the RA preamble includes a series for generating the RA preamble specific to the terminal device, a set of series that can also be used by other terminal devices attempting to connect to the other base station device, and may also include information on the radio resources used to send the RA preamble. The RACH instruction unit 302 may, for example, instruct the connected terminal device to transmit an RA preamble to a candidate base station device for handover via a physical downlink control channel (PDCCH) order. The RACH instruction unit 302 may also, if necessary, instruct the terminal device to retransmit the RA preamble, or, if the RA preamble is configured to be transmitted repeatedly, send an instruction to the terminal device to stop transmitting the RA preamble.

[0040] The inter-base station communication unit 303 communicates with candidate base station devices to which a handover will occur. The inter-base station communication unit 303 can, for example, notify other base station devices that will receive the RA preamble from the currently connected terminal device to send the RA preamble. The inter-base station communication unit 303 is also used, for example, to receive information for the RA procedure at the candidate base station devices to which a handover will occur. The inter-base station communication unit 303 can also receive, for example, messages (message 2 and message 4) that should be sent from the candidate base station device to the terminal device during the RA procedure. Furthermore, the TA value calculated at the candidate base station device to which a handover will occur can be obtained via the inter-base station communication unit 303. The signals received by the inter-base station communication unit 303 can be forwarded to the terminal device via, for example, a RACH instruction unit.

[0041] The handover instruction unit 304 instructs the terminal device to perform a handover when the terminal device is in a state where it should perform a handover, for example, when the reception quality of the radio signal from the other base station device to which the handover is to take place becomes higher than the radio quality between the device and the terminal device by a predetermined level or more. The handover instruction unit 304 transmits a Cell switch command to the terminal device, for example, which includes a cell identifier that can identify the base station device to which the handover is to take place. If the TA value to be used for uplink communication has been determined by the transmission of an RA preamble by the terminal device, that TA value may be included in the command transmitted to the terminal device. This allows the terminal device to transmit uplink user data signals using that TA value immediately after the handover. In this embodiment, an example has been described in which the handover instruction unit 304 transmits the above command after the terminal device has executed the RA procedure, but in some cases, such as in response to a request for uplink communication delay by the terminal device, the above command may be transmitted before the RA procedure is executed. In other words, the RA procedure may be performed before or after the sending and receiving of the handover command.

[0042] The TA maintenance unit 305 manages the TA value that the terminal device should use when transmitting an uplink signal to a candidate base station device for handover. The TA maintenance unit 305 can, for example, set an expiration date and renewal cycle for the TA value, and when the expiration date or renewal cycle arrives, it can instruct the terminal device to retransmit the RA preamble so that the TA value is updated. The TA maintenance unit 305 may also notify the terminal device of the SRS settings at the handover base station device and cause the terminal device to transmit an SRS using those SRS settings to update the TA value. The handover base station device may also have a TA maintenance unit 305, and that base station device may transmit instructions to the terminal device to transmit an RA preamble or an SRS.

[0043] Figure 4 shows an example of the functional configuration of a terminal device. The terminal device includes, 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 Figure 4 shows only the functions particularly relevant 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 and subsequent standards generally have. Also, the functional blocks in Figure 4 are shown schematically, and each functional block may be implemented as an integrated unit or further subdivided. Furthermore, each function in Figure 4 may be implemented, for example, by the processor 201 executing a program stored in the ROM 202 or storage device 204, or by a processor located inside the communication circuit 205 executing predetermined software. Note that the details of the processing performed by each functional unit will not be explained in detail here, and only the general functions will be outlined.

[0044] The RRC setting unit 401 receives RRC layer messages from the connected base station device and performs connection setting processing with the candidate base station device for handover. The RRC setting unit 401 maintains the settings of the currently connected base station device and the candidate base station device for handover in parallel. The RACH instruction receiving unit 402 receives an instruction to transmit the RA preamble from the currently connected base station device. The instruction to transmit the RA preamble may be made in PDCCH order. The RACH instruction receiving unit 402 may also receive instructions to retransmit the RA preamble or to stop transmitting the RA preamble. The RACH processing unit 403 transmits the RA preamble to the candidate base station device for handover in response to receiving the instruction from the RACH instruction receiving unit 402. Here, the sequence for generating the RA preamble and information on the radio resources for receiving the RA preamble at the candidate base station device for handover may be received from the currently connected base station device via the RACH instruction receiving unit 402. The series and radio resource information may be obtained from the system information transmitted by the candidate base station device to which the handover is intended. The handover processing unit 404 changes the connected base station device 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 configuration processing with the candidate base station device to which the handover is intended, as the RRC setting unit 401 performs the configuration processing and the RACH processing unit 403 performs the RACH processing. Therefore, for example, if the received command notifies the allocation of uplink radio resources at the candidate base station device to which the handover is intended, the terminal device can immediately transmit an uplink signal using those radio resources.

[0045] The TA maintenance unit 405 maintains and manages the TA value when it acquires the TA value for a candidate base station device to be handed over. For example, the TA maintenance unit 405 may update the TA value by retransmitting the RA preamble without receiving instructions from the connected base station device or the candidate base station device to be handed over, in response to the expiration of the TA value's validity period. Alternatively, the TA maintenance unit 405 may update the TA value by transmitting a predetermined signal such as an RA preamble or SRS for the candidate base station device to be handed over, in response to instructions from the connected base station device or the candidate base station device to be handed over. Furthermore, the TA maintenance unit 405 terminates the maintenance of the TA value when it receives information from the connected base station device indicating that the maintenance of the TA value should be terminated. Here, the TA maintenance unit 405 may immediately discard the TA value whose maintenance has been terminated, or it may maintain it until its validity period expires. The TA maintenance unit 405 may also maintain and manage the TA value for the original base station device after the handover has been performed. In other words, the TA maintenance unit 405 may maintain the TA value of the currently connected base station device as the TA value of the candidate base station device to which the handover will take place after a handover has been performed from that base station device to another base station device.

[0046] (Process flow) Figure 5 shows an example of the processing flow performed by the wireless communication system of this embodiment. Note that the example shown in Figure 5 is just one example, and each step can be modified as described above. For example, Figure 5 shows an example in which a sequence unique to the terminal device is notified to the terminal device as a sequence for generating the RA preamble to be transmitted to the candidate base station device to be handed over to, but this may be replaced with the procedure described above in which a group of sequences usable by multiple terminal devices are notified. Note that in the example in Figure 5, it is assumed that a connection has been established between the first base station device from which the handover originated and the terminal device.

[0047] The first base station device first sends a HANDOVER REQUEST message to the second base station device, which forms an adjacent cell and is a candidate for the terminal device's handover destination (S501). Upon receiving this message, the second base station device sends a HANDOVER REQUEST ACK message back to the first base station device, containing information such as the communication parameters that the terminal device should use when connecting (S502). Then, the first base station device notifies the terminal device of an RRC Reconfiguration message containing the communication parameters notified by the second base station device (S503). Upon receiving this message, the terminal device sends an RRC Reconfiguration Complete message to the first base station device, completing the configuration process using RRC layer messages. This process eliminates the need for the terminal device to perform RRC layer configuration processing after being instructed by the first base station device to hand over to the second base station device. Note that Figure 5 shows an example where there is only one candidate base station device for the handover destination, but similar configuration processing can be performed for multiple candidate base station devices.

[0048] Subsequently, the terminal device measures the radio signal (e.g., SSB) transmitted from the second base station device, a candidate for handover (S505), and notifies the first base station device of the measurement result (S506). Here, the first base station device may also notify the second base station device of this measurement result. In this case, the second base station device can identify the rough beam to be used for communication with the terminal device. The second base station device may transmit CSI-RS to the terminal device for measurement in order to form a finer beam. In this case, the second base station device may transmit CSI-RS configuration information to the first base station device and have the terminal device transfer this configuration information. The terminal device measures CSI-RS based on the configuration information and reports the measurement result to the connected first base station device. The first base station device then transfers the measurement result to the second base station device. As a result, the second base station device can transmit downlink signals using a fine beam immediately after the terminal device has handed over to it.

[0049] Next, the first base station device instructs the terminal device to transmit an RA preamble to the second base station device (S507). 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 identified, for example, based on system information in the second base station device transmitted from the second base station device and measured by the terminal device. In other words, this information does not have to be obtained from the connected base station device. The sequence to be used when generating the RA preamble may also be identified in advance. In one example, this sequence information 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 is planning to transmit an RA preamble. At this time, the first base station device may notify the second base station device of the sequence that the terminal device will use to generate the RA preamble. This enables the second base station device to detect the RA preamble transmitted by the terminal device. This is just one example; for example, the HANDOVER REQUEST in S501 may implicitly notify the first base station that the terminal device intends to transmit an RA preamble. In this case, the HANDOVER REQUEST ACK in S502 may specify the sequence that the terminal device should use to generate the RA preamble, as indicated by the second base station device. These are just examples; information such as the sequence that the terminal device should use to generate the RA preamble may be shared between the first and second base station devices through other messages. The second base station device calculates the 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). This TA value may be managed by both the first and second base station devices, or by one of them.Furthermore, this TA value may be notified to the first base station device from the second base station device in response to the first base station device deciding to perform a handover of the terminal device. For example, the first base station device may notify the second base station device of the execution of a handover, and in response to that notification, the TA value may be notified to the first base station device from the second base station device.

[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 results (S512). The first base station device may decide whether to perform a handover based on these measurement results (S513). For example, if certain conditions are met, such as when the radio quality of the signal from the second base station device becomes at least a predetermined level higher than the radio quality of the signal from the first base station device, the first base station device may decide to hand over the terminal device to the second base station device. Based on this decision, the first base station device sends a command to the terminal device instructing it to perform a handover (S514). This command may include, for example, the TA value obtained in S510 and information on the uplink resources for signal transmission immediately after the handover. The TA value may be forwarded to the terminal device immediately after being notified to the first base station device in S510, in which case the TA value does not need to be included in this command. The first base station device may also identify information about the uplink resources by querying the second base station device in advance. Alternatively, the first base station device may instruct the terminal device to use a reserved radio resource available at the second base station device without querying. Upon receiving this command, the terminal device may change the connected base station device to the second base station device and send a response signal to the command to the second base station device using the resource specified in the received command (S515). This allows the second base station device to confirm that the terminal device has handed over to it, and then communication of user data may take place between the terminal device and the second base station device (S516). The terminal device may also send user data to the second base station device in addition to or instead of the response signal in S515.

[0051] In this way, according to this embodiment, the time from when the original base station device decides to actually hand over the terminal device to another base station device until the terminal device starts communicating user data with the base station device after the handover can be significantly reduced. Therefore, it becomes 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 embodiments described above, and various modifications and changes are possible within the scope of the gist of the invention.

Claims

1. A terminal device, Receiving means that receives a physical downlink control channel (PDCCH) order from a connected base station device which receives the radio resource control (RRC) layer settings for another base station device that is a candidate for handover from the said base station device, and which instructs the terminal device from the base station device to transmit a random access preamble to the other base station device without performing a handover to the other base station device, A transmission means for transmitting the random access preamble to the other base station device in accordance with the above instructions, An execution means that, upon receiving a command from the base station device instructing a handover to the other base station device, executes the handover to the other base station device, It has, The terminal device is characterized in that the transmitting means retransmits the random access preamble when a signal requesting retransmission of the random access preamble is received from the base station device.

2. The terminal device according to claim 1, characterized in that the receiving means receives instructions to transmit the random access preamble to a plurality of other base station devices by one of the PDCCHs.

3. The receiving means receives from the base station device a sequence unique to the terminal device that the terminal device uses when generating the random access preamble, and information for identifying the radio resources available when transmitting the random access preamble. The transmission means generates the random access preamble using the sequence specific to the terminal device and transmits it on the wireless resource identified by the information. The terminal device according to feature 1.

4. The receiving means receives a timing advance value (TA value) identified by the other base station device based on the random access preamble transmitted by the terminal device, from the base station device or the other base station device. The terminal device according to feature 1.

5. The system further includes means for maintaining the TA value, The terminal device according to claim 4, characterized in that 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 the renewal cycle.

6. The terminal device according to claim 5, wherein the transmitting means transmits the random access preamble to the other base station device as a signal for the predetermined uplink when instructed by the base station device to transmit the random access preamble in response to the expiration of the validity period of the TA value or the arrival of the renewal cycle.

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

8. The terminal device according to claim 5, characterized in that the maintenance means terminates the maintenance of the TA value when the wireless quality between the terminal device and the other base station device falls below a predetermined level.

9. The transmitting means repeatedly transmits the random access preamble at predetermined intervals until a transmission stop instruction for the random access preamble is received from the base station device, and stops transmitting the random access preamble in response to the receipt of the transmission stop instruction. The terminal device according to feature 3.

10. The receiving means receives from the base station device a set of sequences that can be used when generating the random access preamble to be transmitted to the other base station device. The transmission means generates the random access preamble using one sequence selected from the group of sequences and transmits it to the other base station device. The terminal device according to feature 1.

11. The receiving means receives a response to the random access preamble from the base station device or another base station device. When the response is received, the transmitting means includes the identification information of the terminal device. The first message is transmitted to the other base station equipment. The receiving means receives a second message containing the identification information of the terminal device in response to the first message from the base station device or another base station device. The terminal device according to feature 10.

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

13. The transmission means, in response to an instruction to transmit the random access preamble, repeatedly retransmits the random access preamble at predetermined intervals. The receiving means receives a transmission stop instruction for the random access preamble from the base station device, The transmission means stops transmitting the random access preamble in response to the receipt of the transmission stop instruction. The terminal device according to feature 1.

14. A control method performed by a terminal device, The terminal device receives the Radio Resource Control (RRC) layer settings for other base stations that are candidates for handover from the connected base station device, Receiving a physical downlink control channel (PDCCH) order from the base station device 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, Transmit the random access preamble to the other base station device in accordance with the above instructions, When a signal requesting retransmission of the random access preamble is received from the base station device, the random access preamble is retransmitted. When the aforementioned base station device receives a command instructing a handover to the aforementioned other base station device, the handover to the aforementioned other base station device is performed. A control method characterized by including

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