Terminal device, control method, and program for performing handover having improved efficiency

Abstract

To improve communication efficiency when a handover is performed.SOLUTION: A terminal device receives, from a base station device to which it is connected, configuration information on an uplink reference signal for measurement at another base station device that provides a candidate cell for a handover destination, and then receives from the base station device a signal including an instruction to transmit the uplink reference signal and information specifying a beam to be used for transmitting the uplink reference signal, and in response to receiving the signal and before receiving a command instructing a handover to the cell provided by the other base station device from the base station device, transmits the uplink reference signal to the other base station device using the configuration information and the information specifying the beam.SELECTED DRAWING: Figure 6

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 when a handover is performed. [Means for solving the problem]

[0005] A terminal device according to one aspect of the present invention has a receiving means for receiving, from a connected base station device, configuration information of an uplink reference signal for measurement at another base station device that provides a candidate cell for handover, and then receiving from the base station device a signal including an instruction to transmit the uplink reference signal and information specifying the beam to be used for transmitting the uplink reference signal, and a transmitting means for transmitting the uplink reference signal to the other base station device using the configuration information and the information specifying the beam in response to receiving the signal from the base station device and before receiving a command from the base station device instructing a handover to a cell provided by the other base station device, wherein the receiving means further receives the command from the base station device instructing a handover to the cell provided by the other base station device, and the terminal device further has an executing means for executing a handover to the cell provided by the other base station device based on the command being received from the base station device.

[0006] A base station device according to one embodiment of the present invention has a receiving means for receiving from another base station device configuration information for an uplink reference signal for measurement at another base station device that provides a candidate cell for handover to a connected terminal device, and then receiving from the other base station device information specifying a beam to be used for transmitting the uplink reference signal, and a transmitting means for transmitting the configuration information to the terminal device, and then transmitting a signal including an instruction to transmit the uplink reference signal and information specifying the beam, and after transmitting the signal, transmitting a command to the terminal device instructing a handover to a cell provided by the other base station device. Effect of the Invention

[0007] According to the present invention, it is possible to improve communication efficiency when a handover is performed. [Brief description of the drawings]

[0008] [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. [Figure 6] 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

[0009] 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.

[0010] (System Configuration) Fig. 1 shows an example of the configuration of a wireless communication system according to this embodiment. This wireless communication system is a cellular communication system configured 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.

[0011] 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.

[0012] 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 in a radio resource control (RRC) layer (setting using an RRC message) before actually handing over the terminal device 111. That is, in a conventional handover, the terminal device 111 receives a handover instruction from the handover source base station device 101, and the setting in the RRC layer for the handover destination base station device 102, which is performed during the handover process, is notified to the terminal device 111 prior to the timing when the handover process should actually be performed. In this setting, 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, when multiple communication parameters used for communication with the base station device 102 are notified, an identifier (e.g., RRC-TransactionIdentifier or CondReconfigId) uniquely associated with each communication parameter may be included in an RRC message notified to the terminal device 111 or a configuration parameter in the RRC layer. In this case, CondReconfigId is set for each candidate base station device or combination of base station devices as the handover destination. Furthermore, an RRC-TransactionIdentifier may be set when multiple communication parameters are notified for each base station device or combination of base station devices. As an example, CondReconfigId=0 is associated with the base station device 102 that is a candidate base station as the handover destination, and further, when multiple parameters of the base station device 102 are notified, an RRC-TransactionIdentifier (e.g., 0 and 1) may be set for each of the parameters.Furthermore, the handover source base station device 101 may transmit to the terminal device 111 an instruction to measure a synchronization signal (SS) / physical broadcast channel (PBCH) block (SSB) transmitted from the handover destination candidate base station device 102. Note that the SSBs may be transmitted using beams directed in different directions. In this case, a measurement instruction may be transmitted so that the terminal device 111 measures each of the SSBs.

[0013] The terminal device 111 measures the SSB transmitted 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. Then, the terminal device 111 notifies the connected base station device 101 of the measurement result. The base station device 101 may notify the measurement result received from the terminal device 111 to the base station device 102, which is a candidate for the handover destination, which was the measurement target. In this way, when the terminal device 111 actually performs a handover, the base station device 102 can specify the rough direction of the transmission beam to be used for downlink communication (signal transmission) with the terminal device 111. Note that the terminal device 111 may be able to set multiple beams that can be used in downlink communication. In this case, the terminal device 111 may measure the SSB using each of the multiple beams. In this case, the terminal device 111 may specify which of the multiple beams was used to provide good radio quality of the SSB, and may notify the base station device 101 of, for example, the best measurement result. The terminal device 111 can store the beam settings that have provided good radio quality for each of the multiple SSBs in association with the identification information of the SSB. In one example, after the terminal device 111 has handed over to the base station device 102, when the base station device 102 transmits a signal using a beam corresponding to a specific SSB, the base station device 102 can notify the terminal device 111 of the identification information that identifies the specific SSB and instruct the terminal device 111 to communicate using the beam corresponding to the specific SSB.

[0014] The base station device 102 may further notify the terminal device 111 of the transmission setting of channel state information (CSI)-reference signal (RS) in order to specify a fine beam to be used in the downlink. This notification is performed by the base station device 102 notifying the base station device 101 of the transmission setting of 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 performs measurement of the CSI-RS transmitted from the base station device 102 (for example, measurement of reference signal received power (CSI-RSRP), reference signal received quality (CSI-RSRQ), and received signal strength indicator (CSI-RSSI)) 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 a precise beam to be used in 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 shortens the period during which the efficiency of the communication of user data of the terminal device 111 decreases.

[0015] As described above, the terminal device 111 may be able to set a plurality of beams that can be used in downlink communication. In this case, the terminal device 111 may measure the CSI-RS using each of the plurality of beams. In this case, the terminal device 111 may specify which of the plurality of beams was used to obtain good wireless quality of the CSI-RS, and may notify the base station device 101 of the best measurement result, for example. The base station device 102 may notify the terminal device 111 of the configuration of the CSI-RS for each of the plurality of configurable beams, and may cause the terminal device 111 to perform measurements for each of the beams. The terminal device 111 may store the configuration of the beam that can obtain good wireless quality for the CSI-RS corresponding to each of the plurality of beams in the base station device 102, in association with the identification information of the CSI-RS. In one example, after terminal device 111 hands over to base station device 102, when base station device 102 transmits a signal using a beam corresponding to a specific CSI-RS, it can notify terminal device 111 of identification information that identifies the specific CSI-RS and instruct terminal device 111 to communicate using the beam corresponding to the specific CSI-RS.

[0016] 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 in the terminal device 111 as communication parameters before handover. The CSI-RS-Resource includes the time position and frequency position of the radio resource block in which the CSI-RS is transmitted. For example, the terminal device 111 can be instructed to measure a specific CSI-RS radio resource by specifying the CSI-RS-Resource using a CSI-RS-Resource id in a medium access control element (MAC CE) or downlink control information (DCI). In addition, aperiodic or periodic CSI report timing is set in the csi-ReportConfig. For example, the base station device 101 can specify a csi-ReportConfig using a csi-ReportConfig id in a MAC CE or DCI to instruct the terminal device 111 to report the measurement result of the CSI-RS at a specific timing. When performing the above-mentioned measurement configuration for the terminal device 111, the base station device 101 can 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 the one.

[0017] Furthermore, 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 execute a process for establishing uplink synchronization before actually executing a handover. For example, the base station device 101 to which the terminal device 111 is connected transmits an instruction to the terminal device 111 to transmit a random access preamble (RA preamble) to the base station device 102. Then, the terminal device 111 transmits the RA preamble in radio resources prepared for the RA procedure in the base station device 102 according to this instruction. Note that, for example, information on the radio resources and the sequence used to generate the RA preamble may be notified from the base station device 101 to the terminal device 111 during the above-mentioned RRC layer setting process. Furthermore, these pieces of information may be notified to the terminal device 111 when the base station device 101 issues an instruction to transmit the RA preamble. Through this procedure, the RA preamble transmitted from the terminal device 111 is received at 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, the base station device 102 notifies the terminal device 111 of the TA value, for example, by notifying the base station device 101 to which the terminal device 111 is connected, and the base station device 101 transfers the TA value to the terminal device 111. 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 it becomes possible to transmit uplink user data to the base station device 102 without executing an RA procedure again.

[0018] Thereafter, the base station device 101 may notify the terminal device 111 to execute handover at a predetermined trigger, such as when the reception quality of a radio signal from the base station device 101 in the terminal device 111 is deteriorated by a predetermined level compared to the reception quality of a radio signal from the base station device 102. This notification may be performed using a command that may be called a Cell switch command. This command may be transmitted using a 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. Also, when multiple settings are configured in the RRC layer used for communication with the base station device 102, an identifier (for example, RRC-TransactionIdentifier or CondReconfigId) uniquely associated with each setting in the RRC layer may be included in this command. Also, information regarding the beam determined in the base station device 102 and notified to the base station device 101 by performing measurements of SSB and CSI-RS in advance as described above may be notified to the terminal device 111 from the base station device 101. In response to receiving a command from the base station device 101 specifying the base station device 102 as the handover destination, the terminal device 111 changes the connection destination from the base station device 101 to the base station device 102. In this case, since the RA procedure has already been executed as described above, it is not necessary to execute a new RA procedure. Also, since the CSI-RS is measured before the handover as described above, it is not necessary to newly determine a fine beam to be used in downlink communication after the handover. Therefore, the terminal device 111 can communicate user data with the base station device 102 immediately after the handover. Also, when the handover is successful after receiving the command, the terminal device 111 can notify the base station device 102 of the success in an uplink control message such as an RRC message of RRC Reconfiguration Complete or a MAC CE.In this case, the terminal device 111 may include an identifier (for example, an RRC-TransactionIdentifier or a CondReconfigId) associated with communication parameters used for communication with the base station device 102 in the RRC message.

[0019] On the other hand, at this point, a fine beam to be used in uplink communication has not been determined, and in order to use such a fine beam, a setting process needs to be performed subsequently. When setting a fine beam to be used in uplink communication, the terminal device 111 transmits a sounding reference signal (SRS), and the base station device 102, which is a candidate for the handover destination, measures the SRS. Then, based on the measurement result, the base station device 102 can determine a beam for receiving an uplink signal transmitted from the terminal device 111 in communication after the handover of the terminal device 111. Conventionally, such transmission of the SRS and determination of a beam in the uplink are performed by transmitting an SRS transmission instruction from the base station device 102, which is the handover destination, to the terminal device 111 after the handover is completed. For this reason, it is expected that there is a long period of time between the completion of the handover and the determination of a fine beam for the uplink communication, and that communication efficiency during that period will be insufficient. In this embodiment, in consideration of such circumstances, an SRS transmission instruction is transmitted from the base station device 101 to the terminal device 111 while the terminal device 111 is connected to the base station device 101 before handover. This makes it possible to shorten the time until uplink communication using a fine beam becomes possible after handover is completed.

[0020] For example, the base station device 101 may transmit a signal instructing the terminal device 111 to transmit an SRS in the above-mentioned command (Cell switch command). The base station device 101 may also transmit a signal instructing the terminal device 111 to transmit an SRS before the above-mentioned command is issued. In response to receiving this instruction signal, the terminal device 111 transmits an SRS based on the SRS setting for the base station device 102 notified in advance. This makes it possible to determine the beam to be used in uplink communication earlier, compared to the case where an SRS is transmitted in response to an instruction from the handover destination base station device 102 after the handover is completed.

[0021] The SRS configuration is notified from the base station device 101 to the terminal device 111, for example, when the RRC layer is configured for the base station device 102 at the handover destination as described above. Note that this is just an example, and the SRS may be configured separately from the RRC layer configuration. Information on this SRS configuration is notified from the base station device 102 to the base station device 101, and the base station device 101 may notify the notified information on the configuration to the terminal device 111. The SRS configuration includes the number of ports through which the SRS is transmitted, the SRS transmission period, the number of OFDM symbols, parameters related to the SRS bandwidth, parameters related to the SRS hopping, the position of the SRS in the frequency domain, the cyclic shift, the subcarrier interval at which the SRS is transmitted, and an identifier of the sequence used to generate the SRS. The number of SRS ports indicates the number of SRSs transmitted in parallel, and is selected from, for example, 1, 2, or 4. The SRS transmission period includes information specifying, for example, periodic, aperiodic, or semi-persistent. In addition, when periodic or semi-persistent is specified, the SRS transmission cycle further includes information that specifies the time cycle and the time offset from the predetermined timing, for example, in slot units. In the following, it is assumed that semi-persistent is specified and the SRS is transmitted from the terminal device 111 periodically for a certain period. However, this is only an example, and the following discussion can be applied to the case where periodic or aperiodic is specified. The number of OFDM symbols indicates the number of OFDM symbols in which the SRS is transmitted continuously, and can be selected from, for example, one symbol, two symbols, or four symbols. The parameters related to the bandwidth of the SRS, the parameters related to the hopping of the SRS, and the position of the SRS in the frequency domain are parameters that enable the frequency-time resource in which the SRS is transmitted to be specified, for example, by using a table or a formula specified in the standard. The identifier of the sequence used for the cyclic shift and the generation of the SRS is used when the known sequence used for the generation of the SRS is shifted by the time indicated by the cyclic shift to generate the signal sequence of the SRS.The subcarrier spacing at which the SRS is transmitted is set to 2, 4, etc., indicating that the SRS is transmitted at frequency intervals of 2 subcarriers or 4 subcarriers.

[0022] Note that there may be a plurality of SRS settings, and when specifying any one of the plurality of settings, identification information such as SRS Resource Setid or srs-Resourceid is used. That is, between the base station device and the terminal device, an SRS Resource Set associated with one or more srs-Resourceids is defined, and one or more SRS Resource Sets are prepared, and one of them can be specified using SRS Resource Setid or srs-Resourceid. In one example, when two SRS Resource Sets are prepared and four pieces of setting information are prepared for each SRS Resource Set, srs-Resourceid=1 to 4 can be assigned to SRS Resource Set=0, and srs-Resourceid=1 to 4 can be assigned to SRS Resource Set=1. Then, for example, when the base station device 101 instructs to specify one of the settings and transmit the SRS, the SRS Resource Set and srs-Resourceid are specified, and the terminal device 111 can transmit the SRS using the setting information corresponding to the specified identification information. Furthermore, when SRS transmission is instructed in the Cell switch command, the SRS settings may be set in advance as part of the communication parameters used in connection and communication with the handover target candidate base station device 102. Furthermore, when SRS transmission is instructed before the Cell switch command is transmitted or received, the SRS settings may be set in advance as part of the communication parameters used in connection and communication with the handover source base station device 101.

[0023] The above-mentioned SRS configuration may further include configuration information of a beam to be used by the terminal device 111. For example, a beam to be used in the terminal device 111 may be specified by a TCI (Transmission Configuration Indicator). For example, for each of a plurality of beams available to the terminal device 111, information indicating whether or not to use the beam for transmitting SRS (or whether to enable or disable configuration information for forming each of the plurality of beams) may be notified to the terminal device 111 as the SRS configuration. At this time, as one aspect, the validity or invalidity of the TCI state is specified using the TCI corresponding to each of the plurality of beams, thereby indicating whether or not to use the beam corresponding to each TCI for transmitting SRS.

[0024] Also, instead of using TCI, information for identifying a spatial relationship for determining a transmission beam of an SRS may be used. In that case, for example, an SP SRS Resource Set ID or a Resource ID may be used as a parameter indicating the setting information. The Resource ID may include information for identifying a spatial relationship for determining a transmission beam of each SRS. The information for identifying the spatial relationship is, for example, any one of an NZP CSI-RS resource index, an SSB index, and an SRS resource index. This spatial relationship will be described below. For example, as described above, when the terminal device 111 receives an SSB or a CSI-RS transmitted in each of a plurality of beams formed by the base station device 102, the terminal device 111 may store a beam of the own device that has been able to obtain good wireless quality in association with the identification information of the SSB or CSI-RS. In this case, the base station device 102 can indirectly specify a beam to be used in the terminal device 111 by specifying the identification information of the SSB or CSI-RS. For example, the base station device 102 identifies which of a plurality of beams that can be formed in the own device can be used to perform (downlink) communication with good quality based on the reported reception radio quality of the SSB or CSI-RS in the terminal device 111. Then, the base station device 102 notifies the terminal device 111 of identification information (NZP CSI-RS resource index, SSB index) of the SSB or CSI-RS transmitted by the beam for which a measurement result of good radio quality was obtained, and transmits an instruction to the terminal device 111 to transmit the SRS by using the beam of the terminal device 111 stored in association with the identification information. For example, when the channel characteristics of the uplink and downlink are the same (symmetric) (for example, when time division duplex (TDD) is used), a method of specifying the beam of the terminal device 111 by the identification information of the SSB or CSI-RS is effective.Moreover, the base station device 102 can specify an SRS resource index to instruct the use of the same beam as another SRS specified by the SRS resource index. In other words, when an SSB or CSI-RS is set in the spatial relationship of a Resource ID, the terminal device transmits the SRS using the same spatial domain filter as the spatial domain filter for receiving the SSB or CSI-RS, and when another SRS is set in the spatial relationship of a Resource ID, the terminal device transmits the SRS using the same spatial domain filter as the spatial domain filter for transmitting the other SRS.

[0025] For example, when a first CSI-RS is transmitted, the radio quality of the first CSI-RS becomes a sufficiently high value in a beam with TCI=n among a plurality of beams of the terminal device 111, and the value of the radio quality is notified to the base station device 102 via the base station device 101. Also, for example, when a second CSI-RS is transmitted, the radio quality of the second CSI-RS becomes a sufficiently high value in a beam with TCI=m among a plurality of beams of the terminal device 111, and the value of the radio quality is notified to the base station device 102 via the base station device 101. Here, the terminal device 111 associates and stores the identification information of the first CSI-RS with TCI=n, and further associates and stores the identification information of the second CSI-RS with TCI=m. In this case, by notifying the terminal device 111 of an SRS setting indicating that the SRS should be transmitted using a beam corresponding to the first CSI-RS, the terminal device 111 will transmit the SRS using a beam with TCI=n when the setting is used. Also, by notifying the terminal device 111 of an SRS setting indicating that the SRS should be transmitted using a beam corresponding to the second CSI-RS, the terminal device 111 will transmit the SRS using a beam with TCI=m when the setting is used.

[0026] When a transmission instruction of SRS is notified to the terminal device 111 using a Cell switch command, information to be notified to the terminal device 111 in the conventional SP SRS Activation / Deactivation MAC CE can be notified in the command. Note that MAC CE refers to a medium access control-control element. With this information, for example, when a beam is set in advance as setting information of SRS, the setting of the beam can be replaced. For example, in the setting information of SRS notified at the time of setting of the RRC layer as described above, if the SRS is set to be transmitted with a beam corresponding to a specific SSB, the setting can be changed in this information so that the SRS is transmitted with a beam corresponding to another SSB or CSI-RS or a beam used for another SRS. Note that in this case, the terminal device 111 can transmit the SRS by changing only the transmission beam with the information, and using the values ​​before the change for other settings (such as frequency position and sequence). In this case, the Cell switch command may include the cell identifier (SRS Resource Set's Cell ID) of the cell including the SRS resource, the bandwidth part identifier (SRS Resource Set's BWP ID), the SRS Resource Set ID, and the Resource ID as information for specifying the SRS resource used for SRS transmission, and the NZP CSI-RS resource index, the SSB index, and the SRS resource index as information for specifying the beam of the SRS. In addition, at this time, when it is necessary to specify the beam of the SRS using the reference signal of another cell (when the reference signal of another cell is used for spatial relationship derivation), the cell identifier (Resource Serving Cell ID) of the cell including the signal to be referred to for identifying the transmission beam, and the bandwidth part identifier (Resource BWP ID) may also be included in the command. Note that this MAC CE may be used when DL / UL separate TCI is used.On the other hand, when Unified TCI is used, SP / AP SRS TCI State Indication MAC CE or Serving Cell Set based SRS TCI State Indication MAC CE may be used. Therefore, in an environment where Unified TCI is used, information to be notified to the terminal device 111 in SP / AP SRS TCI State Indication MAC CE or Serving Cell Set based SRS TCI State Indication MAC CE may be notified in a Cell switch command. In this case, the Cell switch command may include, as information for specifying an SRS resource used for SRS transmission, a cell identifier (SRS Resource Set's Cell ID) of a cell including an SRS resource, a bandwidth part identifier (SRS Resource Set's BWP ID), an SRS Resource Set ID, a Resource ID, and a TCI state ID as information for specifying an SRS beam. In addition, at this time, if it is necessary to specify the beam of the SRS using the TCI state of another cell (when a reference signal of another cell is used for spatial relationship derivation), the cell identifier (TCI State Serving Cell ID) of the cell that contains the signal to be referenced to identify the transmission beam and the bandwidth part identifier (TCI State BWP ID) may also be included in the command.

[0027] For example, even when SRS transmission is instructed before the Cell switch command is transmitted or received, a similar MAC CE may be transmitted to the terminal device 111. Note that the SP SRS Activation / Deactivation MAC CE or the SP / AP SRS TCI State Indication MAC CE may be transmitted from the base station device 102 to the terminal device 111 via the base station device 101 before the handover, or another MAC CE may be newly defined. However, in this case, since the Cell switch command has not yet been transmitted or received, information indicating which base station device is the SRS transmission destination is notified, for example, by the MAC CE. In response to receiving the MAC CE, the terminal device 111 may start transmitting the SRS for the designated handover destination candidate base station device (for example, the base station device 102) using the beam designated by the MAC CE, based on the SRS transmission settings notified in advance for other settings.

[0028] Also, the terminal device 111 may be instructed to transmit the SRS by specifying the beam to be used, for example, using a physical downlink control channel (PDCCH) / downlink control information (DCI) instead of a MAC CE. For example, a PDCCH order-based SRS transmission instruction may be newly defined, and an instruction to transmit the SRS for the base station device 102 that is a candidate for the handover destination may be notified to the terminal device 111. In this PDCCH / DCI notification, the beam to be used is also specified, so that the beam previously specified when the RRC layer is configured is not used, and the beam newly notified by the PDCCH / DCI is used to transmit the SRS. Note that, even in the case of using the PDCCH / DCI, when an instruction to transmit the SRS is given before the Cell switch command is transmitted and received, identification information indicating the base station device that is a candidate for the handover destination to which the SRS is to be transmitted is notified to the terminal device 111. That is, when the transmission of SRS is instructed when the Cell switch command is transmitted or received, the command specifies the base station device of the handover destination, while the handover destination is not determined before the command is transmitted or received, so the destination of the SRS is specified. Note that the Cell switch command may also notify the terminal device 111 of information specifying the destination of the SRS. Note that in this case, the destination of the SRS is the base station device of the handover destination, but for example, when the RRC layer setting is completed for another base station device that is a candidate for the handover destination after the handover, information specifying the other base station device may be included. In this case, the terminal device 111 can transmit the SRS to the base station device that is a candidate for the handover destination immediately after the handover.

[0029] As described above, the base station device 102 measures the SRS transmitted from the terminal device 111 in response to the SRS transmission instruction issued from the base station device 101 to the terminal device 111 immediately after or before the handover of the terminal device 111. This allows the base station device 102 to determine which beam the terminal device 111 should use in uplink communication after the terminal device 111 connects to the base station device 102 by handover. The base station device 102 measures the SRS based on each of a plurality of settings, and identifies, for example, an SRS setting that can obtain communication quality that satisfies a predetermined standard. Then, the base station device 102 can specify, for example, which beam should be used when transmitting uplink user data, using identification information (for example, SRS Resource Setid and srs-Resourceid) of the SRS transmission setting. The base station device 102 may also specify an indicator (index) that can specify the resource from which the SRS was transmitted. In this case, the terminal device 111 specifies one setting used when transmitting the SRS based on the specified identification information, indicator, etc. Then, the terminal device 111 transmits a physical uplink shared channel (PUSCH) including user data using a beam specified by information included in the one setting. Note that the base station device 102 may specify a beam by TCI or by identification information of SSB or CSI-RS. When a TCI is specified, the terminal device 111 transmits user data using a beam corresponding to the TCI. Also, when the terminal device 111 receives identification information of SSB or CSI-RS, it transmits user data using a beam stored in association with the identification information. Also, a beam used for PUSCH transmission may be specified to the terminal device using a parameter for specifying a transmission beam uniquely determined by the identification information of TCI, SSB, or CSI-RS, for example, an SRS resource indicator, Precoding information and number of layers (TPMI). Note that the beam specification is notified to the terminal device 111 by, for example, DCI.This DCI may be, for example, a DCI of format 0_1.

[0030] In the above embodiment, an example has been described in which an SRS transmission instruction is transmitted and received by MAC CE or DCI. Similarly, an SRS transmission stop instruction can be transmitted and received by MAC CE or DCI. In addition, switching of a beam used when transmitting an SRS may also be instructed by MAC CE or DCI.

[0031] As described above, in this embodiment, the terminal device 111 can quickly use a fine beam in uplink communication after performing handover (cell switch), thereby improving communication efficiency.

[0032] Also, in the above embodiment, an example in which information on beam control of SRS is included in the Cell switch command has been described, but information on beam control of PUCCH may be included in the command. In this case, information to be notified to the terminal device 111 in the conventional PUCCH spatial relation Activation / Deactivation MAC CE may be notified in the Cell switch command. For example, the Cell switch command includes a cell identifier (Serving Cell ID) of a cell including PUCCH, a bandwidth part identifier (BWP ID), a PUCCH Resource ID, information on the beam of PUCCH, and active / inactive information of each PUCCH Spatial Relation Info set for each PUCCH-SpatialRelationInfoId. Before receiving the Cell switch command, one or more PUCCH Spatial Relation Info are set in advance for each PUCCH-SpatialRelationInfoId in the terminal device 111, and information on a reference signal for determining a beam at the time of PUCCH transmission, for example, any one of NZP CSI-RS resource index, SSB index, and SRS resource index is set in the PUCCH Spatial Relation Info. This allows the terminal device 111 to identify the beam to be used for transmitting the PUCCH, using the active / inactive information included in the Cell switch command.

[0033] In the above embodiment, an example in which information on uplink beam control is included in the Cell switch command has been described, but information on downlink beam control may be included in the command. When information on beam control of the downlink channel, PDSCH, and PDCCH is included in the Cell switch command, information to be notified to the terminal device 111 in the conventional TCI States Activation / Deactivation for UE-specific PDSCH MAC CE, Enhanced TCI States Activation / Deactivation for UE-specific PDSCH MAC CE, and TCI State Indication for UE-specific PDCCH MAC CE can be notified in the command. For example, the Cell switch command can be used to enable / disable the TCI state used for communication of the PDCCH or PDSCH using a specified cell identifier (Serving Cell ID) and bandwidth part identifier (BWP ID).

[0034] (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.

[0035] 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 SRS transmission instruction unit 303, an inter-base station communication unit 304, and a handover instruction 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 base station device 102, which is a candidate for the handover destination, is similar to a conventional base station device except that it may transmit information regarding the transmission of SRS to the base station device 101, and is therefore not shown here. Also, FIG. 3 shows only functions that are particularly related to this embodiment, and does not show various other functions that the base station device may have. For example, the base station device naturally has other functions that a base station device that complies with 5G or 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.

[0036] The RRC setting unit 301 notifies, for example, a connected terminal device of RRC layer setting information for another base station device that is a candidate for a handover destination and provides a neighboring cell. For example, the RRC setting unit 301 transmits a handover request to the other base station device that is a candidate for a handover destination, 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. Note that the RRC layer setting information may include a transmission setting of an SRS. The RACH instruction unit 302 instructs the terminal device, for which the RRC layer setting for the other base station device that is a candidate for a handover destination, to transmit an RA preamble to the other base station device. Then, the RACH instruction section 302 receives, for example, a TA value specified in the base station apparatus 102 from the base station apparatus 102, and notifies the terminal apparatus of the TA value, thereby completing the RA procedure.

[0037] The SRS transmission instruction unit 303 notifies the terminal device of an SRS transmission instruction for the other base station device, which is a candidate for the handover destination, to the terminal device for which the RRC layer setting for the other base station device has been completed. For example, the SRS transmission instruction unit 303 may transmit the SRS transmission instruction to the terminal device after the above-mentioned RA procedure is completed and before the transmission of the Cell switch command. Also, the SRS transmission instruction unit 303 may transmit the SRS transmission instruction to the terminal device, for example, in the Cell switch command. The SRS transmission instruction may be notified to the terminal device, for example, by using the MAC CE or DCI. Note that the SRS transmission instruction unit 303 may notify the terminal device of information specifying a beam to be used for transmitting the SRS together with the SRS transmission instruction. For example, the SRS transmission instruction unit 303 notifies the terminal device of information specifying a beam to be used when transmitting the SRS using each of one or more transmission settings. Furthermore, the SRS transmission instruction unit 303 may notify the terminal device of information indicating whether each of a plurality of beams available to the terminal device is used for transmitting the SRS. This notification may be performed, for example, as a notification of a setting of enable / disable of the TCI state. Furthermore, for example, when identification information of a signal to be measured in the terminal device, such as SSB or CSI-RS, and a beam that can receive the signal to be measured with sufficient radio quality are associated and stored in the terminal device, the identification information of the signal to be measured may be notified. In this case, the terminal device transmits the SRS using the beam associated with the identification information of the signal to be measured.

[0038] The inter-base station communication unit 304 communicates with the base station device that is a candidate for the handover destination. The inter-base station communication unit 304 is used, for example, to receive information for an RA procedure in the base station device that is a candidate for the handover destination and information for transmitting an SRS by the terminal device. The inter-base station communication unit 304 may also notify another base station device that is a destination of the RA preamble, for example, to transmit an RA preamble to the connected terminal device. The inter-base station communication unit 304 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 also be acquired via the inter-base station communication unit 304. Note that the signal received in the inter-base station communication unit 304 may be transferred to the terminal device via, for example, the RACH instruction unit 302 or the like. The inter-base station communication unit 304 may also receive an instruction to transmit an SRS to the terminal device from the base station device that is a candidate for the handover destination. In this case, the SRS transmission instruction unit 303 may instruct the terminal device to transmit the SRS based on an instruction received from the base station device of the handover destination candidate. Note that this is just one example, and the inter-base station communication unit 304 may transmit an instruction to cause the connected terminal device to transmit the SRS without being instructed by the base station device of the handover destination candidate, and may notify the base station device of the handover destination candidate that the instruction has been transmitted. Note that the inter-base station communication unit 304 receives information from the base station device of the handover destination candidate that specifies the beam to be used when transmitting the SRS. In this way, the inter-base station communication unit 304 communicates various information related to the RA procedure and SRS transmission related to the handover to the base station device with the base station device of the handover destination candidate.

[0039] The handover instruction unit 305 instructs the terminal device to execute a handover when the terminal device is in a state where it 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 305 transmits, for example, a Cell switch command including a cell identifier or the like capable of identifying the base station device at the handover destination to the terminal device. When the terminal device receives this command, it changes the base station device of the connection destination to the specified base station device and completes the handover.

[0040] FIG. 4 shows an example of the functional configuration of a terminal device. The terminal device has, for example, an RRC setting unit 401, an instruction receiving unit 402, a RACH processing unit 403, an SRS transmission processing unit 404, and a handover processing 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 from the illustration. 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 here in detail, and only the rough functions will be outlined.

[0041] 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 the base station device to which the connection is made. 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. In this embodiment, the RRC setting unit 401 may acquire and hold one or more pieces of setting information for SRS transmission for each of the base station devices to which the connection is made from the base station device to which the connection is made. The instruction receiving unit 402 receives an instruction to transmit an RA preamble or an instruction to transmit an SRS from the currently connected base station device. The instruction to transmit an RA preamble may be issued by the PDCCH order. The instruction to transmit an SRS may be received when a Cell switch command is received or before the command is issued, and a MAC CE or a DCI may be used at that time. In response to the instruction to transmit an RA preamble being received by the 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, the sequence for generating the RA preamble and information on radio resources for receiving the RA preamble in the base station device of the handover destination candidate may be received from the currently connected base station device via the instruction receiving unit 402. Note that the information on the sequence and radio resources may be acquired from system information transmitted by the base station device of the handover destination candidate.

[0042] In response to receiving an SRS transmission instruction by the instruction receiving unit 402, the SRS transmission processing unit 404 transmits an SRS for measurement by a base station device that is a candidate for a handover destination. The SRS transmission processing unit 404 generates an SRS using the setting information received by the RRC setting unit 401, and transmits the SRS using, for example, a beam specified by the SRS transmission instruction received by the instruction receiving unit 402. Note that, for example, when the SRS transmission setting is semi persistent or periodic, the SRS transmission processing unit 404 repeatedly transmits the SRS. In this case, the SRS transmission processing unit 404 can stop transmitting the SRS when, for example, an SRS transmission stop instruction is received by the instruction receiving unit 402. Note that the SRS transmission period may be set in advance (for example, in the SRS transmission instruction), and in this case, the stop instruction may not be received. In addition, for example, the setting information received in advance by the RRC setting unit 401 may include beam setting information, and in this case, the transmission instruction may indicate whether to transmit the SRS using the beam setting or to transmit the SRS using another beam specified in the SRS transmission instruction.

[0043] The handover processing unit 405 changes the destination base station in response to receiving a handover command (Cell switch command) from the connected base station. The terminal device does not need to perform the RRC layer setting process and the RA procedure again with the handover destination candidate base station by the operation of the RRC setting unit 401 and the RACH processing unit 403. For this reason, when the terminal device is notified of the allocation of uplink radio resources in the handover destination candidate base station in the received command, for example, the terminal device can immediately transmit the uplink signal using the radio resources. In addition, the operation of the SRS transmission processing unit 404 allows the handover destination base station device to specify the setting of a fine beam for uplink communication at the time of handover or in a short time after the handover. For this reason, the handover destination base station device can notify the terminal device of information specifying the beam to be used together with the above-mentioned allocation of uplink radio resources.

[0044] (Processing flow) 5 and 6 show an example of the flow of processing executed in the wireless communication system of the present embodiment. FIG. 5 shows an example of the flow of processing when an SRS transmission instruction is notified to a terminal device in a Cell switch command. On the other hand, FIG. 6 shows an example of the flow of processing when an SRS transmission instruction is notified to a terminal device using, as an example, a PDCCH order (DCI) before the command is transmitted. Here, in FIG. 5 and FIG. 6, it is assumed that a connection is established between a handover source base station device and a terminal device. Note that the examples shown in FIG. 5 and FIG. 6 are only examples, and the above-described modifications can be made in each procedure. For example, in the example of FIG. 6, an example is shown in which an SRS transmission instruction is transmitted and received by a PDCCH order, but an individual transmission instruction may be transmitted and received by, for example, a MAC CE.

[0045] First, an example of the process flow in FIG. 5 will be described. A first base station device, which is a handover source, first transmits a HANDOVER REQUEST message to a second base station device (base station device A and base station device B in FIG. 5) that forms an adjacent cell that is a candidate for the handover destination of the terminal device (S501, S503). 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 and SRS setting information to the first base station device (S502, S504). Then, the first base station device notifies the terminal device of an RRC Reconfiguration message including the communication parameters notified from the second base station device (S505). Note that the first base station device may organize separate messages for each of the information on a plurality of candidate base station devices as the handover destination and notify the terminal device of the information, or may notify the terminal device of a message including all of the information. When the terminal device receives this message, it transmits an RRC Reconfiguration Complete message to the first base station device to complete the configuration process using the RRC layer message. This process eliminates the need for the terminal device to perform the RRC layer configuration process after receiving a handover instruction from the first base station device to the second base station device. In addition, the terminal device becomes able to transmit the SRS according to an SRS transmission instruction notified in the handover instruction or before receiving the instruction. Note that, although FIG. 5 shows an example of the flow of the configuration process when there are two candidate base station devices as the handover destination, the same configuration process can be performed for one candidate base station device or for three or more candidate base station devices.

[0046] 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 a sequence to be used by the terminal device to generate an RA preamble and a 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, which 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 an RA preamble and the radio resource to which the RA preamble should be transmitted may be specified in advance. In one example, in S505, the terminal device may be notified of information on the sequence and the radio resource to which the RA preamble should be transmitted. 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 an RA preamble. Also, the first base station device may notify the second base station device of information on the sequence used by the terminal device to generate the RA preamble. This allows the second base station device to detect the RA preamble transmitted by the terminal device. Note that this is just one example, and for example, the HANDOVER REQUEST of S501 or S503 may implicitly notify that the terminal device plans to transmit the 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 of S502 or S504. After that, for example, the second base station device that has received the RA preamble transmits an RA response to the terminal device via the first base station device (S509). The second base station device can identify the TA value when the terminal device is connected to its own device based on the reception timing of the RA preamble, and can notify the terminal device of the TA value by the RA response.The RA procedure includes transmission of a physical uplink shared channel (message 3) from the terminal device to the second base station device after the RA response, and transmission of a contention resolution message (message 4) from the second base station device to the terminal device after message 3 via the first base station device, but these are not shown in Fig. 5. Note that it is sufficient to specify the TA value here, and it is not necessary to transmit message 3 or message 4. A similar RA procedure can also be executed between the terminal device and base station device B (S510 to S512).

[0047] The terminal device measures radio signals (e.g., SSB) transmitted from each of the second base station devices that are candidates for the handover destination, for example, in parallel with the RA procedure, and notifies the first base station device of the measurement result (not shown). Here, the first base station device can 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 can transmit CSI-RS to form a more fine beam and have the terminal device measure it. In this case, the second base station device can 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 makes it possible for the second base station device to transmit downlink signals using a fine beam immediately after the terminal device has handed over to the second base station device.

[0048] Furthermore, the first base station device may determine to perform handover according to the measurement result of the radio signal (for example, SSB) notified from the terminal device. For example, when a predetermined condition is satisfied, such as when the radio quality of the signal from the base station device A 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 determine to handover the terminal device to the base station device A. Based on this determination, the first base station device transmits a Cell switch command instructing the terminal device to execute handover (S514). The terminal device executes handover to the base station device A by this command. Note that this command includes identification information capable of identifying the base station device A as the handover destination. In this processing example, the command further includes SRS transmission instruction information and information specifying the beam to be used when transmitting the SRS. Note that the SRS transmission instruction information may include information indicating which setting information should be used to transmit the SRS among the SRS transmission setting information for the base station device A designated as the handover destination, received in S505. However, in cases where there is only one piece of SRS transmission setting information, information specifying that setting information does not need to be included in the command.

[0049] The transmission setting information of the SRS to be used and information specifying the beam to be used may be acquired from the base station device A of the handover destination (S513). In one example, the base station device A may voluntarily transmit setting information to be used for transmitting the SRS to the first base station device before issuing the command of S514, assuming that the base station device A is the handover destination of the terminal device. In this case, for example, the base station device B may also transmit similar information to the first base station device. Furthermore, the first base station device may notify the base station device A that the terminal device will handover to the base station device A before issuing the command of S514, and the base station device A may transmit setting information to be used for transmitting the SRS to the first base station device in response to the notification. The transmission setting information of the SRS to be used may be specified using, for example, SRS Resource Setid and srs-Resourceid. The information specifying the beam may be information indicating the activation or deactivation of setting information for forming each of a plurality of beams that the terminal device can use. Note that information specifying a beam directly may be notified to the terminal device so as to use a specific beam. For example, the TCI may be used to specify the beam, or when a beam with the best radio quality of the SSB or CSI-RS or a beam exceeding a predetermined quality level is stored in the terminal device in association with information capable of identifying the SSB or CSI-RS (such as identification information or information on the radio resource on which these signals are transmitted), the information capable of identifying the SSB or CSI-RS may be used as information for specifying the beam. When the terminal device receives information capable of identifying the SSB or CSI-RS, it specifies that the beam stored in association with the SSB or CSI-RS is specified. Note that when the transmission setting information of the SRS received in S505 includes first information for specifying a beam, if second information for specifying a beam is received in S514, the first information may be replaced (at least temporarily) by the second information. Then, the terminal device transmits the SRS using the specified beam (S515).

[0050] By measuring this SRS, the base station device A can specify the beam to be used when the terminal device transmits an uplink signal. Then, when transmitting DCI to allocate radio resources for transmitting user data (PUSCH) in the uplink (S516), the base station device A can include information specifying the specified beam to be used. The information specifying the beam here can be, for example, an SRS resource indicator. When the terminal device receives this DCI, it transmits uplink user data using the specified beam and the specified radio resources (S517).

[0051] Conventionally, after a handover command such as S514 is transmitted and the terminal device is connected to base station device A, base station device A transmits an SRS transmission start instruction to the terminal device, and then the SRS is transmitted. In contrast, in this processing example, an SRS transmission instruction is transmitted to the terminal device at the timing when the handover command is issued, so that the terminal device can start transmitting the SRS early. As a result, a fine beam to be used in uplink communication can be quickly determined, and the efficiency of wireless communication can be improved.

[0052] Next, a processing example shown in FIG. 6 will be described. In the processing of FIG. 6, before the cell switch command is transmitted, an SRS transmission instruction is transmitted to the terminal device. In this processing example, the same processing as S501 to S512 in FIG. 5 is also performed first. After that, for example, a first base station device transmits an SRS transmission instruction to the terminal device. For example, the first base station device transmits a PDCCH order (DCI) including an SRS transmission instruction to the base station device A and beam information to the terminal device (S601). Each piece of information transmitted here is the same as each piece of information transmitted to the terminal device in S514 in FIG. 5, and can be configured based on information notified in advance from the base station device A to the first base station device in the same manner as S513. In accordance with this SRS transmission instruction, the terminal device transmits the SRS using the setting information for the base station device A and the designated beam (S602). By measuring this SRS, the base station device A can determine the beam to be used by the terminal device in uplink communication when it is assumed that the terminal device is connected to the base station device itself. Similarly, the first base station device transmits a PDCCH order (DCI) including an SRS transmission instruction and beam information for the base station device B to the terminal device (S603), and the terminal device transmits the SRS using the setting information for the base station device B and the designated beam in accordance with the SRS transmission instruction (S604). The base station device B can also measure the SRS to determine the beam to be used by the terminal device in uplink communication when it is assumed that the terminal device is connected to the base station device itself. The first base station device may transmit an instruction to the terminal device to transmit the SRS for all of the candidate base station devices of the handover destination, or may transmit an instruction to the terminal device to transmit the SRS for only some of the candidate base station devices. For example, the terminal device may continuously perform measurements such as SSB and notify the first base station device of the measurement results, and the first base station device may determine the candidate base station device of the handover destination to which the SRS is to be transmitted according to the measurement results.

[0053] The first base station device may transmit the SRS measurement instruction including information capable of identifying the base station device A or the base station device B, but this information may not necessarily be included. For example, when there is only one second base station device as a candidate for the handover destination, the information for identifying the second base station device may be omitted. Also, a unique identifier may be assigned to each of the transmission setting information of the multiple SRSs related to the multiple base station devices as the candidate for the handover destination, and the identifier may be notified to the terminal device. In this case, since it is sufficient for the terminal device to transmit the SRS for any of the candidate base station devices, it is not necessary to notify the terminal device of the information for which base station device each of the multiple setting information is intended. Also, the information capable of identifying the candidate base station device as the handover destination may be a physical cell identifier (PCI) in one example, but is not limited to this. For example, when the RRC layer is configured, identification information such as the PCI of each base station device may be associated with a specific index and notified to the terminal device, and the specific index may be notified to the terminal device as information capable of identifying the base station device when instructing the transmission of the SRS. For example, 1008 PCIs are prepared in 5G, and one of them is assigned to each base station device. In contrast, the number of candidates for handover destinations is limited, and even if an index that can uniquely identify each of the candidates is assigned, it is possible to express the index with fewer bits than the number of bits required to express the PCI. For this reason, such an index may be used, for example, to reduce the number of bits of the PDCCH order.

[0054] Thereafter, the first base station device transmits a Cell switch command to the terminal device instructing the terminal device to execute the handover in response to the determination that the handover should be performed according to the measurement result of the radio signal (for example, SSB) notified from the terminal device (S605). The terminal device executes the handover to the base station device A by this command. Note that this command includes information on the base station device A indicating the handover destination. Meanwhile, the base station device A has already measured the SRS in S602, and has completed the determination of the beam to be used by the terminal device in the uplink communication. For this reason, the command of S605 does not include an instruction to transmit the SRS, and it is not necessary to transmit an instruction to transmit the SRS after the command of S605. Then, when transmitting DCI to allocate radio resources for transmitting user data (PUSCH) in the uplink (S516), for example, the base station device A may include information specifying the beam to be used specified in S602. When the terminal device receives this DCI, it transmits the uplink user data using the specified beam and the specified radio resources (S517). In this manner, in this processing example, an SRS transmission instruction is sent to the terminal device before a handover command is issued, so that the terminal device can perform uplink communication using a fine beam immediately after handover is performed by that command.

[0055] In the above example, the RA procedure is performed before the transmission and reception of the Cell switch command, but the RA procedure does not necessarily have to be performed at this timing. For example, in the above embodiment, the handover source and handover destination base station devices are described as providing one cell, but these base station devices may provide multiple cells. In this case, if the handover source base station device is not changed and handover to another cell (using a different frequency band, for example) is performed, the RA procedure for establishing uplink synchronization may be omitted. In the above description, the term "base station device" may be read as "cell". That is, in the above description, terms such as "first base station device of handover source" and "second base station device of candidate for handover destination" are used, but these may be read as "first cell of handover source" and "second cell of candidate for handover destination". However, it should be noted that the main entity that transmits and receives signals is the base station device that provides each cell. In the above example, the terminal device transmits the SRS, but a different uplink reference signal from the SRS may be used. In this way, in the above-described embodiment, it is sufficient that the instruction to transmit the SRS is notified to the terminal device after the handover, but in the command instructing the handover or prior to the command, and the transmission of the SRS is started, and various modifications and alterations are possible for other matters.

[0056] As described above, according to this 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 using a fine beam used for uplink communication. This makes it possible to contribute to Goal 9 of the United Nations-led Sustainable Development Goals (SDGs), which is to "build resilient infrastructure, promote sustainable industrialization and foster innovation."

[0057] 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 currently connected base station device, configuration information of an uplink reference signal for measurement in another base station device that provides a candidate cell for a handover destination, and then receiving, from the base station device, a signal including an instruction to transmit the uplink reference signal and information specifying a beam to be used for transmitting the uplink reference signal; a transmitting means for transmitting the uplink reference signal to the other base station device by using the configuration information and information specifying the beam in response to receiving the signal from the base station device and before receiving a command from the base station device instructing a handover to a cell provided by the other base station device; having The receiving means further receives, from the base station device, the command instructing a handover to a cell provided by the other base station device; The terminal device further includes an execution means for executing a handover to a cell provided by the other base station device based on the command being received from the base station device. A terminal device comprising:

2. The terminal device according to claim 1 , characterized in that the information designating the beam is information indicating whether a plurality of beams usable by the terminal device are enabled or disabled.

3. The terminal device according to claim 2 , wherein the information indicating the activation or deactivation of a plurality of beams available to the terminal device is indicated using a Transmission Configuration Indicator (TCI) state.

4. a measurement means for measuring each of a plurality of predetermined downlink signals transmitted from the other base station device by using each of a plurality of beams that the terminal device can use; and a storage means for storing the plurality of predetermined signals in association with any one of the plurality of beams based on the result of the measurement for each of the plurality of predetermined signals, the information designating the beam is information capable of identifying any one of the plurality of predetermined signals, the transmitting means transmits the uplink reference signal using a beam stored in association with the specified signal identified based on the information.

2. The terminal device according to claim 1 .

5. The terminal device according to claim 1 , wherein the uplink reference signal is a sounding reference signal (SRS).

6. The terminal device according to claim 1 , wherein the signal further includes information capable of identifying a cell provided by the other base station device.

7. An index capable of uniquely identifying a cell as a handover destination candidate is assigned to the terminal device; the receiving means receives the signal including the index as information capable of identifying the other base station device.

7. The terminal device according to claim 6,

8. A base station device, a receiving means for receiving, from another base station device, configuration information of an uplink reference signal for measurement in the other base station device that provides a candidate cell as a handover destination of a connected terminal device, and then receiving, from the other base station device, information designating a beam to be used for transmitting the uplink reference signal; a transmitting means for transmitting the setting information to the terminal device, then transmitting a signal including an instruction to transmit the uplink reference signal and information specifying the beam, and transmitting a command to the terminal device after transmitting the signal, the command instructing a handover to a cell provided by the other base station device; A base station device comprising:

9. The base station device according to claim 8, characterized in that the information designating the beam is information indicating whether a plurality of beams usable by the terminal device are enabled or disabled.

10. The base station apparatus according to claim 9 , wherein the information indicating whether a plurality of beams available to the terminal apparatus are enabled or disabled is indicated using a Transmission Configuration Indicator (TCI) state.

11. the terminal device stores a plurality of beams usable by the terminal device in association with each of a plurality of predetermined downlink signals transmitted from the other base station device; the transmitting means transmits to the terminal device, as information for specifying the beam, information capable of identifying any one of the plurality of predetermined signals included in the command; The base station apparatus according to claim 8 .

12. The base station apparatus according to claim 8, wherein the uplink reference signal is a sounding reference signal (SRS).

13. The base station device according to claim 8 , wherein the signal further includes information capable of identifying a cell provided by the other base station device.

14. An index capable of uniquely identifying a cell as a handover destination candidate is assigned to the terminal device; the transmitting means transmits the signal including the index as information capable of identifying the other base station device. The base station device according to claim 13 .

15. A control method executed by a terminal device, comprising: receiving, from a currently connected base station device, configuration information of an uplink reference signal for measurement in another base station device that provides a candidate cell for a handover destination; receiving, after receiving the configuration information, from the base station device, a signal including an instruction to transmit the uplink reference signal and information specifying a beam to be used for transmitting the uplink reference signal; In response to receiving the signal from the base station device and before receiving a command from the base station device instructing a handover to a cell provided by the other base station device, transmitting the uplink reference signal to the other base station device using the configuration information and information specifying the beam; receiving the command from the base station device instructing a handover to a cell provided by the other base station device; performing a handover to a cell provided by the other base station device based on the command being received from the base station device; A control method comprising:

16. A control method executed by a base station device, comprising: receiving, from another base station device, configuration information of an uplink reference signal for measurement in the other base station device that provides a candidate cell as a handover destination of the currently connected terminal device; transmitting the setting information to the terminal device; receiving, after transmitting the configuration information, information designating a beam to be used for transmitting the uplink reference signal from the other base station device; Transmitting a signal including an instruction to transmit the uplink reference signal and information specifying the beam to the terminal device; transmitting, after the transmission of the signal, a command to the terminal device instructing a handover to a cell provided by the other base station device; A control method comprising:

17. A computer installed in the terminal device receiving, from the currently connected base station device, configuration information of an uplink reference signal for measurement in another base station device that provides a candidate cell for a handover destination; receiving, from the base station device after receiving the setting information, a signal including a transmission instruction for the uplink reference signal and information specifying a beam to be used for transmitting the uplink reference signal; in response to receiving the signal from the base station device and before receiving from the base station device a command instructing a handover to a cell provided by the other base station device, transmitting the uplink reference signal to the other base station device by using the configuration information and information designating the beam; receiving, from the base station device, the command instructing a handover to a cell provided by the other base station device; executing a handover to a cell provided by the other base station device based on the command being received from the base station device; Program for.

18. A computer installed in the base station device receiving, from another base station device, configuration information of an uplink reference signal for measurement in the other base station device that provides a candidate cell as a handover destination of the currently connected terminal device; transmitting the setting information to the terminal device; After transmitting the configuration information, receive information designating a beam to be used for transmitting the uplink reference signal from the other base station device; transmitting a signal including an instruction to transmit the uplink reference signal and information specifying the beam to the terminal device; causing the terminal device to transmit a command instructing a handover to a cell provided by the other base station device after the transmission of the signal; Program for.

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