Terminal device that executes handover with improved efficiency, base station device, and control method

Abstract

This terminal device receives, from a base station device being connected, setting information of an uplink reference signal for measurement in another base station device that provides a cell serving as a handover destination candidate, thereafter receives, from the base station device, a command indicating a handover to the cell provided by the other base station device, and including an instruction to transmit the uplink reference signal and information designating a beam to be used for transmission of the uplink reference signal, executes the handover to the cell provided by the other base station device on the basis of the command, and in response to the reception of the command from the base station device, without receiving an instruction to transmit the uplink reference signal from the other base station device, transmits the uplink reference signal to the other base station device using the setting information and the information designating the beam.

Description

Terminal device, base station device, and control method for performing handover with improved efficiency

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

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

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

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

[0005] A terminal device according to one aspect of the present invention has a receiving means for receiving, from a base station device to which it is connected, configuration information for 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 command instructing a handover to a cell provided by the other base station device, the command including an instruction to transmit the uplink reference signal and information specifying a beam to be used for transmitting the uplink reference signal; an executing means for executing a handover to the cell provided by the other base station device based on the command; and a transmitting means for transmitting the uplink reference signal to the other base station device using the configuration information and information specifying the beam in response to receiving the command from the base station device and without receiving an instruction to transmit the uplink reference signal from the other base station device.

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

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

[0008] Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings, in which the same or similar elements are designated by the same reference numerals.

[0009] The accompanying drawings are incorporated in and constitute a part of the specification, illustrate embodiments of the present invention, and together with the description are used to explain the principles of the present invention. Figure 1 is a diagram showing an example of the configuration of a wireless communication system. Figure 2 is a diagram showing an example of the hardware configuration of an apparatus. Figure 3 is a diagram showing an example of the functional configuration of a base station apparatus. Figure 4 is a diagram showing an example of the functional configuration of a terminal apparatus. Figure 5 is a diagram showing an example of the flow of processing executed in the wireless communication system. Figure 6 is a diagram showing an example of the flow of processing executed in the wireless communication system.

[0010] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be combined in any desired manner. Furthermore, the same reference numerals are used to designate identical or similar components, and redundant descriptions will be omitted.

[0011] (System Configuration) Fig. 1 shows an example of the configuration of a wireless communication system according to this embodiment. This wireless communication system is a cellular communication system configured in accordance with a cellular communication standard such as the fifth generation (5G) of the Third Generation Partnership Project (3GPP (registered trademark)) or its successor standard. The wireless communication system includes a base station device 101, a base station device 102, and a terminal device 111. Note that, for simplicity of explanation, 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.

[0012] 1, terminal device 111 is about to move from the range of the cell formed by base station device 101 into the range of the cell formed by base station device 102. At this time, terminal device 111 performs handover to switch the connected base station device from base station device 101 to base station device 102.

[0013] In this embodiment, a handover procedure is performed to shorten the period during which the terminal device 111 is unable to communicate user data due to handover. That is, the handover source base station device 101 performs configuration in the radio resource control (RRC) layer (configuration using an RRC message) before actually handing over the terminal device 111. That is, in a conventional handover, the terminal device 111 receives a handover instruction from the handover source base station device 101, and the RRC layer configuration for the handover destination base station device 102, which is performed during the handover process, is notified to the terminal device 111 prior to the timing when the handover process is actually performed. In this configuration, communication parameters used for connection and communication with the handover destination candidate base station device 102 are notified to the terminal device 111. The communication parameters include, for example, beam configuration in the handover destination candidate base station device 102, parameters for executing a random access procedure, and radio resources (frequency and time resources) to be used. Furthermore, when multiple communication parameters used for communication with the base station device 102 are notified, an identifier (for example, an RRC-TransactionIdentifier or a CondReconfigId) uniquely associated with each of the communication parameters may be included in the RRC message notified to the terminal device 111 or in the configuration parameters in the RRC layer. In this case, a CondReconfigId is set for each candidate base station device or combination of base station devices as the handover destination. Furthermore, when multiple communication parameters are notified for each base station device or combination of base station devices, an RRC-TransactionIdentifier may be set. As an example, when 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 (for example, 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 synchronization signals (SS) / physical broadcast channel (PBCH) blocks (SSBs) transmitted from the handover destination candidate base station device 102. Note that the SSBs may be transmitted using beams directed in different directions. In this case, a measurement instruction may be transmitted so that the terminal device 111 measures each of the SSBs.

[0014] In accordance with a measurement instruction from the currently connected base station device 101, the terminal device 111 measures the SSB transmitted from the base station device 102, which is a candidate for handover, and notifies the currently connected base station device 101 of the measurement results. The base station device 101 may also notify the currently connected base station device 101 of the measurement results received from the terminal device 111 to the currently connected base station device 102, which is a candidate for handover and was the target of the measurement. This allows the base station device 102 to identify the general direction of the transmission beam to be used for downlink communication (signal transmission) with the terminal device 111 when the terminal device 111 actually performs a handover. Note that the terminal device 111 may be able to set multiple beams that can be used for downlink communication. In this case, the terminal device 111 may measure the SSB using each of these multiple beams. In this case, the terminal device 111 may identify which of these multiple beams provided the best SSB wireless quality and 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 wireless quality for each of the multiple SSBs in association with the identification information of the SSBs. 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 predetermined SSB, the base station device 102 can notify the terminal device 111 of the identification information that identifies the predetermined SSB and instruct the terminal device 111 to communicate using the beam corresponding to the predetermined SSB.

[0015] The base station device 102 may further notify the terminal device 111 of the transmission settings of channel state information (CSI)-reference signal (RS) in order to specify a finer beam to be used in the downlink. Note that this notification is performed by the base station device 102 notifying the base station device 101 of the CSI-RS transmission settings, and the base station device 101 transferring the transmission settings to the connected terminal device 111. Upon receiving the transmission settings, the terminal device 111 performs measurements of the CSI-RS transmitted from the base station device 102 (e.g., measurements of reference signal received power (CSI-RSRP), reference signal received quality (CSI-RSRQ), and received signal strength indicator (CSI-RSSI)) in accordance with the information, and notifies the connected base station device 101 of the measurement results. The base station device 101 then transfers the measurement results to the base station device 102. In this way, the base station device 102 can acquire the measurement results of the CSI-RS in the terminal device 111. As a result, the base station device 102 can determine, before the handover actually takes place, the precise beam to be used in downlink communication after the terminal device 111 has performed a handover. This makes it possible to omit the process of determining the beam direction in detail after the handover has taken place, thereby shortening the period during which the efficiency of user data communication for the terminal device 111 decreases.

[0016] As described above, the terminal device 111 may be able to set multiple beams that can be used in downlink communications. In this case, the terminal device 111 may measure the CSI-RS using each of the multiple beams. In this case, the terminal device 111 may identify which of the multiple beams provided good radio quality for 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 CSI-RS settings for each of the multiple configurable beams, and cause the terminal device 111 to perform measurements for each of the beams. The terminal device 111 may store the beam settings that provided good radio quality for the CSI-RS corresponding to each of the multiple beams in the base station device 102, in association with the identification information of the CSI-RS. In one example, after terminal device 111 has handed over to base station device 102, when base station device 102 transmits a signal using a beam corresponding to a specified CSI-RS, it can notify terminal device 111 of identification information that identifies the specified CSI-RS and instruct terminal device 111 to communicate using the beam corresponding to the specified CSI-RS.

[0017] 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 CSI-RS radio resource configuration (CSI-RS-Resource) and a report configuration (csi-ReportConfig) for the base station device 102 that are pre-configured 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, in csi-ReportConfig, aperiodic (aperiodic) or periodic (periodic) CSI report reporting timing is configured. For example, by specifying csi-ReportConfig using csi-ReportConfig id in MAC CE or DCI, it is possible to instruct the terminal device 111 to report the CSI-RS measurement results at a specific timing. In addition, when the base station device 101 performs the above-mentioned measurement configuration on the terminal device 111, it can transmit to the terminal device 111, for example, a cell identifier or the like, included in MAC CE or DCI, as information for the terminal device 111 to identify which base station device's CSI-RS transmission configuration it is.

[0018] Furthermore, the terminal device 111 executes a random access (RA) procedure before it is determined to perform a handover to the base station device 102. That is, the terminal device 111 may perform processing for establishing uplink synchronization before the actual handover is performed. For example, the base station device 101 to which the terminal device 111 is connected transmits an instruction to the terminal device 111 to transmit a random access preamble (RA preamble) to the base station device 102. Then, in accordance with this instruction, the terminal device 111 transmits the RA preamble using radio resources prepared for the RA procedure in the base station device 102. Note that information about these radio resources and the sequence used to generate the RA preamble may be notified to the terminal device 111 by the base station device 101 during, for example, the above-mentioned RRC layer configuration processing. Furthermore, this information may be notified to the terminal device 111 when the base station device 101 issues an instruction to transmit the RA preamble. Through this procedure, the RA preamble transmitted from the terminal device 111 is received by the base station device 102. The base station device 102 can then 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. The base station device 102 then 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 then 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 base station device 101 instructs the terminal device 111 to perform handover to the base station device 102, and can therefore transmit uplink user data to the base station device 102 without re-executing an RA procedure.

[0019] Thereafter, the base station device 101 may notify the terminal device 111 to perform handover at a predetermined trigger, such as when the reception quality of a radio signal from the base station device 101 at the terminal device 111 deteriorates by a predetermined level compared to the reception quality of a radio signal from the base station device 102. This notification may be performed using a command that may be called a Cell switch command. This command may be transmitted using a Medium Access Control Element (MAC CE) or Downlink Control Information (DCI). Note that this command includes identification information of the cell formed by the handover destination base station device 102. Furthermore, when multiple settings are configured in the RRC layer used for communication with the base station device 102, identifiers (e.g., RRC-TransactionIdentifier or CondReconfigId) uniquely associated with each setting in the RRC layer may be included in this command. Furthermore, information regarding the beam determined in the base station device 102 by measuring SSB and CSI-RS in advance as described above and notified to the base station device 101 may be notified from the base station device 101 to the terminal device 111. In response to receiving a command from the base station device 101 specifying the base station device 102 as the handover destination, the terminal device 111 changes its connection destination from the base station device 101 to the base station device 102. In this case, since the RA procedure has already been executed as described above, there is no need to execute a new RA procedure. Furthermore, since CSI-RS measurement has been performed before the handover as described above, there is no need to newly determine the precise beam to be used in downlink communications after the handover. Therefore, the terminal device 111 can communicate user data with the base station device 102 immediately after the handover. Furthermore, if the handover is successful after receiving the command, the terminal device 111 can notify the base station device 102 of the success in an uplink control message such as an RRC message of RRC Reconfiguration Complete or a MAC CE.In this case, the terminal device 111 may include an identifier (for example, RRC-TransactionIdentifier or CondReconfigId) associated with the communication parameters used for communication with the base station device 102 in the RRC message.

[0020] On the other hand, at this point, a fine beam to be used in uplink communication has not been determined, and a subsequent setting process is required to use such a fine beam. 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 handover, measures the SRS. Then, based on the measurement results, the base station device 102 can determine a beam to use when receiving an uplink signal transmitted from the terminal device 111 in communication after handover of the terminal device 111. Conventionally, such SRS transmission and uplink beam determination are performed by transmitting an SRS transmission instruction from the handover destination base station device 102 to the terminal device 111 after handover is completed. For this reason, it is expected that there is a long period of time between the completion of handover and the determination of a fine beam for uplink communication, and communication efficiency during that period will be insufficient. In view of these circumstances, in this embodiment, 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.

[0021] 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). Alternatively, the base station device 101 may transmit a signal instructing the terminal device 111 to transmit an SRS before issuing the above-mentioned command. In response to receiving this instruction signal, the terminal device 111 transmits an SRS based on the SRS settings for the base station device 102 notified in advance. This makes it possible to determine the beam to be used in uplink communication earlier than when an SRS is transmitted in response to an instruction from the handover destination base station device 102 after the handover is completed.

[0022] The SRS configuration is notified from the base station device 101 to the terminal device 111, for example, when configuring the RRC layer for the handover destination base station device 102 as described above. Note that this is just an example, and the SRS configuration may be performed separately from the RRC layer configuration. This SRS configuration information is notified from the base station device 102 to the base station device 101, and the base station device 101 may notify the notified configuration information to the terminal device 111. The SRS configuration includes the number of ports from which the SRS is transmitted, the SRS transmission period, the number of OFDM symbols, parameters related to the SRS bandwidth, parameters related to SRS hopping, the position of the SRS in the frequency domain, a cyclic shift, the subcarrier spacing from 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. Note that when periodic or semi-persistent is specified, the SRS transmission period further includes information specifying the time period and a time offset from a 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 over a certain period. However, this is merely an example, and the following discussion can also be applied when periodic or aperiodic is specified. The number of OFDM symbols indicates the number of OFDM symbols over which the SRS is transmitted consecutively, and can be selected from, for example, 1 symbol, 2 symbols, or 4 symbols. The parameters related to the SRS bandwidth, the parameters related to the SRS hopping, and the position of the SRS in the frequency domain are parameters that enable specifying the frequency-time resource in which the SRS is transmitted using, for example, a table or a calculation formula specified in the standard. The cyclic shift and the identifier of the sequence used to generate the SRS are used when generating the SRS signal sequence by shifting a known sequence used to generate the SRS by the time indicated by the cyclic shift.The subcarrier interval at which the SRS is transmitted is set to 2, 4, or the like, indicating that the SRS is transmitted at a frequency interval of 2 subcarriers or 4 subcarriers.

[0023] Note that there may be multiple SRS settings, and when specifying one of the multiple 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-Resourceid is defined, and one or more SRS Resource Sets are prepared, and one of them is specified using the SRS Resource Setid or srs-Resourceid. In one example, when two SRS Resource Sets are prepared and four pieces of configuration information are prepared in 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 apparatus 101 instructs the terminal apparatus 111 to specify one of the settings and transmit SRS, it can specify the SRS Resource Set and srs-Resourceid, thereby allowing the terminal apparatus 111 to transmit SRS using the configuration information corresponding to the specified identification information. Furthermore, when SRS transmission is instructed in the Cell switch command, the SRS setting may be set in advance as part of the communication parameters used in connection and communication with the handover target candidate base station apparatus 102. Furthermore, when SRS transmission is instructed before the Cell switch command is transmitted or received, the SRS setting may be set in advance as part of the communication parameters used in connection and communication with the handover source base station apparatus 101.

[0024] The above-mentioned SRS configuration may further include configuration information of a beam to be used by the terminal device 111. For example, the beam to be used by 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 to use the beam for SRS transmission (or whether to enable or disable the configuration information for forming each of the plurality of beams) may be notified to the terminal device 111 as the SRS configuration. In this case, as one aspect, whether to use the beam corresponding to each TCI for SRS transmission may be indicated by specifying whether the TCI state is enabled or disabled using the TCI corresponding to each of the plurality of beams.

[0025] Also, instead of using TCI, information for specifying 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 specifying a spatial relationship for determining a transmission beam of each SRS. The information for specifying the spatial relationship is, for example, an NZP CSI-RS resource index, an SSB index, or an SRS resource index. This spatial relationship will be described below. For example, as described above, when the terminal device 111 receives SSBs or CSI-RSs transmitted in each of the multiple beams formed by the base station device 102, the terminal device 111 may store the beams of the own device that have provided good wireless quality in association with the identification information of the SSBs or CSI-RSs. In this case, the base station device 102 can indirectly specify the beams to be used in the terminal device 111 by specifying the identification information of the SSBs or CSI-RSs. For example, based on the reported received wireless quality of the SSBs or CSI-RSs in the terminal device 111, the base station device 102 identifies which of the multiple beams that can be formed by the own device should be used to perform (downlink) communication with good quality. 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 good wireless quality measurement results were obtained, and transmits an instruction to the terminal device 111 to transmit the SRS using the beam of the terminal device 111 that is 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 using the identification information of the SSB or CSI-RS is effective.Furthermore, by specifying an SRS resource index, the base station device 102 can 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.

[0026] For example, when a first CSI-RS is transmitted, it is assumed that, among the multiple beams of the terminal device 111, the radio quality of the first CSI-RS in a beam with TCI=n becomes a sufficiently high value, and the value of that 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, it is assumed that, among the multiple beams of the terminal device 111, the radio quality of the second CSI-RS in a beam with TCI=m becomes a sufficiently high value, and the value of that radio quality is notified to the base station device 102 via the base station device 101. Here, it is assumed that 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.

[0027] When an SRS transmission instruction 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 within that command. Note that MAC CE refers to a medium access control element. This information can replace the beam setting, for example, if a beam setting has been configured in advance as SRS setting information. For example, if the SRS setting information notified during the RRC layer setting as described above is configured to transmit the SRS using a beam corresponding to a specific SSB, this information can change the setting to transmit the SRS using 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 change only the transmission beam using that information, and transmit the SRS using the values ​​before the change for other settings (such as frequency position and sequence). In this case, the Cell switch command may include, as information for specifying the SRS resource to be used for SRS transmission, a cell identifier of the cell including the SRS resource (SRS Resource Set's Cell ID), a bandwidth portion identifier (SRS Resource Set's BWP ID), an SRS Resource Set ID, and a Resource ID, and, as information for specifying the SRS beam, an NZP CSI-RS resource index, an SSB index, and an SRS resource index. Also, at this time, if 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 referenced for specifying the transmission beam, and the bandwidth part identifier (Resource BWP ID) may also be included in the command. Note that this MAC CE can 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 can 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 can be notified in a Cell switch command. In this case, the Cell switch command may include, as information for specifying the SRS resource to be used for SRS transmission, a cell identifier of the cell including the SRS resource (SRS Resource Set's Cell ID), a bandwidth portion identifier (SRS Resource Set's BWP ID), an SRS Resource Set ID, and a Resource ID, and, as information for specifying the SRS beam, a TCI state ID. 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 command may also include the cell identifier (TCI State Serving Cell ID) and bandwidth part identifier (TCI State BWP ID) of the cell including the signal to be referenced for identifying the transmission beam.

[0028] Note that, for example, even when SRS transmission is instructed before a Cell switch command is transmitted or received, a similar MAC CE may be transmitted to the terminal device 111. Note that an SP SRS Activation / Deactivation MAC CE or an 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 handover, or another MAC CE may be newly defined. However, in this case, because the Cell switch command has not yet been transmitted or received, information indicating which base station device is the SRS transmission destination is notified by, for example, that MAC CE. In response to receiving the MAC CE, the terminal device 111 can start transmitting an SRS for the designated candidate handover destination base station device (e.g., the base station device 102) using the beam designated by the MAC CE, with other settings based on the SRS transmission settings notified in advance.

[0029] Furthermore, instead of MAC CE, for example, a physical downlink control channel (PDCCH) / downlink control information (DCI) may be used to instruct the terminal device 111 to specify a beam to be used and transmit an SRS. For example, by newly defining an SRS transmission instruction using a PDCCH order, an instruction to transmit an SRS for a base station device 102 that is a candidate handover destination may be notified to the terminal device 111. By specifying a beam to be used in this notification using the PDCCH / DCI, the beam previously specified when the RRC layer was configured is not used, and the beam newly notified by the PDCCH / DCI is used to transmit the SRS. Note that even when the PDCCH / DCI is used, when an SRS transmission instruction is given before a cell switch command is transmitted or received, identification information indicating a base station device that is a candidate handover destination to which the SRS is to be transmitted is notified to the terminal device 111. That is, when an SRS transmission is instructed when a cell switch command is transmitted or received, the command specifies a handover destination base station device, but since the handover destination has not been determined before the command is transmitted or received, the SRS transmission destination is specified. Note that the cell switch command may also notify the terminal device 111 of information specifying the SRS transmission destination. Note that in this case, the SRS transmission destination is the handover destination base station device, but, for example, if RRC layer configuration has been completed for another base station device that will be a handover destination candidate after the handover, information specifying the other base station device may be included. In this case, the terminal device 111 can transmit an SRS to the base station device that is a further handover destination candidate immediately after the handover.

[0030] As described above, the base station device 102 measures the SRS transmitted from the terminal device 111 in response to the SRS transmission instruction being issued from the base station device 101 to the terminal device 111 immediately after or before 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 communications after the terminal device 111 connects to the base station device 102 via handover. The base station device 102 measures the SRS based on each of multiple settings and, for example, identifies the SRS setting that can obtain communication quality that satisfies a predetermined standard. Then, the base station device 102 can specify which beam to use when transmitting uplink user data, for example, using identification information (e.g., SRS Resource Setid and srs-Resourceid) of the SRS transmission setting. Furthermore, the base station device 102 may specify an indicator (index) capable of identifying the resource on which the SRS is transmitted. In this case, the terminal device 111 specifies one configuration 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 that one configuration. Note that the base station device 102 may specify a beam by TCI or by identification information of an SSB or a CSI-RS. When a TCI is specified, the terminal device 111 transmits user data using a beam corresponding to that TCI. Furthermore, when the terminal device 111 receives identification information of an SSB or a CSI-RS, it transmits user data using a beam stored in association with that identification information. In addition, a beam used for PUSCH transmission may be specified for the terminal device using parameters for identifying a transmission beam uniquely determined by TCI, SSB, and CSI-RS identification information, such as an SRS resource indicator, Precoding information and number of layers (TPMI). Note that the beam designation is notified to the terminal device 111 by, for example, DCI.This DCI may be, for example, a DCI of format 0_1.

[0031] In the above-described embodiment, an example has been described in which an instruction to transmit an SRS is transmitted and received by a MAC CE or a DCI. Similarly, an instruction to stop transmitting an SRS may be transmitted and received by a MAC CE or a DCI. Note that switching of a beam used when transmitting an SRS may also be instructed by a MAC CE or a DCI.

[0032] As described above, in this embodiment, the terminal device 111 can quickly use a fine beam in uplink communication after executing a handover (cell switch), thereby improving communication efficiency.

[0033] In the above-described embodiment, an example has been described in which information on beam control of SRS is included in the Cell switch command, 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 a conventional PUCCH spatial relation activation / deactivation MAC CE can be notified in the Cell switch command. For example, using a Cell switch command, the information includes the cell identifier (Serving Cell ID) of the cell including the PUCCH, a bandwidth portion identifier (BWP ID), a PUCCH Resource ID, information on the PUCCH beam, and active / inactive information for 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 reference signal information for determining the beam at the time of PUCCH transmission, for example, any one of the NZP CSI-RS resource index, SSB index, and SRS resource index, is set in the PUCCH Spatial Relation Info. As a result, the terminal device 111 can specify the beam to be used for transmitting the PUCCH using the active / inactive information included in the Cell switch command.

[0034] Furthermore, in the above embodiment, an example has been described in which information relating to uplink beam control is included in the Cell switch command, but information relating to downlink beam control may also be included in the command. When information related to beam control of the downlink channel, PDSCH, and PDCCH is included in the Cell switch command, information that should 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 portion identifier (BWP ID).

[0035] (Device Configuration) Next, the device configuration will be described. FIG. 2 shows an example of the hardware configuration of a base station device and a terminal device according to this embodiment. In one example, the base station device and the terminal device include a processor 201, a ROM 202, a RAM 203, a storage device 204, and a communication circuit 205. The processor 201 is a computer including one or more processing circuits, such as a general-purpose CPU (Central Processing Unit) or an ASIC (Application Specific Integrated Circuit), and performs the overall processing of the device and each of the above-mentioned processes by reading and executing programs stored in the ROM 202 or the storage device 204. The ROM 202 is a read-only memory that stores information such as programs and various parameters related to the processing performed by the base station device and the terminal device. The RAM 203 functions as a workspace when the processor 201 executes a program and is a random access memory that stores temporary information. The storage device 204 is, for example, a removable external storage device. The communication circuit 205 is, for example, a circuit for wireless communication of 5G or its successor standards. Although FIG. 2 illustrates one communication circuit 205, the base station apparatus and the terminal apparatus may have multiple communication circuits. For example, the base station apparatus and the terminal apparatus may have wireless communication circuits for 5G and its successor standard, and a common antenna for these circuits. The base station apparatus and the terminal apparatus may have separate antennas suitable for each standard. The base station apparatus may also have a wired communication circuit used when communicating with other base station apparatuses or core network nodes. The terminal apparatus may also have a communication circuit conforming to a wireless communication standard other than the cellular communication standard, such as a wireless local area network (LAN) or Bluetooth (registered trademark). The base station apparatus and the terminal apparatus may have separate communication circuits 205 for each of the multiple available frequency bands, or may have a common communication circuit 205 for at least some of these frequency bands.

[0036] FIG. 3 shows an example of the functional configuration of a base station device. The base station device includes, for example, an RRC setting unit 301, 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 handover destination candidate base station device 102 is similar to conventional base station devices except that it may transmit information regarding SRS transmission to the base station device 101, and is therefore not shown here. Also, FIG. 3 shows only functions 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 base station devices compliant with 5G and its successor standards generally have. Also, the functional blocks in FIG. 3 are shown schematically, and the respective functional blocks may be realized as an integrated unit or may be further subdivided. 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 in detail here, and only the general functions will be outlined.

[0037] The RRC setting unit 301, for example, notifies a connected terminal device of RRC layer setting information for another base station device that is a candidate 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 handover destination, acquires communication setting information from the other base station device, and notifies the terminal device of the communication setting information by an RRC layer message. The RRC layer message is an RRC Reconfiguration message. The RRC setting unit 301 then 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 SRS transmission settings. The RACH instruction unit 302 instructs the terminal device, for which RRC layer setting for the other base station device that is a candidate handover destination, to transmit an RA preamble to the other base station device. Then, the RACH instruction unit 302 receives, for example, the TA value specified in the base station device 102 from the base station device 102, and notifies the terminal device of the TA value, thereby completing the RA procedure.

[0038] The SRS transmission instruction unit 303 notifies the terminal device, for which the RRC layer configuration for another base station device that is a candidate for handover has been completed, of an SRS transmission instruction for the other base station device. The SRS transmission instruction unit 303 may transmit the SRS transmission instruction to the terminal device, for example, after the above-mentioned RA procedure is completed and before the transmission of a cell switch command. Furthermore, 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, using MAC CE or DCI. Note that the SRS transmission instruction unit 303 may notify the terminal device, for example, of information specifying a beam to be used for SRS transmission, 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 can notify the terminal device of information indicating whether each of a plurality of beams available to the terminal device is to be used for transmitting the SRS. This notification can be performed, for example, as a notification of the enable / disable setting 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.

[0039] The inter-base station communication unit 304 communicates with a base station device that is a candidate for 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 handover destination and information for transmitting an SRS by the terminal device. The inter-base station communication unit 304 can also notify another base station device that is the destination of an RA preamble, for example, to have the currently connected terminal device transmit an RA preamble. The inter-base station communication unit 304 can also receive, for example, a message (message 2 or message 4) to be transmitted from the base station device that is a candidate for handover destination to the terminal device in the RA procedure. The TA value calculated in the base station device that is a candidate for handover destination can also be acquired via the inter-base station communication unit 304. The signal received by the inter-base station communication unit 304 can be transferred to the terminal device, for example, via 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 a base station device that is a candidate for handover. In this case, the SRS transmission instruction unit 303 may instruct the terminal device to transmit an SRS based on the instruction received from the base station device that is a candidate for handover. Note that this is just one example, and the inter-base station communication unit 304 may transmit an instruction to the connected terminal device to transmit an SRS without relying on an instruction from the base station device that is a candidate for handover, and notify the base station device that is a candidate for handover that the instruction has been transmitted. Note that the inter-base station communication unit 304 may also receive information from the base station device that is a candidate for handover that specifies the beam to be used when transmitting the SRS. In this way, the inter-base station communication unit 304 communicates various information regarding the RA procedure and SRS transmission related to handover to the base station device with the base station device that is a candidate for handover.

[0040] 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 terminal device's handover destination 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 that can identify the base station device at the handover destination to the terminal device. Upon receiving this command, the terminal device changes the base station device to the specified base station device and completes the handover.

[0041] FIG. 4 shows an example of the functional configuration of a terminal device. The terminal device includes, for example, an RRC setting unit 401, an 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 only shows functions particularly related to this embodiment, and various other functions that the terminal device may have are omitted from the illustration. For example, the terminal device naturally has other functions that terminal devices compliant with 5G and subsequent standards generally have. Furthermore, the functional blocks in FIG. 4 are shown schematically, and the respective functional blocks may be realized by being integrated together or may be further subdivided. Furthermore, each function in FIG. 4 may be realized, for example, by the processor 201 executing a program stored in the ROM 202 or the storage device 204, or may be realized, for example, by a processor present within the communication circuit 205 executing predetermined software. Note that the details of the processes performed by each functional unit will not be described here, and only their general functions will be outlined.

[0042] The RRC setting unit 401 performs connection setting processing with a base station device that is a candidate for handover destination by receiving an RRC layer message from a base station device that is a connection destination. The RRC setting unit 401 holds the settings of the currently connected base station device and the settings 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 SRS transmission setting information for each of the candidate base station devices that is a handover destination from the base station device that is a connection destination. 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 a 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 DCI may be used at that time. The RACH processing unit 403 transmits an RA preamble to the base station device that is a candidate for handover destination in response to receiving an instruction to transmit an RA preamble by the instruction receiving unit 402. Here, information on a sequence for generating the RA preamble and radio resources for receiving the RA preamble in the base station device that is a candidate for handover destination can 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 that is a candidate for handover destination.

[0043] 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 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, if 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 (e.g., in the SRS transmission instruction), and in that case, the stop instruction may not be received. Furthermore, for example, the setting information received in advance by the RRC setting unit 401 may include beam setting information, and in that 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.

[0044] The handover processing unit 405 changes the destination base station device in response to receiving a handover command (cell switch command) from the currently connected base station device. Note that, thanks to the operations of the RRC setting unit 401 and the RACH processing unit 403, the terminal device does not need to newly perform RRC layer configuration processing and RA procedures with the candidate handover destination base station device. Therefore, for example, if the received command notifies the terminal device of the allocation of uplink radio resources in the candidate handover destination base station device, the terminal device can immediately transmit uplink signals using those radio resources. Furthermore, thanks to the operation of the SRS transmission processing unit 404, the handover destination base station device can specify a precise beam setting for uplink communication at the time of handover or shortly after handover. Therefore, 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.

[0045] (Processing Flow) Figures 5 and 6 show an example of the processing flow executed in the wireless communication system of this embodiment. Figure 5 shows an example of the processing flow when an SRS transmission instruction is notified to a terminal device in a cell switch command. On the other hand, Figure 6 shows an example of the processing flow when an SRS transmission instruction is notified to a terminal device using, for example, a PDCCH order (DCI) before the command is transmitted. Here, in Figures 5 and 6, it is assumed that a connection has been established between the handover source base station apparatus and the terminal apparatus. Note that the examples shown in Figures 5 and 6 are merely examples, and the above-described modifications can be made to each procedure. For example, although the example in Figure 6 shows an example in which an SRS transmission instruction is transmitted and received using a PDCCH order, the individual transmission instruction may also be transmitted and received using a MAC CE, for example.

[0046] First, an example of the processing flow in FIG. 5 will be described. A first base station device, which is a handover source, first transmits a HANDOVER REQUEST message to second base station devices (base station device A and base station device B in FIG. 5 ) that form neighboring cells that are candidates for the handover destination of the terminal device (S501, S503). Upon receiving this message, the second base station device returns a HANDOVER REQUEST ACK message to the first base station device, including information on communication parameters to be used when the terminal device connects and SRS setting information (S502, S504). The first base station device then notifies the terminal device of an RRC Reconfiguration message that includes the communication parameters and the like notified from the second base station device (S505). Note that the first base station device may compile separate messages for each of the information on multiple candidate base station devices as the handover destination and notify the terminal device of them, or may notify the terminal device of a message that collectively includes all of this information. Upon receiving this message, the terminal device transmits an RRC Reconfiguration Complete message to the first base station device, thereby completing the configuration process using the RRC layer message. This process eliminates the need for the terminal device to perform RRC layer configuration process after receiving a handover instruction from the first base station device to the second base station device. Furthermore, the terminal device becomes able to transmit an SRS in accordance with an SRS transmission instruction notified in the handover instruction or before receiving the instruction. While FIG. 5 shows an example of the flow of the configuration process when there are two candidate base station devices as the handover destination, a similar configuration process can be performed for one candidate base station device or for three or more candidate base station devices.

[0047] Next, the first base station device instructs the terminal device to transmit an RA preamble to the second base station device (S507). Here, this instruction may include information specifying the sequence to be used by the terminal device to generate the RA preamble and the radio resource from which the RA preamble should be transmitted. The radio resource from which the RA preamble should be transmitted may be identified based on system information in the second base station device, for example, transmitted from the second base station device and measured by the terminal device. That is, this information does not need to be acquired from the currently connected base station device. Also, the sequence to be used when generating the RA preamble and the radio resource from which the RA preamble should be transmitted may be identified in advance. In one example, information on the sequence and the radio resource from which the RA preamble should be transmitted may have been notified to the terminal device in S505. The terminal device transmits the RA preamble to the second base station device in accordance with the instruction (S508). At this time, the first base station device may notify the second base station device that the terminal device plans to transmit the RA preamble. Furthermore, the first base station device can notify the second base station device of information on the sequence that the terminal device uses to generate an RA preamble. This enables 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 an RA preamble. In this case, the second base station device may specify the sequence that the terminal device should use to generate an RA preamble in the HANDOVER REQUEST ACK of S502 or S504. Thereafter, 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 a terminal device is connected to the device by the reception timing of the RA preamble, and can notify the terminal device of that 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 via the first base station device after Message 3, but these are not shown in Fig. 5. Note that it is sufficient to specify the TA value here, and transmission of Message 3 or Message 4 is not required. A similar RA procedure can also be performed between the terminal device and base station device B (S510 to S512).

[0048] The terminal device measures radio signals (e.g., SSB) transmitted from each of the second base station devices that are candidate handover destinations, for example, in parallel with the RA procedure, and notifies the first base station device of the measurement results (not shown). Here, the first base station device can notify the second base station device of the measurement results. 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 refined beam and have the terminal device measure it. In this case, the second base station device can transmit CSI-RS configuration information to the first base station device and have the first base station device transfer the configuration information to the terminal device. The terminal device measures CSI-RS based on the configuration information and reports the measurement results to the first base station device with which it is currently connected. Then, the first base station device transfers the measurement results to the second base station device. This allows the second base station device to transmit downlink signals using a refined beam immediately after the terminal device has handed over to the second base station device.

[0049] Furthermore, the first base station apparatus may determine to perform a handover based on the measurement result of a radio signal (e.g., SSB) notified from the terminal apparatus. For example, when a predetermined condition is satisfied, such as when the radio quality of a signal from base station apparatus A becomes higher than the radio quality of a signal from the first base station apparatus by a predetermined level or more, the first base station apparatus may decide to handover the terminal apparatus to base station apparatus A. Based on this determination, the first base station apparatus transmits a Cell Switch command instructing the terminal apparatus to perform a handover (S514). The terminal apparatus executes a handover to base station apparatus A using this command. Note that this command includes identification information that can identify the base station apparatus A as the handover destination. In this processing example, this command further includes SRS transmission instruction information and information specifying the beam to be used when transmitting the SRS. The SRS transmission instruction information may include information indicating which setting information should be used to transmit the SRS from among the SRS transmission setting information for the base station apparatus A designated as the handover destination, received in S505. However, in cases such as when there is only one piece of SRS transmission setting information, information specifying that setting information does not need to be included in the command.

[0050] Note that the SRS transmission setting information to be used and information specifying the beam to be used may be acquired from the base station device A, which is the handover destination (S513). In one example, the base station device A may voluntarily transmit to the first base station device, before issuing the command of S514, setting information to be used for SRS transmission assuming that the base station device A itself will be 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 to the first base station device, setting information to be used for SRS transmission, in response to the notification. The SRS transmission setting information to be used may be specified using, for example, an SRS Resource Setid and an srs-Resourceid. The beam designation information may be information indicating the activation or deactivation of setting information for forming each of multiple beams usable by the terminal device. Note that information designating a beam may be directly notified to the terminal device so that a specific beam is used. For example, the beam designation may be performed using the TCI. Alternatively, if a beam with the best SSB or CSI-RS radio quality 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 resources on which these signals are transmitted), the information capable of identifying the SSB or CSI-RS may be used as the beam designation information. Upon receiving information capable of identifying the SSB or CSI-RS, the terminal device determines that the beam stored in association with the SSB or CSI-RS is designated. Note that if the SRS transmission setting information received in S505 includes first information designating a beam, and second information designating a beam is received in S514, the first information may be (at least temporarily) replaced by the second information. Then, the terminal device transmits the SRS using the specified beam (S515).

[0051] By measuring this SRS, the base station device A can identify a beam to be used when the terminal device transmits an uplink signal. Then, when transmitting DCI to allocate radio resources for transmitting uplink user data (PUSCH) (S516), the base station device A can include information specifying the identified beam to be used. Here, the information specifying the beam can be, for example, an SRS resource indicator. Upon receiving this DCI, the terminal device transmits uplink user data using the specified beam and the specified radio resources (S517).

[0052] Conventionally, after a handover command such as that in S514 is transmitted and the terminal device connects 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 wireless communication can be made more efficient.

[0053] Next, a processing example shown in FIG. 6 will be described. In the processing of FIG. 6, an SRS transmission instruction is transmitted to a terminal device before a cell switch command is transmitted. Note that, in this processing example, the same processing as S501 to S512 of FIG. 5 is first performed. Then, 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 for base station device A and beam information to the terminal device (S601). The information transmitted here is the same as the information transmitted to the terminal device in S514 of FIG. 5, and can be configured based on information previously notified to the first base station device by base station device A, as in S513. In accordance with this SRS transmission instruction, the terminal device transmits an SRS using the setting information for base station device A and the designated beam (S602). By measuring this SRS, base station device A can determine a 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 A. Similarly, the first base station device transmits a PDCCH order (DCI) including an SRS transmission instruction and beam information to base station device B to the terminal device (S603), and the terminal device transmits the SRS using the setting information for base station device B and the designated beam in accordance with the SRS transmission instruction (S604). By measuring this SRS, base station device B can also determine a 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 A. Note that the first base station device may transmit an instruction to the terminal device to transmit an SRS for all of the candidate base station devices as handover destinations, or may transmit an instruction to the terminal device to transmit an 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 a candidate base station device to which to handover to transmit SRS based on the measurement results.

[0054] The first base station device may include information capable of identifying base station device A or base station device B in the SRS measurement instruction when transmitting the SRS measurement instruction, but this information does not necessarily have to be included. For example, if there is only one second base station device as a candidate handover destination, the information identifying the second base station device may be omitted. Alternatively, a unique identifier may be assigned to each of the SRS transmission setting information for the multiple base station devices as candidate handover destinations, and the identifier may be notified to the terminal device. In this case, it is sufficient for the terminal device to transmit an SRS for any of the candidate base station devices, so it is not necessary to notify the terminal device of the information for which each of the multiple setting information is intended. Furthermore, the information capable of identifying the candidate base station device as a handover destination may be, for example, a physical cell identifier (PCI), but is not limited to this. For example, when configuring the RRC layer, identification information such as the PCI of each base station device may be associated with a predetermined index and notified to the terminal device, and the predetermined index may be notified to the terminal device as information capable of identifying the base station device when issuing an SRS transmission instruction. For example, in 5G, there are 1008 PCIs, one of which is assigned to each base station device. In contrast, the number of handover destination candidates 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. Therefore, such an index may be used, for example, to reduce the number of bits of the PDCCH order.

[0055] Thereafter, the first base station apparatus, upon determining that a handover should be performed based on the measurement result of the radio signal (e.g., SSB) notified from the terminal apparatus, transmits a Cell switch command to the terminal apparatus instructing the terminal apparatus to perform the handover (S605). The terminal apparatus executes the handover to base station apparatus A by this command. Note that this command includes information on base station apparatus A indicating the handover destination. Meanwhile, base station apparatus A has already measured the SRS in S602 and has completed determining the beam to be used by the terminal apparatus for uplink communication. Therefore, the command in S605 does not include an instruction to transmit the SRS, and there is no need to transmit an instruction to transmit the SRS after the command in S605. Then, when transmitting DCI to allocate radio resources for transmitting user data (PUSCH) in the uplink (S516), for example, base station apparatus A may include information specifying the beam to be used identified in S602. Upon receiving this DCI, the terminal device transmits uplink user data using the designated beam and the designated radio resource (S517). In this manner, in this processing example, since the SRS transmission instruction is transmitted to the terminal device before the handover command is issued, the terminal device can perform uplink communication using a fine beam immediately after the handover is performed by the command.

[0056] Although the above example illustrates a case in which the RA procedure is performed before transmitting and receiving a cell switch command, the RA procedure does not necessarily have to be performed at this timing. For example, in the above embodiment, the description has been given assuming that the handover source and handover destination base station devices each provide one cell. However, these base station devices may also provide multiple cells. In this case, if the handover source base station device remains unchanged and a handover to another cell (e.g., using a different frequency band) 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, the above description uses terms such as "first handover source base station device" and "second handover destination candidate base station device," but these may be read as "first handover source cell" and "second handover destination candidate cell." However, it should be noted that the entities that transmit and receive signals are the base station devices that provide each cell. In the above example, an example in which the terminal device transmits the SRS has been described, but a different uplink reference signal from the SRS may be used. As described above, in the above 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 before the command, and the transmission of the SRS is started, and various changes and modifications are possible for other matters.

[0057] As described above, according to this embodiment, it is possible to significantly reduce the time from when a handover source base station device decides to actually hand over a 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 "Develop resilient infrastructure, promote sustainable industrialization and foster innovation."

[0058] 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.

[0059] This application claims priority based on Japanese Patent Application No. 2023-125628, filed August 1, 2023, the entire contents of which are incorporated herein by reference.

Claims

1. A terminal device having: 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 command instructing a handover to a cell provided by the other base station device, the command including an instruction to transmit the uplink reference signal and information specifying the beam to be used for transmitting the uplink reference signal; an executing means for executing a handover to the cell provided by the other base station device based on the command; 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 command from the base station device and without receiving an instruction to transmit the uplink reference signal from the other base station device.

2. A terminal device as described in claim 1, wherein the information specifying the beam is information indicating the enabling or disabling of multiple beams that can be used by the terminal device.

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

4. A terminal device as described in any one of claims 1 to 3, further comprising: a measurement means for measuring each of a plurality of specified downlink signals transmitted from the other base station device using each of a plurality of beams available to the terminal device; and a storage means for storing each of the plurality of specified signals in association with one of the plurality of beams based on a result of the measurement for each of the plurality of specified signals, wherein information designating the beam is information capable of identifying one of the plurality of specified signals, and the transmission means transmits the uplink reference signal using the beam stored in association with the specified signal identified based on the information.

5. A terminal device according to any one of claims 1 to 4, wherein the uplink reference signal is a sounding reference signal (SRS).

6. A base station device having: a receiving means for receiving, from another base station device that provides a candidate cell for handover of a connected terminal device, configuration information of an uplink reference signal for measurement at the other base station device, and then receiving information specifying a beam to be used by the terminal device when causing the terminal device to transmit an uplink reference signal using the configuration information; and a transmitting means for transmitting the configuration information to the terminal device, and then, when handing over the terminal device to a cell provided by the other base station device, transmitting a command instructing handover to the cell provided by the other base station device, the command including an instruction to transmit the uplink reference signal and information specifying the beam to be used for transmitting the uplink reference signal.

7. A base station device as described in claim 6, wherein the transmitting means transmits to the terminal device, as information specifying the beam, information indicating the enabling or disabling of multiple beams that can be used by the terminal device, included in the command.

8. The base station device according to claim 7, wherein the information indicating whether a plurality of beams usable by the terminal device are enabled or disabled is indicated using a Transmission Configuration Indicator (TCI) state.

9. A base station device as described in any one of claims 6 to 8, wherein the terminal device associates each of a plurality of specified downlink signals transmitted from the other base station device with one of a plurality of beams that the terminal device can use and stores the associated signals, and the transmitting means transmits to the terminal device information specifying the beam, the information being capable of identifying one of the plurality of specified signals, included in the command.

10. A base station apparatus according to any one of claims 6 to 9, wherein the uplink reference signal is a sounding reference signal (SRS).

11. A control method executed by a terminal device, comprising: 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; after receiving the configuration information, receiving from the base station device a command instructing a handover to a cell provided by the other base station device, the command including an instruction to transmit the uplink reference signal and information specifying a beam to be used for transmitting the uplink reference signal; executing a handover to the cell provided by the other base station device based on the command; and in response to receiving the command from the base station device and without receiving an instruction to transmit the uplink reference signal from the other base station device, transmitting the uplink reference signal to the other base station device using the configuration information and the information specifying the beam.

12. A control method executed by a base station device, comprising: receiving, from another base station device that provides a candidate cell for handover of a connected terminal device, configuration information of an uplink reference signal for measurement at the other base station device; transmitting the configuration information to the terminal device; receiving information specifying a beam to be used by the terminal device when causing the terminal device to transmit an uplink reference signal using the configuration information; and transmitting, when handing over the terminal device to a cell provided by the other base station device, a command instructing handover to the cell provided by the other base station device, the command including an instruction to transmit the uplink reference signal and information specifying the beam to be used for transmitting the uplink reference signal.

13. A program for causing a computer provided in a terminal device to receive, 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; after receiving the configuration information, receiving from the base station device a command instructing a handover to a cell provided by the other base station device, the command including an instruction to transmit the uplink reference signal and information specifying the beam to be used for transmitting the uplink reference signal; executing a handover to the cell provided by the other base station device based on the command; and, in response to receiving the command from the base station device and without receiving an instruction to transmit the uplink reference signal from the other base station device, transmitting the uplink reference signal to the other base station device using the configuration information and the information specifying the beam.

14. A program for causing a computer provided in a base station device to receive, from another base station device that provides a candidate cell for handover of a connected terminal device, configuration information of an uplink reference signal for measurement at the other base station device, transmit the configuration information to the terminal device, receive information specifying a beam to be used by the terminal device when transmitting an uplink reference signal using the configuration information, and transmit, when handing over the terminal device to a cell provided by the other base station device, a command instructing a handover to the cell provided by the other base station device, the command including an instruction to transmit the uplink reference signal and information specifying the beam to be used for transmitting the uplink reference signal.

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