Terminal, base-station device, and control method for performing efficiency-improved cell switching

Abstract

To improve communication efficiency when cell switching is performed.SOLUTION: Receiving, by a terminal, from a connected base-station device, configuration information of an uplink reference signal for measurement in another base-station device providing a cell that is a candidate for a cell switching destination, and then receiving, from the base-station device, a command for instructing cell switching to a cell provided by the other base-station device, the command including an instruction to transmit the uplink reference signal and information for designating a beam to be used for transmission of the uplink reference signal; A cell switching to a cell provided by another base station apparatus is performed based on the command, and after the cell switching is performed, the uplink reference signal is transmitted using the configuration information and the information for designating the beam based on the transmission indication.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a cell switching technique in a cellular communication system. [Background technology]

[0002] In a mobile communication system, as a terminal device moves, a handover is performed to switch the cell (base station device) to which the terminal device is connected. Conventionally, after receiving a handover instruction from a base station device, the terminal device establishes a connection by establishing synchronization and setting up a connection in the cell to which the connection is to be switched. This process for switching the connection takes a certain amount of time, and the terminal device is unable to communicate user data during this process. In contrast, in the technique described in Non-Patent Document 1, the handover source base station device provides the terminal device with communication parameters in the radio resource control (RRC) layer for connection with another 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 device transmits a Layer 1 or Layer 2 command to the terminal device instructing the cell to be switched. In response to receiving the command, the terminal device executes a random access procedure with the base station device at the handover destination, thereby completing the handover without subsequent overlay processing in the RRC layer. This technique can suppress degradation 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 cell switching 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 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 cell switching, and then receiving from the base station device a command instructing cell switching 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 cell switching to the cell provided by the other base station device based on the command, and a transmitting means for transmitting the uplink reference signal using the configuration information and information specifying the beam based on the transmission instruction after the cell switching is executed.

[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 cell switching 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 causing the terminal device to transmit an uplink reference signal; and a transmitting means for transmitting the configuration information to the terminal device, and then, when causing the terminal device to perform cell switching to a cell provided by the other base station device, transmitting a command instructing the terminal device to switch 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. [Effects of the Invention]

[0007] According to the present invention, it is possible to improve communication efficiency when cell switching is performed. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of a wireless communication system. [Figure 2] FIG. 2 illustrates an example of a hardware configuration of the apparatus. [Figure 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. [Figure 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 INVENTION

[0009] Hereinafter, the embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the invention as claimed, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be combined in any desired manner. Furthermore, the same reference numerals are used to designate identical or similar components, and redundant descriptions will be omitted.

[0010] (System Configuration) Fig. 1 shows an example of the configuration of a wireless communication system according to this embodiment. This wireless communication system is a cellular communication system configured in accordance with a cellular communication standard such as the fifth generation (5G) of the Third Generation Partnership Project (3GPP (registered trademark)) or its successor standard. The wireless communication system includes 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.

[0011] 1, terminal device 111 is about to move from within 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 will perform handover to switch the connection destination base station device from base station device 101 to base station device 102.

[0012] In this embodiment, a handover procedure is executed to shorten a 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 executes configuration in a 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 configuration 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 is actually to be 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 communication parameter may be included in an RRC message notified to the terminal device 111 or in a configuration parameter in the RRC layer. In this case, a CondReconfigId is set for each candidate base station device or combination of base station devices as a handover destination. Furthermore, 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 a 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 signal (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 to the terminal device 111 to measure each of the SSBs.

[0013] In accordance with a measurement instruction from the currently connected base station device 101, the terminal device 111 measures the SSB transmitted from the base station device 102, which is a candidate for a handover destination. The terminal device 111 then notifies the currently connected base station device 101 of the measurement results. The base station device 101 may notify the measurement results received from the terminal device 111 to the currently connected base station device 102, which is a candidate for a handover destination and was the subject of the measurement. This allows the base station device 102 to identify the approximate 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 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 identify which of the multiple beams provided the best SSB radio 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 perform communication using the beam corresponding to the predetermined SSB.

[0014] The base station device 102 may further notify the terminal device 111 of the transmission configuration of channel state information (CSI)-reference signal (RS) in order to identify 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 transmission configuration of CSI-RS, and the base station device 101 transferring the transmission configuration to the connected terminal device 111. Upon receiving the transmission configuration, the terminal device 111 performs measurements of the CSI-RS transmitted from the base station device 102 (for example, 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, a 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 takes place, thereby shortening the period during which the efficiency of user data communication for the terminal device 111 decreases.

[0015] As described above, the terminal device 111 may be able to configure multiple beams that can be used in downlink communication. 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 configuration for each of the multiple configurable beams, and cause the terminal device 111 to perform measurements for each of the beams. For the CSI-RS corresponding to each of the multiple beams in the base station device 102, the terminal device 111 may store the beam configuration that provided good radio quality, 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.

[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 CSI-RS radio resource configuration (CSI-RS-Resource) and a report configuration (csi-ReportConfig) for the base station device 102, which 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). Furthermore, the csi-ReportConfig configures aperiodic or periodic CSI report reporting timing. 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. Furthermore, 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 MAC CE or DCI, as information for the terminal device 111 to identify which base station device's CSI-RS transmission configuration this is.

[0017] Furthermore, the terminal device 111 is configured to execute 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 actually performing 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, 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 an 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.

[0018] 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). This command includes identification information of a 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, this command may include an identifier (for example, an RRC-TransactionIdentifier or a CondReconfigId) that is uniquely associated with each setting in the RRC layer. Furthermore, information regarding the beam that was determined in the base station device 102 based on the SSB and CSI-RS measurements performed 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 specifying the base station device 102 as the handover destination from the base station device 101, 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. Also, since CSI-RS measurements have 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. Also, if the handover is successful after receiving the command, the terminal device 111 can notify the base station device 102 of the success in, for example, an RRC message of RRC Reconfiguration Complete or an uplink control message such as 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 subsequent setting process must be performed. 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 result, the base station device 102 can determine a beam for 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.

[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). Alternatively, the base station device 101 may 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 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.

[0021] 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 cycle, 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 cycle includes information specifying, for example, periodic, aperiodic, or semi-persistent. Note that, when periodic or semi-persistent is specified as the SRS transmission cycle, the SRS transmission cycle further includes information specifying the time cycle 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 of time. 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 continuously transmitted, 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 SRS hopping, and the position of the SRS in the frequency domain are parameters that enable the frequency-time resource over which the SRS is transmitted to be specified using, for example, a table or formula specified in the standard. The cyclic shift and the identifier of the sequence used to generate the SRS are used to generate 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 frequency intervals of 2 or 4 subcarriers.

[0022] Note that there may be multiple SRS configurations, and identification information such as an SRS Resource Set ID or an srs-Resource ID is used to specify one of the multiple configurations. That is, an SRS Resource Set associated with one or more srs-Resource IDs is defined between the base station device and the terminal device, and one or more SRS Resource Sets are prepared, and one of the SRS Resource Sets can be specified using an SRS Resource Set ID or an srs-Resource ID. In one example, when two SRS Resource Sets are prepared and four pieces of configuration information are prepared for each SRS Resource Set, srs-Resource IDs 1 to 4 can be assigned to SRS Resource Set=0, and srs-Resource IDs 1 to 4 can be assigned to SRS Resource Set=1, respectively. Then, for example, when the base station device 101 instructs the terminal device 111 to specify one of the configurations and transmit SRS, the base station device 101 specifies the SRS Resource Set and srs-Resource ID, and the terminal device 111 can transmit SRS using configuration 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 to use the beam for SRS transmission (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. In this case, as one aspect, whether to use a beam corresponding to each TCI for SRS transmission may be indicated by specifying whether the TCI state is enabled or disabled using a TCI corresponding to each of the plurality of beams.

[0024] Furthermore, instead of using the TCI, information for specifying a spatial relationship for determining a transmission beam for an SRS may be used. In this case, for example, an SP SRS Resource Set ID or a Resource ID may be used as a parameter indicating the configuration information. The Resource ID may include information for specifying a spatial relationship for determining a transmission beam for 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 a plurality of beams formed by the base station device 102, the terminal device 111 may store beams of the terminal device 111 that have provided good radio quality, in association with identification information of the SSBs or CSI-RSs. In this case, the base station device 102 can indirectly specify a beam to be used in the terminal device 111 by specifying identification information of the SSBs or CSI-RSs. 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 good radio quality measurement results have been 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 identical (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 used for transmitting the other SRS.

[0025] For example, when a first CSI-RS is transmitted, it is assumed that the radio quality of the first CSI-RS in a beam with TCI=n among the multiple beams of the terminal device 111 becomes a sufficiently high value, and that the value of the radio quality is notified to the base station device 102 via the base station device 101. Also, when a second CSI-RS is transmitted, it is assumed that the radio quality of the second CSI-RS in a beam with TCI=m among the multiple beams of the terminal device 111 becomes a sufficiently high value, and that the value of the 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 the identification information of the first CSI-RS with TCI=n and stores it, and further associates the identification information of the second CSI-RS with TCI=m and stores it. 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 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 the command. Note that MAC CE refers to a medium access control 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 so that the SRS is transmitted 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 the 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 used for SRS transmission, a cell identifier (SRS Resource Set's Cell ID) of the cell including the SRS resource, a bandwidth part identifier (SRS Resource Set's BWP ID), an SRS Resource Set ID, and a Resource ID, and, as information for specifying the beam of the SRS, an NZP CSI-RS resource index, an SSB index, and an SRS resource index. Furthermore, at this time, if it is necessary to specify the beam of the SRS using a reference signal of another cell (when a reference signal of another cell is used for spatial relationship derivation), the command may also include a cell identifier (Resource Serving Cell ID) and a bandwidth part identifier (Resource BWP ID) of the cell including a signal to be referenced for identifying the transmission beam. 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 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 can include, as information for specifying an SRS resource to be used for SRS transmission, a cell identifier of a cell including the SRS resource (SRS Resource Set's Cell ID), a bandwidth part identifier (SRS Resource Set's BWP ID), an SRS Resource Set ID, and a Resource ID, and, as information for specifying an SRS beam, a TCI state ID. Also, at this time, if it is necessary to specify the beam of the SRS using the TCI state of another cell (if a reference signal of another cell is used for spatial relationship derivation), the cell identifier (TCI State Serving Cell ID) and bandwidth part identifier (TCI State BWP ID) of the cell containing the signal to be referenced to identify the transmission beam may also be included in the command.

[0027] 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, for example, by the MAC CE. In response to receiving the MAC CE, the terminal device 111 can start transmitting SRS for the designated handover destination candidate base station device (e.g., the base station device 102) using the beam specified by the MAC CE, and based on the SRS transmission settings notified in advance for other settings.

[0028] 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 the beam to be used and transmit the SRS. For example, by newly defining an SRS transmission instruction using a PDCCH order, an instruction to transmit an SRS for the base station device 102 that is a candidate handover destination may be notified to the terminal device 111. By specifying the 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 SRS is transmitted using the beam newly notified by the PDCCH / DCI. Note that even when the PDCCH / DCI is used, when an instruction to transmit an SRS is given before a Cell switch command is transmitted and received, identification information indicating the 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 base station device as a handover destination, but because the handover destination has not been determined before the command is transmitted or received, the SRS transmission destination is specified. Note that information specifying the SRS transmission destination may also be notified to the terminal device 111 in the Cell switch command. Note that in this case, the SRS transmission destination is the base station device as a handover destination, 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.

[0029] As described above, the base station device 102 measures the SRS transmitted from the terminal device 111 in response to an 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 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 to use when transmitting uplink user data, using identification information (e.g., SRS Resource Set ID and srs-Resource ID) of the SRS transmission setting. The base station device 102 may also specify an indicator (index) that can identify the resource from which the SRS is transmitted. In this case, the terminal device 111 identifies 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 the 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 the TCI. Also, 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 the identification information. Also, a beam to be used for PUSCH transmission may be specified to the terminal device using parameters for specifying a transmission beam uniquely determined by identification information of a TCI, an SSB, or a CSI-RS, for example, an SRS resource indicator, Precoding information and number of layers (TPMI), etc. 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-described 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. Note that switching of a beam used when transmitting an SRS may also be instructed by MAC CE or DCI in the same way.

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

[0032] Furthermore, 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 also be included in the command. In this case, information that should be notified to the terminal device 111 in conventional PUCCH spatial relation Activation / Deactivation MAC CE can be notified in the Cell switch command. For example, the Cell switch command includes a cell identifier (Serving Cell ID) of a cell including the PUCCH, a bandwidth part 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 in the terminal device 111 for each PUCCH-SpatialRelationInfoId, and information on a reference signal for determining a beam when transmitting the PUCCH, for example, any one of an NZP CSI-RS resource index, an SSB index, and an SRS resource index, is set in the PUCCH Spatial Relation Info. As a result, the terminal device 111 can identify the beam to be used for transmitting the PUCCH by using the active / inactive information included in the Cell switch command.

[0033] Furthermore, in the above-described embodiment, an example has been described in which information on uplink beam control is included in the Cell switch command, but information on downlink beam control may also 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 terminal device of this embodiment. In one example, the base station device and 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 executed by the base station device and terminal device. The RAM 203 functions as a workspace when the processor 201 executes the 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 nodes in the core network. 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 multiple available frequency bands, or may have a common communication circuit 205 for at least some of these frequency bands.

[0035] 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 base station device 102, which is a candidate for handover, is similar to a conventional base station device 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 are generally possessed by base station devices compliant with 5G and its successor standards. 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 existing 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.

[0036] The RRC setting unit 301, for example, notifies a currently 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. 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 SRS transmission setting. 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 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, for which RRC layer configuration for another base station device that is a candidate for handover, 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. The SRS transmission instruction unit 303 may also 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 using, for example, MAC CE or DCI. The SRS transmission instruction unit 303 may notify, for example, 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 configurations. 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 SRS transmission. This notification can be made, for example, as a notification of a TCI state enable / disable setting. 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 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 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 a destination of an RA preamble, for example, to cause the currently connected terminal device to 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. Note that a signal received in the inter-base station communication unit 304 can be transferred to the terminal device via, for example, the RACH instruction unit 302. The inter-base station communication unit 304 can also receive an instruction to transmit an SRS to the terminal device from the base station device that is a candidate for handover destination. In this case, the SRS transmission instruction unit 303 may instruct the terminal device to transmit SRS based on an 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 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 receives information from the base station device that is a candidate for handover that specifies the beam to be used when transmitting 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.

[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 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 as 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 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. 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 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.

[0041] The RRC setup unit 401 performs connection setup processing with a base station device that is a candidate for handover destination by receiving an RRC layer message from the destination base station device. The RRC setup unit 401 holds the configuration of the currently connected base station device and the configuration of the handover destination candidate base station device in parallel. In this embodiment, the RRC setup unit 401 may acquire and hold one or more pieces of configuration information for SRS transmission for each of the handover destination candidate base station devices from the destination base station device. The instruction receiving unit 402 receives an instruction to transmit an RA preamble and an instruction to transmit an SRS from the currently connected base station device. The instruction to transmit an RA preamble may be issued in 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 MAC CE or DCI may be used at that time. In response to the instruction to transmit an RA preamble 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 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.

[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 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, the instruction receiving unit 402 receives an SRS transmission stop instruction. Note that the SRS transmission period may be set in advance (for example, in the SRS transmission instruction), and in that case, a 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.

[0043] 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 the terminal device does not need to newly perform RRC layer configuration processing and RA procedures with the handover destination candidate base station device, thanks to the operations of the RRC configuration unit 401 and the RACH processing unit 403. Therefore, for example, if the terminal device is notified of the allocation of uplink radio resources in the handover destination candidate base station device in the received command, the terminal device can immediately transmit an uplink signal using the 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.

[0044] (Processing flow) 5 and 6 show an example of a processing flow executed in the wireless communication system of this embodiment. FIG. 5 shows an example of a processing flow 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 a 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 FIGS. 5 and 6, it is assumed that a connection has been established between a handover source base station apparatus and a terminal device. Note that the examples shown in FIGS. 5 and 6 are merely examples, and the above-described modifications can be made to each procedure. For example, while the example in FIG. 6 shows an example in which an SRS transmission instruction is transmitted and received using a PDCCH order, an SRS transmission instruction may also be transmitted and received by, for example, a MAC CE.

[0045] 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). When the second base station device receives this message, it returns a HANDOVER REQUEST ACK message to the first base station device, which includes information on communication parameters to be used when the terminal device connects and SRS setting information (S502, S504). Then, the first base station device 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, 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 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.

[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 the sequence to be used by the terminal device to generate an RA preamble and the radio resource from which the RA preamble is to be transmitted. The radio resource from which the RA preamble is to be transmitted may be identified based on system information in the second base station device that is transmitted from the second base station device and measured by the terminal device, for example. That is, this information does not need to be acquired from the currently connected base station device. Also, the sequence to be used when generating an RA preamble and the radio resource from which the RA preamble is to be transmitted may be identified in advance. In one example, information on the sequence and the radio resource from which the RA preamble is to be transmitted may be 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 an RA preamble. Furthermore, the first base station device may 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 the terminal device is connected to the second base station 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 via the first base station device after message 3, but these steps are not shown in Fig. 5. Here, it is sufficient to specify the TA value, and transmission of messages 3 and 4 is not required. A similar RA procedure can also be performed 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 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 perform measurements. 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.

[0048] Furthermore, the first base station apparatus may determine to perform handover based on the measurement result of the 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 the signal from base station apparatus A becomes higher than the radio quality of the 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 handover (S514). The terminal apparatus executes handover to base station apparatus A using this command. Note that this command includes identification information capable of identifying base station apparatus A, 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. 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 base station apparatus A designated as the handover destination, received in S505. However, in cases where there is only one piece of SRS transmission setting information, the information specifying that setting information does not need to be included in the command.

[0049] 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, base station device A may voluntarily transmit, to the first base station device, setting information to be used for SRS transmission assuming that base station device A will be the handover destination of the terminal device, before issuing the command of S514. In this case, for example, base station device B may also transmit similar information to the first base station device. Furthermore, the first base station device may notify base station device A that the terminal device will be handed over to base station device A before issuing the command of S514, and base station device A may transmit, in response to the notification, setting information to be used for SRS transmission to the first base station device. The SRS transmission setting information to be used may be specified using, for example, an SRS Resource Set ID and an srs-Resource ID. The information specifying the beam 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 specifying a specific beam may also be directly notified to the terminal device. The beam may be designated using, for example, the TCI. Alternatively, if a beam providing 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 resources on which these signals are transmitted), the information capable of identifying the SSB or CSI-RS may be used as information for designating the beam. When the terminal device receives information capable of identifying the SSB or CSI-RS, it 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 second information may (at least temporarily) replace the first information. Then, the terminal device transmits the SRS using the designated beam (S515).

[0050] By measuring this SRS, the base station apparatus A can identify a beam to be used when the terminal apparatus transmits an uplink signal. Then, for example, when transmitting DCI to allocate radio resources for transmitting uplink user data (PUSCH) (S516), the base station apparatus 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 apparatus 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 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.

[0052] 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 also 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 to 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 at S514 of FIG. 5, and can be configured based on information notified in advance from base station device A to the first base station device, as in S513. In accordance with this SRS transmission instruction, the terminal device transmits an SRS using setting information for base station device A and a specified beam (S602). By measuring this SRS, base station device A can determine a beam to be used by the terminal device in uplink communication assuming that the terminal device is connected to the base station device itself. Similarly, the first base station apparatus transmits a PDCCH order (DCI) including an SRS transmission instruction and beam information to the terminal apparatus (S603), and the terminal apparatus transmits the SRS using the setting information for base station apparatus B and the designated beam in accordance with the SRS transmission instruction (S604). By measuring the SRS, base station apparatus B can also determine a beam to be used by the terminal apparatus in uplink communication assuming that the terminal apparatus is connected to the base station apparatus B. Note that the first base station apparatus may transmit an instruction to the terminal apparatus to transmit the SRS to all candidate base station apparatuses as handover destinations, or may transmit an instruction to the terminal apparatus to transmit the SRS to only some of the candidate base station apparatuses. For example, the terminal apparatus may continuously perform measurements such as SSB and notify the first base station apparatus of the measurement results, and the first base station apparatus may determine the candidate base station apparatuses as handover destinations to which the SRS is to be transmitted based on the measurement results.

[0053] The first base station apparatus may transmit an SRS measurement instruction including information capable of identifying base station apparatus A or base station apparatus B, but this information does not necessarily have to be included. For example, if there is only one second base station apparatus as a candidate handover destination, the information identifying the second base station apparatus may be omitted. Alternatively, a unique identifier may be assigned to each of a plurality of pieces of SRS transmission setting information for a plurality of base station apparatuses as candidate handover destinations, and the identifier may be notified to the terminal apparatus. In this case, it is sufficient for the terminal apparatus to transmit an SRS for any of the candidate base station apparatuses, so it is not necessary to notify the terminal apparatus of the information for which each of the plurality of pieces of setting information is intended. Furthermore, the information capable of identifying a candidate base station apparatus as a handover destination may be, for example, a physical cell identifier (PCI), but is not limited to this. For example, during RRC layer configuration, identification information such as the PCI of each base station apparatus may be associated with a predetermined index and notified to the terminal apparatus, and the predetermined index may be notified to the terminal apparatus as information capable of identifying the base station apparatus when issuing an SRS transmission instruction. For example, 1008 PCIs are prepared in 5G, and one of them 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 in the PDCCH order.

[0054] Thereafter, the first base station apparatus, in response to 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 instruct the terminal apparatus to perform the handover (S605). The terminal apparatus performs a 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. For this reason, the command in S605 does not include an instruction to transmit an SRS, and there is no need to transmit an instruction to transmit an SRS after the command in S605. Then, for example, when transmitting DCI to allocate radio resources for transmitting user data (PUSCH) in the uplink (S516), the base station apparatus A can include information specifying the beam to be used identified in S602. Upon receiving this DCI, the terminal apparatus transmits uplink user data using the specified beam and the specified radio resources (S517). In this way, in this processing example, an SRS transmission instruction is sent to the terminal device before a handover command is issued, so that immediately after handover is performed by that command, the terminal device can perform uplink communication using a fine beam.

[0055] 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, but these base station devices may provide multiple cells. In this case, if the handover source base station device remains unchanged and handover to another cell (using, for example, 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 entity that transmits and receives signals is the base station device that provides each cell. Furthermore, in the above example, an example has been described in which the terminal device transmits an SRS, but an uplink reference signal different 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 before 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 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."

[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 connected base station device, configuration information of an uplink reference signal for measurement in another base station device that provides a candidate cell for cell switching, and then receiving, from the base station device, a command instructing cell switching to the 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 execution means for executing cell switching to a cell provided by the other base station device based on the command; a transmitting means for transmitting the uplink reference signal by using the setting information and the information specifying the beam based on the transmission instruction after the cell switching is performed; A terminal device comprising:

2. a measurement means for measuring each of a plurality of predetermined downlink signals transmitted from the other base station device using each of a plurality of beams usable by the terminal device; and a storage means for storing the plurality of predetermined signals in association with any one of the plurality of beams based on the measurement results for each of the plurality of predetermined signals, the information specifying 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 predetermined signal identified based on the information.

2. The terminal device according to claim 1, wherein:

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

4. A base station device, a receiving means for receiving, from another base station device that provides a candidate cell for cell switching 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 the uplink reference signal using the configuration information; a transmitting means for transmitting the setting information to the terminal device, and then, when causing the terminal device to perform cell switching to a cell provided by the other base station device, transmitting a command instructing the terminal device to perform cell switching 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; A base station device comprising:

5. the terminal device stores a plurality of predetermined downlink signals transmitted from the other base station device in association with any one of a plurality of beams usable by the terminal device; the transmitting means includes information capable of identifying any one of the plurality of predetermined signals in the command as information specifying the beam, and transmits the command to the terminal device.

5. The base station device according to claim 4.

6. The base station apparatus according to claim 4 , wherein the uplink reference signal is a sounding reference signal (SRS).

7. A control method executed by a terminal device, comprising: 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 cell switching; receiving, after receiving the setting information, from the base station device, a command instructing cell switching 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; performing a cell switch to a cell provided by the other base station device based on the command; After the cell switching is performed, transmitting the uplink reference signal to the other base station device using the configuration information and information specifying the beam based on the transmission instruction; A control method comprising:

8. A control method executed by a base station device, receiving, from another base station device that provides a candidate cell as a cell switching destination for the currently connected terminal device, configuration information of an uplink reference signal for measurement in the other base station device; transmitting the setting 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 setting information; transmitting a command instructing the terminal device to perform cell switching 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, when causing the terminal device to perform cell switching to a cell provided by the other base station device; A control method comprising:

9. The computer provided 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 cell switching; receiving, after receiving the setting information, from the base station device, a command instructing cell switching 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; performing cell switching to a cell provided by the other base station device based on the command; After the cell switching is performed, the uplink reference signal is transmitted to the other base station device using the configuration information and the information specifying the beam based on the transmission instruction. Program for.

10. A computer provided in the base station device receiving, from another base station device that provides a candidate cell for cell switching of the currently connected terminal device, configuration information of an uplink reference signal for measurement in the other base station device; causing the terminal device to transmit the setting information; receiving information specifying a beam to be used by the terminal device when the terminal device transmits an uplink reference signal using the setting information; When causing the terminal device to perform cell switching to a cell provided by the other base station device, causing the terminal device to transmit a command instructing the terminal device to perform cell switching 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. Program for.

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