Communication terminal, communication system, control method, and program

Abstract

A terminal characterized by comprising a reception means for receiving a reference signal and a transmission means for transmitting feedback information of the reference signal to another communication device, wherein the transmission means selectively transmits either wideband feedback information or subband feedback information to the other communication device on the basis of the reference signal.

Description

Communication terminal, communication system, control method, and program

[0001] The present disclosure relates to a communication terminal, a communication system, a control method, and a program.

[0002] In recent years, the 3GPP (registered trademark) has been formulating specifications for Long Term Evolution (LTE) and next-generation (NR: New Radio). 3GPP is an abbreviation for 3rd Generation Partnership Project.

[0003] In communications (hereinafter referred to as Uu communications) in which a base station controls a user equipment (UE), the UE returns a CSI report to the base station. That is, the UE receives a reference signal and a channel state information (hereinafter referred to as CSI) report transmission instruction transmitted by the base station, and returns a CSI report to the base station based on the received result. In the return method, a wideband of the entire channel band or a subband obtained by dividing the channel is pre-configured in the base station, and the UE returns a CSI report to the base station according to this configuration. The CSI report includes a channel quality indicator (hereinafter referred to as CQI), a precoding matrix index (hereinafter referred to as PMI), etc.

[0004] Meanwhile, a standard specification called Sidelink communication (hereinafter referred to as Sidelink) that does not go through a base station has been established, and this specification realizes direct wireless communication between UEs using an interface called PC5 without going through a mobile communication network (core network). Patent Document 1 invents a method in Sidelink that returns CSI to a UE that transmitted a reference signal based on a reference signal.

[0005] Special Table 2022-526741

[0006] For example, in a radio wave propagation environment where the CQI fluctuation for each subband is small, the difference between the CQI for the subband and the wideband is small. However, even in such an environment, if the CSI report return method of the UE receiving the reference signal is set to the subband, the CSI report for the subband is returned to the UE that transmitted the reference signal. This results in a problem of a large amount of data being returned.

[0007] The present invention has been made in consideration of at least one of the above-mentioned problems, and an object of the present invention is to provide a mechanism that enables efficient transmission of feedback information in accordance with a radio wave propagation environment.

[0008] A communication terminal according to one aspect of the present invention comprises a receiving means for receiving a reference signal and a transmitting means for transmitting feedback information to a transmitter of the reference signal, wherein the transmitting means selectively transmits either wideband feedback information or subband feedback information to the transmitter based on the reference signal.

[0009] According to one aspect of the present invention, it is possible to provide a mechanism that enables efficient transmission of feedback information in accordance with the radio wave propagation environment.

[0010] 1 is a diagram illustrating an arrangement of communication devices in this embodiment. FIG. 2 is a diagram illustrating a functional configuration of a reference signal transmission device in this embodiment. FIG. 3 is a diagram illustrating a functional configuration of a reference signal reception device in this embodiment. FIG. 4 is a diagram illustrating a communication sequence between communication devices in a first embodiment. FIG. 5 is a diagram illustrating a communication flow between communication devices in a first embodiment. FIG. 6 is a diagram illustrating an example of a radio wave propagation environment with large fluctuations in a first embodiment. FIG. 7 is a diagram illustrating an example of a radio wave propagation environment with large fluctuations in a first embodiment. FIG. 8 is a diagram illustrating an example of a radio wave propagation environment with small fluctuations in a first embodiment. FIG. 9 is a diagram illustrating an example of a radio wave propagation environment with small fluctuations in a first embodiment. FIG. 10 is a diagram illustrating an example of a radio wave propagation environment with small fluctuations in a first embodiment. FIG. 11 is a diagram illustrating an example of a slot included in a CSI report when CQI=12 is a threshold in a radio wave propagation environment with small fluctuations in a first embodiment. FIG. 12 is a diagram illustrating an example of a slot included in a CSI report when CQI=12 is a threshold in a radio wave propagation environment with small fluctuations in a first embodiment. 10 is a diagram illustrating an example of a radio wave propagation environment with little fluctuation in a third embodiment.FIG. 11 is a diagram illustrating an example of a radio wave propagation environment with little fluctuation in a third embodiment.FIG. 12 is a diagram illustrating an example of a radio wave propagation environment with little fluctuation in a third embodiment.FIG.

[0011] Each embodiment will be described in detail below with reference to the accompanying drawings. In the following description, the "number ***" in TS*** represents the number of the Technical Specification in the 3GPP standard. In addition, in the accompanying drawings, the same reference number is used for the same or similar configuration, and duplicate explanations may be omitted.

[0012] <First embodiment> [System configuration] Fig. 1 is a diagram showing an example of the arrangement of communication devices according to this embodiment. In Fig. 1, communication device UE-A (101) and communication device UE-B (102) can communicate directly with each other without going through a base station. UE-A (101) is a device that sends a reference signal, and UE-B (102) is a device that receives the reference signal.

[0013] [Functional Configuration of the Device] Next, the functional configuration of the communication device according to this embodiment will be described. Note that the configuration of the functional blocks described below is merely an example. Some (or in some cases all) of the functional blocks described may be replaced with other functional blocks that perform similar functions, some functional blocks may be omitted, or additional functional blocks may be added. Furthermore, one functional block described below may be divided into multiple functional blocks, or multiple functional blocks may be integrated into one functional block.

[0014] 2 is a block diagram showing an example of the functional configuration of a communication device UE-A (101) that transmits a reference signal in this embodiment. In FIG. 2, 201 is a control unit that controls operation. 202 is a storage unit that stores information used by the control unit 201 for control and information related to communication. 203 is a sidelink processing unit between UEs. 204 is a communication unit that transmits and receives information via wireless communication with other devices.

[0015] 3 is a block diagram showing an example of the functional configuration of a communication device UE-B (102) that transmits a reference signal in this embodiment. 301 to 304 in FIG. 3 have the same functions as 201 to 204 in FIG. 2, and therefore description thereof will be omitted. 305 is an index measurement / calculation unit, which is a functional unit that measures the reference signal transmitted by UE-A (101) and measures / calculates an index. The index is, for example, a signal-to-interference-noise ratio (SINR) or a channel quality indicator. The signal-to-interference-noise ratio (SINR) is an index that represents the ratio of the signal levels of a desired signal to an interference signal or noise. The channel quality indicator is a CQI, which is an index that measures the reception quality in a channel. 306 is a threshold determination unit, which is a functional unit that determines whether the difference between subbands is equal to or greater than a first threshold, with respect to the index output by the index measurement / calculation unit 305. That is, it is a functional unit that determines whether the maximum value of the differences between subbands is equal to or greater than a first threshold value for the index for each subband. The difference between subbands is also an index value that represents channel fluctuation. Furthermore, since there are multiple subbands, the maximum value of the differences between subbands means the difference between the minimum and maximum values ​​of the index for each subband. Hereinafter, "the maximum value of the differences between subbands is equal to or greater than a first threshold value for the index for each subband" is also expressed as "the channel fluctuation of the reference signal index is equal to or greater than a first threshold value." This determination may or may not include the first threshold value.

[0016] [Processing Example] The operation of this embodiment will be described with reference to the operation sequence diagram shown in FIG. 4 and the flowchart shown in FIG.

[0017] FIG. 4 shows an example of CSI report transmission in this embodiment. FIG. 5 shows a flowchart of the sequence of FIG. 4 in this embodiment, for each of UE-A (101) and UE-B (102). The processing shown in each flowchart is implemented in UE-A (101) by the control unit 201 executing a control program stored in the storage unit 202. The processing in UE-B (102) is implemented by the control unit 301 executing a control program stored in the storage unit 302. In this case, the processing can be implemented by each of UE-A (101) and UE-B (102) performing calculations and processing of information and controlling each piece of hardware.

[0018] UE-A (101) and UE-B (102) are performing data communication in sidelink communication without using a base station (F401). UE-A (101) transmits a reference signal to UE-B (102) (F402, S501). The reference signal may be a CSI-RS (Channel State Information Reference Signal). UE-B (102) receives the reference signal (S502), measures and calculates the reference signal using the index measurement and calculation unit 305, and outputs an index (S503). Next, it is determined whether the channel fluctuation of the reference signal index at UE-B (102) is equal to or greater than a first threshold (F403, S504). If it is determined that the fluctuation of the reference signal index is equal to or greater than the first threshold, it is determined that the radio wave propagation environment is highly volatile. Furthermore, in S505, it is determined whether or not to transmit in wideband. For example, this determination may be made based on whether or not the device itself has the ability to transmit in wideband. If it is determined that the radio wave propagation environment is highly variable and it is determined that wideband transmission is not to be performed, the process proceeds to S506. The precoding indicator (PMI) and CQI for the subband into which the channel is divided are included in the CSI and transmitted to UE-A (101) (F404, S506).

[0019] On the other hand, if it is determined in S504 that the fluctuation of the reference signal index is less than the first threshold, it is determined that the radio wave propagation environment is one with little fluctuation, and wideband CSI for the entire channel band is sent to UE-A (101) (F404, S507).

[0020] Furthermore, if it is determined in S504 that the fluctuation of the reference signal index is equal to or greater than the first threshold, even if it is determined that the radio wave propagation environment is highly fluctuating, it may be determined in S505 that wideband transmission is to be performed. In this case, similarly, in S507, wideband CSI for the entire channel band is transmitted to UE-A (101) (F404, S507).

[0021] The UE-A (101) determines whether it has received the CSI transmitted by the UE-B (102) (F405, S508). If it has received the CSI, it sends data to the UE-B (102) (F405) based on the received CSI (S509). If it has not received the CSI, it sends data to the UE-B (102) (F405) based on the previous CSI (S510).

[0022] The UE-A (101) may transmit the CSI report transmission instruction to the UE-B (102) at the same time as the reference signal or at a different timing.

[0023] Also, the UE-A (101) may determine whether or not it has received CSI (S508), and if it has not received CSI, it may again transmit a CSI report transmission instruction to the UE-B (102).

[0024] Based on the received CSI, UE-A (101) performs data communication with UE-B (102) (F406, S509).

[0025] In the above F404, S506, and S507, examples of the format of the CSI report sent to UE-A (101) are shown in Tables 1 to 3, and are listed in 3GPP technical specifications TS38.212V17.5.0. Table 1 is the CSI report format for wideband, and Tables 2 and 3 are the CSI report formats for subband.

[0026] For example, in S506, the CSI report is sent to UE-A (101) in the format of Table 2 or Table 3 for the subband, and in S507 in the format of Table 1 for the wideband.

[0027]

[0028]

[0029]

[0030] Next, an example of the processing of S504 to 507 will be explained using the radio wave propagation environment simulation diagrams of Figures 6A to 6C and 7A to 7C. Figures 6A to 6C are examples of radio wave propagation environments with large fluctuations, while Figures 7A to 7C are examples of radio wave propagation environment simulations with small fluctuations. The simulations in each figure use the propagation models TDL (Tapped Delay Line)-C and TDL-D listed in 3GPP Technical Report TR38-901. TDL-C is a non-line-of-sight (NLOS) environment, and TDL-D is a line-of-sight (LOS) environment. The simulation parameters are as shown in Table 4.

[0031]

[0032] FIG. 6A shows the wideband SINR and CQI results for all frames. Because CSI reports are acquired every four slots, the CSI report period, the SINR and CQI data are obtained every four slots. SINR is displayed in decibels (dB). FIGS. 6B and 6C show the SINR and CQI for each subband in slot 0 and slot 4, respectively. In this example, there are 56 resource blocks and the subband size is 4, resulting in a total of 14 subbands. In FIG. 6B, which shows the SINR and CQI for each subband in slot 0 of FIG. 6A, the SINR fluctuates between subbands, ranging from approximately 10 to 15 dB, and the CQI fluctuates between subbands, ranging from 9 to 12. The wideband is the average value of all subbands in slot 0, and the SINR and CQI of slot 0 in FIG. 6A are 13.3 dB and 11, respectively.

[0033] Similarly, in Figure 6C, which shows the SINR and CQI for each subband in slot 4 of Figure 6A, the SINR varies between subbands from about 9 to 17 dB, and the CQI varies from 8 to 13. Also, in Figure 6A, which shows the average values ​​for all subbands in slot 4, the SINR for slot 4 is 13.4 dB, and the CQI is 11. The wideband SINR and CQI for the other slots are similar.

[0034] For example, if the judgment condition in S504 is that the SINR fluctuates by 4 dB or more across all subbands, then in the radio wave propagation environments shown in Figures 6A to 6C, CSI including the CQI and PMI for each subband is sent to UE-A (101) (F404, S506). The case where the SINR fluctuates by 4 dB or more across all subbands corresponds to the case where the difference between the minimum and maximum SINR values ​​for each subband is 4 dB or more. The judgment condition in S504 may be a CQI fluctuation of 3 or more, rather than SINR. Alternatively, a combination of an SINR fluctuation of 3 dB or more and a CQI fluctuation of 2 or more may be used with SINR and CQI.

[0035] Next, examples of radio wave propagation environments with little fluctuation will be explained using Figures 7A to 7C. Similar to Figures 6A to 6C, Figure 7A shows the wideband SINR and CQI results for all frames, while Figures 7B and 7C show the SINR and CQI for each subband in slot 0 and slot 4, respectively.

[0036] In Figure 7B, the SINR is 14.8 to 16.7 dB, and the CQI is 11 to 12, while the CSINR is 12.8 to 15.4 dB, and the CQI is 10 to 12. In Figure 7A, the wideband SINR of slots 0 and 4 is 16 and 14 dB, respectively, and the wideband CQI is 12 and 11, respectively. The SINR and CQI in Figures 7B and 7C show small variations between subbands, and the differences between the wideband SINR and wideband CQI in Figure 7A and those in Figures 7B and 7C are small.

[0037] As in Figures 6A to 6C, when the judgment condition in S504 is that the SINR varies by 4 dB or more across all subbands, CSI including wideband PMI and CQI is sent to UE-A (101) in the radio wave propagation environments of Figures 7A to 7C (F404, S507). As a result, in this example, by sending 14 subband data as one wideband data, the amount of CQI and PMI data in the CSI report to be sent can be reduced to 1 / 14th.

[0038] In this way, according to this embodiment, in a radio wave propagation environment where CQI fluctuations for each subband are small, a UE receiving a reference signal can select a wideband CSI report depending on the reception result of the reference signal, thereby reducing the amount of data to be returned.

[0039] In this embodiment, sidelink communication between devices has been described as an example, but the present invention is not limited to sidelink communication. It is also applicable to cases where UE-A (101) is a base station device, i.e., Uu communication. UE-B (102) measures a reference signal transmitted by the base station and compares the measurement value with a first threshold. As a result of the comparison, fluctuations in the radio wave propagation environment are determined, and if it is determined that the fluctuations are small, it is possible to send a CSI report including wideband PMI and CQI.

[0040] Second Embodiment In the second embodiment, an example of sending a portion of a subband in a CSI report will be described. The functional configuration of a communication device in the second embodiment (FIGS. 2 and 3) is the same as that in the first embodiment, and the sequence and flow are the same as those in FIGS. 4 and 5. The method of sending the subband in S506 differs from that in the first embodiment, and will be described using the simulation examples in FIGS. 8A and 8B.

[0041] 8A and 8B are the same as FIGS. 6B and 6C. For example, if CQI=12 is set as the second threshold, subbands equal to or greater than 12 are included in the CSI report. The subbands in the dashed lines in FIGS. 8A and 8B correspond to this, and in slot 0 of FIG. 8A, subbands 5 to 7, and in slot 4 of FIG. 8B, subbands 5 to 7 and subbands 11 to 13 are equal to or greater than the second threshold. These subband PMIs and CQIs are included in the CSI report and sent to UE-A (101). The second threshold may be specified by SINR, or may be specified by CQI and SINR.

[0042] Alternatively, a predetermined number of subbands with the highest ranking of the subband CQI may be transmitted. As an example, if the predetermined number is 3, subbands 5 to 7 are applicable in FIG. 8A, and subbands 6 and any two of subbands 5 to 7 and 11 to 13 are applicable in FIG. 8B. As in the example described above in relation to the second threshold, these subband PMIs and CQIs are included in a CSI report and transmitted to UE-A (101). Of course, SINR may be used instead of CQI, and weighted CQI and SINR may also be used as the determination criteria.

[0043] Additionally, the CSI report may include the subband number to be sent.

[0044] Third Embodiment In a third embodiment, an example will be described in which the number of resource blocks constituting a subband (subband size) is changed based on an error rate.

[0045] A case where UE-B (102) determines the error rate will be described. In this case, the functional configuration of UE-A (101) may be as shown in FIG. 2, and the functional configuration of UE-B (102) is as shown in FIG. 9. Explanation of the same numbers will be omitted. UE-B (102) additionally includes an error rate measurement unit 905 and an error rate determination unit 906. The error rate measurement unit 905 is a functional unit that measures errors during communication, and the errors are any one of bit errors, frame errors, block errors, packet errors, or the like, or a combination of any two or more of these. The error rate determination unit 906 is a functional unit that compares the error value of the error rate measurement unit 905 with a predetermined third threshold value to determine whether it is larger or smaller.

[0046] The process when UE-B (102) determines the error rate will be described using the process sequence diagram of FIG. 10 and the flowchart of FIG. 11. The data communication in F201 and the reference signal transmission and reception in F202, S501, and S502 are the same as in the first embodiment. As a result of the data communication, UE-B (102) measures the error rate using the error rate measurement unit 905 (S1103). In F1003 and S1104, it is determined whether the subband size can be changed or is necessary. If it is determined that the subband size cannot be changed or is not necessary, the process proceeds to S1109 and S1110, and the same operation as in the first embodiment is performed without changing the subband size. If it is determined that the subband size can be changed and is necessary, a request for a subband size change is made to UE-A (101) (S1105). Upon receiving the subband size change request, UE-A (101) changes the subband size (S1107). The subsequent operations are the same as those in the first embodiment, and therefore will not be described here.

[0047] The two determinations in F1003 and S1104 will be explained.

[0048] The determination is made based on the subband configuration and the third threshold value of the error rate in Table 5 below, which is listed in 3GPP technical specification TS38.214-h20.

[0049] For example, if the number of resource blocks is 80 and the current subband size is 16, and the error rate is equal to or greater than the third threshold, it is determined that it is useful to reduce the subband size, i.e., that a subband resize is useful. In Table 5, when the number of resource blocks is 80, the subband size can be selected from 8 and 16, so in this case, it is determined that a subband resize is necessary and possible. If the error rate is equal to or less than the third threshold, it is determined that it is useful to increase the subband size, i.e., that a subband resize is useful. However, in this example, the subband size cannot be increased beyond 16, so it is determined that a resize is not possible.

[0050]

[0051] Simulation examples of this embodiment are shown in Figures 12A to 12C. The propagation model is TDL-D, similar to Figures 7A to 7C, and the parameters in Table 4 are the same except that the subband size is 8 resource blocks (number of resource blocks = 8). As with Figures 7A to 7C, Figure 12A shows the wideband SINR and CQI results for all frames, while Figures 12B and 12C show the SINR and CQI for each subband in slot 0 and slot 4. The SINR and CQI in Figures 12A to 12C are close to those in Figures 7A to 7C.

[0052] For example, if the subband size is initially 4 resource blocks, when the error rate is equal to or less than the third threshold, the subband size can be changed to 8 resource blocks. In such a radio wave propagation environment with little fluctuation, by changing the subband size to a larger value, it is possible to reduce the data size of the CSI returned to UE-A (101).

[0053] The error rate threshold can be determined by exceeding (falling below) the threshold for a certain period of time, a certain number of times, or a certain percentage. The error rate increase or decrease rate can also be used as the threshold. For example, assume that an error rate increase rate of 2% is set as the third threshold for determination, and the error rate is 5% for the first five slots and 7.5% for the next five slots. In this case, the error rate has increased by 2.5%, which is determined to exceed the third threshold, and it can be determined that a subband size change is necessary.

[0054] The subband size may be other than that shown in Table 5, and may conform to a standard wireless specification or a unique configuration. The UE-A (101) may measure the error rate and change the subband.

[0055] In the third embodiment described above, a modification is possible in which the subband and wideband are switched depending on the error rate, because, for example, when the error rate is small, it is highly likely that the radio wave propagation environment is one with little fluctuation.

[0056] <Other Embodiments> The present invention can also be realized by supplying a program that realizes one or more functions of each of the above-described embodiments to a system or device via a network or a storage medium, and having one or more processors in the computer of the system or device read and execute the program. The present invention can also be realized by a circuit (e.g., an ASIC or FPGA) that realizes one or more functions. Note that ASIC is an abbreviation for Application Specific Integrated Circuit. FPGA is an abbreviation for Field Programmable Gate Array. Furthermore, some or all of the various processes described in the above flowcharts can be realized by using a hardware circuit in cooperation with a processor such as a CPU or MPU.

[0057] Although each embodiment has been described in detail above, it is not limited to the specific embodiment, and various modifications and changes are possible within the scope of the claims. It is also possible to combine all or a plurality of components of the above-described embodiments.

[0058] It should also be understood that each feature described in the foregoing detailed description and illustrated in the accompanying drawings can be considered an independent element, each with its own unique technical contribution.

[0059] It is to be understood that these features, while synergistic in the context of the foregoing detailed description, are not limited to such combinations but may be extracted individually and utilized independently to address particular challenges or enhance particular functions.

[0060] Furthermore, the specific embodiments presented in the foregoing detailed description may include many variations, and these embodiments are exemplary and do not limit the scope, applicability, control, or configuration of the present disclosure in any way.

[0061] Rather, the foregoing detailed description provides those skilled in the art with an example guide for implementing possible embodiments, and it should be understood that various changes may be made in the acts, methods, steps, means, and schemes described in the foregoing detailed description without departing from the scope of the appended claims.

[0062] Thus, one skilled in the art will understand that various operations, methods, steps, means, and schemes in the flowcharts described in this disclosure may be substituted, modified, combined, or eliminated.

[0063] Additionally, other steps, means, and schemes in the various operations, methods, and processes previously described in this disclosure may be substituted, modified, rearranged, decomposed, combined, or eliminated.

[0064] The present invention is not limited to the above-described embodiments, and various modifications and variations can be made without departing from the spirit and scope of the present invention. Therefore, the following claims are appended to apprise the public of the scope of the present invention.

[0065] This application claims priority based on Japanese Patent Application No. 2024-021674, filed February 16, 2024, the entire contents of which are incorporated herein by reference.

Claims

1. A communication device that performs Sidelink communication in accordance with the 3GPP (3rd Generation Partnership Project) standard, characterized by having: a receiving means for receiving a reference signal transmitted from another communication device; and a transmitting means for selectively transmitting either wideband feedback information or subband feedback information to the other communication device based on the reference signal.

2. The communication terminal according to claim 1, characterized in that the transmitting means sends wideband feedback information to the other communication device when the difference between subbands regarding the predetermined index value obtained by measuring the reference signal is less than a predetermined threshold value based on the predetermined index value for each subband.

3. The communication terminal according to claim 2, wherein said transmitting means transmits subband feedback information to said other communication device when said difference is equal to or greater than said predetermined threshold value.

4. The communication terminal according to claim 2, wherein when the difference is equal to or greater than the predetermined threshold, feedback information of a part of the subbands is transmitted to the other communication device.

5. The communication terminal according to claim 4, wherein the feedback information of the partial subbands includes feedback information of subbands whose predetermined index value is equal to or greater than a predetermined value.

6. The communication terminal according to claim 4, wherein the feedback information of the partial subband includes the corresponding subband number.

7. The communication terminal according to claim 2, wherein the predetermined indicator is a signal-to-interference-and-noise ratio, an error rate, or a signal quality indicator.

8. The communication terminal according to claim 1, wherein the feedback information includes a precoding matrix index or a signal quality index.

9. A communication device that performs Sidelink communication in accordance with the 3GPP (3rd Generation Partnership Project) standard, comprising: a receiving means for receiving a reference signal from another communication device; and a transmitting means for sending feedback information to a sender of the reference signal, wherein the transmitting means changes a subband size related to the feedback information of the subband to be sent to the other communication device from a predetermined value based on the reference signal.

10. The communication terminal according to claim 9, wherein the transmitting means increases the subband size beyond a predetermined value when the difference between subbands regarding the predetermined index value obtained for each subband by measuring the reference signal is less than a predetermined threshold.

11. The communication terminal according to claim 10, wherein said transmitting means reduces the subband size to a value smaller than a predetermined value when said difference is equal to or greater than said predetermined threshold value.

12. A communication system comprising a first communication device and a second communication device, wherein the first communication device comprises: a transmitting means for transmitting a reference signal or a data signal; and a changing means for changing a subband size from a predetermined value; the second communication device comprises: a receiving means for receiving a reference signal or a data signal; an error rate measuring means for measuring an error rate based on the reference signal or the data signal; and a change request means for requesting a sender of the reference signal or the data signal to change the subband size; and wherein the changing means of the first communication device changes the subband size based on the change request.

13. The communication system described in claim 12, wherein the second communication device further has a determination means for determining whether a change in subband size is necessary, the determination means determining whether a change in subband size is necessary based on the error rate measured by the error rate measurement means, and the change request means, when the determination means determines that a change in subband size is necessary, requests a source of the reference signal or data signal to change the subband size.

14. A communication system as described in claim 12 or 13, characterized in that the change request means requests that the subband size be reduced when the error rate measured by the error rate measurement means is equal to or greater than a predetermined value, and requests that the subband size be increased when the error rate is less than the predetermined value.

15. The communication system according to claim 12, wherein the errors relating to the error rate include at least one of bit errors, block errors, frame errors, and packet errors.

16. A control method for controlling Sidelink communication compliant with the 3GPP (3rd Generation Partnership Project) standard, comprising: a receiving step of receiving a reference signal from another communication device; and a transmitting step of selectively transmitting either wideband feedback information or subband feedback information to the other communication device based on the reference signal.

17. A program causing a computer to execute a receiving process for receiving a reference signal from another communication device, and a transmitting process for selectively transmitting either wideband feedback information or subband feedback information to the other communication device based on the reference signal.

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