Wireless communication system

Abstract

To make it possible to more appropriately grasp the received signal quality of a transmission frequency in a wireless communication system using the HF band or the like.SOLUTION: A wireless device T transmits a signal quality measurement command to a second receiver at a set transmission frequency. In response to the signal quality measurement command received from the wireless device T, a wireless device R calculates the SNR of a received signal based on a symbol difference of a known signal for channel estimation in the received signal using a demodulation processing / SNR calculation unit 21, converts the received signal from a time axis signal to a frequency axis signal using an FFT unit 22 and calculates the frequency characteristics of the received signal by obtaining received power for each frequency using a frequency characteristic calculation unit 23, and calculates overall received signal quality based on the SNR and the frequency characteristics of the received signal using a measurement response processing generation unit 24.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a wireless communication system that performs wireless communication using the HF (High Frequency) band or the like. [Background technology]

[0002] HF band wireless systems have been put to practical use. One of the features of HF band communication is that it allows long-distance communication by utilizing radio wave reflections in the ionosphere and propagating while repeatedly reflecting between the ionosphere and the earth's surface. The ionosphere is formed by ionizing oxygen, nitrogen, etc. with sunlight, so its state changes greatly depending on the time of day and the season when the influence of the sun is different. Furthermore, at higher frequencies, there is a characteristic that the signal is not reflected depending on the angle of incidence into the ionosphere, and blind zones (skip zones) occur depending on the positional relationship of the transmitter and receiver. For the above reasons, when communicating in the HF band, it was necessary to manually search for and select the optimal frequency, taking into account the time of communication and the positional relationship of the transmitter and receiver.

[0003] The prior art in the technical field of the present invention includes the following: For example, Patent Document 1 discloses a transmitter / receiver for an HF radio line that can automatically set the frequency and transmission speed used on the transmitting station side and the receiving station side without the assistance of an operator. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2002-300072 A Summary of the Invention [Problem to be solved by the invention]

[0005] In HF bands and other communications, the frequency must be selected manually, taking into account the state of the ionosphere, which can be problematic when performing highly urgent communications such as in emergencies. In addition, in the case of a mobile station, the transmitter must always take into account the positional relationship with the receiver, which can be problematic in that it takes time and effort to establish stable communications. For example, when making a voice call using a specific transmission frequency, it is necessary to check the voice quality of the response or the absence of a response, select a different transmission frequency, and try to communicate again, repeating this process until a transmission frequency with good quality is found.

[0006] The present invention has been made in consideration of the above-mentioned conventional circumstances, and aims to make it possible to more appropriately grasp the received signal quality of a transmission frequency in a wireless communication system using the HF band or the like. [Means for solving the problem]

[0007] In order to achieve the above object, a wireless communication system according to an aspect of the present invention is configured as follows: That is, in a wireless communication system in which wireless communication is performed between a first wireless device and a second wireless device, the first wireless device transmits a signal quality measurement command to the second wireless device at a set transmission frequency, and the second wireless device calculates an SNR of a received signal based on a symbol difference of a known signal for channel estimation in the received signal in response to the signal quality measurement command received from the first wireless device, and calculates a frequency characteristic of the received signal by converting the received signal from a time axis signal to a frequency axis signal to obtain a received power for each frequency.

[0008] Here, in the above wireless communication system, the second radio device may be configured to transmit the calculated SNR and the frequency characteristic to the first radio device.

[0009] In the above wireless communication system, the first radio device may be configured to store the SNR and the frequency characteristic received from the second radio device.

[0010] In addition, in the above wireless communication system, the first radio device may be configured to repeatedly transmit a signal quality measurement command while changing a transmission frequency, calculate a received signal quality based on the SNR and the frequency characteristics transmitted from the second radio device in response to the signal quality measurement command, and store the calculated received signal quality in correspondence with the transmission frequency and bandwidth.

[0011] In addition, in the above wireless communication system, the first radio device may be configured to select an optimal transmission frequency, transmission bandwidth, and modulation method for communication with the second radio device based on the received signal quality for each transmission frequency, and apply them to subsequent communication.

[0012] In addition, in the above wireless communication system, the first radio device can be configured to calculate transmission efficiency by multiplying the transmission bandwidth of the signal quality measurement command by the number of multi-level modulation bits of a modulation method calculated from the SNR of the received signal in the second radio device, and to select transmission conditions with high transmission efficiency. Effect of the Invention

[0013] According to the present invention, it becomes possible to more appropriately grasp the received signal quality of a transmission frequency in a wireless communication system using the HF band or the like. [Brief description of the drawings]

[0014] [Figure 1] 1 is a diagram illustrating an example of the configuration of a modulation / demodulation circuit in a wireless communication system according to an embodiment of the present invention. [Diagram 2] FIG. 13 is a diagram illustrating an example of a signal quality measurement request frame. [Diagram 3] FIG. 13 is a diagram showing an example of a signal quality measurement response frame. [Figure 4] FIG. 13 illustrates an example of a communication method instruction frame. [Diagram 5] FIG. 1 is a diagram showing an example of a flow of receiving signal quality measurement. [Figure 6] FIG. 1 is a diagram showing an example of a constellation of received signals with good SNR. [Figure 7]FIG. 1 is a diagram showing an example of a constellation of a received signal with poor SNR. [Figure 8] FIG. 1 is a diagram illustrating an example of a reception spectrum in a good propagation path environment. [Figure 9] FIG. 1 is a diagram illustrating an example of a reception spectrum in a poor propagation path environment. [Figure 10] FIG. 11 is a diagram illustrating an example of a method for determining a transmission condition. [Figure 11] FIG. 13 is a diagram showing an example of a modulation scheme table used to determine a modulation scheme. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] A wireless communication system according to an embodiment of the present invention will be described with reference to the drawings. Fig. 1 shows an example of the configuration of a modulation / demodulation circuit in a wireless communication system according to an embodiment of the present invention. Note that Fig. 1 shows a circuit section related to a first wireless device (hereinafter referred to as "wireless device T") on the transmitting side and a circuit section related to a second wireless device (hereinafter referred to as "wireless device R") on the receiving side as an integrated circuit.

[0016] The modulation / demodulation circuit in FIG. 1 includes a measurement command processing generation unit 11, a modulation processing unit 12, a preamble adding unit 13, a transmission roll-off filter unit 14, a transmission sampling rate conversion processing unit 15, an orthogonal modulation unit 16, an orthogonal demodulation unit 17, a reception sampling rate conversion processing unit 18, a reception roll-off filter unit 19, a synchronization processing / parameter detection unit 20, a demodulation processing / SNR (Signal-to-Noise Ratio) calculation unit 21, an FFT (Fast Fourier Transform) unit 22, a frequency characteristic calculation unit 23, a measurement response processing generation unit 24, and an operating frequency / bandwidth determination unit 25.

[0017] First, the following description focuses on the operation performed when wireless device T commands wireless device R to measure the quality of a received signal. The measurement command processing generator 11 generates signal quality measurement command data for commanding wireless device R to measure the quality of a received signal, and outputs the signal quality measurement command data to the modulation processor 12. The modulation processor 12 modulates transmission data for wireless device R and outputs the data to the preamble adding unit 13. Examples of the transmission data include user data requested to be transmitted by the user of wireless device T, signal quality measurement command data output from the measurement command processing generator 11, signal quality measurement response data output from a measurement response processing generator 24 (described later), and communication method instruction data output from an operating frequency / bandwidth determining unit 25 (described later).

[0018] The preamble adding unit 13 adds a synchronization preamble such as a synchronization pattern for performing synchronization processing in the radio R and reception parameters including a modulation method and settings of transmission data to the front stage of the modulated signal output from the modulation processing unit 12, and outputs the signal to the transmission roll-off filter unit 14. The transmission roll-off filter unit 14 shapes the waveform of the signal from the preamble adding unit 13 using a transmission roll-off filter, and outputs the signal to the transmission sampling rate conversion processing unit 15. The transmission sampling rate conversion processing unit 15 converts the signal from the transmission roll-off filter unit 14 to a sampling frequency of a DAC (Digital to Analog Converter; D / A conversion circuit), and outputs the signal to the quadrature modulation unit 16. The quadrature modulation unit 16 quadrature-modulates the signal from the transmission sampling rate conversion processing unit 15, converts it from a baseband frequency to an intermediate frequency, and outputs the signal. The radio unit T converts the signal output from the quadrature modulation unit 16 from an intermediate frequency to an HF frequency, and transmits it from a transmission antenna.

[0019] Next, the operation of the radio device R when measuring the quality of a received signal according to a command from the radio device T will be described. The radio device R converts the signal received by the receiving antenna from an HF frequency to an intermediate frequency. The orthogonal demodulation unit 17 converts the received signal from the intermediate frequency to a baseband frequency, performs orthogonal demodulation, and outputs the signal to the reception sampling rate conversion processing unit 18. The reception sampling rate conversion processing unit 18 converts the signal from the orthogonal demodulation unit 17 from the sampling frequency of an ADC (Analog to Digital Converter; A / D conversion circuit) to a frequency suitable for demodulation, and outputs the signal to the reception roll-off filter unit 19 and the FFT unit 22.

[0020] The reception roll-off filter unit 19 performs waveform shaping on the signal from the reception sampling rate conversion processing unit 18 using a reception roll-off filter, and outputs the result to the synchronization processing / parameter detection unit 20. The synchronization processing / parameter detection unit 20 detects the preamble position by performing correlation calculation on the signal from the reception roll-off filter unit 19, identifies the start position of the demodulation processing, performs synchronization processing, extracts reception parameters and modulated signals, and outputs them to the demodulation processing / SNR calculation unit 21. The demodulation processing / SNR calculation unit 21 performs demodulation processing on the modulated signal from the synchronization processing / parameter detection unit 20 according to the reception parameters, thereby demodulating the transmission data. Furthermore, when the transmission data demodulated from the reception signal by the demodulation processing / SNR calculation unit 21 is signal quality measurement command data, the demodulation processing / SNR calculation unit 21 calculates the SNR of the reception signal based on the channel estimation error, and outputs the SNR to the measurement response processing generation unit 24. The SNR can be calculated based on, for example, a symbol difference of a known signal for channel estimation in the reception signal.

[0021] The FFT unit 22 converts the signal from the reception sampling rate conversion processing unit 18 from a time axis signal to a frequency axis signal by Fourier transform processing, and outputs the signal to the frequency characteristic calculation unit 23. When the transmission data demodulated from the reception signal by the demodulation processing / SNR calculation unit 21 is signal quality measurement command data, the frequency characteristic calculation unit 23 calculates the reception power for each frequency for the signal from the FFT unit 22, and outputs the calculated power as the frequency characteristic to the measurement response processing generation unit 24. The frequency characteristic of the reception signal can be calculated, for example, from the signal power of each frequency component of the FFT output.

[0022] The measurement response processing generator 24 generates signal quality measurement response data for responding with the output of the demodulation processing / SNR calculation unit 24 (the SNR calculation result of the received signal) and the output of the frequency characteristic calculation unit 23 (the frequency characteristic of the received signal). The signal quality measurement response data generated by the measurement response processing generator 24 is transmitted as transmission data to the wireless device T from the wireless device R to the wireless device T after being processed by the modulation processing unit 12, the preamble adding unit 13, the transmission roll-off filter unit 14, the transmission sampling rate conversion processing unit 15, and the quadrature modulation unit 16 in the same manner as described for the wireless device T.

[0023] Thereafter, in the wireless device T, the received signal including the signal quality measurement response data is processed by the orthogonal demodulation unit 17, the reception sampling rate conversion processing unit 18, the reception roll-off filter unit 19, and the synchronization processing / parameter detection unit 20 in the same manner as the processing described for the wireless device R, and is output to the demodulation processing / SNR calculation unit 21. If the transmission data demodulated from the received signal is signal quality measurement response data, the demodulation processing / SNR calculation unit 21 outputs the data to the operating frequency / bandwidth determination unit 25.

[0024] The operating frequency / bandwidth determination unit 25 analyzes the signal quality measurement response data from the demodulation processing / SNR calculation unit 21, and calculates the received signal quality based on the SNR and frequency characteristics of the received signal of the radio device R for the transmission frequency at the current time. The received signal quality can be calculated by an arithmetic expression such as "Q(f,B)=Q1×SNR+Q2×Fx". Here, in the arithmetic expression, "f" is the measurement frequency, "B" is the bandwidth that can be set, "Q(f,B)" is the received signal quality value under the conditions of the measurement frequency and bandwidth, "SNR" is the SNR calculation result of the received signal, "Fx" is the frequency characteristic of the received signal, and "Q1" and "Q2" are weighting coefficients for calculating the received signal quality. The received signal quality calculated by the operating frequency / bandwidth determination unit 25 is stored in a memory (not shown) in association with the frequency and bandwidth. Thereafter, the operating frequency / bandwidth determination unit 25 generates communication method instruction data for instructing the radio device R to change to the next frequency to be measured, and outputs the data to the modulation processing unit 12. Thereafter, the wireless device T transmits communication method instruction data to the wireless device R in the same procedure as described above, thereby causing the wireless device R to change its own frequency as well.

[0025] After repeating the above-mentioned series of operations for all frequencies to be measured, the operating frequency / bandwidth determination unit 25 of the wireless device T automatically selects (determines) the optimum transmission frequency, transmission bandwidth, and modulation method for the current state of the ionosphere and the positional relationship of transmission and reception (positional relationship between the wireless device T and the wireless device R) based on the reception signal quality, frequency, and bandwidth of each frequency stored in the memory. The operating frequency / bandwidth determination unit 25 further generates communication method instruction data for instructing the wireless device R to change to the selected transmission frequency, transmission bandwidth, and modulation method, and outputs it to the modulation processing unit 12. After that, the wireless device T changes the operating frequency, operating bandwidth, and modulation method of the wireless device R by transmitting the communication method instruction data to the wireless device R in the same procedure as described above, and also changes its own operating frequency, operating bandwidth, and modulation method.

[0026] As described above, the modulation / demodulation circuit of this system, in addition to a general modulation / demodulation circuit, is configured to have a process for generating a command for measuring the received signal quality, a process for calculating the SNR value and frequency characteristics of the received signal upon receiving the command, a process for notifying (responding) the calculated SNR value and frequency characteristics of the received signal to the sender, and a process for selecting (determining) the optimal transmission frequency, transmission bandwidth, and modulation method for communication based on the notified SNR value and frequency characteristics of the received signal for each frequency.

[0027] Fig. 2 shows an example of a signal quality measurement command frame used for transmitting the above-mentioned signal quality measurement command data. Fig. 3 shows an example of a signal quality measurement response frame used for transmitting the above-mentioned signal quality measurement response data. Fig. 4 shows an example of a communication method instruction frame used for transmitting the above-mentioned communication method instruction data.

[0028] The signal quality measurement command frame, the signal quality measurement response frame, and the communication method instruction frame all have a synchronization preamble section, a reception parameter section, and a data section. The synchronization preamble section and the reception parameter section are necessary for wireless communication using a burst signal. The synchronization preamble section is an area for storing a synchronization preamble used to detect a wireless frame. The reception parameter section is an area for storing reception parameters used to demodulate the data section. The data section of the signal quality measurement command frame stores data such as source information and destination information. The data section of the signal quality measurement response frame stores data such as the SNR value of the received signal and the frequency characteristics of the received signal. The data section of the communication method instruction frame stores data such as the changed transmission frequency, transmission bandwidth, and modulation method.

[0029] An example of the flow of receiving signal quality measurement in the wireless communication system of this example is shown in Fig. 5. Here, a case where the receiving signal qualities of three frequencies, frequency A, frequency B, and frequency C, are measured will be described.

[0030] First, the transmitting side (e.g., wireless device T) transmits a signal quality measurement command frame to the receiving side (e.g., wireless device R) on frequency A. At this time, the data section of the signal quality measurement command frame stores source information and destination information. In addition, in order to determine the transmission bandwidth, transmission is performed at the widest possible bandwidth for operation. The transmission process is the same as that of a typical wireless device, and is configured to modulate the reception parameters and transmission data, add a preamble, pass the data through a transmission roll-off filter, convert the sampling rate, and then perform quadrature modulation. When the receiving side receives the signal quality measurement command frame, if the destination information is its own, it starts measuring the received signal quality.

[0031] On the receiving side, the received signal is quadrature demodulated and the sampling rate converted signal is measured. The SNR is calculated for the operating bandwidth based on the channel estimation error for the signal that has passed through a receiving roll-off filter and undergone synchronization processing and parameter detection. For frequency characteristics, the time axis signal is converted to a frequency axis signal by FFT processing the signal after sampling rate conversion processing, and the frequency characteristics of the received signal are calculated based on the signal power of each frequency component.

[0032] After that, the receiving side notifies the transmitting side of the calculated SNR value and frequency characteristics at frequency A. That is, the receiving side transmits a signal quality measurement response frame at transmission frequency A. The data portion of the signal quality measurement response frame stores the SNR value of frequency A and the frequency characteristics of frequency A.

[0033] When the transmitting side receives the signal quality measurement response frame from the receiving side, it calculates the received signal quality for each bandwidth based on the SNR value of frequency A and the frequency characteristics of frequency A, stores the best received signal quality in memory in association with the transmission frequency and bandwidth, and transmits a communication method instruction frame to the receiving side to instruct it to change to the next measurement frequency, transmission frequency B. After transmitting the communication method instruction frame, the transmitting side changes to transmission frequency B. After receiving the communication method instruction frame, the receiving side changes to transmission frequency B. After the transmitting side and receiving side change to measurement frequency B, the same procedure as for measurement frequency A is repeated at measurement frequency B.

[0034] After completing the measurement at the last frequency (frequency C in this example), the transmitter determines the optimal transmission frequency, transmission bandwidth, and modulation method from the received signal quality calculated and stored in memory based on the SNR and frequency characteristics of each frequency measured so far, stores these in a communication method instruction frame and transmits it to the receiver, while changing its own transmission frequency, transmission bandwidth, and modulation method. The receiver changes the transmission frequency, transmission bandwidth, and modulation method according to the received communication method instruction frame, and starts operation.

[0035] An example of a constellation of a received signal with good SNR is shown in Figure 6. Also, an example of a constellation of a received signal with poor SNR is shown in Figure 7. SNR is the signal-to-noise ratio at baseband and is calculated as the logarithmic ratio of the signal power and the noise power.

[0036] Fig. 8 shows an example of a reception spectrum in a good propagation environment. Fig. 9 shows an example of a reception spectrum in a bad propagation environment. The spectrum of the received signal (signal level for each frequency) can be known by converting the time axis signal into a frequency axis signal using FFT processing. Fig. 8 shows a reception spectrum in a good propagation environment, where the frequency characteristics within the band are flat. On the other hand, in Fig. 9, the frequency characteristics within the band drop sharply due to the effects of fading. In such a case, the SNR decreases and demodulation performance deteriorates significantly, so if fading is present within the band, the transmission bandwidth should not be used.

[0037] The communication method (transmission frequency, transmission bandwidth, and modulation method) used in a wireless communication system is determined based on the results of a comprehensive evaluation of the measured SNR and frequency characteristics. Here, the communication method is determined by calculating the transmission efficiency based on each measurement result as follows.

[0038] Regarding the modulation method, for each measured SNR, the modulation method with the maximum number of multi-levels that exceeds the required SNR for each modulation method is selected. For example, if the required SNR for BPSK is 6 dB and the required SNR for QPSK is 9 dB, then QPSK is selected if the measured SNR is 9 dB or higher, and BPSK is selected if it is lower. Regarding the transmission bandwidth, the presence or absence of fading within the band is detected from the frequency characteristics for each transmission bandwidth, and if fading is present, the band is not used. In this case, the fading detection conditions are adjustable.

[0039] FIG. 10 shows an example of a method for determining a communication method. Measurement of the received signal quality is performed for each transmission frequency and transmission bandwidth. At this time, the transmitting side transmits at the maximum bandwidth, and the receiving side measures the received signal quality while changing the bandwidth, and selects the best received signal quality and stores it in memory. Then, the transmission efficiency is calculated by multiplying the transmission bandwidth by the number of multi-level modulation bits of the modulation method calculated from the SNR, and the transmission condition with the highest transmission efficiency is selected. However, if fading is present within the band, the transmission condition will not be selected even if it has the highest transmission efficiency. FIG. 11 shows an example of a used modulation method table, and by using such a used modulation method table, the modulation method according to the SNR value can be uniquely determined.

[0040] As described above, in the wireless communication system of this example, the wireless device T transmits a signal quality measurement command to the second receiver at the set transmission frequency, and the wireless device R calculates the SNR of the received signal based on the symbol difference of a known signal for channel estimation in the received signal by the demodulation processing / SNR calculation unit 21 in response to the signal quality measurement command received from the wireless device T, converts the received signal from a time axis signal to a frequency axis signal by the FFT unit 22, calculates the frequency characteristic of the received signal by obtaining the received power for each frequency by the frequency characteristic calculation unit 23, and calculates the overall received signal quality based on the SNR and frequency characteristic of the received signal by the measurement response processing generation unit 24. Here, the measurement response processing generation unit 24 reduces the received signal quality when there is a steep level fluctuation in the frequency characteristic even if the SNR of the received signal is high. Therefore, according to the wireless communication system of this example, it becomes possible to more appropriately grasp the received signal quality of the transmission frequency.

[0041] In the wireless communication system of this example, the wireless device T is configured to repeatedly transmit the signal quality measurement command while changing the transmission frequency, and to store the received signal quality transmitted from the wireless device R in response to the signal quality measurement command in association with the transmission frequency of the signal quality measurement command. Therefore, the wireless communication system of this example makes it possible to grasp the change in received signal quality due to the difference in transmission frequency.

[0042] In the wireless communication system of this example, the wireless device T is configured to select the optimum transmission frequency, transmission bandwidth, and modulation method for communication with the wireless device R based on the received signal quality for each transmission frequency, and apply them to subsequent communication. Therefore, according to the wireless communication system of this example, subsequent communication can be performed using the optimum transmission frequency, transmission bandwidth, and modulation method for the current state of the ionosphere and the positional relationship of transmission and reception.

[0043] Moreover, in the wireless communication system of this example, the wireless device T is configured to calculate the transmission efficiency by multiplying the transmission bandwidth of the signal quality measurement command by the number of multi-level modulation bits of the modulation method calculated from the SNR of the received signal in the wireless device R, and select the condition with high transmission efficiency. Therefore, according to the wireless communication system of this example, it is possible to suitably select the condition with high transmission efficiency.

[0044] Although the embodiments of the present invention have been described above, these embodiments are merely illustrative and do not limit the technical scope of the present invention. The present invention can take various other embodiments, and various modifications such as omissions and substitutions can be made without departing from the gist of the present invention. These embodiments and modifications are included in the scope and gist of the invention described in this specification, etc., and are included in the scope of the invention described in the claims and their equivalents.

[0045] Furthermore, the present invention can be provided not only as devices such as those described above or as a system composed of these devices, but also as methods executed by these devices, programs for causing a processor to realize the functions of these devices, and storage media for storing such programs in a computer-readable format. [Industrial Applicability]

[0046] The present invention can be used in a wireless communication system that performs wireless communication using the HF band or the like. [Explanation of symbols]

[0047] 11: measurement command processing and generation unit, 12: modulation processing unit, 13: preamble adding unit, 14: transmission roll-off filter unit, 15: transmission sampling rate conversion processing unit, 16: quadrature modulation unit, 17: quadrature demodulation unit, 18: reception sampling rate conversion processing unit, 19: reception roll-off filter unit, 20: synchronization processing / parameter detection unit, 21: demodulation processing / SNR calculation unit, 22: FFT unit, 23: frequency characteristic calculation unit, 24: measurement response processing and generation unit, 25: operation frequency / bandwidth determination unit

Claims

1. In a wireless communication system for performing wireless communication between a first wireless device and a second wireless device, The first radio device transmits a signal quality measurement command to the second radio device at a set transmission frequency; a second radio device that, in response to a signal quality measurement command received from the first radio device, calculates an SNR of the received signal based on a symbol difference of a known signal for channel estimation in the received signal, and calculates frequency characteristics of the received signal by converting the received signal from a time axis signal to a frequency axis signal to obtain received power for each frequency.

2. 2. The wireless communication system according to claim 1, The second radio device transmits the calculated SNR and the frequency characteristic to the first radio device.

3. 3. The wireless communication system according to claim 2, The wireless communication system according to claim 1, wherein the first wireless device stores the SNR and the frequency characteristic received from the second wireless device.

4. 4. The wireless communication system according to claim 3, a first radio device repeatedly transmits a signal quality measurement command while changing a transmission frequency, calculates a received signal quality based on the SNR and the frequency characteristics transmitted from the second radio device in response to the signal quality measurement command, and stores the calculated received signal quality in correspondence with the transmission frequency and bandwidth.

5. 5. The wireless communication system according to claim 4, a first radio device that selects an optimal transmission frequency, transmission bandwidth, and modulation method for communication with the second radio device based on a received signal quality for each transmission frequency, and applies the optimal transmission frequency, transmission bandwidth, and modulation method to subsequent communications.

6. 5. The wireless communication system according to claim 4, a first radio device that calculates transmission efficiency by multiplying a transmission bandwidth of a signal quality measurement command by a number of multi-level modulation bits of a modulation method calculated from an SNR of a received signal at the second radio device, and selects transmission conditions with high transmission efficiency.

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