JP7573936B2 - Wireless communication system - Google Patents

Description

This invention relates to wireless communication systems, and in particular to technology suitable for application to wireless communication systems that use narrowband signals.

In commercial wireless systems that use narrowband signals, new frequency allocations are difficult due to the depletion of frequency bands. For this reason, there are commercial wireless systems in which the same signal is transmitted from multiple base stations to a terminal at the same frequency. However, with this method, if multiple wireless signals are of the same level and out of phase at a certain receiving point, the signal will be lost due to same-wave interference (i.e., beat interference). As a countermeasure against such signal loss, a method is known in which the mapping rules are changed for each base station (Patent Document 1).

JP 2018-166293 A

However, the method described in Patent Document 1 has the problem that it cannot handle situations where two signals with different amplitudes are received.

The object of this invention is to provide a wireless communication system that can demodulate a frequency shift keying modulated wave while suppressing the occurrence of beat interference, regardless of the power ratio of the signals transmitted from each base station.

In order to solve the above problem, the invention described in claim 1 is a wireless communication system comprising a first base station and a second base station each equipped with a wireless communication device that transmits a signal modulated by a frequency shift keying method, and a receiving station that receives a signal transmitted from the first base station and a signal transmitted from the second base station, wherein an offset frequency is set between the frequency in the modulation process of the wireless communication device of the first base station and the frequency in the modulation process of the wireless communication device of the second base station, and the receiving station uses a known signal transmitted during a learning period to determine the propagation environment between the first base station and the receiving station, the propagation environment between the second base station and the receiving station, and the offset frequency, and removes the signal transmitted from one base station from the received signal at the receiving station to obtain the signal transmitted from the other base station.

The invention described in claim 2 is characterized in that in the wireless communication system described in claim 1, the offset frequency is equal to or less than the allowable error for the mapping reference value in the modulation process.

The invention described in claim 3 is characterized in that in the wireless communication system described in claim 1 or 2, the frequency shift keying method is a four-level frequency shift keying method in which the number of multilevels is four.

According to the invention described in claim 1, a certain offset frequency is set for each frequency shift assigned in the modulation process at each base station, and then signals are transmitted from each base station, making it possible to suppress the occurrence of beat interference.

According to the invention described in claim 1, by utilizing a known signal transmitted during the learning period and taking advantage of the fact that the frequency offset between each base station is constant, the signal transmitted from one base station is removed from the received signal at the receiving station to obtain the signal transmitted from the other base station, thereby making it possible to demodulate a frequency shift keying modulated wave while suppressing the occurrence of beat interference, regardless of the power ratio of the signals transmitted from each base station.

According to the invention described in claim 2, the offset frequency is set to an appropriate value, which allows the invention to be applied without any problems and enables good wireless communication.

According to the invention described in claim 3, it is possible to achieve the above-mentioned effects in a wireless communication system that performs wireless communication using a four-level frequency shift keying method.

1 is a diagram showing an overview of a wireless communication system according to an embodiment of the present invention; 2 is a functional block diagram showing a schematic configuration of a modulation section provided in a base station of the wireless communication system of FIG. 1. FIG. 1 is a diagram for explaining an offset frequency added in a base station. 2 is a functional block diagram showing a schematic configuration of a demodulation section provided in a receiving station of the wireless communication system of FIG. 1. 5 is a functional block diagram showing a schematic configuration of an interference canceller unit of the demodulator unit of FIG. 4.

The present invention will be described below based on the illustrated embodiment. Note that the following describes the characteristic configuration of the present invention, and the explanation of the conventional mechanism for wireless communication will be omitted. Also, in each figure, the signal line transmitting the real part (I signal; in other words, the in-phase component, I signal component) and the signal line transmitting the imaginary part (Q signal; in other words, the quadrature component, Q signal component) that make up the complex signal are shown together as a single signal line.

Figure 1 shows an overview of a wireless communication system 1 according to an embodiment of the present invention.

In this embodiment, data is modulated and demodulated by a wireless communication device including a modulator 20 provided in the base stations 2A and 2B and a wireless communication device including a demodulator 30 provided in the receiving station 3, and wireless communication is performed between the base stations 2A and 2B and the receiving station 3. Data modulation and demodulation is performed by the frequency shift keying (FSK: an abbreviation for Frequency Shift Keying) method.

Base stations 2A and 2B generate transmission signals including the information to be transmitted, and transmit the transmission signals to receiving station 3. At this time, base stations 2A and 2B synchronously transmit/radio-transmit the same signal (i.e., broadcast).

The transmission signal sent from base station 2A reaches receiving station 3 via wireless propagation path 5A, and the transmission signal sent from base station 2B reaches receiving station 3 via wireless propagation path 5B.

The receiving station 3 receives signals transmitted from the base station 2A and transmitted via the wireless propagation path 5A, and also receives signals transmitted from the base station 2B and transmitted via the wireless propagation path 5B, performs a predetermined process on the received signals, and outputs the information to be transmitted.

The wireless communication system 1 according to the embodiment includes a first base station (base station 2A) and a second base station (base station 2B), each of which includes a wireless communication device that transmits a signal modulated by a frequency shift keying scheme, and a receiving station 3 that receives a signal transmitted from the first base station (base station 2A) and a signal transmitted from the second base station (base station 2B). An offset frequency Δf is set between the frequency in the modulation process of the wireless communication device of the first base station (base station 2A) and the frequency in the modulation process of the wireless communication device of the second base station (base station 2B). The receiving station 3 uses a known signal X _A ′ transmitted during a learning period to determine a propagation environment H _A between the first base station (base station 2A) and the receiving station 3, a propagation environment H _B between the second base station (base station 2B) and the receiving station 3, and the offset frequency Δf, and removes a signal X _B transmitted from one base station (base station 2B in this embodiment) from a received signal Y at the receiving station 3 to obtain a signal X _A transmitted from the other base station (base station 2A in this embodiment).

Figure 2 is a functional block diagram showing the schematic configuration of a modulation unit 20 provided in base stations 2A and 2B of a wireless communication system 1 according to an embodiment of the present invention.

The mapping unit 21 of the modulation unit 20 provided in the base stations 2A and 2B receives as input a binary data string that has been subjected to predetermined processing (e.g., analog-to-digital conversion processing, error correction coding processing, frequency conversion processing, amplification processing) as necessary for the information/data to be transmitted, and performs modulation mapping processing on the binary data string using the frequency shift keying (FSK) method to generate and output a symbol string.

Specifically, in this embodiment, the mapping unit 21 performs modulation mapping processing in accordance with "Municipal Digital Mobile Communication System (SCPC/4-FSK system)" (Association of Radio Industries and Businesses, Standard Number: ARIB STD-T116), which is a standard for wireless systems using the 4-FSK system among narrowband digital mobile communication systems. In the above standard (ARIB STD-T116), 4-FSK mapping is performed according to the contents shown in Table 1 below.

The mapping unit 21 converts each dibit (in other words, every 2 bits) that constitutes the input binary data string into a symbol according to Table 1, and outputs a symbol string.

The ROF unit 22 has the function of a roll-off filter, receives the symbol sequence output from the mapping unit 21, and performs band limiting processing on the symbol sequence to reduce/eliminate inter-symbol interference before outputting it.

The frequency offset unit 23 receives the symbol sequence output from the ROF unit 22, calculates and outputs a numerical value representing the frequency obtained by adding an offset frequency Δf to the frequency shift value corresponding to each symbol, which is the frequency to be input to the NCO unit 24.

Here, the above-mentioned standard (ARIB STD-T116) specifies that the allowable error in frequency deviation is ±10% or less with respect to the mapping reference value. Taking this into consideration, in this embodiment in which modulation mapping processing is performed according to the above-mentioned standard (ARIB STD-T116), the offset frequency Δf is set in a range in which the absolute value is 31.5 Hz or less.

Based on the above, in this embodiment, specifically, the offset frequency Δf of base station 2A is set to 0 Hz, and the offset frequency Δf of base station 2B is set to 30 Hz (see FIG. 3). Here, the value of the offset frequency Δf added to each frequency deviation value corresponding to each symbol is constant for all frequency deviations.

For example, the offset frequency Δf of base station 2A may be set to 0 Hz and the offset frequency Δf of base station 2B may be set to -30 Hz, or the offset frequency Δf of base station 2A may be set to -15 Hz and the offset frequency Δf of base station 2B may be set to +15 Hz.

The NCO unit 24 has the functionality of a numerically controlled oscillator (NCO: abbreviation for Numerically Controlled Oscillator), receives a numerical value representing a predetermined frequency output from the frequency offset unit 23, and outputs an oscillation signal of a frequency corresponding to the numerical value.

The quadrature modulation unit 25 receives the oscillation signal output from the NCO unit 24, performs quadrature modulation processing using the oscillation signal, and outputs a transmission signal (baseband signal).

The transmission signal output from the quadrature modulation unit 25 is subjected to predetermined processing (e.g., amplification processing, frequency conversion processing) as necessary, and then transmitted/wirelessly transmitted via the antenna 26. At this time, as described above, the base station 2A and the base station 2B transmit/wirelessly transmit the same signal in synchronization.

In this embodiment, a transmission signal X _A (baseband signal) is transmitted from the base station 2A, and a transmission signal X _B (baseband signal) is transmitted from the base station 2B.

The receiving station 3 receives the signal transmitted from the base station 2A and the signal transmitted from the base station 2B via the antenna 31.

Figure 4 is a functional block diagram showing the schematic configuration of the demodulation unit 30 provided in the receiving station 3 of the wireless communication system 1 according to an embodiment of the present invention.

The signal received via the antenna 31 is subjected to predetermined processing (e.g., band limiting, amplification, frequency conversion) as necessary, and then input to the quadrature detection unit 32 of the demodulation unit 30 provided in the receiving station 3.

The quadrature detection unit 32 performs quadrature detection using a local oscillation signal supplied from a local oscillator (not shown), and generates and outputs a complex signal consisting of an in-phase component I signal and a quadrature component Q signal.

The interference canceller unit 4 is a mechanism for removing a signal transmitted from one of the base stations (specifically, base station 2B in this embodiment) from a received signal by utilizing the fact that the frequency offset between base station 2A and base station 2B is constant for signals transmitted from base stations 2A and 2B after a constant offset frequency Δf (specifically, offset frequency Δf = 30 Hz in base station 2B in this embodiment) is added to each frequency shift assigned in the modulation mapping process in each base station 2A, 2B.

Figure 5 is a functional block diagram showing the general configuration of the interference canceller unit 4 in this embodiment.

Here, the transmission signal X _A (baseband signal) transmitted from the base station 2A is expressed by the following formula 1, taking into consideration that the offset frequency Δf is 0 Hz.

Moreover, the transmission signal X _B (baseband signal) transmitted from the base station 2B is expressed by the following formula 2, taking into consideration that the offset frequency Δf is 30 Hz (that is, Δf≠0).

The meanings of the symbols/variables in Equations 1 and 2 are as follows:
X _A : A transmission signal (baseband signal) transmitted from the base station 2A.
X _B : A transmission signal (baseband signal) transmitted from the base station 2B
f _A : Frequency of the transmission signal sent from base station 2A [Hz]
Δf: offset frequency added by base station 2B [Hz]
t: time (sampling time)
j: imaginary unit

The propagation environment H _A in the wireless propagation path 5A from the base station 2A to the receiving station 3 is expressed by the following formula 3, and the propagation environment H _B in the wireless propagation path 5B from the base station 2B to the receiving station 3 is expressed by the following formula 4.

The meanings of the symbols/variables in Equations 3 and 4 are as follows:
H _A : Propagation environment between the base station 2A and the receiving station 3 H _B : Propagation environment between the base station 2B and the receiving station 3 α: Amplitude fluctuation in the propagation environment H _A β: Amplitude fluctuation in the propagation environment H _B Φ _A : Phase fluctuation in the propagation environment H _A Φ _B : Phase fluctuation in the propagation environment H _B

Based on the above, the received signal Y at the receiving station 3 is expressed by the following formula 5. The meanings of the symbols/variables in the following formulas 5 to 11 are the same as those of the symbols/variables in the above formulas 1 to 4.

The interference canceller unit 4 uses a known signal X _A ' included/added to the received signal periodically at a predetermined time interval (or repeatedly at a predetermined timing) to determine and appropriately update the propagation environment H _A between the base station 2A and the receiving station 3, the propagation environment H _B between the base station 2B and the receiving station 3, and the offset frequency Δf [Hz] added by the base station 2B, and obtains the transmission signal X _A transmitted from the base station 2A from the received signal Y (see formula 5 above) at the receiving station 3 based on the above values. Note that the transmission signal X _A transmitted from the base station 2A may be obtained by removing _the transmission signal X _A transmitted from the base station 2B.

Known signal X _A ' (including cases where it is a signal called a reference signal, pilot signal, training signal, etc.) is, for example, a predetermined bit sequence that is determined in advance and shared by both base stations 2A, 2B and receiving station 3, and is stored in a memory unit (not shown) in receiving station 3 for processing in receiving station 3. In the following description, symbols/variables with "'" indicate that they are symbols/variables related to known signal X _A '.

The first division unit 401 of the interference canceller unit 4 receives an input of a received signal Y (see Equation 5 above and Equation 6 below) at the receiving station 3 received via the antenna 31, as well as a known signal _XA ' supplied from a storage unit (not shown), and divides the received signal Y by the known signal _XA ' to output a signal expressed by Equation 7 below. Here, when the transmission signals sent from the base stations 2A and 2B are known signals (in other words, the period, timing), _XA in Equation 6 is _XA ' (this case is called the "learning period").

The LPF unit 402 has a low-pass filter function, receives the signal output from the first division unit 401 (see Equation 7 above), and outputs the propagation environment _H between the base station 2A and the receiving station 3, taking advantage of the fact that, of the signal, the propagation environment _H between the base station 2A and the receiving station 3 is a low frequency component, and the component related to the propagation environment _H between the base station 2B and the receiving station 3 (i.e., the second term in Equation 7 above) is a higher frequency component.

The first propagation environment update control unit 403 receives the propagation environment H between the base station 2A and the receiving station 3 output from the LPF unit 402, and outputs the propagation environment _H during the learning period to the addition unit 404 as the propagation environment H _' for interference cancellation processing, while retaining the propagation environment _H during the period other than the learning period, in other words, until the next learning period.

The first addition/subtraction unit 405 receives an input of the propagation environment _H between the base station 2A and the receiving station 3 output from the LPF unit 402, as well as an input of the signal output from the first division unit 401 (see Equation 7 above), subtracts the signal output from the first division unit 401 from the propagation environment _H ( _H ' in the case of the learning period), and outputs a signal expressed as in Equation 8 below.

The phase angle calculation section 406 receives the signal (see Equation 8 above) output from the first addition/subtraction section 405, and calculates the phase angle θ by taking the arctangent function (ie, tan ⁻¹ ) of the signal.

The delay unit 407 receives the phase angle θ output from the phase angle calculation unit 406, delays the phase angle θ by one sampling period, and outputs the delayed phase angle θ. The phase angle output from the phase angle calculation unit 406 is denoted as " _θt ", and the delayed phase angle output from the delay unit 407 after being delayed by one sampling period with respect to the phase angle _θt is denoted as "θt _-1 ".

The frequency calculation section 408 subtracts the phase angle θ _t-1 output from the delay section 407 from the phase angle θ _t output from the phase angle calculation section 406 to obtain the phase angle change amount Δθ _t in the sampling period. The frequency calculation section 408 further calculates the offset frequency Δf using the relationship Δf = Δθ _t × fs / 2π (where fs is the sampling frequency [Hz]).

The frequency update control unit 409 receives the offset frequency Δf output from the frequency calculation unit 408, and while retaining the offset frequency Δf during the learning period as an offset frequency Δf' for interference cancellation processing during periods other than the learning period, in other words, until the next learning period, calculates and outputs a numerical value representing the offset frequency Δf', which is a frequency to be input to the NCO unit 411.

The amplifier unit 410 has the function of an amplifier, receives a numerical value (signal) representing the offset frequency Δf' output from the frequency update control unit 409, performs a predetermined amplification process, and outputs the result.

The NCO unit 411 has a function of a numerically controlled oscillator, receives a numerical value representing the offset frequency Δf′ output from the amplifier unit 410, and generates and outputs a signal expressed by the following Equation 9 in accordance with the numerical value representing the offset frequency Δf′.

The multiplication unit 412 , the second addition/subtraction unit 413 , the estimation unit 414 , and the second propagation environment update control unit 415 form a processing loop for estimating the propagation environment H _B between the base station 2 B and the receiving station 3 .

The multiplication unit 412 receives the signal output from the NCO unit 411 (see Equation 9 above) as input, and also receives the propagation environment between the base station 2B and the receiving station 3 output from the second propagation environment update control unit 415, multiplies the signal by the propagation environment, and outputs the result.

The second addition/subtraction unit 413 receives the signal output from the first addition/subtraction unit 405 (see Equation 8 above) as an input, and also receives the signal output from the multiplication unit 412, subtracts the two signals, and outputs the difference as an error signal. In other words, the second addition/subtraction unit 413 uses the signal output from the first addition/subtraction unit 405 (see Equation 8 above) as a reference signal, and outputs the difference between the reference signal and the signal output from the multiplication unit 412 as an error signal.

The estimation unit 414 has the function of, for example, a transversal equalizer, and receives an input of the signal output from the NCO unit 411 (see Equation 9 above) as well as the error signal output from the second addition-subtraction unit 413. Based on the error signal output from the second addition-subtraction unit 413, the estimation unit 414 estimates the propagation environment H B (see Equation 4 above) between the base station 2B and the receiving station 3 by using, for example, a least mean square (LMS) _algorithm or the like to converge the signal output from the NCO unit 411 (see Equation 9 above) to the signal output from the first addition-subtraction unit 405 (see Equation 8 above; reference signal).

The second propagation environment update control unit 415 receives an input of the propagation environment _H between the base station 2B and the receiving station 3 output from the estimation unit 414, and outputs the propagation environment H in the learning period as the propagation environment H' for interference cancellation processing to the multiplication unit 412 while holding the propagation environment _H during a period other than the learning period, in other words, until the next learning period. Note that the initial value of _the propagation environment _H when starting the estimation processing (in other words, the convergence processing) in the estimation unit 414 is set to 1, for example.

As a result of the above, the first propagation environment update control unit 403 outputs the propagation environment H _A ' between the base station 2A and the receiving station 3 for interference cancellation processing, the frequency update control unit 409 outputs the offset frequency Δf' added by the base station 2B for interference cancellation processing, and further, the second propagation environment update control unit 415 outputs the propagation environment H _B ' between the base station 2B and the receiving station 3 for interference cancellation processing.

Then, during periods other than the learning period, based on the propagation environment H _A ' between the base station 2A and the receiving station 3, the propagation environment H _B ' between the base station 2B and the receiving station 3, and the value of the offset frequency Δf' added by the base station 2B, the transmission signal X _B (baseband signal) transmitted from the base station 2B is removed from the received signal Y (see equations 5 and 6 above) at the receiving station 3, and the transmission signal X _A (baseband signal) transmitted from the base station 2A is obtained.

Specifically, when the second propagation environment update control unit 415 outputs the propagation environment H _B ' for interference cancellation processing, the multiplication unit 412 outputs a signal expressed by the following formula 10. Note that the NCO unit 411 generates and outputs a signal expressed by the above formula 9 even during periods other than the learning period.

The adder 404 receives the propagation environment H _A ' for interference cancellation processing output from the first propagation environment update control unit 403, and also receives the signal output from the multiplier 412 (see Equation 10 above), adds these two signals together, and outputs a signal expressed as in Equation 11 below.

The second division unit 416 receives as input the received signal Y (see Equation 6 above) and the signal output from the adder unit 404 (see Equation 11 above), divides the received signal Y by the signal output from the adder unit 404, and outputs a transmission signal X _A (baseband signal) transmitted from the base station 2A.

As a result of the above, the interference canceller unit 4 outputs the transmission signal X _A (baseband signal) transmitted from the base station 2 A.

The differential detection unit 33 receives the signal output from the interference canceller unit 4, performs differential detection between consecutive symbols using the signal as input, and generates and outputs a detected signal.

The ROF unit 34 has the function of a roll-off filter with the same characteristics as the ROF unit 22 of the base stations 2A and 2B, receives the detection signal output from the delay detection unit 33, applies band limiting processing to the detection signal, and outputs it.

The hard decision unit 35 and demapping unit 36 receive the detection signal output from the ROF unit 34, and based on the detection signal, perform hard decision and demapping processing on a symbol-by-symbol basis in accordance with the modulation method used in the mapping unit 21 of the base stations 2A and 2B (specifically, in this embodiment, the 4-level FSK method according to the above-mentioned standard (ARIB STD-T116)), and output the dibit value.

The binary data sequence output from the demapping unit 36 is data that reproduces the binary data sequence input to the mapping unit 21 of the base stations 2A and 2B (in other words, a digital signal that has been restored to the original data sequence).

According to the wireless communication system 1 of the embodiment, a constant offset frequency Δf (in the above embodiment, the offset frequency Δf = 30 Hz in base station 2B) is set for each frequency shift assigned in the modulation mapping process in each base station 2A, 2B, and then signals are transmitted from each base station 2A, 2B, making it possible to suppress the occurrence of beat interference.

The wireless communication system 1 according to the embodiment also utilizes a known signal transmitted during the learning period and the fact that the frequency offset between base station 2A and base station 2B is constant, and by removing the signal transmitted from one base station (base station 2B in the above embodiment) from the received signal Y at the receiving station 3 to obtain the signal transmitted from the other base station (base station 2A in the above embodiment), it becomes possible to demodulate the frequency shift keying modulated wave while suppressing the occurrence of beat interference, regardless of the power ratio of the signals transmitted from each base station 2A, 2B.

The wireless communication system 1 according to the embodiment further has the following advantages.
1) It can be applied to the 4-value FSK method, which is being increasingly introduced into commercial wireless systems.
2) The device does not become large, as occurs with transmission diversity.
3) Since the code is not made redundant as in the case of, for example, differential space-time block coding, the information transmission rate does not decrease.
4) Since it is realized by using a frequency offset of several tens of Hz, the frequency utilization efficiency is high.
5) Even if the power ratio of the signals transmitted from each base station is close, no bit errors occur.

The above describes an embodiment of the present invention, but the specific configuration is not limited to the above embodiment, and even if there are design changes within the scope of the invention that do not deviate from the gist of the invention, they are still included in the invention.

Specifically, in the above embodiment, the configuration of the base stations 2A and 2B of the wireless communication system 1 according to the present invention is as shown in FIG. 2, and the configuration of the receiving station 3 is as shown in FIG. 4, but the configuration of the base stations 2A and 2B and the receiving station 3 of the wireless communication system 1 according to the present invention is not limited to the configurations shown in FIG. 2 and FIG. 4, and the configuration of the base stations 2A and 2B and the receiving station 3 may be configured in other ways. In other words, the main point of the wireless communication system 1 according to the present invention is to offset the frequency shift in the modulation mapping process at each base station, and then use a known signal at the receiving station and utilize the fact that the frequency offset is constant to remove the signal transmitted from one base station from the received signal to obtain the signal transmitted from the other base station, and the specific configuration for realizing this main point is not limited to a specific form.

In addition, in the above embodiment, the modulation mapping process is performed according to the "Municipal Digital Mobile Communications System (SCPC/4-level FSK method)" (Association of Radio Industries and Businesses, standard number: ARIB STD-T116), but compliance with the above standard (ARIB STD-T116) is not an essential requirement of this invention, and frequency shift keying may be performed according to other standards or specifications. In addition, the number of multi-levels of frequency shift keying is not limited to 4, and other multi-levels may be used.

REFERENCE SIGNS LIST 1 Wireless communication system 2A Base station 2B Base station 20 Modulation unit 21 Mapping unit 22 ROF unit 23 Frequency offset unit 24 NCO unit 25 Orthogonal modulation unit 26 Antenna 3 Receiving station 30 Demodulation unit 31 Antenna 32 Orthogonal detection unit 33 Differential detection unit 34 ROF unit 35 Hard decision unit 36 Demapping unit 4 Interference canceller unit 401 First division unit 402 LPF unit 403 First propagation environment update control unit 404 Addition unit 405 First addition and subtraction unit 406 Phase angle calculation unit 407 Delay unit 408 Frequency calculation unit 409 Frequency update control unit 410 Amplification unit 411 NCO unit 412 Multiplication unit 413 Second addition and subtraction unit 414 Estimation unit 415: Second propagation environment update control unit 416: Second division unit 5A: Radio propagation path from the base station 2A to the receiving station 3 5B: Radio propagation path from the base station 2B to the receiving station 3

Claims (3)

a first base station and a second base station each including a wireless communication device for transmitting a signal modulated by a frequency shift keying scheme;
a receiving station for receiving a signal transmitted from the first base station and a signal transmitted from the second base station;
an offset frequency is set between a frequency in a modulation process of the wireless communication device of the first base station and a frequency in a modulation process of the wireless communication device of the second base station;
the receiving station obtains a propagation environment between the first base station and the receiving station, a propagation environment between the second base station and the receiving station, and the offset frequency by utilizing a known signal transmitted during a learning period, and removes a signal transmitted from one base station from the received signal at the receiving station to obtain a signal transmitted from the other base station;
1. A wireless communication system comprising: The offset frequency is equal to or smaller than a tolerance for a reference value of mapping in the modulation process.
2. The wireless communication system according to claim 1 . The frequency shift keying method is a four-level frequency shift keying method in which the number of multi-levels is four.
3. The wireless communication system according to claim 1, wherein the first and second communication paths are connected to each other.