Interference detection system, radio equipment, and interference detection method

Abstract

The present invention makes it possible to more accurately detect interference caused by the simultaneous transmission of radio waves of the same frequency by a plurality of wireless stations.　An interference detection system according to an embodiment of the present invention comprises: a main wave estimation unit 10 for estimating a temporary main wave AM-modulated wave AM_m(t) on the basis of a main wave frequency of a reception radio wave Rx(t) including at least a main wave and an AM component (t) of the reception radio wave Rx(t); an adaptive filter unit 20 for removing the temporary main wave AM-modulated wave AM_m(t) from the reception radio wave Rx(t); and an interference detection unit 30 for detecting the occurrence of interference on the basis of a canceller output e(t) of the adaptive filter unit 20.

Description

Interference detection system, radio device, and interference detection method

[0001] The present invention relates to a technique for detecting interference caused by multiple wireless stations simultaneously transmitting radio waves at the same frequency.

[0002] Air traffic control radio communications are prone to problems caused by interference. Interference occurs on the receiving end when different radio stations simultaneously transmit radio waves on the same frequency. Traditionally, the presence or absence of interference has been determined by the human ear. Air traffic control radio communications use amplitude (AM) modulation, and in the event of interference, audio overlap or unusual sounds occur on the receiving end. This has allowed humans to determine the presence or absence of interference based on this information, preventing serious incidents from occurring.

[0003] The following are examples of prior art in the technical field related to the present invention: For example, Patent Document 1 discloses a wireless device system that converts a received signal into a frequency-level relationship by FFT processing, detects peak power from the result, and determines the interference state based on the number of peaks.

[0004] Patent No. 6647424

[0005] When a receiver receives radio waves of the same frequency simultaneously transmitted from multiple radio stations, if there is a large difference in the received power levels of these radio waves (for example, a level difference of 20 dB or more), the impact of audio overlap or abnormal sounds caused by interference becomes extremely small. For this reason, it is nearly impossible for the human ear to determine whether or not there is interference. Furthermore, since methods for determining whether or not there is interference rely solely on hearing, it is easy for interference to be overlooked.

[0006] The present invention has been made in consideration of the above-described conventional circumstances, and aims to enable more accurate detection of interference that occurs when multiple wireless stations simultaneously transmit radio waves at the same frequency.

[0007] In order to achieve the above object, an interference detection system according to one aspect of the present invention has the following technical features: That is, the interference detection system is for detecting interference caused by multiple radio stations simultaneously transmitting radio waves of the same frequency, and is characterized by comprising: a main wave estimation unit that estimates a main wave AM-modulated wave based on the frequency of the main wave in a received radio wave that includes at least the main wave and the AM component of the received radio wave, a filter unit that removes the main wave AM-modulated wave from the received radio wave, and an interference detection unit that detects the occurrence of interference based on the output of the filter unit.

[0008] Here, in the above interference detection system, the main wave estimation unit includes an FFT unit that performs FFT processing on the received radio waves, a main wave frequency detection unit that detects the frequency of the main wave based on the result of the FFT processing, a main wave CW generation unit that generates a main wave CW based on the frequency of the main wave, an AM component extraction unit that extracts the AM component of the received radio waves, and a multiplier that multiplies the main wave CW by the AM component, and can operate to output the multiplication result by the multiplier as the main wave AM modulated wave.

[0009] Furthermore, the interference detection system can operate so that if a peak exists at any frequency in the output of the filter section, it detects that interference has occurred.

[0010] A radio device according to another aspect of the present invention has the following technical features: That is, the radio device has a function of detecting interference caused by multiple radio stations simultaneously transmitting radio waves of the same frequency, and is characterized by comprising: a main wave estimation unit that estimates a main wave AM-modulated wave based on the frequency of the main wave in a received radio wave that includes at least the main wave and the AM component of the received radio wave, a filter unit that removes the main wave AM-modulated wave from the received radio wave, and an interference detection unit that detects the occurrence of interference based on the output of the filter unit.

[0011] An interference detection method according to yet another aspect of the present invention has the following technical features: That is, the interference detection method is for detecting interference caused by multiple radio stations simultaneously transmitting radio waves of the same frequency, and is characterized by comprising the steps of: estimating a main AM-modulated wave based on the frequency of the main wave in received radio waves that include at least the main wave and the AM component of the received radio waves; removing the main AM-modulated wave from the received radio waves; and detecting the occurrence of interference based on the output of the filter unit.

[0012] According to the present invention, it is possible to more accurately detect interference that occurs when multiple wireless stations simultaneously transmit radio waves at the same frequency.

[0013] Fig. 1 is a diagram showing an overview of an air traffic control system to which the present invention is applied. Fig. 2 is a diagram showing an example of the configuration of an interference detection system according to an embodiment of the present invention. Fig. 3 is a diagram showing an example of the waveform of a primary CW(t). Fig. 4 is a diagram showing an example of the convergence transition of the canceller output e(t) when the frequency estimation error of the main wave CW(t) is 0 Hz. Fig. 5 is a diagram showing an example of the convergence transition of the canceller output e(t) when the frequency estimation error of the main wave CW(t) is 4 Hz. Fig. 6 is a diagram showing a comparative example of the spectra of the received radio wave Rx(t) and the canceller output e(t).

[0014] An embodiment of the present invention will be described with reference to the drawings. FIG. 1 shows an overview of an air traffic control system to which the present invention is applied. The illustrated air traffic control system includes stations A and B, which are aircraft, and station C, which is a control station. When stations A and B simultaneously start transmitting radio waves at the same frequency F1 and station C receives these radio waves, the received power level of the signal from station B is 20 dB or more higher than that of the signal from station A. In this case, the signal from station A is buried in the signal from station B, making it difficult for station C to hear the received voice from station B. To address this situation, the present invention provides a technology that can detect the occurrence of interference without relying on hearing. In this specification, the received wave with the higher received power at the receiving end when interference occurs is referred to as the "main wave" and the received wave with the lower received power is referred to as the "interference wave."

[0015] 2 shows an example of the configuration of an interference detection system according to one embodiment of the present invention. The illustrated interference detection system includes a main wave estimator 10, an adaptive filter 20, and an interference detector 30. The main wave estimator 10 includes an FFT unit 11, a main wave frequency detector 12, a main wave CW generator 13, an AM component detector 14, and a multiplier 15. The adaptive filter 20 includes a tap updater 21, an FIR filter 22, and an adder 23.

[0016] First, an overview of the processing performed by the interference detection system of this example will be described. Here, the received radio wave signal Rx(t) (hereinafter referred to as received radio wave Rx(t)) input to the interference detection system is assumed to be a quadrature detection signal (I / Q) consisting of an in-phase component and a quadrature-phase component. t is the sampling time.

[0017] In the interference detection system of this example, a received radio wave Rx(t) including at least a main wave is input to a main wave estimation unit 10. The main wave estimation unit 10 estimates the main wave AM-modulated wave as follows: First, an FFT unit 11 performs FFT (Fast Fourier Transform) on the received radio wave Rx(t), a main wave frequency detection unit 12 detects the main wave frequency, and a main wave CW (Constant Wave) is generated by a main wave CW generation unit 13.

[0018] Furthermore, an AM component extraction unit 14 extracts the AM (amplitude) component of the received radio wave Rx(t), and a multiplier 15 multiplies the AM component(t) by the CW(t) of the main wave. The main wave estimation unit 10 outputs the multiplication result by the multiplier 15 as an estimated main wave AM-modulated wave (hereinafter referred to as a "provisional main wave AM-modulated wave") AM_m(t). Although the received radio wave Rx(t) actually contains an interference wave component, the interference wave can be assumed to be sufficiently small compared to the main wave (for example, a level difference of 20 dB or more), and therefore the provisional AM-modulated wave AM_m(t) can be regarded as approximately the AM-modulated wave of the main wave.

[0019] A one-tap adaptive filter (adaptive filter unit 20 shown in the figure) is configured using the tentative AM-modulated wave AM_m(t) obtained in this way as the desired wave. The adaptive filter unit 20 minimizes the error e(t) using an LMS (Least Mean Square) algorithm, so that the interference wave component from which the main AM-modulated wave contained in the received radio wave Rx(t) has been removed is output from the adaptive filter unit 20 as a canceller output e(t). In other words, the adaptive filter unit 20 performs beat cancellation to cancel the beat (waveform fluctuation) of the received radio wave Rx(t), and the canceller output e(t) is monitored by the interference wave detector 70, making it possible to detect the occurrence of interference with high accuracy.

[0020] Next, the details of the blocks in the main wave estimation unit 10 and the adaptive filter unit 20 will be explained using specific examples. In the following, due to the constraint of the sampling rate of the audio signal used in the simulation = 44.1 kHz, this will be downsampled to 1 / 5, 8.802 kHz, for explanation. Note that each signal in the following explanation is a complex signal (A real +j×A image )

[0021] (a) FFT Processing and Main Wave CW Generation Processing The FFT unit 11 performs FFT processing (windowing not required) on the received radio wave Rx(t) including at least the main wave. Hereinafter, each subcarrier component (complex signal) resulting from the FFT processing will be represented as FFT(n). Here, n indicates the subcarrier number and is an integer value ranging from 1 to 4096. In this simulation, FFT processing is performed with a sampling frequency of 8.802 kHz and an FFT size of 4096. In this case, the frequency resolution (subcarrier spacing) is approximately 2.15 Hz.

[0022] Based on the FFT(n) output from the FFT unit 11, the main frequency detection unit 12 detects the subcarrier number n with the maximum power. max Next, the main wave CW generating unit 13 calculates the subcarrier number n max Here, n max13 subcarriers centered on the subcarrier select_SC (=FFT(n max −6) to FFT(n max +6)) shall be selected.

[0023] Next, the main wave CW generating unit 13 performs iFFT (inverse Fast Fourier Transform) processing on the selected subcarrier select_SC to generate a primary CW(t) (t=1 to 4096). Here, it is assumed that the iFFT processing is performed with an iFFT size of 4096.

[0024] Figure 3 shows an example of waveforms of the I and Q values ​​of the primary CW(t). In order to remove interference bands and AM components, the primary CW(t) is generated using only selected subcarriers select_SC rather than all subcarriers, so the primary CW(t) does not have a constant envelope, as shown in Figure 3. Note that Figure 3 shows the case where the CW frequency is not equal to 8.802 kHz / 4096.

[0025] Next, the main wave CW generating unit 13 performs differential detection on one sample using data from the generated primary CW(t) excluding both attenuated sides, to obtain the phase rotation amount delayed_phase per sample. Here, differential detection is performed using data within the dashed rectangle shown in Figure 3, specifically, data from sampling time t = 256 to 3840.

[0026] Next, the main wave CW generating unit 13 calculates the angular velocity ω per sample by performing an ATAN (arc tangent) transformation on the phase rotation amount delayed_phase, and then generates CW(t) by performing an exp(jωt) transformation. The angular velocity ω calculated in the above calculation is used in the subsequent processing.

[0027] (b) Extraction process of AM component of received radio wave Rx(t) The AM component extraction unit 14 calculates the absolute value of the received radio wave Rx(t) at each sampling time t to find the AM component(t) of the received radio wave Rx(t). That is, the calculation AM component(t) = abs(Rx(t)) is performed. Here, abs(Rx(t)) is a function that finds the absolute value of Rx(t).

[0028] (c) Generation process of temporary main wave AM-modulated wave The temporary main wave AM_m(t) is calculated by multiplying the main wave CW(t) by the AM component (t) in the multiplier 15. That is, the calculation AM_m(t) = AM component (t) × CW(t) is performed.

[0029] (d) Adaptive Filter Processing The adaptive filter unit 20 performs adaptive filter processing based on the received radio wave Rx(t) and the tentative main AM-modulated wave AM_m(t). Specifically, the tap update unit 21 generates a tap coefficient h(t) for one tap based on the tentative main AM-modulated wave AM_m(t) and the canceller output e(t-1) from one time instant earlier, and sets (updates) the tap coefficient of the FIR filter 22. Next, the FIR filter 22 multiplies the tentative main AM-modulated wave AM_m(t) by the tap coefficient h(t) to calculate the equalization output C(t). After that, the adder 23 adds the received radio wave Rx(t) and the equalization output C(t), and outputs the result as the canceller output e(t).

[0030] Here, each signal used in the above processing is a complex number and can be expressed as follows: Rx(t)=Rx real (t) + j × Rx image (t) AM_m(t)=AM_m real (t) + j × AM_m image (t) h(t)=h real (t) + j × h image (t) C(t)=C real (t) + j × C image (t) e(t)=e real (t) + j × e image (t) It is assumed that the initial value of the tap coefficient is h(0)=0+j×0.

[0031] The equalization output C(t), the canceller output e(t), and the tap coefficient h(t) are calculated by the following calculations: C(t) = AM_m(t) × h(t), e(t) = Rx(t) - C(t), and h(t) = h(t) + μe(t) × AM_m(t). * Note that * represents a complex conjugate. μ (step gain) is generally set to 1.0 or less, but in this example it is set to 0.1.

[0032] Figures 4A and 4B show examples of the convergence transition of the canceller output e(t). Here, the acquisition conditions are a main wave frequency of 2 kHz, an interference wave frequency of 200 Hz, and a reception level difference between the main wave and the interference wave (hereinafter referred to as the "D / U ratio") of 40 dB. Figure 4A shows an example in which the frequency estimation error (corresponding to ω above) of the main wave CW(t) is 0 Hz, while Figure 4B shows an example in which the frequency estimation error of the main wave CW(t) is 4 Hz. In Figure 4A, the LMS algorithm causes the main wave level to become almost zero after approximately 50 sample updates, leaving only the interference wave remaining. On the other hand, in Figure 4B, although the main wave level attenuates, the main wave remains even after that (after 50 samples). This is thought to be because the LMS algorithm devotes its efforts to correcting the phase rotation due to the frequency estimation error in addition to canceling the main wave.

[0033] Next, beat cancellation by the adaptive filter unit 20 is evaluated. Here, the FFT size is 4096, and a Hanning window is used as the window function. Figure 5 shows a comparative example of the spectrum of the received radio wave Rx(t) before beat cancellation and the canceller output e(t) after beat cancellation. In this figure, the spectral waveforms before and after beat cancellation have been adjusted so that the zero frequency is at the center.

[0034] Before beat cancellation, the D / U ratio of the interference wave peak was about 10 dB due to the bandwidth broadening caused by AM modulation. After beat cancellation, however, the D / U ratio of the interference wave peak improved to about 40 dB (although the absolute amount was attenuated by about 10 dB due to beat cancellation).

[0035] As shown in FIG. 5 , the spectrum waveform after beat cancellation clearly shows a peak of the interference wave at a frequency (−200 Hz in the illustrated example) different from the peak frequency of the main wave (100 Hz in the illustrated example). Therefore, the occurrence of interference can be detected by monitoring the canceller output e(t) using the interference detection unit 30 and determining whether a peak exists at any frequency. The interference detection unit 30 detects, for example, a frequency at which a level fluctuation equal to or greater than a predetermined threshold occurs as the peak frequency of the interference wave and determines that interference has occurred. Here, the canceller output e(t) is obtained by removing the main wave component from the received radio wave Rx(t), but it is possible that the main wave component remains. Therefore, the occurrence of interference may be detected by confirming that the peak frequency detected from the canceller output e(t) is different from the peak frequency of the main wave.

[0036] Note that beat cancellation in the adaptive filter unit 20 causes a peak that appears to be intermodulation to occur at a frequency that is symmetrical to the peak frequency of the interference wave relative to the peak frequency of the main wave. In the example of Figure 5, an intermodulation peak occurs at the intermodulation frequency = 100 Hz + abs (100 Hz - (-200 Hz)) = 100 Hz + 300 Hz = 400 Hz. However, since the purpose of beat cancellation is to check whether there are peaks other than the main wave (check for the presence of interference waves), the presence of intermodulation does not pose a problem (i.e., there is no problem even if intermodulation is detected).

[0037] As described above, the interference detection system of this example comprises a main wave estimation unit 10 that estimates a main wave AM-modulated wave AM_m(t) based on the main wave frequency in received radio waves Rx(t) that include at least the main wave and the AM component (t) of the received radio waves Rx(t), an adaptive filter unit 20 that removes the main wave AM-modulated wave AM_m(t) from the received radio waves Rx(t), and an interference detection unit 30 that detects the occurrence of interference based on the canceller output e(t) of the adaptive filter unit 20.

[0038] More specifically, the main wave estimation unit 10 includes an FFT unit 11 that performs FFT processing on the received radio wave Rx(t), a main wave frequency detection unit 12 that detects the main wave frequency based on the result of the FFT processing, a main wave CW generation unit 13 that generates a main wave CW(t) based on the main wave frequency, an AM component extraction unit 14 that extracts the AM component(t) of the received radio wave, and a multiplier 15 that multiplies the main wave CW(t) by the AM component(t), and is configured to output the multiplication result by the multiplier 15 as a main wave AM-modulated wave AM_m(t). Furthermore, the interference detection unit 20 is configured to detect the occurrence of interference when a peak exists at any frequency in the canceller output e(t) of the adaptive filter unit 20.

[0039] This configuration makes it possible to more accurately detect interference caused by multiple radio stations simultaneously transmitting radio waves on the same frequency, thereby more reliably preventing problems caused by interference in air traffic control radio communications, etc.

[0040] 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 on various other embodiments, and various modifications such as omissions and substitutions can be made without departing from the spirit of the present invention. These embodiments and modifications thereof are included in the scope and spirit of the invention described in this specification, etc., and are included in the invention described in the claims and their equivalents.

[0041] Furthermore, the present invention can be provided not only as devices such as those described above or as systems composed of these devices, but also as methods executed by these devices, programs for realizing the functions of these devices using a processor, and storage media for storing such programs in a computer-readable manner.

[0042] The present invention can be used in systems such as air traffic control systems, where it is important to detect interference caused by multiple radio stations simultaneously transmitting radio waves at the same frequency.

[0043] 10: Main wave estimation unit, 11: FFT unit, 12: Main wave frequency detection unit, 13: Main wave CW generation unit, 14: AM component extraction unit, 15: Multiplier, 20: Adaptive filter unit, 21: Tap update unit, 22: FIR filter, 23: Adder, 30: Interference detection unit

Claims

1. An interference detection system for detecting interference caused by multiple radio stations simultaneously transmitting radio waves of the same frequency, comprising: a main wave estimation unit that estimates a main wave AM modulated wave based on the frequency of the main wave in a received radio wave that includes at least the main wave and the AM component of the received radio wave; a filter unit that removes the main wave AM modulated wave from the received radio wave; and an interference detection unit that detects the occurrence of interference based on the output of the filter unit.

2. In the interference detection system described in claim 1, the main wave estimation unit comprises an FFT unit that performs FFT processing on the received radio waves, a main wave frequency detection unit that detects the frequency of the main wave based on the result of the FFT processing, a main wave CW generation unit that generates a main wave CW based on the frequency of the main wave, an AM component extraction unit that extracts the AM component of the received radio waves, and a multiplier that multiplies the main wave CW by the AM component, and the multiplication result by the multiplier is output as the main wave AM modulated wave, characterized in that the interference detection system.

3. An interference detection system as claimed in claim 1 or 2, characterized in that the interference detection section detects the occurrence of interference when a peak exists at any frequency in the output of the filter section.

4. A radio device having a function for detecting interference caused by multiple radio stations simultaneously transmitting radio waves of the same frequency, comprising: a main wave estimation unit that estimates a main wave AM modulated wave based on the frequency of the main wave in a received radio wave that includes at least the main wave and the AM component of the received radio wave; a filter unit that removes the main wave AM modulated wave from the received radio wave; and an interference detection unit that detects the occurrence of interference based on the output of the filter unit.

5. An interference detection method for detecting interference caused by multiple radio stations simultaneously transmitting radio waves of the same frequency, comprising the steps of: estimating a main wave AM modulated wave based on the frequency of the main wave in a received radio wave that includes at least the main wave and the AM component of the received radio wave; removing the main wave AM modulated wave from the received radio wave; and detecting the occurrence of interference based on the output of the filter section.

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