JPH04360441A - Interference compensation system - Google Patents

Abstract

PURPOSE:To allow the system to cope with a broad band interference and to improve an interference resistance characteristic by suppressing an interference wave with a band stop filter, compensating the distortion to be produced with a base band equalizer and implementing error correction decoding. CONSTITUTION:An input digital signal fd from a terminal 1 at a sender side is subject to error correction coding by a coder 2, a carrier is modulated by a modulator 4 and the result is sent to a transmission line 5. A receiver side receives the signal and an interference detector 3 detects a frequency and a band width of an interference wave and sends the result to a band stop filter 6 as interference wave information 14. After the interference wave is eliminated based on the information, the result is demodulated by a demodulator 7. Then distortion generated by the band stop filter is equalized and compensated by a base band equalizer 8 and the result is corrected and decoded by an error correction decoder 9 based on the logic corresponding to error correction coding. Thus, the system copes with an interference wave without revision of an error correction coding and a center frequency and the interference wave immunity is improved.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference compensation system suitable for a digital wireless communication system under Co-Channel interference in which a narrowband interference wave falls within the band of a desired wave.

0002

[Background Art] In order to improve the Co-Channel interference resistance, the required band and center frequency of the transmission signal can be changed depending on the interference wave frequency, or only the interference wave component can be extracted using an auxiliary antenna and then synthesized in reverse phase. There is a method in which the interference wave component is suppressed using a low-pass filter or a high-pass filter on the receiving side, a method in which the interference wave component is suppressed using a band-elimination filter, etc.

[0003] Changing the required band and center frequency of the transmission signal has the problem of requiring changes to the parameters of the existing system.

[0004] In the method of extracting only interference wave components using an auxiliary antenna or the like and performing anti-phase synthesis, it is difficult to compensate for interference waves and multiple interference waves arriving from the same direction as the desired wave.

[0005] When suppressing interference wave components using a low-pass filter or high-pass filter on the receiving side, or when suppressing interference wave components using a band-stop filter, interference waves arriving from the same direction as the desired wave or multiple interference can be suppressed. Although it is possible to compensate for the waves, the suppression distorts the desired waves, resulting in deterioration in transmission quality.

[0006] In the case of low-pass filters or high-pass filters, they are effective against relatively wide band interference, but
As the interference wave approaches the carrier frequency of the desired wave, the amount of suppression increases and the deterioration of transmission quality increases.

[0007] In the case of a band-stop filter, it is possible to cope with interference waves located anywhere within the desired wave band, but the deterioration of transmission quality increases as the stop band increases for relatively wide-band interference.

[0008]

[Problems to be Solved by the Invention] The present invention provides a system that does not use an auxiliary antenna or the like without changing the error correction coding rate or center frequency, and provides a band rejection filter at the position where interference waves exist on the receiving side. The aim is to suppress interference waves by using the RFID and compensate for the distortion of the desired wave caused by the suppression using an equalizer, thereby dealing with relatively wideband interference, and further improving anti-interference characteristics by error correction. .

[0009]

[Means for Solving the Problems] In order to achieve this object, the present invention provides a signal obtained by applying error correction coding to a digital input signal to be transmitted on the transmitting side to generate a carrier wave having a frequency fc of a desired wave. The receiving side has means for detecting the frequency and bandwidth of the interference wave that falls within the band of the desired wave, and the carrier frequency fc of the desired wave and the interference wave. The difference Δf from the wave frequency and the interference wave bandwidth BW are obtained, and the obtained fc +Δ
After removing the interference wave with an IF band band rejection filter that performs band rejection with a stop band width BW centered on The signal equalized by the baseband equalizer that compensates for distortion is subjected to error correction decoding using logic corresponding to the error correction encoding.

[0010] Furthermore, a band rejection filter on the receiving side,
The arrangement order of the demodulator and equalizer can be changed as appropriate.

[0011]

[Operation] In the present invention, on the transmitting side, the speed fd (
bit/sec) digital input signal at a coding rate (n
-1)/n (n is a natural number) error correction code, and the resulting signal is serial-parallel converted to provide two speed fb
(fb = n/2(n-1)fd), and this signal is waveform-shaped by a roll-off filter with a roll-off rate α (0<α<1).The signal has a frequency fc.
(Hz) carrier wave is orthogonally modulated, fc − (1 + α)
A quadrature modulation signal having a band from fb /2 to fc + (1+α) fb /2 is transmitted.

[0012] On the receiving side, an interference wave detector provided on the receiving side measures the frequency position Δf and bandwidth BW of the interference wave, and an IF band for band rejection with a stopband width BW centered around the obtained fc + Δf is detected. After removing the interference wave received by a band-elimination filter, the demodulated signal is equalized by a baseband equalizer that compensates for the distortion of the desired wave distorted by the IF band-elimination filter, and the above-mentioned error is removed from the obtained signal. Error correction decoding is performed using logic corresponding to correction encoding.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1A, 1B, and 1C are block diagrams of a communication device embodying the present invention.

In FIG. 1A, on the transmitting side, a digital signal to be transmitted is input to terminal 1. The speed of this digital input signal is fd (bit/sec). This signal at terminal 1 is input to error correction encoder 2 . This error correction encoder 2 has a coding rate (n-1)/n (n is a natural number)
is a convolutional encoder. The output of this error correction encoder 2 is serial-parallel converted, and the speed fb (bit/s
ec) (fb=n/2(n-1)fd) A signal whose waveform is shaped by a roll-off filter 3 with a roll-off rate α (0<α<1) is input to a four-phase phase modulator 4, A carrier wave of frequency fc is orthogonally modulated. This four-phase phase modulator 4
The output is transmitted to the wireless transmission path 5.

On the receiving side, the interference wave detector 13 detects the frequency component and bandwidth of the interference wave added to the desired wave, and detects the interference wave indicative of the frequency difference Δf between the interference wave frequency and the desired wave carrier and the interference wave bandwidth BW. The wave information 14 is obtained and transmitted to the IF band rejection filter 6. Here, the detection of interference waves is
There are two methods: one using the IF band received signal and the other using the baseband demodulated signal; either method may be used in the present invention. In the IF band rejection filter 6, this interference wave information 1
4, frequency characteristics are set such that band rejection of the stop band BW is performed around fc +Δf. The signal on the wireless transmission path 5 has a frequency difference fc obtained from the interference wave information 14.
The baseband band passes through the IF band band rejection filter 6 which eliminates the stop band BW around +Δf, is demodulated by the four-phase phase demodulator 7, and compensates for the desired wave distorted by the IF band band rejection filter 6. input to converter 8. The output of this baseband equalizer 8 is input to an error correction decoder 9. This error correction decoder 9 is a decoder having the same logic as the error correction encoder 2 on the transmitting side, and performs error correction on the equalized output signal of the baseband equalizer 8 and outputs a digital output signal to the terminal 10. Send. The speed of this output signal is equal to the communication speed fd of the input signal.

FIG. 1B is the same as FIG. 1A except for the order of the IF band band rejection filter 6, the IF band equalizer 7a, and the four-phase phase demodulator 8a.

In FIG. 1C, a four-phase phase demodulator 6b,
It is the same as FIG. 1A except for the order of the baseband band rejection filter 7b and the baseband band equalizer 8b.

The feature of the present invention is that the desired wave signal spectrum distorted by the band rejection filter for suppressing interference waves is extremely similar to the signal spectrum under frequency selective fading. The distortion of the desired wave caused by band rejection is compensated for by an equalizer, and the deterioration is further improved by applying error correction.

FIG. 2 is a diagram schematically showing the signal waveform and signal spectrum at each position of the interference compensation method according to the embodiment of FIG. 1A. 2(a) is an output signal waveform diagram (eye pattern) of the error correction encoder 2, and FIG. 2(b) is an input signal waveform diagram (eye pattern) of the baseband equalizer 8.
c) is an output waveform diagram (eye pattern) of the baseband equalizer 8. 2(d) is the output signal spectrum of the four-phase phase modulator 3, and FIG. 2(e) is the narrowband Co-Chan
The received signal spectrum in the presence of nel interference, same (
f) is the output signal spectrum of the IF band-stop filter. FIG. 2(g) shows the frequency characteristics of the IF band rejection filter.

FIG. 3 shows the interference wave detector 13 used in this example.
FIG. 3 is a diagram showing a signal spectrum at each position. Figure 3 (a
) is the received signal spectrum input to the interference wave detector 13, and FIG. 3(b) is the FFT (Fast Fourier
This is a received signal spectrum obtained by Pk is the average power of the k-th FFT series, N
is the number of FFT points. This interference wave detector converts the received signal (including both desired waves and interference waves) arriving from the wireless transmission path into the frequency domain using FFT, which converts the time domain signal into the frequency domain signal. death,
The standard deviation σ of the average power of the converted received signal is multiplied by a constant (A
Set a threshold value for Aσ (multiplied by The interference wave frequency Δf is determined from the frequency, and the BW is determined from the number of points exceeding the threshold value, and the results are output as interference wave information 14.

FIG. 4 is a diagram schematically showing the ability of a band rejection filter to follow fluctuations in interference waves in this interference compensation method.

FIG. 4(a) shows the center frequency of the filter following Δf of the interference wave information 14 obtained from the interference wave detector 13, and FIG. 4(b) shows the interference wave information obtained from the interference wave detector 13. 14 shows how the rejection bandwidth of the filter follows the BW of 14. Furthermore, in this interference compensation system, both Δf and BW can be tracked simultaneously.

FIG. 5 is a block diagram of the baseband equalizer 8 used in this embodiment. This equalizer is a two-dimensional baseband linear adaptive equalizer whose tap data and tap coefficients are complex numbers.
The number of taps is 17, and the adaptive algorithm is LMS (Leas).
tMean Square) method was adopted. The equalizer output is output from the previous stage of the discriminator and is configured to provide soft decision information to the error correction decoder 9.

[0023] In such a digital signal transmission system,
As mentioned above, under narrowband Co-Channel interference, the interference wave is suppressed by a band-elimination filter that performs band-elimination around fc + Δf according to the frequency position Δf of the interference wave, and the resulting distortion of the desired wave is By compensating with an equalizer and further improving the deterioration by error correction, signal transmission with a practical bit error rate can be performed.

Next, the results of testing this will be explained. The test used the system shown in Figure 1. Interference waves and noise are mixed into the transmission line 5 from another interference source 11 and a noise generator 12 to create a state similar to a practical radio line. The position and amount of interfering waves and the amount of noise are variable.

The speed of the digital signal input to terminal 1 is fd = 20 Mbit/sec, the roll-off rate of the waveform shaping roll-off filter is α = 0.5, and the frequency position of the narrow band Co-Channel interference ( When the distance from the carrier frequency) is Δf = 8.8 MHz, fc +Δ
The signal power at f was suppressed by DP=13 dB, and the stopband was set to BW=9.77 MHz. In the interference wave detector, the number of FFT points is N=128, and the threshold setting constant is A=2.8. The equalizer is a two-dimensional baseband linear adaptive equalizer whose tap data and tap coefficients are complex numbers.The number of taps is 17, and the adaptive algorithm is LMS (Least Mean
Square) method was adopted. The equalizer output outputs the front stage of the discriminator and provides soft decision information to the error correction decoder. As the error correction code and decoding method, a convolutional code/3-bit soft-decision Viterbi decoding method with a coding rate of R=1/2 and a constraint length of K=4 was used.

FIG. 6 shows the test results in graph form. Figure 6
is a diagram showing the characteristics of an example of the present invention and a comparative example, with the horizontal axis representing the signal-to-noise power ratio and the vertical axis representing the code error rate. Curve A is the test result of the embodiment of the present invention when the desired wave to interference wave power ratio is 3 dB. Curve B is an actual measurement result of a comparative example when only error correction without band rejection and equalization is applied at a desired wave to interference wave power ratio of 3 dB. Curve T is a theoretical value when there is no interference wave and only error correction without band rejection or equalization is applied.

That is, curve A is the result of measuring the bit error rate from terminal 1 to terminal 10 by transmitting and receiving digital signals according to the above parameters in the embodiment of the present invention and varying the amount of mixed noise. Curve B is an example of a conventional method in which transmission power, interference wave power, and information transmission amount are equal to those of the embodiment of the present invention shown in curve A, and in which band rejection and equalization are not performed using only error correction. Comparing curves A and B, the deterioration from the theoretical value T in terms of signal-to-noise power ratio required to obtain a bit error rate of 10-4 is approximately 1.
2 dB, and about 6.6 dB for curve B, and the method of the present invention reduces the deterioration by about 5.4 dB compared to the conventional method.

[0028] In this way, the system of the present invention uses narrowband Co
- When channel interference exists, it is possible to realize a digital transmission system with a code error rate superior to conventional systems.

As described above, in the embodiment, a roll-off filter is used for waveform shaping, a four-phase phase modulator is used as a modulation method, and an IF band rejection filter has a suppression amount of 13 dB and a rejection band of 9.7.
7MHz, the number of FFT points in the interference wave detector is 128, the constant for setting the threshold is 2.8, the baseband band equalizer is a two-dimensional baseband linear adaptive equalizer, the number of taps is 17, and the adaptive algorithm. In the LMS method, the equalizer output is used as the front stage of the discriminator and soft decision information is output, and the error correction method is convolutional coding with a coding rate of 1/2, constraint length of 4, and 3-bit soft decision Viterbi decoding method. Although the present invention has been described as above, the present invention can be similarly realized using combinations of other waveform shaping filters, modulation methods, band rejection filters, interference wave detectors, equalizers, and error correction methods.

[0030]

[Effects of the Invention] As explained above, according to the present invention,
When narrowband Co-Channel interference exists, the interference wave is suppressed by applying a band rejection filter according to the frequency position of the interference wave, and at the same time, the suppression generates distortion of the desired wave as occurs under frequency-selective fading. Even if I let you,
By compensating for distortion of this desired wave using an equalizer and applying error correction, it is possible to obtain a digital transmission line with a practical code error rate. The present invention applies to Co-Channel
It is highly effective when applied to digital wireless communication systems under interference.

[Brief explanation of the drawing]

FIG. 1A is a block diagram of a communication method according to an embodiment of the present invention.

FIG. 1B is a block diagram of a communication method according to an embodiment of the present invention.

FIG. 1C is a block diagram of a communication method according to an embodiment of the present invention.

FIG. 2 is a diagram showing signal waveforms and signal spectra of various parts in the present invention.

FIG. 3 is a configuration diagram of an interference wave detector in the present invention.

FIG. 4 is a diagram showing the followability of a stopband filter to fluctuations in interference waves.

FIG. 5 is a configuration diagram of a two-dimensional baseband linear adaptive equalizer according to an embodiment of the present invention.

FIG. 6 is a diagram showing test results of the present invention.

[Explanation of symbols]

1 Terminal 2 into which the digital signal to be transmitted is input
Error correction encoder 3 Waveform shaping filter 4 Four-phase phase modulator 5 Wireless transmission line 6 IF band rejection filter 7 Four-phase phase demodulator 8 Baseband band equalizer 9 Error correction decoder 10 Received output signal is transmitted Terminal 11
Interference source 12 Noise source 13 Interference wave detector 14 Interference wave information 7a IF band equalizer 8a Four-phase phase demodulator 6b Four-phase phase demodulator 7b Baseband band band rejection filter 8b
baseband equalizer

Claims (3)

[Claims] Claim 1: In a system for transmitting and receiving digital signals, on the transmitting side, a signal obtained by applying error correction coding to the digital signal to be transmitted is determined at the frequency fc of the desired wave.
modulate the carrier wave and transmit the resulting modulated wave,
The receiving side has means for detecting the frequency and bandwidth of the interference wave that falls within the band of the desired wave, and detects the difference Δf between the carrier frequency fc of the desired wave and the frequency of the interference wave and the interference wave bandwidth BW. After removing the interference wave with an IF band band rejection filter that performs band rejection with a stop band width BW centered around the obtained fc +Δf, the received signal is demodulated and the obtained signal is passed through the IF band band rejection filter. An interference compensation method characterized in that error correction decoding is performed using logic corresponding to the error correction coding on a signal equalized by a baseband equalizer that compensates for the desired wave distorted by the interference correction method. Claim 2: In a system for transmitting and receiving digital signals, on the transmitting side, a signal obtained by applying error correction coding to the digital signal to be transmitted is determined at the frequency fc of the desired wave.
modulate the carrier wave and transmit the resulting modulated wave,
The receiving side has means for detecting the frequency and bandwidth of the interference wave that falls within the band of the desired wave, and detects the difference Δf between the carrier frequency fc of the desired wave and the frequency of the interference wave and the interference wave bandwidth BW. After removing the interference wave with an IF band band rejection filter that performs band rejection with a stop band width BW centered around the obtained fc +Δf, the obtained signal is distorted by the IF band band rejection filter to form the desired wave. An interference compensation method characterized by demodulating a signal equalized by an IF band equalizer that compensates for the interference, and performing error correction decoding on the obtained signal using logic corresponding to the error correction encoding. 3. In a method for transmitting and receiving digital signals, on the transmitting side, a signal obtained by applying error correction coding to the digital signal to be transmitted is used to determine the frequency fc of the desired wave.
modulate the carrier wave and transmit the resulting modulated wave,
The receiving side has means for detecting the frequency and bandwidth of the interference wave that falls within the band of the desired wave, and detects the difference Δf between the carrier frequency fc of the desired wave and the frequency of the interference wave and the interference wave bandwidth BW. After demodulating the received signal and removing the interference wave with a baseband band-stop filter that performs band-stopping on the obtained signal with a stopband width BW centered around Δf, the obtained signal is converted into the baseband signal. An interference compensation method characterized by performing error correction decoding using logic corresponding to the error correction encoding on a signal equalized by a baseband equalizer that compensates for the desired wave distorted by the band rejection filter. .