CN110113278A - A kind of Modulation Mode Recognition method based on all-digital receiver - Google Patents

Abstract

The Modulation Mode Recognition method based on all-digital receiver that the invention discloses a kind of.The present invention estimates carrier frequency and three dB bandwidth the following steps are included: step 1, do FFT transform to the measurement and control signal that accepting device receives；Step 2 completes down coversion, filtering and resampling using the carrier frequency that estimation obtains；Step 3 passes through phase-locked loop tracking frequency offset to the low frequency signal after Orthogonal Decomposition, distinguishes fsk signal and AM_FM, MTONE_FM signal using frequency-tracking curve；Step 4 identifies AM_FM, MTONE_FM signal by doing FFT transform to the frequency-tracking curve for including modulation intelligence.The present invention has operand simple, easy to accomplish on hardware.The present invention can accurately be judged in all-digital receiver the narrowband FM signal of fsk signal and complex modulated that recognition correct rate is better than based on traditional signal transient frequency spectrum discerning method.

Description

A Modulation Method Identification Method Based on All-Digital Receiver

technical field

The invention is mainly aimed at signal detection and frequency estimation, and mainly relates to a modulation mode identification method based on an all-digital receiver.

Background technique

In the actual electronic reconnaissance environment, the transmitted signal is often accompanied by a mixture of wideband digital modulation signal and narrowband signal, which requires research on the modulation method identification of the received communication signal. And in the case that the various parameters of the signal (signal bandwidth, modulation mode) are unknown, it makes the signal reconnaissance and identification more difficult.

The currently used modulation method identification methods mainly include the identification method research based on signal cyclic spectrum, the identification method research based on signal high-order cumulant, the identification method based on signal constellation diagram, and the identification method based on instantaneous frequency spectrum. Although the identification method based on the instantaneous frequency spectrum is simple to operate, it cannot realize the modulation mode identification of the composite modulated narrow-band FM signal, and is easily affected by factors such as noise, and often requires a higher signal-to-noise ratio for identification. The modulation identification method based on the signal constellation diagram is a kind of PSK/QAM signal modulation identification. In order to realize the modulation identification, the accurate estimation of the carrier frequency of the received signal and the complete synchronization of the signal must be completed. Research on communication signal modulation recognition based on signal cyclic spectrum analysis, by extracting the variance feature of the signal cyclic spectrum envelope amplitude to classify and identify, and using the method of high-order statistical features, the modulation mode identification is realized through the cumulant parameters of each order. These two methods have a large amount of calculation, the calculation process of parameter extraction is difficult, and the real-time online analysis of signals is poor, so it is difficult to apply in actual engineering projects.

Based on the above analysis, it can be seen that the current modulation recognition methods are mainly aimed at PSK/QAM signals, and for the recognition of narrow-bandwidth analog-digital mixed signals, the recognition performance of the current recognition methods drops rapidly. Moreover, the current recognition algorithm has a large amount of calculation and is difficult to implement on hardware.

Contents of the invention

The purpose of the present invention is to realize the signal detection of FSK, AM_FM and MTONE_FM aiming at the deficiencies of the prior art. The invention proposes a modulation mode identification method based on an all-digital receiver. The algorithm of the invention has better identification performance, and can distinguish composite modulated narrow-band FM signals, and the complexity of the algorithm is not high, and can be realized on hardware.

The technical solution adopted by the present invention to solve its technical problems comprises the following steps:

Step 1. Perform FFT transformation on the measurement and control signal received by the receiving device, and estimate the carrier frequency and 3dB bandwidth;

Step 2, using the estimated carrier frequency to complete down-conversion, filtering and re-sampling;

Step 3, track the frequency offset through the phase-locked loop for the low-frequency signal after the orthogonal decomposition, and use the frequency tracking curve to distinguish the FSK signal from the AM_FM and MTONE_FM signals;

Step 4. Identify the AM_FM and MTONE_FM signals by performing FFT transformation on the frequency tracking curve containing the modulation information.

Step 1 is specifically implemented as follows:

1-1. Perform 1024-point FFT transformation on the received signal, find out the abscissa value corresponding to the maximum amplitude P, and calculate the signal frequency f ₀ ;

1-2. Find the noise floor SNR _est of the frequency corresponding to the maximum point, and calculate the threshold value of the 3dB bandwidth. The frequency range corresponding to the part higher than the threshold value is the bandwidth threshold of the signal:

The specific implementation of step 2 is as follows:

2-1. According to the calculated signal frequency f ₀ , the signal is down-converted, low-pass filtered, and CIC extracted to reduce the data rate;

This process is expressed as:

According to the result of the initial estimation of the frequency of the received signal, the signal is mixed, and the sum frequency component is filtered out by low-pass filtering, and the in-phase signal and quadrature signal of the FM composite modulation signal are obtained, which can be expressed as:

2-2. According to the 3dB bandwidth, distinguish between narrowband (AM_FM, MTONE_FM) signals and wideband FSK signals. If it is a narrowband signal, it needs to decimate the sampling rate by 500 times of CIC to reduce the sampling rate, and if it is a broadband signal, it needs to decimate the sampling rate by 2 times of CIC.

Step 3 is specifically implemented as follows:

3-1. The quadrature decomposed signal is multiplied by the initial frequency of the local voltage-controlled oscillator, and low-pass filtered to obtain the in-phase component and the quadrature component. At this time, the signal I and Q channels can be expressed as:

In the formula, T _S is the sampling interval of the signal, K _f is the modulation frequency deviation, Δf is the initial frequency difference between the down-converted signal and the VCO, and m(t) is the modulation signal;

f _c represents the carrier frequency of the signal, and n represents the number of sampling points;

Take these two quadrature signals as the input signals of the phase detector. The phase discrimination equation can be expressed as:

Because arctangent and division operations are difficult to implement on hardware, the following phase identification equation can be used instead:

3-2. The phase error information is obtained by the phase detection equation. The phase detection error result is filtered by the second-order loop to filter out the sum frequency component, and the difference frequency component is used as the input voltage of the voltage-controlled oscillator. After the loop is stable, the error approaches zero. Implement frequency tracking.

Step 4 is specifically implemented as follows:

4-1. The carrier synchronization loop based on the all-digital phase-locked loop tracks the frequency information, and the AM and MTONE signals are modulated on the frequency, so the frequency tracking curve reflects the modulation information, and the frequency information contained in the tracking of the phase-locked loop The signal is transformed by FFT, and the modulation mode identification can be realized according to the different spectrum characteristics of the modulation information. The frequency tracked by the FSK signal will jump, and the narrowband (AM_FM, MTONE_FM) signal and the wideband FSK signal can be distinguished by tracking the frequency combined with the signal's spectrogram.

4-2. The frequency information of the AM_FM signal is tracked by the carrier loop, and the AM signal is modulated on the frequency. By performing FFT operation on the frequency information, an obvious spectral peak can be obtained. By performing FFT operation on the frequency information of the MTONE_FM signal, because the MTONE signal is modulated on the frequency, it can be seen from the simulation results that there are multiple spectral peaks. Obviously, the AM_FM signal and the MTONE_FM signal can be distinguished by searching for the number of spectral peaks. The specific method is to calculate the average value of the corresponding signal spectrum, and find out the peak value which is much larger than the average value, so as to determine the number of frequency impulse components.

The beneficial effects of the present invention are as follows:

1. The present invention is aimed at the situation where the reconnaissance signal parameters are unknown, such as carrier frequency, modulation mode, etc. are all unknown. Modulation identification can be realized.

2. The present invention contains modulation information through the frequency information tracked by the phase-locked loop, and realizes the modulation mode identification of the composite modulation signal (AM_FM, MTONE_FM), and the identification rate is higher than the commonly used identification method based on the instantaneous frequency spectrum of the signal.

3. The present invention has a simple calculation amount, is easy to realize by hardware, can be well used in actual engineering projects, and has high practical significance.

Description of drawings

Fig. 1 is a block diagram of a modulation identification method based on a carrier synchronization loop to extract user information in the present invention;

Figure 2 3dB bandwidth estimation flow chart;

Figure 3 FSK signal carrier synchronization frequency offset tracking curve;

Figure 4 Frequency information of AM_FM signal carrier synchronous tracking;

Fig. 5 Frequency information of MTONE_FM signal carrier synchronous tracking;

Fig. 6 FFT spectrum diagram of frequency signal containing modulation information tracked by AM_FM signal;

Fig. 7 FFT spectrum diagram of frequency signal containing modulation information tracked by MTONE_FM signal;

Detailed ways

Specific embodiments of the present invention will be further described below in conjunction with the accompanying drawings.

As shown in Figure 1, a modulation mode recognition method based on an all-digital receiver, the specific implementation process is as follows:

Step 1. Perform FFT transformation on the measurement and control signal received by the receiving device, and estimate the carrier frequency and 3dB bandwidth. The specific implementation flow chart is shown in Figure 2;

Step 2, using the estimated frequency to complete down-conversion, filtering and resampling, etc.;

Step 3, track the frequency offset through the phase-locked loop for the low-frequency signal after the orthogonal decomposition, and use the frequency tracking curve to distinguish the FSK signal from the AM_FM and MTONE_FM signals as shown in Figure 2, Figure 3, and Figure 4 respectively;

Step 4. Identify the AM_FM and MTONE_FM signals by performing FFT transformation on the frequency tracking curve containing modulation information, as shown in Figures 5 and 6.

Step 1 is specifically implemented as follows:

1-1. Perform 1024-point FFT transformation on the received signal, find out the abscissa value corresponding to the maximum amplitude value P, and calculate the carrier frequency f _c ;

1-2. Find the noise floor SNR _est of the corresponding frequency at the maximum point, and the flow chart of 3dB bandwidth estimation is shown in Figure 2. From this, the threshold value of the 3dB bandwidth can be calculated, and the frequency range corresponding to the part higher than the threshold value is the bandwidth of the signal;

The specific implementation of step 2 is as follows:

2-1. According to the calculated carrier frequency, the signal is orthogonally decomposed into two channels of I and Q, and the mixed signal is filtered out by low-pass filtering, and then extracted by CIC to reduce the data rate;

This process can be expressed as:

According to the result of the initial estimation of the frequency of the received signal, the signal is mixed and the sum frequency component is filtered out by a low-pass filter, and the in-phase and quadrature signals of the FM composite modulation signal can be expressed as:

2-2. According to the 3dB bandwidth, distinguish between narrowband (AM_FM, MTONE_FM) signals and wideband FSK signals. If it is a narrowband signal, it needs to decimate the sampling rate by 500 times of CIC to reduce the sampling rate, and if it is a broadband signal, it needs to decimate the sampling rate by 2 times of CIC.

The specific implementation of step 3 is as follows:

3-1. The quadrature-decomposed signal is multiplied by the initial frequency of the local voltage-controlled oscillator to obtain an in-phase component and a quadrature component. At this time, the signal I channel and Q channel can be expressed as:

In the formula, T _S is the sampling interval of the signal, K _f is the modulation frequency deviation, Δf is the initial frequency difference between the down-converted signal and the VCO, and m(t) is the modulation signal.

Take these two quadrature signals as the input signals of the phase detector. The phase discrimination equation can be expressed as:

Because arctangent and division operations are difficult to implement on hardware, the following phase identification equation can be used instead:

3-2. The phase error information is obtained by the phase detection equation. The phase detection error result is filtered by the second-order loop to filter out the sum frequency component, and the difference frequency component is used as the input voltage of the voltage-controlled oscillator. After the loop is stable, the error approaches zero. Implement frequency tracking.

Step 4 is specifically implemented as follows:

4-1. The carrier synchronization loop based on the all-digital phase-locked loop tracks frequency information, as shown in Figure 3, Figure 4, and Figure 5. AM and MTONE signals are modulated on frequency, so the frequency tracking curve reflects the modulation information. By performing FFT transformation on the signal containing frequency information tracked by the phase-locked loop, the modulation mode identification can be realized according to the different spectrum characteristics of the modulation information. The frequency tracked by the FSK signal will jump, and the narrowband (AM_FM, MTONE_FM) signal and the wideband FSK signal can be distinguished by tracking the frequency combined with the signal's spectrogram.

4-2. The frequency information of the AM_FM signal is tracked by the carrier loop, and the AM signal is modulated on the frequency. By performing FFT operation on the frequency information, an obvious spectral peak can be obtained, as shown in Figure 6. By performing FFT operation on the frequency information of the MTONE_FM signal, because the MTONE signal is modulated on the frequency, it can be seen from the simulation results that there are multiple spectral peaks, as shown in Figure 7. Obviously, the AM_FM signal and the MTONE_FM signal can be distinguished by searching for the number of spectral peaks. The specific method is to calculate the average value of the corresponding signal spectrum, and find out the peak value which is much larger than the average value, so as to determine the number of frequency impulse components.

Example:

The signals are modulated signals sent by China Electronics Technology Group Corporation using Agilent E4438C, and AM_FM, MTONE_FM, and FSK signals are collected by an AD sampling board with a sampling frequency of 200M. The bandwidth of the FSK signal is 6.4MHz, and the bandwidth of the AM_FM and MTONE_FM signals is 200kHz.

Firstly, the input signal is transformed by FFT to obtain the spectrum diagram of the signal, the carrier frequency of the signal is estimated and the 3dB bandwidth is found, and the narrowband and broadband signals are distinguished. Next, the estimated carrier frequency is used as the down-conversion frequency to complete the down-conversion, CIC extraction and low-pass filtering. If it is a narrowband signal, the sampling frequency needs to be decimated by CIC500 times to reduce the sampling rate and sent to the carrier synchronization loop. If the broadband signal needs to be extracted twice by CIC, it is sent to the carrier synchronization loop to track the frequency information. In the identification of the modulation mode, the FSK signal and the AM_FM and MTONE_FM signals can be distinguished according to whether the bandwidth and the tracked frequency information jump. Because the tracked frequency curve reflects the information of the modulated signal, the narrowband FM signal of complex modulation can be distinguished by performing FFT transformation on the frequency information containing the modulated information. According to the test results, the test results of the method of the present invention correspond to 95.5%, 98.5% and 100 respectively when the signal-to-noise ratio is 2dB, 4dB and 6dB. However, it is difficult to distinguish these two signals by the commonly used instantaneous spectrum modulation identification method at present.

The result of the embodiment shows that the identification algorithm of the narrow-bandwidth signal modulation mode based on the all-digital receiver of this patent has the advantages of simple computation, easy hardware implementation, and fast simulation operation speed. Compared with the instantaneous spectrum feature recognition method, the patented algorithm can recognize composite modulated narrow-band FM signals, and has relatively high practical significance.

Finally, it should be noted that the purpose of the disclosed embodiments is to help further understand the present invention, but those skilled in the art can understand that various replacements and modifications can be made without departing from the spirit and scope of the present invention and the appended claims. It is possible. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the protection scope of the present invention is subject to the scope defined in the claims.

Claims (1)

1. A modulation method identification method based on an all-digital receiver, characterized in that it may further comprise the steps: Step 1. Perform FFT transformation on the measurement and control signal received by the receiving device, and estimate the carrier frequency and 3dB bandwidth; Step 2, using the estimated carrier frequency to complete down-conversion, filtering and re-sampling; Step 3, track the frequency offset through the phase-locked loop for the low-frequency signal after the orthogonal decomposition, and use the frequency tracking curve to distinguish the FSK signal from the AM_FM and MTONE_FM signals; Step 4, identify the AM_FM and MTONE_FM signals by performing FFT transformation on the frequency tracking curve containing the modulation information; Step 1 is specifically implemented as follows: 1-1. Perform 1024-point FFT transformation on the received signal, find out the abscissa value corresponding to the maximum amplitude P, and estimate the signal frequency f ₀ ; 1-2. Find the noise floor SNR _est of the frequency corresponding to the maximum point, and calculate the threshold value of the 3dB bandwidth. The frequency range corresponding to the part higher than the threshold value is the bandwidth threshold of the signal: Step 2 is specifically implemented as follows: 2-1. According to the calculated signal frequency f ₀ , the signal is down-converted, low-pass filtered, and CIC extracted to reduce the data rate; This process is expressed as: According to the result of the initial estimation of the frequency of the received signal, the signal is mixed, and the sum frequency component is filtered out by low-pass filtering, and the in-phase signal and quadrature signal of the FM composite modulation signal are obtained, which can be expressed as: 2-2. According to the 3dB bandwidth, distinguish the narrowband (AM_FM, MTONE_FM) signal and the wideband FSK signal; if the narrowband signal needs to decimate the sampling rate by 500 times of CIC to reduce the sampling rate, if it is a broadband signal, it needs to decimate the sampling rate by 2 times of CIC ; Step 3 is specifically implemented as follows: 3-1. The signal after quadrature decomposition is multiplied by the initial frequency of the local voltage-controlled oscillator, and low-pass filtered to obtain the in-phase component and quadrature component; at this time, the signal I and Q can be expressed as: In the formula, T _S is the sampling interval of the signal, K _f is the modulation frequency deviation, Δf is the initial frequency difference between the down-converted signal and the VCO, and m(t) is the modulation signal; f _c represents the carrier frequency of the signal, and n represents the number of sampling points; The two orthogonal signals are used as the input signal of the phase detector; the phase detection equation can be expressed as: Because arctangent and division operations are difficult to implement on hardware, the following phase identification equation can be used instead: 3-2. The phase error information is obtained by the phase detection equation. The phase detection error result is filtered by the second-order loop to filter out the sum frequency component, and the difference frequency component is used as the input voltage of the voltage-controlled oscillator. After the loop is stable, the error approaches zero. Realize frequency tracking; Step 4 is specifically implemented as follows: 4-1. The carrier synchronization loop based on the all-digital phase-locked loop tracks the frequency information, and the AM and MTONE signals are modulated on the frequency, so the frequency tracking curve reflects the modulation information, and the frequency information contained in the tracking of the phase-locked loop According to the different characteristics of the modulation information spectrum, the modulation mode identification can be realized; while the frequency tracked by the FSK signal will jump, and the narrowband (AM_FM, MTONE_FM) signal can be distinguished by tracking the frequency combined with the signal spectrum diagram. Broadband FSK signal; 4-2. The frequency information of the AM_FM signal is tracked by the carrier loop, and the AM signal is modulated on the frequency. By performing FFT operation on the frequency information, an obvious spectral peak can be obtained; and by performing FFT operation on the frequency information of the MTONE_FM signal , because the MTONE signal is modulated on the frequency, it can be seen from the simulation results that there are multiple spectral peaks; obviously, the AM_FM signal and the MTONE_FM signal can be distinguished by searching the number of spectral peaks; the specific method is to calculate the average of the corresponding signal spectrum value, find the peak value that is much larger than the average value, so as to determine the number of frequency impulse components.