A very low frequency modulation feature extraction method based on deskewing processing

Abstract

The application discloses a very low frequency modulation feature extraction method based on a de-skew processing. The method firstly performs LOFAR spectrum on a scattering signal received by a single array element to obtain a frequency variation rate and a modulation frequency; selects a signal time length T, a time delay tau and a signal initial time t0, intercepts a target signal corresponding to the time length and a reference signal corresponding to the time delay; multiplies the target signal and the reference signal to obtain a demodulation signal, and then performs Fourier transform on the demodulation signal to obtain frequency domain information of the demodulation signal, draw a demodulation spectrum and preliminarily read the modulation frequency; the demodulation spectrum of N array elements is obtained again according to the above method; the demodulation spectrums of the N array elements are superimposed to obtain a total demodulation spectrum, and a very low frequency modulation frequency is read from the spectrum; a corresponding relationship between a modulation depth and a demodulation depth is calculated; and a very low frequency modulation feature is obtained. The application can effectively remove the influence of Doppler frequency shift on the modulation feature extraction; the corresponding relationship between the demodulation depth and the modulation depth is obtained, so that the water surface / water target is distinguished by using the modulation depth subsequently.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of underwater acoustic signal processing, and particularly relates to a very low frequency modulation feature extraction method based on dechirp processing. BACKGROUND

[0002] When an underwater moving body is sailing, it will be affected by the periodic fluctuation force generated by the sea current, thereby generating a periodic motion with extremely low frequency. This periodic motion will excite the surrounding fluid medium to generate water gravity waves with the same frequency. The water gravity waves will be coupled with the low-frequency sound emitted by the active sound source, generating forward scattering sound signals carrying modulation features (1 Semenov AG. Ultralow frequency fields of moving bodies [M]. 2017: 1-240.). In 2021, Wang Huan et al. verified the existence of very low frequency water gravity waves through theory and experiment (2 Wang H, Fang E, Wu M, et al. Ultra Low Frequency Wave Produced by Underwater Oscillating Sphere and Its Measurement [J]. Journal of Marine Science and Engineering, 2021, 9(12): 1317.), but did not further study the modulation effect. In 2023, Jing Chenxuan (3 Jing Chenxuan, Shi Shengguo, Yang Desen, et al. Study on the mechanism and characteristics of underwater low-frequency oscillating vortex field acoustic scattering modulation [J]. Acta Physica Sinica, 2023, 72(01): 221-235.) et al. studied the mechanism and spatiotemporal frequency characteristics of the acoustic scattering modulation field of underwater low-frequency oscillating vortex field. So far, the research on underwater very low frequency modulation acoustic scattering has focused more on the generation mechanism, and there is no related research on the extraction of very low frequency modulation features.

[0003] When a bistatic sonar is used to detect an underwater moving body, the relative motion between the target and the receiving array will inevitably cause Doppler shift. The Doppler shift and the very low frequency modulation may appear in the scattering signal at the same time. The Doppler shift and the modulation frequency are extremely close in value, and direct envelope spectrum extraction cannot achieve the extraction of modulation features, so it is necessary to remove the Doppler shift first. The dechirp technology is mainly used in the field of radar signal processing, and is used to demodulate linear frequency modulation signals to obtain the distance information of the target. Since the speed of underwater target motion is not large, the Doppler shift generated during the target's crossing of the transmitting-receiving line is similar in form to the linear frequency modulation signal.

[0004] Based on this special scenario and the previous theoretical and methodological research, the application uses the principle of dechirp processing to remove the influence of the Doppler shift in the scattering signal, and then realizes the extraction of the very low frequency modulation features. SUMMARY

[0005] The application aims to provide a very low frequency modulation feature extraction method based on desquamation processing, which extracts the modulation features carried in the target forward scattering signal with Doppler shift, including modulation frequency and modulation depth.

[0006] The application achieves the goal by the following technical solutions:

[0007] A very low frequency modulation feature extraction method based on desquamation processing, the specific steps are as follows:

[0008] Step 1: LOFAR spectrum is made for the scattering signal received by a single element, and the frequency change rate and modulation frequency are estimated;

[0009] Step 2: According to step 1, the signal duration T, time delay τ and signal initial time t0 are selected, and the target signal with corresponding duration and the reference signal with corresponding time delay are intercepted;

[0010] Step 3: According to step 2, the target signal and the reference signal are conjugated to obtain the demodulation signal;

[0011] Step 4: According to step 3, the Fourier transform is made on the demodulation signal to obtain the frequency domain information of the demodulation signal, and the demodulation spectrum is drawn to preliminarily read the modulation frequency;

[0012] Step 5: Steps 2 to 4 are repeated for the received signals of N elements to obtain the demodulation spectrum of N elements;

[0013] Step 6: The demodulation spectrums of N elements obtained in step 5 are superimposed to obtain a total demodulation spectrum, and the very low frequency modulation frequency is read from the spectrum;

[0014] Step 7: The corresponding relationship between the modulation depth and the demodulation depth is calculated;

[0015] Step 8: The very low frequency modulation features are comprehensively obtained according to the modulation frequency obtained in step 6 and the demodulation depth obtained in step 7.

[0016] Further, the modulation depth of the received signal of the single element in step 1 is m, 0

[0017] Further, the signal from t0 to t0+T is intercepted as the target signal, and the signal from t0+τ to t0+τ+T is intercepted as the reference signal in step 2.

[0018] Further, the received scattering signal is simplified as a linear frequency modulation signal carrying modulation, that is:

[0019] r(t) = exp[j - 2π(f l + 0.5kt)t]

[0020] + m - exp[j - 2π(f l - f0+ 0.5kt)t]

[0021] + m - exp[j - 2π(f l + f0+ 0.5kt)t]

[0022] wherein the frequency modulation range of the linear frequency modulation pulse is f l - f h , the pulse width is T, the linear frequency modulation slope is k = (f h - f l ) / T, f0 is the modulation frequency, m is the modulation depth, and 0 < m < 1; the time delay difference between the reference signal and the target signal is τ, and the form of the reference signal is:

[0023] r ref (t) = exp[j - 2πf l (t - τ) + jπk(t - τ) 2 ]

[0024] + m - exp[j - 2π(f l - f0)(t - τ) + jπk(t - τ) 2 ]

[0025] + m - exp[j - 2π(f l + f0)(t - τ) + jπk(t - τ) 2 ]

[0026] The formula of the linear frequency modulation signal carrying modulation is multiplied by the conjugate of the formula of the reference signal to obtain:

[0027] y(t) = r(t) - r ref * (t)

[0028] = exp[j - 2πkτt] exp[j - 2π(f l τ - 0.5kτ 2 )] - [1 + m 2 - exp(-j - 2πf0τ) + m 2 - exp(j - 2πf0τ)]

[0029] + m - exp[j - 2π(kτ - f0)t] exp[j - 2π(f l τ - 0.5kτ 2 )] - [1 + exp(-j - 2πf0τ)]

[0030] + m·exp[j·2π(kτ+f0)t]exp[j·2π(f l τ-0.5kτ 2 )]·[1+exp(j·2πf0τ)]

[0031] + m 2 ·exp[j·2π(kτ-2f0)t]exp[j·2π(f l τ+f0τ-0.5kτ 2 )]

[0032] + m 2 ·exp[j·2π(kτ+2f0)t]exp[j·2π(f l τ-f0τ-0.5kτ 2 )]

[0033] The linear frequency modulation signal carrying the modulation is demodulated to obtain a single frequency signal carrying two modulation frequencies, and since m is small, m 2 is smaller, and the kτ-2f0 and kτ+2f0 components are ignored; the obtained carrier is a single frequency amplitude modulation signal, which is the demodulation signal.

[0034] Further, compared with the single-element demodulation spectrum, the total demodulation spectrum has obvious modulation components and is easy to read the modulation frequency when the signal-to-noise ratio is low.

[0035] Further, the amplitudes of the frequency components after demodulation in step 7 have a certain quantitative relationship, and when the influence of the phase is ignored, the amplitude of the kτ component corresponds to the size of (1+2m 2 ), the amplitudes of the kτ-f0 component and the kτ+f0 component correspond to the size of 2m, and the amplitudes of the kτ-2f0 component and the kτ+2f0 component correspond to the size of m 2 ; the average difference between the modulation component and the peak value of the array modulation spectrum is defined as the demodulation depth, that is:

[0036]

[0037] where Δ is the difference caused by the phase.

[0038] The beneficial effects of the present application are:

[0039] 1. The present application can effectively remove the influence of Doppler frequency shift on modulation feature extraction;

[0040] 2. The present application can accurately obtain the modulation frequency;

[0041] 3. The application can obtain the correspondence between demodulation depth and modulation depth, facilitating subsequent water surface / underwater target discrimination using modulation depth.

[0042] 4. The calculation method of the application is simple, and the calculation result is accurate. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 is a flow chart of the modulation feature extraction method.

[0044] Figure 2 The LOFAR spectrum of the single-element signal is given.

[0045] Figure 3 The demodulation spectrum of the single-element signal is given.

[0046] Figure 4 The total demodulation spectrum of all element signals is given.

[0047] Figure 5 The total demodulation spectrum with a modulation depth of 0.05 is given.

[0048] Figure 6 The total demodulation spectrum with a modulation depth of 0.5 is given.

[0049] Figure 7 The demodulation under different modulation depths is compared. DETAILED DESCRIPTION

[0050] The application will be further described below in combination with the drawings.

[0051] The very low frequency modulation feature extraction method based on desloping processing of the application has the following specific steps:

[0052] Step 1: Perform LOFAR spectrum on the single-element received scattering signal, and estimate the frequency change rate and modulation frequency;

[0053] The modulation depth of the single-element received signal is m (0 < m < 1), and the modulation frequency is f0. Due to the movement of the target, it will bring Doppler shift. By performing LOFAR spectrum, the approximate range of the frequency change rate and the modulation frequency can be read from the spectrum. In the application, the modulation depth is 0.2, the modulation frequency is 0.25 Hz, and the single-element received scattering signal LOFAR spectrum obtained by simulation is as shown in Figure 2 Due to the existence of Doppler shift, the received signal form is close to a linear frequency modulation signal, the frequency change rate is about 0.006 Hz / s, and the modulation frequency is less than 0.5 Hz.

[0054] Step 2: Select the signal length T, the time delay τ and the initial time t0 of the signal, and intercept the corresponding length of the target signal and the corresponding time delay of the reference signal;

[0055] When the signal length is 100s and the time delay is 0.5s, the signal segment with the modulation phenomenon is cut according to the LOFAR spectrum, the signal segment of 200s-300s is taken as the target signal, and the signal segment of 200.5s-300.5s is taken as the reference signal.

[0056] Step 3: The target signal is multiplied by the conjugate of the reference signal to obtain a demodulated signal.

[0057] Since the signal with Doppler frequency shift is very close to the linear frequency modulation signal, the received scattering signal can be simplified as a linear frequency modulation signal carrying modulation, that is,

[0058]

[0059] wherein it is assumed that the frequency modulation range of the linear frequency modulation pulse is f l -f h , the pulse width is T, the linear frequency modulation slope is k = (f h -f l ) / T, f0 is the modulation frequency, m is the modulation depth, and 0 < m < 1. Assuming that the time delay difference between the reference signal and the target signal is τ, the reference signal is in the form of:

[0060]

[0061] The equation (1) is multiplied by the conjugate of the equation (2) to obtain:

[0062]

[0063] The linear frequency modulation signal carrying modulation can obtain a single-frequency signal carrying two modulation frequencies after demodulation, but since m is small, m 2 is smaller, and the kτ-2f0 and kτ+2f0 components can be almost ignored.

[0064] The conjugate of the target signal and the reference signal is multiplied in the application to obtain an amplitude modulation signal with a single carrier as the demodulated signal.

[0065] Step 4: Fourier transform is performed on the demodulated signal to obtain the frequency domain information of the demodulated signal, and the demodulated spectrum is drawn to preliminarily read the modulation frequency;

[0066] The single-element demodulated spectrum obtained in the application is shown in Figure 3 , and the modulation frequency of about 0.25Hz can be clearly seen.

[0067] Step 5: Steps 2 to 4 are repeated to obtain the demodulated spectrum of all elements.

[0068] Repeat steps 2 to 4 for the received signals of N elements to obtain N demodulation spectra. In the simulation of the present application, 32 elements are used, and thus 32 single-element demodulation spectra are obtained.

[0069] Step 6: Superimpose the 32-element demodulation spectra obtained in step 5 to obtain a total demodulation spectrum, from which the very low frequency modulation frequency can be read.

[0070] The total demodulation spectrum of the present application is shown in Figure 4 Compared with the single-element demodulation spectrum, the modulation component is more obvious, and the modulation frequency is easier to read. When the signal-to-noise ratio is low, the advantage of the total demodulation spectrum will be more obvious.

[0071] Step 7: Calculate the correspondence between the modulation depth and the demodulation depth.

[0072] There is a certain quantitative relationship between the amplitudes of the frequency components after demodulation. When the influence of the phase is ignored, the amplitude of the kτ component corresponds to the size of (1+2m 2 ), the amplitudes of the kτ-f0 component and the kτ+f0 component correspond to the size of 2m, and the amplitudes of the kτ-2f0 component and the kτ+2f0 component correspond to the size of m 2 In the simulation of the present application, the received signals with modulation depths of 0.05 and 0.5 are simulated, and the array demodulation spectra are shown in Figure 5 and Figure 6 The comparison of the demodulation conditions of different modulation depths is shown in Figure 7 The average difference between the modulation component and the peak value of the array modulation spectrum is defined as the demodulation depth, i.e.:

[0073]

[0074] where Δ is the difference caused by the phase, which is determined by the actual situation.

[0075] Step 8: According to the modulation frequency obtained in step 6 and the demodulation depth obtained in step 7, the very low frequency modulation characteristics are comprehensively obtained.

[0076] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. For those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A very low frequency modulation feature extraction method based on de-skewing, characterized by: The specific steps are as follows: Step 1: Calculate the LOFAR spectrum of the scattered signal received by a single array element to estimate the frequency change rate and modulation frequency; Step 2: According to step 1, select the signal duration T, delay τ and the initial time of the signal t0, and intercept the target signal of the corresponding duration and the reference signal of the corresponding delay; Step 3: According to step 2, the target signal is conjugate-multiplied by the reference signal to obtain the demodulated signal; Step 4: According to step 3, perform Fourier transform on the demodulated signal to obtain the frequency domain information of the demodulated signal, draw the demodulation spectrum, and preliminarily read the modulation frequency from it; Step 5: Repeat steps 2 to 4 for the received signals of N array elements to obtain the demodulated spectra of the N array elements. Step 6: Superimpose the N array element demodulation spectra obtained in step 5 to obtain a total demodulation spectrum, and read the VLF modulation frequency from the spectrum; Step 7: Calculate the corresponding relationship between modulation depth and demodulation depth; Step 8: Based on the modulation frequency obtained in step 6 and the demodulation depth obtained in step 7, the very low frequency modulation characteristics are obtained.

2. The method for extracting very low frequency modulation features based on de-skewing according to claim 1, wherein: In step 1, the modulation depth of the received signal of a single array element is m, 0<m<1, and the modulation frequency is f0. Due to the movement of the target, Doppler frequency shift will be caused; a LOFAR spectrum is made, and the frequency change rate and the range of the modulation frequency are read from the spectrum.

3. The method for extracting very low frequency modulation features based on de-skewing according to claim 1, wherein: In step 2, the signal from t0 to t0+T is intercepted as the target signal, and the signal from t0+τ to t0+τ+T is intercepted as the reference signal.

4. The method for extracting VLF modulation features based on de-skewing according to claim 1, wherein: The step 3 simplifies the received scattered signal into a linear frequency modulation signal carrying modulation, namely: r(t)=exp[j·2π(f l +0.5kt)t] +m·exp[j·2π(f l -f0+0.5kt)t] +m·exp[j·2π(f l +f0+0.5kt)t] Among them, the frequency modulation range of the linear frequency modulation pulse is f l -f h , the pulse width is T, then its linear frequency modulation slope is k=(f h -f l ) / T, f0 is the modulation frequency, m is the modulation depth, and 0<m<1; the time delay difference between the reference signal and the target signal is τ, then the form of the reference signal is: r ref (t)=exp[j·2πf l (t-τ)+jπk(t-τ) 2 ] +m·exp[j·2π(f l -f0)(t-τ)+jπk(t-τ) 2 ] +m·exp[j·2π(f l +f0)(t-τ)+jπk(t-τ) 2 ] Multiplying the formula for the linear FM signal carrying the modulation conjugate with the formula for the reference signal yields: y(t)=r(t)·r ref * (t) =exp[j·2πkτt]exp[j·2π(f l t-0.5kt 2 )]·[1+m 2 ·exp(-j·2πf0τ)+m 2 ·exp(j·2πf0τ)] +m·exp[j·2π(kτ-f0)t]exp[j·2π(f l t-0.5kt 2 )]·[1+exp(-j·2πf0τ)] +m·exp[j·2π(kτ+f0)t]exp[j·2π(f l t-0.5kt 2 )]·[1+exp(j·2πf0τ)] +m 2 ·exp[j·2π(kτ-2f0)t]exp[j·2π(f l τ+f0τ-0.5kτ 2 )] +m 2 ·exp[j·2π(kτ+2f0)t]exp[j·2π(f l τ-f0τ-0.5kτ 2 )] After demodulation, the linear frequency modulation signal carrying modulation obtains a single-frequency signal carrying two modulation frequencies. Since 0<m<1, the kτ-2f0 and kτ+2f0 components are ignored; the obtained carrier is a single-frequency amplitude modulation signal, which is the demodulated signal.

5. The method for extracting very low frequency modulation features based on de-skewing according to claim 1, wherein: Compared with the demodulation spectrum of a single array element in step 6, when the signal-to-noise ratio is low, the modulation component of the total demodulation spectrum is obvious and the modulation frequency is easy to read.

6. The method for extracting very low frequency modulation features based on de-skewing according to claim 1, wherein: There is a certain quantitative relationship between the amplitudes of the frequency components after demodulation in step 7. When the influence of the phase is ignored, the amplitude of the kτ component is (1+2m 2 ), the magnitude of the kτ-f0 component and the kτ+f0 component is 2m, and the magnitude of the kτ-2f0 component and the kτ+2f0 component is m 2 ; Define the average difference between the modulation component and the peak value of the array modulation spectrum as the demodulation depth, that is: Among them, Δ is the difference caused by the phase effect.

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