Sub-band synthesis method based on scattering center classification

Abstract

The invention provides a sub-band synthesis method based on scattering center classification, and the method comprises the steps: carrying out the estimation and compensation of a linear phase between sub-bands, and extracting a scattering center from a sub-band echo after the linear phase compensation. And the scattering centers in each sub-band are divided into inherent scattering centers and unique scattering centers according to the relative distance parameters of the scattering centers. A new fixed phase estimation and compensation method is introduced for an inherent scattering center to compensate an additional phase difference introduced by scattering characteristic fluctuation and an original fixed phase difference. Corresponding full-band echoes are generated for different types of scattering centers and are superposed, and finally target full-band echoes are obtained. According to the method, the characteristic that the target electromagnetic scattering characteristic fluctuates in the ultra-wideband frequency band range is fully considered, and a new fixed phase estimation and compensation method is introduced to compensate an extra phase item caused by fluctuation of the target scattering characteristic, so that the final precision of sub-band synthesis is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of radar signal processing, and in particular relates to a sub-band synthesis method based on a GTD model and a Root-MUSIC algorithm. Background Art

[0002] Based on radar theory, the range resolution of the target depends on the bandwidth of the radar signal. The relative bandwidth of the traditional radar transmission signal is usually less than 10% of the carrier frequency, so it is difficult to achieve a high range resolution. In view of the limitation of radar bandwidth, there are two ways to obtain ultra-wideband UWB radar signals. One method is to directly build UWB radar hardware, which will lead to a significant increase in system complexity and design costs. Another effective method to generate UWB signals is sub-band synthesis technology. It is a signal processing method that fuses two or more radar sub-band echo signals into an ultra-wideband radar echo signal, thereby avoiding the high cost of building a true UWB radar and further improving the range resolution of radar targets.

[0003] With the development of subband synthesis technology, there are linear phases and fixed phases between subbands, which need to be estimated and compensated. Subband synthesis methods can be summarized into two categories: model-based algorithms, including traditional spectrum estimation algorithms and sparse representation algorithms, and data-driven algorithms. Spectral estimation algorithms are widely used in subband synthesis processing due to their low computational complexity. Analyzing the existing subband synthesis methods, it can be seen that the existing methods all assume that the electromagnetic scattering characteristics of the target remain consistent throughout the entire synthesis frequency band, thus obeying the electromagnetic scattering model under the same parameters. However, in actual situations, the electromagnetic scattering characteristics of the target fluctuate within the ultra-wideband frequency band, which will affect the final accuracy of subband synthesis. Summary of the invention

[0004] The technical problem to be solved by the present invention is to analyze the cause of the fluctuation of target scattering characteristics within the ultra-wideband frequency domain, thereby affecting the sub-band synthesis accuracy, and then propose a sub-band synthesis method for improving the accuracy of sub-band synthesis.

[0005] In order to solve the above technical problems, the present invention conducts detailed modeling of the phase mismatch between multiple sub-bands based on the GTD model, and deduces that the fluctuation of the electromagnetic scattering characteristics of the target will introduce an additional phase difference on the basis of the original fixed phase between the sub-bands, thereby reducing the accuracy of sub-band integration. The technical solution adopted for this analysis is a sub-band integration method based on scattering center classification to estimate and compensate for the additional phase difference introduced by the fluctuation of the target scattering characteristics, thereby improving the accuracy of sub-band integration, which specifically includes the following steps:

[0006] Estimate and compensate the linear phase between sub-bands;

[0007] Extract the electromagnetic scattering center parameters of the target based on the sub-band echo after linear phase compensation;

[0008] The scattering centers in each sub-band are divided into inherent scattering centers and unique scattering centers, and the inherent scattering center processing step or the unique scattering center processing step is entered into the corresponding step;

[0009] Intrinsic scattering center processing steps: calculate a new fixed phase based on the inherent scattering center and compensate it to compensate for the additional phase difference introduced by the fluctuation of the scattering characteristics and the original fixed phase difference, and restore the phase correlation of the inherent scattering center between sub-bands; then reconstruct the sub-band echo of each inherent scattering center after restoring the phase correlation to obtain the reconstructed sub-band echo of the inherent scattering center; finally, obtain the full-band echo of the inherent scattering center by performing sub-band synthesis on the reconstructed sub-band echo of the inherent scattering center;

[0010] Unique scattering center processing steps: performing sub-band extrapolation on the unique scattering center in each sub-band to generate full-band echoes of the unique scattering center;

[0011] Sub-band integration step: superimpose the full-band echo of the inherent scattering center with the full-band echo of the unique scattering center to obtain the superimposed full-band echo, and then use the original sampling data of each sub-band and the blank frequency bands between the sub-bands to use the corresponding part of the superimposed full-band echo to form the final target full-band echo.

[0012] Specifically, the electromagnetic scattering center parameters of the target are extracted based on the GTD model and the Root-MUSIC algorithm. The electromagnetic scattering center parameters include the order of the GTD model, the scattering intensity and the relative distance of the scattering center.

[0013] The present invention first takes into account the characteristic that the linear phase between sub-bands will cause the one-dimensional range image of the sub-band to shift left and right, and first estimates and compensates the linear phase between sub-bands. Once the linear phase between sub-bands is compensated, the peak points of the one-dimensional range image of the sub-band will be aligned and correspond one to one. Then, based on the GTD model, the scattering center is extracted from the sub-band echo after the linear phase is compensated, and the scattering center in each sub-band is divided into two categories according to the relative distance parameter of the scattering center: the inherent scattering center whose relative distance parameter is within plus or minus one distance resolution and the unique scattering center whose relative distance parameter is not within plus or minus one distance resolution. A new fixed phase estimation and compensation method is introduced for the inherent scattering center to compensate for the additional phase difference introduced by the fluctuation of the scattering characteristics and the original fixed phase difference. Finally, the scattering centers of different categories are respectively integrated to generate the corresponding full-band echo, and superimposed to obtain the final ultra-wideband echo.

[0014] The beneficial effect of the present invention is that it fully considers the fluctuation of target electromagnetic scattering characteristics within the ultra-wideband frequency band, introduces a new fixed phase estimation and compensation method to compensate for the additional phase terms caused by the fluctuation of target scattering characteristics, thereby improving the final accuracy of sub-band synthesis. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a block diagram of the sub-band integrated signal processing flow.

[0016] Figure 2 The flowchart of the improved sub-band synthesis method based on the GTD model and the Root-MUSIC algorithm is shown in the embodiment.

[0017] Figure 3 For the experimental goal.

[0018] Figure 4 This is a comparison of the accuracy of the present invention and the traditional sub-band synthesis method. DETAILED DESCRIPTION

[0019] The specific implementation modes of the present invention are described clearly and completely below in conjunction with the accompanying drawings so that those skilled in the art can better understand the invention.

[0020] The Geometrical Theory of Diffraction (GTD) model is a high-frequency approximate theory in electromagnetic wave propagation and scattering analysis and an important tool in computational electromagnetics.

[0021] Figure 1 The figure shows the three key steps of the traditional sub-band synthesis method: The first step is sub-band coherent processing, that is, estimating and compensating the linear phase difference and fixed phase difference between the sub-band echoes. After compensation, the sub-bands are phase aligned. The second step is the full-band GTD model parameter estimation. The third step is to estimate the full-band echo signal of the target based on the full-band GTD model parameters and the original sub-band echo data. Interpolation is performed between sub-band 1 and sub-band 2, and sub-band 1 and sub-band 2 are extrapolated to obtain the full-band echo. In the first step of the traditional sub-band synthesis method, it is generally assumed that the scattering characteristics of different sub-bands are approximately the same, that is, different sub-bands conform to a unified GTD parameter model, so the phase difference between sub-bands is only linear phase and fixed phase.

[0022] However, when the electromagnetic scattering characteristics of the target fluctuate within the ultra-wideband frequency range, the sub-bands will no longer obey the unified GTD parameter model.

[0023] Based on the basic radar theory and GTD model, the sub-band echo can be expressed as:

[0024] ;

[0025] In the above formula, n is the sampling point number, subband number i=1,2, represents the representation of subband i echo at the nth sampling point, represents the representation of subband 2 echo at the nth sampling point, is the GTD model order of subband i, represents the amplitude of the sub-band i echo at the mth scattering center, represents the phase of the subband i echo at the mth scattering center, represents the frequency dependence factor of the subband i echo at the mth scattering center, is the relative distance of the sub-band i echo at the mth scattering center. The relative distance is the distance between the scattering center and the radar minus the distance between the reference point and the radar. Generally, the actual distance of the radar detecting the target is taken. For example, if the target is at 1000m, and there are three scattering centers on the target at 905m, 1001m, and 1005m respectively, then the relative distance is -5m, 1m, and 5m. is the speed of light, is the starting frequency, is the frequency sampling interval, is the pole of the mth scattering center of the sorted sub-band i echo, is the amplitude of the mth scattering center of the subband i echo after sorting, exp is the natural exponential function, and j is the imaginary unit. The amplitude of the scattering center is the complex amplitude. At this time, taking subband 1 as the reference, there is a linear phase between subband 1 and subband 2 With a fixed phase , as shown below:

[0026] ;

[0027] and Respectively represent the number of sampling points of sub-band 1 and sub-band 2, is the number of sampling points of the full-band echo. The poles and amplitudes of the scattering centers of sub-band 1 and sub-band 2 are shown below:

[0028] ;

[0029] When the target scattering characteristics fluctuate, at the same relative distance The amplitude phase and frequency dependence factors of the scattering center under the parameters will no longer remain unchanged. Compared with the scattering center amplitude It can be seen from the phase term that if and is variable, which will introduce additional phase difference superimposed on the original fixed phase The additional phase term is given by:

[0030] ;

[0031] The new fixed phase for:

[0032] ;

[0033] From the above derivation, it can be seen that the change of the scattering characteristics of the target will introduce an additional phase term on the original fixed phase to form a new fixed phase, and each scattering point is different.

[0034] The Root-MUSIC algorithm is an improved version of the classic MUSIC (Multiple Signal Classification) algorithm. It significantly improves the computational efficiency and parameter estimation accuracy by replacing spectral search with polynomial root finding.

[0035] Based on the Root-MUSIC algorithm, the linear phase and new fixed phase of the sub-band echo considering the fluctuation of the target electromagnetic scattering characteristics are estimated and compensated. The detailed steps are as follows:

[0036] Step 1: Estimation and compensation of linear phase based on the minimum entropy principle: Assume that the true linear phase between subbands is , represents the linear phase to be estimated and compensated. To compensate Subband 2 echo after that. The sub-band 2 echo is then inverse Fourier transformed to obtain a high-resolution one-dimensional range image of sub-band 2. , perform inverse Fourier transform on the sub-band 1 echo to obtain the high-resolution one-dimensional range image of sub-band 1, which is The amplitudes are superimposed to obtain a one-dimensional range image amplitude sequence after linear phase compensation between sub-bands. :

[0037] ;

[0038] Its amplitude distribution for:

[0039] ;

[0040] Calculating Information Entropy :

[0041] ;

[0042] When the linear phase compensation The closer to the true linear phase , the one-dimensional range images of the two sub-bands have the highest overlap, corresponding to the minimum information entropy. Therefore, the information entropy to be compensated can be obtained by minimizing the following formula:

[0043] ;

[0044] At this time, the compensation estimate is Compensation is obtained on the callback of sub-band 2 Subband 2 echo after .

[0045] Step 2: Scattering center parameter extraction and classification: After estimating and compensating the linear phase, the sub-band echo is obtained based on the GTD model and Root-MUSIC algorithm. and The scattering center of the target is extracted from the image. Then, based on the relative distance parameter of the scattering center The scattering centers are classified. The classification criteria are: the relative distance parameters of the scattering centers within plus or minus one distance resolution unit are sub-band intrinsic scattering centers ISC, and those exceeding plus or minus one distance resolution unit are unique scattering centers USC. At this time, the poles and amplitudes of the scattering centers are: They are also divided into two categories: and .

[0046] In the traditional subband synthesis method, the fixed phase estimation calculation is as follows:

[0047] ;

[0048] Where, angle is the phase angle function; There are two problems in the above calculation. One is the one-to-one correspondence problem of the scattering center, and the other is that the influence of the fluctuation of the target scattering characteristics is not considered. Because the fluctuation of the scattering characteristics will introduce additional phase terms: Therefore, the fixed phase is different for each scattering center. Therefore, it is necessary to perform fixed phase compensation for each scattering center.

[0049] Step 3: Calculate the fixed phase difference for the intrinsic scattering center, as shown in the following formula: The calculation method at this time includes both the original fixed phase and the additional phase term introduced by the fluctuation of the target scattering characteristics :

[0050] ;

[0051] For the intrinsic scattering center, the new fixed phase estimated by compensation is:

[0052] ;

[0053] Step 4: Based on the amplitude of the pole and scattering center of the intrinsic scattering center after recovering the phase correlation , Reconstruct sub-band echoes from intrinsic scattering centers and :

[0054] ;

[0055] in, is the number of intrinsic scattering centers.

[0056] Step 5: Perform joint parameter estimation on the echo of the inherent scattering center in the above formula to obtain the full-band scattering center pole With amplitude , thus obtaining the full-band echo of the intrinsic scattering center :

[0057] ;

[0058] Step 6: Perform a 2-fold frequency extrapolation on the unique scattering center to obtain the full-band echo of the unique scattering center of sub-band 1 and sub-band 2 and :

[0059] ;

[0060] Step 7: Superimpose the full-band echo of the inherent scattering center with the full-band echo of each sub-band unique scattering center to obtain the superimposed full-band echo :

[0061] ;

[0062] In order to reduce the error, the original sampling data of sub-band 1 and sub-band 2 are used, and the full-band echo is used in the blank frequency band between sub-band 1 and sub-band 2. The corresponding part of the target full-band echo is finally obtained as follows:

[0063] ;

[0064] So far, the improved subband synthesis method based on the GTD model and the Root-MUSIC algorithm has been completed. Next, in combination with the actual subband parameters, each subband is synthesized according to the above method to compare the accuracy of the subband synthesis, thereby reflecting the improvement of the subband synthesis accuracy by the present invention.

[0065] First, set the sub-band comprehensive full frequency range to: 4.5-8.5GHz, sub-band 1 to 4.5-5.5G, sub-band 2 to: 7.5-8.5GHz, and blank band to: 5.5-7.5GHz. Both radars use linear frequency modulation signals as radar transmission signals, with a bandwidth of 1GHz and a pulse of 10us. Figure 3 As shown. The scattering field data is measured in a microwave darkroom. Based on the scattering field data of the target and the radar transmission signal, each sub-band echo of the target and the ideal full-band echo are generated. At the same time, the original linear phase and fixed phase between sub-bands are set as follows: , .

[0066] based on Figure 2 Steps 1 to 7 shown in the figure process the sub-band echoes to obtain the full-band echoes after sub-band integration. The correlation coefficient is introduced to measure the accuracy of sub-band integration, as shown below: For an ideal full-band echo, is the full-band echo obtained by sub-band integration. At the same time, it can be seen from the following formula that the larger the correlation coefficient, the higher the accuracy of sub-band integration, and its maximum value is 1.

[0067] ;

[0068] is the 2 norm. Compared with the traditional method, the correlation coefficient results are as follows Figure 4 As shown in the figure, the correlation coefficients of the methods proposed in the present invention are all higher than 0.9, which is a great improvement over the traditional methods. For example, under the condition of 15dB signal-to-noise ratio, the correlation coefficient of the present invention is improved by 25% compared with the traditional method. As can be seen from the above, the present invention fully considers the influence of the fluctuation of the target scattering characteristics on the sub-band synthesis, and the new fixed phase difference estimation and compensation method based on the Root-MUSIC algorithm effectively improves the accuracy of the sub-band synthesis.

[0069] Although the above describes the illustrative specific embodiments of the present invention to facilitate those skilled in the art to understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations using the concept of the present invention are protected.

Claims

1. A sub-band synthesis method based on scattering center classification, characterized in that: The following steps are involved: Estimate and compensate the linear phase between sub-bands; Extract the electromagnetic scattering center parameters of the target based on the sub-band echo after linear phase compensation; The scattering centers in each sub-band are divided into inherent scattering centers and unique scattering centers, and the inherent scattering center processing step or the unique scattering center processing step is entered into the corresponding step; Intrinsic scattering center processing steps: calculate a new fixed phase based on the inherent scattering center and compensate it to compensate for the additional phase difference introduced by the fluctuation of the scattering characteristics and the original fixed phase difference, and restore the phase correlation of the inherent scattering center between sub-bands; then reconstruct the sub-band echo of each inherent scattering center after restoring the phase correlation to obtain the reconstructed sub-band echo of the inherent scattering center; finally, obtain the full-band echo of the inherent scattering center by performing sub-band synthesis on the reconstructed sub-band echo of the inherent scattering center; Unique scattering center processing steps: performing sub-band extrapolation on the unique scattering center in each sub-band to generate full-band echoes of the unique scattering center; Sub-band integration step: superimpose the full-band echo of the inherent scattering center with the full-band echo of the unique scattering center to obtain the superimposed full-band echo, and then use the original sampling data of each sub-band and the blank frequency bands between the sub-bands to use the corresponding part of the superimposed full-band echo to form the final target full-band echo.

2. The method according to claim 1, characterized in that: The linear phase between sub-bands is estimated and compensated based on the minimum entropy theorem.

3. The method according to claim 2, characterized in that: Estimation of Linear Phase Between Subbands Based on Minimum Entropy Theorem The specific method is: ； Among them, argmin is the minimum solution function, represents the linear phase variable, , ; is the amplitude distribution of the one-dimensional range image amplitude sequence after linear phase compensation between sub-bands; for Information entropy.

4. The method according to claim 1, characterized in that: The electromagnetic scattering center parameters of the target are extracted based on the geometric diffraction theory GTD model and the Root-MUSIC algorithm. The electromagnetic scattering center parameters include the order of the GTD model, the scattering amplitude and the relative distance of the scattering center.

5. The method according to claim 4, characterized in that: The scattering centers in each sub-band are divided into intrinsic scattering centers and unique scattering centers based on the relative distances of the electromagnetic scattering centers in each sub-band.

6. The method according to claim 5, characterized in that The scattering center whose relative distance is within plus or minus one distance resolution is the intrinsic scattering center; The scattering center whose relative distance exceeds the range resolution by ±1 is a unique scattering center.

7. The method according to claim 4, characterized in that: The method to calculate the new fixed phase based on the intrinsic scattering center is: Calculate the additional phase term introduced by the fluctuation of the target scattering characteristics at the mth intrinsic scattering center : ； Where angle is the phase angle function, is the amplitude of the subband 2 echo at the mth intrinsic scattering center, is the amplitude of the subband 1 echo at the mth intrinsic scattering center; At the mth intrinsic scattering center, the amplitude after compensation is obtained with a new fixed phase for: ； Where exp is the natural exponential function and j is the imaginary unit.

8. The method according to claim 7, characterized in that: Reconstruct the subband echo of the intrinsic scattering center based on the pole of the mth intrinsic scattering center of subband 1 and subband 2 after restoring the phase correlation and the amplitude of the scattering center and : ； in, is the number of intrinsic scattering centers, and Respectively represent the number of sampling points of sub-band 1 and sub-band 2, N is the number of sampling points of the full-band echo, is the amplitude of the subband 1 echo at the mth intrinsic scattering center, is the amplitude of the subband 2 echo after compensation at the mth intrinsic scattering center, and is the pole of sub-band 1 echo and sub-band 2 echo at the mth intrinsic scattering center, and n is the sampling point number.

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