Shifting sideband splitting beam forming method

Abstract

The invention belongs to the technical field of active sonar detection, and particularly relates to a shift sideband split beam forming method. According to the method, shift sideband beam forming and split beam forming are combined, and beam forming is performed on a baseband signal with a low sampling rate, so that the operand is effectively reduced, and a narrower beam opening angle and a lower sidelobe level are obtained. According to the method, the calculation amount of receiving processing is reduced, a narrow beam opening angle is obtained, and more useful target beam information is obtained through a method of increasing the number of beams.

Description

Technical Field

[0001] The present invention belongs to the technical field of active sonar detection, and is a technology for appropriately processing a received signal of a multi-element linear array to form a maximum output in a predetermined direction, thereby suppressing signals and interference in other directions, and particularly relates to a shifted sideband splitting beam forming method. Background Art

[0002] At present, in the field of sonar detection, conventional beamforming algorithms usually use a weighted-delay-sum algorithm of time domain signals. This algorithm has the advantages of wide bandwidth, simple structure and easy implementation. However, since the delay compensation accuracy of the time domain depends on the sampling rate of the received signal, a higher sampling frequency (5-10 times the Nyquist frequency) is required to achieve better performance, and a higher data transmission rate is required. Especially for high-frequency sonars with higher signal frequencies, the disadvantages of conventional beamforming are more obvious. In addition, the horizontal opening angle of the beam of conventional beamforming is too large, which is not conducive to accurate target positioning. Summary of the invention

[0003] The object of the present invention is to provide a shifted sideband splitting beamforming method to reduce the amount of calculation of receiving processing and obtain a narrower beam opening angle, while obtaining more useful target beam information by increasing the number of beams.

[0004] In order to solve the above technical problems, the present invention provides a shifted sideband splitting beamforming method, which converts the high sampling rate array element signal into a low sampling rate complex sideband signal by shifted sideband beamforming, and then splits the low sampling rate complex sideband signal to form a beam, thereby effectively reducing the amount of calculation and obtaining a narrower beam opening angle and a lower sidelobe level.

[0005] The shifted sideband beamforming includes: receiving complex sideband signals of N array element signals after AD conversion and digital down-conversion, the complex sideband signals x1(t)…x N (t) real part and x1(t)…x N (t) imaginary part and After multiplication and addition, it is used as the input of the complex splitting beamformer, where t i is the time delay in the beam direction, i = 1…N; the center frequency of the complex sideband signal is f0-f x ;

[0006] The split beam forming includes: dividing a receiving array for receiving complex sideband signals into two left and right sub-arrays, generating left and right beams for the same wave arrival direction by conventional beam forming, and then summing and subtracting them according to the following algorithm formula (1), and performing a nonlinear summing operation, and utilizing the cancellation principle to effectively reduce the beam opening angle and the sidelobe level under the condition of a certain array size;

[0007] The algorithm formula is:

[0008] S Hyper (t)=[(|S L (t)|+|S R (t)|) n -|S L (t)-S R (t)| n ] 1 / n (1);

[0009] Where n∈[0.3,1]; n is the splitting index, which is used to adjust the output beam main lobe width and side lobe height; S L (t) is the left submatrix, S R (t) is the right submatrix.

[0010] Preferably, Chebyshev weighting is also included to weight the amplitude of each array element; the Chebyshev weighting calculation steps are as follows:

[0011] 1) Based on the given main-sidelobe ratio A and the number of array elements N, calculate X0 according to formula (2);

[0012]

[0013] Where n = N-1;

[0014] 2) Calculate α from X0 using formula (3);

[0015]

[0016] 3) Calculate the weight coefficient relative to α using formula (4);

[0017]

[0018] Among them, the weight of array element 1 is specified is the relative weight of array element k, k=2..N, S=0..k-2.

[0019] Preferably, by first using to modulate the array element signal, and then the real part and the imaginary part of the complex product are filtered by a low-pass filter to obtain the complex sideband signals of the N array element signals.

[0020] Preferably, the method receives the complex sideband signal after AD conversion and digital down-conversion, and after processing, forms beam domain data to output to the target discrimination signal processing module.

[0021] Preferably, the original sampling rate of the array element domain signal is 5 MHz, and the sampling rate is reduced to 96 kHz after digital down-conversion; the carrier center frequency is f0-f x , the low-pass filter order is 416, the downsampling factor is 52, and the splitting index n∈[0.3,1].

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention provides a beam forming method for active sonar receiving signals, which receives the echo signal of the active sonar, generates a beam domain signal with a higher signal-to-noise ratio and a narrower beam opening angle through real-time calculation, and sends it to subsequent active signal processing, target detection and tracking modules to achieve the purpose of active sonar detecting targets. The main features of the present invention are low computational complexity, and can effectively control the beam opening angle and sidelobe amplitude, improve the signal-to-noise ratio of the received signal, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a process flow chart of the present invention.

[0025] Figure 2 FIG. 4 is a structural diagram of the shifted sideband beamforming of the present invention.

[0026] Figure 3 It is a schematic diagram of the split beam processing method of the present invention.

[0027] Figure 4 This is the unweighted directivity diagram of the present invention when n=1.

[0028] Figure 5 This is the directivity diagram of the split weighting when n=0.7 according to the present invention.

[0029] Figure 6 This is the directivity diagram of the present invention with conventional weighting and splitting without weighting when n=0.7.

[0030] Figure 7 This is the unweighted directivity diagram of the present invention when n=0.8.

[0031] Figure 8 This is the azimuth-amplitude diagram of the present invention.

[0032] Fig. 9 This is the azimuth-distance diagram of the present invention. DETAILED DESCRIPTION

[0033] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. The advantages and features of the present invention will become more apparent from the following description. It should be noted that the accompanying drawings are in very simplified form and in non-precise proportions, and are only used to conveniently and clearly assist in explaining the purpose of the embodiments of the present invention.

[0034] like Figure 1 As shown, the shifted sideband split beamforming combines the shifted sideband beamforming and the split beamforming, that is, by frequency-converting the high sampling rate array element signal into a low sampling rate complex sideband signal by the shifted sideband beamforming, and then performing split beamforming on the low sampling rate complex sideband signal, the amount of calculation is effectively reduced, and a narrower beam opening angle and a lower sidelobe level are obtained. The method of the present invention receives the complex sideband signal after AD conversion and digital down-conversion, and after processing, forms beam domain data and outputs it to the signal processing module such as target discrimination.

[0035] The embodiment of the present invention studies the shifted sideband splitting beamforming algorithm, and performs Matlab simulation according to the typical sonar receiving array type and working parameters, simulates the generation of 96 input signals, performs digital down-conversion and beamforming, and outputs 180 beam data.

[0036] (1) Shifted sideband beamforming:

[0037] The digital down-converted array element signal is converted into a complex sideband signal, which greatly reduces the input sampling rate of the beamforming. For a single-frequency signal, if the demodulation frequency is the same as the signal frequency, the signal spectrum is shifted down to become a complex DC signal. If the complex demodulation element and the AD converter have DC drift, it will cause errors in the two orthogonal components, thereby affecting the output of the beamforming. Therefore, it is possible to choose to move the input signal spectrum to an intermediate frequency (f0-f x ) can avoid the above problems.

[0038] The structure of shifted sideband beamforming is as follows: Figure 2 As shown. Among them, x1(t)…x N (t) real part, x1(t)…x N (t) The imaginary part is the signal after AD conversion, that is, the complex sideband signal of the N-channel array element signal (first use e -j2π(f0-fx)t To modulate the array element signal, the real and imaginary parts of the complex product are then filtered with a low-pass filter). The center frequency of the complex sideband signal is (f0-f x ). Then, x1(t)…x N (t) real part, x1(t)…x N (t) imaginary part and e -j2π(f0-fx)ti (i=1…N,t i The delay in the direction of beam formation) is multiplied and added and then used as the input of the complex splitting beamformer.

[0039] (2) Split beam forming:

[0040] Split beamforming is a high-resolution beamforming algorithm that divides the receiving array into two left and right sub-arrays. It generates left and right beams for the same wave direction of arrival using conventional beamforming. Then, it sums and subtracts them according to formula (1) and performs nonlinear summation operations. By using the cancellation principle, it can effectively reduce the beam angle and sidelobe level under the condition of a certain array size. The algorithm structure is shown in Figure 3 .

[0041] S Hyper (t)=[(|S L (t)|+|S R (t)|) n -|S L (t)-S R (t)| n ] 1 / n (1)

[0042] Where n is the splitting index, which is used to adjust the output beam main lobe width and side lobe height, usually n∈[0.3,1].

[0043] (3) Chebyshev weighted sidelobe suppression:

[0044] Chebyshev weighting is to weight the amplitude of each array element. The purpose of weighting is to reduce the sidelobe height as much as possible without changing or changing the main lobe width very little, so as to achieve the purpose of single direction and reduce the appearance of false targets. The calculation steps of Chebyshev weighting are as follows.

[0045] 1) Based on the given main-sidelobe ratio A and the number of array elements N, calculate X0 according to formula 2;

[0046]

[0047] Where n = N-1;

[0048] 2) Calculate α from X0 using formula (3);

[0049]

[0050] 3) Use formula (4) to calculate the weight coefficient relative to α.

[0051]

[0052] Among them, the weight of array element 1 is specified is the relative weight of array element k, k=2..N, S=0..k-2.

[0053] (4) Simulation effect:

[0054] The analog receiving array type is: horizontal linear array, the number of array elements is 96, the working OS rate is f0 and the array element spacing is d, the number of beams formed is 180, covering a 90° detection sector, and the beam spacing is 90° / 180. The original sampling rate of the array element domain signal is 5MHz, and the sampling rate is reduced to 96kHz after digital down-conversion. The carrier center frequency is f0-f x , the low-pass filter order is 416, the downsampling multiple is 52, and the splitting index n∈[0.3,1]. Different beamforming directivity diagrams are shown in Figures 4 to 7 The beam performance parameters are shown in Table 1.

[0055] Table 1 Simulation results

[0056]

[0057] From the above analysis, we can see that when using shifted sideband splitting beamforming and selecting splitting index n = 1, without weighting, the beam opening angle and sidelobe level meet the general use requirements. In order to leave a certain margin for the sidelobe level, the splitting index n = 0.8 or even n = 0.7 can be selected. In addition, from Figure 5 and Figure 6 It can be seen that Chebyshev weighting is applicable to shifted sideband conventional beamforming, but not to shifted sideband split beamforming.

[0058] The simulated target azimuth is -0.25° (90th beam position), and the signal-to-noise ratio is 10 (adjustable). The detection waveform is a single-frequency signal (CW) with a pulse width of 0.2ms (adjustable). Select n=0.8, and the azimuth-amplitude diagram and azimuth-range diagram of the unweighted shifted sideband splitting beam formation are as follows: Figure 8 and Fig. 9 Shown: From Figure 8 and Fig. 9 It can be seen that the use of shifted sideband splitting beamforming can effectively control the beam angle and sidelobe amplitude and improve the signal-to-noise ratio.

[0059] (5) Conclusion:

[0060] Using the shifted sideband splitting beamforming method of the present invention:

[0061] 1) The beam angle is reduced from 0.89° in conventional beamforming to 0.24°;

[0062] 2) The sidelobe level is reduced from -13.37dB in conventional beamforming to -23.41dB;

[0063] 3) The signal-to-noise ratio of the beam domain signal is greatly improved, and the signal quality is significantly improved, which is very beneficial for post-processing such as target identification.

[0064] The above description is only a description of the preferred embodiments of the present invention, and is not intended to limit the scope of the present invention. Any changes or modifications made by a person skilled in the art in the field of the present invention based on the above disclosure shall fall within the scope of protection of the claims.

Claims

1. A shifted sideband splitting beamforming method, characterized in that: This method uses shifted sideband beamforming to convert high sampling rate array element signals into low sampling rate complex sideband signals, and then splits the low sampling rate complex sideband signals to form beams, effectively reducing the amount of calculation and obtaining a narrower beam opening angle and a lower sidelobe level. The shifted sideband beamforming includes: receiving complex sideband signals of N array element signals after AD conversion and digital down-conversion, the complex sideband signals x1(t)…x N (t) real part and x1(t)…x N (t) imaginary part and After multiplication and addition, it is used as the input of the complex splitting beamformer, where t i is the time delay in the beam direction, i = 1…N; the center frequency of the complex sideband signal is f0-f x ; The split beam forming includes: dividing a receiving array for receiving complex sideband signals into two left and right sub-arrays, generating left and right beams for the same wave arrival direction by conventional beam forming, and then summing and subtracting them according to the following algorithm formula (1), and performing a nonlinear summing operation, and utilizing the cancellation principle to effectively reduce the beam opening angle and the sidelobe level under the condition of a certain array size; The algorithm formula is: S Hyper (t)=[(|S L (t)|+|S R (t)|) n -|S L (t)-S R (t)| n ] 1 / n (1); Where n∈[0.3,1]; n is the splitting index, which is used to adjust the output beam main lobe width and side lobe height; S L (t) is the left submatrix, S R (t) is the right submatrix.

2. A shifted sideband splitting beamforming method as claimed in claim 1, characterized in that: It also includes Chebyshev weighting, which is used to weight the amplitude of each array element; the Chebyshev weighting calculation steps are as follows: 1) Based on the given main-sidelobe ratio A and the number of array elements N, calculate X0 according to formula (2); Where n = N-1; 2) Calculate α from X0 using formula (3); 3) Calculate the weight coefficient relative to α using formula (4); Among them, the weight of array element 1 is specified is the relative weight of array element k, k=2..N, S=0..k-2.

3. The shifted sideband splitting beamforming method according to claim 1, characterized in that: By first using to modulate the array element signal, and then the real part and the imaginary part of the complex product are filtered by a low-pass filter to obtain the complex sideband signals of the N array element signals.

4. The shifted sideband splitting beamforming method according to claim 1, characterized in that: The method receives the complex sideband signal after AD conversion and digital down-conversion, and after processing, forms beam domain data and outputs it to the target discrimination signal processing module.

5. The shifted sideband splitting beamforming method according to claim 1, characterized in that: The original sampling rate of the array element domain signal is 5MHz, and the sampling rate is reduced to 96kHz after digital down-conversion; the carrier center frequency is f0-f x , the low-pass filter order is 416, the downsampling factor is 52, and the splitting index n∈[0.3,1].

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