A linear phase modulation method and system based on a linear receiver array

Abstract

This application provides a linear phase modulation method and system based on a linear receiving array, aiming to solve the problems of traditional underwater acoustic imaging systems being complex, having low resolution, and being unable to image moving targets in real time. The method uses an angular domain bandpass filter to filter and linearly phase-modulate the echo signal of a uniform acoustic wave with the same wavefront phase as the transmitted wavefront from the transmitting transducer at the receiving array. Then, through linear superposition, it modulates the wave into a spiral wave with a linear phase to the incident angle of the plane wave in each direction. The system includes a transmitting transducer for transmitting pulses, a linear receiving transducer array for receiving echoes, and an array signal processor for executing the above modulation method. This invention requires only a single sound source and only one transmission and reception cycle to perform real-time high-resolution underwater acoustic imaging of both stationary and moving targets, significantly simplifying the system while generating spiral waves of a much higher order than existing technologies, thereby constructing high-resolution underwater acoustic images.

Description

Technical Field

[0001] This application pertains to underwater acoustic imaging technology, specifically relating to a linear phase modulation method and system based on a linear receiving array. Background Technology

[0002] Currently, in underwater acoustic imaging, the wavefront of the transmitted signal is generally an equiphase surface; the wavefront of the near-field transmitted signal is spherical, while the wavefront of the far-field transmitted signal is planar. The received signal is beamformed using conventional beamforming methods, and the beam angular resolution is limited by the aperture of the receiving array. In recent years, improving the beam angular resolution by spatially differentiating the wavefront of the transmitted signal, thus giving different weights to different regions of the radiation field, has become a cutting-edge research direction in underwater acoustic imaging. The orthogonality between vortex-phase helical waves with different topological charges is widely used in high-resolution underwater acoustic imaging research.

[0003] A transmitting transducer emits vortex-phase helical acoustic waves with different topological charges, and a receiving transducer array receives the echoes of vortex-phase helical waves with corresponding topological charges. By repeatedly transmitting and receiving vortex-phase helical waves with different topological charges, and utilizing the orthogonality between these waves, signals incident from different directions can be separated, thus obtaining robust, high-resolution underwater acoustic images. However, this imaging method has a significant drawback: it requires both the transducer and the target to remain stationary, and can only perform staring imaging on stationary targets, not real-time underwater acoustic imaging of moving targets.

[0004] To improve imaging speed, an improved scheme has emerged: transmitting multiple orders of orbital angular momentum signals carried on different carrier frequencies in parallel using a transmitting array. The receiving transducer separates the echoes from different carrier frequencies by filtering, thus acquiring different orders of orbital angular momentum echo signals simultaneously. Although this improves imaging speed, this scheme is limited by the bandwidth of the transmitting and receiving transducers, and the number of orders of orbital angular momentum signals transmitted in parallel is limited (at most only a dozen or so), thus the improvement in image angular resolution is also relatively limited.

[0005] Therefore, there is an urgent need for a method that can perform real-time high-resolution underwater acoustic imaging of stationary and moving targets through a single transmission and reception. Summary of the Invention

[0006] The purpose of this application is to overcome the shortcomings of existing underwater acoustic imaging systems, such as complexity, low resolution, and inability to image moving targets in real time.

[0007] To achieve the above objectives, this application proposes a linear phase modulation method based on a linear receiving array. This phase modulation method obtains the phase difference of the incident plane wave in the azimuth direction by performing linear phase modulation on a snapshot data after a single transmission by the transmitting transducer and reception of its echo signal by the linear receiving array, so as to perform real-time underwater acoustic imaging of stationary and moving targets.

[0008] The specific steps include:

[0009] Step 1: The transmitting transducer emits uniform sound waves with the same wavefront phase, and receives the echo signal through a linear receiving array;

[0010] Step 2: Filter the echo signal, the filtering is performed by... A angular domain bandpass filter divides the imaging sector into an equal number of imaging sub-sectors;

[0011] Step 3: Combine the snapshot data obtained after each filtering process with... ( Multiply by , and perform linear phase modulation; where, For topological load number, Angle of incidence;

[0012] Step 4: Linearly superimpose all the modulated snapshot data to obtain a spiral wave modulated from plane waves incident from various directions.

[0013] Preferably, the number of the angular domain bandpass filters... .

[0014] The angular passband center angle range of the aforementioned angular domain bandpass filter is: And the range of the angle passband bandwidth is .

[0015] The stated angular passband width is designed based on the half-peak width angle of the main lobe of a conventionally formed beam. Figure 2 The center angle of the angular passband of the angular domain bandpass filter is The angle passband bandwidth is The imaging sector of the linear receiving array is symmetrical about the normal to the transducer radiating surface, and the typical incident angle of the acoustic wave is... The imaging sector is If the angle of incidence of the sound wave is... , then ( , Multiplying the incident plane wave (called the topological charge number) by a snapshot of the incident plane wave on a linear receiving array is equivalent to linearly phase-shifting the incident plane wave. Angle of incidence Different plane waves have different phase shifts, achieving linear phase modulation within 90° of the imaging sector of the linear receiving array. The linear phase modulation of the 90° imaging sector of the linear receiving array becomes a quarter-sector of a complete 360° vortex phase modulation.

[0016] This application also proposes a linear phase modulation system based on a linear receiver array, comprising:

[0017] A transmitting transducer, wherein the transmitting transducer is used to transmit equal-phase wavefront acoustic pulses with equal amplitude;

[0018] A receiving transducer array, wherein the receiving transducer array is a linear receiving array, is used to receive echo signals;

[0019] An array signal processor, for processing the aforementioned echo signal, including an angular domain bandpass filter, and configured to perform the linear phase modulation method as described in claim 1.

[0020] An angular bandpass filter is a matrix filter designed with the incident angle of a plane wave as the only variable. It can be designed using the angular stopband constraint condition and the angular passband maximum error minimization optimization criterion. The minimum bandwidth of the angular passband is limited by the aperture of the receiving array, just like the half-width of the main lobe of a conventional beamforming filter.

[0021] To test the function of this invention in modulating the phase of in-phase plane waves incident from various directions into a linear relationship with the incident angle, plane waves with constant amplitude were incident on a linear receiving array at different angles, and the amplitude and phase of the output relative to the input plane wave were measured. The results show that the linear phase modulation method of the linear receiving array of this invention is functionally correct and has high accuracy.

[0022] Compared with the prior art, the advantages of the present invention are:

[0023] 1. Existing technology generates vortex-phase helical sound waves through phased control of multiple emitting sound sources, which is suitable for generating topological charge numbers of... The vortex phase spiral wave requires at least The invention uses a single sound source to generate vortex phase modulation at the receiving end through signal processing. Similar to traditional underwater acoustic imaging, only one sound source is needed for the sound transmission.

[0024] 2. The linear phase spiral signal generated by this invention has a much higher order than the vortex phase spiral acoustic wave generated by the prior art. As a result, the incident signal plane wave has a larger phase difference in the azimuth direction, which is beneficial for obtaining higher acoustic image angular resolution in subsequent underwater acoustic imaging.

[0025] 3. Compared with existing technologies that can only perform staring imaging of stationary targets by transmitting vortex-phase spiral acoustic waves with different topological charge numbers, this invention obtains the phase difference of incident plane waves in the azimuth direction by performing linear phase modulation of different topological charge numbers through a snap at the receiving end. This invention can not only perform staring imaging of stationary targets, but also perform real-time underwater acoustic imaging of moving targets. Attached Figure Description

[0026] Figure 1 This refers to the equally spaced linear receiving array model involved in the embodiments of the present invention;

[0027] Figure 2 This is the amplitude-angle response curve of the angular domain bandpass filter involved in the embodiment of the present invention;

[0028] Figure 3 The imaging sector involved in the embodiment of the present invention is divided into 91 sub-sectors;

[0029] Figure 4 This is a flowchart of the linear phase modulation process for the received signal involved in the embodiments of the present invention;

[0030] Figure 5-1 This is related to the embodiments of the present invention. The amplitude of the output plane wave after linear phase modulation;

[0031] Figure 5-2 This is related to the embodiments of the present invention. The amplitude of the output plane wave after linear phase modulation;

[0032] Figure 5-3 This is related to the embodiments of the present invention. The amplitude of the output plane wave after linear phase modulation;

[0033] Figure 6-1 This is related to the embodiments of the present invention. Phase shift-incident angle relationship curve of a plane wave after time-linear phase modulation;

[0034] Figure 6-2 This is related to the embodiments of the present invention. Phase shift-incident angle relationship curve of a plane wave after time-linear phase modulation;

[0035] Figure 6-3 This is related to the embodiments of the present invention. The phase shift-incident angle relationship curve of a plane wave after time-linear phase modulation. Detailed Implementation

[0036] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0037] Take, for example, an equally spaced linear receiving array consisting of 48 array elements.

[0038] like Figure 1 As shown, for a linear receiver array with 48 equally spaced elements, if... A far-field plane wave signal source from one direction If the data is incident on this receiving array, then the snapshot data received by these 48 array elements can be written in matrix form as follows:

[0039] (1)

[0040] in, yes Given the array manifold matrix, if the spacing between adjacent elements of the receiving array is half the wavelength of a plane wave, then the array manifold vector... , ,in Indicates transpose; yes 3D signal source vector; yes 3D noise vector.

[0041] For the snapshot data shown in equation (1), if filtered by a matrix filter, the filter coefficients are... complex matrix Then the filter output It can be represented as:

[0042] (2)

[0043] in, and These are the filtered array manifold matrix and the noise data matrix, respectively.

[0044] To enable the designed matrix filter to have angular domain filtering capabilities, the angular passband is configured according to requirements. Signals can pass through the sector without distortion, while in the corner stopband... The signal is suppressed within the sector, which is the complex coefficient matrix of the designed matrix filter. and array manifold vector satisfy:

[0045] (3)

[0046] Design the coefficient matrix of the corner domain filter. It is generally impossible to fully satisfy equation (3), but by borrowing the optimization method in FIR digital filter design, the coefficient matrix of the angular domain filter can be designed using the optimization design criterion. To satisfy equation (3) as much as possible.

[0047] If we remember the angle band The array manifold vectors inside are Angular stopband The array manifold vectors inside are The noise attenuation rate within the corner stopband is The optimization design problem of designing an angular domain bandpass filter using the angular stopband constraint condition and the angular passband maximum error minimization criterion can be expressed as:

[0048] (4)

[0049] in Let Frobenius norm be represented. Equation (4) can be transformed into the standard form of a second-order cone programming problem, and then the coefficient matrix can be solved using the sedumi() function in the SeDuMi package embedded in the MATLAB toolbox. .

[0050] Design a specific angular domain bandpass filter using the method described above. Figure 1 The model shown consists of 48 equally spaced linear receiver arrays, with the spacing between adjacent array elements equal to half the wavelength of the incident plane wave. The angular passband of the angular domain bandpass filter is taken as... The angle stopband is taken as The stopband attenuation is The corner passage has 41 arrayed manifold vectors. The angle damper has 174 arrayed manifold vectors. The optimization design problem can be expressed as equation (4), which is transformed into the standard form of a second-order cone programming problem. The complex coefficient matrix of the angular domain bandpass filter can be solved using the sedumi() function in MATLAB. A plane wave is incident at an arbitrary angle on... Figure 1 The linear receiving array, with its 48 array elements, receives snapshot data represented as complex vectors. Its conjugate complex vector is Then complex vector The L2 norm is The amplitude of the plane wave is Using the aforementioned corner stopband constraints and the corner passband maximum error minimization criterion, a corner domain bandpass filter is designed. With the input plane wave amplitude remaining constant, the output plane wave amplitude of the corner domain bandpass filter varies with the incident angle of the input plane wave. The ratio of the output plane wave amplitude to the input plane wave amplitude, with the maximum value normalized, yields the curve showing the relationship between the output plane wave amplitude and the incident angle. Figure 2 As shown, the center angle of the passband is The angle passband bandwidth is The amplitude fluctuation of the plane wave within the passband is less than The amplitude attenuation of the plane wave in the corner stopband reaches .from Figure 2 It can be seen that the relationship between the plane wave amplitude output by the designed angular domain bandpass filter and the incident angle meets the design requirements.

[0051] Using the same angular domain bandpass filter design method described above, in the linear array imaging sector... every Design a corner-domain bandpass filter. These 91 corner-domain bandpass filters can be used to perform corner-domain bandpass filtering on the snapshot data of the 48 elements of a linear receiving array, thereby improving... The imaging sector is divided into 91 imaging sub-sectors, such as Figure 3 As shown.

[0052] exist Figure 1 In the middle, a column has an amplitude of 1 and a frequency of plane wave With the angle of incidence Incident on a 48-element, equally spaced linear receiving transducer array, with the spacing between adjacent elements being [missing information]. Then, the baseband signal of a snapshot array element field data output by the receiving array can be expressed as: ,in This indicates transpose.

[0053] If the incident plane wave Phase shift Then, a snapshot baseband signal output by the linear receiver array and The corresponding expression can be represented as: To increase the different incident angles The difference in phase shift of the incident plane wave will Multiply by an integer Phase shift obtained For each angle of incidence plane wave The baseband data modulation of the element domain of the aforementioned receiving array is different. times Phase shift obtained with The corresponding signal can be represented as: .

[0054] If the angle of incidence Discretization, for each discrete incident angle The baseband data generated by the plane wave in the receiving array element domain is processed. Phase shift modulation, which modulates each discrete incident angle Phase shift of baseband data in the element domain caused by plane waves Linear phase modulation of the incident plane wave can be obtained by linear superposition.

[0055] like Figure 4 As shown, the outputs of the 91 angular domain bandpass filters are respectively compared with... Multiply, then linearly superimpose, and the output is That is, the topological load number is Linear phase spiral acoustic wave incident on Figure 1 The baseband signal of a snapshot array element domain data is output from a linear receiving array. Through linear phase modulation of the signal, the... The in-phase plane wave incident on the imaging fan is twisted into a linear-phase spiral wave, realizing linear phase modulation of the incident plane wave at the receiving end.

[0056] according to Figure 4 The plane wave linear phase modulation process, from the incident angle from to plane wave every The generated array element domain data was tested, with the amplitude of the incident plane wave remaining constant for a given topological charge number. The output snapshot data is a complex vector. Its conjugate complex vector is Then complex vector The L2 norm is The corresponding amplitude of the plane wave is For topological load number When the values ​​are 30, 50, and 70 respectively, through Figure 4 The signal processing flow shown performs linear phase modulation, and the output plane wave amplitude is as follows: Figure 5-1 , Figure 5-2 and Figure 5-3 As shown in the figure, the green dashed line represents the plane wave amplitude. and . Figure 5-1 , Figure 5-2 and Figure 5-3 This indicates that, from to A plane wave of constant amplitude incident on the imaging fan-shaped surface passes through, as... Figure 4 After linear phase modulation of the process shown, the amplitude fluctuation of the output plane wave is within 2dB.

[0057] Under the condition of constant input plane wave amplitude, the fluctuation of output plane wave amplitude after linear phase modulation was tested at different incident angles. Similarly, the relationship between the phase shift of the output plane wave relative to the input plane wave and the incident angle after linear phase modulation can be tested. For an incident angle of... plane wave A 2048-point inverse Fourier transform is performed on the snapshot data generated by the equally spaced linear receiving array to obtain its angular spectrum. The index of the maximum value of the angular spectrum and its corresponding phase shift are identified. Then, a 2048-point inverse Fourier transform is performed on the snapshot data output after linear phase modulation to obtain the output angular spectrum. The phase shift corresponding to the index of the peak value of the input angular spectrum is identified. The difference between the output and input plane wave phase shifts is the phase shift of the plane wave before and after linear phase modulation. Topological charge number The relationship between the phase shift of the plane wave and the incident angle of the sound wave was simulated and tested at angles of 1, 30, and 70, respectively. Figure 6-1 , Figure 6-2 and Figure 6-3 As shown, Figure 6-1 It is the topological load number This is a curve showing the relationship between the phase shift of the plane wave and the incident angle of the sound wave before and after linear phase modulation (time 1). The incident angle of the sound wave is from... Change to Plane wave phase shift from linear change to The green dashed line in the figure represents the phase shift of the plane wave. and . Figure 6-2 and Figure 6-3 These are the topological charge numbers. The curves show the relationship between the plane wave phase shift and the incident angle of the sound wave before and after linear phase modulation at 30° and 70°. The incident angle of the sound wave is from... Change to The phase shift of a plane wave also changes linearly; the green dashed line in the figure represents the phase shift of the plane wave. and As can be seen from Figures 5 and 6, after linear phase modulation, plane waves incident from all directions within the imaging sector can pass smoothly, the plane wave amplitude remains almost unchanged, the phase shift of plane waves incident from different directions is linearly proportional to the incident angle, and the phase shift of plane waves after higher-order linear phase modulation still maintains a good linear relationship with the incident angle.

[0058] from Figure 5-1 , Figure 5-2 and Figure 5-3 It can be seen that at the incident angle of the sound wave and The amplitude fluctuation of the output plane wave is larger in the vicinity than at the intermediate incident angle. Figure 6-1 , Figure 6-2 and Figure 6-3 This indicates that at the incident angle of the sound wave and Nearby, the phase shift of the plane wave deviates by and There exists a so-called endpoint effect. This endpoint effect can be overcome by designing a sector larger than the imaging sector. In the case of this application, this can be achieved by designing... and The imaging sector is used to overcome the endpoint effect.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application do not depart from the spirit and scope of the technical solutions of this application, and should all be covered within the scope of the claims of this application.

Claims

1. A linear phase modulation method based on a linear receiving array, characterized in that, The phase modulation method obtains the phase difference of the planar wave in the azimuth direction by linearly modulating a snapshot data after the transmitting transducer transmits and the linear receiving array receives the echo signal, so as to realize real-time underwater acoustic imaging of the static target and the moving target; The specific steps include: Step 1, the transmitting transducer transmits uniform sound waves with the same wave front phase, and the linear receiving array receives the echo signal; Step 2, filtering the echo signals, the filtering being through a single angular domain band-pass filter, dividing the imaging fan into equal imaging sub-fans; Step 3, multiply each filtered snapshot data with , to perform linear phase modulation; wherein, is the topological charge, , is the incident angle; Step 4, linearly superimposes the modulated snapshot data to obtain the spiral wave modulated by the planar wave incident from each direction.

2. The linear phase modulation method of claim 1, wherein, The topological charge number The range of values is to .

3. The linear phase modulation method of claim 2, wherein, The topological charge number values of 30, 50 and 70.

4. The linear phase modulation method of claim 1, wherein, The spiral wave has a linear phase with the incident angle.

5. The linear phase modulation method of claim 1, wherein, The number of the angular domain band-pass filters .

6. The linear phase modulation method of claim 1, wherein, The angular passband center angle range of the angular domain band-pass filter is -45°~45°, and the angular passband bandwidth range is 2.6°~3.7°.

7. The linear phase modulation method of claim 1, wherein, The imaging sector is symmetric about the normal line of the linear receiving array.

8. A linear phase modulation system based on a linear receiving array, characterized in that, It comprises: a transmitting transducer, which is used for transmitting equal-amplitude equal-phase wave front sound pulses; a receiving transducer array, which is a linear receiving array and is used for receiving echo signals; an array signal processor, which is used for processing the echo signals and comprises an angular domain band-pass filter and is configured to perform the linear phase modulation method according to claim 1.

9. The linear phase modulation system of claim 8, wherein, The angular domain band-pass filter is a matrix filter.

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