Synthetic aperture imaging method for orthogonal frequency division multiplexing signals based on backward projection

Abstract

The invention discloses a synthetic aperture imaging method of an orthogonal frequency division multiplexing signal based on backward projection, and belongs to the technical field of radar imaging. According to the method, in the process of carrying out synthetic aperture imaging by utilizing OFDM signals, firstly, range compression is carried out, range direction compression is carried out by adopting a matched filtering thought, then range direction interpolation is carried out, range direction interpolation is carried out by adopting a frequency domain zero filling thought, then an imaging area is divided, the distance from each azimuth moment to a radar grid point is calculated, and the distance between each azimuth moment and the radar grid point is calculated. And finally, phase compensation and coherent superposition are carried out on the data at each azimuth moment, and the spatial position of the target in the imaging area is reconstructed point by point. According to the method, each pixel point in the imaging area can be accurately calculated, the spatial positions of all targets in the imaging area are reconstructed, the imaging precision is improved, the image quality is improved, and the good imaging effect is achieved under the near-field condition.

Description

Technical Field

[0001] The invention belongs to the technical field of radar imaging, and in particular relates to a synthetic aperture imaging method of an orthogonal frequency division multiplexing signal based on back projection. Background Art

[0002] Synthetic aperture radar (SAR) is a technology that uses radar systems to obtain high-resolution images. It is widely used in fields such as earth observation, military reconnaissance, meteorological monitoring and disaster warning. Its core principle is to synthesize a "virtual antenna" that is larger than the actual antenna through the movement of the radar antenna relative to the target, thereby improving the resolution of the radar. It can achieve all-day and all-weather regional monitoring imaging. In recent years, with the development of information technology, wireless communication and radar detection have become similar in frequency band usage, hardware architecture and signal processing. Taking into account spectrum efficiency and cost-effectiveness, the demand for radar communication integration technology continues to grow.

[0003] Orthogonal frequency division multiplexing transmits data by decomposing broadband signals into multiple orthogonal narrowband subcarriers. It has the advantages of strong anti-multipath interference capability, high spectrum utilization, anti-inter-symbol interference (ISI), and easy integration with multiple-input multiple-output (MIMO) systems. This enables OFDM to provide efficient and reliable transmission in complex wireless channel environments and is widely used in wireless communication technology. Therefore, OFDM signals have become one of the best candidate signals for radar communication integration. When using OFDM signals for synthetic aperture imaging, the range Doppler algorithm (RDA) is generally used to implement it. The range Doppler algorithm has insufficient imaging accuracy and resolution in some application scenarios that require higher imaging accuracy and higher resolution. RDA is based on the far-field assumption, and the distance transformation between the target and the radar can be represented by a linear model. In near-field imaging, the target's distance change rate is large. The assumption of RDA does not hold under near-field conditions and is not suitable for near-field imaging. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and provide a synthetic aperture imaging method of orthogonal frequency division multiplexing signal based on back projection, which is a high-precision and high-resolution imaging method based on back projection and has good imaging effect under near-field conditions. In the process of synthetic aperture imaging using OFDM signals, firstly, distance compression is performed, and the idea of ​​matched filtering is used to perform distance compression, and then distance interpolation is performed, and the idea of ​​frequency domain zero filling is used to perform distance interpolation, and then the imaging area is divided, the distance to the radar grid point at each azimuth moment is calculated, the corresponding data is found, and finally the data at each azimuth moment is phase compensated and coherently superimposed, and the spatial position of the target in the imaging area is reconstructed point by point, which is an accurate method for synthetic aperture imaging using OFDM signals.

[0005] The technical problem proposed by the present invention is solved in this way:

[0006] A synthetic aperture imaging method for orthogonal frequency division multiplexing signals based on back projection comprises the following steps:

[0007] Step 1: After removing the center carrier frequency of the echo data of SAR imaging based on OFDM signal, perform range Fourier transform; perform Fourier transform on the range reference signal s0(τ); perform conjugate multiplication of the two Fourier transformed signals to obtain the range compression frequency domain signal S r (f τ , t);

[0008] Step 2: Signal S r (f τ , t) perform distance interpolation and inverse Fourier transform to obtain the time domain signal s after distance interpolation r (τ, t);

[0009] Step 3, select the imaging area, set the center of the imaging area, the imaging distance width and the imaging azimuth width, and divide the imaging area into grids;

[0010] Step 4: For each azimuth moment, calculate the distance R between the radar platform and the grid point in the i-th row and j-th column. ij , calculate the round trip delay The values ​​of i and j can traverse all grids in the imaging area; for the grid in the i-th row and j-th column, the time domain signal s collected at the current azimuth time r Find the delay t in (τ, t) ij data;

[0011] Step 5: At the current azimuth moment, phase compensation is performed on the data found in each grid. The phase compensation amount Then coherent superposition is performed as the imaging result.

[0012] Furthermore, the specific process of step 1 is:

[0013] The range reference signal s0(τ) is expressed as:

[0014]

[0015] Among them, w r is the distance window function, τ is the fast time variable, R min is the shortest distance between the imaging area and the radar, c is the speed of light, N is the number of OFDM signal subcarriers, 0≤k≤N-1, S k is the information transmitted by the kth subcarrier, j is the imaginary symbol, T is the radar pulse width;

[0016] Echo data of SAR imaging based on OFDM signal r (τ, t) is expressed as:

[0017]

[0018] Among them, w a is the azimuth envelope window function, t is the slow time variable, g m is the target reflection coefficient, f c is the carrier frequency, v is the speed of the radar platform, R0 is the shortest distance between the radar and the target, t c is the beam center deviation time;

[0019] After removing the center carrier frequency, s r (τ, t) becomes:

[0020]

[0021] Respectively r (τ, t) and s0(τ) are transformed to the distance domain and conjugate multiplied to obtain the frequency domain signal S after distance compression. r (f τ , t):

[0022]

[0023] Among them, W r (f τ ) is the frequency domain expression of the distance window function, f τ is a fast time frequency domain variable, sinc is the sinc function, 0≤n≤N-1, S n The information transmitted by the nth subcarrier.

[0024] Furthermore, the specific implementation process of step 2 is:

[0025] The frequency domain signal S after distance compression r (f τ , t) is filled with zeros, and then the inverse Fourier transform is performed to obtain the time domain signal s after range interpolation r (τ, t);

[0026] S k Use 1 and -1 for encoding. r (f τ , t) Lieutenant General Denoted as a constant, the interpolated time domain signal s r (τ, t) is expressed as:

[0027]

[0028] Among them, p r is the distance impulse response amplitude function, and is the sinc function.

[0029] Furthermore, in step 5, for any grid point position (x0, y0) in the imaging area, the imaging result after coherent superposition is expressed as I(x0, y0):

[0030]

[0031] Where M is the number of pulses emitted during SAR imaging.

[0032] The beneficial effects of the present invention are:

[0033] The method described in the present invention can accurately calculate each pixel point in the imaging area, reconstruct the spatial position of all targets in the imaging area, improve imaging accuracy, improve image quality, obtain images with higher accuracy and higher resolution, and also have good imaging effects under near-field conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 It is a schematic diagram of the process of the method of the present invention;

[0035] Figure 2 It is a single point target imaging diagram when the distance interpolation value is 32 times in the method described in the embodiment;

[0036] Figure 3 It is a distance slice diagram of a single point target when the distance interpolation value is 32 times in the method described in the embodiment;

[0037] Figure 4 It is a azimuth slice diagram of a single point target when the range interpolation value is 32 times in the method described in the embodiment;

[0038] Figure 5 A distribution diagram of multiple target points in the method described in the embodiment;

[0039] Figure 6 This is a multi-point target imaging diagram when the range interpolation value is 32 times in the method described in the embodiment. DETAILED DESCRIPTION

[0040] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0041] This embodiment provides a synthetic aperture imaging method based on back-projection orthogonal frequency division multiplexing signals, and its flow chart is as follows: Figure 1 As shown, the following steps are included:

[0042] Step 1: SAR imaging echo data s based on OFDM signal r After removing the center carrier frequency from (τ, η), Fourier transform is performed in the range direction; Fourier transform is performed on the range reference signal s0(τ), and conjugate multiplication is performed to obtain the range compression signal S r (f τ , t);

[0043] Radar reference signal s0(τ):

[0044]

[0045] Among them, w r is the distance window function, τ is the fast time variable, R min is the shortest distance between the imaging area and the radar, c is the speed of light, N is the number of OFDM signal subcarriers, 0≤k≤N-1, S k is the information transmitted by the kth subcarrier, j is the imaginary symbol, T is the radar pulse width;

[0046] Echo data of SAR imaging based on OFDM signal r (τ, t) is expressed as:

[0047]

[0048] where w a is the azimuth envelope window function, t is the slow time variable, g m is the target reflection coefficient, f c is the carrier frequency, v is the speed of the radar platform, R0 is the shortest distance between the radar and the target, t c is the beam center deviation time;

[0049] After removing the center carrier frequency, s r (τ, t) becomes:

[0050]

[0051] Respectively r (τ, t) and s0(τ) are transformed to the distance domain and conjugate multiplied to obtain the frequency domain signal S after distance compression. r (f τ , t):

[0052]

[0053] Among them, W r (f τ ) is the frequency domain expression of the distance window function, fτ is a fast time frequency domain variable, sinc is the sinc function, 0≤n≤N-1, S n The information transmitted by the nth subcarrier.

[0054] Step 2: Frequency domain signal S r (f τ , t) perform distance interpolation;

[0055] Step 2 is implemented as follows:

[0056] The frequency domain signal S after distance compression r (f τ , t) is filled with zeros, and then the inverse Fourier transform is performed to obtain the time domain signal s after range interpolation r (τ, t);

[0057] S k Use 1 and -1 for encoding. It can be approximated as a constant, and the time domain signal s after distance interpolation r (τ, t) is expressed as:

[0058]

[0059] Among them, p r is the distance impulse response amplitude function, and is the sinc function.

[0060] Step 3, select the imaging area, set the center of the imaging area, the imaging range width and the imaging azimuth width, divide the imaging area into grids, each grid corresponds to a pixel in the SAR image;

[0061] Step 4: For each azimuth moment, calculate the distance R between the radar platform and the grid point in the i-th row and j-th column. ij , and calculate the round trip delay The values ​​of i and j can traverse all grids in the imaging area; for the grid in the i-th row and j-th column, the time domain signal s collected at the current azimuth time r Find the delay t in (τ, t) ij Due to the limitation of sampling rate, according to the delay t ij It is not always possible to find the corresponding data, but only to approximate the critical value. r (τ, t) is interpolated to estimate the critical value to improve the accuracy;

[0062] Step 5: At the current azimuth moment, phase compensation is performed on the data found in each grid. The phase compensation amount Then coherent superposition is performed as the imaging result.

[0063] For any grid point (x0, y0) in the imaging area, the imaging result after coherent superposition is expressed as I(x0, y0):

[0064]

[0065] Where M is the number of pulses emitted during SAR imaging.

[0066] The method of the present invention reconstructs the spatial positions of all targets in the imaging area by distance compression, distance interpolation, dividing the imaging area into grid points, calculating the distance between each azimuth moment and the pixel point, calculating the two-way time delay, finding the data of the corresponding distance in the data of the current azimuth moment, and then performing phase compensation and coherent superposition. Each of the present invention only involves Fourier transform and time delay calculation, so the implementation is also very simple.

[0067] In the method described in this embodiment, the specific settings of the simulation parameters are as follows: the near-field imaging mode of the linear OFDM SAR is adopted, the distance between the radar and the center of the scene is 100m, the radar movement speed is 20m / s, the central carrier frequency of the transmitted signal is 26GHz, the range interpolation multiple is 32 times, and the signal bandwidth is 250MHz.

[0068] Figure 2 This is an imaging diagram of a single point target when the range interpolation is 32 times in the method described in this embodiment. The range interpolation multiple is 32 times. The data found during back projection is highly accurate and the spatial position of the point target is accurately reconstructed. The slant distance from the center of the scene is 100m. Under near-field conditions, the imaging effect is good.

[0069] Figure 3 For the single-point target range slice when the range interpolation is 32 times, the range interpolation is 32 times, and the accuracy of finding the corresponding data during back projection is high; Figure 4 This is a single point target azimuth slice when the range interpolation value is 32 times in the method described in this embodiment. The point target range slice and azimuth slice PSLR and ISLR are shown in Table 1:

[0070] Table 1 PSLR and ISLR table of point target range slice and azimuth slice

[0071]

[0072] From the data in the table, it can be seen that the imaging method has good resolution.

[0073] Figure 5 This is a multi-point target distribution map, showing the distribution of 9 point targets in the center of the scene.

[0074] Figure 6This is a multi-point target imaging image when the range interpolation is 32 times, and the slant distance of the scene center is 100m. The spatial position of each point is accurately reconstructed. By using range interpolation, the data accuracy is increased. The slant distance of the scene center is 100m, and high-precision and high-resolution imaging can be achieved under near-field conditions.

[0075] In summary, judging from the processing results, the method provided by the present invention has practical value.

Claims

1. A synthetic aperture imaging method for orthogonal frequency division multiplexing signals based on back projection, characterized in that: The following steps are involved: Step 1: After removing the center carrier frequency of the echo data of SAR imaging based on OFDM signal, perform range Fourier transform; perform Fourier transform on the range reference signal s0(τ); perform conjugate multiplication of the two Fourier transformed signals to obtain the range frequency domain azimuth time domain signal S after range compression r (f τ , t); Step 2: Signal S r (f c , t) perform range interpolation and inverse Fourier transform to obtain the time domain signal s after range interpolation r (τ, t); Step 3, select the imaging area, set the center of the imaging area, the imaging distance width and the imaging azimuth width, and divide the imaging area into grids; Step 4: For each azimuth moment, calculate the distance R between the radar platform and the grid point in the i-th row and j-th column. ij , calculate the round trip delay The values ​​of i and j can traverse all grids in the imaging area; for the grid in the i-th row and j-th column, the time domain signal s collected at the current azimuth time r Find the delay t in (τ, t) ij data; Step 5: At the current azimuth moment, phase compensation is performed on the data found in each grid. The phase compensation amount Then coherent superposition is performed as the imaging result.

2. The synthetic aperture imaging method of orthogonal frequency division multiplexing signal based on back projection according to claim 1, characterized in that: The specific process of step 1 is: The range reference signal s0(τ) is expressed as: Among them, w r is the distance window function, t is the fast time variable, R min is the shortest distance between the imaging area and the radar, c is the speed of light, N is the number of OFDM signal subcarriers, 0≤k≤N-1, S k is the information transmitted by the kth subcarrier, j is the imaginary symbol, T is the radar pulse width; Echo data of SAR imaging based on OFDM signal r (τ, t) is expressed as: Among them, w a is the azimuth envelope window function, t is the slow time variable, g m is the target reflection coefficient, f c is the carrier frequency, v is the speed of the radar platform, R0 is the shortest distance between the radar and the target, t c is the beam center deviation time; After removing the center carrier frequency, s r (τ, t) becomes: Respectively r (τ, t) and s0(τ) are transformed to the distance domain and conjugate multiplied to obtain the frequency domain signal S after distance compression. r (f τ , t): Among them, W r (f τ ) is the frequency domain expression of the distance window function, f τ is a fast time frequency domain variable, sinc is the sinc function, 0≤n≤N-1, S n The information transmitted by the nth subcarrier.

3. The synthetic aperture imaging method of orthogonal frequency division multiplexing signal based on back projection according to claim 2, characterized in that: The specific implementation process of step 2 is: The frequency domain signal S after distance compression r (f τ , t) is filled with zeros, and then the inverse Fourier transform is performed to obtain the time domain signal s after range interpolation r (τ, t); S k Use 1 and -1 for encoding. r (f τ , t) Lieutenant General Denoted as a constant, the time domain signal s after distance interpolation r (τ, t) is expressed as: Among them, p r is the distance impulse response amplitude function, and is the sinc function.

4. The synthetic aperture imaging method of orthogonal frequency division multiplexing signal based on back projection according to claim 3, characterized in that: In step 5, for any grid point position (x0, y0) in the imaging area, the imaging result after coherent superposition is expressed as I(x0, y0): Where M is the number of pulses emitted during SAR imaging.

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