A MIMO-SAR signal transmission and echo separation method and device

Abstract

The present invention discloses a MIMO-SAR signal transmission and echo separation method and device, belonging to the field of signal processing technology. The method includes two processes: a signal transmission process, using a reversible coding matrix to perform preliminary phase encoding on the waveforms transmitted by antennas in different elevation directions, and again performing azimuth phase encoding on the waveforms transmitted by antennas in different azimuth directions between different transmission pulses; a signal receiving process, performing azimuth DBF on the aliased signal formed by the reception, preliminarily separating the aliased echoes in the azimuth frequency domain, and then compensating for the errors between the azimuth transmission channels, and finally using the inverse matrix of the coding matrix to multiply the preliminarily separated echo matrix to achieve phase decoding, so that the aliased echoes in the elevation direction can be effectively separated. This scheme can effectively separate the echoes of scenes with undulating terrain, and has no requirements for the orthogonality of signals transmitted by different antennas, and single-base waveforms can be directly applied.

Description

Technical Field

[0001] The present invention belongs to the technical field of signal processing, and in particular relates to a MIMO-SAR (Multiple-Input Multiple-Output Synthetic Aperture Radar) signal transmission and echo separation method and device. Background Art

[0002] As an active microwave radar, synthetic aperture radar can conduct ground observation under various adverse meteorological conditions, and is not affected by time and weather. High-resolution remote sensing images of the earth's surface obtained by transmitting microwave signals can provide data support and guarantee for tasks such as natural resource monitoring, geological disaster warning, geological exploration, maritime management, and land surveying and mapping. With the continuous expansion of demand for earth remote sensing applications (such as three-dimensional imaging, moving target detection), as well as wider surveying and mapping bands and higher resolutions, SAR needs to make full use of waveform and spatial resources to improve radar system performance.

[0003] The MIMO concept was first proposed in the field of wireless communications. The multi-antenna configuration can increase channel capacity while providing excellent noise and narrowband fading resistance. The MIMO concept has also been applied to the radar field, bringing about a huge improvement in radar performance. As an active imaging radar system, the multiple-input multiple-output synthetic aperture radar can fully configure the antenna's transmit / receive channels. Compared with traditional SAR systems, MIMO-SAR systems can obtain more degrees of freedom in elevation and azimuth, thereby breaking through the antenna configuration constraints of traditional SAR systems. This makes MIMO-SAR very attractive for high-resolution wide-band imaging, ground moving target detection, interferometric and tomographic scanning applications, and downward 3-D imaging. In addition, the MIMO-SAR system can adapt to multi-modal operation requirements and can provide opportunities to alleviate or even resolve the conflicting user needs to date by flexibly configuring system resources.

[0004] For multiple-input multiple-output synthetic aperture radar, the biggest technical challenge at present is to effectively separate the aliased echoes emitted by different transmitting antennas without losing imaging performance. Unfortunately, the most promising short-time offset orthogonal scheme and segmented phase coding scheme both require the use of elevation digital beamforming (DBF) technology to ensure echo separation, which greatly increases the complexity of the system. It not only requires sacrificing the degrees of freedom in the elevation direction, but also requires high-precision digital elevation information, which is not suitable for real-time generation. In addition, the undulating terrain will cause great interference to the estimation of the incoming wave direction, and it is impossible to repeatedly suppress the interference signal in the echo.

[0005] In summary, how to get rid of the pitch-axis digital beamforming technology and separate the signal transmitted by a single channel from the aliased echoes is a difficult problem that needs to be solved urgently. Summary of the invention

[0006] To solve the above technical problems, the present invention provides a MIMO-SAR signal transmission and echo separation method and device, which can effectively separate the echoes of terrain undulating scenes, and has no requirements on the orthogonality of signals transmitted by different antennas, and a single base waveform can be directly applied.

[0007] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0008] A MIMO-SAR signal transmission and echo separation method comprises the following steps:

[0009] Step 1: Use the coding matrix to perform preliminary phase coding on the waveforms transmitted from the antennas in different elevation directions;

[0010] Step 2: Perform azimuth phase encoding on the waveforms transmitted from antennas in different azimuths between different transmission pulses;

[0011] Step 3: Perform azimuth DBF on the aliased signal formed by the reception to preliminarily separate the aliased echoes in the azimuth frequency domain;

[0012] Step 4: For the echoes mixed in the azimuth frequency domain that are initially separated, compensate the error between the antenna transmission channels in the azimuth direction;

[0013] Step 5: using the inverse matrix of the encoding matrix and the aliased echo vector after error compensation between channels to achieve phase decoding;

[0014] Step 6: Extract the signal transmitted by a single channel from the decoded echo signal vector.

[0015] On the other hand, the present invention provides a MIMO-SAR signal transmission and echo separation device, comprising:

[0016] The phase encoding module is used to perform preliminary phase encoding on the waveforms transmitted by antennas in different elevation directions using a coding matrix; and to perform azimuth phase encoding on the waveforms transmitted by antennas in different azimuth directions between different transmission pulses;

[0017] The echo separation and compensation module is used to perform azimuth DBF on the aliased signals formed by the reception, and preliminarily separate the echoes aliased in the azimuth frequency domain; for the preliminarily separated echoes aliased in the azimuth frequency domain, compensate the error between the antenna transmission channels in the azimuth direction;

[0018] The phase decoding module is used to realize phase decoding by using the inverse matrix of the encoding matrix and the aliased echo vector after error compensation between channels; and extract the signal transmitted by a single channel from the decoded echo signal vector.

[0019] In a third aspect, the present invention provides an electronic device, comprising: one or more processors; a memory for storing one or more programs; wherein, when the one or more programs are executed by the one or more processors, the one or more processors implement the aforementioned MIMO-SAR signal transmission and echo separation method.

[0020] In a fourth aspect, the present invention provides a computer-readable storage medium having executable instructions stored thereon, which, when executed by a processor, enables the processor to implement the aforementioned MIMO-SAR signal transmission and echo separation method.

[0021] The beneficial effects of the present invention are:

[0022] The technical solution proposed by the present invention completely gets rid of the use of pitch DBF in echo separation, and the signal waveform used for encoding has no requirements on the orthogonality of the transmitted signal, and echo separation has no additional requirements on the number of antennas. The echo separation step hardly increases the complexity of the system, and can greatly reduce the deployment cost of the MIMO-SAR system. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a flow chart of a MIMO-SAR signal transmission and echo separation method of the present invention;

[0024] Figure 2 Schematic diagram of a point target signal transmitted at row 1 and column 1 and received at row 1 and column 1 in an embodiment;

[0025] Figure 3 It is a cross-sectional diagram of a point target signal transmitted at the 1st row and the 1st column and received at the 1st row and the 1st column in the embodiment;

[0026] Figure 4 Schematic diagram of a point target signal transmitted at the 2nd row and the 1st column and received at the 1st row and the 1st column in the embodiment;

[0027] Figure 5 This is a cross-sectional diagram of a point target signal transmitted at the 2nd row and 1st column and received at the 1st row and 1st column in the embodiment. DETAILED DESCRIPTION

[0028] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0029] Different from the common distributed MIMO radar system, the present invention focuses on the co-located MIMO radar system, whose TX channel and RX channel are much smaller than the distance of the far-field target. Limited by the spacecraft platform, the TX and RX channels of the single-base MIMO-SAR system usually share the same antenna array using a TX-RX switch. When receiving, each MIMO-SAR antenna will record the ground scattered echo (effective signal) emitted by its own TX channel and the scattered echo emitted by the remaining antennas, and the scattered echo emitted by the remaining antennas is considered to be an interference signal (an unfocused response to the main waveform of the transmission). Generally, different from the communication system, the interference energy widely distributed in the time and frequency domain will cause false distributed targets in the image, greatly losing the imaging performance of the system, and this energy cannot be ignored. Therefore, under the premise of not losing imaging performance, fully separating the echoes of different transmitting antennas is the first step in the realization of the MIMO-SAR system. At the same time, the waveform used by the MIMO-SAR system must be suitable for the real transmitter / receiver architecture. So far, there is no solution and available practical hardware that can achieve echo separation without sacrificing system resources. It can be seen that echo separation is the most challenging problem in building a MIMO-SAR system.

[0030] Based on this, Figure 1 As shown, the present invention provides a MIMO-SAR signal transmission and echo separation method. First, in the process of signal transmission, a reversible coding matrix is ​​used to perform preliminary phase coding on the waveforms transmitted by antennas in different elevation directions, and the waveforms transmitted by antennas in different azimuth directions are again phase-coded in azimuth between different transmission pulses. In the process of signal reception, azimuth DBF is performed on the aliased signal formed by reception, and the aliased echoes in the azimuth frequency domain are preliminarily separated, and then the errors between azimuth transmission channels are compensated, and finally the inverse matrix of the coding matrix is ​​multiplied with the preliminarily separated echo matrix to realize phase decoding, so that the aliased echoes in elevation direction can be effectively separated.

[0031] Specifically, the steps include:

[0032] Step 1: Use the coding matrix to perform preliminary phase coding on the waveforms transmitted from the antennas in different elevation directions;

[0033] Step 2: Perform azimuth phase encoding on the waveforms transmitted from antennas in different azimuths between different transmission pulses;

[0034] Step 3: Perform azimuth DBF on the aliased signal formed by the reception to preliminarily separate the aliased echoes in the azimuth frequency domain;

[0035] Step 4: For the echoes mixed in the azimuth frequency domain that are initially separated, compensate the error between the antenna transmission channels in the azimuth direction;

[0036] Step 5: using the inverse matrix of the encoding matrix and the aliased echo vector after error compensation between channels to achieve phase decoding;

[0037] Step 6: Extract the signal transmitted by a single channel from the decoded echo signal vector.

[0038] Furthermore, the step 1: using a reversible coding matrix to perform preliminary phase coding on the waveforms transmitted by antennas in different elevation directions includes expanding the radar waveform to be transmitted into a set of signal bases, which are expressed as:

[0039] (1)

[0040] in, represents the monobasic waveform used for coding, represents a vector consisting of a single base waveform, Represents the fastest time in distance direction. Represents the number of signals used for encoding, and also represents the dimension of the vector. Represents transposition. For a single-base waveform, the signal is first phase-coded modulated in the elevation direction, which can be expressed as:

[0041] (2)

[0042] in, Represents the signal matrix transmitted by each sub-antenna, It is a carefully designed phase matrix for encoding. The phase elements in the matrix are Represents the signal encoded for the signal transmitted by the transmitter in the mth row and nth column. It is worth noting that the phase coding matrix The elements in can be selected arbitrarily, as long as they satisfy the constructed phase matrix The reversible condition is sufficient.

[0043] Furthermore, in step 2, the waveforms transmitted by antennas in different azimuths are again azimuthally phase-encoded between different transmission pulses, including, in combination with inter-pulse modulation, multiplying the inter-pulse phase modulation factor by the transmission signal between each pulse, so as to realize azimuth phase encoding of the transmission waveform between different pulses of the SAR transmission signal. Specifically, the baseband signal transmitted by the sub-antenna in the mth row and nth column between the kth pulses is It can be expressed as:

[0044] (3)

[0045] in, , m, n and k , Represents a set of integers. represents the radiation waveform of the nth column and the pulse-to-pulse phase modulation factor of the kth PRI, represents the signal of the mth row and nth column transmitter after preliminary phase encoding. Assume that the pulse repetition frequency PRF of the system is defined as ,but Defined as:

[0046] (4)

[0047] Where I represents the number of waveforms that can be modulated by APC, j represents the imaginary unit, and PRI represents the pulse repetition interval, which is equal to .

[0048] Furthermore, in step 3, azimuth DBF is performed on the aliased signal formed by the reception, and the aliased echoes in the azimuth frequency domain are initially separated, including:

[0049] The echo is fast Fourier transformed in azimuth, and the slant angle-Doppler frequency of the echo obtained by transmitting the nth column of antennas in the lth Doppler subband is It can be expressed as:

[0050] (5)

[0051] represents the azimuth frequency, represents the wavelength of the signal, Represents the flight speed of MIMO-SAR, represents the frequency shift added to the nth column of transmitted signal echo in the range-Doppler frequency domain, and has:

[0052] (6)

[0053] Represents the coefficient of variation of the nth column of transmitted signal echo in the lth Doppler subband in the range-Doppler frequency domain. Similar to the DBF in the elevation direction, when there is an angle difference between the echoes, the interference signal can be eliminated by the zero-point suppression technology, thereby retaining the target signal. Therefore, the filter of the azimuth DBF based on the least squares method can be expressed as:

[0054] (7)

[0055] The superscript H indicates conjugate transpose, and the superscript -1 indicates inversion. is the azimuth subaperture array steering vector, representing the squint angle corresponding to the frequency deviation of each echo in the Doppler domain, and is:

[0056] (8)

[0057] in, Denotes the desired weighting for separating the Doppler subbands, and in order to separate the aliased echoes obtained by transmitting the nth column antenna, we have:

[0058] (9)

[0059] in, , ,and Indicates the number of receiving antennas in azimuth, the transmit-receive channel spacing and the operating wavelength, angle set contains useful signals ( ) and interference signals ( ), Represents the transmission channel number, Represents the receiving channel number, Represents the number of azimuth antennas, for The elements in for The superscript parameter in .

[0060] After azimuth DBF processing, the signal transmitted by the nth antenna can be preliminarily separated from the aliased echo. ,Right now:

[0061] (10)

[0062] Represents azimuth time. represents the demodulation phase matrix, which is the conjugate of the modulation matrix ( ). and They represent the azimuth Fourier transform and the inverse azimuth Fourier transform, respectively. Represents the aliased echo signal of the i-th row of receiving antennas in the l-th Doppler sub-band. Represents the set of Doppler subbands.

[0063] Furthermore, compensating the error between the azimuth transmission channels in step 4 includes:

[0064] Before performing elevation phase demodulation, it is necessary to compensate for the difference between the antenna's transmission channels in azimuth. Assuming that the target echo is located in the kth column, the echo after channel compensation is It is expressed as:

[0065] (11)

[0066] in, represents the aliased echo transmitted by the antenna in the nth column and received by the antenna in the i-th row and the b-th column, Represents the total length of the antenna in azimuth.

[0067] Furthermore, in step 5, the inverse matrix of the coding matrix is ​​finally multiplied by the preliminarily separated echo matrix to achieve phase decoding, which includes:

[0068] In order to further separate the aliased row signals, a phase encoding matrix is ​​required in the elevation direction. The inverse of , according to the least squares criterion, constructs the demodulation weight vector ,Right now:

[0069] (12)

[0070] have:

[0071] (13)

[0072] Where P is the expected vector matrix for echo separation, which can be expressed as:

[0073] (14)

[0074] The aliased echo vector after azimuth DBF processing and channel compensation Multiply it with the demodulation weight vector, that is:

[0075] (15)

[0076] Represents the demodulated signal vector without aliasing received by the i-row antenna.

[0077] Furthermore, separating the signal transmitted by a single channel from the aliased echo in step 6 includes:

[0078] After azimuth DBF and elevation decoding, the single-channel response transmitted by the mth row and nth column antenna and received by the ith row and kth column antenna can be reconstructed. , which can be expressed as:

[0079] (16)

[0080] in, , is the matrix The element in the kth column of .

[0081] Finally, the single channel response can be separated from the aliased echo, and in subsequent processing, the existing mature imaging algorithms (such as RD, CS, WK) can be used to restore the backscattering information of the illuminated scene.

[0082] Example

[0083] In order to verify the effectiveness of the invention, a set of waveform parameters is used for simulation verification in this embodiment. In this embodiment, there are two types of transmission signals, s1 is a positive frequency modulation signal, s2 is a negative frequency modulation signal, and two transmitting antennas are arranged in azimuth and two in elevation, for a total of four. The matrix for phase encoding the signal basis is:

[0084]

[0085] So the point target signal emitted by the first row and first column and received by the first row and first column is imaged by the CS algorithm as follows: Figure 2 As shown in the figure, the peak lobe ratio in the range direction is -13.2461dB, the integrated side lobe ratio is -9.9936 dB, and the resolution broadening coefficient is 0.9989. The peak lobe ratio in the azimuth direction is -13.2599dB, the integrated side lobe ratio is -9.9905dB, and the resolution broadening coefficient is 0.9981. The profile is shown in the figure. Figure 3 shown.

[0086] The result of imaging the point target signal emitted by the 2nd row and 1st column and received by the 1st row and 1st column through the CS algorithm is as follows: Figure 4 As shown in the figure, the peak lobe ratio in the range direction is -13.2467dB, the integrated side lobe ratio is -9.9741 dB, and the resolution broadening factor is 0.9983. The peak lobe ratio in the azimuth direction is -13.2565dB, the integrated side lobe ratio is -9.9659dB, and the resolution broadening factor is 0.9972. The profile is shown in the figure. Figure 5 shown.

[0087] In summary, this method can successfully separate aliased echoes, and various imaging indicators are consistent with theoretical indicators, which proves the correctness and practicality of this scheme.

[0088] On the other hand, the present invention provides a MIMO-SAR signal transmission and echo separation device, comprising:

[0089] The phase encoding module is used to perform preliminary phase encoding on the waveforms transmitted by antennas in different elevation directions using a coding matrix; and to perform azimuth phase encoding on the waveforms transmitted by antennas in different azimuth directions between different transmission pulses;

[0090] The echo separation and compensation module is used to perform azimuth DBF on the aliased signals formed by the reception, and preliminarily separate the echoes aliased in the azimuth frequency domain; for the preliminarily separated echoes aliased in the azimuth frequency domain, compensate the error between the antenna transmission channels in the azimuth direction;

[0091] The phase decoding module is used to realize phase decoding by using the inverse matrix of the encoding matrix and the aliased echo vector after error compensation between channels; and extract the signal transmitted by a single channel from the decoded echo signal vector.

[0092] In a third aspect, the present invention provides an electronic device, comprising: one or more processors; a memory for storing one or more programs; wherein, when the one or more programs are executed by the one or more processors, the one or more processors implement the aforementioned MIMO-SAR signal transmission and echo separation method.

[0093] In a fourth aspect, the present invention provides a computer-readable storage medium having executable instructions stored thereon, which, when executed by a processor, enables the processor to implement the aforementioned MIMO-SAR signal transmission and echo separation method.

[0094] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A MIMO-SAR signal transmission and echo separation method, characterized in that: The steps include: Step 1: Use the coding matrix to perform preliminary phase coding on the waveforms transmitted from the antennas in different elevation directions; Step 2: Perform azimuth phase encoding on the waveforms transmitted from antennas in different azimuths between different transmission pulses; Step 3: Perform azimuth DBF on the aliased signal formed by the reception to preliminarily separate the aliased echoes in the azimuth frequency domain; Step 4: For the echoes mixed in the azimuth frequency domain that are initially separated, compensate the error between the antenna transmission channels in the azimuth direction; Step 5: using the inverse matrix of the encoding matrix and the aliased echo vector after error compensation between channels to achieve phase decoding; Step 6: Extract the signal transmitted by a single channel from the decoded echo signal vector.

2. A MIMO-SAR signal transmission and echo separation method according to claim 1, characterized in that: The step 1 comprises: expanding the radar waveform to be transmitted into a set of signal bases, and when the SAR transmits the signal, performing preliminary phase encoding on the signal to be transmitted using a reversible phase encoding matrix.

3. A MIMO-SAR signal transmission and echo separation method according to claim 2, characterized in that: The step 2 includes, in combination with inter-pulse modulation, multiplying the inter-pulse phase modulation factor with the transmission signal between each pulse, so as to achieve azimuth phase encoding of the transmission waveform between different pulses of the SAR transmission signal.

4. A MIMO-SAR signal transmission and echo separation method according to claim 3, characterized in that: The step 3 includes, after receiving the aliased echo signal, performing azimuth DBF processing on the aliased echo signal in the range-Doppler domain to preliminarily separate the aliased echoes in the azimuth frequency domain.

5. A MIMO-SAR signal transmission and echo separation method according to claim 4, characterized in that: The step 4 includes compensating the echoes mixed in the azimuth frequency domain that are initially separated. Assuming that the target echo is located in the kth column, the echoes that have undergone channel compensation are It is expressed as: （11） in, represents the aliased echo transmitted by the antenna in the nth column and received by the antenna in the i-th row and the b-th column, and represent azimuth Fourier transform and inverse azimuth Fourier transform, respectively. Represents distance to fast time, represents the azimuth time, j represents the imaginary unit, k represents the pulse index, Represents the total length of the antenna in azimuth. represents the flight speed of MIMO-SAR, represents the azimuth frequency, Represents the number of azimuth antennas.

6. A MIMO-SAR signal transmission and echo separation method according to claim 5, characterized in that: The step 5 includes inverting the phase matrix used for transmit coding, constructing a demodulation weight vector according to the least squares criterion, and multiplying the demodulation weight vector with the aliased echo vector after inter-channel error compensation to achieve phase decoding.

7. A MIMO-SAR signal transmission and echo separation method according to claim 6, characterized in that: The signal transmitted by a single channel extracted from the decoded echo signal vector in step 6 includes the single channel response received by the antenna in the i-th row and the k-th column It is expressed as: （16） in, For the matrix The element in the kth column of represents the decoded, alias-free signal vector received by the i-row antenna.

8. A MIMO-SAR signal transmission and echo separation device, characterized in that: include: A phase encoding module is used to perform preliminary phase encoding on waveforms transmitted from antennas in different elevation directions using a coding matrix; Performing azimuth phase encoding on waveforms transmitted from antennas in different azimuths between different transmission pulses; The echo separation and compensation module is used to perform azimuth DBF on the aliased signals formed by reception, and preliminarily separate the aliased echoes in the azimuth frequency domain; For the echoes mixed in the azimuth frequency domain that are initially separated, the error between the antenna transmission channels in the azimuth direction is compensated; A phase decoding module, used to implement phase decoding by using the inverse matrix of the encoding matrix and the aliased echo vector after error compensation between channels; The signal transmitted by a single channel is extracted from the decoded echo signal vector.

9. An electronic device, characterized in that: include: one or more processors; A memory for storing one or more programs; When one or more programs are executed by the one or more processors, the one or more processors implement the MIMO-SAR signal transmission and echo separation method according to any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that: Executable instructions are stored thereon, and when the instructions are executed by the processor, the processor can implement the MIMO-SAR signal transmission and echo separation method described in any one of claims 1 to 7.

Images