CN113655443A - Low-frequency band SAR radio frequency interference suppression method for broadband digital television signal - Google Patents

Low-frequency band SAR radio frequency interference suppression method for broadband digital television signal Download PDF

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CN113655443A
CN113655443A CN202110988151.2A CN202110988151A CN113655443A CN 113655443 A CN113655443 A CN 113655443A CN 202110988151 A CN202110988151 A CN 202110988151A CN 113655443 A CN113655443 A CN 113655443A
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frequency
interference
signal
sub
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田卫明
谢鑫
胡程
邓云开
董锡超
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Beijing Institute of Technology BIT
Chongqing Innovation Center of Beijing University of Technology
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Beijing Institute of Technology BIT
Chongqing Innovation Center of Beijing University of Technology
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/023—Interference mitigation, e.g. reducing or avoiding non-intentional interference with other HF-transmitters, base station transmitters for mobile communication or other radar systems, e.g. using electro-magnetic interference [EMI] reduction techniques
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/02—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S13/00
    • G01S7/36—Means for anti-jamming, e.g. ECCM, i.e. electronic counter-counter measures

Abstract

The invention discloses a low-frequency band SAR radio frequency interference suppression method aiming at broadband digital television signals, which comprises the following steps: step one, estimating interference dominant frequency; step two, frequency spectrum segmentation; step three, sub-band characteristic subspace decomposition; step four, splicing frequency spectrums; the method can effectively inhibit the broadband interference of the digital television signal type.

Description

Low-frequency band SAR radio frequency interference suppression method for broadband digital television signal
Technical Field
The invention belongs to the technical field of synthetic aperture radars, and particularly relates to a low-frequency-band SAR radio frequency interference suppression method for broadband digital television signals.
Background
The low-frequency band synthetic aperture radar has low central frequency and is usually positioned in a P wave band, and electromagnetic waves in the wave band can penetrate through leaf clusters and shallow earth surface to reconnaissance and detect hidden targets. Therefore, the low-frequency-band synthetic aperture radar becomes an important reconnaissance tool in the modern military field, and can play an important role in the civil fields of environmental resource reconnaissance, topographic mapping and the like.
A large number of signals of broadcasting, television, various mobile communications and the like exist in the working Frequency band of the low-Frequency synthetic aperture radar, and these signals enter a radar receiver along with echo signals of targets in a scene, so that serious Radio Frequency Interference (RFI) is caused. The power of the radio frequency interference, especially the broadband interference of a digital television signal type, is usually much higher than that of the target echo, so that the scene target echo information is annihilated, and the imaging quality of the low-frequency-band synthetic aperture radar is seriously affected. Therefore, effective suppression of radio frequency interference is an important prerequisite for normal operation of a low-frequency-band synthetic aperture radar system.
Since the beginning of the study of radio frequency interference in the 90 s of the 20 th century, radio frequency interference has been modeled basically as interference in the form of a narrow band, i.e. a superposition of a plurality of sinusoidal signals of different amplitudes, frequencies, initial phases. The modeling mode for the broadband interference is mainly to assume that the interference signals in the broadband form are chirp signals and sinusoidal modulation signals, and the broadband interference in the two forms can be converted into narrowband interference suppression in an instantaneous frequency spectrum, so that the suppression is performed by a narrowband interference suppression method. For wideband interference of digital television signal types, model establishment and suppression method research are lacked.
The broadband interference in the form of digital television signals is represented as a wider strip in a time-frequency domain, and the broadband interference still exists in the instantaneous frequency spectrum of the broadband interference, so that the traditional narrowband interference suppression method has a limited effect on the broadband interference of the digital television type. The wideband interference of the digital television signal type is continuous in the time domain, and thus will last for the entire sampling duration after entering the radar receiver and will be present in each azimuth sampled signal. Therefore, there is a need to provide an effective interference suppression method for radio frequency broadband interference of the digital television signal type.
Disclosure of Invention
In view of this, the present invention provides a method for suppressing low-frequency band SAR radio frequency interference for wideband digital television signals, which can effectively suppress wideband interference of the type of the digital television signals.
The technical scheme for realizing the invention is as follows:
the low-frequency band SAR radio frequency interference suppression method aiming at the broadband digital television signal comprises the following steps:
estimating an interference signal from an original echo signal by using a characteristic subspace decomposition method, and performing frequency estimation on the estimated interference signal;
secondly, performing range-direction FFT on the original echo signal to obtain a range frequency domain spectrum of the original echo signal; according to the position of the estimated main frequency of the interference signal in the frequency spectrum, carrying out frequency spectrum segmentation on the distance frequency domain frequency spectrum to obtain frequency spectrums of a plurality of sub-bands;
performing distance-oriented IFFT on the divided sub-band frequency spectrum to obtain sub-band echo data, estimating an interference signal of the sub-band echo data by using a characteristic sub-space decomposition method for the sub-band echo data, and subtracting the interference signal of the sub-band echo data from the sub-band echo data to obtain the sub-band echo data subjected to interference suppression processing;
performing distance direction FFT on the sub-band echo data after the interference suppression processing to obtain a sub-band frequency spectrum, and performing frequency domain splicing processing to obtain a complete broadband signal frequency spectrum after the interference suppression; and performing range-wise IFFT on the obtained complete broadband signal spectrum after the interference suppression to obtain an echo signal after the interference suppression.
Further, the estimating of the interference signal by using the feature subspace decomposition method for the original echo signal in the step one specifically includes:
an original echo signal matrix is obtained from an original echo signal, eigenvalue decomposition is carried out on a covariance matrix constructed by the original echo signal matrix, an interference subspace is constructed through main eigenvalues and corresponding eigenvectors thereof, then the original echo signal matrix is projected to the interference subspace, and estimation of a separated interference signal can be obtained.
Further, the frequency estimation method in the step one adopts a method of estimating signal parameters by a rotation invariant technology.
Further, in the fourth step, a wideband synthesis method under a chirp step frequency system is adopted as the method for splicing the sub-band frequency spectrum after the interference suppression processing, including but not limited to a frequency domain splicing method.
Furthermore, the operations in the first step to the fourth step are not only suitable for the ultra-wideband chirp signals, but also suitable for the chirp step frequency signals, and can be applied to each sub-band for processing and then performing wideband synthesis, and can also be applied to performing interference suppression processing on the wideband signals after the wideband synthesis.
Has the advantages that:
the invention provides a low-frequency-band synthetic aperture radar radio frequency interference suppression method for broadband digital television signals, which is based on a characteristic subspace decomposition method, and is used for detecting positions with interference in broadband frequency spectrum by using a frequency estimation method and then implementing effective segmentation to obtain more accurate interference signal estimation and improve the interference suppression effect. The method can be applied to the situation that only broadband digital television signal interference exists, and has universality on the situation that both broadband interference and narrowband interference exist simultaneously.
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FIG. 1 is a flow chart of the present invention.
Detailed Description
The invention is described in detail below by way of example with reference to the accompanying drawings.
The invention provides a low-frequency band SAR radio frequency interference suppression method aiming at broadband digital television signals, which comprises the following steps as shown in figure 1:
s1 estimating interference dominant frequency
Suppose the number of distance sampling points of the original echo signal is NrThe original echo data at any azimuth time is expressed as a vector
Figure BDA0003231491580000041
For echo vector
Figure BDA0003231491580000042
Performing time sliding recording with length L to obtain M L-dimensional subvectors, wherein M is Nr-L +1, representing the k-th subvector as Xk＝[x(k),x(k+1),···,x(k+L-1)]TThen, the M subvectors are arranged into an L × M echo data matrix X, namely:
Figure BDA0003231491580000043
l is the dimension of the autocorrelation matrix, which is moderate in value. If the value of L is too small, the characteristic value is not sufficiently decomposed, so that the frequency resolution is too low, the loss of useful signals can be caused during data projection, and the main lobe of an imaging result is reduced and raised; if the value of L is too large, on one hand, the calculation amount is increased, and on the other hand, if L is too large, which means that M is small, the number of samples of the autocorrelation matrix used for estimation is reduced, which causes a statistical error of the autocorrelation matrix, thereby reducing the estimation accuracy of the interference subspace and affecting the effect of interference suppression. In the actual data processing process, the value of L needs to satisfy the estimation condition M is more than or equal to 2L, when N isrWhen the value is larger, taking the calculation amount and the estimation precision into consideration, the L is generally 128; when N is presentrSmaller, usually according to NrIs selected, the amount of computation and the estimation accuracy are taken into account, L is usually 2m. Wherein m is NrThe exponent of the higher power of 2 is reduced by 2, and the autocorrelation matrix is constructed from the obtained echo data matrix:
Figure BDA0003231491580000051
then to
Figure BDA0003231491580000052
And (3) carrying out characteristic value decomposition:
Figure BDA0003231491580000053
wherein Λ ═ diag (λ)1,λ2,…,λL) L eigenvalues are arranged in descending order of λ1＞λ2＞…＞λLU is a feature vector U consisting of L correspondences1,U2,...,ULFormed matrix, i.e. U ═ U1,U2,...,UL]。
Then r (r < L) obvious large eigenvalues are found out from the L eigenvalues, and the corresponding eigenvectors are expanded into an interference subspace:
J＝span{U1,U2,...,Ur},1≤r＜L
then, each sub-vector X in the echo data matrix X is processedk(k is more than or equal to 1 and less than or equal to M) is projected to an interference subspace, so that the estimation of an interference signal can be obtained
Figure BDA0003231491580000054
The method for estimating the signal parameters based on the rotation invariant technology estimates the frequency of the interference signal:
if the frequency of each interference signal is used to construct matrix A
Figure BDA0003231491580000055
The matrix C can be found such that J ═ AC, C is a non-singular matrix.
The two subspaces of A and J are constructed separately as follows:
Figure BDA0003231491580000056
A2＝A1d, wherein
Figure BDA0003231491580000057
Is represented by A2,A1The rotation operator in between.
The diagonal elements of the diagonal matrix are eigenvalues, and the corresponding frequencies of the eigenvalues are just the interference signal frequencies to be estimated.
At this time A2,A1Unknown, J2,J1Are known. Then there is
J2＝[0 IL-1]J＝[0 IL-1]AC
＝A2C＝A1DC＝[IL-1 0]ADC
＝[IL-1 0]JC-1DC＝J1C-1DC
Let phi become C-1DC, Φ can be regarded as J2,J1The rotation operator in between. And Φ and D are similar matrices and thus their eigenvalues are the same.
J2＝J1Φ, at this point, the least squares solution of Φ is solved:
Figure BDA0003231491580000061
calculating its characteristic value
Figure BDA0003231491580000062
The frequencies corresponding to these eigenvalues are exactly the interference signal frequencies to be estimated. Extracting the phase of the characteristic value, wherein the phase is distributed in [ -pi, pi [ -pi [ ]]Which needs to be converted to correspond to the frequency of the interfering signal.
S2, spectrum segmentation: and performing range-direction FFT on the obtained radar echo signal to obtain a frequency spectrum of a range frequency domain of the radar echo signal. According to the position of the estimated main frequency of the interference signal in the distance frequency spectrum, the distance frequency domain frequency spectrum is subjected to frequency spectrum division to obtain the frequency spectrums of a plurality of sub-bands, and the purpose is to reduce broadband interference in one sub-band as much as possible, so that the accuracy of main eigenvalue judgment is improved, and an interference subspace is constructed more accurately. The distance direction points of the sub-band frequency spectrums can be consistent or different, namely, a uniform division mode or a non-uniform division mode can be adopted.
S3, sub-band characteristic subspace decomposition: and performing range-oriented IFFT on each obtained sub-band to obtain echo data of the sub-band. According to the method described in S1, a feature subspace decomposition is performed for each subband to obtain an estimate of the interference data. Subtracting the interference number from the raw echo dataAccordingly, the echo data after removing the interference can be obtained as
Figure BDA0003231491580000063
Dividing M subvectors
Figure BDA0003231491580000064
The echo data matrix obtained by the permutation is:
Figure BDA0003231491580000065
last pair of
Figure BDA0003231491580000066
The signal vector after interference suppression can be obtained by performing data rearrangement, namely performing the reverse process of sliding recording
Figure BDA0003231491580000071
Observing the echo data matrix X, it can be seen that the matrix elements equal to the row-column coordinates and the row-column coordinates are echo data at the same distance and time, that is, the elements on the secondary inverse diagonal, and averaging the echo data to obtain the echo value of each distance to the sampling point, wherein the expression is as follows:
Figure BDA0003231491580000072
s4, spectrum splicing: and performing distance direction FFT on the sub-band echo data after the interference suppression processing to obtain a sub-band frequency spectrum, and performing frequency domain splicing processing to obtain a complete broadband signal frequency spectrum after the interference suppression. In the embodiment of the invention, the frequency domain splicing method is only used for obtaining the complete broadband frequency spectrum signal after interference suppression, and the broadband synthesis operation can be finished by referring to the broadband synthesis method under other linear frequency modulation stepped frequency systems to obtain the broadband signal frequency spectrum.
And performing range-wise IFFT on the obtained complete broadband signal spectrum after the interference suppression to obtain an echo signal after the interference suppression.
The process of the invention is not only suitable for the ultra-wideband linear frequency modulation signal, but also suitable for the linear frequency modulation step frequency signal, can be applied to each sub-band for processing and then carrying out wideband synthesis, and can also be applied to the interference suppression processing on the wideband signal after the wideband synthesis.
The process of the invention can be applied to the situation that only broadband digital television signal interference exists, and has universality to the situation that broadband interference and narrow-band interference exist simultaneously.
In summary, the above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims (5)

1. A low-frequency band SAR radio frequency interference suppression method for broadband digital television signals is characterized by comprising the following steps:
estimating an interference signal from an original echo signal by using a characteristic subspace decomposition method, and performing frequency estimation on the estimated interference signal;
secondly, performing range-direction FFT on the original echo signal to obtain a range frequency domain spectrum of the original echo signal; according to the position of the estimated main frequency of the interference signal in the frequency spectrum, carrying out frequency spectrum segmentation on the distance frequency domain frequency spectrum to obtain frequency spectrums of a plurality of sub-bands;
performing distance-oriented IFFT on the divided sub-band frequency spectrum to obtain sub-band echo data, estimating an interference signal of the sub-band echo data by using a characteristic sub-space decomposition method for the sub-band echo data, and subtracting the interference signal of the sub-band echo data from the sub-band echo data to obtain the sub-band echo data subjected to interference suppression processing;
performing distance direction FFT on the sub-band echo data after the interference suppression processing to obtain a sub-band frequency spectrum, and performing frequency domain splicing processing to obtain a complete broadband signal frequency spectrum after the interference suppression; and performing range-wise IFFT on the obtained complete broadband signal spectrum after the interference suppression to obtain an echo signal after the interference suppression.
2. The method for suppressing low-frequency SAR radio frequency interference for wideband digital television signals according to claim 1, wherein the method for estimating the interference signal by using feature subspace decomposition for the original echo signal in the step one specifically comprises:
an original echo signal matrix is obtained from an original echo signal, eigenvalue decomposition is carried out on a covariance matrix constructed by the original echo signal matrix, an interference subspace is constructed through main eigenvalues and corresponding eigenvectors thereof, then the original echo signal matrix is projected to the interference subspace, and estimation of a separated interference signal can be obtained.
3. The method for suppressing low-frequency SAR radio frequency interference for wideband digital television signals as claimed in claim 1, wherein the frequency estimation method in step one employs a method of estimating signal parameters by a rotation invariant technique.
4. The method for suppressing low-frequency SAR radio frequency interference for wideband digital television signals according to claim 1, wherein the splicing method for the sub-band spectrum after the interference suppression processing in step four employs a wideband synthesis method under a chirp step frequency system, including but not limited to a frequency domain splicing method.
5. The method for suppressing low-frequency-band SAR radio frequency interference for wideband digital television signals according to claim 1, wherein the operations of the first to fourth steps are not only suitable for ultra-wideband chirp signals, but also suitable for chirp step frequency signals, and can be applied to each sub-band for processing respectively and then performing wideband synthesis, and also can be applied to perform interference suppression processing on wideband signals after wideband synthesis.
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