CN110398718B - Radio frequency interference suppression method based on FRFT frequency estimation subspace - Google Patents
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Abstract
The invention provides a radio frequency interference suppression method based on an FRFT frequency estimation subspace. Specifically, calculating the optimal order of FRFT; calculating the FRFT of a frame of original data under the optimal order through a kernel function to obtain an original signal of an FRFT frequency domain; detecting radio frequency interference of the FRFT frequency domain signal; if the FRFT frequency domain signal has radio frequency interference, obtaining an FRFT time domain signal through inverse Fourier transform; constructing an orthogonal projection matrix by using a FRFT frequency estimation subspace method, and carrying out projection transformation on FRFT time domain signals to inhibit radio frequency interference; performing fast Fourier transform to obtain an FRFT frequency domain signal after radio frequency interference suppression; and carrying out p-order inverse FRFT conversion on the FRFT frequency domain signal to obtain an original signal subjected to radio frequency interference suppression. The method can simultaneously inhibit the steady-state radio frequency interference and the unsteady-state radio frequency interference, can keep the integrity of useful signals on the basis of fully inhibiting the radio frequency interference, increases the effective detection distance of the radar, and improves the accuracy of the radar in ocean dynamics parameter inversion and target detection.
Description
Technical Field
The invention belongs to the field of radar signal processing, and particularly relates to a radio frequency interference suppression method based on an FRFT frequency estimation subspace.
Background
The high-frequency ground wave radar acquires the distribution information of ocean surface dynamic parameters (wind, wave, current and the like) in a non-contact mode, and is important detection equipment applied to ocean radar environment monitoring. Compared with the traditional fixed-point and calibration observation, the high-frequency ground wave radar has the advantages of all weather and wide coverage area, and can obtain complete kinetic parameter space-time change information of the observed sea area. Compared with satellite observation, the method has the advantages of time and space resolution, and is not influenced by weather factors such as rain and fog. China has vast ocean territory, and high-frequency radar remote sensing can provide a feasible technical detection means for maritime economic activities, navigation safety and national ocean equity maintenance.
The high-frequency ground wave radar works in a short wave band (3-30 MHz), and a large amount of radio frequency interference existing in the frequency band is one of main factors influencing the working performance of the high-frequency ground wave radar. The radio frequency interference comprises man-made interference such as short wave communication, broadcasting stations and other non-cooperative high frequency radar signals, and natural interference such as cosmic noise, thunder and lightning. Compared with a target and sea surface scattered signal, the radio frequency interference energy is usually higher by several orders of magnitude, and the radio frequency interference energy enters a radar receiver and is mixed with a useful signal, so that the effective detection distance of the radar is reduced, the detection precision of the radar is reduced, and the receiver is overloaded in severe cases, so that the radar cannot work normally.
The currently proposed radio frequency interference suppression algorithm is mainly divided into: based on the self-adaptive beam forming algorithm (based on the characteristic that the incoming wave direction of the radio frequency interference is basically kept unchanged), the orthogonal projection algorithm (based on the correlation of the radio frequency interference in distance and direction), and the time domain detection zero setting, then restoring the algorithm by interpolation or prediction. The adaptive beamforming algorithm has a poor effect of suppressing the radio frequency interference with time varying incoming wave direction, and cannot suppress the radio frequency interference located in the main lobe. Orthogonal projection algorithms based on distance coherence generally need to obtain a plurality of distance unit signals without sea clutter as training matrixes, however, for high-frequency ground wave radar observed in a short distance, the training matrixes are difficult to obtain, and the algorithms generally have large attenuation on useful signals. However, the time domain detection zero-setting algorithm is often unfamiliar with the continuous occurrence of interference.
Fractional fourier transform (FRFT) is a generalized form of fourier transform and can be viewed as chirp orthogonal basis decomposition. FRFT can be understood as rotation of a time-frequency two-dimensional plane, has good aggregation effect on linear frequency modulation signals under the optimal order (corresponding to a rotation angle), and is widely applied to the fields of quantum mechanics, optics and signal processing.
Disclosure of Invention
The present invention has been made in view of the above problems, A method for suppressing radio frequency interference of a high-frequency ground wave radar based on an FRFT frequency estimation subspace is provided. The method is not affected by disturbance of the direction of an interference incoming wave, can inhibit the interference to the maximum extent, simultaneously avoids attenuation of effective signals, and provides a quick and effective radio frequency interference resisting method for the existing high-frequency ground wave radar system, thereby increasing the radar detection distance and improving the radar detection precision.
The technical scheme of the invention is a radio frequency interference suppression method based on FRFT frequency estimation subspace, which comprises the following steps:
in the step 1, the method comprises the following steps of, calculating the optimal order of FRFT;
in the step 3, the step of, detecting radio frequency interference of the FRFT frequency domain signal;
step 7, performing p-order inverse FRFT conversion on the FRFT frequency domain signal subjected to the radio frequency interference suppression to obtain an original signal subjected to the radio frequency interference suppression;
preferably, the optimal order of calculating FRFT in step 1 is:
wherein p is the optimal order of FRFT, f s Representing the sampling frequency, and B representing the sweep frequency bandwidth of the radar;
preferably, the one-frame original signal in step 2 is x (t);
in step 2, the FRFT frequency domain signal is:
wherein, K p (t, u) is a kernel function,
α =2p/π represents the rotation angle of the time-frequency two-dimensional plane, u represents the FRFT frequency;
preferably, the detecting the radio frequency interference of the FRFT frequency domain signal in step 3 is:
for signals transformed to FRFT frequency domain, i.e. X p (u) taking dB value:
X pd (u)＝20·lg(abs[X p (u)])
wherein abs [. Cndot ] represents the amplitude, the median amplitude A:
A＝median[X pd (u)]
wherein mean [. Cndot. ] represents the median magnitude. Setting a proper threshold k based on A, wherein the threshold k is 10-15dB larger than A in general;
if the amplitude of the original signal converted to the FRFT frequency domain is not higher than the threshold value, judging that the FRFT frequency domain signal is not interfered;
if the amplitude of the FRFT frequency domain signal is higher than a threshold value k, judging that the FRFT frequency domain signal has radio frequency interference;
preferably, the step 4 of obtaining the FRFT time-domain signal by inverse fourier transform is:
x p (v)＝IFT[X p (u)]
wherein IFT [. Cndot. ] represents an inverse Fourier transform;
preferably, the step 5 of constructing the orthogonal projection matrix by using the FRFT frequency estimation subspace method specifically includes:
firstly, an orthogonal projection matrix H is constructed p ：
H p ＝I-D(D H D) -1 D H
Wherein, [ ·] -1 Representing a matrix inversion operation; [. The] H Conjugate transpose of a representation matrixCalculating;
D＝[d 1 ,d 2 ,…,d n ]FRFT frequency domain vector d representing radio frequency interference i Formed matrix, d i Expressed as:
wherein,
f p,i represents the frequency of the radio frequency interference corresponding to the ith spectral point in the FRFT frequency domain, round [ ·]Expressing rounding, wherein M is the number of sampling points of one sweep frequency;
due to the influence of sampling intervals in the FRFT discretization calculation process, the radio frequency interference cannot be represented as a single peak in an FRFT frequency domain, but occupies a plurality of frequency units and can be obtained by searching spectral peaks with the amplitude higher than a threshold value k;
the step 5 of suppressing the radio frequency interference of the FRFT frequency domain signal specifically includes:
the data vector after the radio frequency interference suppression is:
x′ p ＝H p x p
wherein x is p ＝[x p (1),x p (2),…,x p (M)] T
Preferably, the FRFT frequency domain signal after the radio frequency interference suppression in step 6 is:
X′ p (u)＝FFT[x′ p (v)]
wherein FFT [. Cndot. ] represents a fast Fourier transform;
preferably, in step 7, the original signal after the radio frequency interference suppression is:
wherein,
in step 3, radio frequency interference is detected by using a fixed threshold method. Because the radio frequency interference energy is very strong, and after the radio frequency interference energy is subjected to the FRFT conversion of the optimal order, the radio frequency interference energy is gathered into a plurality of peaks, the amplitude of the peaks is usually higher than that of sea clutter and background noise by a plurality of orders of magnitude, a dB value is taken for an FRFT frequency domain signal, the median thereof is used as a threshold reference, and a proper threshold range is set. And regarding the signals which are not higher than the threshold, the radio frequency interference does not exist, and the subsequent processing is not carried out. Otherwise, the interference is considered to exist, and the step 4 is carried out.
In step 3, the frequency of the spectrum point corresponding to the radio frequency interference is obtained by searching the peak of which the FRFT frequency domain is higher than the threshold value.
In step 5, these frequency points are used to construct an orthogonal projection matrix, and then the original signal transformed to the FRFT transform time domain by one inverse FFT is projected on the matrix, so that the radio frequency interference is suppressed and the loss of effective signals can be avoided.
Compared with the prior art, the invention has the advantages that:
the radio frequency interference suppression method based on the FRFT frequency estimation subspace can gather the radio frequency interference through the FRFT, and the radio frequency interference can be effectively suppressed on the premise that the original target signal and the sea clutter signal are reserved to the maximum extent. This can be done also for interference and signal superposition, which most radio frequency interference algorithms cannot achieve.
The radio frequency interference suppression method based on the FRFT frequency estimation subspace has the advantages that the suppression performance is not influenced by the incoming wave direction and frequency change of radio frequency interference and is not limited by antenna array elements, the application range is wide, and the performance is stable.
Drawings
FIG. 1: working principle block diagram of high frequency ground wave radar;
FIG. 2: obtaining a three-dimensional data block through two times of FFT;
FIG. 3: the radio frequency interference enters a radar receiver, is mixed with a local oscillation signal and is filtered in a low-pass mode;
FIG. 4: the method of the invention is a flow chart;
FIG. 5: a distance-Doppler spectrum comparison graph before the suppression of original signals containing radio frequency interference;
FIG. 6: a distance-Doppler spectrum comparison graph after the original signal containing the radio frequency interference is suppressed;
FIG. 7 is a schematic view of: doppler spectrograms before and after the 3 rd distance element signal radio frequency interference suppression;
FIG. 8: doppler spectrogram before and after 15 th range element signal radio frequency interference suppression.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
The working principle block diagram of the high-frequency ground wave radar is shown in fig. 1 (1 is a receiving antenna, 2 is a receiving signal, 3 is a local oscillator signal, 4 represents mixer mixing, 5 represents low-pass filtering, 6 is a/D sampling, 7 represents solution distance, and 8 represents solution doppler): after the received signals of each channel (L channels) are mixed with the local oscillation signals, the baseband signals are obtained through low-pass filtering high-frequency components and grating lobes. Sampling the baseband signal for M times to obtain a corresponding fast time domain signal, and performing discrete Fourier transform (FFT) solution distance (each frequency point corresponds to a distance element) on the fast time domain signal. And after coherent accumulation of N sweep periods, obtaining a time sequence with the length of N corresponding to each distance element. And performing FFT on the slow time domain signal corresponding to each range bin to obtain the Doppler spectrum of the range bin. This process is performed for all range bins within the detection range, resulting in a range-doppler spectrum. As shown in fig. 2, the three-dimensional data block is obtained by two FFTs, wherein 1 represents an array element, 2 represents a pulse, and 3 represents a range gate.
Before entering the radar receiver, the radio frequency interference may be equivalent to a superposition of several single frequency signals:
I i (t)＝∑A i (t)exp[j2πf i t+jφ i ]
wherein A is i (t)、f i And phi i Respectively representing the amplitude, frequency, and initial phase of the ith interference.
FIG. 3 shows an RF interference entering a radar receiver and a local oscillator signal, wherein the carrier frequency is f 0 The chirp slope is k, the mixing and low pass filtering are shown schematically. After low-pass filtering, the radio frequency interference becomes a linear frequency modulation signal with the same modulation frequency as the local oscillator. The radio frequency interference only appears in a specific time of one frequency sweeping period, and the expression is as follows:
I io (t)＝A i (t)exp[j2πΔft+jπkt 2 +jφ i ],t 1 ≤t≤t 2
wherein Δ f = (f) i -f 0 ) Is the difference between the radio frequency interference and the radar carrier frequency;
and
respectively the start and end times within one sweep after the radio frequency interference enters the receiver.
The FRFT transform has an aggregation effect on the chirp signals, and the aggregation effect is the best at the best order, theoretically, a single spectral line, while the sea clutter and the target signals are broadened. The invention utilizes the characteristic of FRFT transformation and combines a method for estimating subspace based on FRFT frequency to construct an orthogonal projection matrix to inhibit radio frequency interference.
The following describes an embodiment of the present invention with reference to fig. 1 to 8, which is a method for rf interference suppression based on FRFT frequency estimation subspace, including the following steps:
the optimal order of calculating the FRFT in step 1 is:
wherein p is the optimal order of FRFT, f s Representing the sampling frequency, and B representing the sweep frequency bandwidth of the radar;
f in actually measured selected radar parameters s For 1988Hz, B for 30KHz, p was calculated to be-0.0421.
in step 2, the frame of original signal is x (t);
in step 2, the FRFT frequency domain signal is:
wherein, K p (t, u) is a kernel function,
α =2p/π represents the rotation angle of the time-frequency two-dimensional plane, u represents the FRFT frequency;
for signals transformed to FRFT frequency domain, i.e. X p (u) taking dB value:
X pd (u)＝20·lg(abs[X p (u)])
wherein abs [. Cndot ] represents the amplitude of the solution, solving for the median amplitude A:
A＝median[X pd (u)]
wherein mean [. Cndot. ] represents the median magnitude. Setting a proper threshold k based on A, wherein the threshold k is 10-15dB larger than A in general;
if the amplitude of the original signal converted to the FRFT frequency domain is not higher than the threshold value, judging that the FRFT frequency domain signal is not interfered;
if the amplitude of the FRFT frequency domain signal is higher than a threshold value k, judging that the FRFT frequency domain signal has radio frequency interference;
the step 4 of obtaining the FRFT time domain signal through inverse Fourier transform is as follows:
x p (v)＝IFT[X p (u)]
wherein IFT [. Cndot. ] represents an inverse Fourier transform;
firstly, an orthogonal projection matrix H is constructed p ：
H p ＝I-D(D H D) -1 D H
Wherein, [ ·] -1 Representing a matrix inversion operation; [. For] H Representing a conjugate transpose operation of a matrix;
D＝[d 1 ,d 2 ,…,d n ]FRFT frequency domain vector d representing radio frequency interference i Formed matrix, d i Expressed as:
wherein,
f p,i represents the frequency of the radio frequency interference corresponding to the ith spectral point in the FRFT frequency domain, round [ ·]Expressing rounding, wherein M is the number of sampling points of one sweep frequency;
due to the influence of sampling intervals in the FRFT discretization calculation process, the radio frequency interference cannot be represented as a single peak in an FRFT frequency domain, but occupies a plurality of frequency units and can be obtained by searching spectral peaks with the amplitude higher than a threshold k;
the step 5 of suppressing the radio frequency interference of the FRFT frequency domain signal specifically includes:
the data vector after the radio frequency interference suppression is:
x′ p ＝H p x p
wherein x is p ＝[x p (1),x p (2),…,x p (M)] T
the FRFT frequency domain signal after the radio frequency interference suppression in step 6 is:
X′ p (u)＝FFT[x′ p (v)]
wherein FFT [. Cndot. ] represents a fast Fourier transform;
step 7, performing p-order inverse FRFT conversion on the FRFT frequency domain signal subjected to the radio frequency interference suppression in the step 6 to obtain an original signal subjected to the radio frequency interference suppression;
the original signal after the radio frequency interference suppression in step 7 is:
wherein,
fig. 5 is a range-doppler spectrum before the radio frequency interference suppression, and fig. 6 is a range-doppler spectrum after the radio frequency interference suppression, in which many strip-shaped radio frequency interferences are greatly suppressed and the signal-to-noise ratio is improved.
FIG. 7 is a 3 rd range bin Doppler spectrum after RFI suppression, i.e., a short range bin Doppler spectrum; FIG. 8 is a 15 th range bin Doppler spectrum, i.e., a long range bin Doppler spectrum. Therefore, the interference intensity is obviously weakened and the signal-to-noise ratio is improved at-0.5 Hz, 0.7Hz and the like which are concentrated in radio frequency interference.
It should be understood that parts of the specification not set forth in detail are well within the prior art.
It should be understood that the above description of the preferred embodiments is given for clarity and not for any purpose of limitation, and that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (5)
1. A radio frequency interference suppression method based on FRFT frequency estimation subspace is characterized by comprising the following steps:
step 1: calculating the optimal order of FRFT;
step 2: calculating the FRFT of a frame of original data under the optimal order through a kernel function to obtain an original signal of an FRFT frequency domain;
and step 3: detecting radio frequency interference of the FRFT frequency domain signal;
and 4, step 4: if the FRFT frequency domain signal has radio frequency interference, obtaining an FRFT time domain signal through inverse Fourier transform;
and 5: constructing an orthogonal projection matrix by using a FRFT frequency estimation subspace method to inhibit radio frequency interference of FRFT frequency domain signals;
step 6: performing fast Fourier transform on the FRFT time domain signal subjected to the radio frequency interference suppression to obtain an FRFT frequency domain signal subjected to the radio frequency interference suppression;
and 7: performing p-order inverse FRFT conversion on the FRFT frequency domain signal subjected to the radio frequency interference suppression to obtain an original signal subjected to the radio frequency interference suppression;
the step 3 of detecting the radio frequency interference of the FRFT frequency domain signal comprises the following steps:
for signals transformed to FRFT frequency domain, i.e. X p (u) taking dB value:
X pd (u)＝20·lg(abs[X p (u)])
wherein abs [. Cndot ] represents the amplitude, the median amplitude A:
A＝median[X pd (u)]
wherein, mean [. Cndot. ] represents the median amplitude, and an appropriate threshold k is set by taking A as a reference, and the threshold k is usually 10-15dB greater than A;
if the amplitude of the original signal transformed to the FRFT frequency domain is not higher than the threshold value, judging that the FRFT frequency domain signal is not interfered;
if the amplitude of the FRFT frequency domain signal is higher than a threshold value k, judging that the FRFT frequency domain signal has radio frequency interference;
the step 4 of obtaining the FRFT time domain signal through inverse Fourier transform is as follows:
x p (v)＝IFT[X p (u)]
wherein IFT [. Cndot. ] represents an inverse Fourier transform;
step 5, the construction of the orthogonal projection matrix by using the FRFT frequency estimation subspace method specifically comprises the following steps:
firstly, an orthogonal projection matrix H is constructed p ：
H p ＝I-D(D H D) -1 D H
Wherein, [ ·] -1 Representing a matrix inversion operation; [. The] H Representing a conjugate transpose operation of a matrix;
D＝[d 1 ，d 2 ，...，d n ]FRFT frequency domain vector d representing radio frequency interference i A matrix of formations, d i Expressed as:
wherein,
f p，i represents the frequency of the radio frequency interference corresponding to the ith spectral point in the FRFT frequency domain, round [ ·]Expressing rounding, wherein M is the number of sampling points of one sweep frequency;
due to the influence of sampling intervals in the FRFT discretization calculation process, the radio frequency interference cannot be represented as a single peak in an FRFT frequency domain, but occupies a plurality of frequency units and can be obtained by searching spectral peaks with the amplitude higher than a threshold value k;
the step 5 of suppressing the radio frequency interference of the FRFT frequency domain signal specifically includes:
the data vector after the radio frequency interference suppression is:
x′ p ＝H p x p
wherein x is p ＝[x p (1)，x p (2)，…，x p (M)] T 。
2. The FRFT frequency estimation subspace-based radio frequency interference suppression method of claim 1, wherein:
the optimal order of calculating the FRFT in step 1 is:
wherein p is the optimal order of FRFT, f s Representing the sampling frequency and B representing the swept bandwidth of the radar.
3. The FRFT-frequency estimation subspace-based radio frequency interference suppression method of claim 1, wherein:
in step 2, the frame of original signal is x (t);
in step 2, the FRFT frequency domain signal is:
wherein, K p (t, u) is a kernel function,
α =2p/π represents the rotation angle of the time-frequency two-dimensional plane, u denotes the FRFT frequency.
4. The FRFT-frequency estimation subspace-based radio frequency interference suppression method of claim 1, wherein:
the FRFT frequency domain signal after the radio frequency interference suppression in step 6 is:
X′ p (u)＝FFT[x′ p (v)]
in the formula, FFT [. Cndot. ] represents a fast Fourier transform.
5. The FRFT-frequency estimation subspace-based radio frequency interference suppression method of claim 1, wherein:
the original signal after the radio frequency interference suppression in step 7 is:
wherein,
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| CN117784033B (en) * | 2023-12-28 | 2024-07-23 | 北京航空航天大学 | Vehicle radar interference filtering method based on fractional Fourier transform |
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