CN109407055B - Beamforming method based on multipath utilization - Google Patents

Abstract

The invention discloses a beam forming method based on multipath utilization, which mainly solves the problem that multipath coherent signals cannot be effectively received in a low-altitude multipath environment in the prior art. The method comprises the following implementation steps: the array radar acquires echo signal data and solves a covariance matrix R of a received signal _xx (ii) a Conducting Toeplitz covariance matrix reconstruction on the received signal covariance matrix to obtain a new covariance matrix R; performing eigenvalue decomposition on the reconstructed covariance matrix to obtain a noise subspace E _N (ii) a Forming a spatial spectrum function S (theta) according to the noise subspace; according to the spatial spectrum function, finding out an angle corresponding to a peak point of a spectrum peak, namely an angle of an estimated target; and taking the estimated target angle as a beam direction to obtain beam forming. The method of the invention uses the estimated target angle as the beam direction to form the beam, effectively uses the multipath coherent information, improves the signal-to-multi-noise ratio of the echo signal, and can be used for effectively receiving the multipath coherent signal in the low-altitude multipath environment.

Description

Beamforming method based on multipath utilization

technical field

The invention belongs to the technical field of radar, and in particular relates to a beam forming method, which can be used for effectively receiving multipath coherent signals in a low-altitude multipath environment.

Background technique

When the radar is performing low-altitude detection, the echo signal not only includes the direct wave signal, but also includes the multipath wave signal. Under the influence of the multipath effect, the direct wave and the multipath wave are mutually coherent source signals, and the multipath coherent interference will cancel the desired signal, resulting in a sharp decline in the signal-to-noise ratio, and the performance of the traditional beamforming algorithm will decline rapidly or even fail. The effect seriously affects the detection performance of low-elevation-angle targets. In order to solve this problem, the multipath interference beamforming algorithm has been widely studied. At present, according to the different processing methods for multipath interference, it can be divided into multipath interference suppression beamforming algorithm and multipath signal receiving beamforming algorithm.

The multipath interference suppression beamforming algorithm mainly achieves effective reception of desired signals through decoherence processing or linear constraints. Among them, the typical decoherence processing algorithm is the spatial smoothing algorithm, which can effectively realize the decoherence processing, but sacrifices the effective aperture of the array and has poor robustness.

The multipath signal receiving beamforming algorithm is to jointly receive multipath signals, and the purpose is to make full use of multipath signal information. The joint reception of multipath signals is achieved by imposing the worst performance constraint on the desired signal and multipath interference, and it has good robustness, but the algorithm needs to estimate the direction of multipath interference, and there is a problem when the input SNR The problem of performance degradation and low output signal-to-noise ratio when increasing.

Contents of the invention

The purpose of the present invention is to address the deficiencies of the above-mentioned existing methods, and propose a method for radar beamforming based on multipath utilization in a low-altitude multipath environment, so as to effectively utilize multipath information for beamforming and improve the signal density of echo signals. noise ratio.

To achieve the above object, technical solutions of the present invention include as follows:

(1) The array radar receives the signal and obtains the echo data X, which includes the direct wave signal, multipath signal and noise;

(2) Calculate the covariance matrix R _xx according to the echo data X;

(3) Perform Toeplitz matrix reconstruction on the covariance matrix R _xx to obtain a new covariance matrix R:

3a) Take out the diagonal elements parallel to the main diagonal of the covariance matrix R _xx in turn, and calculate its average value r(-k):

Among them, N is the number of array elements, k=0,1,...,N-1;

3b) According to the average value r(-k), the covariance matrix R after reconstruction of the Toeplitz matrix is obtained:

(4) According to the reconstructed covariance matrix R, calculate the direction of arrival

4a) Perform eigenvalue decomposition on the reconstructed covariance matrix R to obtain the noise subspace E _N :

R＝E _S Σ _S E _S ^H +E _N Σ _N E _N ^H ,

Among them, ( ) ^H is the conjugate transpose operation, E _S refers to the signal subspace, Σ _S refers to the diagonal matrix composed of the large eigenvalues of R, and Σ _N refers to the diagonal matrix composed of the small eigenvalues of R;

4b) Project the search vector a(θ) to the noise subspace E _N , and calculate the spatial spectral function S(θ):

Among them, a(θ) represents the steering vector arriving from the direction of θ;

4c) Find the angle corresponding to the peak point of the spectral peak from the spatial spectral function S(θ), which is the direction of arrival

作为波束指向，得到波束形成y：(5) The target angle estimated in step (4)

As the beam pointing, the beamforming y is obtained:

表示从

方向到达的导向矢量。Among them, W is the weighting vector,

means from

The steering vector for the direction of arrival.

Compared with the prior art, the present invention has the following advantages:

1. The target search range is more accurate

The present invention greatly reduces the search range of the target by using the estimated direction-of-arrival angle of the target as the beam pointing, so that the search range of the target is more accurate.

2. Can effectively receive multipath coherent signals

In order to weaken the influence of multipath effect on beamforming, the traditional method adopts the method of suppressing multipath effect. The present invention makes full use of incoming wave information of multipath coherent signals, carries out covariance matrix reconstruction and MUSIC angle estimation on echo signals, uses estimated target angle as beam pointing, and performs beamforming, thus in the case of low-altitude multipath It can effectively solve the problem of multipath coherent signal reception, and has a wide range of applications.

Simulation results show that the present invention obviously improves the SNR of the echo signal.

Description of drawings

Fig. 1 is the realization flowchart of the present invention;

Fig. 2 is when signal-to-noise ratio is-10dB, simulates the angle estimation result figure after using Toeplitz matrix reconstruction decoherence in the present invention;

Fig. 3 is when the signal-to-noise ratio is -10dB, the directional gain comparison diagram of the simulated conventional beamforming method and the method of the present invention;

Fig. 4 is the method of using Toeplitz matrix reconstruction decoherence in the present invention to carry out DOA estimation to the target, and the result figure of the number of simulated estimated angles changing with the input signal-to-noise ratio;

Fig. 5 is a comparison chart of the simulation output SNR with the input SNR by using the conventional beamforming method, the adaptive beamforming method and the beamforming method of the present invention.

Detailed ways

With reference to Fig. 1, the concrete realization steps of the present invention are as follows:

Step 1: Obtain radar echo data.

Assuming that the receiving array is a uniform linear array with N array elements, the array element spacing d is half a wavelength, and the signal source is the synthesis of direct wave and multipath wave, then the array radar receives the signal, and the obtained echo data X includes the direct wave signal, multipath wave path signal and noise, which is expressed as follows:

为直达波与多径波的延迟相位，S为载波信号，P为高斯白噪声。Among them, Γ is the reflection coefficient of the ground or the sea surface, and its value is Γ=-1 under horizontal polarization and flat land and sea surface conditions, a(θ _t ) is the steering vector in the direction of the direct wave, a(θ _r ) Steering vectors for multipath wave directions,

is the delayed phase of the direct wave and the multipath wave, S is the carrier signal, and P is Gaussian white noise.

Step 2: According to the echo data X, calculate the covariance matrix R _xx :

R _xx =E[XX ^H ]

Among them, (·) ^H is the conjugate transpose operation, and E[·] is the mean value operation.

Step 3: Perform Toeplitz matrix reconstruction on the covariance matrix R _xx to obtain a new covariance matrix R.

3a) Take out the diagonal elements parallel to the main diagonal of the covariance matrix R _xx in turn, and calculate its average value r(-k):

Among them, N is the number of array elements, k=0,1,...,N-1;

3b) According to the conjugate inversion of the average value r(-k): r(-k)=r ^* (k), 2N-1 average values are calculated:

{r(-N+1),r(-N+2),...,r(0),...r(N-2),r(N-1)}

Among them, (·) ^* is the conjugate operation;

3c) According to the obtained 2N-1 average values, the covariance matrix R after reconstruction of the Toeplitz matrix is obtained:

Step 4: According to the reconstructed covariance matrix R, calculate the direction of arrival

4a) Perform eigenvalue decomposition on the reconstructed covariance matrix R to obtain the noise subspace E _N :

R＝E _S Σ _S E _S ^H +E _N Σ _N E _N ^H ,

Among them, E _S refers to the signal subspace, Σ _S refers to the diagonal matrix composed of large eigenvalues of R, and Σ _N refers to the diagonal matrix composed of small eigenvalues of R;

4b) Project the search vector a(θ) to the noise subspace E _N , and calculate the spatial spectral function S(θ):

其中d为阵元间距，λ为信号波长，(·)^T表示转置操作；where a(θ) represents the steering vector arriving from the direction θ,

Where d is the array element spacing, λ is the signal wavelength, ( ) ^T represents the transpose operation;

4c) Find the angle corresponding to the peak point of the spectral peak from the spatial spectral function S(θ), which is the direction of arrival

Complete radar low-altitude target coherent source DOA estimation.

作为波束指向，得到波束形成y。Step 5: Estimate the target angle in step (4)

As beam pointing, beamforming y is obtained.

得到

方向到达的导向矢量

5a) According to the estimated target angle

get

Steering vector for direction arrival

Among them, d is the array element spacing, λ is the signal wavelength, ( ) ^T represents the transpose operation;

5b) Obtain the beamforming output y:

Beamforming is to use a beam of a certain shape to pass useful signals or signals that require directions, and to suppress interference that does not require directions. By weighting and summing the outputs of each array element, the antenna array beam can be "steered" to within a period of time In one direction, the beamforming output y can be obtained:

y = W ^H X,

Wherein, W is an array weighting vector;

作为波束指向，使

得到波束形成y：5c) will

as beam pointing so that

Get beamforming y:

Among them, (·) ^H is the conjugate transpose operation.

The effects of the present invention can be further verified by the following simulation experiments.

1. Experimental scene:

Taking a uniform linear array with 32 array elements as an example for simulation, the array element spacing is half wavelength, the carrier frequency is f ₀ =4GHz, the antenna height is 29m, the target height is 142m, the target distance is 10Km, and the bandwidth is 50MHz, the distance difference between the multipath wave signal and the direct wave signal is 0.8236m, the reflection coefficient amplitude is 0.9, the phase is 160°, the angle of arrival of the direct wave signal is 0.6475°, the angle of arrival of the multipath wave signal is -0.9797° The incoming wave angle difference between the wave signal and the multipath wave signal is 1.6272°.

2. Experimental content and analysis of experimental results:

In experiment 1, under the condition that the number of snapshots is 256 and the signal-to-noise ratio is -10dB, the target direction of arrival is estimated by Toeplitz covariance matrix reconstruction and MUSIC spectrum estimation method, and the results are shown in Figure 2; The estimated target angle is used as beam pointing to perform beamforming, and the comparison chart of directional gain between the conventional beamforming method and the beamforming method of the present invention is simulated, and the result is shown in FIG. 3 .

It can be seen from Figures 2 and 3 that the estimated angle is estimated to be 0.044°, compared with the direct wave angle of 0.6475° and the multipath wave angle of -0.9797°, the angle after decoherence is closer to the direct wave angle; compared with the conventional beamforming method In comparison, the present invention uses the estimated target angle as the beam pointing to perform beamforming, so that it has a higher directional gain in the target direction and can receive direct wave signals better.

In experiment 2, under the condition that the number of snapshots is 1024, the input signal-to-noise ratio ranges from -25dB to 10dB, and the step size is 2dB, the direction of arrival of the target is estimated by Toeplitz covariance matrix reconstruction and MUSIC spectrum estimation method. Carry out 100 Monte-Carlo simulation experiments, simulate and estimate the change of the number of angles with the input signal-to-noise ratio, and the results are shown in Figure 4.

It can be seen from Figure 4 that when the input signal-to-noise ratio ranges from -25dB to -14dB, the estimated angle after reconstruction and decoherence through the Toeplitz matrix is between the angle of arrival of the direct wave signal and the angle of arrival of the multipath wave signal, and The number is 1. When the input signal-to-noise ratio ranges from -14dB to 10dB, the estimated angle after reconstruction and decoherence through the Toeplitz matrix is between the angle of arrival of the direct wave signal and the angle of arrival of the multipath wave signal, and the number is 2.

Experiment 3, under the condition that the number of snapshots is 1024, the range of input signal-to-noise ratio is -25dB to 10dB, and the step size is 2dB, conventional beamforming, adaptive beamforming and the multipath beamforming method of the present invention are used respectively , the simulated output SNR varies with the input SNR, and Monte-Carlo simulation experiments are carried out 100 times, and the results are shown in Fig. 5 .

It can be seen from FIG. 5 that the output signal-to-noise ratio performance of the beamforming method of the present invention increases with the increase of the signal-to-noise ratio. Compared with conventional beamforming and adaptive beamforming methods, the output SNR of the beamforming method of the present invention is higher than that of the former two methods.

Claims (3)

1. A beamforming method based on multipath utilization, comprising the following: (1) The array radar receives the signal and obtains the echo data X, which includes the direct wave signal, multipath signal and noise; (2) Calculate the covariance matrix R _xx according to the echo data X; (3) Perform Toeplitz matrix reconstruction on the covariance matrix R _xx to obtain a new covariance matrix R: 3a) Take out the diagonal elements parallel to the main diagonal of the covariance matrix R _xx in turn, and calculate its average value r(-k):

Among them, N is the number of array elements, k=0,1,...,N-1; 3b) According to the average value r(-k), the covariance matrix R after reconstruction of the Toeplitz matrix is obtained:

(4) According to the reconstructed covariance matrix R, calculate the direction of arrival

4a) Perform eigenvalue decomposition on the reconstructed covariance matrix R to obtain the noise subspace E _N : R＝E _S Σ _S E _S ^H +E _N Σ _N E _N ^H , Among them, (·) ^H is the conjugate transpose operation, E _S refers to the signal subspace, Σ _S refers to the large eigenvalue matrix of R, and Σ _N refers to the small eigenvalue matrix of R; 4b) Project the search vector a(θ) to the noise subspace E _N , and calculate the spatial spectral function S(θ):

Among them, a(θ) represents the steering vector arriving from the direction of θ; 4c) Find the angle corresponding to the peak point of the spectral peak from the spatial spectral function S(θ), which is the direction of arrival

(5) The direction of arrival estimated in step (4)

As the beam pointing, the beamforming y is obtained:

Among them, W is the weighting vector,

means from

The steering vector for the direction of arrival. 2. The method according to claim 1, wherein the echo data X obtained by the receiving array in (1) is expressed as follows:

Among them, Γ is the reflection coefficient of the ground or the sea surface, and its value is Γ=-1 under the conditions of horizontal polarization and flat land and sea surface, a(θ _t ), a(θ _r ) are the direct wave direction and multiple Steering vector in radial wave direction,

is the delayed phase of the direct wave and the multipath wave, S is the carrier signal, and P is Gaussian white noise. 3. The method according to claim 1, wherein the covariance matrix R _xx obtained in the step (2) is expressed as follows: R _xx =E[XX ^H ] Among them, E[·] is the mean value operation.

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