Study On Adaptive Beamforming For Multipath Signal Utilization

As an important part of array signal processing technology,adaptive beamforming technology has developed rapidly and widely used in military and national economic fields such as radar,communication,medical treatment,electronic countermeasures,and geological exploration.Compared with traditional beamforming methods,adaptive beamforming technology has good spatial resolution,high processing gain,and interference suppression capabilities.In practice,the adaptive beamforming system needs to deal with various complex situations,among which the influence of the widespread multipath phenomenon on the adaptive beamforming algorithm cannot be underestimated.When the desired signal has multipath propagation,the performance of the optimal adaptive beamforming algorithm for ordinary non-coherent signals drops sharply or even fails.The algorithms such as de-coherence,matrix reconstruction,and super-resolution estimation with excellent theoretical performance are suitable for multipath environments,but they still have engineering application defects such as energy loss of multipath signals,aperture reduction,complex structure,and large amount of calculation.Aiming at the problems of expected signal cancellation and multipath signal energy loss faced by existing classical adaptive beamforming methods in multipath environment,this paper relies on the National Natural Science Foundation of China to develop adaptive beamforming for multipath signal utilization method research.The main research work and innovation points of the paper are summarized as follows:1.Aiming at the difficulty of estimating the desired signal in the practical application of Minimum Mean Square Error(MMSE)beamformer,an adaptive beamforming algorithm using subarray cross-correlation multipath is proposed.In the case where the direct wave angle is known,through blocking preprocessing and Minimum Variance Distortionless Response(MVDR)beamforming in uniformly divided sub-arrays,the desired signal is obtained with noise,and used as the desired signal of another subarray.Utilizing the uncorrelated characteristics of the noise between sub-arrays,sub-array beamforming is performed based on the MMSE criterion,and the sub-beams are synthesized to obtain the beamforming weight vector of the full array.Theoretical analysis and simulation results show that the algorithm can solve the problem of adaptive beamforming desired signal cancellation in multipath environment,and can simultaneously receive the signals of each path of the desired signal,realizing multipath utilization to maximize the output signal-to-interference-noise ratio.Compared with available methods,the beamformer performs better in small samples and low signal-to-noise ratio(SNR).What’s more,the computation complexity is reduced.2.Aiming at the complex actual scene where both the desired signal and the interference have multipath propagation,a beamforming method is proposed based on oblique projection.The available methods and subarray cross-correlation multipath beamforming algorithms are mostly aimed at scenarios where only the desired signal has multipath.However,in practice,the interference may also has multipath,which leads to poor robustness of the beamforming algorithm.In this complex practical scenario,in order to estimate desired signal’s composite steering vector of the MVDR optimal beamformer,the irrelevant components in the covariance matrix are removed by using the Toeplitz property of the non-multipath interference and noise covariance matrix,and a coherent covariance matrix including multipath coherent components is obtained.Based on the property that the composite steering vector of the coherent signal locates in the signal subspace of the coherent covariance matrix and the original covariance matrix at the same time,according to the correlation between the Direction of Arrival(DOA)steering vector and the composite steering vector,the DOA matching relationship of each source and the composite steering vector of the expected signal are determined for oblique projection beamforming.Oblique projection multipath utilization beamforming method overcomes the performance degradation of some available algorithms in the presence of multipath coherent interference.Theoretical analysis and simulation experiments prove that the algorithm can efficiently utilize multipath signals while suppressing interference without aperture loss.3.Aiming at the problem of estimating the interference-plus-noise covariance matrix and the desired signal’s composite steering vector of the MVDR optimal beamformer in multipath scenarios,a multipath utilization beamforming method is proposed based on subspace projection.In the multipath scenario,the algorithm constructs an oblique projection matrix firstly from other directions to the direction of the direct wave of the desired signal through the estimated value of DOA.Then,the oblique projection matrix and the signal covariance matrix are used to estimate the composite steering vector of the desired signal,and the beamforming is performed based on the MVDR criterion.Theoretical analysis and simulation results show that this method avoids the problems of signal cancellation and multipath energy loss effectively,and the composite steering vector can be obtained without estimating the reflection coefficient of multipath signals,which greatly reduces the computational complexity.In addition,the method does not need to estimate the interference-plus-noise covariance matrix,and is not affected by SNR,having strong robustness.