Robust Beamforming Research With Strong Interference

In order topromote the ability that sonar system detect far field target under strong interference and in complex underwater acoustical environment,the thesis takes a deep exploration into the way beams that have robust spatial filtering performances form,which combines latest research results on adaptive beamforming technology and matrix filtering technology,taking interference rejection capability as an important standard of estimating robustness.At the same time,we take a research on the form of robust far field beam under near field interference.1.A method to form beams is put forward to increasethe robustness of algorithm,aiming at solving the contraction between robustness and interference rejection capability in the way of robust adaptive beamforming,by establishing preprocessing matrix trough quadratic feature decomposition and decreasing the composition of desired signal among received data.First of all,by analyzing the robustness principle of robust adaptive beamforming,we come up with the idea to raise the robustness trough weakening the composition of desired signal among training data;Furtherly,making use of signal subspace of covariance matrix receiving data,direction vector that interference roughly come from,as well as particular attenuation function,build preprocessing matrixes containing possible desired signal from the cut off training data;At last,obtain the final right of beamforming using opposite angle loading method.As we can see in a computer simulation experiment,when the training data does not contain desired signal and no error exists in it,this method has a similar performance with other opposite angle loading adaptive beamforming method.While there are errors in orienting vector or observing direction,this method will be more capable of rejecting interference.2.To solve the contradiction between low-dimensional design and filtering performance in matrix filtering technology,a low-dimensional filtering matrix design method with strong interference suppression capability is proposed in combination with K-L transform.Firstly,the the oretical derivation gives the relationship between the number of larger eigenvalues of the filter matrix and the passband bandwidth of the filter matrix.Secondly,the thresholds for comparing the size of eigenvalues are given by using the stopband attenuation.Finally,the method of projection to the eigenvector matrix corresponding to the large eigenvalues is used to obtain the low-dimensional spatial filter matrix which finally has less dimension than the number of elements,and the columns of the filter matrix are orthogonal to avoid the failure of the filter matrix to correlate the white noise space.The computer simulation experiment shows that,the low-dimensional filter matrix obtained by this method has the same filtering performance as the filter matrix with the number of array elements equal to the number of dimensions;It has better spatial filtering performancecompared with orthogonal filter matrixof the same dimension;It also has a smaller passband response error compared to an adaptive filter matrix with the same null steering width and depth.3.When it comes to that the interference is located in the near field region and the target signal is in the far field,let's take a research into the far field beamforming problem.Firstly.the approximate analytic expression of the correlation between the spherical wave and the plane waveguide vector under the condition that the linear array is given.Through the analysis of this equation,the effect near field interferencehas on the far field beamforming,as well as the relationship between the array aperture,theposition of interference and the wavelength of signal can be figured out.(1)Aiming at the standard adaptive beamforming technology,combined with the result derived by the principle of stationed phase,the expression of the far-field adaptive beam response under near-field interference is given.From this formula,we can see that therelationship between the way near-field interference response for the far-field adaptive beam,the array aperture,the near-field interference position,the signal wavelength and the far field desired direction.Using this relationship,according to the specific application background,reasonable array layout can weaken or even avoid near field interference to the far field beam.Far more,when the near-field beamforming method is applied to the near-field interference,the simulation results show that the robust adaptive beamforming method can effectively suppress near field interference while achieving far-field beamforming,the effect of interference on the far field beam is almost the same as that of the previous analysis:(2)Aiming at the matrix filtering technology under near field interference,in combination with the result derived by the principle of stationed phase,the influence of near field null steering on theresponse of far-field beam is given.The size of the influence and array aperture,near field interference position,signal wavelength and design parameters of filtering matrix,and furthermore,the application of low-dimensional orthogonal filtering matrix under near-field interference is given.The computer simulation experiment proves tha the method can form the null steering in the pre-set near field region and generate beam responses thatmeet the designed parameters in the far field.And the simulation results show that the influence of the near field zero region on the far field beam is almost the same as tha of the previous analysis,which can be weakened by reasonable array layout and matrix parameter design.