Adaptive Beamforming For Broadband Phased Array

With the application field of array signal processing and the application background of broadband signals becoming more and more extensive,traditional narrowband digital beamformers can no longer meet the increasing demands.Therefore,the research on digital beamforming for broadband phased array becomes more and more important.Starting from the common signal and interference model of broadband phased array,this thesis studies the adaptive beamforming structure for broadband signals: space-time structure and frequency domain DFT structure,and their related criteria and algorithms.Based on them,the broadband subarray adaptive beamforming structure for large scale arrays is studied and the interference suppression performance of beamformers under different structures is analyzed.For cases where the look direction of the array is towards off-broadside(not perpendicular to the array for a linear array),the influence of presteering on the incident angle of interferences is derived and a linearly constrained minimum variance(LCMV)algorithm based beamformer which has no need for presteering is studied.Finally,in order to increase the degrees of freedom required to protect the signal of interest without loss of interference suppression performance,an improved beamformer without pre-steering is studied.The main contents of this thesis are organized as follows:Firstly,due to the variability of the frequency of broadband signals,the received signals of broadband phased arrays are directly modeled by the time delay,and the accurate delay between array elements is obtained by using fractional delay filters.Several commonly used broadband signal models and beamforming performance measures are also introduced.Secondly,this thesis introduces and studies the adaptive beamforming theory of broadband phased array from two aspects of beamforming structure and array structure.The effectiveness of the space-time structure for broadband phased arrays is verified by comparing with the pure spatial structure.The beamforming performance of two frequency domain DFT structures is also studied and compared through the directivity pattern.Then the broadband subarray beamforming structure is studied,and the purpose and significance of different subarray partitioning methods are verified by simulations.Thirdly,for the case where the broadband signal of interest is incident from offbroadside direction,the relevant formulas of pre-steering are derived and the equivalent incident angle of the interference is verified by simulations.A beamforming structure based on LCMV algorithm without pre-steering is studied,and verified by simulation that it can obtain a good gain for the broadband signal of interest in off-broadside directions.Finally,based on the beamforming structure without pre-steering,an improved constraint design with higher degree of freedom is proposed.After low-rank approximation,the signal of interest can be better preserved by the designed constraint without increasing the cost of the array.Meanwhile,the effectiveness of the design is verified by comparative simulations.