Research On Key Techniques Of Beamforming And DOA Estimation Based On Time-Modulated Array

In recent years,Time Modulated array(TMA)has attracted much attention in the signal processing field.Compared with the traditional phased array,TMA adopts rf switch instead of the phase shifter,realizes beamforming by periodically modulating rf switch,and adopts single channel output to extract harmonic signal with filter.Compared with the phased array,TMA has a simpler hardware structure and can realize pattern synthesis or beamforming by designing switching time series.Due to the periodic modulation of the RF switch,TMA generates a series of harmonic beams,which can transmit different signals in different directions,and has broad prospects in the field of communication or detection.Because of these advantages,TMA has been extensively studied.In this paper,based on TMA,beamforming and DOA estimation techniques are studied in-depth,and a series of new methods are proposed for various problems.The main contents and innovations of this paper are as follows:1.Beamforming based on time-modulated uniform linear arrayThis paper mainly studies beamforming technology of time-modulated linear array to optimize TMA array design in large-scale array scenario and reduce system complexity and cost.In view of the problems of large TMA array in full array,such as multiple rf branches,complex system structure and high cost,the optimization algorithm can be used to reduce the number of antennas to further simplify the structure and reduce the cost on the premise of meeting the performance requirements.To solve the above problems,based on time-modulated sparse array,an algorithm is proposed to optimize the number of array elements under the requirements of specified sidelobe and beam width.By designing fitness function and optimizing switch sequence using genetic algorithm,fundamental beamforming is realized,and multiple beamforming is realized using ±1 harmonic and fundamental wave.In order to improve the system complexity and cost when the number of antennas increases,STMPA and ESTMPA with improved structure are proposed.By improving the I/Q modulator design,on the one hand,the array aperture is increased,thus improving the beam performance;on the other hand,the system structure is simplified,and the hardware complexity and cost are reduced.At the same time,the improved system structure maintains high efficiency and balance between performance and cost.2.Nonuniform linear array beamforming based on time modulationBased on the time modulation array,a variety of beams with special shapes are studied to adapt to different application scenarios,and the application design is carried out.In order to reduce the number of harmonics that can be used in practical engineering due to the progressively decreasing harmonic energy generated by TMA,a bidirectional beamforming algorithm based on time modulation array is proposed to increase the number of harmonic beam pointing.By introducing time offset,the simultaneous scanning of double time series is realized,and the +1 harmonic with double direction is generated.By introducing fundamental component and combining with genetic algorithm,the three-direction beamforming is realized and the number of directions is further extended.To improve the beam performance of TMA with limited array aperture,a time-modulated linear array super-beamforming algorithm is proposed.By dividing the array into left and right subarrays for time modulation,the left and right beams are obtained,and then the sum and difference beams are obtained by the sum and difference beams,and finally the super beam is obtained by the sum and difference beams.Compared with the traditional beam,the beam width of the super beam is narrower and the sidelobe is lower,and the array structure does not need to be significantly modified,only the received signals of the left and right subarrays need to be processed.In this paper,a super beamforming system based on ESTMPA is proposed for automotive radar,which can generate narrow beam with low sidelobe based on low complexity system structure and extremely small array aperture.At the same time,the bandwidth of the transmitted signal is 8 times higher than that of the traditional structure,which can further improve the resolution of the system.3.Time modulated planar array beamformingBased on time modulation planar array,this paper puts forward the innovation in theory and application.The traditional time-modulated array harmonic beamforming method scans each element one by one.For linear array,the total modulation time is not long,but for planar array,the total modulation time increases exponentially when the number of elements increases.At the same time,because the traditional method scans the array elements sequentially,the efficiency of planar array will decrease greatly as the number of elements increases.Aiming at the above problems,a harmonic beamforming method based on block subarray is proposed.By dividing the planar array into multiple subarrays and scanning at the same time,the total modulation cycle time is reduced and the system efficiency is improved.Due to the decrease of the total modulation time,the requirements of hardware performance such as RF switch,controller and filter are reduced,and the hardware cost of the system is further reduced,or the signal with larger bandwidth can be transmitted without changing the system structure.Aiming at beam focusing in wireless charging,near field beamforming is realized based on time modulated array,time modulated L-shaped array is proposed,and its signal model in near field is deduced.Adaptive beamforming algorithm is used to realize near-field beamforming based on time-modulated L-shaped array,which can achieve beam focusing effect at specified coordinate position.4.DOA estimation based on time modulation arrayThe DOA estimation technology of time-modulated array is mainly studied,and various methods are proposed to improve the PERFORMANCE of TMA DOA estimation from the aspects of accuracy,degree of freedom and direction finding range.In order to simplify the structure of TMA array and improve the degree of freedom of TMA array,time modulation was introduced into the mutual array,and the concept of time modulated mutual array was proposed.Based on the time modulated mutual array(TMCA),time modulated extended mutual array(TMECA)was further proposed.Based on TMCA and TMECA,the signal model and method to realize DOA estimation are derived.Compared with time-modulated linear array,the DOA estimation accuracy is improved when the number of elements is the same.To solve the problem of fixed direction finding range of TMA,a method based on optimized harmonics was proposed to extend the Angle range of direction finding.By optimizing the switching time,the Angle interval between harmonics is increased and the range of direction finding is enlarged.Aiming at the problem that the error of direction finding algorithm increases when the incident Angle is exactly at the Angle of harmonic main beam,a new correction method is proposed to improve the accuracy of direction finding.In the 7-element time-modulated linear array,the range of direction finding is increased from±59° to ±75°,and the high accuracy of direction finding is guaranteed.For the underdetermined DOA estimation problem of TMA,a method based on high-order harmonic cumulants is proposed.The degree of freedom of the array is increased by high-order cumulative processing of the harmonic signals of the array,and the incident signals with more than the number of elements can be detected.The expressions of higher order cumulants and covariance matrices of order 2,4 and 6 are derived,and further extended to order 2q.This method is based on the traditional TMA structure,and has the advantages of low complexity,low cost and high precision under low SNR.