Study Of Focusing And Detection Method For Targets Based On Time Reversal Technique

The propagation of Electromagnetic wave will be affected by multipath effect.Because different propagation paths have different time delay and energy attenuation levels for signals,the coherent superposition of signals at the receiver is reduced,and non-coherent superposition is increased.The result is that the signal received by receiver is distorted seriously and the interference is obvious.The multipath effect has a particularly serious impact on target detection and imaging of modern radar systems,and limits its detection distance and usage scenario.Combining the time reversal technology with the method of traditional radar target detection,using the self-adaptive focusing of the time reversal electromagnetic wave to make the antenna bea m focus on the direction of targets,thereby improving the signal-to-noise ratio of the echo signal and the probability of target detection,and whether the time reversal beam can accurately focus on the target position,is the key to apply this method.Thesis based on the basic principle of time reversal focuses,combined with array antenna beam forming and sparse array antenna grating lobe elimination technique,the application of time reversal method in far-field target focusing and detection is studied.The main research work of the paper is as follows.First,the methods of near-field and far-field beam focusing for single target in Multipath environment are studied separately,a algorithm of time reversal operator decomposition focusing for multiple targets selective focusing is proposed as the problem that the time reversal focusing is not effective when facing multiple targets,by analyzing the transmitted signal and the received signal of different antenna elements the cross pulse response is obtained,then the cross pulse response is used to construct the time reversal operator matrix.The eigenvector we obtained By decomposing the time reversal operator matrix contains the information of the target's directional.By selecting different eigenvecto rs as the excitation of antenna we can make the antenna beam focus on different targets which we want.Then the problems of time reversal focusing and the factors that influence the time reversal focusing are analyzed with simulation calculations.Then,the application of time reversal focusing technique in ultra-sparse array antenna is studied.In order to solve the problem of serious grating lobes in the time reversal beam of ultra-sparse array antenna,a method of grating lobe elimination based on frequency selective cancellation is proposed.By selecting appropriate frequency points in the frequency band of the time reversal signal,the distributions of the direction coefficient of the array antenna under different frequency points are calculated respec tively.The direction of the main lobe in the direction coefficient is constant,but the direction of the grating lobe will varies with frequency,By multiplying the direction coefficient of different frequency points,the main lobe is enhanced and the grating lobe is weakened and a better elimination effect of the grating lobes is achieved.Finally,the feasibility of determining the azimuth angle of targets by calculating the direction of the main lobe in the direction coefficient of the antenna and grati ng lobe elimination method is verified by simulation experiment.