The Robust Performance Of Near Field Multi-beam Forming

In array signal processing system,far field assumption greatly reduce the degree of difficulty to beamforming,while when the signal source is located in near field,signal model is sphere wave instead of plan wave model,for example vehicle system,telephone conference,communication system in small closed space and so on.So,far field assumption can't be used in near field of array.Otherwise,output performance of array will become worse even failure.Meanwhile with increasing development of electronic information technology,the electromagnetic environment became more and more complicated than ever before,so how the multiple target signals are effectively distinguished,identified,monitored in complex environment is being a severe test of modern sonar,radar,and all kinds of reconnaissance system.Now,the main technology which simultaneously obtains wide airspace coverage and high sensitivity is multi-beam forming technique.Thus multi-beam forming technology is being a hot research point,and the studies are becoming necessary.Most existing approaches for multi-beam forming use algorithm in sub-array respectively and switch the direction of beam to realize multi-area coverage.In this paper,we develop a method for multi-beam based on the traditional algorithm LCMV(Linear constrained minimum variance)which can produce multi-beam simultaneously out of sub-array.Thought LCMV is the most common existing method to overcome the problem of uncertainty in the signal look direction,its performance in near-field multi-beam can't realize what we desired.So,we improve this algorithm to provide robustness against steering vector error and interference.This improved LCMV(USC-LCMV)algorithm upon an uncertain set for near-field multi-beam is presented in this paper.This algorithm can accurately distinguish and effectively receive desired signals of multiple directions in complex environment without changing the structure of array.Through simulation results,we can see that the improved algorithm USC-LCMV can direct its main beams toward actual signal directions,so,USC-LCMV can be expected to provide sufficient robustness improvements.Then,array's spatial resolution direction(SRP)is a direct restriction to resolution performance of beamforming algorithm.When interference is on main lobe of signal,adaptive beamforming algorithm is not efficient.In the last part of this thesis,space-time USC-LCMV based on optimal space-time processer is used in near-field beamforming.This algorithm which combines area space and time domain has spatial and time resolution,it achieves space time filtering.The weight vector solved by space-time steering vector is not only about attitude angle but also about frequency,so if the frequencies of desired signal and interference signal are very different,space-time USC-LCMV can still receive target signal when the interference is nearby target signal,ensures great output.Form simulation,we can see that space-time USC-LCMV has excellent robustness to spatial resolution of array.