Research On Time Modulated Array:Theroy And Its Application In Direction Finding

With the rapid development of modern electronic technology,most conventional wireless systems such as radar and communication put forward new demands including multifunction,high-performance,and low complexity on their antenna systems.The conventional antenna system will find it difficult to meet the rapidly changing requirements.As a novel antenna with low cost and low complexity,time modulated array(TMA)has attracted a great interest by the researchers in recent years.TMA is a novel antenna which adds the periodic RF switches in the RF front-end.By keeping the duty cycles of pulse in different channels consistent with the ratio of amplitude weighting in the corresponding channels of conventional antenna array,TMA can synthesis the same pattern as that of conventional antenna array.Compared with three dimensions of conventional antenna in space,TMA is also called four-dimension antenna since the time factor is introduced into the design of antenna array as an independent dimension.Meanwhile,periodic modulation will produce many harmonic components which contain different information and these harmonic components can be used for direction finding,beam scanning,SDMA and so on.Although TMA has the advantage of simple structure and low cost,its drawback should not be ignored.Conventional TMA has the problems of lower energy utilization and instantaneous performance instability,and there are some limitations including low accuracy and bandwidth in the application of direction finding using TMA.To address these concerns,this paper proposed a structure to improve the efficiency and stability of conventional TMA,and proposed some methods to improve the accuracy of direction finding and broad the range of bandwidth.The main content of this thesis consists of the following four parts:1.In order to overcome the low-efficiency problem caused by the energy absorption of feeding network,this paper presents a novel design of TMA by using a reconfigurable divider/combiner(RPDC)for improving the array's efficiency.With the proposed structure,the power absorbed during OFF state can be evenly transferred to other array elements during ON state.The improved pulse sequences strategy is derived to control the radiation pattern of TMA using RPDC,and the corresponding radiation energy expressions are derived.To experimentally verify the feasibility of the proposed method,two TMA using absorption switches and RPDC are constructed and compared to verify the effectiveness of the proposed method.2.To improve the instability of instantaneous performance in conventional TMA,this paper proposes an optimizing strategy whose goal is to improve the instantaneous stability of array based on the TMA using RPDC.By combining the stability of RPDC's instantaneous efficiency,the strategy can increase the stability of array's gain.Meanwhile,the optimizing results show that compared with the existing results,the proposed method can effectively reduce the sideband level for the flexibility of RPDC.3.In order to solve low-accuracy problem of direction finding using TMA,this paper proposes two methods to improve the accuracy of direction finding in both time and frequency domains.By accumulating the multiple pulses in the time domain,the accuracy of phase gradient detection can be effectively improved.The method can achieve highaccuracy OAM phase gradient detection and decrease bit error rate in the coding and decoding system using OAM modes.The use of multiple harmonic estimation can fully utilize the information of high order harmonic components in the frequency domain.The equivalence of the best linear unbiased estimator and generalized least squares estimator is proved and lower computation is needed for multiple harmonic estimation.Finally,a test system is constructed to verify the effectiveness of multiple harmonic estimation.4.To address the bandwidth limitation of direction finding technology using TMA,this paper proposes two direction-finding methods for linear frequency modulation(LFM)signals based on the Fourier transform and pulse compression.By analyzing the signals' characteristics in the time-frequency domain,the direction-finding expression is derived with Fourier transform.Meanwhile,within the direction-finding method with pulse compression,the limitation of transmitting signal and the direction-finding expression of echo signal are also given.Finally,two test systems are constructed to verify the effectiveness of two methods.