Research On Satellite-Borne Adaptive Beamforming And DOA Estimation Based On Signal’s Time Domain Characteristics

With the advantages of short propagation delay,low path loss and easy miniaturization of user terminals,low-orbit satellite(LEO)communication systems have attracted widespread attention in recent years.Its low orbital altitude and low communication elevation angle make it more susceptible to various kinds of interferences from ground communication facilities,causing serious impact on the communication performance.As an effective space domain signal processing method,array signal processing is widely used in LEO communication systems.Beamforming and direction of arrival(DOA)estimation are two important branches of array signal processing.By weighting the received data of array elements,beamforming can receive the designed signal with high gain while suppress the interferences and background noise terms.DOA estimation is mainly to estimate the direction of incident signals and provide a priori information for beamforming.The performance of both is usually limited by the physical aperture of the antenna array.However,in LEO platforms,the design of antenna arrays should keep lightweight and miniaturized,and it is difficult to enhance beamforming or DOA estimation performance by expanding the size of antenna arrays.To address above problems,adaptive beamforming and DOA estimation methods based on the time domain characteristics of received signals are discussed in this paper.In the context of LEO communication applications,we mainly focus on the robust widely linear beamforming method based on the noncircular characteristics and the DOA estimation method in the CDMA system based on the correlation characteristics of spreading sequence.To improve the robustness against large errors,a widely linear(WL)beamformer based on the worst-case performance optimization(WCPO)method with multiple uncertainty sets is proposed.The corresponding problem is a multi-constrained nonconvex optimization problem,and an iterative method based on the semidefinite relaxation is proposed to solved it.Simulation results demonstrate that the proposed method has great robustness against noncircular coefficient estimation errors,DOA estimation errors,and array imperfections.And in comparison with several representative robust WL beamformers,the proposed method can achieve better performances,especially under large error conditions.To reduce the computational complexity of the user-by-user DOA estimation in CDMA systems,a simplified ESPRIT method is proposed.According to the great correlation characteristics of the spread spectrum sequence,the despread signal for a particular user can be approximated regarded as a single source signal combined with some noise.Thus,the point division or least square method are used to replace the eigendecomposition operation in the standard ESPRIT method.And then the simplified ESPRIT method is extended to the standard hexagonal array by spatial mapping and virtual elements filling.Simulation results demonstrate that the proposed simplified ESPRIT method has a very similar performance to the standard ESPRIT method but the computational complexity is greatly reduced.