Research On Improved MVDR Beamforming For Suppressing Multimode Clutter In MIMO-OTHR

Over-the-Horizon-Radar(OTHR)uses high-frequency electromagnetic waves to propagate through the atmospheric ionosphere to detect targets with a longer line of sight.However,due to the propagation of electromagnetic waves through the ionosphere Multi-mode clutter will be generated,which will seriously affect the detection of the target.Multi-input multi-output(MIMO)technology is applied to OTHR to construct MIMO-OTHR,so that it can make full use of spatial information to achieve non-causal adaptive beamforming,so as to achieve the effect of suppressing multimode clutter.MIMO-OTHR realizes the detection of slow-moving ship targets in the outer sea with a line-of-sight distance of 1000?4000km through the propagation of high-frequency electromagnetic waves in the ionospheric refraction and back-scattering path.In the process of detecting the target,multiple scattering detection paths will be returned,which will cause the occurrence of multi-mode clutter.Coupled with the influence of ionospheric phase pollution,the first-order Bragg peak of multimode clutter will spread on the Doppler spectrum,thus forming a Spread Doppler Clutter(SDC),and its SDC clutter Affect the detection performance of OTHR on the target of slow-moving ships on the sea surface.In MIMO-OTHR,the minimum variance undistorted response(MVDR)is usually used to solve the impact of the ionospheric multimode clutter in the received echo on the detection of the target.However,due to the omnidirectional transmission of MIMO-OTHR,the inaccurate estimation of the clutter noise covariance matrix and the too few training samples in the limited coherent accumulation time,the traditional MVDR output signal to noise ratio(SCNR)is low.In response to these problems,this thesis improves the MVDR algorithm from two aspects,the main work includes:(1)Improve the traditional MVDR algorithm in MIMO-OTHR to improve its multi-mode clutter suppression capability.First,based on the preprocessing of the transmit beamspace using the second order cone programming,the transmission energy is concentrated to the main mode angle;then,the array smoothing is performed at the receiving end of the array to suppress the expected signal of the main mode and improve the estimation accuracy of the noise covariance matrix;Finally,the use of MVDR beamforming double iteration algorithm to reduce the need for computation and training samples.The effectiveness of the algorithm is verified by simulation,and its performance is analyzed.The simulation results show that:the improved MVDR algorithm can effectively suppress multimode clutter;with fewer training samples and a smaller amount of calculation,a higher output SCNR can be obtained.(2)In order to further improve the ability of multi-mode clutter suppression,a new radar system of FDA-MIMO-OTHR is constructed by introducing the technology of frequency diversity array(FDA)on the basis of the research of clutter suppression in MIMO-OTHR,so as to improve the ability of multi-mode clutter suppression in range dimension.First,based on the pre-processing of the transmit beamspace using a non-uniform frequency offset,the transmission energy is concentrated to the main mode angle;then,the array smoothing is performed at the receiving end of the array to suppress the expected signal of the main mode and improve the clutter noise covariance matrix estimation Accuracy;Finally,the use of MVDR beamforming dual iteration algorithm to reduce the need for computation and training samples,verify its effectiveness through simulation,and analyze the performance,The simulation results show that the improved MVDR algorithm can effectively suppress the multi-mode clutter under the construction of FDA-MIMO-OTHR new radar system;with less training samples and less computation,it can obtain higher output SCNR,and achieve the actual situation with the distance dimension dependence.