Study On Beamforming Method In MIMO Radar Under Jamming Environment

In view of the increasingly complex electromagnetic jamming environment and the increasingly strong jamming combat capability,robust and reliable jamming suppression methods can effectively improve the target detection and tracking performance of radar system under complex electromagnetic jamming environment.But most of the jamming suppression methods are passive countermeasure based on adaptive receiving beamforming technique at the receiver,which may not be able to effectively suppress the jamming signal in practical applications.Aim at this,radar system should adopt more active countermeasure at the transmitter,so as to consume the ability of electronic jamming and further reduce the possibility of jamming signal entering the receiver,and closely combine with passive countermeasure to fully tap the potential anti-jamming ability of radar system.In addition,each transmitting antenna of multiple input multiple output(MIMO)radar can transmit different waveform,which makes it have more transmitting degrees of freedom,so this dissertation will take the MIMO radar as a platform to study the transmit beampattern design,robust adaptive receiving beamforming as well as joint transmitting and receiving beamforming based on subarray partition,so as to meet the requirements of anti-jamming as well as target detection and tracking ability of modern radar system as far as possible.The main research contents of this dissertation are as follows:1.The transmit beampattern design method is studied.Several classical transmit beampattern design methods are briefly introduced.Aiming at the problem that the existing transmit beampattern design methods are sensitive to the selection of 3d B mainlobe width,a low sidelobe transmit beampatten design method for MIMO radar with mainlobe shape-preserving is proposed.The method can effectively avoid the phenomenon of mainlobe splitting by constraining the mainlobe shape of transmit beampattern,and significantly reduce the sidelobe level of transmit beampattern by using a modified matrix to modify the non-diagonal elements of transmit waveform covariance matrix.Simulation results show that the transmit beampattern synthesized by this method not only has the desired mainlobe shape,but also has lower sidelobe level.In addition,in order to effectively suppress the sidelobe nonhomogeneous clutter,a transmit beampattern design method for MIMO radar based on prior information is proposed.Based on the prior information of target echo,this method can estimate the average power of clutter-plus-noise in the received echoes by transmitting a set of known waveforms,and then establish the optimization model for transmit waveform covariance matrix by maximizing the ratio of the difference between target echo power and clutter-plus-noise average power to target echo power.Simulation results demonstrate that the transmit beampattern synthesized by this method can form null in the strong clutter direction,thus efficiently improving the signal-to-clutter-plus-noise ratio of the received echoes.2.The robust adaptive receiving beamforming method is studied.Diagonal loading method is one of the most widely used robust adaptive receiving beamforming methods,which can effectively improve the robustness of adaptive receiving beamformer,but how to determine the optimal loading factor is still the core problem and difficulty.Aim at this,a robust adaptive receiving beamforming method for MIMO radar based on diagonal loading technique is proposed.Compared with other methods of determining loading factor,the method not only does not need to specify any uncertain parameters that are difficult to determine in practice,but also is easy to calculate.And the obtained loading factor can effectively guarantee that the eigenvector corresponding to target signal participates in the calculation of weight vector.Moreover,the dimension of virtual steering vector of MIMO radar is relatively large in practical applications,which can bring a lot of computation burden to adaptive receiving beamforming algorithm,so a low-complexity robust adaptive receiving beamforming method for MIMO radar based on covariance matrix and steering vector estimation is proposed,so as to effectively improve the computational efficiency of adaptive receiving beamforming algorithm and the robustness of adaptive receiving beamformer.This method replaces the sample covariance matrix by the Kronecker product of the reconstructed transmit covariance matrix and the reconstructed receive covariance matrix,and estimates the desired signal steering vector by maximizing the output signal-to-interference-plus-noise ratio of adaptive receiving beamformer,which can greatly reduce the computational complexity of adaptive receiving beamforming algorithm.3.The joint transmitting and receiving beamforming method based on subarray partition is studied,so as to give full play to the spatial degrees of freedom of the transmitter and receiver.In the case of a single target signal,a joint transmitting subarray partition and beamforming design method based on two-dimensional phased-MIMO radar is proposed.In this method,the transmitting array of MIMO radar system is equally partitioned into a number of nonoverlapping subarrays,and the optimization model for subarray structure of transmitting array,transmit beamformer weight vector of each subarray and receive beamformer weight vector is established by maximizing the output signal-to-interference-plus-noise ratio of receiving beamformer under certain constraint conditions.Furthermore,for the scenario of multiple target signals,a joint transmitting subarray partition and beamforming method for active jamming suppressing in phased-MIMO radar is proposed.This method determines the number of antenna elements in each subarray according to the transmit energy ratio required by each target signal.At the transmitter,the transmit beampattern nulling method is utilized to deal with active jamming signal such as suppressed jamming signal and deception jamming signal,or the orthogonal transmitting method is utilized to contend with deception jamming signal depending on the transmit waveform of radar system.And at the receiver,the adaptive beamforming technique is utilized to suppress the jamming signal entering the receiver.Simulation results show that both the transmit beampattern and the transmit-receive beampattern designed by the above two methods have lower peak sidelobe level,and can form null in the jamming signal direction,thus effectively improving the signal-to-interference-plus-noise ratio of the received echoes and the receiving beamformer.