| It has always been a key issue in the radar field to ensure survival and accomplish the probing task in the complex electromagnetic environment.At present,many new changes have taken place in the electromagnetic environment.On the one hand,with the development of society,there are many tall buildings and iron towers in the urban environment,and the communication equipment increases sharply,which inadvertently introduces more interference sources to radar detection.On the other hand,radar jamming technology has developed from simple means such as noise blocking jamming and placing human jamming sources to deceptive jamming or complex forms of coexistence of multiple jamming.The new electromagnetic environment situation puts forward higher requirements for the antijamming ability of radar.It is difficult for traditional radar to take into account all kinds of radar probing tasks while suppressing interference.Because each array element can transmit signals with different forms,multiple input multiple output(MIMO)radar has the ability called waveform diversity,which makes it one of the radar systems with the most flexible working mode and the highest degree of optimization freedom.Compared with the traditional phased array radar,the advantage of waveform diversity enables MIMO radar to mobilize more radar resources to suppress interference and accomplish radar probing tasks such as target detection and multi-target tracking.Therefore,this dissertation studies the waveform design method of MIMO radar from the aspects of transmit nulling,spatial synthetic signal optimization,joint transmit and receive optimization and countering deceptive jamming.Specifically,the main work and innovations of this paper can be summarized as follows:1.As the location of target and sidelobe interference changes in real time,the beam directions and null points of the transmit beampattern should also be adjusted in time.To solve the above problems,this paper proposes two real-time multi-beam beampattern transmit nulling methods under constant modulus constraint.The first method constructs a constant modulus waveform matrix with multi-beam beampattern in real time based on the idea of base-beam,and then uses the block coordinate descent algorithm to fine tune the waveform phase to achieve the effect of transmit nulling.This method has simple operation and good real-time performance.However,the influence of fine-tuning waveform phase on the initial multibeam beampattern is uncontrollable,which may lead to the distortion of mainlobe and the elevation of sidelobe level.Based on the above reasons,this paper further proposes a transmit nulling method based on the common-null-points transmit beamformer.In this method,the constant modulus waveform optimization is reduced to several parallel "phaseonly" transmit beamformer designs,so as to achieve the purpose of real-time optimization.These transmit beamformers have different beam directions,but set notches in the same directions.Therefore,the synthesized transmit beampattern has the effect of multiple beams and multiple null points.The computational complexity of the proposed methods is much lower than that of the existing algorithms.Simulation experiments show that both methods can fast realize the transmit nulling with multi-beam beampattern.2.In complex electromagnetic environment,the power spectrum shape and spectrum compatibility of mainlobe direction spatial synthetic signal are of great significance to MIMO radar.Aiming at the joint optimization of multi-beam beampattern and spatial synthetic signal,two cost functions are proposed in this paper for the power spectrum shaping and spectrum compatibility design of spatially synthesized signals,respectively.In order to reduce the calculation as much as possible,the multi-beam beampattern design scheme proposed in this paper is to make the desired beam approach only a few "key points" to achieve the maximum transmit gain in the target direction.Under this beampattern design strategy,the two cost functions proposed can be unified into the same problem model.Then,an algorithm based on alternating direction multiplier method is proposed to obtain the constant modulus waveform,and the computational complexity and convergence of the proposed algorithm are analyzed.Considering the better electromagnetic compatibility performance of wideband radar,the proposed cost functions and algorithm are extended to wideband MIMO array,and good results are achieved.3.Aiming at the problem of target detection in passive interference environment,the joint optimization of MIMO radar transmit waveform and receive filter based on interference prior information is considered.Considering the optimization scale and the computation resources at receiver,the existing space-time filter is equivalently divided into the cascade of spatial filter and temporal filter.This signal processing structure requires less computational complexity and less data memory space.Based on this signal processing scheme,under the constraint of peak to average ratio,a cost function is proposed to maximize the output signal-to-interference-noise ratio and minimize the temporal integral sidelobe level.Then,aiming at the situation of temporal unmatched filter and temporal matched filter,this paper proposes two joint transmit and receive optimization methods based on sequential optimization framework and majorization-minimization optimization framework respectively.Compared with the existing methods,the proposed joint optimization methods show better computational efficiency and optimization performance.4.Deceptive jamming can not only cover up the real target,but also produce false peaks to mislead radar.Aiming at the deceptive jamming of intermittent sampling and repeated forwarding,an anti-jamming scheme based on joint design of transmitter and receiver is proposed in this paper.This jamming signal can be considered as the superposition of multiple target echoes with different Doppler frequencies.Based on this discovery,a jamming parameter acquisition method based on intra-pulse Doppler is proposed.The jamming parameters are obtained by transmitting the waveform with thumbtack ambiguity function and using the echo processing method of parallel Doppler channel.Then,based on the known jamming parameters,the waveform with inter-pulse Doppler shift is actively transmitted to realize the decorrelation between the jamming signal and the target signal in the slow-time dimension,so that the identification and suppression of the jamming signal can be realized.Considering the multi-beam advantage of MIMO radar,this paper implements the anti-jamming scheme in the wideband MIMO array system,and successfully extends the proposed anti-jamming scheme to the multi-beam case. |
