| Each transmitting array element of MIMO radar transmits orthogonal waveforms,forms a low gain wide beam in airspace,and performs waveform diversity at the receiving end to realize multi beam formetion and complete all target search in airspace.MIMO radar has very important application value because of its good target detection performance,high degree of freedom and strong anti-jamming ability.In recent years,electronic jamming technology has developed rapidly,the deceptive jamming style is becoming more and more complex,especially the deceptive jamming in the main lobe direction,which is also an extremely severe challenge to MIMO radar.Element pulse coding(EPC)technology is a new MIMO waveform design technology proposed by the research group in recent years.Based on the orthogonal signals of the baseband,the coding design is carried out between different pulses and different transmitting arrays element.The coding design introduces new pulse dimension information,which has brought new breakthroughs in anti deceptive interference,distance ambiguity resolution and other technologies.Similar to MIMO radar,the performance of EPC-MIMO radar also depends on the design of orthogonal waveform.Considering that it is often faced with high-speed moving targets in practical engineering applications,the phase coded signal is sensitive to doppler frequency shift,and the velocity of various targets varies greatly,speed compensation will greatly increase the processing complexity of search radar.However,LFM signals with better doppler tolerance can only generate two sets of orthogonal signals with positive and negative frequency modulation under the constraints of bandwidth and time-width and keeping the time-frequency distribution as a unique straight line,which can not meet the requirement of more than 2degrees of freedom corresponding to multiple transmissions.Based on the frequency modulation signal,a segmented frequency modulation(Segmented FM)orthogonal waveform is proposed.The waveform is optimized by nonlinear programming and monte carlo tree intelligent search algorithm respectively,which realizes good autocorrelation and cross-correlation performance and robust doppler tolerance.Finally,the element pulse coding multiple input multiple output(EPC-MIMO)radar system is constructed,and the optimized SLFM signal is transmitted.The measured results verify the effectiveness of the optimized waveform.The measured results verify the effectiveness of the optimized waveform.The main contents of this paper are as follows:1.The transmitting and receiving signal model of EPC-MIMO radar is studied.On the basis of baseband orthogonal signal,the transmitting element and pulse are space-time coded so that the receiver can distinguish the echo signals of different transmitting pulse.The transmit,receive and transmit receive joint pattern formulas of EPC radar are introduced,and the influence of coding coefficient on pattern is analyzed.Then,the generation and signal model of main lobe deceptive jamming are introduced.The main lobe multi false target deceptive jamming is to modulate the intercepted radar signal and forward it across pulses.The echo signal processing flow of EPC-MIMO receiver is given.Subsequently,the processing results of each stage are simulated and analyzed,including multi waveform separation depending on signal orthogonality and its unique element pulse decoding.2.Aiming at the problem that it is difficult to balance the tolerance of doppler and the degree of freedom of orthogonal multiple emission,a nonlinear programming segmented frequency modulation signal(NLP-SFM)optimization method is proposed.Simulation results verify the performance of several traditional pulse compression signals.For obtain good orthogonal performance and better doppler tolerance at the same time,a segmented linear frequency modulation(SLFM)signal with random positive and negative frequency modulation slope is proposed based on the excellent doppler tolerance of frequency modulation signals.Each signal is divided into multiple segments,and each segment is a small bandwidth frequency modulation signal.The frequency modulation slope of each segment is random and the phase continuity of two adjacent segments is guaranteed.Multiple waveforms were obtained by optimizing SLFM signals with the goal of cross-correlation.Because of the inherent nature of LFM signal,although its orthogonality and doppler tolerance are good,the autocorrelation peak sidelobe is relatively high.Therefore,the design of segmented nonlinear frequency modulation(SNLFM)signal is optimized.After optimization,the waveform cross-correlation performance decreases within an acceptable range,and the autocorrelation peak sidelobe level can be reduced to-25 d B.3.Aiming at the problem that the NLP-SFM optimization problem is non convex,the solution of nonlinear programming is easy to fall into the local optimal solution,and the initial value setting seriously affects the optimization result.The research on using monte carlo tree search optimizate segmented linear frequency modulation signal(MCTS-SLFM)is proposed.The steps of MCTS-SLFM optimization method are designed.Through Selection,Expansion,Simulation and Backpropagation four steps to establish a complete asymmetric binary tree.The number of optimized waveforms determines the number of tree layers,and the optimal node is found in each layer as the inflection point of waveform segmentation.Compared with NLP method,MCTS-SLFM method is easier to obtain the optimal solution.Simulation results show that the orthogonality of waveform is further reduced by 1d B,and the doppler tolerance is also improved.4.Participated in the construction of EPC-MIMO radar system,used NLP-SLFM waveform as baseband signal to generate EPC-MIMO transmitting signal,and carried out field tests to obtain measured data.The overall design scheme of radar is introduced,and the test scheme and specific steps are described in detail.Then,detailed explain the some sub module of the radar system.Finally,the echo signal containing moving target and interference signal is processed whitch verify the validity of segmented frequency modulation EPC-MIMO waveform. |
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