Study On MIMO Radar Transmit Waveform Design Based On LFM Signal

Compared with the phased-array radar,multiple-input multiple-output(MIMO)radar transmits independent waveforms from different antennas and thus obtains the advantage of waveform diversity.With the development of radar technology and the complication of detection environment,simple working mode cannot complete the complex mission of radar anymore.Radar should satisfy the various requirements in many aspects,including the adaptability of environment,ability of multi-mode and detection of diverse targets.To achieve these advantages,transmit waveform design is executed for MIMO radar to improve the corresponding performance,such as the transmit energy location,pulse compression property,sensitivity to Doppler frequency,etc.Due to the excessive degree of freedom,phase coding signal is widely applied to waveform design methods,whereas it has the disadvantage of Doppler sensitivity which leads to the difficulty in target searching,especially the moving targets with high velocity.Thus,it is necessary to employ linear frequency modulation(LFM)signal for waveform design,so that the property of good Doppler tolerance is obtained.This dissertation focuses on the orthogonal and correlated waveform set design according to the practical requirement of performance.The main content is organized as follows:1.The characteristics of LFM signal are analyzed,and the basic principle and design methods of waveform design based on LFM signal are introduced.Firstly,the frequency modulation,pulse compression property and the ambiguity function of the LFM signal are explored and compared with that of the phase coding signal,which exhibits the features of LFM signals.Then,with the case of some typical waveforms,the waveform design methods based on LFM signal are introduced according to the sorts,which includes the basic principles,waveform characteristics,design algorithm and actual problems.This part is the basis of the following study on waveform design based on LFM signal.2.A research on the orthogonal discrete frequency coding waveform with LFM(DFCWLFM)design is made.In order to obtain orthogonal DFCW-LFM signals with low autocorrelation sidelobe level and crosscorrelation level,the correlation function is studied,and a genetic simulated annealing algorithm(GASA)based on genetic algorithm(GA)and simulated annealing(SA)is used to design DFCW-LFM waveform set.Simulation results indicate that the designed waveform set has good correlation property.In addition,to increase the degree of freedom,a modified DFCW-LFM design method is proposed.Phase coding waveform is applied to the subpulses of DFCW-LFM,thus forming a compound waveform,and our GASA algorithm is also employed to its optimization.Compared with the existing DFCW-LFM,design results show that the performance of the proposed compound waveform in correlation is improved.3.Aiming at solving the problems in conventional orthogonal LFM waveforms,the theoretical analysis and design methods are studied.Firstly,considering the critical role the pulse compression property and Doppler tolerance of spatial synthesized signals plays in MIMO radar,a detailed analysis is made using orthogonal LFM signals.Secondly,to suppress the high sidelobes of pulse compression result dramatically,two orthogonal LFM waveform set design methods are proposed.One is the joint optimization of frequency codes and initial phases.Different algorithms are adopted for frequency code design and initial phase optimization simultaneously.The resulting LFM waveforms have low autocorrelation sidelobe level in the directions of interest,perfect transmit beampattern and good Doppler tolerance.The other is an orthogonal LFM waveform set design method with various frequency steps.The optimization model is established based on the temporal property and spatial property jointly,which results in the optimal frequency steps and initial phases.The transmitted power of the designed waveform set is approximately equal in all directions,and the spatial synthesized signals have good correlation properties at little expense of Doppler tolerance.4.The issue of correlated waveform set design based on LFM signal is focused on.Based on the signal model of frequency diverse LFM waveforms,the effect of waveform parameters have on transmit beampattern is analyzed,and then a correlated LFM waveform set design method and a method for correlated waveform set design using LFM basic beams are proposed accordingly.The former adopts different optimization strategies of frequency steps and initial phases for single beam design and multiple beam design respectively.The obtained LFM waveforms have some superiorities over the present waveforms,such as constant-envelope and easy-generation.The latter utilizes the idea that a beampattern can be synthesized by some basic beams.The transmit waveforms are composed of the designed LFM waveform sets whose beampatterns match the basic beams.The designed waveform set guarantees both the beampattern matching performance and Doppler tolerance.5.Considering the high correlation sidelobes and difficult-generation of orthogonal frequency division multiplexing LFM(OFDM-LFM)signal,the topic of waveform design is studied.Firstly,an OFDM-LFM waveform diversity design method with correlation interference suppression is proposed.The reason for high correlation sidelobes is explored,and two new design schemes are presented.Via designing the various subchirp durations or subcarrier bandwidths specially,the high sidelobes are depressed and the correlation property is improved.Secondly,a large time-bandwidth product orthogonal waveform set design method is proposed.Based on the cost function established by depressing the correlation sidelobe level,the subpulse durations of the waveforms are optimized by sequential quadratic programming(SQP).Simulation results show that the designed waveforms have good correlation property.