OFDM-LFM Waveform Design For Clutter Suppression

Clutter suppression has always been a difficult problem in the field of radar signal processing.Traditional clutter suppression methods mainly focus on signal processing at the radar receiver,which belongs to a passive anti-clutter way and makes it difficult for the radar to adapt to complex and ever-changing detection environments.In order to enhance the adaptability of radar to complex detection environments,the exploration of radar system degrees of freedom has been extended from the receiving end to the transmitting end,making radar waveform design a research focus.Waveform design,aided with the prior information of the target and environment,fully utilizes the freedom of the radar transmitter,thus achieving active clutter suppression and improving the detection performance of the radar system.The basic principle of waveform design for clutter suppression is the redistribution of emission energy by optimizing some waveform parameters based on prior information of the target and environment.The ideal energy allocation result is to reduce the emission energy in areas with strong clutter,and concentrate the emission energy in areas with weak clutter and target,thereby achieving the effect of clutter suppression.The selection of basic waveforms plays a very important role in waveform design,as it can affect the performance of optimized waveforms.Due to the combination of the characteristics of both LFM signals and OFDM modulation,orthogonal frequency division multiplexing linear frequency modulation(OFDM-LFM)signals not only have the advantages of the former in terms of large time-bandwidth product,high spectral efficiency,and high Doppler tolerance,but also have the characteristics of the latter including frequency agility,strong spectral dynamic allocation energy,and high degrees of freedom,making it a good choice for anti-clutter waveform design.Therefore,this article conducts research on anti-clutter waveform design methods based on OFDM-LFM signals.In order to improve the detection performance of radar,this article focuses on the problem of target detection in clutter environments and studies the anti-clutter OFDM-LFM waveform design algorithms for SISO and MIMO radar systems.The main contributions and innovations of the article are as follows:1.To solve the problem of clutter suppression in single target detection for SISO radar,a method including the solution to the optimal transmission waveform spectrum and the spectral approximation is proposed.Firstly,the optimal transmission waveform spectrum under the maximum SCNR criterion is solved by using prior information of the target and clutter.Then,an OFDM-LFM waveform design method based on multiple groups of subchirp durations optimization is proposed.The simulation results show that the multi-group optimization method can solve the problem that the single-group optimization method cannot achieve the spectrum shaping of an OFDM-LFM waveform with multiple groups of subcarriers and achieve clutter suppression by redistributing the emission energy in the frequency domain.Compared with the traditional OFDM-LFM waveform with uniform sub-chirp durations and the waveform with optimized single group of subchirp durations,the waveform optimized by the proposed method has higher output SCNR and greatly reduces the high sidelobes in the autocorrelation of the traditional waveform.2.To solve the problem of non-uniform clutter suppression in single target detection for MIMO radar,an indirect OFDM-LFM waveform design method is proposed,which includes two steps: the solution to the optimal spectrum based on SCA-ADMM algorithm and optimization of multiple groups of subchirp durations of OFDM-LFM waveform.The iterative algorithm based on SCA-ADMM can effectively solve the optimal spectrum,where SCA and ADMM can respectively achieve the transformation from an original non-convex problem to a convex problem and quick solution to multiple subproblems.Simulation results show that the optimized waveform obtained by the proposed method can redistribute lower emission energy in the frequency band and spatial azimuth with strong clutter,while concentrate emission energy in the frequency band and azimuth with weak clutter and target,thus achieving joint clutter suppression in the spatial and frequency domains.In addition,compared with the general and single-group optimized waveforms,the proposed multi-group optimized waveforms not only concentrate more energy at the target position in the spatial and frequency domains,but also have lower sidelobes in the received beampattern,indicating that the proposed algorithm can more effectively suppress clutter components in the echo,thus achieving higher SCNR.3.To solve the problem of non-uniform clutter suppression in single target detection for MIMO radar and further improve the output SCNR,a method of jointly designing OFDM-LFM waveforms and receive filter based on ISO algorithm is proposed.Unlike the two-step and indirect optimization method of solving the optimal spectrum first and then performing spectral approximation,this method directly optimizes the subchirp durations of OFDM-LFM waveforms by using ISO algorithm,and also designs the corresponding receive filter based on the optimized waveform.Simulation results show that compared with the method that only optimizes the transmitting waveform and the two-step optimization method,the proposed method has a higher output SCNR and further improves the clutter suppression performance of the radar system.4.To solve the problem of non-uniform clutter suppression in multi-target detection for MIMO radar,a method of jointly designing OFDM-LFM waveforms and receive filter based on multi-parameter optimization is proposed.In this method,the transmitting waveform and receive filter are alternately optimized to maximize the minimum SCNR.In each iteration,the OFDM-LFM waveform spectra are optimized by sequentially optimizing multiple groups of subchirp durations,frequency codes,and chirp rate sign codes by using the ISO algorithm,the genetic algorithm based on Grefenstette encoding,and the genetic algorithm based on traditional binary encoding,respectively.Simulation results show that the proposed method mainly concentrates energy on the azimuth and frequency band of the target to be detected after beamforming and filtering,and can achieve joint clutter suppression in the spatial and frequency domains.In addition,compared with the waveform design method that only optimizes multiple groups of subchirp durations,the method of jointly optimizing single group of subchirp durations,frequency codes and chirp rate sign codes,the proposed method has higher SCNR,thus improving the multi-target detection performance of target.