Research On Fourth-order Cumulants Beamforming And Its Application In Underwater Targets Detection

Due to all kinds of complicated influences of sea environment, how to detect andrecognize underwater target is still one of the essentials in underwater acoustic signalprocessing. How to obtain target echo data with high SRR (Signal-to-Reverberation Ratio)(SNR：Signal-to-Noise Ratio) is the key issue to improve the ability of underwater targetsdetection and the accuracy of recognition in complex environments. Methods to suppressreverberation interference of active sonar and obtain target echo with high SRR data havebeen proposed in this paper, which are typical underwater mines detection problems.Firstly, array signal processing method, which is used to form narrow beams in spacedomain by small size arrays, realizes both target DOA (direction of arrival) estimation andreverberation suppression. Simultaneously, basing on the characteristic differences betweentarget echo and reverberation, a signal processing method is taken to filter reverberation.Adopting twice suppression of reverberation interference in both spatial domain andtransform domain, the purpose to improve the target echo SRR can be achieved.The differences on models and characteristics of three types of signals (the target echo,reverberation, noise) related to active sonar target detection are analyzed and summarized inthis paper. And a suppression scheme of reverberation interference is provided which is also atheoretical basis for the subsequent processing.In order to balance high spatial beamforming resolution and robust performance, thefourth-order cumulants which has the array aperture extension characteristics is used.Combined with minimum redundancy array structure, a fourth-order cumulants beamformingmethod based on minimum redundancy array is proposed. The method uses only a few arrayelements to achieve high spatial resolution, and reduces matrix complexity of data matrix ofthe fourth-order cumulants from the original M~2×M~2to M×(2M1). The theoreticalgain of fourth-order cumulants beamforming processing is calculated in this paper, whichproved that under some special noise distribution, there is a critical SNR between the gain offourth-order cumulants beamforming and that of conventional beamforming. At the same time,the performances of the fourth-order cumulants beamforming are fully discussed, includingresolution, snapshot influence, error performances. In order to verify the spatial resolution ofthe fourth-order cumulants beamforming, the experiment data from lake is processed, resultsof which proved the fourth-order cumulants beamforming method, which has higher spatialresolution and better background suppression ability, is better than the conventional beamforming method.To fulfill the requirements of broadband detection, the fourth-order cumulantsbeamforming combined with the fractional Fourier transform is extended to realize DOAestimation of LFM signal, and the performance of the method is also analyzed. At the sametime, based on the aggregation differences between target echo and reverberation afterfractional Fourier transform, the reverberation can be filtered in transform domain. The singleLFM signal, highlight model signal added with real reverberation signal are processed, andthe results showed that the reverberation interference can be filtered by the fractional Fouriertransform effectively, and the SRR of the target echo can be further raised. Taking fractionalFourier transform as a link, method combined target DOA estimation with anti-reverberationis given in this paper.The active sonar sea target detection data is dealt with to validate the characteristics ofthe proposed method finally. Both the target DOA estimation of single pulse and sailing matchresults in particular angle are presented in the paper. The probability density of realreverberation is analyzed and fitted. Moreover, under given reverberation background, thefourth-order cumulants beamforming processing gain is calculated.Then the fractional Fouriertransform domain reverberation filtering method is adopted for the main beam output signal.Based on the SRR definition in the paper, the averages of SRR processing gain under twograzing angles are obtained.According to target detection demands of underwater active sonar, a method by usingonly a small apture of array obtaining narrow beamwith is proposed. Related researches inthis paper can simplify the complexity of acoustic detection and signal processing, and can bewidely applied to other types of underwater targets detection.