| With the ever increasing national attention to the establishment of marine powerful nation and the rapid development of underwater acoustic technologies,it is significantly urgent needs for ocean explorations and coastal defense construction that how to improve the capability of the detection and direction of arrival(DOA)estimation of remote targets from the marine underwater acoustic system.In this dissertation,in-depth research about critical technologies related to wideband DOA estimation with deficient snapshots and finite snapshots,and the end-fire targets DOA estimation,and underdetermined wideband DOA estimation method with nested arrays(NA),are carried out using a combination of theoretical analysis,computer simulation and experimental verification,to solve the problem of sensing capability of underwater targets.This research results in important theoretical significance and practical application impact for signal detection and DOA estimation,and provides technical guidelines for design and development of marine underwater acoustic systems.The main results and achievements are summarized as follows:1.An enhanced DOA estimation method by using atomic norm minimization(ANM)is proposed to improve the poor performance of incoherent signal subspace DOA estimation methods given deficient snapshots.Thanks to the decent Vandermonde structure provided by the manifold matrix of each sub-band bin,the Toeplitz-structured covariance matrix of the signals of interest can be reconstructed by solving an ANM problem.Then,the eigenvalue decomposition of the recovered covariance matrix allows to well separate the signal subspace and the noise subspace,which enables to retrieve the angles of targets at individual sub-bands.Finally,the estimation results of all sub-bands are averaged out to output the final estimation angle.Simulation results show that the proposed method can effectively improve the performance of DOA estimation compared with the traditional incoherent signal subspace methods.2.A new wideband DOA estimation method via joint sub-band atomic norm minimization is proposed to solve the problem of DOA estimation of wideband signals with coherent processing given deficient snapshots.In this method,a focus matrix obtained by RSS algorithm is formed and adopted to align each sub-band signal to a predefined reference frequency.Then,the focused sub-band signals are combined to form a fat measurement matrix,which leads to a structure-based optimization problem where a low-rank covariance matrix with Toeplitz structure is reconstructed via semi-definite programming(SDP).Simulation results verify the correctness and effectiveness of the proposed method.In addition,the proposed method can effectively estimate coherent source signals.3.A novel wideband DOA estimation method based on covariance matrix reconstruction and denoising is proposed to solve the wideband DOA estimation over finite snapshots.A focused coherent covariance matrix is obtained by rotating every sub-band bin to a reference frequency bin and then applying an eigenvalue decomposition on the focused coherent covariance matrix.Thanks to the value distribution of the eigenvalues,a low-rank enhancement is achieved by a denoising operation through structure-based optimization,which leads to improved performance of subsequent MUSIC.Simulation results show that the proposed method outperforms the spatial smoothing MUSIC method(SS-MUSIC)and Top-MUSIC at low SNRs and small angle separations.4.A new DOA estimation for wideband signals with A-shaped array is proposed to obtain high performance of DOA estimation without expanding array aperture and increasing array elements,when the signals come from the direction of end-fire or from a direction close to endfire.To do so,an A-shaped array is first proposed with desired array structure,followed with the derivation of corresponding mathematical model for wideband signals.Further,to suppress the interferences,an MVDR beam forming method based on sub-band decomposition is applied to estimate the spatial spectrum accurately,and the target is finally estimated by locating the position of spectral peaks.This approach is motivated from an important fact that the A-shaped array can enlarge the angle scanning range to 360 degrees.Simulation results show that the proposed solution can effectively detect and estimate the signal from the direction of end-fire.Moreover,the proposed solution can obtain finer resolution for multiple signals from the nearby direction of end-fire at low SNRs.5.Aiming at addressing the problem of fewer physical sensors than signals in DOA estimation,we propose a novel underdetermined DOA estimation method for wideband signals equipped with NA,which utilizes ANM to seek and obtain the optimal Toeplitz covariance matrix.A virtual array formed by the difference co-array is used as a substitute for the physical array geometry,which enlarges the array aperture and increases the degrees of freedom(DOF).We then build a virtual signal mode through vectorizing the covariance matrix of each frequency bin,and collecting the average vector by focusing technique,and building a signal model with single snapshot based on the virtual array.A target detection method under the underdetermined wideband virtual array is proposed by combining the forward smoothing method and criterion in information theory.In the virtual array signal model over a single snapshot,the array steering vectors exhibit Vandermonde structure,which leads to the Toeplitz covariance matrix recovery via the ANM with improved detection and estimation accuracy.Simulation results show that the proposed detection method enhances the probability of successful signal detection,and outperforms other underdetermined DOA estimation methods in terms of higher estimation accuracy at low SNRs and over small number of snapshots.6.Upon the theoretical analysis and computer simulation evaluation,a lake test system for A-shaped array is constructed,on which the relevant lake test research is carried out.The field test results verify the effectiveness and efficiency of the proposed wideband DOA estimation method based on an A-shaped array,The lake test results are consistent with the simulation results,which sheds lights on its promising applications in practice. |
