Research Of Focusing And Imaging With Time-reversal Mirror

Ocean is underwater acoustic channel propagate signal from source to receiver. It is important for signal processor to know how to compensate for aberrations caused by inhomogeneities in the propagation medium. Time reversal is a unique self-adaptive focusing technique developed from phase conjugation using in nonlinear optics and ultrasonic fields. It can avoid imaging aberration of inhomogeneous media in acoustic and realize self-adaptive focusing without the prior knowledge of the media and the transducer. The foundamental theory to study is expained first in this paper and we introduce the application of the time reversal mirror in the domain of the medince and underwater acoustic and so on, then adaptive focusing and imaging on time reversal mirror are studied:(1) The self-adaptive focusing effects of time reversal mirror (TRM) are studied based on time-reversal focusing theory. The simulation on self-adaptive focusing for time reversal is carried out in typical underwater experiment and corresponding theoretical analysis is also conducted. The simulation results of TRM show that focusing gain can be obtained using the coherence of multi-path channel. Focusing effect with a TRM through random medium is much better than that obtained in homogeneous medium.(2) Focusing performance of TRM in inhomogeneous medium is studied. The experiment research on time reversal focusing is carried out in a virtual water tank and corresponding theoretical analysis is also conducted. Using the coherence of multi-path, the effective aperture of TRM is enlarged and spatial processing gain can be obtained more than lOdB. Focusing effect with a TRM through inhomogeneous medium is much better than that obtained in homogeneous medium.(3) An eigenvalue decomposition algorithm of time reversal operator is proposed. The time reversal operator can be decomposed for ideally resolved scattered target. Each of its eigenvectors of non-zero eigenvalue provides the focusing phase law of the corresponding target for transducer arrays. The algorithm realizes selective detection and focusing and provides theoretical foundation for distinguishing multiple targets and selective focusing.(4) A detection method for LFM signals based on TRM and Wigner-Hough Transform is proposed. The simulation on time frequency analysis is carried out in a virtual water tank. A signal is received passively by a TRM, time-reversed and then re-transmitted into the same medium. The re-transmitted signal is received near the source and WHT time frequency analysis is performed. Then comparison of detection performance of WVD and WHT on LFM signal is made. The simulation results show that WVD of multiple component LFM signal causes cross-term interference signals at the low frequency and corresponding high frequency segments of every scattering point. This new method can suppress multi-scattering, reduce cross-term and enhance the detection performance of double component LFM signal.(5) A new imaging algorithm is proposed which is motivated by TRM in random medium. DOA-AT imaging algorithm performs as the following steps: arrival time analysis of the echo received from the scatters; singular value decomposition of the array response matrix in frequency domain; combination of DOA estimation of target function in the domain and arrival time estimation that results in DOA-AT estimation algorithm.The range estimation of the imaging target is improved.