时间反转镜聚焦和成像技术研究
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摘要
海洋是目标信号传至接收点所经的声信道,如何克服传播介质不均匀及信道的多途特性对接收信号的影响,是声呐信号处理的关键之一。时间反转法是从光学中的相位共轭法演化而来的一种自适应聚焦方法,它具有不需要介质不均匀性的先验信息,自适应修正各种多途引起的畸变,实现自适应聚焦的独特优点。本文首先对研究的基本理论作简单的阐述,并且介绍了时间反转镜在医学、水声等领域的实际应用。然后全文就时间反转镜自适应聚焦和成像作如下研究：
(1)分析了时间反转自适应聚焦原理,在典型的水文条件下进行了时间反转镜自适应聚焦的仿真研究并进行了相应的理论分析。仿真结果表明,在随机媒质中,时间反转镜利用水声信道的相干多途特性获得聚焦增益,聚焦效果明显优于均匀媒质。
(2)研究了非均匀媒质中时间反转镜的聚焦性能问题。在虚拟水槽内进行了时间反转聚焦的仿真实验研究并进行了相应的理论分析。理论和仿真实验结果表明,在非均匀媒质中,由于利用水声信道的相干多途特性,时间反转镜的有效孔径增大,获得空间处理增益10dB以上；非均匀媒质聚焦效果明显优于均匀媒质。
(3)提出一种时间反转算子特征值分解算法。对于可分辨的理想散射目标,该算法将时间反转算子特征值分解,每个非零特征值对应的特征向量为换能器阵列提供相应目标的聚焦相位信息,实现选择性检测和聚焦,这为时间反转镜区分多目标和选择聚焦提供了理论依据。
(4)提出一种基于时间反转镜和Wigner-Hough变换的线性调频信号检测方法。在虚拟水槽内进行时频分析的仿真研究,将时间反转镜被动接收的信号进行时间反转再发射到同一媒质中,在信号源处将接收到的信号进行WHT时频分析,同时比较了WVD分布和WHT变换的LFM信号检测性能。仿真结果表明,对于多分量LFM信号,WVD在每个散射信号的低频和高频段产生交叉干扰项,影响对LFM信号的检测。时间反转镜结合WHT分析能够抑制多途效应,减少交叉项,提高双分量LFM信号的检测性能。
(5)提出在随机媒质中采用时间反转镜超声成像的DOA-AT新算法。新算法通过对散射中接收信号的到达时间和响应矩阵频域奇异值进行分解,将时域内目标函数DOA估计与到达时间估计结合起来,使成像目标的纵坐标估计得到明显改善,对目标检测能力增强。
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.
(1)分析了时间反转自适应聚焦原理,在典型的水文条件下进行了时间反转镜自适应聚焦的仿真研究并进行了相应的理论分析。仿真结果表明,在随机媒质中,时间反转镜利用水声信道的相干多途特性获得聚焦增益,聚焦效果明显优于均匀媒质。
(2)研究了非均匀媒质中时间反转镜的聚焦性能问题。在虚拟水槽内进行了时间反转聚焦的仿真实验研究并进行了相应的理论分析。理论和仿真实验结果表明,在非均匀媒质中,由于利用水声信道的相干多途特性,时间反转镜的有效孔径增大,获得空间处理增益10dB以上；非均匀媒质聚焦效果明显优于均匀媒质。
(3)提出一种时间反转算子特征值分解算法。对于可分辨的理想散射目标,该算法将时间反转算子特征值分解,每个非零特征值对应的特征向量为换能器阵列提供相应目标的聚焦相位信息,实现选择性检测和聚焦,这为时间反转镜区分多目标和选择聚焦提供了理论依据。
(4)提出一种基于时间反转镜和Wigner-Hough变换的线性调频信号检测方法。在虚拟水槽内进行时频分析的仿真研究,将时间反转镜被动接收的信号进行时间反转再发射到同一媒质中,在信号源处将接收到的信号进行WHT时频分析,同时比较了WVD分布和WHT变换的LFM信号检测性能。仿真结果表明,对于多分量LFM信号,WVD在每个散射信号的低频和高频段产生交叉干扰项,影响对LFM信号的检测。时间反转镜结合WHT分析能够抑制多途效应,减少交叉项,提高双分量LFM信号的检测性能。
(5)提出在随机媒质中采用时间反转镜超声成像的DOA-AT新算法。新算法通过对散射中接收信号的到达时间和响应矩阵频域奇异值进行分解,将时域内目标函数DOA估计与到达时间估计结合起来,使成像目标的纵坐标估计得到明显改善,对目标检测能力增强。
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.
引文
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