Xionghou Liu

Low complexity MIMO sonar imaging using a virtual sparse linear array

A multiple-input multiple-output (MIMO) sonar can synthesize a large-aperture virtual uniform linear array (ULA) from a small number of physical elements. However, the large aperture is obtained at the cost of a great number of matched filters with much heavy computation load. To reduce the computation load, a MIMO sonar imaging method using a virtual sparse linear array (SLA) is proposed, which contains the offline and online processing. In the offline processing, the virtual ULA of the MIMO sonar is thinned to a virtual SLA by the simulated annealing algorithm, and matched filters corresponding to inactive virtual elements are removed. In the online processing, outputs of matched filters corresponding to active elements are collected for further multibeam processing and hence, the number of matched filters in the echo processing procedure is effectively reduced. Numerical simulations show that the proposed method can reduce the computation load effectively while obtaining a similar imaging performance as the traditional method.

Low Sidelobe Range Profile Synthesis for Sonar Imaging Using Stepped-Frequency Pulses

A stepped-frequency sonar imaging method with low range sidelobe peak (SLP) is proposed. Two linear frequency modulation (LFM) pulses with partly overlapped bands and opposite modulation directions are used; and both auto- and cross-correlation functions are taken to synthesize the low SLP range profile. With the presence of echo misalignment, a 2-D searching method is given to find the optimal signal parameters at the lowest SLP. Numerical simulations show that the proposed method can suppress the range SLP effectively and requires only a half instantaneous system bandwidth, when compared with the traditional method using a single LFM pulse.

Modeling the vertical directionality of wind-generated noise in deep ocean using Pekeris-branch-cut-based normal modes

Vertical directionality is an important property of wind-generated noise. In this paper, we extend the vertical directionality model of wind-generated noise in [12] from shallow water to deep ocean. In order to treat the overhead and distant noise simultaneously, we propose to use Pekeris-branch-cut-based normal modes to represent the Greens functions from noise sources to receivers. Comparing with the situation in shallow water, we find that the diagonal dominance of the noise modal covariance is weaker in deep ocean. In our model, both diagonal elements and a plenty of off-diagonal elements near the main diagonal line are involved into the modeling progress to ensure the accuracy. The errors could be introduced by neglecting the off-diagonal elements are simulated and analyzed. Using wave propagation theory, we give a reasonable explanation of why the diagonal dominance of the noise modal covariance matrix is weaker in deep ocean.

Two-step optimal subarray detection based on matched-field processing

The optimal detector based on matched-field processing with a large-aperture vertical linear array (VLA) suffers from the so called “snapshot deficient problem”, which leads to an inaccurate estimate of the noise covariance matrix and hence a degraded detection performance. In this paper, to improve the detection performance with deficient snapshots, a two-step sub-array detector (SAD) is proposed. Due to the reduction on the hydrophone number in the two-step processing, the proposed SAD improves the estimate of the noise covariance matrix. Thus, SAD has a better detection performance than the optimal full array detector (FAD) in a shallow-water correlated noise environment with short snapshots. Numerical simulation results demonstrate the effectiveness of the proposed SAD.

Range sidelobe suppression for FD-MIMO sonar imaging using multi-ping amplitude weighting

The frequency diverse multiple-input multiple-output (FD-MIMO) sonar can use a group of small-bandwidth FD pulses to synthesize a large bandwidth signal, which brings a higher range resolution than traditional methods. However the range sidelobe level (SLL) is high due to the cross-correlation function (CCF) interferences and the spectrum leakage between auto-correlation functions (ACFs) of transmitting waveforms. To suppress the range SLL, we propose a multi-ping amplitude weighting method using a set of FD continuous wave (FDCW) pulses. In our method these FDCW pulses are divided into several groups, and each group is emitted in the corresponding ping. To suppress CCFs, the band gap between FDCW pulses transmitted in a ping is set to be relatively large. Furthermore, to suppress the spectrum leakage, the amplitude weights calculated via an optimization problem are applied to ACFs of all FDCW pulses. Via numerical simulations, we show the proposed method can suppress the range SLL of FD-MIMO sonar effectively.

Using double-ping frequency diverse mimo sonar to improve angle and range resolution

A high angle and range resolution imaging method using a double-ping frequency diverse multiple-input multiple-output (MIMO) sonar is proposed. The proposed MIMO sonar is composed of a uniform linear array (ULA) and two transmitting elements located at the two ends of the receiving ULA. The interelement spacing of the two transmitting elements is equal to the produce of the inter-element spacing and the element number of the receiving ULA. We show that such array configuration is able to ensure a high angle resolution without increasing the array physical size. At the same time we use a pair of frequency diverse linear frequency modulation (FD-LFM) pulses to achieve a high range resolution. Based on the array configuration and the transmitting waveforms, we give the double-ping signal transmitting mode and the echo processing procedure. Via numerical simulations we show that the proposed double-ping MIMO sonar imaging method can improve the angle and range resolutions simultaneously.

Underwater high-resolution range-dimension imaging using comb spectrum signal

The range resolution of an imaging sonar is difficult to be improved due to the restriction of the instantaneous bandwidth of the sonar system. To increase the range resolution of the imaging sonar, an imaging method with the use of the comb spectrum signal, which is composed of a set of continuous wave (CW) pulses with different center frequencies, is proposed. After the round-trip propagation these CW components, although sharing the same propagation range, have different phase shifts due to the frequency offset between them. In other words, the phase differences occur at different ranges. Thus, the beamformer (which is traditionally applied to the angle dimension to filter out echoes reflected back from a certain angle) can be utilized to process these CW components in the range dimension. We call such beamformer the range-dimension beamformer. Similar to the adaptive beamformer applied to the angle dimension, the range-dimension adaptive beamformer is proposed to obtain a high range resolution. Furthermore, we give the scanning vector, the covariance matrix, and the expression of the range-dimension adaptive beamformer. Via numerical examples we show that the proposed method can increase the range resolution effectively under the restriction of a certain instantaneous system bandwidth.

Array gain of an eigenvalue beamformer in a continental slope waveguide

The acoustic environment of the continental shelf slope waveguide is more complex than those with flat bottoms. For a long receiving array in this area, the amplitude and phase of the received signals will be distortional, leading an attenuation in the spatial correlation of the sound field and a degradation in the array gain of beamformers. This paper shows the performances of three beamformers: the conventional beamformer (CBF), the minimum variance distortionless response beamformer (MVDR-BF) and the eigenvalue beamformer (EBF) by means of the array gain in the continental shelf slope waveguide, via theoretical analyses and computer simulations. A horizontal uniform linear array (ULA) with a wide-aperture in the slope region receiving signals from a shallow water source is considered. The array gains of CBF, MVDR-BF and EBF are compared, and the results show that EBF demonstrates higher array gains compared to CBF and MVDR-BF.