Shaohao Zhu

Study of robust and high-gain beamforming based on diagonal reducing

Ocean ambient noise mainly includes uncorrelated noise and correlated noise. The uncorrelated noise only affects the diagonal elements in the noise covariance matrix, and it can be suppressed by subtracting a value on the diagonal elements. Utilizing this feature, a high array-gain beamforming based on diagonal reducing is presented. Meanwhile, the robustness becomes poor. So, the array weight norm constraint is used to ensure the robustness of this method, and the threshold is selected by some criterions. Then, the diagonal reducing value is calculated by the newton iteration method. After subtracting the diagonal reducing value, the covariance matrix is used in the minimum variance distortionless response (MVDR) beamformer. Both the numerical simulations and experimental results show that the proposed method can improve the array gain and the resolution capacity of multi-targets and it also can provide a tradeoff between array gain and robustness by chanOcean ambient noise mainly includes uncorrelated noise and correlated noise. The uncorrelated noise only affects the diagonal elements in the noise covariance matrix, and it can be suppressed by subtracting a value on the diagonal elements. Utilizing this feature, a high array-gain beamforming based on diagonal reducing is presented. Meanwhile, the robustness becomes poor. So, the array weight norm constraint is used to ensure the robustness of this method, and the threshold is selected by some criterions. Then, the diagonal reducing value is calculated by the newton iteration method. After subtracting the diagonal reducing value, the covariance matrix is used in the minimum variance distortionless response (MVDR) beamformer. Both the numerical simulations and experimental results show that the proposed method can improve the array gain and the resolution capacity of multi-targets and it also can provide a tradeoff between array gain and robustness by changing the diagonal reducing value.ging the diagonal reducing value.

Direction-of-arrival estimation using high-order superdirective beamforming for a circular hydrophone array

This paper presents a method of direction-of-arrival estimation using the high-order superdirective beamforming for a circular hydrophone array. The weighting vectors with different orders used for DOA estimation can be obtained using the previously proposed eigenbeam decomposition and synthesis theory, which can provide frequency-invariant beampatterns in a wide frequency band. The DOA estimation result of this method is the sum of estimations with different orders and the robustness decreases with an increase of the number of order. The reduced-rank technique is applied to achieve robust DOA estimations by truncating some error-intolerable high order estimation results and reserving the low order ones. Simulation results show that the DOA method proposed in this paper can improve the angular resolution (when the sensor space to wavelength ratio is smaller than 1/2) compared to the conventional beamforming (CBF) method. An experiment was conducted in the South China Sea with a 12-hydrophone circular array, and the results show effectiveness and validity of the proposed method.

Depth-dependence and vertical directionality of wind-driven ambient noise in deep ocean

The received wind driven noise field can be described as a sum of fields radiated from many random sources on the sea surface. Wind driven noise source levels can be modeled by empirical relationship with local wind speed. A method for modeling wind-driven ambient noise based on ray approach in deep ocean is presented. Simulations are performed to demonstrate the validity of the method. The wind-driven ambient noises are attributed to not only the near noise sources but also distant noise sources. The depth dependence and vertical directionality of ambient noise caused by wind agitation are analyzed by using the proposed modeling method, and vertical directionality frequency dependence is also described. In addition, the relationship between the logarithm of wind speed and noise levels is also analyzed at near surface and bottom depth. The power of wind speed with respect to received noise levels is equal to that of wind speed with respect to noise source levels approximately.

Experimental study of superdirective frequencyinvariant beamforming for a circular hydrophone array

This paper first presents a superdirective frequency-invariant beamforming method for circular sensor arrays. The weighting vector of this method is in close-form, which is derived using the criterion of minimizing the mean square error between the desired and synthesized beampatterns. A detailed experimental study of this superdirective frequency-invariant beamforming method is then followed, which is used to further demonstrate its performance. In the experiment, the superdirective frequency-invariant broadband beamformers with a specific response are designed so that they can be applied to effectively suppress the ambient noise. The experimental data are processed in the time domain in which finite impulse response filters are designed according the obtained superdirective frequency-invariant broadband beamformers. The results show that this superdirective frequency-invariant beamforming method provides good signal-to-noise ratio enhancement.

Effects of eddy on close range DOA estimation

In this paper, Argo data combined with altimetry data were used to investigate the effect of eddy on Sound Speed Profile (SSP) in Northwestern Pacific Ocean. A combination of the closed contours method and the Okubo-Weiss method is used to detect the eddy. Statistics of SSP anomalies induced by eddy have been calculated by removing the climatology from Argo profiles in eddy. Then effects of eddy on close range Direction of Arrival (DOA) estimation have been studied with a conventional beamforming method. Results suggest that the DOA estimation performance may deteriorate due to presence of the eddy. The deterioration occurs when the receiver is placed near the direct path range. This is because the direct path range is sensitive to the SSP anomalies and the presence of the eddy may lead to the receiver falling in/out of the direct path range.

Effects of hydrophone gain errors and self-noise on the DOA performance

This paper studies the effects of hydrophone gain errors and self-noise on the direction-of-arrival (DOA) performance of three methods: the conventional beamforming (CBF) method, the minimum variance distortionless response (MVDR) method and the multiple signal classification (MUSIC) method. Simulations for a 16-hydrophone uniform circular array whose diameter is 2 m are provided in three different scenarios, in which the incident signal is supposed to be a narrow-band signal. The results show that both the gain errors and the self-noise make the main-lobes of the beam patterns wide and the side-lobes high, and the self-noise caused by data channels has less influence on the DOA performance than the gain errors caused by the external pressure. Experiments with a 16-hydrophone uniform circular array at a depth of 3500 m in the South China Sea show that the three methods cannot obtain the direction of a real target at a low signal to noise ratio (SNR) when two hydrophones have large self-noise. By contrast, when the amplitude of each channel signal is normalized individually, the direction of the real target can be estimated using the above-mentioned three methods. If gain errors of some hydrophones are large, amplitude normalization can eliminate the errors very well. Consequently, amplitude normalization can reduce the negative effects of hydrophone gain errors and self-noise and improve the DOA performance.