Constrained Channel Estimation Methods in Underwater Acoustics

Underwater acoustic signal processing aims to reconstruct the shallow water acoustic channel from both direct arrival and delayed multipath reflections. This enables accurate acoustic communications in shallow water, such as between underwater autonomous vehicles (UAVs), which are essential for coastal surveillance and other applications. In this work, we take a previously implemented algorithm for channel estimation and apply practical constraints motivated by shallow water acoustic physics. We base our estimation constraints on the physical properties of the rapidly fluctuating reflections from the moving sea surface and rough sea bottom. Our work aims to reduce the computation time and prediction error in the channel estimation using our constraints for real time applications

Multiple and single snapshot compressive beamforming

For a sound field observed on a sensor array, compressive sensing (CS) reconstructs the direction-of-arrival (DOA) of multiple sources using a sparsity constraint. The DOA estimation is posed as an underdetermined problem by expressing the acoustic pressure at each sensor as a phase-lagged superposition of source amplitudes at all hypothetical DOAs. Regularizing with an $\ell_1$-norm constraint renders the problem solvable with convex optimization, and promoting sparsity gives high-resolution DOA maps. Here, the sparse source distribution is derived using maximum a posteriori (MAP) estimates for both single and multiple snapshots. CS does not require inversion of the data covariance matrix and thus works well even for a single snapshot where it gives higher resolution than conventional beamforming. For multiple snapshots, CS outperforms conventional high-resolution methods, even with coherent arrivals and at low signal-to-noise ratio. The superior resolution of CS is demonstrated with vertical array data from the SWellEx96 experiment for coherent multi-paths.Comment: In press Journal of Acoustical Society of Americ

Environmental model-based time-reversal underwater communications

Advances in underwater acoustic communications require the development of methods to accurately compensate channels that are prone to severe double spreading of time-varying multipath propagation, fading and signal phase variations. Assuming the environmental information as a key issue, this work aims to improve communications performance of single-input-multiple-output transmission systems in such channels through the enhancement of their estimates used for equalization. The acoustic propagation physical parameters of the environment between the source and the receivers are considered in the process. The approach is to mitigate noise e ects in channel identi cation for Passive Time-Reversal (PTR), which is a low complexity probe-based refocusing technique to reduce time spreading and inter-symbol interference. The method Environmental-based PTR (EPTR) is proposed that, inspired by matched eld inversion, inserts physics of acoustic propagation in the channel compensation procedure through ray trace modeling and environmental focalization processing. The focalization is the process of tweaking the environmental parameters to obtain a noise-free numerical model generated channel response that best matches the observed data. The EPTR performance is tested and compared to the pulse-compressed PTR and to the regularized `1-norm PTR. The former is based on classical `2-norm channel estimation and the latter, inspired by compressive sensing, uses weighted `1-norm into the `2-norm estimation problem to obtain improved estimates of sparse channels. Successful experimental results were obtained with the proposed method for signals containing image messages transmitted at 4 kbit/s from a source to a 16-hydrophones vertical array at 890 m range during the UAN'11 experiment conducted o the coast of Trondheim (Norway). The scienti c contributions of this work are (i) the understanding of the process of employing physical modeling and environmental focalization to equalize and retrieve received messages in underwater acoustic communications, thus exploiting the sensitivity of environmental parameters in order to adapt a communications system to the scenario where it is used; and (ii) the presentation of a new PTR-based method that focuses environmental parameters to model suitable noise-free channel responses for equalization and whose real data results were successful for a set of coherent signals collected at sea. The proposed method is a step forward to a better understanding on how to insert physical knowledge of the environment for equalization in digital underwater acoustic communications

Analysis of and techniques for adaptive equalization for underwater acoustic communication

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2011Underwater wireless communication is quickly becoming a necessity for applications in ocean science, defense, and homeland security. Acoustics remains the only practical means of accomplishing long-range communication in the ocean. The acoustic communication channel is fraught with difficulties including limited available bandwidth, long delay-spread, time-variability, and Doppler spreading. These difficulties reduce the reliability of the communication system and make high data-rate communication challenging. Adaptive decision feedback equalization is a common method to compensate for distortions introduced by the underwater acoustic channel. Limited work has been done thus far to introduce the physics of the underwater channel into improving and better understanding the operation of a decision feedback equalizer. This thesis examines how to use physical models to improve the reliability and reduce the computational complexity of the decision feedback equalizer. The specific topics covered by this work are: how to handle channel estimation errors for the time varying channel, how to use angular constraints imposed by the environment into an array receiver, what happens when there is a mismatch between the true channel order and the estimated channel order, and why there is a performance difference between the direct adaptation and channel estimation based methods for computing the equalizer coefficients. For each of these topics, algorithms are provided that help create a more robust equalizer with lower computational complexity for the underwater channel.This work would not have been possible without support from the O ce of Naval Research, through a Special Research Award in Acoustics Graduate Fellowship (ONR Grant #N00014-09-1-0540), with additional support from ONR Grant #N00014-05- 10085 and ONR Grant #N00014-07-10184

Inversion for subbottom sound velocity profiles in the deep and shallow ocean

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2005This thesis investigates the application of acoustic measurements in the deep and shallow ocean to infer the sound velocity profile (svp) in the seabed. For the deep water ocean, an exact method based on the Gelfand-Levitan integral equation is evaluated. The input data is the complex plane-wave reflection coefficient estimated from measurements of acoustic pressure in water. We apply the method to experimental data and estimate both the reflection coefficient and the seabed svp. A rigorous inversion scheme is hence applied in a realistic problem. For the shallow ocean, an inverse eigenvalue technique is developed. The input data are the eigenvalues associated with propagating modes, measured as a function of source-receiver range. We investigate the estimation of eigenvalues from acoustic fields measured in laterally varying environments. We also investigate the errors associated with estimating varying modal eigenvalues, analogous to the estimation of time-varying frequencies in multicomponent signals, using time-varying autoregressive (TVAR) methods. We propose and analyze two AR sequential estimators, one for model coefficients, another for the zeros of the AR characteristic polynomial. The decimation of the pressure field defined in a discrete range grid is analyzed as a tool to improve AR estimation. The nonlinear eigenvalue inverse problem of estimating the svp from a sequence of eigenvalues is solved by iterating linearized approximations. The solution to the inverse problem is proposed in the form of a Kalman filter. The resolution and variance of the eigenvalue inverse problem are analyzed in terms of the Cramer-Rao lower bound and the Backus-Gilbert (BG) resolution theory. BG theory is applied to the design of shallow-water experiments. A method is developed to compensate for the Doppler deviation observed in experiments with moving sources.I am grateful for the support of my work provided by the WHOI Academic Programs Office and the Office of Naval Research

Effects of errorless learning on the acquisition of velopharyngeal movement control

Session 1pSC - Speech Communication: Cross-Linguistic Studies of Speech Sound Learning of the Languages of Hong Kong (Poster Session)The implicit motor learning literature suggests a benefit for learning if errors are minimized during practice. This study investigated whether the same principle holds for learning velopharyngeal movement control. Normal speaking participants learned to produce hypernasal speech in either an errorless learning condition (in which the possibility for errors was limited) or an errorful learning condition (in which the possibility for errors was not limited). Nasality level of the participants’ speech was measured by nasometer and reflected by nasalance scores (in %). Errorless learners practiced producing hypernasal speech with a threshold nasalance score of 10% at the beginning, which gradually increased to a threshold of 50% at the end. The same set of threshold targets were presented to errorful learners but in a reversed order. Errors were defined by the proportion of speech with a nasalance score below the threshold. The results showed that, relative to errorful learners, errorless learners displayed fewer errors (50.7% vs. 17.7%) and a higher mean nasalance score (31.3% vs. 46.7%) during the acquisition phase. Furthermore, errorless learners outperformed errorful learners in both retention and novel transfer tests. Acknowledgment: Supported by The University of Hong Kong Strategic Research Theme for Sciences of Learning © 2012 Acoustical Society of Americapublished_or_final_versio

OBTAINING A REFERENCE FOR CALIBRATING BROADBAND MULTIBEAM SEABED BACKSCATTER

Standard calibration procedures for multibeam sonars currently only address the fidelity of the bathymetric data. Equivalent effort is needed to ensure that the acquired seabed backscatter strength measurements are referenced to a similarly precise level. This thesis presents an operational method for acquiring reference seabed backscatter data utilizing multiple pre-calibrated split beam echo sounders covering a wide range (45-450 kHz) of frequencies. This is needed to cover the full range of frequencies utilized by multi-sector multibeam systems operating in continental shelf depths.The method considers both the frequency and the angle of incidence dependence of the backscatter strength of a homogenous seafloor region. By using a mechanically rotated plate, the split beam transducers, once calibrated, are able to collect the absolute angular response curve of the seafloor for any frequency within the bandwidth of interest. This thesis addresses the design, implementation and required processing to deliver the curves of selected areas. Although not part of this research, the next step would be to calibrate the desired multibeam echosounder for backscatter by comparing the results obtained by the systems over the same seafloor area. The results obtained, reveal one of the most complete pictures of the continuous variation of the seabed backscatter angular response from 45 to 400 kHz. Significantly, this extends well above the 100 kHz level that normally defines the upper end of surface scattering model fidelity. As the chosen sites cover the main range of expected marine sediment types (gravel to mud), trends in both frequency and grazing angle are apparent that might impact the choice of frequency used in multi-spectral backscatter imaging

OBTAINING A REFERENCE FOR CALIBRATING BROADBAND MULTIBEAM SEABED BACKSCATTER

Standard calibration procedures for multibeam sonars currently only address the fidelity of the bathymetric data. Equivalent effort is needed to ensure that the acquired seabed backscatter strength measurements are referenced to a similarly precise level. This thesis presents an operational method for acquiring reference seabed backscatter data utilizing multiple pre-calibrated split beam echo sounders covering a wide range (45-450 kHz) of frequencies. This is needed to cover the full range of frequencies utilized by multi-sector multibeam systems operating in continental shelf depths.The method considers both the frequency and the angle of incidence dependence of the backscatter strength of a homogenous seafloor region. By using a mechanically rotated plate, the split beam transducers, once calibrated, are able to collect the absolute angular response curve of the seafloor for any frequency within the bandwidth of interest. This thesis addresses the design, implementation and required processing to deliver the curves of selected areas. Although not part of this research, the next step would be to calibrate the desired multibeam echosounder for backscatter by comparing the results obtained by the systems over the same seafloor area. The results obtained, reveal one of the most complete pictures of the continuous variation of the seabed backscatter angular response from 45 to 400 kHz. Significantly, this extends well above the 100 kHz level that normally defines the upper end of surface scattering model fidelity. As the chosen sites cover the main range of expected marine sediment types (gravel to mud), trends in both frequency and grazing angle are apparent that might impact the choice of frequency used in multi-spectral backscatter imaging