Shallow Water Acoustic Channel Research Articles

Shallow water acoustic channel modeling with adaptive communications

The underwater acoustic channel is known to be severely bandwidth limited due to sound attenuation by sea water, and interaction with the ocean surface and bottom. Yet, shallow water acoustic channels at high frequencies are little understood, particularly in shallow water environments, and hence the quest for achieving a viable adaptive communication solution has been a challenge that perplexed scientists for a long time. In this abstract, we first take Hodson River estuary as an example to investigate the characterizations of shallow water environments, which mainly comprises the evaluation of key channel parameters such as the scattering function, Doppler shift, coherent bandwidth, coherent time, 2D (i.e., Time-Frequency) time-variant channel impulse response (CIR). The study will also cover channel fading statistics, and water conditions that affect the CIR (e.g., bubble plumes and mddium inhomogeneities). Finally, the models developed will be used to evaluate the achievable performance of channel estimation and adaptive communication systems in shallow water acoustic media.

Volterra series-based non-linearity analysis of shallow water acoustic channels

Most of existing underwater acoustic (UWA) communication systems are based on the linear UWA channels. However, some environmental factors (e.g., bubble plumes) in complicated shallow water environments will contribute to the non-linearity of channels. Therefore, In order to fully understand the properties of shallow water acoustic channels, and develop more bandwidth-efficient communication systems in complicated shallow water environments, we adopt the Volterra series to analyze the non-linearity of shallow water acoustic channels for the first time, and its completed theoretical derivations will be presented. Volterra series combines the representations of a nonlinear system without memory and a linear, casual system with memory to describe a nonlinear system with memory. Generally speaking, the central problem in using a Volterra approach to the analysis of nonlinear channels with momory consists of estimating the Volterra kernels, which represent a nonparametric characterization of the channel.

Low Consumption Communications Over Shallow Water Acoustic Channels

Acoustic or ultrasonic transmission is the preferred alternative for underwater communications. Power is not an unlimited resource in the underwater environment. Moreover the underwater acoustic channel is subject to severe slow and fast fading. High transmission power is needed so that a fair BER is achieved. A power control policy is, therefore, of paramount importance. This communication explores the benefits of using power control in underwater communications, when gamma/Ricean channels are considered. Simulation results are presented that suggest which channels are more appropriate for the power control to be applied, and estimate the amount of expected power saving.

Performance of least mean square equalizer in shallow water acoustic communication channel

In shallow water, a transmitted signal is severely influenced by sea surface and bottom boundaries. Every signal to receiver except the signals through the water medium experiences a time-variant scattering in the sea surface and grazing-angle-dependent bottom reflection loss in the bottom. Consequently, the performance of underwater acoustic communication systems is degraded, and high-speed digital communication is disrupted by inter-symbol interference (ISI) effect. In this study, least mean square (LMS) algorithm equalizer is adopted to cancel out ISI effect. The equalizer is implemented in underwater acoustic communication system. Experiment is conducted in 26 m depth littoral ocean and the standard Lenna image which consists of 50x50 pixels and 8 bits per pixel is transmitted. It is verified that equalization is an efficient way to achieve high transmission rate in shallow water acoustic communication channel. This work was supported by Research Programs of Hanwha Corporation 2011.

Effect of wind-generated bubble plumes on shallow water acoustic channels

Wind blowing over the sea surface generates many dramatic effects. One of the most important effects is bubble plumes generated by breaking waves, which have major effects on high frequency acoustic propagation, sound speed profile and ambient noise due to the sound scattering and absorption properties of bubbles. On one hand, we will introduce several theoretical models of bubble plumes; on the other hand, we will propose how to calculate the acoustic signal attenuation and sound speed anomaly due to wind-generated bubble plumes. Simulation results will show the shallow water acoustic channel impulse response and sound speed anomaly with the effect of bubble plumes.

Experimental studies of support vector machine based blind equalization for shallow water channels

Due to extended multi-path spread and rapidly changing characteristics, shallow water acoustic channels pose excessive difficulties to the design of high performance underwater communication systems. While classic blind equalization algorithms such as the constant modulus algorithm (CMA) offer potential solutions to tackle the ISI (inter symbol interference) in underwater scenarios without training sequence, slow convergence rate as well as low noise tolerance limits their practical applications. In this paper, blind equalizer based on support vector machine (SVM) is adopted, which makes use of the excellent generalization ability of SVM to accelerate the convergence, and addresses the carrier phase error with embedded phase lock loop (PLL). Experimental SVM blind equalization results conducted in physical shallow water channels show significant performance improvements, demonstrating the effectiveness of the proposed method.

Performance simulation and implementation of gold sequences in underwater acoustic CDMA

Underwater Acoustic Network (UAN) is becoming a hotter research area of underwater acoustic communication recently. Underwater acoustic channels are restricted by narrow bandwidth, long delay and high ambient noise, so Code Division Multiple Access (CDMA) appears to be a promising multiple access technique for UAN. However, due to low underwater data rate, only short pseudo-random sequences are available. Comparing to m sequences, Gold sequences with the same length show the same cross-correlation properties but enable networks to support more users. This paper examines the performance of short Gold sequence in underwater acoustic channel, to determine whether it is a good choice for UAN. Specifically, the paper makes contributions on the following fronts: (1) Based on a Rayleigh fading shallow water acoustic channel model, the performance of bit error rate (BER) of a 63-bit Gold-sequence up to 65 users CDMA is simulated and evaluated in MATLAB platform. (2) Underwater CDMA nodes using Gold-sequence Direct Sequence Spread Spectrum (DSSS) modulation is implemented with National Instruments' new embedded device (cRIO9076). An experiment at sea will be conducted at Xiamen Port to confirm the better performance of short Gold sequences in underwater CDMA.

Iterative Receiver in Time–Frequency Domain for Shallow Water Acoustic Channel

Inter-symbol interference (ISI) caused by multi-path propagation, especially in shallow water channel, degrades the performance of underwater acoustic (UWA) communication systems. In this paper, we combine soft minimum mean squared error (MMSE) equalization and the serially concatenated trellis coded modulation (SCTCM) decoding to develop an iterative receiver in time–frequency domain (TFD) for underwater acoustic point to point communications. Based on sound speed profile (SSP) measured in the lake and finite-element ray (FER) tracing method (Bellhop), the shallow water channel is constructed to evaluate the performance of the proposed iterative receiver. The results suggest that the proposed iterative receiver can reduce the calculation complexity of the equalizer and obtain better performance using less receiving elements.

Iterative Receiver in Time–Frequency Domain for Shallow Water Acoustic Channel

Inter-symbol interference (ISI) caused by multi-path propagation, especially in shallow water channel, degrades the performance of underwater acoustic (UWA) communication systems. In this paper, we combine soft minimum mean squared error (MMSE) equalization and the serially concatenated trellis coded modulation (SCTCM) decoding to develop an iterative receiver in time–frequency domain (TFD) for underwater acoustic point to point communications. Based on sound speed profile (SSP) measured in the lake and finite-element ray (FER) tracing method (Bellhop), the shallow water channel is constructed to evaluate the performance of the proposed iterative receiver. The results suggest that the proposed iterative receiver can reduce the calculation complexity of the equalizer and obtain better performance using less receiving elements.

A geometric mixed norm approach to shallow water acoustic channel estimation and tracking

The shallow water acoustic channel is challenging to estimate and track due to rapid temporal fluctuations of its large delay spread. However, the impulse response and representations of its time-variability often exhibit a sparse structure that can be exploited to improve estimator performance. We propose a sparse reconstruction of the shallow water acoustic channel that employs a novel optimization metric combining the complex square root of the channel coefficients and a non-convex complex function based on the L2 estimation error. Our mixed norm formulation is mathematically equivalent to conventional L2 constrained L1 minimization, but fundamentally different in the non-convex topology we employ to solve for and track the optimal coefficients in real time directly over the complex field. Our estimation and tracking algorithm is designed for robustness with respect to the ill-conditioned nature of the data matrix, can smoothly handle different levels of sparsity, and is modeled to include delays due to multi-path and the Doppler spread induced by the channel. We present numerical evidence over simulated as well as field data to compare the performance of our method to conventional sparse reconstruction techniques.

Attack Simulation Model and Channel Statistics in Underwater Acoustic Sensor Networks

In recent years, underwater acoustic wireless sensor networks have been used in many areas. There have been many field trials of acoustic propagation models and statistics for shallow water conditions. However, field trials are limited environmentally and, hence, not widely accepted. Simulations of the impulse response of a shallow underwater acoustic channel allows less expensive system tests that are reproducable. This paper presents a shallow water acoustic channel model based on the actual acoustic propagation characteristics with path attenuation, ambient noise, multiple paths, and Doppler effects. The second-order statistical characteristics of the simulation model are verified with the autocorrelations and crosscorrelations of the quadrature components and the complex envelopes of channel impulse responses. The channel model is implemented in Matlab with the results showing that the absorption coefficient and path losses are both dependent on the frequencies and propagation distances and that the path gain can be improved with Light of Sight (LOS) and short range acoustic propagation. Analysis of the channel impulse response and the frequency response that the zero-order Bessel function of first kind can be used to describe the correlation functions for the impulse response. The shallow underwater acoustic channel is time-varying and can not be modeled as a wide-sense stationary-uncorrelated scattering channel.

Effectiveness of sparse optimization techniques against rapid fluctuations in the shallow water acoustic channel

The shallow water acoustic channel exhibits rapid temporal fluctuations due to fluid and platform motion as well as reflection from moving surfaces. Tracking the time-varying channel delay spread effectively for subsequent equalization is an open signal processing challenge. The delay-Doppler spread function characterizes this time-variability over a selected range of Doppler frequencies but itself does not exhibit stationary behavior over longer time intervals. Typically the delay-Doppler spread function and sometimes the delay spread itself follow a sparse distribution where most of the energy is concentrated in a few significant components distributed sparsely over a larger support. A variety of sparse optimization techniques have recently been proposed in the compressive sensing literature to efficiently track sparsely distributed coefficients. However, the ill-conditioned nature of the delay-Doppler spread estimation problem, coupled with the necessity to track the complex-valued coefficients directly in real time, renders direct application of traditional sparse sensing techniques infeasible and intractable in the shallow water acoustic paradigm. The talk will provide a synopsis of well-known and recently proposed sparse optimization techniques, with focus on mixed norm algorithms, along with a comparative analysis of these techniques in the shallow water acoustic paradigm over simulated and experimental field data.

Iterative Multiuser Detection for Underwater Acoustic Channels

Two different synchronous downlink multiple-access schemes, interleave-division multiple access (IDMA) and code-division multiple access (CDMA), are considered for pragmatic underwater communications channels exhibiting extended multipath spread and time variability. Two single-element DFE-IDMA and DFE-CDMA receivers are proposed that utilize chip-level adaptive decision feedback equalization (DFE) and carrier phase tracking along with iterative interference cancellation (IC) and channel coding. The receiver equations describing the detection algorithms are derived and their performance is investigated and compared. To track and compensate for the channel effects, the DFE and carrier phase tracking units are jointly optimized based on the mean square error (MSE) criterion and adapted iteratively by exchanging soft information in terms of log-likelihood ratio (LLR) estimates with the turbo processing stage. The detection is implemented by using soft chip cancellation to remove multiple-access interference (MAI) effects between users. The performance of the proposed receiver structures is investigated and compared in short-range shallow-water acoustic channels using offline processing of signals acquired during sea trials in the North Sea. Results for synchronous multiuser scenarios, with two and four users at an effective rate of 439.5 b/s per user, demonstrate that the proposed DFE-based IDMA and CDMA receivers can provide bit error rate (BER) performances of approximately 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> at an average signal-to-interference-and-noise ratio (SINR) of 11 dB . The experimental results also demonstrate that these direct adaptive receivers have better performance and significantly mitigate the bit errors associated with the channel estimation (CE)-based IDMA and CDMA receivers with Rake reception while maintaining lower complexity. Furthermore, in some cases, the receivers with partial knowledge of the interleavers' patterns or codes can still achieve performance comparable to those with full knowledge.

Implementation of an Impulse Noise Suppression Method in OFDM Underwater Acoustic Communication System

This paper describes the characteristics of the impulse noise in the shallow water acoustic channel, and its influence on the bit error rate (BER) performance of the Orthogonal Frequency Division Multiplexing (OFDM) system. The simulation has been carried out to analyze the performance of the joint suppression of impulse noise in time and frequency domain in OFDM underwater acoustic communication system. Data processing of the actual sea trial in Xiamen Harbor shows that the method has some impressive effect on impulse noise in shallow water acoustic OFDM system, and improves the BER performance on different levels.

Statistical Modeling of MIMO Mobile-to-Mobile Underwater Channels

This paper proposes a geometry-based statistical model for wideband multiple-input-multiple-output (MIMO) mobile-to-mobile (M-to-M) shallow-water acoustic (SWA) multipath fading channels. Based on the reference model, the corresponding MIMO time-frequency correlation function, Doppler power spectral density, and delay cross-power spectral density are derived. These statistics are important tools for the design and performance analysis of MIMO M-to-M SWA communication and sonar systems. Finally, the derived statistics are compared with the experimentally obtained channel statistics, and close agreement is observed.

Adaptive Channel Estimation for Multiuser Underwater Communication Systems

Problem statement: Multiple Access Interference (MAI) signals and poor estimation of the unknown channel parameters in the presence of limited training sequences are two of the major problems that degrade the systems’ performance. Two synchronous multiuser receivers with Rake reception of Interleave Division Multiple Access (IDMA) and Code Division Multiple Access (CDMA) systems, in conjunction with channel estimation, are considered for communication over different short range shallow water acoustic channels. Approach: The proposed hard/soft chip channel estimation and carrier phase tracking are jointly optimized based on the Mean Square Error (MSE) criterion and adapted iteratively by the reconstructed MAI signal. This is generated from exchanged soft information in terms of Log-Likelihood Ratio (LLR) estimates from the single-users’ channel decoders. The channel parameters and error estimation are used to enable the chip cancellation process to retrieve an accurate measurement of the detrimental effects of Intersymbol Interference (ISI) and MAI. Results: The performance of the proposed receiver structures with small processing gain are investigated and compare with 2 and 4 synchronous users using memoryless Quadrature Phase-Shift Keying (QPSK) at an effective rate of 439.5 bps per user. Conclusion: The results demonstrate that the performance is limited by MAI and ISI signals and the IDMA performance outperforms long code CDMA and short code CDMA.

Modeling a shallow water acoustic communication channel using environmental data for seafloor sensor networks

AbstractDevelopment of communication channels for underwater sensor networks holds many unique challenges. Communication near the bottom of the ocean is no exception as the effects of reflection and refraction greatly affect how acoustic waves travel between a source and an intended receiver. Deployment and testing in the ocean are difficult and expensive; thus there is a strong reliance on models to aid in design and development of a potential network. Since each ocean region can present very unique challenges, it is of great value to model an environment based on real environmental parameters whenever available. A well prepared channel model will provide the ability to show channel capacity as it relates to node positions, as well as showing the performance of modulation techniques to an environment with propagation characteristics and path arrivals. This channel model will also be implementable into a simulation package to allow for high quality simulation of higher level protocols. The proposed method has proved to be a useful tool in modeling a particular environment and provides insight into underwater sensor node placement and modulation. Copyright © 2009 John Wiley &amp; Sons, Ltd.

An adaptive sparse sensing approach to shallow water acoustic channel estimation and tracking.

The shallow water acoustic channel is challenging to estimate and track due to the ill‐conditioned nature of the problem, the need to optimize over a complex field, and the time‐varying nature of the channel coefficients. We have previously presented a geometric mixed norm approach to estimate and track the channel delays and delay‐Doppler spread function coefficients that exploit the often sparse distribution of the channel coefficients. In this work, we present the effectiveness of our approach over a range of field data collected at 15 m depth over ranges of 60, 200, and 1000 m at various wind conditions. We also compare the performance of the estimation and tracking algorithm against other sparse sensing approaches to shallow water acoustic channel estimation, e.g., the L1‐regularized least squares algorithm. We show that the choice of the sparsity factor λ, which is an essential design parameter in all sparse sensing methods, plays a critical role in the effectiveness of sparse reconstruction of real...

Advances in Signal Processing for Maritime Applications

The maritime domain continues to be important for our society. Significant investments continue to be made to increase our knowledge about what “happens” underwater, whether at or near the sea surface, within the water column, or at the seabed. The latest geophysical, archaeological, and oceanographical surveys deliver more accurate global knowledge at increased resolutions. Surveillance applications allow dynamic systems to be accurately characterized. Underwater exploration is fundamentally reliant on the effective processing of sensor signal data. All maritime applications face the same difficult operating environment: fading channels, rapidly changing environmental conditions, high noise levels at sensors, sparse coverage of the measurement area, limited reliability of communication channels, and the need for robustness and low energy consumption, just to name a few. There are obvious technical similarities in the signal processing that have been applied to different measurement equipment, and this special issue aims to help foster cross-fertilization between these different application areas. The articles in this special issue cover the following topics: First, underwater acoustics: “Underwater broadband source localization based onmodal filtering and features extraction”, “Simulation of matched field processing localization based on EMD denoising and Karhunen-Loeve expansion in underwater waveguide environment,” “A relative-localization algorithm using incomplete pair-wise distance measurements for underwater applications,” “Acoustic particle detection with the ANTARES detector,” “Masking of time-frequency patterns in applications of passive underwater target detection,” “An underwater acoustic implementation of DFT-spread OFDM,” “Low complexity iterative receiver design for shallow water acoustic channels,” and “Automatic indexing and content analysis of whale recordings and XML representation.” Second, underwater nonacoustics: “Silent localization of underwater sensors using magnetometers”, “Hausdorff-based RC and IESIL combined positioning algorithm for underwater geomagnetic navigation”, and “Realistic subsurface anomaly discrimination using electromagnetic induction and an SVM classifier”. Third, radar: “CFAR detection from non-coherent radar echoes using bayesian theory”, “Artificial neural network-based clutter reduction systems for ship size estimation in maritime radars.” Fourth, optics: “An evaluation of pixel-based methods for the detection of floating objects on the sea surface,” “Statistical real-time model for performance prediction of ship detection from micro-satellite electro-optical imagers,” “Techniques for effective optical noise rejection in amplitude-modulated laser optical radars for underwater three-dimensional imaging,” “Underwater image processing: state of the art of restoration and image enhancement methods,” and “A fully automated method to detect and segment a manufactured object in an underwater color image.”

Low Complexity Iterative Receiver Design for Shallow Water Acoustic Channels

An adaptive iterative receiver structure for the shallow underwater acoustic channel (UAC) is proposed using a decision feedback equalizer (DFE) and employing bit-interleaved coded modulation with iterative decoding (BICM-ID) in conjunction with adaptive Doppler compensation. Experimental results obtained from a sea trial demonstrate that the proposed receiver not only reduces inherent problem of error propagation in the DFE but also improves its convergence, carrier phase tracking, and Doppler estimation. Furthermore, simulation results are carried out on UAC, modelled by utilizing geometrical modelling of the water column that exhibits Rician statistics and a long multipath spread resulting in severe frequency selective fading and intersymbol interference (ISI). It has been demonstrated that there is a practical limit on the number of feedback taps that can be employed in the DFE and data recovery is possible even in cases where the channel impulse response (CIR) is longer than the span of the DFE. The performance of the proposed receiver is approximately within 1 dB of a similar system employing DFE and turbo code, however, at a significantly reduced computational complexity and memory requirements, making our system attractive for real-time implementation.