Shallow Water Acoustic Communication Research Articles

Underwater Acoustic Propagation Physics and Signal Processing Techniques for Shallow-Water Acoustic Communications Special Issue

Underwater Acoustic Propagation Physics and Signal Processing Techniques for Shallow-Water Acoustic Communications Special Issue

Combining sparse recovery approaches with underwater acoustic channel models for robust communications in the shallow water paradigm

Shallow water acoustic communication techniques are fundamentally challenged by the rapidly fluctuating multipath effects due to oceanic phenomena such as surface wave focusing, specular reflections from the moving sea surface, Doppler effects due to fluid motion as well as sediment-dependent absorption from the sea bottom. Several signal processing techniques have been recently proposed that specialize in recovering the shallow water acoustic channel components using compressing sampling and mixed norm optimization theory. However, these novel techniques are typically agnostic of underlying underwater acoustic propagation phenomena. Furthermore, state-of-the-art in shallow water channel estimation typically does not account for the three-way uncertainty principles governing the localization of sparsity, time, and frequency for the time-varying shallow water acoustic channel. This talk will focus on recent techniques proposed in this domain, their relative benefits and shortcomings; as well as offer new insights into how we can combine knowledge of basic underwater acoustic propagation physics with state-of-the-art in sparse sensing and related techniques to achieve robust shallow water channel estimation. The talk will also provide an overview of equalization techniques that can be harnessed with channel estimation techniques proposed.

Frequency-selective fading statistics of shallow-water acoustic communication channel with a few multipaths

The bit error rate of an underwater acoustic communication system is related to multipath fading statistics, which determine the signal-to-noise ratio. The amplitude and delay of each path depend on sea surface roughness, propagation medium properties, and source-to-receiver range as a function of frequency. Therefore, received signals will show frequency-dependent fading. A shallow-water acoustic communication channel generally shows a few strong multipaths that interfere with each other and the resulting interference affects the fading statistics model. In this study, frequency-selective fading statistics are modeled on the basis of the phasor representation of the complex path amplitude. The fading statistics distribution is parameterized by the frequency-dependent constructive or destructive interference of multipaths. At a 16 m depth with a muddy bottom, a wave height of 0.2 m, and source-to-receiver ranges of 100 and 400 m, fading statistics tend to show a Rayleigh distribution at a destructive interference frequency, but a Rice distribution at a constructive interference frequency. The theoretical fading statistics well matched the experimental ones.

2P6-1 Fading Statistics Characterization of Shallow water Acoustic Communication Channel

2P6-1 Fading Statistics Characterization of Shallow water Acoustic Communication Channel

Multipath Effects on High-Frequency Coherent Acoustic Communications in Shallow Water

Shallow-water acoustic communication channel, referred to as a multipath-limited channel, produces inter-symbol interference that poses a significant obstacle to reliable communication. Accordingly, signal-to-multipath ratio (SMR), rather than signal-to-noise ratio (SNR), becomes an important factor affecting communication performance. However, it is difficult to estimate SMR from measured communication data, especially at higher frequency (>10 kHz) because many arrivals scattered from rough ocean boundaries produce a significant intrapath time spreading, which acts as random noise in communication. In this paper, the energy fraction of the channel impulse response existing in one symbol duration is proposed as a parameter for estimating the quality of shallow-water communication channels. This parameter is compared with the bit-error-rate performance for data acquired in shallow water off the south coast of Korea, where the water depth is 45 m and the bottom consists of sandy clay sediment. The results imply that the energy fraction in one symbol duration may be used as a parameter for describing shallow-water communication channels and applied to the quick decision of a symbol or bit rate in a shallow-water field for reliable underwater communication.

Time-frequency localization issues in the context of sparse process modeling

Practical applications in acoustics such as shallow water acoustic communications often involve non-stationary processes that follow a time-varying sparse support. A classic example is acoustic scatter due to multiple reflections at the moving ocean surface and sea bottom, localized in the time-frequency domain as the sparsely distributed and time-varying Delay-Doppler spread function. In this work, we connect time-frequency localization of non-stationary processes with related issues in adaptive sparse sensing in the context of modeling and tracking the Delay-Doppler spread function. To this end, we provide an overview of methodological advances in adaptive sparse sensing techniques, and compare them over experimental field data collected as 15 m depth, 200 m range and moderate to rough sea conditions. We also comment on other adaptive techniques, such as least-squared error minimization algorithms, and discuss their extensions to the sparse sensing domain. We also explore the uncertainty principle underlying time-frequency representations, particularly in terms of how it influences the related and occasionally competing challenges to sparse process modeling: (i) restricted isometry criteria (RIP) for precise sparse reconstruction and (ii) choice of temporal window to localize the non-stationary Delay-Doppler spread function.

Rate Compatible LDPC Codes Design for Shallow Water Acoustic Communications

The fast temporal variations shallow water acoustic (SWA) channels need the temporal variations channel coding scheme at the acceptable cost. Hence, flexible channel coding rate, adjusted according to different ocean channel characteristics, should be desired in the design of practical error control SWA communication system. In this paper, we propose rate-compatible LDPC (RC-LDPC) codes to improve the communication system reliability for SWA communications. The proposed SWA system adopting RC-LDPC codes consists of three important preprocessing: channel state information (CSI) estimator, signal-to-noise ratio (SNR) estimator and RC-LDPC pattern. For the estimation error, we define and derive the sensitivity to imperfect CSI and imperfect SNR. Then the design of RC-LDPC codes and SWA channel profile are described. Furthermore, the RC-LDPC performance pattern is given and sensitivity to estimation error of CSI and SNR are analyzed via simulation. It is shown that RC-LDPC codes have good performances with wide range of rates in SWA channels. Finally, coding rate distributions of RC-LDPC codes in different SNR at BER below 10-4 for SWA channel are investigated.

Design of RC-LDPC Codes and Its Application for Shallow Water Acoustic Communications

Design of RC-LDPC Codes and Its Application for Shallow Water Acoustic Communications

Sequential analysis for underwater communications

The state-of-the-art in high-rate, single-carrier wideband signaling for acoustic communications is represented by the decision feedback equalizer (DFE). Though often effective, its performance is far from the optimal obtained from soft decision maximum a posteriori probability (MAP) and maximum-likelihood sequence detectors (MLSD). While these algorithms have optimal performance, their complexity increases exponentially with the duration of the channel impulse response. In practice, such methods are only used for multicarrier modulation, after mode filtering or other form of channel shortening, time-spatial pre-processing equalization. The optimal joint channel estimation and data decoding algorithm is derived and analyzed. The combination of pre-processing, channel response shortening equalization, and joint channel and data recovery have shown excellent performance in shallow water acoustic communications experiments. A sequential estimation alternative to MLSD-based decoding is “almost” as effective in a probability sense, given a modest increase in signal-to-noise ratio (SNR). That approach combines very high performance with a small computational burden relative to the MLSD approach. The practical result of the paper is the investigation of the replacement of the DFE with a sequential implementation (FANO) of a likelihood sequence estimator. Comparative performance of the two using at-sea experiments in very shallow water is provided.

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.

Multi-input multi-output multicarrier acoustic communications in shallow water

Recently multi-carrier orthogonal frequency division multiplexing (OFDM) communications, popularly used in wireless channels, has been introduced in underwater acoustic channels with a large delay spread, as an alternative to the typical single carrier approaches. In this paper, we investigate multiple-input/multiple-output (MIMO) OFDM communications which can effectively increase the data rate in band-limited underwater channels. The performance of MIMO OFDM communications will be illustrated using the data collected from the KAM11 experiment conducted in shallow water, west of Kauai, Hawaii, which involved multiple transmit and receive arrays with different bandwidths, inter-element spacings, and apertures at various ranges between them up to 10 km.

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.

A new signal processing method for multipath compensation in shallow underwater acoustic communication.

The most difficult problem in shallow underwater acoustic communications is considered to be the the time spread inherent to acoustic propagation in guide. The standard approach is to design an equalizer that attempts to correct for these propagation effects. In the presentation, a novel approach to multipath compensation in shallow water acoustic communication is proposed. The method is based on a dedispersion transformation technique, which was proposed in our previous paper [N. Wang, 9th Western Pacific Acoustic Conference, Beijing (2009)]. After dedispersion transform to a long range acoustic propagation signal, the field sampling data of a single hydrophone are transformed to several normal modes with the same waveforms. The dispersion of each modes, especially higher modes, can be removed after the transform. Then the computation cost of equalization can be decreased dramatically. The proposed approach requires less prior environmental information except an approximate beta‐value of waveguide invariant. The validity of the present approach is verified in simulation.

Long-range multi-carrier acoustic communications in shallow water based on iterative sparse channel estimation

Long-range orthogonal frequency division multiplexing (OFDM) acoustic communications is demonstrated using data from the Kauai Acomms MURI 2008 (KAM08) experiment carried out in about 106 m deep shallow water west of Kauai, HI, in June 2008. The source bandwidth was 8 kHz (12-20 kHz), and the data were received by a 16-element vertical array at a distance of 8 km. Iterative sparse channel estimation is applied in conjunction with low-density parity-check decoding. In addition, the impact of diversity combining in a highly inhomogeneous underwater environment is investigated. Error-free transmission using 16-quadtrative amplitude modulation is achieved at a data rate of 10 kb/s.

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 & Sons, Ltd.

Investigation of joint channel estimation and data decoding for underwater acoustic communications with multi‐carrier modulation.

The current state‐of‐the‐art in high‐rate acoustic communications is represented by adaptive multi‐channel equalization of single‐carrier wideband signals. An alternative technique with significant potential for achieving high bit rates over multipath‐distorted (frequency selective) channels is multi‐carrier modulation (MCM). There are obvious technical advantages of MCM. Each frequency channel of MCM can be transmitted using a relatively narrow‐band, high‐efficiency transducer, and low‐power amplifier, which allows achieving a very broad bandwidth. Use of narrow‐band channels increases the tolerance for time synchronization errors in multipath array processing. Each frequency channel has a relatively low delay spread when measured in symbol units and can be equalized with low‐complexity algorithms and simplified array processing. The reduction of the ISI span in each sub‐channel allows for the application of soft decision decoders optimal for joint channel estimation and data recovery such as maximum‐likelihood sequence detectors (MLSDs) and adaptive BCJR MAP algorithm. The adaptive form of the MAP algorithm is considered. The combination of channel shortening equalization and a practical MLSD algorithm with dual loop feedback and pre‐survivor processing were tested in shallow water acoustic communications experiments and shown a very good performance. [Work supported by ONR Grant N00014‐07‐1‐0229.]

Investigation of Mode Filtering as a Preprocessing Method for Shallow-Water Acoustic Communications

Acoustical array data from the 2006 Shallow Water Experiment (SW06) was analyzed to show the feasibility of broadband mode decomposition as a preprocessing method to reduce the effective channel delay spread and concentrate received signal energy in a small number of independent channels. The data were collected by a vertical array, which spans the water column from 12-m depth to the bottom in shallow water 80 m in depth. Binary-sequence data were used to phase-shift-keyed (PSK) modulate signals with different carrier frequencies. No error correction coding was used. The received signals were processed by a system that does not use training or pilot signals. Signals received both during periods of ordinary internal wave activity and during a period with unusually strong internal wave solitons were processed and analyzed. Different broadband mode-filtering methods were analyzed and tested. Broadband mode filtering decomposed the received signal into a number of independent signals with a reduced delay spread. The analysis of signals from the output of mode filters shows that even a simple demodulator can achieve a low bit error rate (BER) at a distance 19.2 km.

Frequency-domain channel estimation and equalization for shallow-water acoustic communications

A new frequency-domain channel estimation and equalization (FDE) scheme, combined with a new group-wise phase correction scheme, is proposed for single-carrier (SC) underwater acoustic communications systems employing single transducer and multiple hydrophones. The proposed SC-FDE scheme employs a 2 N -point Fast Fourier Transform (FFT) to estimate and equalize the channel in frequency domain, where N is the number of symbols in a data block. Both the frequency-domain channel estimation and equalization are designed by the linear minimum mean square error criterion. Initial channel estimation is performed by a pilot signal block and later updates are achieved using the detected data blocks. The proposed phase correction scheme utilizes a few pilot symbols in each data block to estimate the initial phase shift and then correct it for the block to combat the large phase rotation due to the instantaneous Doppler drifts in the acoustic channels. Time-varying instantaneous phase drifts are re-estimated and compensated adaptively by averaging the phase variation across a group of symbols. The proposed SC-FDE and phase correction method is applied to the AUVFest’07 experimental data measured off the coast of Panama City, Florida, USA, June 2007. With the Quadrature Phase Shift Keying (QPSK) modulation and a symbol rate of 4 ksps, the proposed scheme achieves an average uncoded bit error rate on the order of 1×10 −4 for fixed-to-fixed channels with the source–receiver range of 5.06 km. For the moving-to-fixed source–receiver channels where the source–receiver range is 1–3 km, the multipath delay spread is 5 ms, the average Doppler shifts are ±20 Hz, and the maximum instantaneous Doppler drifts from the mean is ±4 Hz, the proposed scheme achieves an average uncoded bit error rate on the order of 1×10 −3 .

Ocean surface degradation of shallow water acoustic communication.

The accurate prediction of underwater acoustic communication system performance at high frequencies is essential to system risk mitigation prior to deployment. The deterioration of acoustic coherence by ocean surface dynamics will be predicted via three dimensional (3‐D) ray tracing with the inclusion of interaction with a realistic dynamic ocean surface. Acoustic communication performance limits at high frequencies for most fixed platform systems are dominated by multipath interaction with the spectrally rich heaving ocean. The high‐fidelity Wave‐watch III surface spectra model is used here with a wave‐number integration approach for surface time series modeling. A fully 3‐D ray tracing model is used to model the propagation of sound under a dynamic surface boundary at the time and spatial scales that dominate coherence degrading effects for underwater acoustic communications. The proposed method will serve the rapid development of operating regimes for high‐frequency underwater acoustic communication systems. Proposed applications include distributed underwater sensor networks and persistent deployable low‐cost systems that meet a wide range of underwater sensing needs. This work seeks to solve problems in deploying advanced communication systems by predicting the degradation of signals due to ocean variability and automatically selecting appropriate signaling rates, schemes, and constants. [This work was supported by ONR.]

Energy Consumption Model for a Broadband Shallow-Water Acoustic Communications Network

A multistatic active sonar system with several widely distributed sensors must share information between the sensors. If acoustic communication is the means used to share information, it can consume a significant fraction of the system's total energy budget. In this communication, an energy consumption model is developed for a shallow-water acoustic communications network. The model includes the environmental factors like the sound-speed profile in the water column and the composition of the seabed. The model uses the waveguide invariant concept to incorporate efficiently the broadband nature of the communications signals. Numerical results demonstrate how relaying messages between intermediate sensors can save substantial energy compared to direct communications. The calculations also show that energy consumption can vary by more than an order of magnitude depending on the seabed composition.