Communication In Shallow Water Research Articles

High-rate synthetic aperture communications in shallow water

Time reversal communication exploits spatial diversity to achieve spatial and temporal focusing in complex ocean environments. Spatial diversity can be provided easily by a vertical array in a waveguide. Alternatively, spatial diversity can be obtained from a virtual horizontal array generated by two elements, a transmitter and a receiver, due to relative motion between them, referred to as a synthetic aperture. This paper presents coherent synthetic aperture communication results from at-sea experiments conducted in two different frequency bands: (1) 2-4 kHz and (2) 8-20 kHz. Case (1) employs binary-phase shift-keying modulation, while case (2) involves up to eight-phase shift keying modulation with a data rate of 30 kbits/s divided by the number of transmissions (diversity) to be accumulated. The receiver utilizes time reversal diversity combining followed by a single channel equalizer, with frequent channel updates to accommodate the time-varying channel due to coupling of space and time in the presence of motion. Two to five consecutive transmissions from a source moving at 4 kts over 3-6 km range in shallow water are combined successfully after Doppler compensation, confirming the feasibility of coherent synthetic aperture communications using time reversal.

Reflections on the early days of parametric array research and development.

Following Westervelt’s classic paper (1960/1963), there was a period of laboratory exploration, involving small‐scale experiments and theoretical modeling, with Orhan Berktay’s early work anticipating the significance of several practical applications. Robert Mellen organized the First International Symposium on Nonlinear Acoustics (ISNA) in 1968, which the preset author was privileged to attend. Research interest and momentum began to build, expanding to field and seagoing experiments, as well as computational modeling. Some of the seminal milestones and achievements of this period are identified and illustrated. These include theoretical work, which enabled new experiments, and vice versa, facilitated by research tool engineering. A number of classic problems were studied, including parametric transmission and the effects of finite amplitude attenuation. Applications to air, water, and sediment research evolved, including propagation and backscattering measurements, subbottom profiling, and modal effects and communication in shallow water. Parametric reception became a hot topic, along with its intricate signal processing. Focusing of finite amplitude beams was developed, involving the use of lenses, also utilized for steering and scanning parametric beams. The bandwidth capability of parametric interaction was recognized, as were parametric transients for echo‐ranging. Imaging applications emerged for both harmonic and difference frequency radiations, and a spin‐off to biomedical ultrasonics was forged. By the mid‐1980s, a dozen ISNA symposia had been held and the field had blossomed.

A CDMA-based Medium Access Control for UnderWater Acoustic Sensor Networks

UnderWater Acoustic Sensor Networks (UW-ASNs) consist of sensors and Autonomous Underwater Vehicles (AUVs) performing collaborative monitoring tasks. In this article, UWMAC, a distributed Medium Access Control (MAC) protocol designed for UW-ASNs, is introduced. The proposed MAC protocol is a transmitter-based Code Division Multiple Access (CDMA) scheme that incorporates a novel closed-loop distributed algorithm to jointly set the optimal transmit power and code length. CDMA is the most promising physical layer and multiple access technique for UW-ASNs because it is robust to frequency-selective fading, it compensates for the effect of multipath at the receiver, and it allows receivers to distinguish among signals simultaneously transmitted by multiple devices. UW-MAC aims at achieving three objectives, i.e., guarantee i) high network throughput, ii) low channel access delay, and iii) low energy consumption. It is demonstrated that UW-MAC simultaneously achieves these three objectives in deep water communications (where the ocean depth is more than 100 m), which are usually not severely affected by multipath. In shallow water communications, which may be heavily affected by multipath, it dynamically finds the optimal trade-off among these objectives according to the application requirements. UW-MAC is the first protocol that leverages CDMA properties to achieve multiple access to the scarce underwater bandwidth, while other protocols tailored for this environment have considered CDMA merely from a physical layer perspective. Experiments show that UW-MAC outperforms many existing MAC protocols tuned for the underwater environment under different architecture scenarios and simulation settings.

Feasibility of global-scale synthetic aperture communications

A recent paper [Higley et al., J. Acoust. Soc. Am. 118, 2365-2372 (2005)] investigated synthetic aperture communications in shallow water exploiting the relative motion between a source and a receiver. This letter presents a feasibility study of synthetic aperture communications at global distances using the 57 Hz acoustic data from the Heard Island Feasibility Test conducted in January 1991 [Baggeroer and Munk, Phys. Today 45, 22-30 (1992)]. Specifically, a reception at Ascension Island, about 9200 km from the source ship moving at about 3 kt near Heard Island, is analyzed by treating the 255-digit m-sequence tomography signal as a binary-phase shift-keying communication signal with an information rate of 11.4 bitss. The performance using a single receiver combining three consecutive receptions spaced 5.2 min ( approximately 470 m) apart indicates that synthetic aperture acoustic communications is feasible at global distances.

Investigation of modal processing for low frequency acoustic communications in shallow water

Acoustic receptions at a vertical line array collected during the SW06 experiment were processed to show the feasibility of broadband mode decomposition as a preprocessing method to shorten time-spread and concentrate received signal energy in a small number of independent channels. The vertical array spanned the water column from 12 m depth to the bottom. PSK m-sequence modulated signals with different carrier frequencies were transmitted from a distance of 19.2 km. Signals were processed for both the case of ordinary internal waves activity and the case with abnormally strong internal wave solitons. The measured sound velocity and known bottom properties were accounted for. Mode filtering was based on a broadband pseudo-inverse processing of the received VLA signals. The broadband mode filtering decomposed the received signal into a number of independent signals with a reduced time-spread. The constellation of signals from the output of mode filters showed that a simple demodulator can achieve a high quality reception. Even during strong internal waves activity the acoustic energy was concentrated in a small number of the first acoustical modes. The receiver estimated the mode-time intensity distribution and used the strongest modes for demodulation. High quality reception is demonstrated with the data.

Reconstruction of temporal invariants of the time‐reversal operator for communication in shallow water

Time‐reversal techniques allow focusing in waveguides with application for detection or telecommunication (Kupperman et al., J. Acoust. Soc. Am. 103(1), 25‐40, (1998)). It has been shown that the DORT method (French acronym for Decomposition of the Time Reversal Operator) can be used to achieve detection and selective focusing on pointlike scatterers in a waveguide (Mordant et al., J. Acoust. Soc. Am. 105, 2634‐2642 (1999)). However, the decomposition is done in frequency domain providing eigenvectors V(ω) of undetermined phase. In order to achieve temporal focusing, it is necessary to built 'temporal eigenvectors' V(t) as an appropriate combination of the eigenvectors V(ω). Due to dispersion, the reconstruction of the temporal eigenvectors is difficult in waveguides. One solution was proposed by Mordant et al., using the symmetry of the array response matrix and assuming continuous frequency dependence of the invariants. However this method does not work if eigenvalues crossings occur. Furthermore, it cannot be applied to distinct arrays of transmitters and receivers. We propose a simple method to reconstruct the temporal invariants by using the back propagation of eigenvector in free space. Application to data from a laboratory experiment and from the DOREV 2005 shallow water experiment are presented.

Model for energy consumption by a broadband shallow-water acoustic communications network

A multistatic active sonar system with several widely distributed sensors must share information between the sensors. Acoustic communication is a viable method to share information and is particularly appropriate when the sensors are mobile. A potential drawback is that a significant fraction of the total energy budget for the system might be consumed just by sending information between the sensors. In the present work, an energy consumption model is developed that is appropriate for incoherent acoustic communication in shallow water. The model builds upon the foundation established by Sozer et al. [IEEE J. Oceanic Eng. 25, 72–83 (2000)], but uses an improved sub-model for acoustic propagation. The sub-model includes environmental factors like the sound speed profile in the water column and the composition of the seabed. By exploiting the waveguide invariant concept, the broadband nature of the communications signals can be included efficiently in the calculation. 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. [Work supported by ONR.]

Multiuser Communications Using Passive Time Reversal

A recent paper (Song , IEEE Journal of Oceanic Engineering, vol. 31, no. 2, pp. 170-178, 2006) demonstrated multiple-input-multiple-output (MIMO) communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes passive time reversal communications which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4-km range in a 120-m-deep water using quaternary phase-shift keying (QPSK) modulation, achieving an aggregate data rate of 6 kb/s. Moreover, the same data rate has been achieved at 20-km range by three users using 16 quadrature amplitude modulation (16-QAM).

Multiuser communications using passive time reversal

A recent paper [Song et al., IEEE J. Ocean. Eng. 31, 165–173 (2006)] demonstrated multipleinput/multiple-output (MIMO) multi user communications in shallow water using active time reversal where the time reversal array (i.e., base station) sent different messages to multiple users simultaneously over a common bandwidth channel. Passive time reversal essentially is equivalent to active time reversal with the communications link being in the opposite direction. This paper describes MIMO passive time reversal communications, which enables multiple users to send information simultaneously to the time reversal array. Experimental results at 3.5 kHz with a 1-kHz bandwidth demonstrate that as many as six users can transmit information over a 4-km range in 120-m-deep shallow water using QPSK modulation, achieving an aggregate data rate of 6 kbits/s. Moreover, the same data rate has been achieved at 20 km range by three users using 16-QAM modulation.

Synthetic aperture time-reversal communications in shallow water: Experimental demonstration at sea

Time reversal has been shown as an effective way to focus in both time and space. The temporal focusing properties have been used extensively in underwater acoustics communications. Typical time-reversal communication experiments use vertical transducer arrays both to increase the signal-to-noise ratio and decrease the temporal sidelobes created in the time reversal process. Comparable temporal focusing is achieved using a horizontal array. In this paper, synthetic aperture time-reversal communications are accomplished, requiring only two transducers (one transmitter and one receiver). Deriving results from an at-sea experiment, this work confirms the viability of synthetic aperture time-reversal communications.

Basic Research on Time-Reversal Waves in Deep Ocean for Long Acoustic Communication

We have studied the focusing property of time-reversal waves and its application to acoustic communication in shallow water. In this study, this focusing property in the deep ocean and its application to long horizontal acoustic communication are discussed. The results are as follows. Even if a time-reversal array (TRA) does not expand from the sea surface to the seabed, time-reversal signals converge, and the focusing property is not significantly affected by the depths of the focus and TRA. Then, it is revealed that by using time reversal, it is possible to enssure communication channel over a long range, through the simulation. In phase modulation, time reversal can demodulate almost only by itself, while in amplitude and phase modulation, an adaptive filter to compensates further.

Basic study on active time reversal communication in shallow water

Basic study on active time reversal communication in shallow water

Adaptive Array Processing for High-Speed Acoustic Communication in Shallow Water

A novel multichannel signal-processing technique, capable of achieving stable high-speed underwater acoustic communication with a fairly low complexity of implementation, is presented. The approach is to split the space and time processing into two separate suboptimal processes to reduce computational complexity and instabilities associated with large tap vectors at large spread factors or BL products. The first suboptimal process starts with the eigen-spatial decomposition of the acoustic channel into multiple coherent orthogonal paths. Only the most energetic coherent paths are retained. For each selected path, individual synchronization is performed, the mean Doppler shift is estimated, and the message is decoded. The message binary information obtained for each individual path is recombined using maximal-ratio combining (MRC) to generate a decision binary sequence along with an array of quality factors, or metrics, associated with each bit. Each metric corresponds to the signal-to-noise-and-interference ratio (SNIR) associated with each bit. The second suboptimal process is a linear multichannel equalizer (LME). The mean Doppler estimate of the most energetic coherent path is used to resample each input signal of the array that go into the LME, to reduce the rate of fluctuation of the resulting effective channel. These resampled input signals appear as if they have passed through a channel with a lower B (Doppler spread), which allows the final space-time processor (the LME) to operate in a stable manner. The decision binary sequence and the array of quality metrics obtained at the output of the MRC are used as inputs in the decision process of the LME. If the metric associated with a decision bit exceeds a computed threshold, this bit, obtained at the output of the first suboptimal process, is used as a decision input to the LME. This technique is applied to achieve high-speed acoustic communication in very shallow water using coherent modulation techniques. As compared with coherent beamforming alone, the proposed technique causes a significant increase in SNIR, a significant decrease of the bit-error rate, and provides an estimate of the BL product. Experimental results demonstrate that stable acoustic communication can be achieved at 16000 coded bits per second (b/s) over 3.2 km in 40 ft of water and in sea-state 2 conditions. The fading properties of the channel are measured. Experimental results show that the BL product can vary by a decade in 116 ms and by two decades within minutes, from 0.001 to 0.1. The real-time analysis shows a strong correlation between the communication performance, the spread factor, and the spatial coherence of the channel.

A new multichannel spatial diversity technique for long-range acoustic communications in shallow water

A new multichannel spatial diversity technique has been developed for underwater acoustic communications in very shallow waters. This technique combines a novel synchronization method with maximum-likelihood symbol estimation. It was tested with a multichannel Mills-Cross receiver using various numbers of elements. The FAU general purpose acoustic modem source transmitted messages using four types of frequency-hopped multiple-frequency-chirp-keying (FH-MFSK) modulation: 4 hops at 221 coded bits per second (cps), 2 hops at 442 cps, or no hopping at 1172 cps. These types of modulation allowed for robust data transmission in adverse environment. The modem operated between 16 and 32 kHz at 192-dB source level, at ranges from 1 to 5 km in 40 ft of water. Using only four channels of the Mills-Cross receiver array, messages coded at 1172 cps were received with a frame error rate (FER) of 0% at a range of 3 km. In the same four-channel configuration, messages coded at 221 cps were received with no frame error at 5 km. The experimental results matched the performance predictions from the Crepeaus model. This reliable and computation-efficient method can be implemented on new generations of embedded acoustic modems, such as the four-channel FAU acoustic modem, and can provide significant improvements in communication performance.

A comparison of normal mode and parabolic equation range-dependent propagation models as tools for acoustic communication in shallow water

Accurate prediction of the effects of a shallow-water propagation channel on acoustic signal distortion would aid in the optimal design of shallow-water communication systems. It has been shown that the correlation between modeled (based on a parabolic equation approach) and measured ocean responses decreases significantly with increasing propagation range because of the mismatch in relative amplitudes and time delays between the modeled and measured bottom-interacting arrivals. This raises the question whether the discrepancies are related to the particular modeling algorithm or to insufficient information about the propagation channel. A new normal mode range-dependent algorithm is used to simulate the acoustic response for the source–receiver configuration used in an experiment in the Atlantic Ocean at the Southern end of the Blake Plateau in which a 25–150 Hz linear, frequency-modulated source signal is received on a 15-element vertical array. The normalized correlation coefficients between source and receiver signals as a measure of signal distortion are computed as functions of receiver depth and propagation range and compared for both modeling algorithms. The influence of the inaccuracy of the bottom parameters, particularly the compressional to shear wave conversion in determining high distortion areas, is also discussed.

High-speed acoustic communication in shallow water using multiple coherent path beamformer technique

A multiple coherent space-time processing method is used for acoustic communication at high data rate in very shallow water, between 16 and 32 kHz. The multiple coherent path beamformer (MCPB) method for processing the signals uses an adaptive processor that forms beams in the direction of a collection of coherent signals representing the strongest path. The source is the FAU Utility Acoustic Modem used for acoustic networking during AUV operations. The system encodes data using coherent modulation schemes with coded rates varying between 4000 and 48<th>000 bps. The receiver system is composed of a 64-channel Mills–Cross array attached to an FAU embedded 64-channel acquisition system. Results already obtained show that the MCPB can decode data reliably at 3200 m for rates up to 32<th>000 bps, with a platform speed of 2–3 knots and a sea-state of 1–2. For a source level of 186 dB, BPSK encoding and 62.5&♯61549;s symbols (16<th>000 bps), the MCPB achieves a bit error rate of 0.2% without any Forward Error Coding. These results demonstrate the long-range and high-data rate transmission ability of the system, and make possible real-time transmission of side-scan data and other low-resolution video images.

Environmental factors that affect acoustic propagation and underwater communications in the surf zone

Acoustic signals propagating through the surf zone are strongly influenced by scattering from surface gravity waves, reflection from the sea-floor and absorption and scattering by the clouds of bubbles entrained by breaking surf. Shoaling gravity waves create focal regions in surface-reflected wavefronts, and introduce systematic variations in wavefront travel times. Both of these surface effects are important to the design and operation of very shallow water communication systems. To study them, an experiment was conducted 40 m north of Scripps Pier in November of 2000. Single cycle acoustic pulses centered at 10 kHz and phase modulated pseudorandom sequences were repetitively transmitted over a 40 m path to a 3-element vertical hydrophone array in 5 m deep water. Environmental characterization was provided by a 10-element pressure array mounted along the propagation path to measure the shoaling gravity wave field. The results of this experiment and additional experimental data collected at Scripps Pier during the Fall of 2000 over several regimes of environmental conditions (the SZATE experiment) will be discussed. [Work supported by ONR.]

Stochastic simulation of acoustic communication in turbulent shallow water

This paper presents a stochastic model of a turbulent shallow-water acoustic channel. The model utilizes a Monte Carlo realization method to predict signal transmission conditions. The main output from the model are statistical descriptions of the signal-to-multipath ratio (SMR) and signal fading. Probability density functions of signal envelope are evaluated by Pearsons's Skew-Kurtosis Chart, generally predicting Ricean fading. Dynamic calculations of SMR by the model overcome the main inconveniences of deterministic calculations, providing smooth instead of noisy curves as a result. Dynamic calculations of SMR and fading are concluded to provide more intelligible and realistic results than deterministic calculations.

Spatio-temporal processing of coherent acoustic communication data in shallow water

Achieving reliable underwater communication in shallow water is a difficult task because of the random time-varying nature of multipath propagation. When the product of Doppler-related signal bandwidth spread and multipath-related time spread of the channel is larger than one, some types of adaptive signal processing may not work very well. In this paper, various methods of coherent space-time processing are compared for a condition of a marginally overspread channel operating at 50 kHz. Various combinations of suboptimal spatially adaptive and time adaptive methods are considered. The coherent path beamformer (CPB) and recursive least squares (RLS) adaptive beamformer, both in combination with RLS time filtering, are analyzed. Also considered in the analysis is the combined RLS space-time optimal adaptive processor. Many experiments using broad-band phase-shift-keyed transmissions in shallow water have been conducted to provide data for testing these various processing methods. Because of the rapid time variation of the multipath, the product of bandwidth spread and time spread at this test site approached unity. In this environment, a suboptimal approach consisting of the adaptive beamformer followed by RLS equalization reduced reverberation and transmission errors.

Characterization of the underwater channel for acoustic communications

In recent years, acoustic underwater (AUW) communications have known a huge increase in research and development due to the growing needs of wireless transmission systems. While mobile radio channels fit well within the model of Rayleigh fading, this model is often used for the design of communication systems. However, many experimentations carried out by our laboratory (LIS ex CEPHAG) with the French Navy or within European Community MAST projects (LORACOM and PARACOM) have shown that this model do not fit for AUW transmissions. The AUW channel is well modeled, using ray tracing, by great deterministic propagation paths (macro-multipath) associated with random fluctuations (micro-multipaths). Experimental results will be presented. The slow variations of the deterministic part may be due to internal waves, the little, fast variations of the received energy by appearance or disappearance of micro-paths. A random channel modeling developed for LORACOM project (Contract No. MAS2-CTG4-0080) agrees with this behavior. Different AUW medium characterizations depending on the channel geometry (shallow or deep water communications), on the frequencies (between 1 and 10 kHz), and on the range (1 to 100 km) will be discussed. Spatial coherence results will also be presented.