Underwater Acoustic Communication System Research Articles

Design of a symmetric multicarrier modulation for underwater acoustic communications

Mobile underwater acoustic (UWA) channels always exhibit large delay spread and Doppler spread. Multicarrier modulation is an efficient technique for the communications over multipath channels. However, the orthogonality of the transmit signals, e.g., OFDM signals, will be destroyed by the Doppler spread and interference will be produced. Thus, an equalizer is required in the receiver. For this reason, we argue that multicarrier modulation with Gaussian pulse (symmetric in time and frequency domains) as the transmit filter is more suitable for the UWA communication systems than the OFDM. The key problem for the multicarrier modulation is to design/optimize the number of carriers and the parameter alpha in the Gaussian pulse which will greatly affect the system performance. In this paper, we investigate the optimization problem for four kinds of channels: AWGN channel, frequency-selective channel, time-selective channel and doubly selective channel. Channel information is assumed known in the transmitter by feedback. Analytical results are obtained and the numerical examples are given. To achieve the cost function, a 2D MMSE-DFE equalizer is used. Finally, we propose the balanced design for the practical engineering.

Doppler compensated underwater acoustic communication system

Spread spectrum methods are used in communication systems to provide a low probability of intercept in hostile environments and multiple access capability in systems shared by many users as well as to provide high processing gain in channels where the transmitted signal is distorted by multipath effects. Such systems serve to be an effective tool for underwater telemetry environments, where multipath propagation effect and Doppler spreading is seen to be more predominant. This paper describes the implementation of a Doppler compensated underwater telemetry system based on CDMA technique. The system consists of multiple CDMA transmitters and a phase locked loop based carrier recoverable CDMA receiver. The effects of the Doppler shift can be compensated by the carrier recovery subsystem in the demodulator, based on PLL technique, which extracts the carrier frequency/phase and simultaneously demodulates the signal. The decision device in the receiver consists of a PN sequence generator as well as a bank of correlators, which are used to determine the data transmitted. The system simulation has been implemented in MATLAB. The advantage of this system is that multiple transmitting stations can transmit simultaneously to a central receiver, thereby increasing the system throughput.

A frequency-modulated-carrier digital communication technique for multipath underwater acoustic channels

Along with interferences, multipath propagation and, in particular, its instability in shallow sea is the main obstacle to increasing the digital data transfer rate in underwater acoustic communication systems. Given these conditions, the use of simple waveforms, for which the product of bandwidth by the length of the processing interval is close to unity, makes testing the quality of the communication channel much more difficult and strongly limits the possibilities of devices that compensate for current waveform distortions, such as, for example, equalizers. In multipath channels with random parameters, various complex waveforms with different spectrum spreading techniques had been used for years. Complex waveforms with a large base (time-bandwidth product) offer advantages in efficiently suppressing narrowband interferences and providing asynchronous multiple-access communications. This paper proposes a new complex waveform, which uses sequences of chirp pulses. On receiving, this waveform can be divided into individual responses through converting the delays of these individual responses to frequency offsets. After a special frequency conversion of the received signal, the multipath responses are separated in the frequency domain by conventional band-pass filtering.

The impact of bubbles on underwater acoustic communications in shallow water environments

Two characteristics of the acoustic channel that significantly impact the performance of underwater acoustic communications systems are the total energy throughput of the channel and the delay spread of the channel impulse response. These characteristics impact both coherent (e.g., phase shift keyed) and incoherent (e.g., frequency shift keyed) systems. Experimental data are presented from the surf zone and shallow-water environments demonstrating the impact of bubbles on these characteristics. A novel statistical analysis utilizing a variance reduction ratio is used to identify the measurable environmental variables (e.g., significant wave height) that can be used to best predict the impact of the bubbles. [Work supported by ONR Ocean Acoustics.]

Examination of time-reversal acoustics in shallow water and applications to noncoherent underwater communications.

The shallow water acoustic communication channel is characterized by strong signal degradation caused by multipath propagation and high spatial and temporal variability of the channel conditions. At the receiver, multipath propagation causes intersymbol interference and is considered the most important of the channel distortions. This paper examines the application of time-reversal acoustic (TRA) arrays, i.e., phase-conjugated arrays (PCAs), that generate a spatio-temporal focus of acoustic energy at the receiver location, eliminating distortions introduced by channel propagation. This technique is self-adaptive and automatically compensates for environmental effects and array imperfections without the need to explicitly characterize the environment. An attempt is made to characterize the influences of a PCA design on its focusing properties with particular attention given to applications in noncoherent underwater acoustic communication systems. Due to the PCA spatial diversity focusing properties, PC arrays may have an important role in an acoustic local area network. Each array is able to simultaneously transmit different messages that will focus only at the destination receiver node.

Further investigations into performance metrics for underwater communications

Determining the relationship between the performance of an underwater acoustic data communications system and the operating environmental conditions is a problem that continues to plague researchers. The complexity of the time-varying channel is difficult to measure and model. Therefore an approach that uses metrics measured from data collected at sea to characterize the channel is attractive. As expected, preliminary assessments on limited data have shown that performance depends not only on environmental conditions, but also on system implementation. By extracting a variety of metrics, a better understanding of the subset that discriminate between good and bad performance can be developed. Also by analyzing the relationship between certain metrics and performance, system limitations can be identified for re-evaluation. For example, a surprising result of the initial assessment of performance using a multichannel decision feedback equalizer on real data showed that sparseness of multipath arrivals may be an arbiter of performance [Richman et al., J. Acoust. Soc. Am. 110, 2619 (2001)]. Therefore changes to the algorithm that allows for sparse arrivals may improve performance. In this paper, a larger number of metrics from greater quantities of real-data and system configurations are measured and evaluated against equalizer results.

Application of time reversal to underwater acoustic communications

Underwater acoustic communication systems must mitigate the intersymbol interference caused by the time-varying multipath dispersion. Time-reversal (TR), or phase-conjugation, focuses energy back to its origin despite the complexity of the propagation channel. The TR properties of dispersion reduction and the mitigation of the effects of channel fading make time-reversal a potentially attractive component of a coherent communications system. An overview of TR ocean acoustics concepts illustrated with data is presented as related to underwater acoustic communications. Experimental results of TR and control transmissions of binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) communication sequences are also presented together with measured bit error rates. These results will be discussed in relation to environmental fluctuations and their effects on the time-reversal focus. Extended duration (tens of minutes) time-reversal pulse transmissions will be examined as well as the time-evolving characteristics of the channel impulse response. [Work supported by ONR.]

Performance metrics for underwater communications: An initial assessment

The relationship between the performance of an underwater acoustic data communications system and operating conditions is investigated. There has been much work devoted toward this end focused on developing mathematical models of the underwater acoustic channel. However, the underwater acoustic environment is a complicated one, and current models do not fully account for the challenging, time-varying conditions. The analysis here is based on actual data transmissions collected during several at-sea tests involving a variety of operating conditions. Metrics have been developed that attempt to characterize properties relating to signal-to-noise ratio, Doppler, multipath spread, channel coherence, and channel complexity. By applying these metrics to data transmissions in conditions with different water depths, carrier frequencies, and source/receiver geometries, a better understanding can be developed of the factors that determine the performance of an underwater communications system. The ultimate goal of this work is to provide a systematic prediction method by computing those metrics that are the true arbiters of performance. This work will also aid in the development and validation of mathematical models of the underwater acoustic environment by providing practical measures with which to compare the simulated channels.

Using continuous additive training sequences for high rate, Doppler tolerant communications

An underwater acoustic communications system designed to transmit 20 kbit/s over ranges from 100 m up to 5 km between a semi-autonomous underwater vehicle and a surface vessel moving with up to 20 knots relative velocity is presented. Under these conditions, attention must be given not only to the steady-state equalization capability of the receiver, but also to its ability to respond to sudden changes in channel profile due to sudden movements of the remote vehicle. In addition to conventional packet-based transmissions, a novel continuous transmission system was also implemented during the experimental stages. Instead of using distinct channel probe, training and data blocks, a continuous training sequence was added on top of a continuous data transmission. Advantages of continuous additive training sequences (CATS) such as an improved tolerance to burst errors, greater data throughput and flexible re-synchronization ability will be highlighted. These are balanced against the disadvantages of increased receiver complexity and compromised ambient noise performance. Results of several sea-trials will be presented to verify the performance of the system at short and long ranges using both static and dynamic platforms. [Work sponsored by DERA Winfrith, UK.]

An experimental investigation on a direct blind equalizer for underwater acoustic multiple FIR channel

A blind equalizer removes the distortion caused by intersymbol interference (ISI) without the need for training sequences. In order to get the complete phase information of the ISI, high-order statistics have to be used to find the equalizer taps. However, L. Tong et al. have shown that most channels can be identified from the second-order cyclostationary statistics through fractionally spaced equalizers. This result has stimulated considerable research activities in fractionally spaced channel identification and equalization. In this paper a single-input multi-output (SIMO) channel blind equalizer for an underwater acoustic communication system is considered. A direct blind equalization scheme was investigated by simulations, which is a novel unsupervised adaptive equalizer and was suggested by J. Labat first for single-input single-output (SISO). An interactive matched processing (MP) is added for the SIMO case. The work verified the prediction that the linear equation form of the direct blind equalizer lends itself naturally to adaptive solutions and optimum combinations of the equalizer outputs and improved performance over existing algorithms in terms of mean-square error and probability of error in the equalized data.

Improved RLS algorithm for time-variant underwateracoustic communications

An extension to the concept of the standard recursive least squares (RLS) algorithm is considered. The improved RLS algorithm is described and has been implemented using experimental data from an underwater acoustic communication system. Results obtained show that the improved RLS algorithm has faster convergence and improved tracking performance than both the standard RLS and least mean squares (LMS) algorithms.

Measured channel characteristics and the corresponding performance of an underwater acoustic communication system using parametric transduction

The authors present the results of experiments carried out in the Mediterranean Sea using parametric sonar to establish underwater acoustic communication. Short duration, pulsed carrier signals were used to estimate the impulse response of the channel and these observations were related to the performance of a parametric communication system operating under the same conditions. The statistical characteristics and spectra of the amplitude and phase fluctuations were used to analyse the pulsed response over short observation periods. Two distinctive types of channels were encountered during the experiment: one characterised by highly fluctuating multipath, the other predominantly a single path. Typically, the amplitude fluctuations experienced by the main path were Rician in nature with a spectral content in the sub-Hertz range, whilst the phase fluctuations tended towards more Gaussian-like behaviour with a comparable frequency spectrum. Under these conditions a high data rate differential phase shift keyed (DPSK) telemetry link was established over a 1.7 km path. The ensuing performance of the communication system, characterised by the bit error rate for various symbol rates, is presented and demonstrates the viability of a parametric telemetry system.

Telesonar channel estimation and adaptation

Sound is the best form of energy for broadcasting communication signals in the ocean. However, the physical channel is impaired by nonwhite noise, frequency-selective absorption, a refractive medium, a sloping seafloor, a dynamic sea-surface, and scattering at the boundaries. Temporal variability of these influences and motion of the link terminals introduce fluctuations in the received signal and noise fields. These oceanographic and geometric factors produce measurable effects on underwater acoustic communication systems. The relevant direct measures of these channel effects include multipath spreading, Doppler shifts and spreading, coherence time, and noise statistics. Measuring and understanding these effects support optimization of both modulation and demodulation parameters. Such channel adaptation strategies enhance quality of service for the communication link. Theory, simulation, and ocean experiment illustrate link improvements. [Work funded by ONR 32, the SSC-SD Independent Research Program, and the Navy SBIR Program.]

Optimal use of spatial bandwidth in underwater acoustic communications to enhance bandwidth efficiency

The operation of underwater acoustic communication systems is often directly or indirectly limited by the available temporal bandwidth with performance measures, such as range and information rate, closely tied to the system bandwidth efficiency. Spatial diversity has typically been viewed as a secondary channel feature, either introducing undesirable intersymbol interference or, more favorably, offering a mechanism to offset fading. It is proposed to treat spatial diversity as spatial bandwidth and demonstrate higher bandwidth efficiency with existing power and temporal bandwidth constraints. Several spatial modulation algorithms are explored, including time variant transmitter beamforming where both waveform and choice of projector weights denote information, a spatial analog to multi-carrier modulation where each propagation path corresponds to a carrier, and a coordinated multi-user scenario where each projector emits independent information. Performance of each algorithm is shown to be superior to conventional phase-coherent modulation techniques that make no explicit use of spatial bandwidth. Results of a small-scale field experiment will also be given.

Digital underwater acoustic voice communications

This paper describes the design of an underwater acoustic diver communication system controlled by a digital signal processor. The speech signal transmission rate is compressed by using linear predictive coding (LPC) and the extracted parameters are transmitted through the water to a synchronized receiver by employing digital pulse position modulation (DPPM). The pulse position in each time frame is estimated by an energy detection and decision algorithm which enables the received LPC parameters to be recovered and used to synthesize the speech signal.

Recent advances in high-speed underwater acoustic communications

In recent years, underwater acoustic (UWA) communications have received much attention as their applications have begun to shift from military toward commercial. Digital communications through UWA channels differ substantially from those in other media, such as radio channels, due to severe signal degradations caused by multipath propagation and high temporal and spatial variability of the channel conditions. The design of underwater acoustic communication systems has until recently relied on the use of noncoherent modulation techniques. However, to achieve high data rates on the severely band-limited UWA channels, bandwidth-efficient modulation techniques must be considered, together with array processing for exploitation of spatial multipath diversity. The new generation of underwater communication systems, employing phase-coherent modulation techniques, has a potential of achieving at least an order of magnitude increase in data throughput. The emerging communication scenario in which the modern underwater acoustic systems mill operate is that of an underwater network consisting of stationary and mobile nodes. Current research focuses on the development of efficient signal processing algorithms, multiuser communications in the presence of interference, and design of efficient modulation and coding schemes. This paper presents a review of recent results and research problems in high-speed underwater acoustic communications, focusing on the bandwidth-efficient phase-coherent methods. Experimental results are included to illustrate the state-of-the-art coherent detection of digital signals transmitted at 30 and 40 kb/s through a rapidly varying one-mile shallow water channel.

A coupled bispectral, temporal and spatial coherence function of the pressure field, scattered from a moving sea surface

Fluctuations in the scattering of high-frequency sound from the moving sea surface is of significance, particularly in underwater acoustic communication systems using adaptive methods. Surface scattering may statistically be described using coherence functions, especially for higher frequencies when the coherent part of the pressure field is virtually nonexistent. Previous studies have presented coherence functions as a function of either spatial and temporal variations of the channel, but with a fixed signal carrier frequency or two signal frequencies and temporal channel variations (or some of the several possible Fourier transform duals). Here, a coupled bispectral, temporal and spatial coherence function is presented. The coherence function is derived from the pressure field, given by the two-dimensional Kirchhoff–Helmholtz integral for two monochromatic tones evaluated at separate receiver positions. The channel variations are caused by a wind-driven, gravity-wave dispersed sea surface with a Pierson–Moskowitz spectrum. The derivation of the coherence function involves numerical integration. Numerical results are compared to earlier model data from the literature. [Work sponsored by the Danish Technical Research Council.]

Experimental realization of ASK underwater digital acoustic communications system using error correcting codes

An ASK digital underwater acoustic communications system has been designed and tested to investigate the influence of error correcting codes on the performance of the system. The underwater communications channel is a multi-path fading channel and has been modelled accordingly, and the error probability was estimated under varying operating conditions. This paper describes the experimental investigations which confirm the presence of certain error probability levels. An improvement in the system performance when using error correcting coding techniques has been achieved. The ASK modulation scheme has been used in the system for ease of implementation. The tests were carried out in an outdoor swimming pool to simulate short range channels with severe multi-path effects.

Reduction of bit-error rates by adaptive equalization for a 500-kbit/s underwater acoustic communication system

For the video transmission from untethered subsea robots, an acoustic communication system using the 16-ary quadrature amplitude modulation (16QAM) method is being developed to perform at a capacity of 500 kbit/s at a maximum transmission range of 60 m. To improve the quality of the transmitted information, an adaptive equalization technique will be introduced to this system. In this study, an adaptive equalizer based on a baseband decision-directed equalization method is realized as a computer program. The optimal parameters of the equalizer such as the number of taps and step sizes are determined by computer simulations. In order to estimate the performance of the acoustic communication system with an adaptive equalizer, random code data modulated by 16QAM are transmitted to a receiver located 10–60 m from the source through a surface channel, and stored in a digital memory bank for the off-line processing of an adaptive equalizer. Results show that the bit-error rates in the output of the receiver are about 10−4 at a range of 60 m because of the channel distortion caused by multipath interference, the attenuation characteristics of the channel, etc. However, the bit-error rates in the output of the adaptive equalizer are reduced to less than 10−7 in the 10- to-60-m range. [Conducted as part of the R&D Program of the Large-Scale Project “Advanced Robot Technology” by the Agency of Industrial Science and Technology, Ministry of International Trade and Industry.]

Long‐range underwater acoustic communciations—A review

Long‐range underwater acoustic communication systems are constrained by the properties of the medium. In particular, the non‐uniform (frequency‐dependent) attenuation of the signal amplitude, coupled with multipath and Doppler‐induced time and frequency spreading, limit long range communications to low frequencies (less than 500 Hz), relatively low data rates (of the order of bits per second), fairly narrow frequency bands. Methods to overcome, or mitigate, the medium‐induced signal degradations are based on attempts to optimize the waveform/receiver structures to the characteristics of the propagation channels. Implicit diversity, provided by convolutional coding and interleaving, frequency‐hopping spread spectrum techniques, and Adaptive RAKE‐like receivers provide considerable signal protection against multipath fading and interference.