Spatial diversity equalization applied to underwater communications

Underwater acoustic digital communication is difficult because of the nature of the fading multipath channels. Digital signal processing, such as adaptive equalization, is known to greatly improve the communication data rate by limiting intersymbol interference (ISI). However, existing underwater acoustic equalization studies are limited to single-channel techniques, and spatial diversity processing is limited to selection or combining. In this paper, we design minimum mean-square error (MMSE) equalizers jointly among all spatial diversity channels. We call this spatial diversity equalization (SDE). Results are based on a very sparse vertical array in a midrange underwater acoustic channel. We study the effect of element number and placement, the length of the equalization filters, and linear feedforward versus nonlinear decision feedback algorithms. A suboptimum equalizer combiner (EC) is studied to alleviate the computational intensity of JCE. We first design the system for a known acoustic channel; later, some results are verified using adaptive algorithms. Results are presented both in terms of the mean-square error (MSE) and the probability of a symbol error. The latter is important as it is the ultimate interest for a digital communication system. We found that system performance improves rapidly with an increase in the number of spatial channels     

Acoustic telemetry--An overview

Acoustic telemetry from underwater submersibles and sensors has been pursued ever since it was recognized that the ocean could support signal transmission. While it has been evident that some form of communication is possible, the ocean has proved to be a distressingly difficult medium in which to achieve high data rates. High data rate transmission requires a wide bandwidth which is severely constrained in the ocean because of the absorption of high-frequency energy. Moreover, the ocean is a very reverberant environment with both time and frequency spreading of signals; this further limits data transmission rates. The net effect of the bandwidth and reverberation constraints has led to either acoustic telemetry systems with low data rates or to the use of tethered systems. Over the years, various forms of acoustic communication systems have been developed. These have included direct AM and SSB for underwater telephones, FM for sensor data, FSK and DPSK for digital data, and parametric sonars for narrow-beam systems. As offshore operations have increased, several other systems have been proposed and/or built to respond to particular needs. In this paper, we review the underwater channel and the limitations that it imposes upon acoustic telemetry systems. We then survey some of the systems that have been built (excluding military systems) and indicate how they use various communication system principles to overcome these limitations.     

A Variable-Rate Spread-Spectrum System for Underwater Acoustic Communications

A key research area in underwater acoustic (UWA) communication is the development of advanced modulation and detection schemes for improved performance and range-rate product. In this communication, we propose a variable-rate underwater data transmission system based on direct sequence spread spectrum (DSSS) and complementary code keying (CCK), particularly for shallow-water acoustic channels with severe multipath propagation. We provide a suboptimum receiver that consists of a bidirectional decision feedback equalizer (BiDFE) to cancel both postcursor and precursor intersymbol interference (ISI). We also develop iterative signal processing and time-reversal (TR) diversity processing to mitigate the effect of error propagation in BiDFE. We present performance analysis on bit error rate (BER) for different data rates. Our works show that this new variable-data-rate DSSS-CCK is a suitable candidate for UWA communications over varying channel conditions and distance.      

Multichannel processing of broad-band multiuser communicationsignals in shallow water acoustic channels

High-throughout multiple-access communication networks are being considered for use in underwater acoustic channels. Bandwidth limitations of underwater acoustic channels require receivers to process broad-band communications signals in the presence of several active users. To deal with the resulting multiple-access interference in addition to high intersymbol interference, the spatial variability of ocean multipath is exploited in a multichannel multiuser receiver. Two configurations of such a receiver, a centralized and a decentralized one, are presented in fully adaptive modes of operations. While greatly reducing intersymbol and multiple-access interference, spatial diversity implies high increase in adaptive multiuser receiver complexity. To reduce the complexity of the optimal multichannel combiner, spatial structure of multipath is exploited. The complexity of resulting adaptive decentralized multichannel multiuser receiver is reduced at almost no cost in performance. Comparison of proposed multichannel receivers in an experimental shallow water channel demonstrates superior performance of spatial signal combining. The use of multiple input channels is shown to provide high level of tolerance for the near-far effect in both centralized and decentralized receivers. Decentralized receiver with reduced-complexity combining is found to satisfy the performance/complexity trade-off required for practical receiver realization in shallow water networks     

Phase-coherent digital communications for underwater acousticchannels

High-speed phase coherent communications in the ocean channel are made difficult by the combined effects of large Doppler fluctuations and extended, time-varying multipath. In order to account for these effects, we consider a receiver which performs optimal phase synchronization and channel equalization jointly. Since the intersymbol interference in some underwater acoustic channels spans several tens of symbol intervals, making the optimal maximum-likelihood receiver unacceptably complex, we use a suboptimal, but low complexity, decision feedback equalizer. The mean squared error multiparameter optimization results in an adaptive algorithm which is a combination of recursive least squares and second-order digital phase and delay-locked loops. The use of a fractionally spaced equalizer eliminates the need for explicit symbol delay tracking. The proposed algorithm is applied to experimental data from three types of underwater acoustic channels: long-range deep water, long-range shallow water, and short-range shallow water channels. The modulation techniques used are 4- and 8-PSK. The results indicate the feasibility of achieving power-efficient communications in these channels and demonstrate the ability to coherently combine multiple arrivals, thus exploiting the diversity inherent in multipath propagation     

Compound compensation strategies for wireless data communicationsover the multimodal acoustic ocean waveguide

Underwater acoustic communications (UAC) at the reverberation-limited range results in severely distorted information signals. Wide-band signals are subject to both intermodal and intramodal-type of dispersions. The underwater acoustic channel impulse response and the sidelobes strongly depend on the waveguide structure and the source and receiver positions. The motion and displacement from this position, as well as other environmental variabilities impose a real-time adaptivity for the receiver operation to keep track of the fluctuations. To increase the system's reliability and data rate, there is a need to employ adaptive equalizers and diversity techniques to improve the margins against noise, and intersymbol interference (ISI). Blind adaptive equalization (BAE) is the ideal adaptive compensation when operating point-to-multipoint networks, and centralized communication systems in general. Inherent optimum multiple resonant modes within the ocean acoustic waveguide can be exploited judiciously via a new proposed parallel data multicarrier modulation (MCM) scheme by sending data over the multiple subcarriers. MCM might eventually obviate equalization which introduces higher-order computational complexity to the receiver. The above modulation eliminates multipaths and allows operation at multiples of the single-carrier transmission rate. The system's immunity to distortions such as ISI, fast fades, and impulsive noises, is increased due to incorporation of symbol guard space. Direct comparisons with single carrier schemes (such as higher-order statistics (HOS)-based equalization) are of great interest, since the proposed new receiver configuration has low-complexity to provide a compact, portable and low-power practical acoustic modem. Finally, network topology issues are considered to determine optimum network architectures for underwater acoustic LANs. A central topology (CT) supported by BAE and MCM transmission is proposed     

Low complexity adaptive equalizers for underwater acoustic communications

Interference signals due to scattering from surface and reflecting from bottom is one of the most important problems of reliable communications in shallow water channels. To solve this problem, one of the best suggested ways is to use adaptive equalizers. Convergence rate and misadjustment error in adaptive algorithms play important roles in adaptive equalizer performance. In this paper, affine projection algorithm （APA）, selective regressor APA（SR-APA）, family of selective partial update （SPU） algorithms, family of set-membership （SM） algorithms and selective partial update selective regressor APA （SPU-SR-APA） are compared with conventional algorithms such as the least mean square （LMS） in underwater acoustic communications. We apply experimental data from the Strait of Hormuz for demonstrating the efficiency of the proposed methods over shallow water channel. We observe that the values of the steady-state mean square error （MSE） of SR-APA, SPU-APA0 SPU-normalized least mean square （SPU-NLMS）, SPU-SR-APA0 SM-APA and SM-NLMS algorithms decrease in comparison with the LMS algorithm. Also these algorithms have better convergence rates than LMS type algorithm.     

Spatial modulation experiments in the underwater acoustic channel

A modulation technique for increasing the reliable data rate achievable by an underwater acoustic communication system is presented and demonstrated. The technique, termed spatial modulation, seeks to control the spatial distribution of signal energy such that the single physical ocean channel supports multiple parallel communication channels. Given a signal energy constraint, a communication architecture with access to parallel channels will have increased capacity and reliability as compared to one with access to a single channel. Results from two experiments demonstrate higher obtainable data rates and power throughput for a system employing spatial modulation than for one that does not. The demonstrated benefits were characterized by an equivalent SNR gain of over 5 dB in the first experiment. In the second experiment, using two element source and receiver arrays with apertures of 0.9 m, a coherently modulated signal was shown to offer nearly 50% greater capacity by using spatial modulation than by using temporal modulation alone.     

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     

水声通信中基于小波变换的图像编码研究

提出了一种高误比特率传输条件下的图像编码方法，它适用于水声信道的图像传输。针对水下图像的特点，选取合适的小波基和变换参数对图像进行离散小波变换；依据小波系数的能量分布特性，对不同的子带采用不同的量化和定长编码，编码率为0．8比特／像素。水声通信试验表明，在传输误比特率达到10^-2时，仍能得到可接受的图像质量。     

Estimation of Rapidly Time-Varying Sparse Channels

The estimation of sparse shallow-water acoustic communication channels and the impact of estimation performance on the equalization of phase coherent communication signals are investigated. Given sufficiently wide transmission bandwidth, the impulse response of the shallow-water acoustic channel is often sparse as the multipath arrivals become resolvable. In the presence of significant surface waves, the multipath arrivals associated with surface scattering fluctuate rapidly over time, in the sense that the complex gain, the arrival time, and the Dopplers of each arrival all change dynamically. A sparse channel estimation technique is developed based on the delay-Doppler-spread function representation of the channel. The delay-Doppler-spread function may be considered as a first-order approximation to the rapidly time-varying channel in which each channel component is associated with Doppler shifts that are assumed constant over an averaging interval. The sparse structure of the delay-Doppler-spread function is then exploited by sequentially choosing the dominant components that minimize a least squares error. The advantage of this approach is that it captures both the channel structure as well as its dynamics without the need of explicit dynamic channel modeling. As the symbols are populated with the sample Dopplers, the increase in complexity depends on the channel Doppler spread and can be significant for a severely Doppler-spread channel. Comparison is made between nonsparse recursive least squares (RLS) channel estimation, sparse channel impulse response estimation, and estimation using the proposed approach. The results are demonstrated using experimental data. In training mode, the proposed approach shows a 3-dB reduction in signal prediction error. In decision-directed mode, it improves significantly the robustness of the performance of the channel-estimate-based equalizer against rapid channel fluctuations.     

Performance limitations in underwater acoustic telemetry

Performance limitations in digital acoustic telemetry are addressed. Increases in computational capabilities have led to a number of complex but practical solutions aimed at increasing the reliability of acoustic data links. These solutions range from ocean-basin scale data telemetry to video-image transmission at a few hundred yards' distance. The opportunity to implement highly complex tasks in real time on modest hardware is a common factor. The data rates range from 1 to 500 kb/s and are much slower than satellite channels, while acceptable system complexity is higher than virtually any other channel with comparable data throughput. The basic performance bounds are the channel phase stability, available bandwidth, and the channel impulse response fluctuation rate. Phase stability is of particular concern for long-range telemetry, channel fluctuation characteristics drive equalizer, and synchronizer design; the bandwidth limitation is a direct constraint on data rate for a given signaling method     

Correlation-based decision-feedback equalizer for underwater acoustic communications

The purpose of this paper is to develop a decision-feedback equalizer (DFE) using a fixed set of parameters applicable to most shallow oceans with minimal user supervision (i.e., a turn key system). This work is motivated by the superior performance [bit error rate (BER)] of the multichannel DFE compared with other methods, such as passive-phase conjugation (PPC), at the same time noting its sensitivity to different acoustic environments. The approach is to couple PPC, utilizing its adaptability to different environments, with a single-channel DFE. This coupling forms an optimal processor for acoustic communications in theory, but it has never been implemented in practice. By coupling with DFE, the method achieves the same spatial diversity as conventional multichannel DFE, without requiring a large number of receivers as does PPC. The correlation-based DFE in terms of the autocorrelation functions of the channel impulse responses summed over the receiver channels (the Q function) is derived. This paper shows in terms of waveguide physics, further supported by real data, the many desirable features of the Q function that suggest, given adequate sampling of the water column, a general applicability of the correlation-based equalizer to different environments, irrespective of the sound speed profiles, bottom properties, and source-receiver ranges/depths. This property can be expected to hold approximately for a small number of receivers with spatial diversity. This paper demonstrates the robustness of the new equalizer with moving source data despite the range change (which modifies the impulse response) and symbol phase change due to time-varying Doppler.     

Performance of sequential decoding of convolutional codes overfully fading ocean acoustic channels

Sequential decoding of long-constraint convolutional codes is shown to be a feasible technique for digital data telemetry over realistic marine acoustic channels. A computational bound for sequential decoding over a fading dispersive channel is derived for hardlimiting and quantizing decoders. The results indicate that a minimum of 8 dB of bit SNR (signal-to-noise ratio) is required for sequential decoder operation. Simulations indicate that 14-dB bit SNR results in simple and feasible implementations. Diversity methods for coded transmissions over Rayleigh fading channels are examined. The optimal diversity level for minimum error probability of uncoded systems and the diversity level of minimizing the sequential decoder computational load are derived and shown to be different, with the latter requiring a higher order of diversity. Performance differences between fixed-diversity and optimal-diversity systems are presented     

High-Rate Communication for Underwater Acoustic Channels Using Multiple Transmitters and Space–Time Coding: Receiver Structures and Experimental Results

In this paper, we consider the use of multiple antennas and space-time coding for high data rate underwater acoustic (UWA) communications. Recent advances in information theory have shown that significant capacity gains can be achieved by using multiple-input-multiple-output (MIMO) systems and space-time coding techniques for rich scattering environments. This is especially significant for the UWA channel where the usable bandwidth is severely limited due to frequency-dependent attenuation. In this paper, we propose to use space-time coding and iterative decoding techniques to obtain high data rates and reliability over shallow-water, medium-range UWA channels. In particular, we propose to use space-time trellis codes (STTCs), layered space-time codes (LSTCs) and their combinations along with three low-complexity adaptive equalizer structures at the receiver. We consider multiband transmissions where the available bandwidth is divided into several subbands with guard bands in between them. We describe the theoretical basis of the proposed receivers along with a comprehensive set of experimental results obtained by processing data collected from real UWA communications experiments carried out in the Pacific Ocean. We demonstrate that by using space-time coding at the transmitter and sophisticated iterative processing at the receiver, we can obtain data rates and spectral efficiencies that are not possible with single transmitter systems at similar ranges and depths. In particular, we have demonstrated reliable transmission at a data rate of 48 kb/s in 23 kHz of bandwidth, and 12 kb/s in 3 kHz of bandwidth (a spectral efficiency of 4 bs^-1Hz^-1) at a 2-km range.     

Multichannel Detection for Wideband Underwater Acoustic CDMA Communications

Direct-sequence (DS) code-division multiple access (CDMA) is considered for future wideband mobile underwater acoustic networks, where a typical configuration may include several autonomous underwater vehicles (AUVs) operating within a few kilometers of a central receiver. Two receivers that utilize multichannel (array) processing of asynchronous multiuser signals are proposed: the symbol decision feedback (SDF) receiver and the chip hypothesis feedback (CHF) receiver. Both receivers use a chip-resolution adaptive front end consisting of a many-to-few combiner and a bank of fractionally-spaced feedforward equalizers. In the SDF receiver, feedback equalization is implemented at symbol resolution, and receiver filters, including a decision-directed phase-locked loop, are adapted at the symbol rate. This limits its applicability to the channels whose time variation is slow compared to the symbol rate. In a wideband acoustic system, which transmits at maximal chip rate, the symbol rate is down-scaled by the spreading factor, and an inverse effect may occur by which increasing the spreading factor results in performance degradation. To eliminate this effect, feedback equalization, which is necessary for the majority of acoustic channels, is performed in the CHF receiver at chip resolution and receiver parameters are adjusted at the chip rate. At the price of increased computational complexity (there are as many adaptive filters as there are symbol values), this receiver provides improved performance for systems where time variation cannot be neglected with respect to the symbol rate [e.g., low probability of detection (LPD) acoustic systems]. Performance of the two receivers was demonstrated in a four-user scenario, using experimental data obtained over a 2-km shallow-water channel. At the chip rate of 19.2 kilochips per second (kc/s) with quaternary phase-shift keying (QPSK) modulation, excellent results were achieved at an aggregate data rate of up to 10 kb/s     

水声信道中一种抗多途跳频通信的研究

水声通道系统是当代海洋开发和海洋环境立体监测中的重要技术组成部分，广泛应用于海洋监测、海洋资源勘探和开发等方面。然而，水声信道的随机起伏、时－空－频变的多途特征使水声通信技术成为当代最为复杂的通信技术之一。本文介绍近年来在高速水声通信方面的一些研究进展，讨论了一种利用高速数字信号处理器（DSP）实现水声信道跳频通信的方案，并探讨了该方案在调制信号设计及信号处理实现上所采用的关键技术。     

Spatial diversity processing for underwater acoustic telemetry

A large increase in the reliability of shipboard or stationary underwater acoustic telemetry systems is achievable by using spatially distributed receivers with aperture sizes from 0.35 to 20 m. Output from each receiver is assigned a quality measure based on the estimated error rate, and the data, weighted by the quality measure, are combined and decoded. The quality measure is derived from a Viterbi error-correction decoder operating on each receiver and is shown to perform reliability in a variety of non-Gaussian noise and jamming environments and reduce to the traditional optimal diversity system in a Gaussian environment. The dynamics of the quality estimator allow operation in the presence of high-power impulsive interference by exploiting the signal and noise differential travel times to individual sensors. The spatial coherence structure of the shallow water acoustic channel shows relatively low signal coherence at separations as short as 0.35 m. Increasing receiver spacing beyond 5 m offers additional benefits in the presence of impulsive noise and larger-scale inhomogeneities in the acoustic field. A number of data transmission experiments were carried out to demonstrate system performance in realistic underwater environments     

Performance Enhancement of Multiuser Passive Time Reversal Communication Based on Space-Time Block Coding

Reliable,with high data rate,acoustic communication in time-varying,multipath shallow water environment is a hot research topic recently.Passive time reversal communication has shown promising results in improvement of the system performance.In multiuser environment,the system performance is significantly degraded due to the interference among different users.Passive time reversal can reduce such interference by minimizing the cross-correlated version of channel impulse response among users,which can be realized by the well-separated users in depth.But this method also has its shortcomings,even with the absence of relative motion,the minimization sometimes may be impossible because of the time-varying environment.Therefore in order to avoid the limitation of minimizing the cross-correlated channel function,an approach of passive time reversal based on space-time block coding (STBC) is presented in this paper.In addition,a single channel equalizer is used as a post processing technique to reduce the residual symbol interference.Experimental results at 13 kHz with 2 kHz bandwidth demonstrate that this method has better performance to decrease bit error rate and improve signal to noise ratio,compared with passive time reversal alone or passive time reversal combined with equalization.     

The state of the art in underwater acoustic telemetry

Progress in underwater acoustic telemetry since 1982 is reviewed within a framework of six current research areas: (1) underwater channel physics, channel simulations, and measurements; (2) receiver structures; (3) diversity exploitation; (4) error control coding; (5) networked systems; and (6) alternative modulation strategies. Advances in each of these areas as well as perspectives on the future challenges facing them are presented. A primary thesis of this paper is that increased integration of high-fidelity channel models into ongoing underwater telemetry research is needed if the performance envelope (defined in terms of range, rate, and channel complexity) of underwater modems is to expand