Analysis of channel effects on direct-sequence and frequency-hoppedspread-spectrum acoustic communication

Multiuser underwater acoustic communication is one of the enabling technologies for the autonomous ocean-sampling network (AOSN). Multiuser communication allows vehicles, moorings, and bottom instruments to interact without human intervention to perform adaptive sampling tasks. In addition, multiuser communication may be used to send data from many autonomous users to one buoy with RF communications capability, which will then forward the information to shore. The two major signaling techniques for multiuser acoustic communication are phase-shift keying (PSK) direct-sequence spread-spectrum (DSSS) and frequency-shift keying (FSK) frequency-hopped spread-spectrum (FHSS). Selecting between these two techniques requires not only a study of their performance under multiuser conditions, but also an analysis of the impact of the underwater acoustic channel. In the case of DSSS, limitations in temporal coherence of the channel affect the maximum spreading factor, leading to situations that may be better suited to FHSS signals. Conversely, the multipath resolving properties of DSSS minimize the effects of frequency-selective fading that degrade the performance of FSK modulation. Two direct-sequence receivers potentially suitable for the underwater channel are presented. The first utilizes standard despreading followed by decision-directed gain and phase tracking. The second uses chip-rate adaptive filtering and phase tracking prior to despreading. Results from shallow water testing in two different scenarios are presented to illustrate the techniques and their performance     

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 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.      

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     

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     

Improvement of Time-Reversal Communications Using Adaptive Channel Equalizers

The spatial and temporal focusing properties of time-reversal methods can be exploited for undersea acoustic communications. Spatial focusing mitigates channel fading and produces a high signal-to-noise ratio (SNR) at the intended receivers along with a low probability of interception elsewhere. While temporal focusing (compression) reduces significantly intersymbol interference (ISI), there always is some residual ISI depending upon the number of transmitters, their spatial distribution (spatial diversity), and the complexity of the channel. Moreover, a slight change in the environment over the two-way propagation interval introduces additional ISI. Using multilevel quadrature amplitude modulation (M-QAM) in shallow water, we demonstrate that the performance of time-reversal communications can be improved significantly by cascading the received time series with an adaptive channel equalizer to remove the residual ISI     

Exploiting phase fluctuations to improve temporal coherence

A special-purpose definition is proposed for phase fluctuations to overcome the obstacle of unpredictable dynamic changes in the phase angle. This definition implies a specific time history for each phase sample and any deviation is termed a phase fluctuation. Its application to acoustic data led to the development of a technique for temporally aligning the phase angles of the acoustic pressure phasors. This alignment process transforms the signal phasors to the real half-space of a rotated complex plane, while the corresponding noise is distributed with random phase angles. Signal processing conducted in the rotated plane improves the temporal coherence of the signals without significantly altering the incoherence of the noise. Coherent attenuation and cancellation of signals is common with temporal coherence and vector averaging. These were eliminated when the aligned-phase angles were substituted for the original unaligned phase angles. Thus, the transformation produces a net temporal coherence gain. Furthermore, it significantly improves the robustness of the signal processor to source and receiver motion. An automatic identifier of signals in the transformed plane also is introduced. Signal identification is based on aligned-phase angle temporal coherence, which significantly improves identification of signals. Results are included for both ocean and atmosphere acoustic data.     

An Iterative Equalization and Decoding Approach for Underwater Acoustic Communication

In this paper, we present an iterative approach for recovering information sent over a shallow underwater acoustic (UWA) communication channel. The procedure has three main tasks: estimation of channel model parameters (CMPs), channel equalization, and decoding. These tasks are performed cyclicly until the algorithm converges. Information bits are convolutionally encoded, punctured and permuted, mapped into quaternary phase-shift keying (QPSK) symbols, linearly modulated, and transmitted through a downward-refracting ocean waveguide. Training symbols are prepended to the transmitted sequence for initial estimation of CMPs. Our algorithm processes data from a single receive sensor. Data are received on a vertical array and the performance of the algorithm for each sensor in the array is examined. There is negligible Doppler spread in the received data. However, difference between transmitter and receiver clocks as well as slight motion of the receive array produce a nonnegligible compression of the received signals. Consequently, there is observable Doppler “shift.” Nonuniform resampling of the data produces time series we model as the output of a linear time-invariant system. Resampling and CMP estimation are done iteratively, in conjunction with equalization and decoding. The algorithm successfully processes the data to yield few or no information bit errors.      

Experimental detection and reception performance for uplink underwater acoustic communication using a remote, in-air, acousto-optic sensor

Covert communications between underwater and aerial platforms would increase the flexibility of surface and air vehicles engaged in undersea warfare by providing a new netcentric warfare communications capability and could have a variety of commercial and oceanographic applications. Research into an acousto-optic sensor shows promise as a means for detecting acoustic data projected toward the water surface from a submerged platform. The laser-based sensor probes the water surface to detect perturbations caused by an impinging acoustic pressure field. Experimental studies were conducted to demonstrate acousto-optic sensor feasibility for obtaining accurate phase preserved recordings of communication signals across the air-water interface. The recorded surface velocity signals were transferred to an acoustic communications receiver that used conventional acoustic telemetry algorithms such as adaptive equalization to decode the signal. The detected, equalized, and decoded bit error rate performance is presented for hydrostatic and more realistic, hydrodynamic water surface conditions.     

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     

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     

Adaptive multiuser detection for underwater acoustical channels

An underwater acoustic local area network (ALAN) provides multipoint-to-point telemetry between many high-rate, ocean-bottom sensors and a central, surface-deployed receiver in the 10-30 kHz vertical acoustical channel. Ocean-bottom modems initiate the transmission process by requesting data channel time slots via a common narrow-band request channel. Request packets overlap in time and frequency in this channel, and the throughput and average transmission delay rely heavily on the successful resolution of the request packet collisions. This paper presents the design, analysis, and experimental demonstration of a request channel receiver capable of resolving collisions between several asynchronous and cochannel packets. The receiver algorithm differs from standard capture schemes (by demodulating the data from both strong and weak transmitters), conventional spread-spectrum receivers (by overcoming the near-far problem), and existing multiple-access demodulation techniques (by adapting to the number of interfering signals, and the unknown phase, Doppler, amplitude, and timing of each signal in the collision). The receiver demodulates the collided packets by decision-directed techniques through a novel method of estimating the interference for each user which minimizes error propagation due to inaccurate tentative decisions. An inwater experiment illustrates that this technique is extremely desirable for collision resolution in underwater acoustic local area networks, and also for underwater autonomous vehicles with both sidescan sonar as well as acoustic telemetry links     

Fluctuations I: spatial and temporal scales of importance for low-frequency propagation

Over the past 5 to 10 years much of the work in fluctuations has been broadly exploratory, intended to characterize observable time and space scales of fluctuation and to identify the environmental phenomena responsible. Much of Project MIMI (Miami/Michigan), with both fixed-system and moving-source experiments, has been devoted to this task. Although the data base now established is useful in providing direction for future work that is more application-oriented, more survey data of this type are needed, particularly at very low acoustic frequencies. More information is required, for example, on the space and time scale of fluctuations which degrade performance in array reception and which influence phase coherence at multisite receivers. A better definition is needed also of the relative importance of temporal and spatial variability in the medium for fluctuations in signals from moving sources. This paper presents data and discussion pertinent to these needs.     

Analysis of shallow-water experimental acoustic data including acomparison with a broad-band normal-mode-propagation model

Channel temporal variability, resulting from fluctuations in oceanographic parameters, is an important issue for reliable communications in shallow-water-long-range acoustic propagation. As part of an acoustic model validation exercise, audio-band acoustic data and oceanographic data were collected from shallow waters off the West Coast of Scotland. These data have been analyzed for temporal effects. The average impulse response for this channel has been compared with simulations using a fast broad-band normal-mode propagation model. In this paper, we also introduce a novel technique for estimating and removing the bistatic reverberation contribution from the data. As propagation models do not necessarily account for reverberation, it has to be extracted from the signals when comparing measured and modeled transmission loss     

水声信道高速率数据传输技术

本文介绍近年来水声信道高速率数所传输技术的一些研究进展，并结合本所研究的水声数据遥测，数字语音通讯和视频图像传输实验样机，讨论了具有抗多途干扰的声传输系统在调制信号设计及信号处理上所采用的关键技术。     

CSI feedback-based CS for underwater acoustic adaptive modulation OFDM system with channel prediction

In this paper, we investigate the performance of adaptive modulation （AM） orthogonal frequency division multiplexing （OFDM） system in underwater acoustic （UWA） communications. The aim is to solve the problem of large feedback overhead for channel state information （CSI） in every subcarrier. A novel CSI feedback scheme is proposed based on the theory of compressed sensing （CS）. We propose a feedback from the receiver that only feedback the sparse channel parameters. Additionally, prediction of the channel state is proposed every several symbols to realize the AM in practice. We describe a linear channel prediction algorithm which is used in adaptive transmission. This system has been tested in the real underwater acoustic channel. The linear channel prediction makes the AM transmission techniques more feasible for acoustic channel communications. The simulation and experiment show that significant improvements can be obtained both in bit error rate （BER） and throughput in the AM scheme compared with the fixed Quadrature Phase Shift Keying （QPSK） modulation scheme. Moreover, the performance with standard CS outperforms the Discrete Cosine Transform （DCT） method.     

Effects of internal waves on sound pulse propagation in the Straitsof Florida

An unexplained result of broad-band transmission experiments made more than ten years ago by DeFerrari in the Straits of Florida (center frequency ~500 Hz, bandwidth ~100 Hz, water depth ~200-m, range ~20 km) is that the measured pulse response functions failed to show the expected multipath replicas of the transmitted pulse and instead were smeared into a single broad cluster (duration ~50-~350 ms) in which the unresolved multipaths fluctuated rapidly in geophysical time (coherence time ≪12 min) leaving only a relatively stable envelope that is useful for oceanographic inversion. It is demonstrated here that the effects of internal waves on sound pulse propagation in the Straits of Florida can explain these observed results, and it is suggested that similar instabilities of acoustic multipaths due to internal waves are to be expected in other shallow-water propagation conditions. The demonstration is based on numerical simulations with the broad-band UMPE acoustic model that includes multiple forward scattering from volume inhomogeneities induced by internal wave fluctuations that are described by a broad spectrum of excitation. The simulated temporal variability, stability, and coherence of acoustic pulse arrivals are displayed on geophysical time scales from seconds to many hours and are qualitatively in agreement with the measured data in the Straits of Florida     

基于Visual C和PC的软件水声调制解调系统

介绍了一种DS/DBPSK调制解调技术方案,该方案采用了具有较强抗干扰、抗衰落和抗多径性能的扩频技术,并结合与信道适应性较好、不需载波恢复的差分相位调制技术,对浅海水声信道造成的干扰进行抑制.在此方法上,基于Visual C(VC)及PC平台设计了一个通用性好的软件水声调制解调系统,利用双缓冲邋和多线程技术机制实现算法实时处理,同时可通过串口通信提供上层应用接口.实验系统验证了扩频和差分调制技术在浅海水声通信中的性能,为水声组网提供了底层支持.海试实验结果表明了系统的有效性.     

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。     

A simulation tool for high data-rate acoustic communication in ashallow-water, time-varying channel

The paper discusses the development of a simulation tool to model high data-rate acoustic communication in shallow water. The simulation tool is able to generate synthetic time series of signals received at a transducer array after transmission across a shallow-water communication channel. The simulation tool is suitable for testing advanced signal processing techniques for message recovery. A channel model has been developed based on the physical aspects of the acoustic channel. Special emphasis has been given to fluctuations of the signal transmission caused by time-varying multipath effects. At shorter ranges, the temporal variations are dominated by acoustic scattering from the moving sea surface. Therefore, the channel model produces a coherence function which may be interpreted as a time-varying reflection coefficient for the surface scattered acoustical path. A static, range-independent ray model identifies the significant multipaths, and the surface path is modulated with the time-varying reflection coefficient. The advantages and limitations of the channel model are discussed and assumptions necessary to overcome the limitations are emphasised. Based on the assumptions, an algorithm has been developed and implemented to model how a binary message will be modulated when transmitted by a transducer, is distorted in the channel and finally is received by a transducer array