Josko Catipovic

Phase-coherent digital communications for underwater acoustic channels

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. >

Adaptive multichannel combining and equalization for underwater acoustic communications

A theoretically optimal multichannel receiver for intersymbol interference communication channels is derived, and its suboptimal versions with linear and decision feedback equalizer are presented. A practical receiver based on any of these structures encounters difficulties in the underwater acoustic channels in which the extended time‐varying multipath is accompanied by phase instabilities. A receiver that overcomes these problems by jointly performing adaptive mean‐squared error diversity combining, multichannel carrier‐phase synchronization and decision feedback equalization is proposed. Its performance is demonstrated on the experimental telemetry data from deep and shallow water long‐range acoustic channels. Presented results indicate superior quality of coherent PSK and QAM reception obtained through joint equalization of very few channels.

Analysis of the impact of channel estimation errors on the performance of a decision-feedback equalizer in fading multipath channels

A coherent receiver with a decision-feedback equalizer (DFE) operating on a Rayleigh fading channel under a suitable adaptive algorithm is considered. In the analysis of a DFE, a common assumption is that the receiver has perfect knowledge of the channel impulse response. However, this is not the case in practice, and for a rapidly fading channel, errors in channel tracking can become significant. We analyze theoretically the impact of these errors on the performance of a multichannel DFE. The expressions obtained for the achievable average MPSK bit error probabilities depend on the estimation error covariance. In order to specify this matrix, we focus on a special case when a Kalman filter is used as an optimal channel estimator. In this case, the probability of bit error can be assessed directly in terms of channel fading model parameters, the most interesting of which is the fading rate. Our results show the penalty imposed by imperfect channel estimation, as well as the fading-induced irreducible error rates. >

Algorithms for joint channel estimation and data recovery-application to equalization in underwater communications

Methods for joint ocean-channel estimation and data recovery are derived using an optimal, maximum likelihood (ML) estimation criterion. The resulting ML problems may be complex, thus iterative algorithms are used, e.g. the expectation-maximization (EM) algorithm. The different methods correspond to different assumptions about the ocean channel. The theoretical derivation of these methods as well as preliminary results on simulated ocean data experiments are presented. >

Reduced‐complexity spatial and temporal processing of underwater acoustic communication signals

Multichannel processing of high‐speed underwater acoustic communication signals requires computationally intensive receiver algorithms. The size of adaptive filters, determined by the extent of ocean multipath, increases with signaling rate and limits system performance through large noise enhancement and increased sensitivity of computationally efficient algorithms to numerical errors. To overcome these limitations, reduction in receiver complexity is achieved by exploiting the relationship between optimal diversity combining and beamforming. Under relatively simple conditions, two adaptive receivers, one based on diversity combining which does not rely on any spatial signal distribution, and the other based on optimal beamforming, are shown to achieve the same performance. The beamforming approach, however, leads to a receiver of lower complexity. Carrying these observations over to a general case of broadband transmission through an unknown channel, a fully adaptive receiver is developed which incorpor...

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. >

An integrated approach to multiple AUV communications, navigation and docking

In this paper we report on our progress in two topics related to the development of an autonomous oceanographic sampling network. The first topic deals with the use of a flexible DSP system on the Odyssey class AUV which provides acoustic communications and ultra-short baseline navigation, while the second topic details our efforts in integrating these capabilities for the purpose of autonomous vehicle docking. We present an algorithm for homing in on a beacon and our results of testing this approach at sea. We show how our docking approach may be extended to allow coordinated multiple vehicle operations and demonstrate this approach for the case of two vehicles conducting a coordinated survey. We also include results from the at-sea tests of our acoustic communications system.

Performance of high‐rate adaptive equalization on a shallow water acoustic channel

Multipath propagation in underwater acoustic channels causes intersymbol interference in the transmission of digital communication signals. An increase of the transmission rate on a multipath channel results in longer intersymbol interference, which ultimately limits the performance of a phase‐coherent digital communication system. Recent experimental results, however, show a seemingly surprising result: an increase in transmission rate resulted in improved system performance. An explanation for this phenomenon is found in the time variation of the ocean multipath. In strongly fluctuating shallow water channels, higher transmission rates allow for more frequent sampling of the rapidly varying channel, thus resulting in a better tracking capability of the receiver. Experimental results obtained in shallow water show a substantial improvement in performance of QPSK coherent detection over a 1‐mile range, as the data rate is increased from 5 to 20 kilobits per second. A theoretical analysis based on stochast...

Design and performance analysis of a Digital Acoustic Telemetry System for the short range underwater channel

This paper presents the design and performance description of a microprocessor based Digital Acoustic Telemetry System (DATS). The system was used to transmit data over short range underwater paths and to measure the fading characteristics of a group of CW tones. This work reports on the test results from Woods Hole Harbor. The system has also been used for under-ice propagation experiments in Lake Caanan, NH, and in the Marginal Ice Zone near Svalbard. Geometries chosen for the tests are representative of the short range marine work site paths for which high data rate communications systems are desirable.

Asynchronous Multiuser Reception for OFDM in Underwater Acoustic Communications

Recently significant progress has been made on point-to-point underwater acoustic communications, and the interest has grown on the application of those techniques in multiuser communication settings, where the asynchronous nature of multiuser communication poses a grand challenge. This paper develops a time-asynchronous multiuser reception approach for orthogonal frequency-division multiplexing (OFDM) transmissions in underwater acoustic channels. The received data burst is segmented and apportioned to multiple processing units in an overlapped fashion, where the length of the processing unit depends on the maximum asynchronism among users on the OFDM block level. Interference cancellation is adopted to reduce the interblock interference between overlapped processing units. Within each processing unit, the residual inter-block interference from multiple users is aggregated as one external interference which can be parameterized. Multiuser channel estimation, data detection, and interference mitigation are then carried out in an iterative fashion. Simulation and emulated experimental results demonstrate the robustness of the proposed receiver with signal asynchronism among multiple users in both time-invariant and time-varying environments. It is observed that the receiver decoding performance degrades as the channel time variation and the maximum relative delay among users increase.