A.E. Adams

Performance of coherent PSK receivers using adaptive combining, beamforming and equalisation in 50 km underwater acoustic channels

During the first phase of trials, undertaken in the Mediterranean by the MAST2 project LORACOM, PSK modulated data was transmitted over a 50 km underwater acoustic channel. The objectives of LORACORM are to achieve reliable underwater acoustic transmission over long ranges for applications such as remote instrumentation and ocean buoy monitoring. The test data were received on both a short vertically aligned transducer array (AV1), and on a number of widely spaced hydrophones in the water column (F1). This paper describes the results of processing this data using a number of digitally implemented coherent receiver structures based on adaptive equalisation, adaptive beamforming, and adaptive multichannel combining.

Protocols for sub-sea communication networks

The design of multi-node sub-sea acoustic communication networks is in its infancy. Particularly, efficient protocols are needed that reserve the maximum proportion of available communication time for data transmission, as opposed to network control. However, these protocols must retain the capability to cope with the volatile behavior experienced in underwater channels. This paper presents protocols designed to meet these criteria.

A blind multichannel combiner for long range underwater communications

This paper presents the development and performance of blind algorithms for a spatial diversity scheme to enable reliable data telemetry over a long range underwater acoustic channel. A number of Bussgang based stochastic gradient algorithms were tested for this multipath channel with additive white and coloured shipping noise. Both simulation and real experimental tests have shown that a significant improvement is obtained by utilising the spatial diversity of the long range channel and the ability of the combiner to perform joint equalisation and carrier phase tracking.

Spread-spectrum based adaptive array receiver algorithms for the shallow-water acoustic channel

In this paper we propose spread spectrum-based adaptive array receiver structures for reliable low data-rate multi-access communication in the shallow-water acoustic channel. The performance of the proposed receiver algorithms is evaluated by employing both offline processing and simulation. Results indicate that multi-access error-free reception is attainable in network scenarios where transmission is multipath impaired.

Further results from the SAMI synthetic aperture sonar

This paper presents two techniques which may enhance the usefulness of synthetic aperture sonar, specifically multi-ping and Vernier techniques. A more efficient method of synthetic aperture processing is introduced, which was useful in processing the Vernier data in real-time. Results from the sea-trials held in May 1996 are presented and the authors conclude that multi-ping and Vernier techniques have brought benefits to the SAMI synthetic aperture system.

Analysis of experimental shallow water network channel and theoretical channel model

This paper describes the testing of an experimental water acoustic network communication that was conducted at the North Sea, Netherlands. The North Sea is a shallow channel that exhibits multipath propagation that results in signal fading and phase fluctuations at the receiver. The Doppler effect is another phenomenon that is observed due the movement of both the source and the receiver that is caused either by the swelling of the wave fronts or the mechanical movement of the source. Sources of background noise are highly variable from a mixture of environmental and man-made noises. Situations of multiuser paths with range to depth ratio of 28:1 and 277:1 respectively are examined and the results from the experimental data are then compared to a simulated theoretical model. The simulated channel is characterized by using ray theory and extending it to a multipath expansion for a series of reflections resulting in multipath arrival at the receiver.

Modeling of the non-linear acoustic response of bubbles by Volterra series analysis

The ability to determine the size distribution of a field of bubbles has a wide range of practical applications. Bubbles have traditionally been detected by using either optical or acoustic techniques. Bubbles exhibit strong acoustic resonance properties dependent on their size. By probing a field of bubbles it is possible to form an estimate of the range of sizes present by analysing the scattered data. There are a number of acoustic bubble sizing methods currently available, some of which exploit the nonlinear dynamic behaviour of the bubble. By modeling this behaviour, improvements may be made to the estimation techniques. This paper presents estimates for the quadratic transfer function of a single bubble based on the Volterra series. A method of estimating the Volterra kernels using a neural network is presented which significantly reduces the quantity of data required to form an estimate of the transfer function.

Doppler compensation for underwater acoustic communications

Doppler shift in underwater acoustic communications presents a major challenge to modern designers. Carrier tracking and symbol synchronisation are severely affected by Doppler shift induced by relative motion of the transmitting and receiving structures. This paper presents an efficient Doppler compensation system that can be used as a generic pre-processor to conventional equalisation and beamforming structures. The rationale behind the choice of the design parameters is reported as well as the experimental system performance based on results from trials in the North Sea.

Analysis of a multi-element multi-user receiver for a shallow water acoustic network (SWAN) based on recursive successive interference cancellation (RSIC) technique

A novel technique based on recursive successive interference cancellation (RSIC) is proposed for cancelling intersymbol interference (ISI) and multiple access interference (MAI), which adversely affect the performance of shallow water acoustic networks (SWAN). Conventional structures employing decision feedback equalisation (DFE) are known to be effective for intersymbol interference reduction. However, if there are one or more interfering users, the DFE performs inadequately in extracting the useful signal for the weaker user. The performance is further degraded due to power control inefficiency. The simulation results presented in this paper demonstrate the ability of the RSIC structure in mitigating the effect of MAI, ISI and power control inefficiency.

High resolution analysis of ultra-shallow water acoustic travel time and gain perturbations

From the analysis of the direct path gain, this paper shows how it is possible to separate artefact effects from effects induced by the channel. The method proposed is based on the combination of the impulse response and the Wigner-Ville time frequency distribution. The article demonstrates the influence of swell on the travel delay of an underwater acoustic wave.