Shallow Water Acoustic Networks Research Articles

Comparison of DS-CDMA and MC-CDMA techniques for dual-dispersive fading acoustic communication networks

Recently, code division multiple access (CDMA) techniques have been utilised to achieve reliable multiuser communication in asynchronous shallow-water acoustic networks. Two different communication techniques, are considered. Both have their origins in spread spectrum principles and can offer significant benefits in such underwater environments, which are characterised by limited transmission bandwidth and are required to operate asynchronously with low signal-to-noise ratios. In direct-sequence (DS)-CDMA, spread data are transmitted at a single carrier frequency. In contrast, in multi-carrier (MC)-CDMA a set of carrier frequencies is employed to achieve frequency diversity. The proposed adaptive receiver architectures, based on the minimisation of the mean square error (MSE), integrate the functions of multi-access interference cancellation, equalisation and phase-carrier tracking. The performance of DS and MC-CDMA has been evaluated and compared under different simulation scenarios with respect to the multipath delay, Doppler broadening and number of users, indicating the effectiveness of each system in an underwater environment. Finally, analytical derivation of the BER performance of the MMSE multicarrier receiver in a Rayleigh-fading environment is presented.

Multicarrier code division multiple access (MC-CDMA) technique for underwater acoustic communication networks using short spreading sequences

Code division multiple access (CDMA) techniques have recently been utilised to achieve reliable multiuser communication in asynchronous shallow water acoustic networks. The authors consider this type of communication technique, which has its origins in spread spectrum principles. The performance of multicarrier (MC) CDMA for short length complex spreading sequences, which demonstrate both good cross-correlation and auto-correlation properties, has been evaluated by means of computer simulations. In MC-CDMA, a set of carrier frequencies is employed to achieve frequency diversity. The proposed adaptive receiver architectures integrate the functions of despreading, multi-access and narrowband interference cancellation, equalisation, and phase-carrier tracking. Results show the effectiveness of the system in a rapidly time-varying, frequency-selective channel.

Performances of multi-element multi-user detection strategies in a shallow-water acoustic network (SWAN)

A multi-element receiver strategy is proposed in this paper for a multi-user shallow-water acoustic network (SWAN). The base station receiver, equipped with prior knowledge of the synchronization and training sequences of all intended users, has the task of demodulating the received signals of each user independent of the presence of other users. The adopted receiver strategy enables robust communications through the challenging underwater environment which is limited by both environmental and system factors. The channel is characterized by inter-symbol interference due to multipath propagation and multiple access interference. In this paper, we propose a number of multi-user detection receiver structures employing adaptive decision feedback equalization and spatial diversity to mitigate the effect of these two types of interference. Computer simulations and experimental sea trials conducted in the North Sea in 1999 were used to test the receiver strategies' performance for a two user near far scenario. Amongst a number of strategies tested, the structure based on recursive successive interference cancellation demonstrated improved performance overall.

Shallow water acoustic networks

Underwater acoustic networks are generally formed by acoustically connected ocean bottom sensor nodes, autonomous underwater vehicles (AUVs), and surface stations that serve as gateways and provide radio communication links to on-shore stations. The quality of service of such networks is limited by the low bandwidth of acoustic transmission channels, high latency resulting from the slow propagation of sound, and elevated noise levels in some environments. The long-term goal in the design of underwater acoustic networks is to provide for a self-configuring network of distributed nodes with network links that automatically adapt to the environment through selection of the optimum system parameters. This article considers several aspects in the design of shallow water acoustic networks that maximize throughput and reliability while minimizing power consumption.

Underwater acoustic networks

With the advances in acoustic modem technology that enabled high-rate reliable communications, current research focuses on communication between various remote instruments within a network environment. Underwater acoustic (UWA) networks are generally formed by acoustically connected ocean-bottom sensors, autonomous underwater vehicles, and a surface station, which provides a link to an on-shore control center. While many applications require long-term monitoring of the deployment area, the battery-powered network nodes limit the lifetime of UWA networks. In addition, shallow-water acoustic channel characteristics, such as low available bandwidth, highly varying multipath, and large propagation delays, restrict the efficiency of UWA networks. Within such an environment, designing an UWA network that maximizes throughput and reliability while minimizing the power consumption becomes a very difficult task. The goal of this paper is to survey the existing network technology and its applicability to underwater acoustic channels. In addition, we present a shallow-water acoustic network example and outline some future research directions.