Digital communication using acoustic modems is the method of choice for exchanging data among distant or highly mobile equipment used in various underwater activities. However, achieving efficient communication in this environment is challenging due to severe distortions that affect the transmitted signals as they undergo multiple reflections and refractions in their propagation path. Attempts to overcome these impairments in high data rate coherent modems haven’t been entirely satisfactory, thus hampering their widespread adoption.
Recently, much attention has been devoted to the use of OFDM (Orthogonal Frequency Division Multiplexing) modulation for wireless and cable applications as a way of approaching channel capacity with simple transmitter/receiver architectures. In OFDM the message stream is divided into many parallel lower rate streams that modulate a set of partially overlapping orthogonal carriers. Since longer symbols are less sensitive to multipath, equalization requirements may be considerably relaxed on each sub carrier. This feature is quite appealing in underwater communications, where highly complex filters used for equalization constitute the main computational bottleneck.
Although preliminary studies on the use of OFDM for underwater coherent communication have been published, the analyses are rather superficial and should mainly be regarded as proof of concept. The present proposal will address issues that are particularly relevant in an underwater environment:
1- Channel identification and equalization are extremely important in underwater communication because multipath propagation may induce channel responses lasting hundreds of milliseconds. Unlike terrestrial OFDM applications, frequency-selective channels have to be explicitly considered. Recently developed blind or semi-blind identification techniques should be applicable under such conditions, thus reducing the need for pilot tones.
2- Significant Doppler shift may be induced in acoustic waveforms even by relatively slow emitter/transmitter motion caused by waves and currents. Performance studies for terrestrial OFDM have shown that accurate tracking of average Doppler is required to ensure low intercarrier interference. Average and differential Doppler compensation has not been studied in detail for single-carrier communications, but it will likely play an important role in underwater OFDM systems. An approach based on simple ray propagation models will be used to predict the evolution of Doppler in each path and guide the tracking algorithms.
