Webb Research Corporation (WRC) and Woods Hole Oceanographic Institution (WHOI) plan to develop on a modified Orthogonal Frequency Division Multiplexing (OFDM) modulation for mobile time-variable underwater acoustical channel. The OFDM and COFDM (coded OFDM) systems are well known as very attractive system for multi-path underwater acoustic communications channels. There were a few attempts to realize OFDM systems for underwater communications. The traditional approach expects the symbol duration of the transmitted signal to be larger then channel time spread and scattering shows as deep amplitude fading, which needs code protection from burst errors. Nevertheless there are no good examples of practical reliable working OFDM systems in the real ocean situations. All advantages of OFDM systems disappear in a time-variable channel, when channels begin to interfere due to Doppler frequency spread. Exploiting the main advantage of usual OFDM to simplify signal in a single frequency channel of broadband multi-channel system, a new approach suggests to use much shorter pulses partially spread in time and to apply more complicated soft decision multi-path decoders such as linear and decision feedback (DFE) equalizers and maximum-likelihood sequence detection (MLSD) algorithm. The expected result is a more robust and reliable system in time-variable or mobile channels. The proposal includes three stages: estimation of mobile channel statistical parameters using computer models of sound propagation; comparing different approaches for real time channel pulse response estimation; numerical simulation of proposed algorithms of signal receiving. The investigation will obtain and test innovative algorithms for an underwater acoustic communication modem, which can be used not only for high data rate communication at short range, but for long-range low data rate communication as well.
Keywords: Acoustic Communications, Underwater Acoustic, Long-Range Sound Propagation, Undersea Gliders.
