Experimental assessment of OFDM and DSSS modulations for use in littoral waters underwater acoustic communications

Robust and reliable horizontal long-range acoustic communications in littoral water environment (water depth /spl sim/100 m; range over 10 km) is known to be a very challenging task. Most commercially available acoustic modems (COTS) therefore use classical modulation schemes, such as FSK (frequency shift keying), with limited data rate for the ranges of interest. This paper addresses experimental results recently obtained with two other approaches which both aim at providing robust transmissions with higher bit rate: first is a set of OFDM (orthogonal frequency division multiplex) modulations in which data are transmitted by modulating a large number of orthogonal carriers, second approach is an application of DSSS (direct sequence spread spectrum) techniques where the transmitted symbols are spread over the used bandwidth with a pseudo-noise binary.