Characterization of Multipath Acoustic Channels in very Shallow Waters for Communications

The motivations for studying the channel characteristics of a very shallow water environment are twofold: one, to account for the poor performance of commercial off the shelf (COTS) underwater acoustic modems in warm, very shallow waters and two, to design realistic channel models and communication systems which are able to perform in such an environment. This paper presents an experimental analysis of medium frequency (9-28 kHz) channel measurements in very shallow waters (15-30 m) for transmission distance ranging from 80 m to 4 km in the coastal seas of Singapore. The channel probe signals are mainly binary phase shift keying (BPSK) modulated m-sequences. They provide a large bandwidth-duration signal that is ideal for delay-Doppler measurements, giving adequate delay and Doppler resolution. Our channel measurements and analysis have shown that delay and Doppler spread decreases as the distance increases. This implies that at longer distances (up to 4 km), the channel is able to support higher bit rates. In contrary, COTS modems generally degrade to lower bit rates when the transmission range increases in our local shallow water environment.