Underwater spread-spectrum communications

For low data rate communications, spread spectrum techniques can be used in the underwater medium. They fight intersymbol interference caused by multipath propagation. They have to be used if the signal-to-noise ratio (SNR) is inferior to 0 dB. They are particularly interesting for anti-jamming or discretion purposes. The low SNR technique is also resorted to in case of peak-limited energy (and long range propagation) or energy saving (immersed autonomous transmitter). We present and discuss first the method used which consists in transmitting pseudo-noise codes or `signatures´. Binary sequences from the Gold and Kasami families were chosen to increase the data rate. In this case, the decision is taken after multi-correlation. New direct sequences of variable lengths and conceived at LETI-CEA (France) are also selected. They are particularly interesting in a multi-user environments (CDMA for example). The results obtained for two Mediterranean communications situations between a boat and the military site of Cap Ferrat (near Nice) are presented. For the last experiment, a 20-km communication was maintained for several minutes, using the new LETI 72-chip sequences, with an error rate inferior to 0.2% and a very low SNR, about -14 dB (7 bits/s). Due to close multipaths, the fading phenomenon is a delicate problem for underwater communications. To combat fading, spatial diversity seems a better solution than time or frequency diversity. Results obtained using seven receivers placed between 100 and 300 m are presented. Different combining methods were tested: selection, maximal ratio combining... . The best results were obtained using a few sensors