Robust shallow water acoustic communications based upon orthogonal sequences and real-time channel identification

Shallow water acoustic communication (water depth <100 m; range/depth ratio/spl sim/100) is addressed. Under these conditions the main difficulties are related to the time/Doppler spread and the time variability of the channel. A new signalling scheme, especially designed for robustness in these conditions, is addressed. It is based upon transmission of "quasi orthogonal" sequences. A fixed reference sequence is transmitted permanently for real-time channel identification; simultaneously, and in the same frequency band, one or several sequences are transmitted to carry the useful information. Samples of results on signals recorded in the North Sea (water depth <20 meters, range up to 10 km) in April/May 1999 during the ROBLINKS (European MAST project) experiments are finally presented. They clearly demonstrate the ability (providing sufficient SNR) of the addressed scheme to transmit data up to 1 kbits/s.