Experimental Studies of Underwater Acoustic Communications over Multipath Channels

Underwater acoustic experiments were conducted in the Trondheim fjord between fixed transmitters and receivers separated by 2.3 km, 4.3km and 8.0 km. Direct-sequence spread spectrum signals were used and binary information was modulated between two consecutive m-sequences, and recovered by cross-correlating between the two consecutive m-sequences. With the spreading gain, this scheme is robust to SNR fluctuations. Time-updated channel impulse responses were estimated by a periodically transmitted measuring sequence, and fast variations of multipath channels are shown also in the stationary communication scenarios. The results show that the performance is limited by time delay spread caused by multipath.