Experimental studies of underwater acoustic communication in Trondheim fjord

Direct-sequence spread-spectrum signal was used for communication tests over underwater channels in Trondheim fjord. Differential binary phase shift keying was utilized between two adjacent symbols. To the receiver, a method uses a time updated channel impulse response estimation to recover differential phase modulated information, and it takes the estimation from the previous symbol as the match filter. The effectiveness of this method is ensured by the coherence between two consecutive symbols over time varying channels. This method is insensitive to multipath patterns, and it does not require time synchronization as precise as the convention de-spread method does. In our experiments, good performance was achieved, even in low SNR tests. The performance loss at high SNR in the experiments was caused by long time delay spread. Late-arriving paths from the previous symbol were buried in the current symbol during time-windowing process, and the late-arriving paths might decrease the magnitude of the differential phase information. In this situation, it is prone to cause errors.