Phase coherent communication in the presence of ship noise

A sea trial with a communication system using phase shift keying has been conducted. The communication system uses bit rates of 1.5-5 kbit/sec over a range up to approximately 3 km. The propagation paths are in the wake of the surface vessel thus the communication channel is heavily influenced by flow noise and propeller noise. The most characteristic feature of this noise are high energy spikes. These are believed to be created by the propeller cavitation. The impulse response measurements of the acoustic communication channel show some time varying multipath. This motivates a receiver capable of handling multipath channels with a few different propagation paths. The noise characteristics make a receiver especially sensitive to feedback errors. Thus emphasis is on utilizing receiver structures that are robust to feedback error propagation. Both decision feedback equalizers and maximum a posteriori receivers using the Viterbi algorithm have been used on the collected data. A performance gain has been found for the latter class of receivers, especially in the presence of heavy ship noise. Bit error rates from the experiment as a function of time and channel events are presented