Signaling techniques for underwater acoustics communications. Tonal and spread spectrum frequency shift key systems are compared

This study compares two sophisticated signaling schemes to compensate for distortions in underwater acoustic channels, where the channel rate of change approaches the reciprocal of the multipath time spread. The first scheme uses tonal pulses much longer than the channel multipath spread. The second (commonly called RAKE) uses spread spectrum pulses to resolve the multipath and an adaptive receiver to combine the multipath echoes coherently. Analysis shows that the RAKE scheme (1) performs considerably better for slowly fading channels and (2) can use a smaller decision interval and can therefore tolerate higher fade rates.