Swept Carrier Acoustic Underwater Communications

This paper describes a novel idea of reducing multipath interference in underwater acoustic communication systems. The proposed idea is based on the observation that the time separation between direct and multipath waveforms can be utilized to suppress the multipath interference. A proposed system employs a carrier with instanteous frequency swept periodically in time over a broad frequency range. This frequency range is much larger than the bandwidth occupied by a message signal. On the other hand the reciprocal of sweeping rate is much smaller compared to the message bandwidth. The carrier is modulated by a message resulting in a time varying instantaneous narrow-band spectrum of the transmitted signal. The received signal is demodulated by a band-pass tracking receiver and suppression of the out-of-band multipath spectrum is accomplished. A simple model for the multipath channel has been proposed and related to the system operation and performance. Some results and conclusions from tests performed on the prototype are presented.