Ultrasonic link for improved incoherent data transmission in horizontal shallow water channels

Abstract

An acoustic telemetry link may be disturbed or even collapse due to non-stable intersymbol interferences resulting from the effect that transmitted signal takes numerous, time-varying different-length paths to receiver. The complex time variations of multipath arrivals create an extremely hard problem for equalizers used for processing of conventional frequency-constant carrier signals. Our studies on underwater communication of dolphins revealed several solutions to overcome these problems. It turned out that chirping and singing is most essential in this conjunction. Applied to technical telemetry, underwater communication could be substantially improved by the implementation of the method called Sweep-Spread Carrier Communication. In this method, carrier signal consists of a succession of sweeps, which cause permanent rapid fluctuation of signal frequency. When converting received signal into constant intermediate frequencies, multipath arrivals can be resolved and separated not as traditionally in time domains, applying complex equalizers, but-much easier-in frequency domains by means of usual band-pass filters. As the experimental results show, this kind of signal processing improves statistical channel properties, providing increased data rates and high transmission stability, which were confirmed in validation experiments recently carried out in shallow fresh and sea water channels.