Sweep-spread carrier for underwater communication over acoustic channels with strong multipath propagation.

Multipath propagation causes the transmitted signal to take numerous, time-varying different-length paths to the receiver. Exploitation of conventional frequency-constant carrier signals for communication over underwater acoustic channels typically shows that intricate mutual time variations of multipath arrivals create an extremely hard problem for equalizers to compensate for nonstable intersymbol interactions. Communication over such channels can be, however, substantially improved by using a new method based on the implementation of a sweep-spread carrier (S2C). Such a carrier consists of a succession of sweeps, which cause permanent rapid fluctuation of signal frequency. Apart from some additional useful effects such as providing multiuser access and reducing influence of narrow-band noise, the method provides significant processing improvement enabling clear separation of multipath arrivals by converting their time delays into their frequency reallocations--the steeper the sweeps, the better the multipath resolution that can be achieved. When converting the signal into constant intermediate frequencies, the best suitable multipath arrival can be separated not as traditionally in time domains, applying complex equalizers, but in frequency domains by means of usual band-pass filters. High transmission stability of this approach was confirmed in both computer simulations as well as in validation experiments carried out since summer 1999.