Implementation of a sweep-spread function for communication over underwater acoustic channels

Communication over underwater acoustic channels with strong, extended multipath can be 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 multi-user access and reducing influence of narrow-band noise, the sweep-spreading represents a powerful tool enabling separation of signal multipaths by converting their time delay into frequency split of individual multipath arrivals. The steeper the sweeps, the better multipath resolution can be achieved. After signal despreading, i.e. conversion into a constant intermediate frequency, the best suitable multipath arrival (or even several of them in parallel) can be separated and analyzed. In this way, all signal parameters can be recovered with high quality. The high performance and transmission stability of this approach could be confirmed in computer simulations as well as in validation experiments with horizontal transmission in natural shallow water channels carried out since summer 1999.