Multipath compensation for underwater acoustic communication

The internal stratification of the ocean causes multipath effects such as distortion and severe frequency dependent fades. A multipath medium results in temporal spread of the transmitted pulses as well as time-variant propagation delays and attenuation factors. In underwater acoustics communications applications, the operational frequency range often lies between 1.5 kHz-1.5 MHz. The technique proposed in the paper to mitigate the multipaths, employs a joint optimization criterion on the time, space end autocorrelation domain simultaneously. A combination of self-optimized receiver and spatial diversity suits appropriately for the case of "very shallow", as well as "extremely shallow" channels with a range to depth ratio larger than 10:1. The blind equalization algorithm is able to recover the phase even in the case of non-minimum phase systems, which is a difficult task for "very shallow" ocean channels subject mainly to surface reverberation. Multilevel modulation schemes can be employed such as high-level multiple phase shift keying modulation MPSK, in order to obtain a high data-rate acoustic telemetry link. However the modulation complexity is bounded by the induced intersymbol interference and the reduction of the noise margins. Finally, simulation results with QPSK signaling are presented to verify efficiency at low signal-to-reverberation ratios.<<ETX>>