Differential frequency hopping performance in Doppler-inducing underwater acoustic communication channels

The relationships between the parameters of Doppler spread-inducing underwater environments and the bit-error performance of differential frequency hopping (DFH) modulation in the underwater acoustic channel are characterized. Wind speed determines the nature of the effect that the water surface imposes on the acoustic DFH wavefoms propagating underwater. Low wind speeds result in an essentially flat, low-absorption sea surface. In this regime, strong surface reflections and little frequency spreading make intersymbol interference (ISI) the dominant effect on the received waveforms. At high wind speeds, the higher density of air bubbles in the surface layer absorbs almost all energy incident on the surface, resulting in no surface reflections reaching the receiver. In this regime, the surface has little effect on the received signal, either in the form of ISI or frequency spreading. The intermediate ranges of wind speed, with a mix of ISI and surface-induced Doppler spread, pose the most challenging conditions. Simulations and at-sea experiments show that recent algorithmic improvements to the receiver make DFH robust to a variety of environmental conditions and that DFH modulation parameters can be easily adapted to a variety of operationally relevant scenarios based on environmental information.