On the Capacity of a Class of Acoustic Channels

We consider a class of underwater acoustic communication channels where each propagation path can be characterized by a complex-valued Gaussian fading process. The capacity of such channels is computed and analyzed using three power allocation strategies: water-filling, uniform, and on-off uniform power allocation across the signal bandwidth. Our analysis considers the effects of imperfect channel estimation, delayed feedback, and pilot overhead, which contribute to about 1 bps/Hz loss from 4 bps/Hz at 20 dB SNR for the experimental channel. We find that given the long feedback delays associated with acoustic channels, all-on uniform power allocation which does not require feedback and is simple to implement emerges as a justified practical solution that outperforms the other strategies. Furthermore, when considering acoustic-specific propagation effects, such as frequency-dependent attenuation and colored noise, considerable gain can be achieved by selecting the frequency band according to the attenuation pattern and the available transmit power, e.g. at least 6 dB gain for a 10 km link when compared to transmission over a preselected frequency band of 10 kHz 15 kHz.