Acoustic channel equalization results for the ASIMOV high-speed coherent data link

Equalization results are provided for an underwater acoustic coherent communication system operating at 30 kbps in a vertical channel. This is one of two data links that are used in project ASIMOV to convey data between an autonomous underwater vehicle and an autonomous surface craft. The two vehicles cooperate to remain vertically positioned while executing missions at sea, thereby establishing a communication channel where acoustic transmission at high data rates is possible. The receiver structure and algorithms must be kept relatively simple to enable real-time operation, but tolerance to moderate intersymbol interference and channel fluctuations is required. Experimental results are presented for timing and carrier recovery, as well as channel equalization using several reference-driven and blind algorithms. Blind equalization algorithms are unsuitable for more complex ocean channels, but under these favorable transmission conditions the performance degradation relative to their reference-driven counterparts is quite small. Channel characterization confirms that, although intersymbol interference has a short time span and may easily be compensated, error rates remain relatively high as a result of strong noise contamination in the received signal.