Equalization and coding for coherent communication in the ASIMOV vertical link

This paper analyzes the performance of a coherent modem designed for high-speed vertical transmission at 30 kbit/s from an underwater vehicle to a surface craft. Hardware and power constraints require the modem to be based on a single fixed-point DSP, thus restricting the choice of signal processing algorithms for real-time operation. Under these conditions, high data rates can only be achieved with very favourable communication channels that result from the particular geometric configuration. Channel delay and Doppler spreads under different sea states are characterized using at-sea data collected during the ASIMOV 2000 mission. Several algorithms are tested for adaptation of the equalizer at the receiver, and it is found that simple LMS-type algorithms perform almost as well as more complex alternatives that are known to have superior convergence and tracking properties. The system also includes signal-space coding blocks that are used to lower the error rate without paying in bandwidth or transmitted power. The error probabilities of coded and uncoded modulation are estimated and compared, confirming that coding provides a real, albeit small, performance gain.