Impact of real modem characteristics on practical underwater MAC design

The grand challenges posed by the adverse acoustic channels have been extensively studied in medium access control (MAC) design for underwater acoustic networks (UWANs). In recent years a bunch of MAC protocols have been proposed and analyzed to address the long propagation delay and constraint power supply problems. However, the challenges from the real acoustic modem characteristics are still neglected in the current research. Based on our experiments and sea trial tests with different modems - Teledyne Benthos modem, WHOI MicroModem and high speed UConn OFDM modem, we discover two important real system issues and discuss their significant effect on MAC design in UWANs. Experiment and simulation results reveal that the long preamble sequence feature of acoustic modems harm most existing MAC protocols with short packets, such as acknowledgment, notification, reservation used control packets. Other than reservation based methods are vulnerable to the long transmission time and busy terminal problem, the random access based protocols can benefit from them. As a result, Aloha based protocols yield encouraging throughput performance in UWANs and are reconsidered as viable solution for underwater MAC. Based on the analysis, simulation and experiment results, we also provide discussion on the direction of future MAC designs.