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D-sync: Doppler-based time synchronization for mobile underwater sensor networks

Time synchronization is an essential service in underwater networks, required for many functionalities such as MAC, sleep-scheduling, localization, and time-stamping of sensor events. However, there exist two fundamental challenges to underwater synchronization, namely, large propagation delays and substantial node mobility during the synchronization process. While existing underwater time sync solutions have been proposed to address these challenges, they rely on heavy signaling, which is undesirable due to high energy costs. In this paper, we introduce a powerful new approach that incorporates physical layer information, namely an estimate of the Doppler shift. Large Doppler shift has been identified as a major challenge to underwater communication, and current systems implement sophisticated solutions to estimate and track such Doppler shift for each data exchange. While an impediment to communication, we will show that the Doppler shift contains highly useful information that can be leveraged to greatly improve time synchronization. Specifically, it provides an indication of the relative motion between nodes. Our new protocol, called D-sync, strategically exploits this feature to address the timing uncertainty due to node mobility. As such, D-sync can handle substantial mobility, without making any assumptions about the underlying motion, and without extensive signaling. Simulation results show that D-sync significantly outperforms existing time synchronization both in terms of accuracy and energy.