MU-Sync: A time synchronization protocol for underwater mobile networks

Abstract

Although there are numerous time synchronization algorithms recently proposed for terrestrial wireless sensor networks, none of these could be directly applied to underwater acoustic sensor networks. This is because they typically assume that the propagation delay is negligible, which is not the case in underwater. Furthermore, the sensor nodes in underwater tend to have some degree of mobility due to wind or ocean current, which complicates the problem even more by introducing time-varying delay. In this paper, we propose a cluster-based synchronization algorithm for underwater acoustic mobile networks, called "MU-Sync". Our design avoids frequent re-synchronization by estimating both the clock skew and offset. As underwater mobile networks experience both time-varying and long propagation delay, previous works that estimate the clock skew using a single least square error linear regression tend to be inaccurate. In the MU-Sync, the clock skew is estimated by performing the linear regression twice over a set of local time information gathered through message exchanges. The first linear regression enables the cluster head to offset the effect of long and varying propagation delay; the second regression in turn obtains the estimated skew and offset. With the help of MAC-level time stamping, we can further reduce the nondeterministic errors that are commonly encountered by those synchronization algorithms that rely on message exchanges. Copyright 2008 ACM.