Research On Time Synchronization Algorithm In Multi-hop Underwater Sensor Networks

Time synchronization is the fundamental problem in distributed sensor networks. On one hand, time stamp is an important element of sensing data, and the clock accuracy determines the values of sensing data. For example, in target tracking, sensors in different areas sense the interesting object and record the present time of object in its local clock. So, the difference of sensors’clock determines accuracy of speed evaluation by the time stamps of reporting messages from different sensor nodes. On the other hand, time synchronization is the basis for sensor networks to collaborate. Basic protocols, such as TDMA, data fusion, localization, collaborative sleep, require precise synchronization among sensor nodes.Comparing to territorial sensor networks, UWSNs has unique characteristics of long propagation delay and restricted mobility, caused by acoustic communication and underwater environment. The two unique characteristics make the design of protocol to synchronize UWSNs more challenging. Moreover, acoustic communication is high energy consumed. In order to guarantee the quality of communication, the acoustic modem amplifier and transducer usually need 10W level energy to transmit a signal. So, the design of synchronization protocol should consider the problem of energy consumption.Existing UWSN time synchronization protocols focus on single-hop synchronization. For multi-hop scenarios, all these protocols can only be applied in hop by hop manner. The hop by hop manner makes time cost to synchronize the whole networks high, and the synchronization precision will come down caused by error accumulation.In this paper, we focus on synchronization in multi-hop UWSNs, and propose the solution called "MulSync". By using concurrent communication mechanism and broadcast nature of acoustic signals, MulSync computes the skew and offset of unsynchronized nodes by using three linear regression to fuse the time information from different nodesThe contributions of this paper are as follows:1) MulSync cut down time cost and message overhead by using concurrent synchronization communication mechanism and exploiting the broadcast nature of acoustic signals.2) By using three linear regressions to fuse multiple synchronization messages from different beacon nodes and through diverse paths, Mulsync solves the decrease of synchronization precision as the hop increase in some sense.3) The simulation results demonstrate that MulSync achieves high-level synchronization precision at lower message overhead and time cost, comparing to MU-Sync.