A Multi-Stage Method for Time Synchronization in Acoustic Underwater Communications

Time synchronization techniques for underwater communications face challenging issues: time-varied channel, and strong color noise. In addition, since underwater channels are low in bandwidth and prone to low signal to noise ratio, one would prefer to employ a synchronization technique with low overhead. Previous synchronization techniques used preamble to synchronize, which consumes the bandwidth. In this paper, we propose a time synchronization method using the guard interval (GI) in an OFDM symbol. Herein, identical GIs are inserted at the head and tail of an OFDM symbol. At the receiver side, our method checks the difference and similarity between the heading and tailing GIs, from which it can determine the beginning of the OFDM symbol. The proposed method used an iterative technique to magnify the difference between the real signal and the background noise, so that in case of low SNR, the receiver still can perform synchronization. Simulation results show that the proposed method can effectively synchronize when the SNR is low.