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Channel variations caused by motility would introduce inter-carrier interference (ICI), which degrades channel estimation and data detection in underwater orthogonal frequency division multiplexing (OFDM) systems. To deal with this problem, a joint time-varying channel estimation and inter-carrier interference mitigation method is proposed in this paper, where time domain channel estimation and interference auto-cancellation are merged into an iterative scheme. Meanwhile, linear approximation is used to track the time-varying channel while interference auto-cancellation is achieved via pre-coding to reduce the sensibility of OFDM systems to frequency shifts. Unlike frequently-used opposite or conjugate mapping, which leads to an inevitable decrease in frequency utility ratio, differential coding is introduced for pre-coding without adding redundancy. Simulations show that the proposed algorithm can accurately estimate channel state information (CSI), and robustly mitigate ICI over time-varying underwater acoustic channels (UWAC).

TDMA is an ideal choice for solving the data collision in underwater acoustic communication networks, but it needs the global clock synchronization, which is hard to realize in underwater environment. Further, the asymmetrical energy consumption of the main node and the sub nodes will reduce the network lifetime. To address these problems, we presented a relative clock based and energy sense MAC protocol. In this protocol, global clock synchronization is not needed, all nodes mapped the network schedule on its local clock according to the relative propagation delay and worked on it. The gateway is changed dynamically according to its energy level, in order to prolong the network lifetime. The performance and the engineering feasibility of this protocol were demonstrated by simulations and lake trail.