Time Reversal Based MAC for Multi-Hop Underwater Acoustic Networks

Constrained-energy underwater acoustic nodes are typically connected via a multi-hop underwater acoustic network (MHUAN) to cover a broad marine region. Recently, protocols for efficiently connecting such nodes have received considerable attention. In this paper, we show that the time reversal (TR) process plays an important role in the medium access control (MAC) because of its physical capability to exploit the multi-path energy from the richly scattering underwater environment, as well as to focus the signal energy in both spatial and temporal domains. In MHUANs, with severe multi-path propagation at the physical layer, the active TR process spatially focuses the signals to the location of the intended receiver; this significantly diminishes the interference among parallel links. We propose an active TR-based MAC (TRMAC) protocol for MHUANs, with the aim of minimizing collision and maximizing channel utilization simultaneously. Furthermore, by considering the impact of the cross correlation between different links on the TR-based medium access, we derive the threshold of the link cross correlation to resolve collision caused by the high cross correlation between realistic links. We perform simulations using the OPNET and BELLHOP environments, and show that the proposed TRMAC results in significantly improved throughput, decreased delay, and reduced data drop ratio in MHUANs.