  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
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 TR-based
MAC results in significantly improved throughput, decreased delay, and reduced
data drop ratio in MHUANs.
