Impact and Analysis of Spatial Correlation on Slotted Based MAC in UANs

The propagation delay is non-negligible in underwater acoustic networks (UANs) since the propagation speed is five orders of magnitude smaller than the speed of light. In this case, space and time factors are strongly coupled to determine the arrival time of a received packet, since the propagation delay depends on the spatial distribution of nodes in the network. Slotted based medium access control allows only packet sending at the beginning of a slot, which removes the time uncertainty to some extent and decouples space and time factors. More importantly, simultaneous transmissions are potential to be successful in UANs due to the difference of inter-node propagation delay. Thus, the slotted MAC is a promising approach to exploit the long propagation delay in UANs. However, different from terrestrial radio networks, packets sent at different slots may also collide at a receiver due to the long propagation delay, which is called inter-slot collisions. A guard time should be used to extend the length of the time slot, in order to remove inter-slot collisions. Considering that a long slot length may degrade the channel utilization, this paper tries to study the collisions with respect to the slot length in UANs. We find that the packet collision is highly determined by the maximal inter-node distance difference. We then derive the closed-form expression for the probability of successful transmissions in uniformly distributed network. The simulation results by NS3 verify the the trade-off between collision and channel utilization, and there exists an optimal slot length to maximize network throughput.