Impact and analysis of space-time coupling on slotted MAC in UANs

TL;DR

This paper investigates how space-time coupling affects slotted MAC protocols in underwater acoustic networks, revealing collision dynamics, spatial reuse opportunities, and optimal slot and guard interval configurations for improved network performance.

Contribution

It provides a detailed analysis of space-time coupling effects on collision behavior and proposes optimal slot and guard interval settings for UANs.

Findings

Inter-slot and intra-slot collisions can occur, with inter-slot spanning multiple slots.

Collision-free regions exist with guard intervals longer than a packet duration.

Maximum throughput is achieved with guard intervals shorter than a packet duration.

Abstract

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 collisions of packet transmissions. To this end, this paper analyzes the impact of spatial-time coupling on slotted medium access control (MAC). We find that both inter-slot and intra-slot collisions may exist, and the inter-slot collision may span multiple slots. The sending slot dependent interference regions could be an annulus inside the whole transmission range. It is pointed out that there exist collision-free regions when a guard interval larger than a packet duration is used in the slot setting. In this sense, the long slot brings spatial reuse in a transmission range. However, we further find that the successful transmission probabilities and throughput are the same for the slot lengths of one packet duration and two packet durations. Simulation results show that the maximum successful transmission probability and throughput can be achieved by a guard interval less than a packet duration, which is much shorter than the existing slot setting in typical Slotted-ALOHA. Simulations also show that the spatial impact is greater for vertical transmission than for horizontal transmissions due to the longer vertical transmission range in three-dimensional UANs.