Spatial-reuse TDMA for Large Scale Underwater Acoustic Multi-hop Grid Networks

In contrast to terrestrial wireless network, where the propagation delay is almost negligible when compared to the packet duration (i.e., the duration of the signal carrying a packet), in underwater acoustic networks it is not rare to observe a propagation delay equal to, or even greater than, the packet duration, as the signal travels at the speed of sound that is orders of magnitude smaller than the speed of light. On the one hand, this phenomenon makes carrier sense-based Medium Access Control (MAC) protocols, such as Carrier Sense Multiple Access (CSMA), almost ineffective in underwater acoustic networks [1]. In fact, CSMA outperforms other contention-based protocols such as Aloha and Slotted Aloha only if its vulnerable time, that in CSMA is equal to the propagation delay, is at least 100 times smaller than the packet duration. Moreover, the standard Time Division Multiple Access (TDMA) is very inefficient, as each time slot should account for a guard time equal to the maximum propagation delay to avoid collisions between consecutive slots. On the other hand, the large propagation delay can be exploited by scheduling simultaneous transmissions between adjacent nodes without causing collisions at the receiver. In this paper we design a MAC scheme that leverages on the propagation delay and on the near-far interference to maximize the packet transmissions in large underwater acoustic grid networks, and we compare its performance with other MAC protocols implemented in the DESERT Underwater Framework [2].