  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.
