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.

翻译：水下声学网络（UANs）中传播延迟不可忽略，因为传播速度比光速低五个数量级。在此情况下，空间和时间因素强烈耦合，共同决定数据包传输的碰撞情况。为此，本文分析了空时耦合对时隙介质访问控制（MAC）的影响。我们发现存在时隙间碰撞与时隙内碰撞，且时隙间碰撞可能跨越多个时隙。发送时隙相关的干扰区域可能是整个传输范围内的环形区域。研究表明，当时隙设置中使用大于数据包持续时间的保护间隔时，存在无碰撞区域。在此意义上，长时隙可在传输范围内实现空间复用。然而，我们进一步发现，当时隙长度为单个数据包持续时间和两个数据包持续时间时，成功传输概率和吞吐量相同。仿真结果表明，通过设置小于数据包持续时间的保护间隔（远小于典型时隙ALOHA的现有时隙设置）可达到最大成功传输概率和吞吐量。此外，仿真还显示，由于三维UANs中垂直传输范围更长，垂直传输的空间影响比水平传输更大。