Due to the high propagation latency and high power consumption of acoustic communications, scheduling techniques designed for terrestrial radio-based systems, may not be suitable for underwater acoustic sensor networks (UWASN). In this paper, we consider how to time schedule each link in a single broadcast domain. We show that, unlike its terrestrial RF counterpart, this problem is NP-complete, and the hard-to-approximate ratio is presented. Due to the intractability and inflexibility of centralized scheduling policies, and the high communication energy overhead of reservation-based strategies, we further investigate the performance of an ALOHA-like access scheme, which is distributed, randomized and requires no topology knowledge. According to our analysis, although the random scheduling policy that picks transmission times uniformly in a given interval is throughput optimal for terrestrial radio-based systems, it performs poorly in underwater acoustic networks. We thereby seek for the throughput-optimal, distributed random policy by solving a nonlinear optimization problem. We present an extensive comparison between this policy and the uniform one, with respect to different packet lengths, scheduling length, and network density. We show that the optimal solution offers substantial improvements in throughput, particularly for long packets.
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