Delay efficient sleep scheduling in wireless sensor networks

Medium access techniques for wireless sensor networks raise the important question of providing periodic energy-efficient radio sleep cycles while minimizing the end-to-end communication delays. This study aims to minimize the communication latency given that each sensor has a duty cycling requirement of being awake for only 1/k time slots on an average. As a first step we consider the single wake-up schedule case, where each sensor can choose exactly one of the k slots to wake up. We formulate a novel graph-theoretical abstraction of this problem in the general setting of a low-traffic wireless sensor network with arbitrary communication flows and prove that minimizing the end-to-end communication delays is in general NP-hard. However, we are able to derive and analyze optimal solutions for two special cases: tree topologies and ring topologies. Several heuristics for arbitrary topologies are proposed and evaluated by simulations. Our simulations suggest that distributed heuristics may perform poorly because of the global nature of the constraints involved. We also show that by carefully choosing multiple wake-up slots for each sensor significant delay savings can be obtained over the single wake-up schedule case while maintaining the same duty cycling. Using this technique, we propose algorithms that offer a desirable bound of d + O(k) on the delay for specialized topologies like the tree and grid and a weaker guarantee of O((d + k) log n) for arbitrary graphs, where d is the shortest path between 2 nodes in the underlying topology and it is the total number of nodes.