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uScan: a lightweight two-tier global sensing coverage design

Wireless Sensor Networks (WSNs), consisting of thousands of low-cost sensor nodes, have been used in many application domains such as military surveillance [1], habitat monitoring [4] and scientific exploration [6]. Limited power supplies and difficulties in harvesting ambient energy make energy conservation a critical issue to address. As one of solutions, energyefficient sensing coverage extends system lifetime by leveraging on the redundant deployment of sensor nodes. Existing algorithms [5, 7, 8, 9] are designed to be well distributed and localized with solid performance gains. While the state-of-the-art is encouraging, we believe there are some aspects that need further investigation. In most algorithms, extending system lifetime is achieved essentially through coordination among neighboring nodes. The local node density, therefore, imposes a theoretical upper bound on the system lifetime, if a continuous sensing coverage or a partial coverage is required. Such a bound can be surpassed through global scheduling. However, the overhead of global scheduling would increase significantly if the coordination among the nodes goes beyond the neighborhood. To address these issues, we introduce a two-tier global scheduling method, called uScan. At the first level, coverage is scheduled to activate different portions of an area. We propose an optimal scheduling algorithm to minimize area breach. At the second level, sets of nodes are selected to cover active portions. Interestingly, we show that it is possible to obtain optimal set-cover results in linear time if the layout of areas satisfies certain conditions. We have implemented and evaluated our design on the Berkeley TinyOS/Mote platform [2], using 30 MicaZ motes. The results indicate that uScan is a lightweight solution with significant energy savings, compared with localized solutions.