Achieving energy-synchronized communication in energy-harvesting wireless sensor networks

With advances in energy-harvesting techniques, it is now feasible to build sustainable sensor networks to support long-term applications. Unlike battery-powered sensor networks, the objective of sustainable sensor networks is to effectively utilize a continuous stream of ambient energy. Instead of pushing the limits of energy conservation, we aim to design energy-synchronized schemes that keep energy supplies and demands in balance. Specifically, this work presents Energy-Synchronized Communication (ESC) as a transparent middleware between the network layer and MAC layer that controls the amount and timing of RF activity at receiving nodes. In this work, we first derive a delay model for cross-traffic at individual nodes, which reveals an interesting stair effect. This effect allows us to design a localized energy synchronization control with O(d³) time complexity that shuffles or adjusts the working schedule of a node to optimize crosstraffic delays in the presence of changing duty cycle budgets, where d is the node degree in the network. Under different rates of energy fluctuations, shuffle-based and adjustment-based methods have different influences on logical connectivity and cross-traffic delay, due to the inconsistent views of working schedules among neighboring nodes before schedule updates. We study the trade-off between them and propose methods for updating working schedules efficiently. To evaluate our work, ESC is implemented on MicaZ nodes with two state-of-the-art routing protocols. Both testbed experiment and large-scale simulation results show significant performance improvements over randomized synchronization controls.