One of the main concerns in traditional Wireless Sensor Networks (WSNs) isenergy efficiency. In this work, we analyze two techniques that can extendnetwork lifetime. The first is Ambient \emph{Energy Harvesting} (EH), i.e., thecapability of the devices to gather energy from the environment, whereas thesecond is Wireless \emph{Energy Transfer} (ET), that can be used to exchangeenergy among devices. We study the combination of these techniques, showingthat they can be used jointly to improve the system performance. We consider atransmitter-receiver pair, showing how the ET improvement depends upon thestatistics of the energy arrivals and the energy consumption of the devices.With the aim of maximizing a reward function, e.g., the average transmissionrate, we find performance upper bounds with and without ET, define both onlineand offline optimization problems, and present results based on realisticenergy arrivals in indoor and outdoor environments. We show that ET cansignificantly improve the system performance even when a sizable fraction ofthe transmitted energy is wasted and that, in some scenarios, the onlineapproach can obtain close to optimal performance.
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