Abstract-As battery-powered mobile devices become more popular and energy hungry, wireless power transfer technology receives intensive interests, as it allows the power to be transferred from a charger to ambient devices wirelessly. The existing studies mainly focus on the power transfer efficiency but overlook the health impairments caused by RF exposure. In this paper, we study the Safe Charging Problem (SCP) of scheduling power chargers so that more energy can be received while no location in the field has electromagnetic radiation (EMR) exceeding a given threshold Rt. We prove that SCP is NP-hard and propose a solution which provably outperforms the optimal solution to SCP with a relaxed EMR threshold (1 − )Rt. Testbed results based on 8 Powercast TX91501 chargers validate our results. Extensive simulation results show that the gap between our solution and the optimal one is only 6.7% when = 0.1, while a naive greedy algorithm is 34.6% below our solution.

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Abstract-One fundamental question for wireless power transfer technology is the energy provisioning problem, i.e., how to provide sufficient energy to mobile rechargeable nodes for their continuous operation. Most existing works overlooked the impacts of node speed and battery capacity. However, we find that if the constraints of node speed and battery capacity are considered, the continuous operation of nodes may never be guaranteed, which invalidates the traditional energy provisioning concept. In this paper, we propose a novel metric -Quality of Energy Provisioning (QoEP) -to characterize the expected portion of time that a node sustains normal operation by taking into account node speed and battery capacity. To avoid confining the analysis to a specific mobility model, we study spatial distribution instead. As there exist more than one mobility models corresponding to the same spatial distribution, and different mobility models typically lead to different QoEPs, we investigate upper and lower bounds of QoEP in 1D and 2D cases. We derive tight upper and lower bounds of QoEP for 1D case with single source, and tight lower bounds and loose upper bounds for general 1D and 2D cases with multiple sources. Finally, we perform extensive simulations to verify our theoretical findings.Index Terms-Quality of energy provisioning, wireless power transfer, mobility.

Abstract-Wireless power transfer technology is considered as one of the promising solutions to address the energy limitation problems for end-devices, but its incurred potential risk of electromagnetic radiation (EMR) exposure is largely overlooked by most existing works. In this paper, we consider the Safe Charging with Adjustable PowEr (SCAPE) problem, namely, how to adjust the power of chargers to maximize the charging utility of devices, while assuring that EMR intensity at any location in the field does not exceed a given threshold Rt. We present novel techniques to reformulate SCAPE into a traditional linear programming problem, and then remove its redundant constraints as much as possible to reduce computational effort. Next, we propose a distributed algorithm with provable approximation ratio (1 − ǫ). Through extensive simulation and testbed experiments, we demonstrate that our (1 − ǫ)-approximation algorithm outperforms the Set-Cover algorithm by up to 23%, and has an average performance gain of 41.1% over the SCP algorithm in terms of the overall charging utility.

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