Abstract: Radio frequency (RF)-based wireless energy transfer is one of promising techniques to power the next-gene-ration wireless sensor networks (WSNs).Due to the very limited energy harvesting rate,the RF energy harvesting WSNs usually operate in the ultra low duty cycle mode.So one fundamental issue of this kind of WSNs is how to deploy a given number of energy transmitters (ETs) so as to maximize the minimum duty cycle of all nodes.Firstly,this paper analyzed this optimal ET placement problem so as to deeply and theoretically understand it.Then the greedy approach and the particle swarm optimization (PSO) based approach were presented for this problem.Simulation results show that the proposed approaches can significantly improve the duty cycle when compared to the even placement of ETs.

Key words: Wireless sensor networks,RF energy harvesting,Energy transmitter placement,Duty cycle optimization