**Abstract**

Recently, lots of works have been done on the optimal power management of wireless devices. This leads to the main idea of ambient energy harvesting. Among various energy harvesting approaches, one is to use radio waves existing in the ambient environment for battery charging, called RF energy harvesting. In this chapter, in order to improve the RF energy harvesting performance, we utilize spectrum sensing to allow the wireless devices to select the frequency band with maximum power that exceeds a predefined threshold to charge the device (this power threshold can be determined according to battery type and its required charging power) and the device can use this power for battery charging. Also, a novel voltage multiplier circuit is proposed. By means of simulations and experimental tests, it can be seen that after detection of our desired 1 mW RF signal, system output power is about 532*μ* W and 450*μ* W in simulation and practical situations respectively.

**Keywords:** energy harvesting, voltage multiplier, OFDM, spectrum sensing
