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Radio frequency energy harvesting (RFEH) is an energy conversion technique employed for converting energy from the electromagnetic (EM) field into the electrical domain (i.e., into voltages and currents). In particular, RFEH is a very appealing solution for use in body area networks as it allows low-power sensors and systems to be wirelessly powered in various application scenarios. Extracting energy from RF sources sets a challenging task to designers and researchers as they find themselves at the interface between the electromagnetic fields and the electronic circuitry. Therefore, knowledge from both domains is required in order to design a high-performance RF energy harvester. In this chapter, the fundamentals, limitations, trade-offs, and challenges of RF energy harvesting are introduced both on system and circuit levels. Several design techniques, such as passive voltage boosting and impedance matching, are presented. State-of-the-art solutions are presented, their advantages and limitations are described, and their figure-of-merit discussed. The rectifier non-linear impedance and performance with respect to frequency, aspect ratio, threshold voltage, loading conditions, and input power variations are discussed. Moreover, the impedance variation introduced by (flexible) antennas for wearable sensors is presented and a technique to overcome the effects of impedance changes is presented. The choice of the antenna-rectifier interface impedance is discussed as it plays a crucial role in the harvester sensitivity and power conversion efficiency optimization. In the last subsection an example of an RFEH design is furnished, starting from the system level specification requirements of a wearable sensor. Finally, a co-design is described to optimize the antenna-rectifier interface. |
