RF energy harvesting networks: Existing techniques and hardware technology

Radio frequency (RF) energy harvesting is the process by which radiative electro-magnetic waves, typically from 3 kHz to 300 GHz, are captured, converted, stored and used to operate usually low-energy consumption devices ranging from wearable electronics to sensor networks. RF signals can be primarily generated from two sources: dedicated and ambient RF sources. The former can be deployed to provide energy when predictable supply is expected, usually from license-free frequency bands of radio spectrum. The latter refer to RF transmitters originally not intended for energy transfer, such as TV towers, radio towers, and Wi-Fi routers. Until recently, most implementations of networks powered by RF energy were demonstrated through experiments and prototype setups as proof-of-principles and to show potential practical applications. Now, with the ubiquitousness of Wi-Fi and the advance in CMOS technology and supercapacitors, RF-powered devices have found various applications and are now used or offered in commercial products. The aim of this book chapter is to provide a basic understanding of RF energy harvesting (RFEH) techniques, describe the underlying electronics hardware design, show the current state-of-the-art applications and commercially available products, and finally expand to future applications and challenges ahead.