I. Introduction
Nowadays, as the energy crisis and its impacts are gradually increasing, the demand of ambient green energy is becoming essential for the sustainability in energy provision both to the industry and to the public. For this purpose, modern research has been focused to harvesting energy from available ambient resources (solar, thermal, wireless RF, etc). Among them, the harvesting of RF energy has several advantages. RF energy is free and continuously present in the environment, coming from public telecommunication networks. Its single drawback is its low power level, as its power density within an urban area is about 0.2nW/cm2-1μW/ cm2. For all that, it seems to be an attractive candidate to the emergent low-power technologies. The upward trend of Internet of Everything (IoE) guides us to a world where billions of objects will need autonomous and embedded electronic sensors connected over public or private networks. The massive utilization of energy standalone sensors could make their energy supply a major problem, due not only to the great number of them and the required energy amount but also to the possibility of their positions’ access to be limited (bridges, remote or hazardous positions etc). They are exactly these applications at which the utilization of RF harvested energy could be the solution to the problem. At first the majority of modern sensors operate with electric power of microwatt level and thus the RF power collected from ambient could be enough for them. In addition, the RF energy harvesting technology could be used to wirelessly activate electronic sensors or recharge their batteries, thus avoiding their replacement and eliminating human intervention.