# A Power Processing Circuit for Indoor Wi-Fi Energy Harvesting for Ultra-Low Power Wireless Sensors
## 1 Introduction  
The latest development of low power consumption electronic device has contributed to a variety of energy smart wireless sensors. The wireless sensor collects data such as temperature, capacitance, vital signs, etc. Table 1 shows some low power wireless sensors and their typical power consumptions presented by \(P_{\textit{cons}}\). Mostly, the power requirements for the listed sensors are supplied by batteries. However, using batteries as an energy source has some disadvantages in the practical world, especially the replacement can be an issue when the stored energy is depleted. Alternatively, energy harvesting can be used as a new approach to powering wireless sensors. The energy harvesting system scavenges power from the ambient environment of wireless sensors, with popular ambient energy sources including thermal, vibrational, solar, electromagnetic, etc. And radio power transmitters can be used as an energy source for developing radio frequency energy harvesting systems (RFEHS), especially in urban areas where the transmitted radio frequency (RF) power is widely available at any time.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \(P_{\textit{cons}}\) & **110 nW** & **270 nW** & **450 nW** & **1000 nW** \\ \hline \multirow{3}{*}{
\begin{tabular}{c} Temperature \\ Sensor [1] \\ \end{tabular} } & Capacitive Sensor & Biomedical Sensor & Actuator Network \\  & & [2] & [3] & [4] \\ \hline \hline \end{tabular}
\end{table}
Table 1: State of the art wireless sensor power consumption.  
There is some drawback when RF sources such as Wi-Fi are used as an energy source for an RFEHS. The drawback is regarding power density, especially in an indoor area, where it is often very low [5]. Various RF power density levels for an outdoor environment in [6] have been summarized for several isotropic transmitters. In an indoor environment, a power density of \(1\)\(\upmu\)W/cm\({}^{2}\) at 2.45 GHz spectrum has been used to drive low-power low duty cycle applications [7]. Previous work on RFEHS has focused on single-band [8; 9; 10] and multi-band [11; 12] operating frequencies. For multi-band frequencies energy harvesters, the amount of energy at the input can be high. However, due to the complexity of the multi-band system and the power drop at various rectifier diodes and power processing circuits, the radio frequency-to-direct current (RF-to-DC) conversion efficiency is often lower than the single-band RFEHS.  
This research proposes a power processing circuit for a 2.45 GHz Wi-Fi indoor energy harvesting system which can accept the operational input power as low as \(-50\) dBm. The proposed circuit acts also as an extension of a successful research reported in [13] with reduced charging time of the storage device. In this research, the analysis of the proposed power processing circuit with the other components of an RFEHS will be discussed using an equivalent circuit. A two-port network will be analyzed to derive the relevant equations for the equivalent circuit. Then simulation analysis on the impedance matching of the rectification circuit will show the effect of diode parameters on the operation of the proposed power processing circuit. At the end of this research, the power processing circuit will be tested in a practical environment to demonstrate its actual performance with all the components of the proposed RFEHS.