# A Power Processing Circuit for Indoor Wi-Fi Energy Harvesting for Ultra-Low Power Wireless Sensors
## 9 Conclusions  
This paper presents a method for designing a complete power processing circuit for an indoor RF energy harvesting system. The analytical analysis of the power processing circuit attached to an antenna and a PMU has been conducted based on an equivalent circuit. The impedance matching between the energy harvester, matching circuit and rectifier circuit have been analysed. From the simulation analysis, it has been demonstrated that the selected Schottky diode used in designing the proposed power processing circuit gave the best performance compared to other diodes. The power processing circuit works well with Wi-Fi power typically at \(-20\) dBm, which can even go down to as low as \(-50\) dBm. The overall RF-to-DC power efficiency of \(30\%\) is obtained from the proposed  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Comp**} & **This Work** & \multirow{2}{*}{I191 **2014**} & \multirow{2}{*}{I201 **2013**} & \multirow{2}{*}{I21 **2010**} & \multirow{2}{*}{I221 **2013**} & \multirow{2}{*}{I231 **2014**} \\  & & & & & & & \\ \hline Rect. Topology & Proposed & \multirow{2}{*}{\(\star\)} & Series & Series & \multirow{2}{*}{\(\star\)} & \multirow{2}{*}{\(\star\)} & Doubler \\ Rectenna Topology & & & & & & & \\ No. of RF band & Single & Multi & Single & Double & Double & Five \\ \hline RF bands (GHz) & \(2.45\) Wi-Fi & \(0.9,18,2,1,\) & \(0.45\) & \(0.9,18\) & \(0.9,2.0\) & \(0.267,0.40,0.79,\) \\ \hline \multirow{2}{*}{RF-to-DC Efficiency} & \(6\)–\(30\%\) & \(15\%\) & \(45\%\) & \(4\%\) (for each) & \(9.1\%\) and \(8.9\%\) & \(19\%,20\%,16\%,5\%\) \\  & \(-20\) dBm & \(-20\) dBm & \(-10\) dBm & band) \(12\) dBm & \(-19\) dBm & \(-10\) dBm \\ \hline \multirow{2}{*}{RF source and distance} & \multirow{2}{*}{Ambient RF} & Dedicated RF 50 m & Dedicated RF 50 m & Dedicated RF 3.75 m & - & Dedicated 4 m \\ \hline \hline \end{tabular}  
* A bandpass filter for a single frequency; *
* Multi-bandpass filter for multiple single frequencies; **
* A bandpass filter for multiple single freq;
* Multi-bandpass filter for multiple single freq. using four-dipole antenna; *+ Multi-bandpass filter for multiple single freq.; no physical connection between two different RF channels; *+ Multi-bandpass filter for multiple single freq.  
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Table 6: System performance comparison.  
\begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**Single Band**} & \multicolumn{1}{c}{**2.45 GHz, Wi-Fi**} \\ \hline DC output voltage & & \\ Using generator for 3 h & & \\ \(P_{in}=-50\) dBm & \(50\) mV \\ \(P_{in}=-40\) dBm & \(100\) mV \\ \(P_{in}=-30\) dBm & \(300\) mV \\ \(P_{in}=-20\) dBm & \(400\) mV \\ \hline The circuit power consumption & \multirow{2}{*}{\(18\) nW} \\ \(P_{in}=0\) dBm & & \\ \hline The circuit equivalent impedance & \(50\) k\(\Omega\) \\ \hline In indoor environment & & \\ (RF power density = \(0.1\) mW/m\({}^{2}\)) & \(3\) uW \\ DC output power & \(30\%\) \\ RF-to-DC conversion efficiency & & \\ \hline \hline \end{tabular}
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Table 5: The proposed system key features.  
energy harvesting system. The final measurement result has demonstrated that the proposed system can charge up a 200 mF supercapacitor in 20 hours, and the stored energy of 0.11 J can be used to drive low power wireless sensors.  
**Acknowledgments:** The main funding for the research works is from the Postgraduate Research Student Support (PReSS account), The University of Auckland, New Zealand.  
**Author Contributions:** [PERSON] conceived and designed the experiments; [PERSON] performed the experiments; [PERSON] analyzed the data; [PERSON] contributed analysis tools; [PERSON] wrote the paper; [PERSON] proposed the research topic and supervised the research progress.  
**Conflicts of Interest:** The authors declare no conflict of interest.