# RF Energy Harvesting Wireless Communications:
## 2 Urban and Semi-Urban RF Environment
### RF Distribution Features
#### 2.2.2 Nonuniform Spectrum Distribution  
Due to the broadcast nature of wireless communications, RF signals radiated by different telecommunication systems coexist in the air. The strength of signals from different RF sources can have a big difference, as listed in Table 1. According to the measurements for ambient RF power in Boston [23], the highest power density (84 nW/cm\({}^{2}\)) that appears around 1800 MHz is three orders of magnitude higher than the power density (0.18 nW/cm\({}^{2}\)) measured around 2.4 GHz.  
The difference of RF power density on the spectrum is mainly caused by the huge difference in the transmission power and deployment density of different RF facilities. For instance, the transmission power of a cellular microcell base station is usually from 1 W to 5 W [30], which can cover only hundreds of meters. Hence, the deployment density of microcell stations is usually high to provide a reliable mobile communication service. As a contrast, the transmission power of a TV tower can be as high as 100 kW on the VHF band or 5 MW on the UHF band in order to cover users within 97 km [41]. Therefore, the distribution of TV towers can be sparse.  
Due to the nonuniform distribution of RF power density on the spectrum, an RF-EHWC device needs to select a proper frequency band to harvest energy. For instance, in the center of a city, it may  
\begin{table}
\begin{tabular}{|c|c|c|c|} \hline Source & \multicolumn{2}{c|}{Conditions} & \multicolumn{2}{c|}{Density} \\ \hline \multirow{4}{*}{\begin{tabular}{} \end{tabular} } & \multirow{2}{*}{Morming} & Unshaded & 20 mW/cm\({}^{2}\) \\  & & Shaded by tree & 3 mW/cm\({}^{2}\) \\ \cline{3-4}  & & Unshaded & 60 mW/cm\({}^{2}\) \\  & & Shaded by tree & 10 mW/cm\({}^{2}\) \\ \hline \multirow{6}{*}{\begin{tabular}{} \end{tabular} } & DTV & 470–610 MHz & 0.89 nW/cm\({}^{2}\) \\  & GSM900 (MT) \({}^{\ddagger}\) & 880–915 MHz & 0.45 nW/cm\({}^{2}\) \\  & GSM900 (BT) \({}^{\ddagger}\) & 920–960 MHz & 36 nW/cm\({}^{2}\) \\  & GSM1800 (MT) \({}^{\ddagger}\) & 1710–1785 MHz & 0.5 nW/cm\({}^{2}\) \\  & GSM1800 (BT) \({}^{\ddagger}\) & 1805–1880 MHz & 84 nW/cm\({}^{2}\) \\  & 3G (MT) \({}^{\ddagger}\) & 1710–1785 MHz & 0.46 nW/cm\({}^{2}\) \\  & 3G (BT) \({}^{\ddagger}\) & 2110–2170 MHz & 12 nW/cm\({}^{2}\) \\  & WiFi & 2.4–2.5 GHz & 0.18 nW/cm\({}^{2}\) \\ \hline \multirow{2}{*}{\
\begin{tabular}{} \end{tabular} } & Human body & 10 K temp diff & 0.22 nW/cm\({}^{2}\) \\  & (4 \(\times\) 8 cm) & 35 K temp diff & 0.47 nW/cm\({}^{2}\) \\ \hline \multirow{2}{*}{
\begin{tabular}{} \end{tabular} } & \multicolumn{2}{c|}{Push bottom [39]} & 2 mJ/N \\  & Human biomechanics [40] & 7.34 nW/cm\({}^{3}\) \\ \hline \multicolumn{4}{|c|}{\({}^{\ddagger}\) BT, Base transmitter. \({}^{\ddagger}\) MT, Associated mobile transmitter.} \\ \end{tabular}
\end{table}
Table 1: The power density of different energy sources.  
Figure 5: Nonuniform distribution of RF facilities around the center of Berlin, German [36].  
be far from a TV tower but surrounded by multiple microcell mobile base stations. In this case, harvesting energy in the cellular band is a wise choice. By contrast, if an RF-EHWC device is closer to a TV, it will be better to harvest energy in the TV band.