# RF Energy Harvesting Wireless Communications:
## 2 Urban and Semi-Urban RF Environment
### RF Distribution Features
#### 2.2.1 Nonuniform Spatial Distribution  
The intensities of RF energy in different places have high heterogeneity due to the following factors:  
* _High spreading loss_: As measured in [30], the total power density on \(900\,\mathrm{MHz}\) and \(1800\,\mathrm{MHz}\) can exceed \(50\,\mathrm{mW}/\mathrm{m^{2}}\) when the measurement site is \(10\,\mathrm{m}\) from a cellular base station. However, this  
Figure 2: The limitation of MPE recommended by US FCC in 2013. The figure is created based on Table 1 in [27]. Solid lines and dot lines represent the MPE for controlled exposure \({}^{\ddagger}\) and uncontrolled exposure \({}^{\ddagger}\), respectively. \({}^{\ddagger}\) Controlled exposure: Persons who are exposed as a consequence of their employment, provided those persons are fully aware of the potential for exposure and can exercise control over their exposure. \({}^{\ddagger}\) Uncontrolled exposure: Members of the general public who are exposed as a consequence of their employment may not be fully aware of the potential for exposure or cannot exercise control over their exposure. value quickly decreases to around \(0.05\,\mathrm{mW}/\mathrm{m}^{2}\) when measured \(50\,\mathrm{m}\) away from the base station. The authors also measured the RF power density between \(100\,\mathrm{MHz}\) and \(3\,\mathrm{GHz}\) at different places in Houston and demonstrated the results in Figure 3. As shown in the figure, the KETH-TV signal on \(540\,\mathrm{MHz}\) was around \(-30\,\mathrm{dBm}/\mathrm{MHz}\) in Quail Run, Houston, which is about \(1\) mile away from the KETH-TV tower. In Sharpstown, Houston, which is about \(10\) miles away from the TV tower, the strength of RF signal on \(540\,\mathrm{MHz}\) reduced to \(-50\,\mathrm{dBm}/\mathrm{MHz}\) owning to the high spreading loss. In this situation, the TV signal on \(540\,\mathrm{MHz}\) becomes too weak to be harvested by most of the RF-EHWC devices [31]. _Block of obstacles_: Large obstacles can block an RF wave with a short wavelength. Mountains, slopes, and buildings that have a much larger size than the wavelength of radio signals can directly affect the spatial distribution of RF power density. As demonstrated in Figure 4, the signal attenuation is large but smooth around Houston (plain area); by contrast, it has dramatic fluctuations around Boston (hilly area). In addition to geography, buildings can also change the distribution of RF energy in the air. The results reported in [32] indicate that the power density in an indoor site is at least one order of magnitude lower than that in an outdoor one when two measurement sites have the same distance to a cellular base station. _Nonuniform deployment of RF facilities_: The number of RF facilities deployed in an area is usually related to the population density. Consequently, the nonuniform distribution of population causes a nonuniform deployment of RF facilities, which results in the nonuniformity of RF power density in the environment. In Figure 5, we show the distribution of RF facilities in Berlin, which is published by the German Federal Network Agency in 2017 [36]. Figure 3 also demonstrates the heterogeneity of spectrum distribution resulted from the nonuniform deployment of RF facilities. Since the KloI-FM and KETH-TV stations are built near Quail Run, Houston, we can  
Figure 4: The influence of geography on the distribution of RF power density in Houston (plain area) and Boston (hilly area) drawn with TV Fool [33]. The effective radiated power (EPR) and the height above average terrain (HAAT) of TV station placed at Houston are \(1\,\mathrm{kW}\) and \(580\,\mathrm{m}\), respectively [34]. The corresponding parameters of TV station located at Boston are \(1.35\,\mathrm{kW}\) and \(390\,\mathrm{m}\), respectively [35]. Figure 3: The RF power density measured at different places in Houston, TX. observe strong RF strength on 101 MHz and 540 MHz bands in that area.