## 4 Discussion
### Relationship with Population Density  
The relationship between the data measured over a wide area and the population density was discussed, referring to the literature [33]. [PERSON] et al. measured the transmission power level (dBm/MHz) from a mobile phone terminal while driving in order to investigate the radio interference from the ground system to the satellite system in the development of a shared system of land and satellite communication systems [33]. They reported that the transmission power level is inversely proportional to the power of the population density.  
In this work, a similar investigation was carried out using the results of the 2020 National Census [34]. We used the reference area mesh provided by the Japanese government [31] to divide the measurement area into a 1 km\({}^{2}\) mesh. Information such as population density [34] is linked to this mesh so that we can estimate how many people live in the area mesh we measured. Figure 7 contains a scatter plot that shows the relationship between population density and E-field strength for mobile phone base stations, including 5G (a), only 5G (b), and for broadcast transmission towers of FM/TV (c) in metropolitan Tokyo and Chiba Prefecture. The horizontal and vertical axes show the population density and the E-field strength, respectively. For mobile phone base stations, including 5G (Figure 7a), there are proportional relationships with the power of the population density, although there are large variations. E-field strengths of only 5G (Figure 7b) are characterized by a small amount of data and concentration in densely populated areas such as 10,000 per km\({}^{2}\). On the other hand, there is no significant relationship for broadcasting towers.  
Figure 7: Relationship between population density and E-field strength. Left, Tokyo; right, Chiba. (a) Mobile (including 5G); (b) mobile (5G); (c) FM/TV.  
For mobile phone base stations, there are cases where the E-field strength is high in areas with low population density. For example, it has been confirmed that there are commercial facilities where almost no people live, while many people work there during the day. For broadcasting in Tokyo, the E-field strength is concentrated at 70 to 110 \(\mathrm{dB\mu V/m}\) with a population density near 10,000 \(1/\mathrm{km^{2}}\), which can be considered to be due to the existence of the Tokyo Skytree.  
The regression line of the distribution for mobile phone base stations in Figure 7a was calculated to have a slope of about 0.5 in all prefectures. On the other hand, according to the literature [32], the transmission power from mobile phone terminals decreases with a slope of \(-0.38\) when the population density increases. In this measurement, it was found that the E-field strength from mobile phone base stations increases with almost the same absolute value of the slope against the population density. That is, although [32] shows the results of the 3 rd generation mobile phone system, the relationship between the two results is confirmed. The main reason that the E-field strength from mobile phone base stations has a positive relationship with population density seems to be that there are more mobile phone base stations in densely populated areas.