## 4 Propagation Loss Calculation Using the Improved TWPE Model
### Verification of TWPE  
Propagation losses from Shanghai-Wuxi using the same environment settings and radar parameters as intercity links of Hangzhou-Wuxi are shown in Figure 17. Different LC types were considered to evaluate the PL distribution on the S-W link using the TWPE model, as shown in Figure 17a,b. The AREPS result with the IGBP LC type is shown in Figure 17c. The range-dependent PLs at a ducting layer of 60 m on the S-W link of different LCs using TWPE and AREPS are compared in Figure 17d. The propagation distance from Shanghai to Wuxi is about 115 km. Moreover, the terrain relief of the intercity link between Shanghai to Wuxi was kept below 10 m, which fits the application condition of the paraxial approximation of PWE. Based on the plain cover characteristics of East China, the land types of terrain on this intercity link are classified into grassland and water bodies at first. We conducted WAPE considering terrain relief, tropospheric turbulence, and dielectric permittivity on the first assumption and the land types of IGBP from MODIS data. From Figure 17a,b, we used the IGBP cover type form MODIS with a resolution of 250 m, the PL distribution basically remains at the same level as the conducted form TWPE with LC type using grassland and water bodies. Figure 17d shows the PL distribution at the transmitting height of 60 m in the trapping layer of the surface-based duct. The PL at a height of 60 m using the LC type of IGBP from MODIS between 60 and 105 km is larger than that of using the grassland and water bodies as terrain features. On the remaining links, the PL using the two LC classifications are reversed or on the same level. AREPS uses the RPO model to divide the long-distance ducting propagation into four models to improve the calculation speed and facilitate engineering applications. There was not much to consider on the tropospheric scatter and terrain scatter in AREPS. Therefore, the PL trend was similar but AREPS result was 20 dB lower than TWPE result. Given that the industry can achieve GPS receivers with tracking sensitivity below \(-\)160 dBm, at the height of the surface-based duct within the receiver, the realistic LC classification could be an essential environmental condition to evaluate whether CCI would happen on this intercity link. The possibility of CCI could be an essential decision support of 5G base station deployment. Propagation loss from Jiaxing to Wuxi using the same environment settings and radar parameters, the same as in the previous section, are shown in Figure 18. Different LC types were considered to evaluate the PL distribution on the J-W link using TWPE model, as shown in Figure 18a,b. The AREPS result with the IGBP LC type is shown in Figure 18c. The range-dependent PLs at a ducting layer of 60 m on the J-W link of different LCs using TWPE and AREPS are compared in Figure 18d. The distance of the intercity link from Jiaxing to Wuxi was about 93 km. Moreover, the terrain relief was around 20 m, besides a peak, which had a maximum height of 80 m. The peak was not high enough to cut off ducting and diffraction, and the radio wave elevation angle fit the maximum limit of TWPE. From Figure 18b, we can see that when we considered the LC types more specifically, the PL on the intercity link Jiaxing-Wuxi increased slightly. Due to the limitation of the RPO model in AREPS using a range-independent RO model in the range-dependent environment, in extreme cases this can cause discontinuities along the RO/PE boundary [28]. This discontinuity occurred in the peak of the intercity link of Jiaxing-Wuxi. The two classifications of LC types do not give obvious differences probably due to the classifications of grasslands and waterbodies being close to those of the real LC type distributions on the intercity link. It can be seen from Figure 9d that the real LC data on the Jiaxing-Wuxi link extracted by the IGBP LC classification also consisted of most WAT--17 in IGBP LC1--and several other kinds of vegetation categories.