## 3 Methodology
### Structured Model Reconstruction
#### 3.3.3 G Signal Intensity Simulation  
In the 5G era, 5G devices have design features that support higher signal frequency and the shorter wavelength to generate faster transmission speed; however, this leads to diminished capability of penetrating through walls [48]. With 80% of today's businesses occurring indoors, setting up large-scale small base stations to increase signal intensity has become a common occurrence. However, due to the complexities of the indoor environment (e.g., occlusion problems), network construction has become a challenging undertaking.  
In this study, the output structured models have three properties: semantic, geometric, and connection types. These models are made up of basic building elements, such as the ceilings, floors, walls, windows, doors and pillars, which have direct effect on signal propagation in the real world. Thus, the structured model can become an important tool in analyzing 5G signal simulation.  
According to the standard of 3 GPP [49], the non-line-of-sight signal propagation loss model of indoor space is expressed:  
\[L_{f_{5},dB}=32.4+31.9\text{-lg}(d_{p})+20\text{-lg}(f) \tag{11}\]  
where \(L_{f_{5},dB}\) is the propagation loss; \(d_{p}\) is the max propagation distance \((100~{}m)\); and, \(f\) is frequency of the electromagnetic wave \((0.5~{}GHz-100~{}GHz)\). The formula suggests that greater propagation loss occurs with larger wave frequency or with longer propagation distance. In an ideal indoor environment (no attenuation losses), when the frequency remains constant, the propagation loss increases with increasing distance, which then decreases the power received by the receiver.  
The indoor environment is comprised of open cubicles, walled offices, open areas, corridors, etc. In this study, the 5G base stations were assumed to be located at the height of 2 m, near the ceilings. The ray-tracing solution is adopted to provide a detailed multipath and accurately simulate the spatial variation. Figure 14 illustrates the principle of single signal propagation, where the intensity multipath results from the reflection of walls and transmission of openings.  
In our experiment, the signal propagation intensity was simulated in the structured model. Three base stations were mounted at the corridor and a room, as shown in Figure 15a. Every base  
Figure 14: Principle of signal propagation (the signal intensity changes from strong to weak that corresponds to color from red to blue).  
Figure 13: The rooms with topological relationship (the solid lines are connected by the doors, and the dotted lines are the connected by adjacent walls).  
station was assumed to be at the center of a sphere, randomly launching 150 rays. Using a frequency of 100 GHz, the intensity was calculated within a range of 100 m using the signal propagation loss model (see Figure 15b). The profile provided an effective means of measuring the changes of signal intensity and became a useful tool in visualization and inspection of 3D interpolation results. Therefore, the interpolation method of Inverse Distance Weight method (IDW) used in simulating the intensity profile can be calculated with:  
\[P=\sum_{i=1}^{n}\varepsilon_{i}P_{i}\ \ \varepsilon_{i}=\frac{\frac{1}{(D_{i})^{ \#}}}{\sum\limits_{i=1}^{n}\frac{1}{(D_{i})^{\#}}} \tag{12}\]  
where the intensity value \(P\) of the interpolation point is defined as the weighted average value of known point intensity value \(P_{i}\); \(D_{i}\) is the Euclidean distance from interpolation point to its nearest sampling point; and, \(m\) is the power exponent. Finally, the profile result is shown in Figure 15c.