## 3 Results
### Line-of-Sight Analysis for Underground Mine  
The developed technology was applied to the Samdo mine in Dogye-eup, Samcheoksi, Gangwon-do, South Korea. The Samdo mine is an underground mine that produces high-quality limestone for steelmaking using the board-and-pillar method. As mining and blasting operations are continuously performed at the Samdo mine, its environment must be monitored, and communication devices must be installed. The total length of the Samdo mine drift is over 60 km. However, in this study, the experiments were conducted at a blasting site approximately 2 km away from the mine adit. Underground mine sites are dark and humid; consequently, accurately determining communicable areas using only human judgment is a challenging feat. Therefore, in this study, prior to installing ZigBee-based devices, the developed LOS analysis method was applied to identify areas where communication was possible. A 3D model of the mine drift was constructed using terrestrial LiDAR, and an experiment was performed on a specific section close to the blasting point. Figure 6a shows the results of SCI analysis for the target area; here, points with relatively narrow communicable areas are shown in purple, whereas points with wider communicable areas are shown in red. Figure 6b shows the communicable area at point A with a low SCI, whereas Figure 6c shows the communicable  
Figure 5: ZigBee-based IoT devices and experiment results. (**a**) Coordinator device and router device and (**b**) RSSI test result. area at point B with a high SCI. Evidently, the communicable area of point B is wider than that of point A. Moreover, point C in Figure 6d is located close to point B but has a relatively narrow communication area and lower SCI value. A large space was selected to install the coordinator device to avoid interference from haulage trucks. The LOS analysis results at that location (point C) are shown as number 1 in Figure 7. The first monitoring device was installed inside the mine (point 1). In this process, a location within the communicable area of the coordinator device with a high SCI was selected. A total of 29 installation points were selected in this manner, and the LOS analysis results for some of them are shown in Figure 7. Figure 8 shows the results of visiting the Samdo mine and scanning the surrounding area using a portable LiDAR (VLP-16, Velodyne, San Jose, CA, USA) at the selected points. Figure 9 shows the images captured with an optical camera from the same point toward the next installation point. The orientations of the 3D models presented in Figure 7 were adjusted for comparison with those shown in Figures 8 and 9. The areas marked with red boxes in Figure 7 correspond to those shown in Figures 8 and 9. By applying LOS analysis to the 3D model prior to onsite work, devices were quickly installed onsite. The installation process is summarized as follows. 1. Provisional setting of the starting point considering the surrounding environment;
2. Determination of the starting point considering the SCI results for the target area;
3. Analysis of LOS at the starting point;
4. Determination of the secondary installation point, i.e., a point where the SCI is high and the LOS is connected from the starting point;
5. LOS analysis at the secondary installation points;
6. Determination of the third installation point and repetition of the same process;
7. Investigation of LOS in the field using portable LiDAR and detailed positioning. Figure 6: Results of LOS analysis based on a 3D model of the Samdo mine target area. (**a**) Smooth communication index (SCI), (**b**) communicable area at point A with a low SCI, (**c**) communicable area at point B with a high SCI, and (**d**) communicable area at point C.  
A total of 29 ZigBee-based devices were installed in this manner, and as they were installed in an underground space, the distance between the modules was set as approximately 30 m. Figure (a)a shows a monitoring module developed using the Xbee 3 module and ADXL335 accelerometer to measure the vibration.