## 4 Discussion  
In this study, an LOS analysis technology and SCI applicable to 3D models were proposed and applied to underground mine sites. Thus, appropriate points were determined for installing IoT monitoring devices in advance. This developed technology is highly compatible with popular 3D model formats and can be easily applied. However, it has some limitations which must be overcome.  
In this study, LOS analysis was performed quickly using the ray-casting algorithm in the Python Open3D library. Rapid analysis is possible through efficient programming; however, for commercialization, many tests must be conducted in diverse areas. In addition, the calculation speed can be improved further by modifying the code.  
The SCI concept was proposed by calculating the average distance between points where the LOS was connected to the target point. However, a more appropriate metric may be capable of representing this concept compared with the average distance. For example, the overall visible area can be calculated. However, as this involves more complicated calculations and considerations, in this study, a method that allowed for simple and fast calculations was employed. In addition to the wide communicable area, various conditions, such as the path or direction of device installation and the surrounding environment, can also be considered. Therefore, a more reasonable SCI must be considered by proposing various indicators through further research.  
In this study, radio waves were assumed to travel in a straight line. However, electromagnetic waves do not propagate only in a straight line. In reality, an ellipsoidal  
Figure 10: Results of underground mine field test. (**a**) Vibration monitoring module made using Xbee 3 and ADXL335 accelerometer and (**b**) example of vibration measurement result (modified from [PERSON] et al. [5]).  
Figure 9: Images captured at the installation locations of IoT devices using an optical camera. Numbers 1 to 6 are the results for some points where monitoring devices were installed.  
communication space called a Fresnel zone is formed between the transmitter and receiver owing to refraction [26]. Therefore, in the future, we plan to conduct additional research on communicable areas and improve the algorithm by considering refraction phenomena. In underground mines surrounded by internal walls, the effect of reflection may be greater than that of refraction. Various experiments are required to quantify this phenomenon, which is a difficult task. For example, propagation channel modeling could be a solution to improve the performance in this study. When channel state information for communication devices is available, transfer characteristics, such as signal constellation and allocated power, can be considered [27]. To increase applicability in actual mines, these algorithms must be applied in 3D to accurately predict the strength of radio waves for communication. However, as the application of IoT technology to underground mines and facilities increases, additional research on this subject will be necessary. And it is necessary to verify this through strength measurement experiments.  
In this study, the technology was applied to areas where 3D models have already been built; however, it could also be applied to unexplored closed spaces where development continues. For example, the communication environment can be analyzed for hypothetical scenarios that consider changes in the 3D model. Additionally, the 3D models and analysis results can be updated periodically for areas where mining occurs and the structure of the mine drift changes.