## 1 Retrospective: The Development of Intelligent Ventilation Systems in Underground Mines
[19] developed the Automatic Ventilation System Adjustment Software based on the requirement of ventilation change in the ventilation system, and [PERSON] et al. [20] proposed a method for the accurate monitoring of tunnel wind speed by a long-span ultrasonic linear wind speed sensor based on a time difference method. Meanwhile, [PERSON] et al. [21], [PERSON] et al. [22] and [PERSON] et al. [23] optimized the ventilation planning algorithm and finished the simulation process. What's more, [PERSON], [PERSON] and [PERSON] [24] used the Internet of Things technology in order to improve the information transmission and monitoring function of mine ventilation in 2019. Some of the research results are based on the original research, while experts have made some improvements, such as algorithm optimization or combination with advanced information transmission technology, such as Internet of Things technology. Furthermore, the development of some modeling tools, such as the aforementioned 3D VentCloud, also lays the foundation for the realization of intelligent mine ventilation. Perspective: Is ZigBee WSN Technology the Best Choice for Intelligent Ventilation Systems in Underground Mines? As previously mentioned, [PERSON] et al. [16; 17] indicated that the narrow space of an underground environment significantly enhances the signal intensity, and as a result ZigBee WSN technology can have a great advantage in the underground environment. In fact, in the narrow environment of the underground mines, short-distance transmission systems such as ZigBee WSN technology can play the greatest role and reduce energy consumption. As a result, ZigBee WSN technology is gradually used in monitoring and alarm systems in many mines. As early as in 2008, [PERSON] et al. [25] applied ZigBee WSN technology to underground mine safety monitoring systems. They used the ZigBee WSN technology to collect temperatures, humidity and methane from underground coal mines. Then, the ZigBee WSN technology transferred data to ARM-based information processing terminals [26], and interestingly, [PERSON], [PERSON] and [PERSON] [27] designed a high-performance wireless robot network communication system for underground mines based on ZigBee WSN technology. Evidently, the application of ZigBee WSN technology is becoming more and more popular for underground mines, as it has good information collection capabilities, such as harmful gas concentration and temperature, which can provide signals for intelligent mine ventilation systems. As a result, in the future, ZigBee WSN technology is likely to become a medium for information transmission in intelligent mine ventilation systems because of its three characteristics: (1) suitability for the underground mines; (2) low energy consumption; (3) good performance for the information collection. However, along with the continuous development of science and technology, the world has gradually entered the era of 5G technology. It is reported [28, 29] that in 2019 Ambra chose Ericsson, as part of a global cooperation agreement, to provide 5G network solutions for automated ventilation systems, which shows that the 5G technology has been applied in underground mines in recent years. 5G technology has the advantages of being able to track vehicles as well as excavators in real time, monitor the underground environment and to remotely control machines such as vehicles, drilling rigs, ventilation systems and other equipment. At the same time, 5G technology [30] has an excellent performance in preventing underground casualties and post-accident (mine fire, traffic accident or gas poisoning) search and rescue. However, at this stage, the 5G technology is obviously not perfect. For example, its functional area coverage is small, so if we want to apply 5G technology to intelligent ventilation of the underground mines in the future, the 5G technology would need to be further optimized.