## 1 Introduction  
Collisions not only between pieces of equipment but also between equipment and mine workers frequently occur inside underground mines, which are narrow and dark [1]. According to mine disaster statistics issued by the Centers for Disease Control and Prevention (CDC), accidents involving mining equipment inside underground mining sites in the United States have accounted for 37 deaths since 1984, and 16 workers have been killed in crashes involving shuttle cars and buckets between 2000 and 2010 [2]. Moreover, nearly 47% of all deaths that have occurred in underground mines between 2011 and 2015 have been caused by collisions between equipment and workers [3]. To prevent the occurrence of said collisions between pieces of equipment as well as between equipment and workers inside underground mines, proximity warning systems (PWSs) involving the use of electromagnetic- or magnetic-field sensors have been developed and used in the past [4]. A PWS provides collision risk warnings when workers and equipment come within a certain distance of each other [5]. Similar to PWSs, other proposed technologies intended for collision prevention include proximity detection systems (PDSs), which are devices that detect the approach of a piece of equipment or a worker through use of sensors, and collision avoidance systems (CASs), which involve the use of devices that effect maneuvers to avoid the occurrence of potentially fatal collisions [5,6]. The National Institute for Occupational Safety and Health (NIOSH) in the United States has performed numerous studies concerning the development of PWSs for mining sites. [PERSON] [7] performed an experiment to detect obstacles in blind spots in the driver's field of view via the use of various sensors attached to a 50-ton dumping truck. Based on the results of these experiments, he subsequently analyzed the reliability and false alarm rate of the proximity-detection technology using sensors [8], [PERSON] and [PERSON] [9] developed a PWS using radio-frequency identification (RFID) technology. Using a set frequency of 315 MHz, the said system received radio signals from tags through a reader and provided progressive collision-risk signals (far, medium-range, and near) for objects approaching blind spots. Commercialized RFID-based PWSs currently include the underground collision avoidance system (UCAS) by Becker Mining Systems [10] and IMPACT, developed by Mine Site Technologies [11]. [PERSON] [4] developed a hazardous area signaling and ranging device (HASARD) capable of detecting the approach of equipment and workers towards a continuous mining machine (CMM) by means of a magnetic transceiver. This system was capable of detecting a 60-kHz band magnetic field transmitted by wire-loop antennas at the receiver end. In the event of a collision risk, the system provided equipment operators with visible, audible, and vibrational signals. Other PWSs employing magnetic-field sensors include the PBE Group's proximity alert system [12], Strata's HazardAvert [13], and NAUTILUS International's buddy proximity detection system [14]. Recently, attempts have been made to develop PWSs using Bluetooth low energy (BLE), which corresponds to Bluetooth 4.0 wireless communication technology. Based on the IEEE 802.15.1 communication standard corresponding to the 2.4-GHz frequency band, the use of BLE serves to reduce power consumption as well as packet size, thereby enabling faster data transmission [15; 16]. [PERSON] et al. [17] developed PWS using Bluetooth beacons for preventing collisions between workers and equipment used at construction sites. Bluetooth beacons are devices that periodically transmit BLE signals. PWS devices developed in Reference [17] received signals from multiple Bluetooth beacons installed on dumping trucks and wheel loaders via the use of a smartphone, and provided workers with collision warnings. [PERSON] et al.