# Smart Helmet-Based Personnel Proximity Warning System for Improving Underground Mine Safety
## 2 Design of the Proximity Warning System (PWS) Based on Bluetooth Beacons and Smart Helmets
### Design of BLE Transmission Units Using Bluetooth Beacon  
Bluetooth beacons periodically transmit information, including the general-purpose unique identifier of the beacon and media access control (MAC) address through the BLE signal. The intensity of the BLE signal transmitted by the Bluetooth beacon is expressed as Tx power, and the unit is dBm. The received intensity of the BLE signal can be quantified using the RSSI value. RSSI is represented in a negative form by a value between \(-99\) dBm and \(-35\) dBm. The propagation distance of the BLE signal may vary depending on the signal transmission intensity and direction of the signal propagation of the Bluetooth beacon. An increase in the BLE signal transmission intensity increases the signal propagation distance. The signal propagation direction is bidirectional, and the signal can be spread uniformly in all directions, but this limits the propagation distance. The BLE signal is first propagated relative to the Bluetooth beacon when the signal is transmitted as the directional signal. The change in RSSI according to the BLE signal transmission intensity and the direction of the radio wave of the Bluetooth beacon was previously analyzed [46].  
Bluetooth beacons can communicate with peripheral devices in three ways: point-to-point, broadcast, and mesh. The point-to-point method is a method of exchanging data by pairing the master device transmitting a large amount of data and the slave device receiving data at 1:1. The broadcast method is a method in which the observer receives information when the broadcast periodically transmits its ID information to the peripheral  
Figure 1: Overview of personal proximity warning system (PWS) using smart helmet.  
devices. Bluetooth beacons are mainly broadcast missions, and observers mainly use PCs and smartphones. The mesh method is connected to several master and slave devices [46].  
In this study, RECO beacons (Perples, Seoul, Korea) were used as BLE transmission devices. RECO beacons are certified by institutions in Korea, the United States, Europe, and Japan and meet global beacon standards (Table 1). Figure 2 shows examples of heavy equipment and vehicles at the mine site with RECO beacons. A Bluetooth beacon was installed on the back of the room mirror on the front of the truck, and a Bluetooth beacon was provided on the front of the heavy equipment. The Bluetooth beacons set the directional signal such that the signal could be propagated further. The signal transmission strength and period of the beacons were set to \(-4\) dBm and \(1\) s, respectively.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Item** & **Value** \\ \hline Dimensions (Diameter \(\times\) Height) & 45 mm \(\times\) 20 mm \\ Weight & 11.6 g (0.4 oz) \\ Processor & 32-bit ARM® Cortex®-M0 \\ Battery & CR2450 Lithium Coin Battery (3 \(V\), 620 mAh) \\ Casing & Acrylonitrile Butadiene Byrene (ABS) Plastic \\ Chipset & Nordic rrf51822 \\ Thermal Resistance & 93 \({}^{\circ}\)C (200 \({}^{\circ}\)F) \\ Operating Temperature & \(-10\)–\(60\)\({}^{\circ}\)C (14–140 \({}^{\circ}\)F) \\ Wireless Technology & Bluetooth 4.0 (i.e., BLE or Bluetooth® Smart) \\ Signal range & 1 m-70 m (3.2 ft–230 ft) \\ Signal transmission period & Min (10 ms), Max (2 s) \\ Transmission power & Min (\(-16\) dBm), Max (4 dBm) \\  & Korea Certification (KC) \\ South Korea & Federal Communication Commission (FCC) \\ Certification & USA & Conformité Européene (CE) marking \\ Europe & Ministry of Internal Affairs and \\ Japan & Communications (MIC) of Japan \\ \hline \hline \end{tabular}
\end{table}
Table 1: Specifications of the RECO beacon [47].  
Figure 2: Bluetooth beacons attached to trucks **(a,b)** and excavators **(c,d)**.