## 4 Description of the Proposed System  
The proposed system is based on a telecommunications network that has the three fundamental layers within the network design: access, distribution, and core. Each layer can be considered as a well-defined structured module, with specific functions. The three-layer model ensures that the network maintains sufficient strength and flexibility to provide critical services as well as allowing for growth and changes that occur over time [42]. In the monitoring center which is within the core layer, a router is able to provide connectivity to the high-speed backbone (Internet) so that the data collected by the location system and stored in the server database is available at any time and in any place. In addition, the core layer through the router is interconnected with the distribution routers through an optical fiber link. Distribution routers interact between the access layer and the core layer to provide high availability to the end user and a route to the core layer. The access layer represents the perimeter of the network, where traffic enters or leaves the network. The main function of the VLC access points is to provide the users access to the network. The VLC access points are connected to the distribution routers, which could implement technologies for quality of service and network security. To provide a high data rate and a wider coverage area, as well as to ensure the availability of the link at all times, a traditional cellular structure is the chosen option. It is assumed that inside the underground tunnel, VLC access points are placed on the roof along the entire underground tunnel, where each of them is composed of an LED and a photodetector with coordinates \(\left(\left(x_{i}\right),\left(y_{i}\right),\left(h_{i}\right)\right)\)The LEDs are used to provide illumination as well as communication in the optical cell formed by the light beam. The newly approved IEEE 802.15.7-2018 standard for local and metropolitan area networks for short-range OWC systems describes the physical layer and the data link layer of VLC; covering topics such as: network topology, addressing, collision prevention, among others [43]. Based on the standard, the chosen topology for the present work is a star topology, in which communication is established between several mobile devices and a particular VLC access point. Networks with a star topology operate independently of other networks using the same topology. This is achieved by choosing a wireless personal area network (WPAN) identifier that is not being used by another network within the coverage area. Once the WPAN identifier is chosen, the VLC access point allows mobile devices to join the network. The standard defines the multiple access with carrier monitoring and collision avoidance (CSMA/CA) as the technique used to access the channel [43]. By using non-beacon-enabled CSMA/CA, the transmission medium is shared by several mobile users at the same time. Every time the device tries to communicate, it has to wait for a random off-time period. If the carrier-sense mechanism finds the channel available to communicate, then the device transmits the data frame. Otherwise, if the channel is busy, the mobile device waits for another random period before trying to access the channel again. Once the signal has been sent from the VLC access points \(i\), and is captured by the VLC mobile device \(p\) mounted on the miner's safety helmet, a digital processing unit will be responsible for determining the received power of the signal. Then, the distance between the LED and photodetector is given as:  
\[d(i,p)=\left(\frac{Pt_{i}T_{s}(\Psi)g(\Psi)A_{t}(m_{1}+1)(h_{i}-h_{p})^{m_{1}+1 }}{2\pi\text{Pr}_{(i,p)}}\right)^{\frac{1}{1}/m_{1}+3}. \tag{9}\]  
and depends on the received power (\(Pr\)), transmitted power (\(Pt\)), as well as other channel param. In real conditions, this distance is estimated based on the received and transmitted power, and certain channel param; therefore, channel estimation is required.