## 4 Description of the Proposed System
An experimental method to estimate the channel param is based on locating the photodetector in different places and measuring the received power. To calculate the position, the VLC mobile device, through the digital processor, implements the trilateration technique. This is done to locate the miners within the different cells, and send the calculated position to the monitoring center by means of the VLC access point with which there is coordination. The benefit of the proposed method in this system is the 3-dimension localization for multiple miners because it introduces and additional variable to the analysis in comparison to the traditional 2-dimension systems; therefore, a lower average localization error is expected. The proposed algorithm in this manuscript is capable of localizing several miners using a VLC 3 dimensional trilateration localization scheme in any sector of the the mine, even when two or more miners are located in the same sector and at the same time. Figure 2 shows the presented model for placing the LEDs and photodetectors. For illustration purposes, Figure 2 depicts the proposed system for a cell of three LEDs and two photodetectors (miners). Different number of LEDs can be used in each cell, but at least three LEDs are needed for the proposed 3D localization scheme in each cell. Figure 3 shows the general block diagram of the proposed VLC localization system. The data is transmitted by the LEDs and is received by the photodetectors. The received signal is affected by the optical channel. Two filtering processes are used in the receiver to prepare the useful data for the localization algorithm. The first step is optical filtering, which it is used to eliminate the extra lights outside of the visible light range. The second step is the use of a bank of finite impulse response (FIR) frequency digital filters to separate the carrier tones related to each LED. Then, an amplifier is considered to increase the power of the received signals. Finally, the amplified signal is ready to be used in the localization algorithm. The proposed localization algorithm is based on the estimated power of the received signal of each carrier tone. Figure 4 shows the basic steps of the localization algorithm in the receiver side. The received signal by the photodetector is the superposition of three LED signals, and each LED transmits the data on a different carrier tone. Therefore, a proper method to separate the signals in the photodetector consists on the use of a bank of FIR filters in the frequency-domain bank. Next, the power of each carrier tone is calculated because the localization method is based on the estimated power of each LED. Then, a sphere can be considered, and it is centered in the location of the LED for which all points in the sphere have the same power as the photodetector. This process is done for the two other carrier tones because the received signal in the photodetector is the superposition of the three LEDs. Finally, three spheres are plotted by centering of locations of the LEDs based on the power calculation of the received signals. In the ideal case, these three spheres should have intersection in just one point, and it should be in the location of the photodetector. But these three spheres have an intersection area, not just a point, because of noise and reflection in the environments. The point in this area, which has the closest distance to all of the three spheres, is considered as the estimated location of the photodetector. The localization error is described as the euclidean distance between this estimated location and the real one. Figure 4: Steps of the proposed localization algorithm in the receivers side. Figure 3: General block diagram of the proposed VLC system.