## 3 ICI Problem in Underground Mining VLC Environments and ADR-Based Solutions
### Contextualization of the ICI Problem in Underground Mining VLC Environments  
In a noisy VLC scheme such as a mining environment, there must be multiple \(T_{x}\), because it is necessary to have full lighting and communication coverage in the scenarios studied. Figure 4 presents an example where two adjacent optical cells are shown, in which there is an overlapping area in their respective coverage areas. Therefore, the ICI in these schemes is always present. To present this problem, we must identify the most important parameters involved in the existence of ICI. According to Figure 4, we will consider two \(T_{x}\) that maintain a distance \(d_{T_{x}}\) from each other. In addition, it is considered \(\phi_{1/2}\) and the height between the roof where the \(T_{x}\) are located and the mining floor, called \(h\).  
The optical cells produced by \(T_{x_{1}}\) and \(T_{x_{2}}\), assuming that all the \(T_{x}\) of the mining scenario have the same physical parameters, cover a circular area of radius \(r_{0}\). By trigonometric relationship, \(r_{0}\) acquires the form of  
\[r_{0}=h\ tan(\phi_{1/2}). \tag{13}\]  
Therefore, in order for the coverage areas of the optical cells to overlap and produce ICI, it must be met that \(d_{T_{x}}<2r_{0}\), in other words, the distance between the \(T_{x}\)s is less than the diameter of the optical cell.  
In general, in a mining VLC scenario, there are several \(T_{x}\)s that produce optical cells that overlap each other; as such, the ICI is a challenge that must be addressed to improve the efficiency of the underground VLC mining system. To contribute to the mitigation of this impairment, the use of  
Figure 3: Block diagram of the underground mining VLC system.  
multiple PDs in \(R_{x}\) is proposed to reduce the ICI, which are located in geometric distributions, termed as ADR. Benefits of applying these structures in \(R_{x}\) can be summarized as follows.  
1. The geometric structure of the ADR means that the PDs located there point in different directions and at different angles; this diversifies the reception of the optical signals.
2. ADRs provide a fixed coverage area that is based on the FoV of each PD. This becomes narrower when the number of PDs is increased. This benefit allows reduction of the LoS ICI caused by neighboring \(T_{x}\)s.
3. Being light reflections of a dispersive nature, having a narrower FoV, the ICI NLoS can be reduced, because most of the light components received by reflections would be rejected.
4. By increasing the number of PDs in \(R_{x}\), according to their location in the ADRs, greater granularity can be obtained, which improves the ability to suppress the ICI. However, among the factors that should be considered when choosing the geometric distribution and the number of PDs to use, must be included the complexity of the ADR design, the cost and processing time of the received signals, and the physical manufacturing cost of these models.  
Based on these benefits and features, we present below the design and analysis of two ADR schemes, one of pyramid structure, and a new scheme based on a hemi-dodecahedral distribution.