## 3 Results and Discussion
### Simulation Parameters
In datasheets such as [42], the normalized SPD \(\Phi_{\text{norm}}(\lambda)\) is usually presented in such a way that \(\max_{\lambda}\Phi_{\text{norm}}(\lambda)=1\). For a desired total transmitted optical power \(P\), SPD \(\Phi(\lambda)\) can be calculated with the following expression:  
\[\Phi(\lambda)=P\frac{\Phi_{\text{norm}}(\lambda)}{\int\Phi_{\text{norm}}( \lambda^{\prime})\,d\lambda^{\prime}}\,, \tag{98}\]  
such that \(\int\Phi(\lambda)\,d\lambda=P\) holds. \(\Phi_{1,\,\text{norm}}(\lambda)\), \(\Phi_{2,\,\text{norm}}(\lambda)\), and \(\Phi_{3,\,\text{norm}}(\lambda)\) used in this work are presented in Figure 4a. Note that Equation (98) implies that \(P\) does not need to be specified to calculate the channel DC components, i.e.,  
\[H_{\text{opt}}=\frac{P_{R}}{P_{T}}=\frac{\int\mathcal{A}(\lambda)\Phi(\lambda) \,d\lambda}{\int\Phi(\lambda)\,d\lambda}=\frac{\int\mathcal{A}(\lambda)\Phi_{ \text{norm}}(\lambda)\,d\lambda}{\int\Phi_{\text{norm}}(\lambda)\,d\lambda}\,, \tag{99}\]  
and  
\[H_{\text{el}}=\frac{I_{\text{ph}}}{P_{T}}=\frac{\int R(\lambda)\mathcal{A}( \lambda)\Phi(\lambda)\,d\lambda}{\int\Phi(\lambda)\,d\lambda}=\frac{\int R( \lambda)\mathcal{A}(\lambda)\Phi_{\text{norm}}(\lambda)\,d\lambda}{\int\Phi_{ \text{norm}}(\lambda)\,d\lambda}\,. \tag{100}\]  
The optical filters are the same as those used in the experimental CSK setup described in [10; 24], whose spectral transmittance can be found in [47] and is presented in Figure 4b. The spectral responsivity of the PD is also the same as the one used in [10; 24] and can be found in [48] and is presented in Figure 4c. Shotcrete is a special process used for mine support as part of the new Austrian tunneling method (NATM) [49], in which concrete is sprayed onto the tunnel walls. In consequence, concrete walls are ubiquitous in modern mines. The spectral reflectance of the concrete walls was taken from [50], assuming a high-water content concrete, and is presented in Figure 4d. The dust was assumed to be coal dust, with a refractive index taken from [30], found empirically in a coal mine in China, and presented in Figure 4e. The particle size distribution parameters of the coal dust were taken from [34], where empirical data from coal mines in the former Soviet Union were used. The work of [34] presents a wide range of possible coal dust concentrations based on different activities in the mine. A concentration of \(W_{p}=5000\) mg/m\({}^{3}\) was selected, which is about the highest concentration that can be obtained in coal mines without dust prevention measures according to [30] and is much higher than the maximum of 4 mg/m\({}^{3}\) defined by the China National Health and Wellness Committee [30]. However, [34] reports dust concentrations of up to 30 g/m\({}^{3}\) under certain mining activities. We use this high concentration value in the simulations to better exemplify the effect of dust in the communication channel. The obstacle dimensions were selected to be uniformly distributed in the ranges of \([0,1]\) m and \([0,2]\) m for the width and height, respectively. Additionally, the obstacle location was selected to be uniformly distributed in ranges of \([0,3]\) m and \([1,2]\) m for the x and y coordinates, respectively, meaning that shadowing only occurs in one half of the tunnel in our simulation.