## 2 System and Channel Model
### Atmospheric Turbulence Model  
The parameter \(I_{a}\) follows the \(M\)(alaga) model with a PDF specified by [PERSON] et al. in [11]:  
\[f_{I_{a}}(I_{a})=A_{(\mathbb{N}\,\mathrm{or}\,\Re)}\sum_{(\mathbb{N}\,\mathrm{ or}\,\Re)}a_{k(\mathbb{N}\,\mathrm{or}\,\Re)}\,I_{a}^{\frac{a+k-2}{2}}K_{a-k}(2 \sqrt{B_{(\mathbb{N}\,\mathrm{or}\,\Re)}I_{a}}), \tag{3}\]  
where the subscripts \(\mathbb{N}\) or \(\Re\) relate to the type of parameter \(b\) being a natural or a real number [12]. Thus, for \(b\in\mathbb{N}\), the summation of Equation (3) corresponds to \(\sum_{(\mathbb{N})}\left[\cdot\right]\equiv\sum_{k=1}^{b}\left[\cdot\right]\), while the other parameters are given as:\[\begin{array}{l}A_{(\mathbb{R})}\,=\,\frac{2a^{\frac{d}{2}}(kc)^{b+\frac{c}{2}}}{c ^{\frac{d+2}{2}}}\,\Gamma(a)\,(bc+\Omega)^{b+\frac{c}{2}}},\\ B_{(\mathbb{R})}\,=\,\frac{ab}{bc+\Omega},\\ a_{k(\mathbb{R})}\,=\,\left(\begin{array}{c}b-1\\ k-1\end{array}\right)\,\frac{(bc+\Omega)^{1-\frac{b}{2}}}{(k-1)!}\left(\frac {\Omega}{c}\right)^{k-1}\left(\frac{a}{b}\right)^{\frac{k}{2}}.\end{array} \tag{4}\]  
On the other hand, for \(b\in\Re\), the summation is specified as \(\sum_{(\mathbb{R})}\left[\cdot\right]\equiv\sum_{k=1}^{\infty}\left[\cdot\right]\), while the remaining parameters are determined by:  
\[\begin{array}{l}A_{(\mathbb{R})}\,=\,\frac{2\,a^{\frac{d}{2}}(bc)^{b}}{c^{ \frac{d+2}{2}}\,\Gamma(a)\,(bc+\Omega)^{b}},\\ B_{(\mathbb{R})}\,=\,\frac{a}{c},\\ a_{k(\mathbb{R})}\,=\,\frac{(b)_{k-1}(a)c^{\frac{b}{2}}}{[(k-1)!]^{2}c^{k-1}( k+\Omega)^{k-1}},\end{array} \tag{5}\]  
where \((b)_{k}\) denotes the Pochhammer symbol (Equation (06.10.02.0001.01) of Wolfram Function site in [13]). Moreover, \(K_{\wp}(\cdot)\) is the _v-th_ order modified Bessel function of the second kind (Equation (8.432.2) of [PERSON] and Ryzhik in [14]),and \(\Gamma(\cdot)\) is the gamma function (Equation (8.310.1) [14]). We should recall here, according to the definition of the \(M\)(alaga) distribution, that the observed field at the receiver side is supposed to consist of three terms, i.e., the line-of-sight (LOS) component, the component coupled to LOS that is quasi-scattered forward by eddies on the propagation axis, and a scatter component that is statistically independent from the previous two terms, owing to the energy scattered to the receiver side by off-propagation axis eddies [11]. Thus, \(a\) reflects a positive parameter related to the effective number of large-scale cells characterizing the scattering process, and \(c\) equals \(2b_{0}(1\,-\,\rho)\)[12]. The values of \(b_{0}\) and \(\rho\) depend on the total scatter components, while \(\Omega\) is the average optical power of the coherent contribution, which consists of LOS and scattering terms coupled to the LOS term. Specifically, the parameter \(\rho\), where \(0\leq\rho\leq 1\), indicates the relationship between the two scattering components of the \(M\)(alaga) model, representing the amount of scattering power coupled to the LOS component. Meanwhile, \(b\) corresponds to the amount of fading. Table 1 indicates that the \(M\)(alaga) distribution has the concrete advantage of unifying most of the well-known turbulence models reported by [PERSON] et al. in [11].