## 5 Numerical Results
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Optical wavelength (\(\lambda\)) & 1550 nm \\ Symbol duration (\(\tau\)) & 1 us \\ PD efficiency (\(\eta\)) & 0.5 \\ IRS panel energy reflection efficiency (\(\zeta\)) & 1 \\ End-to-end distance (\(d_{e2e}\)) & 800 m \\ Horizontal angle of emitted beam (\(\theta_{L}\)) & \(\pi/4\) \\ Horizontal angle of IRS panel (\(\theta_{IRS}\)) & \(\pi/10\) \\ Horizontal angle of PD panel (\(\theta_{P}\)) & \(\pi/3\) \\ PD aperture radius (\(r_{p}\)) & 2.5 cm \\ Area of an IRS unit & \(10\times 10\) cm\({}^{2}\) \\ Maximum number of iterations (\(T_{\max}\)) & 30 \\ Scintillation parameters (\(\alpha,\beta\)) & 1, 1 \\ Background radiation energy per bit (\(E_{nb}\)) & \(-170\) dBJ \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters of the system. In Figure 5, we simulate the MSEs of several baseline schemes alongside our proposed method with varying numbers of IRS units. Notably, the MSE for all four schemes decreases as the number of IRS units increases. However, for the randomized IRS configuration scheme, the downward trend becomes less obvious after \(N=32\). This occurs because the randomized configuration fails to effectively leverage the additional IRS units to enhance communication performance. As the number of IRS units increases significantly, the  
Figure 4: Comparison of normalized MSE of different schemes, where \(M=16,K=4\) and \(N=64\). Figure 3: Convergence performance comparison of the proposed AO algorithm and the scheme that only optimizes the precoding matrix. potential benefits diminish due to the ineffective scheduling of the IRS. In contrast, the MSE for our proposed algorithm demonstrates a consistent and significant decline with the increasing number of IRS units. This trend is expected, as a greater number of IRS units expands the feasible space of the NLOS channel matrix, allowing our IRS configuration scheme to fully capitalize on the benefits provided by the additional units and thereby enhancing the overall system performance. Additionally, Figure 6 presents the BER performance of our AO algorithm. It is evident that, as the number of emitting LEDs increases, the average BER for users decreases significantly. Specifically, when the transmission energy per bit is \(E_{b}=-163\) dBJ, the BER performance of a \(16\times 4\) system can reach \(10^{-5}\). These results demonstrate the effectiveness of our precoding design scheme, which leverages diversity gain to enhance the system's resilience against turbulent fading. We also investigated the BER performance of the system with varying numbers of IRS units (i.e., 12, 24, 40, and 60), as depicted in Figure 7. It is evident from the figure that, when the number of IRS units is small, the NLOS channel provided by the IRS struggles to achieve satisfactory communication performance. Specifically, the BER does not exhibit a significant downward trend with increasing transmission energy per bit, \(E_{b}\). For instance, with only 12 IRS units, even at a transmitted signal energy per bit of \(E_{b}=-163\) dBJ, the average BER remains around \(10^{-1}\). However, as the amount of IRS units rises, the system's average BER exhibits a rapid decline with a rising \(E_{b}\), demonstrating that our proposed method effectively configures all IRS units to achieve excellent BER performance. Specifically, when 60 IRS units are configured and the transmitted signal energy per bit, \(E_{b}\), is \(-163\) dBJ, the average BER of the system can reach \(10^{-5}\). Figure 5: Comparison of normalized MSE of different schemes under different numbers of IRS units, where \(M=16,K=4\) and \(E_{b}=-170\) dBJ. In addition, considering the influence of environmental factors and uncertainties, we also design experiments to verify the robustness of the algorithm. As shown in Figure 8, we explored the effect of turbulence intensity on the average BER performance of the proposed  
Figure 6: Average BER performance of AO algorithm under different transmitting LEDs, where \(N=36\).