## 6 Simulation Results and Analysis  
This section presents simulation results of the GSTBC-EFOGSM FSO system with varying parameters. The performance of the proposed system was compared to that of the EFOGSM, OGSM, and OSM systems. Monte-Carlo simulations were employed to validate the numerical results. Under the M-distributed integrated atmospheric channel, each parameter selection of system is shown in Table 2. \begin{table}
\begin{tabular}{c c c} \hline \hline
**System Parameters** & **Symbol** & **Value** \\ \hline Light wavelength & \(\lambda\) & \(1550\) nm \\ \hline Link distance & d & \(1000\) m \\ \hline the beam bandwidth & \(w_{2}/r\) & \(25\) \\ \hline the jitter error & \(\delta_{s}/r\) & \(3\) \\ \hline Path loss & \(h_{t}\) & \(0.937\) \\ \hline a positive parameter related to effective large-scale cells in the & \(\alpha\) & \(6.37727\) \\ scattering process & & \\ \hline a natural number representing the number of fading parameters & \(\beta\) & \(5\) \\ \hline represents the amount of scattered power coupled with the LOS & \(\eta\) & \(0.5\) \\ \hline The average power of the total scatter components & \(2b_{0}\) & \(0.5\) \\ \hline the average optical power of the LOS term & \(\Omega\) & \(0.5\) \\ \hline The difference between the determined phase of LOS and coupled & \(\phi_{\rm A}-\phi_{\rm B}\) & \(\pi/2\) \\   to the LOS scattering term & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Selection of simulation parameters for GSTBC-EFOGSM FSO system. Figure 3 shows the data transmission rate \(R_{p}^{{}^{\prime}}\) simulation for different optical spatial modulation schemes with different number of lasers \(\rm{N_{t}}\). For OSM, OGSM and the main signal constellation of EFOGSM and GSTBC-EFOGSM, the modulation order of M-PAM is M = 8. 4-PAM was used as a secondary signal constellation for EFOGSM and GSTBC-EFOGSM. When \(\rm{N_{t}}=10\), the figure illustrates that the transmission rates for OSM, OGSM, EFOGSM, and GSTBC-EFOGSM schemes were 6 bpcu, 8 bpcu, 14 bpcu and 10.5 bpcu, respectively. The data transmission rate of GSTBC-EFOGSM and EFOGSM schemes was higher than that of OSM and OGSM. In addition, Figure 3 shows that the data transfer rate improvement of OSM and OGSM was limited when \(\rm{N_{t}}\) increased. The transmission rate of EFOGSM and GSTBC-EFOGSM could be made faster with an increase in \(\rm{N_{t}}\). This is because all possible laser combinations were fully utilized in GSTBC-EFOGSM and EFOGSM, and the transmission rate was improved by the change of the digital signal constellation and the number of lasers. Although the transmission rate of GSTBC-EFOGSM was slightly lower than that of EFOGSM, this is because the GSTBC-EFOGSM system divided the input bit into K blocks and performed EFOGSM modulation at the same time, so the transmission rate of GSTBC-EFOGSM system was lower than that of EFOGSM system using only one modulation. However, GSTBC-EFOGSM had a higher and faster transmission rate than other OSM schemes, and also had higher diversity gain than EFOGSM in performance. Therefore, by comparing these schemes, we can see that GSTBC-EFOGSM has a higher data transmission rate. Figure 4 shows the simulation graph of ABER performance comparison of GSTBC-EFOGSM, EFOGSM and OGSM schemes at the same data transmission rate. In order to achieve a data transmission rate of 9 bpcu, the GSTBC-EFOGSM system used \((M,N_{t},K,T)=(16,16,3,4)\) and \((M,N_{t},K,T)=(256,16,4,8)\), respectively, when the coding ratio R = 0.75 and 0.5. The accuracy of numerical calculations was confirmed by the results of MC simulation. From the simulation results, we can see that the GSTBC-EFOGSM scheme had better ABER performance than EFOGSM and OGSM. When the Signal-to-Noise Ratio (SNR) of GSTBC-EFOGSM R = 0.75 was 13 dB, the obtained ABER was about \(10^{-10}\). In order to obtain the same ABER, the SNR under EFOGSM and OGSM schemes were  
Figure 3: Data transmission rate \(R_{p}^{{}^{\prime}}\) for different OSM with different number of lasers \(\rm{N_{t}}\). 40 dB and 45 dB, respectively.