## 6 Simulation Results and Analysis
The EFOGSM scheme outperformed OGSM, because the laser combination of EFOGSM changed, which was fully conducive to the redundancy of laser combination so that the same data could be transmitted through multiple lasers. The GSTBC-EFOGSM scheme performed better than the other regular OSM schemes because the GSTBC technique was added to the EFOGSM scheme, which achieved a significant enhancement in performance over the EFOGSM scheme alone. Furthermore, the simulation results demonstrated that in order to achieve an ABER of \(10^{-20}\) orders of magnitude, the SNR required for the GSTBC-EFOGSM system was 23 dB and 36 dB for coding ratios of 0.75 and 0.5, respectively. That is, when the coding ratio was changed from 0.75 to 0.5, the SNR of the system increased by 13 dB. By increasing the coding rate, the cost of engineering implementation can be effectively reduced, channel coding redundancy can be decreased and transmission efficiency can be improved. In this system, R = 0.75 is the more suitable option. Thus, through a comparison of these schemes, it becomes apparent that the GSTBC-EFOGSM scheme with R = 0.75 provides better performance gains. Figure 5 describes the ABER simulation diagram of GSTBC-EFOGSM and EFOGSM schemes under different data transmission rate \(R_{p}^{{}^{\prime}}\). The coding ratio of GSTBC-EFOGSM was set to 0.75. In order to achieve data transmission rates of 7 bpcu, 9 bpcu and 12 bpcu, respectively, \((M,N_{t},K,T)=(8,16,3,4)\), \((M,N_{t},K,T)=(16,16,3,4)\) and \((M,N_{t},K,T)=(64,16,3,4)\) were used in the proposed GSTBC-EFOGSM system, while \((M,N_{t})=(7,4)\), \((M,N_{t})=(8,4)\) and \((M,N_{t})=(32,4)\) were used in the EFOGSM system. The MC simulation results were used to confirm the accuracy of the numerical calculations. As shown in Table 3, when SNR = 30, the ABER of EFOGSM and GSTBC-EFOGSM systems gradually increased with the increase in Rp, and when the value of fixed Rp was 7 bpcu, the ABER of GSTBC-EFOGSM system was significantly smaller than that of EFOGSM system. Therefore, it can be concluded that increasing the transmission rate will also reduce the anti-interference ability of the system and increase the ABER of the system, so it is not possible to blindly increase the transmission rate of the system and thus lose the ABER of  
Figure 4: ABER comparison of GSTBC-EFOGSM, EFOGSM and OGSM schemes at the same data transmission rate. the system. Furthermore, the implementation of GSTBC coding technique substantially diminished the ABER of the GSTBC-EFOGSM system in contrast to the EFOGSM system. For instance, in achieving a \(10^{-10}\) ABER, the GSTBC-EFOGSM system yielded a coding gain of roughly 26 dB. This outcome highlights the superiority of utilizing the GSTBC technique in FSO systems. Figure 6 depicts the ABER simulation of the GSTBC-EFOGSM system with R = 0.75, featuring different photodetector numbers. The simulation results exhibited a satisfactory agreement with the MC simulation results, thereby verifying the accuracy of the proposed approach. As illustrated in Figure 6, when the photodetector number was Nr = 4, the system necessitated an SNR of 32 dB to achieve an ABER of \(10^{-15}\). However, when the number of photodetectors was appropriately augmented, for example, when Nr = 5, 6 and 7, the SNE of the system was 24 dB, 20 dB and 18 dB, respectively, for attaining the same ABER. Thus, the system attained performance gains of 8 dB, 12 dB and 14 dB, respectively. This outcome indicates that increasing the photodetector number can effectively enhance the system's performance. Moreover, the decline trend of the system's ABER became more pronounced with an increase in the photodetector number, signifying that a greater number of photodetectors leads to more prominent performance advantages for the system. Figure 5: ABER comparison between GSTBC-EFOGSM and EFOGSM schemes under different data transmission rates \(R_{p}\).