## 6 Simulation Results and Analysis
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Rp** & **EFOGSM** & **GSTBC-EFOGSM** \\ \hline
7 bpcu & \(7.46\times 10^{-9}\) & \(1.09\times 10^{-23}\) \\
9 bpcu & \(1.49\times 10^{-8}\) & \(4.64\times 10^{-21}\) \\
12 bpcu & \(2.39\times 10^{-7}\) & \(1.48\times 10^{-18}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Table of ABER performance comparison between EFOGSM and GSTBC-EFOGSM with SNR = 30. Figure 7 shows the energy efficiency of different optical spatial modulation systems under the condition of M = 256. In the optical spatial modulation mapping, the required transmission bits of the system part were transmitted in the spatial domain to improve the energy efficiency in the communication transmission. As shown in Figure 7, when N\({}_{\rm{r}}\) = 6, the energy efficiency of OSM, OGSM, EFOGSM and GSTBC-EFOCSM iwa 24%, 33%, 87% and 50%, respectively. For N\({}_{\rm{r}}\) = 8, OSM, OGSM, EFOGSM and GSTBC-EFOCSM can achieve 27%, 38%, 112% and 67% energy savings, respectively. The energy efficiency of EFOGSM and GSTBC-EFOCSM was significantly higher than that of other conventional OSM schemes, and this high performance was a feature brought by the low number of bits transmitted by the system communication. This is because EFOGSM and GSTBC-EFOCSM schemes used two signal constellations in the modulation process. Therefore, their spatial domain used two sets of laser mapping combination to transmit information bits, while other optical spatial modulation schemes only used one set of laser mapping combination scheme to index information. Although the energy efficiency of GSTBC-EFOCSM was slightly lower than that of EFOGSM, it was also 1.5 times of that of OGSM. Moreover, the coding technology added in GSTBC-EFOCSM brought about a stronger overall performance of the system. Therefore, by comparing these schemes, we can see that GSTBC-EFOCSM has higher energy efficiency. The computational complexity analysis of the ML decoder in the receiver for various optical spatial modulation systems is illustrated in Figure 8. All schemes had an R\({}_{\rm{P}}\) of 9 bpcu and used the same number of lasers. Figure 8 shows that when N\({}_{\rm{r}}\) = 4, the computational complexities of EFOGSM, GSTBC-EFOCSM, OSM and OGSM schemes were 864, 12,096, 18,432 and 36,864, respectively. The EFOGSM scheme obviously greatly reduced the receiver computational complexity of the conventional OSM scheme, which was reduced by more than 90% compared with OSM and OGSM. The reason for this is that EFOGSM offered orthogonal signals that minimized the complexity of the ML decoder by transmitting two distinct signal constellations. Therefore, the computational complexity of the receiver was significantly lower than that of other OSM schemes. Although GSTBC  
Figure 6: ABER comparison of the GSTBC-EFOCSM R = 0.75 system with different receiver photodetectors N\({}_{\rm{r}}\). EFOGSM was less complex than EFOGSM, it was 34% less complex than OSM and 67% less complex than OGSM. Moreover, GSTBC-EFOGSM had the performance advantage of diversity gain compared to EFOGSM, which also compensated for its slightly higher complexity. Therefore, by comparing these schemes, it is clear that GSTBC-EFOGSM has lower receiver computational complexity. Figure 8: Computational complexity of the receiver under different optical spatial modulation systems. Figure 7: Energy efficiency \(\eta\) under different optical spatial modulation systems.