## 3 Results and Discussion  
In this section, the results from the modeling of the proposed MDM-OCDMA-FSO system using MNZCC code are presented and discussed under different scintillation effects and clear air weather conditions using OptiSystem v.19 simulation software. We considered two cases for the transmission link. In the first case, the FSO range was fixed at 100 m, whereas the multimode fiber length was varied from 320 m to 420 m. In another case, the MMF range was fixed at 100 m and the FSO range was varied from 1.5 to 2.2 km. Figure 3 presents the BER performance of the proposed MDM-OCDMA-FSO system under clear weather conditions. The scintillation is considered ideal in this case. With increase in fiber length, the BER value increases and, hence, the system performance decreases for all users. User 1, User 2, User 3 and User 4 achieve a BER of less than \(10^{-3}\) at the MMF lengths of 380, 375, 365 and 380 m, respectively, for Donut mode 0, as shown in Figure 3a. Similarly, for Users 5 to 8 operating on Donut mode 1, an acceptable BER \(10^{-3}\) is achieved for User 5, User 6, User 7 and User 8 at the fiber lengths of 360, 385, 380 and 390 m, respectively, as shown in Figure 3b. The measured eye spectrums at 320 m fiber length show the successful transmission of all the users. Further, it is observed that performance of the hybrid fiber-FSO link is restricted due to the presence of fiber non-linearities and noise. Thus, the obtained faithful MMF fiber range is 380 m with a 100 m FSO range, in the presence of fiber interference and an FSO weak turbulence effect. Figure 4 shows the measured BER plots where the MMF length is fixed at 100 m and the FSO range is varied. The figure shows that User 1, User 2, User 3 and User 4 achieve a BER of less than \(10^{-3}\) at the MMF lengths of 2.05 km, 2.07 km, 2.10 km and 2 km, respectively, as shown in Figure 3a. Similarly, as shown in Figure 3b, an acceptable BER of \(10^{-3}\) is achieved for User 5, User 6, User 7 and User 8 at FSO ranges of approximately 2.05 km, 2.07 km, 1.95 km and 1.9 km. From Figures 3 and 4, it can be seen that users which are operated on Donut mode 0 achieve good BER compared to users which are operated on Donut mode 1. Furthermore, the performance of the proposed OCDMA-based MDM-FSO-MMF was also investigated under the impact of scintillations. It is shown that the performance of the hybrid fiber-FSO link is limited due to the presence of FSO channel interference and turbulence effects. Therefore, the maximum achieved FSO range  
Figure 3: Measured BER at fixed FSO link of 100 m under clear air conditions. (**a**) Users 1 to 4 operating on Donut mode 0 and with corresponding eye patterns at 320 m fiber length, (**b**) users 5 to 8 operating on Donut mode 1 with corresponding eye patterns at 320 m fiber length. is 2.10 km with 100 m MMF fiber, in the presence of fiber interference and an FSO weak turbulence effect. For the impact of scintillations, both gamma-gamma and log-normal modeling are considered. In the case of gamma-gamma modeling, Figure 5a,b present the BER performance of the proposed OCDMA-based MDM-FSO-MMF system using MNZCC code over a fixed 300 m fiber along with a variable FSO range. Eight users at Donut mode 0 and Donut mode 1 under both weak (10\({}^{-17}\) m\({}^{-2/3}\)) and strong turbulences (10\({}^{-13}\) m\({}^{-2/3}\)) are considered. It can be seen that, as the atmospheric turbulence strength increases, the BER values again increase over a varied FSO range from 700-2100 m. The figure also shows that User 1 and User 5 in the system at Donut mode 0 and 1, respectively, exhibit the best performance out of all eight users under both weak and strong turbulences. The value of an acceptable BER (\(\leq\)10\({}^{-3}\)) is computed for User 1, User 2, User 3 and User 4 under the impact of weak turbulences at the FSO transmission ranges of 2100, 1800, 1900 and 1500 m, respectively.