# Relay Selection Schemes for FSO Communications over Turbulent Channels
## 4 Simulation Results
This SNR gain is due to the selection of the best relay depending on the long distance, R-D, which has the dominant effect. Using three relays and with link type 500-500 m, the ABER versus SNR for moderate turbulence is plotted in Figure 6. It is clear that S-R-D scheme has higher SNR gain than S-R and R-D schemes by 4.5 dB and 4.9 dB, respectively. This SNR gain is due to the selection of the best relay that happens depending on the long distance, S-R-D, which has the dominant effect. Figure 4: Average bit error rate (ABER) vs. SNR of Free Space Optics (FSO) over L-N channel for weak turbulence with distance 400–600 m, and the number of relays = 3, and w\({}_{z}\) = 2 m.  
Figure 5: ABER vs. SNR of FSO over L-N channel for moderate turbulence with distance 400–600 m, the number of relays = 3, and w\({}_{z}\) = 2 m.  
The first hop is affected by turbulence more than the second hop as the turbulence increases with the propagation distance assuming that all other parameters are fixed in the 600 m-400 m scheme, as depicted in Figure 7. The S-R scheme, which activates a relay per time slot according to the first hop channel effects, is the best scheme when compared with the other schemes, as it works to improve the first hop SNR gain, which has the dominant effect on the overall system. The obtained results show that the S-R scheme has higher SNR gain than the S-R-D and R-D schemes by 0.5 dB and 6.9 dB, respectively. This SNR gain is due to the selection of the best relay depending on the long distance, S-R, which has the dominant effect. Figures 8-10 show the great effect of the pointing error on the link compared to the weak pointing error effect on Figures 5-7. The S-R-D scheme is the best choice in case of high pointing error effect. It has higher SNR gain than R-D and S-R schemes for different hops distance. The utilization is as follows. If we have high pointing error as a result of vibration of building or wind, we use S-R-D, especially if we don't have an automatic pointing control. This SNR gain is because the pointing error in this condition is higher than the effect of turbulence and the pointing error is not a function of distance. So, it has the same effect on the two hops. Figure 6: ABER vs. SNR of FSO over L-N channel for moderate turbulence with distance 500–500 m, the number of relays = 3, and w\({}_{\rm z}\) = 2 m.  
Figure 7: ABER vs. SNR of FSO over L-N channel for moderate turbulence with distance 600–400 m, the number of relays = 3, and w\({}_{\rm z}\) = 2 m.  
Figure 8: ABER vs. SNR of FSO over L-N channel for moderate turbulence with distance 400–600 m, the number of relays = 3, and w\({}_{z}\) = 0.5 m.  
Figure 10: ABER vs. SNR of FSO over L-N channel for moderate turbulence with distance 600–400 m, the number of relays = 3, and w\({}_{z}\) = 0.5 m.  
Figure 9: ABER vs. SNR of FSO over the L-N channel for moderate turbulence with distance 500–500 m, the number of relays = 3, and w\({}_{z}\) = 0.5 m.  
Figure 8 shows that the S-R-D scheme has higher SNR gain than the R-D and S-R schemes by 2.8 dB and 6.9 dB, respectively, due to the high mitigation of pointing error effect by the S-R-D scheme. In Figure 9, it is noted that the S-R-D scheme has higher SNR gain than the R-D and S-R schemes by 5.7 dB and 5.7 dB, respectively, because the relay is in the middle of the total distance between the source and destination. Figure 10 shows that the S-R-D scheme has higher SNR gain than the S-R and R-D schemes by 3.2 dB and 6.9 dB, respectively, over a strong turbulent channel. Figures 11-13 study the effect of strong turbulence with weak and strong pointing error. When comparing Figure 6, considering moderate turbulence, with Figure 11, and accounting for strong turbulence, the obtained results show that S-R-D needs only more 3 dB to achieve the same ABER while R-D and S-R need 10 dB to achieve the same ABER. Moreover, Figure 11 shows that the S-R-D scheme has higher SNR gain than the R-D and S-R schemes by 11.3 dB. This is due to the high mitigation of the strong turbulence effect by the S-R-D scheme.