# Effects of Atmospheric Turbulence on Optical Wireless Communication in NEOM Smart City
## 7 Results and Discussion
The results obtained are in agreement with the experimental work in [63], which showed the increase of scintillation with increasing of temperature for the wavelength of 632 nm. For example at \(T=25~{}^{\circ}\)C and \(T=30~{}^{\circ}\)C, the scintillation \(\sigma_{I}^{2}=1.4\) and \(\sigma_{I}^{2}=2.1\) respectively. This result is twice the value we obtain for the same meteorological parameters for November and June, as the wavelength we used is 1550 nm. Figure 7 shows the variation in the beam spreading at different transmission distances for \(\lambda=1550\) nm. This value of the wavelength was selected owing to the corresponding resilience to atmospheric turbulence. The range of beam spreading was calculated using Equation (8), the maximum \(\sigma_{I}^{2}\) in summer (June), and minimum value in winter (February). Figure 7 shows a comparison of the beam spreading in strong turbulence during summer, moderate turbulence during winter, and in a scenario without turbulence. The higher scintillation in summer leads to approximately 28% higher beam spreading than that in the winter season. This shows the effect of temperature on the beam spreading, which agrees with the results in [63]. Furthermore, the spot size of the beam depends on the transmission distance. Increasing distance leads to higher beam spreading. However, the spot size of the received signal remains stable up to 300 m, and the atmospheric turbulence leads to negligible effects. Figure 5: Variation in the Refractive Index Structure with the Wind Speed and Temperature. The BER of transmission depends on the average received power and scintillation strength. Assuming the target of BER as \(10^{-9}\), Figure 8 shows that on-off keying (OOK) outperforms higher-order modulation, and it also is a cheap and simple modulation technique, which is the reason that we chose it as the proposed modulation technique. Figure 9 illustrates the validation of the employed analytical model against the simulation results. Figures 10 and 11 show the results of a MATLAB simulation conducted to examine the BER performance of the FSO system at different SNRs in both seasons at different propagation distances (from 100 m to 1000 m). The BER performance in winter, with the LN channel used to model the moderate turbulence, and in summer, with the G-G channel used to model the strong turbulence, respectively. Results show that under large propagation distances and strong turbulence, high power is required to achieve the target BER. Moreover, the required SNR is similar for distances ranging from 100 to 300 m in both seasons. Figure 6: Comparison of Scintillation Index for Different Wavelengths at L = 1000 m.  
Figure 7: Comparison of the Beam Spreading for Different Transmission Distances. Figure 8: BER Performance for the Proposed System using Different Modulation Order for M-ary PAM in Winter, Modeled using the LN Channel, for 1 km. Figure 9: Comparison of BER Determined Analytically and Through Simulations for Gamma–Gamma and Log-Normal Channels. Figure 12 and 13 show the availability of the proposed system for winter and summer, respectively. Results show that to achieve availability of 99.9 %, a power margin of 9 dB and 17.38 dB must be achieved for winter and summer, respectively. Figure 11: BER Performance for the Proposed System using OOK in Summer, Modeled using the G-G Channel, for Different Propagation Distances. Figure 10: BER Performance for the Proposed System using OOK in Winter, Modeled using the LN Channel, for Different Propagation Distances.