**6. Simulation results of BER and SNR**

**Figure 28.** Received power (dBm) versus average visibility (km).

202 Contemporary Issues in Wireless Communications

**Figure 29.** Received power (dBm) versus link range (km).

Figure 29 shows the received power versus the link range. As the link range between trans‐ mission and receiver increases, the received power decreases. At the distance of 0.5 km, the received power for the wavelength of 1550 nm is of –20.3 dBm where for the others two are of –21.7 dBm. However, in the distance of 5 km, the received power reaches –36 dBm for wavelength of 780 nm and –34.1 dB for the wavelength of 850 nm. For three study cases, the study was done to improve the efficiency of FSO systems, the wavelength of 1550 nm for three cases must be used and the distance between transmitter and receiver should be reduced.

The data was taken from the Civil Aviation and Meteorology Authority and the Yemeni Meteorological Service. The work includes the analysis of these real data. The purpose here is to discuss the relationship for calculating the variance, SNR, and BER for a range of parameters. We used the wavelengths of 850, 1000, and 1550 nm. Particular attention was given to the 1550 nm wavelength since it is commonly used as the 3rd window of optical communication backbone links. Moreover, being significantly bigger than visible wavelengths, the human retina in particular and the components of the eye in general are less sensitive to the 1550 nm wavelength. Thus, this wavelength is appropriate for eye safety.

Figure 30 illustrates the log intensity fluctuations versus the link range between transmitter and receiver for three values of wavelengths. The log intensity fluctuation depends on the wavelength and increases with the propagation distance. As the transmission range increases the variance (atmospheric turbulence) increases too. For a 2000 m transmission range, the variance is about 0.17 for the wavelength of 850 nm, 0.12 for 1000 nm, and 0.075 for 1550 nm. For a 4000 m transmission range, the variance is about 0.56 for 850 nm, 0.42 for 1000 nm, and 0.25 for 1550 nm. These results show that the use of a wavelength of 1550 nm can reduce the variance "atmospheric turbulence" effect on the FSO systems [37].

**Figure 30.** Intensity fluctuations against transmission range.

Figure 31 indicates the comparison between the beam spreading on a distance l from the transmitter in case of atmospheric turbulence and in case without atmospheric turbulence. The spot size of the beam at the transmitter (with the distance l = 0) equals 0.008 m. At the distance 200 m, the spot size of the beam is ω (l) = 0.015 m in case of absent turbulence and ωeff (l) = 0.015 m in case of turbulences. At the distance 5000 m, the ω (l) = 0.31 m and ωeff (l) = 0.33 m. From the above results, we conclude that the expansion of the spot size of the beam depends on the distance between sender and receiver as indicated on Fig.32, and on the atmospheric turbulence along the transmission range as indicated on Fig. 33. The higher the turbulence is, the greater the expansion of the beam size is. Figure 34 shows the SNR versus the transmission range of 0 to 4500 m. As the link range between the transmitter and receiver increases, the SNR decreases. This means that the increment of link range is able to decrease the transmission quality and efficiency of FSO systems. At a low range of 200 m, the SNR is about 74 dB for 850 nm, 77 dB for 1000 nm, and 82 dB for 1550 nm. For 4000 m, the SNR is about 18 dB for 850 nm, 21 dB for 1000 nm, and 26 dB for 1550 nm.

**Figure 31.** Beam spreading versus transmission range.

**Figure 32.** Waist of beam with turbulence versus scintillation.

**Figure 33.** SNR versus transmission range.

turbulence along the transmission range as indicated on Fig. 33. The higher the turbulence is, the greater the expansion of the beam size is. Figure 34 shows the SNR versus the transmission range of 0 to 4500 m. As the link range between the transmitter and receiver increases, the SNR decreases. This means that the increment of link range is able to decrease the transmission quality and efficiency of FSO systems. At a low range of 200 m, the SNR is about 74 dB for 850 nm, 77 dB for 1000 nm, and 82 dB for 1550 nm. For 4000 m, the SNR is about 18 dB for 850 nm,

21 dB for 1000 nm, and 26 dB for 1550 nm.

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**Figure 31.** Beam spreading versus transmission range.

**Figure 32.** Waist of beam with turbulence versus scintillation.

Figure 34 shows the BER versus the transmission range. As the link range between transmis‐ sion and receiver increases, the BER increases too. At 2500 m link range, the BER is about 10– 4 for 850 nm, 10–6 for 1000 nm, and 10–9 for 1550 nm. At 4000 m, the BER is 10–2 for 850 nm, 10– 3 for 1000 nm, and 10–4 for 1550 nm. If we want an acceptable communication BER of 10–9, the maximum link range between transmitter and receiver should be about 1600 m for 850 nm, 1900 m for 1000 nm, and 2500 m for 1550 nm.

**Figure 34.** BER versus transmission range.

**Figure 35.** SNR with and without turbulence, SNRref and SNR, respectively, versus transmission range.

Figure 35 indicates SNR versus expansion of the beam size resulting from air turbulence. For a beam size of ωeff (l) = 0.015 m, the SNR = 64 dB and SNReff = 62 dB, but for ωeff (l) = 0.33 m, the SNR = 4.7 dB and SNReff = 3.4 dB. From these results, we conclude that when the beam expands, the loss in terms of the beam intensity increases. This leads to the decrease in the SNR value, and therefore the BER increases as indicated in Fig. 36. For a spot size of 0.015 m, the BER = 10–115, and when the spot size of the beam is 0.33 m, the BER increases up to 10–5 approximately. From the results above, we conclude that the narrow beam shows a limited effect of the atmospheric turbulence on the intensity.

Figure 37 shows the BER versus link range between transmitter and receiver. This figure graphically represents the BER as a function of the irradiance variance. For 3500 m, BER is 10–6 for the SNR and 10–5 for the SNReff. For an irradiance variance 0.05 the BER 10–6 for the SNR and 10–5 for the SNReff. From the results obtained, we conclude that to improve the performance of the FSO transmission systems, it is recommended to shorten the link range between transmitter and receiver. Another improvement of the signal quality offered by the FSO systems includes using the 1550 nm wavelength. The SNR of FSO systems with 1550 nm wavelength is higher than that corresponding to 1000 and 850 nm wavelengths. To reduce the atmospheric turbulence effects on FSO systems, we suggest using the 1550 nm wavelength. Moreover, for the 1550 nm wavelength, the allowable power is largely higher compared to smaller wavelengths (about 50 times compared to 850 nm). This shows that the system operates well during heavier attenuation of the atmosphere since we can safely increase power at the source.

**Figure 36.** BER versus waist of beam with turbulence.

**Figure 37.** BER versus transmission range for SNR and SNReff.
