**5. Optical link budget**

After illustrating the geometric loss, total attenuation, and haze effects on the FSO in Sana'a, Aden and Taiz cities, we return to the link budget of FSO systems. This section concentrates on received power versus low and average visibility, and link range. Table 11 illustrates the main FSO link parameters. We note that all the given values in the following table are presumed to calculate the received power for three cases as presented in Figs. 27−29.


**Table 11.** Optical link budget parameters.

**Figure 26.** Atmospheric attenuation (dB) versus low visibility (km) for Taiz city.

**scattering coefficient/km-1 atmospheric attenuation/dB from to from to**

780 nm 11.37 0.45 49.4 2 850 nm 10.99 0.41 47.7 1.8 1550 nm 8.69 0.21 37.7 0.94

780 nm 44. 8 0.45 194.4 1.95 850 nm 43.8 0.41 190.2 1.8 1550 nm 37.6 0.22 163.5 0.94

780 nm 44.8 0.7 194.4 3.1 850 nm 43.8 0.65 190.2 2.8 1550 nm 37.6 0.37 163.5 1.6

**Table 10.** Results of scattering coefficient and atmospheric attenuation at low visibility for Sana'a, Aden and Taiz cities.

After illustrating the geometric loss, total attenuation, and haze effects on the FSO in Sana'a, Aden and Taiz cities, we return to the link budget of FSO systems. This section concentrates on received power versus low and average visibility, and link range. Table 11 illustrates the

**city wavelength**

200 Contemporary Issues in Wireless Communications

Sana'a

Aden

Taiz

**5. Optical link budget**

The results presented in Fig. 27 show the relationship between received power for three different wavelengths and low visibility. As seen in Fig. 27, received power increases with the increment of low visibility. We note that the obtained received power at the wavelength of 1550 nm is the best as compared to the other two. For example, the received power curve for the wavelength of 1550 nm increases from −67 dBm at the distance of 0.6 km to –27 dBm at the distance of 5 km. However, we note that the receiving power is reduced for other two wavelengths of 780 and 850 nm. As shown in Fig. 28, the received power at wavelength of 1550 nm shows the best compared to other two wavelengths. While the received powers at the wavelengths of 780 and 850 nm are, lower.

**Figure 27.** Received power (dBm) versus low visibility (km).

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

**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.
