*4.1.2.1. Total attenuation during hazy days*

Figure 15 shows total attenuation at low visibility. We used Eq. (8) to plot Fig. 15. Here, we assume that link range is 1 km. When visibility is 0.8 km, total attenuations are 31.8, 31 and 26.4 dB at wavelengths of 780, 850 and 1550 nm, respectively. And when visibility is 5 km, total attenuation are 16.8, 16.4, and 15.4 dB for wavelengths of 780, 850 and 1550 nm, respectively. It is suggested that the more visibility is the least effects of total attenuations on FSO perform‐ ance.

**Figure 15.** Total attenuation (dB) versus low visibility (km).

Figure 16 indicates the total attenuation versus average visibility. When visibility is 6.4 km, total attenuation is 15.96, 15.8 and 14.9 dB at wavelengths of 780, 850 and 1550 nm, respectively. Note that when visibility is 9.7 km, total attenuation are of 15.39, 15.28 and 14.7 dB at wave‐ lengths of 780, 850 and 1550 nm, respectively. Based on the previously mentioned, we conclude that total attenuation at wavelength 1550 nm is less than that at wavelengths of 780 and 850 nm. Therefore, to reduce the effect of total attenuation during hazy days, we use the wave‐ length of 1550 nm.

**Figure 16.** Total attenuation (dB) versus average visibility (km).

*4.1.2. Total attenuation*

192 Contemporary Issues in Wireless Communications

part of FSO design.

Section 4.1.1.

ance.

*4.1.2.1. Total attenuation during hazy days*

**Figure 15.** Total attenuation (dB) versus low visibility (km).

Total attenuation depends on attenuation resulted from hazy and rainy days and geometric loss. The attenuation in hazy days depends on visibility, while during rainy days it would be determined by rainfall rate. Visibility range changes with the quantity and density of particles, such as fog, haze and dust attached to air. The higher the density of these particles is, the less visibility is and total attenuation increases. The density of these particles is not fixed. It keeps varying with time and place as well as rainfall. The quantity and density of these particles are unpredictable, and visibility and rainfall rate are also uncontrollable. Thus, they are all not

We can control the value of geometric loss, because it depends on fixed parameters such as transmitter diameter and receiver apertures, transmission range, and beam divergence. During the design of FSO system, geometric loss must be at minimum to reduce the effect of total attenuation on FSO system. In this part, we used design 2 as demonstrated in Table 6 to calculate total attenuation because this design geometric loss is less as described above in

Figure 15 shows total attenuation at low visibility. We used Eq. (8) to plot Fig. 15. Here, we assume that link range is 1 km. When visibility is 0.8 km, total attenuations are 31.8, 31 and 26.4 dB at wavelengths of 780, 850 and 1550 nm, respectively. And when visibility is 5 km, total attenuation are 16.8, 16.4, and 15.4 dB for wavelengths of 780, 850 and 1550 nm, respectively. It is suggested that the more visibility is the least effects of total attenuations on FSO perform‐

**Figure 17.** Total attenuation (dB) versus link range (km).

Figure 17 represents total attenuation versus link range. From this figure, we found that total attenuation directly proportions with link range. When link range is 0.5 km, total attenuation is 17.3, 16.9 and 14.6 dB at wavelengths of 780, 850 and 1550 nm, respectively. In addition, when link range is 5 km, total attenuation becomes 115.0, 111.8 and 88.4 dB at wavelengths of 780, 850 and 1550 nm, respectively. Therefore, to reduce the effect of total attenuation on FSO, the distance between the transmitter and receiver shall be small. Figure 18 shows the relation‐ ship between total attenuation and laser beam divergence for three wavelengths. With increasing the beam divergence, the total attenuations are increased for three cases as dem‐ onstrated in Fig. 18.

**Figure 18.** Total attenuation (dB) versus laser beam divergence (mrad).

That means when the beam divergence at 1 mrad the total attenuations 32, 31, and 26 for wavelengths 780, 850, and 1550 nm, respectively. While at beam divergence of 10 mrad, we noticed that the total attenuation was increased 51.1, 50.8, and 46 dB for three previously indicated wavelengths. Therefore, to reduce atmospheric attenuation, the beam divergence should be small in accordance with the previous results. Table 8 shows the results of total attenuation for design parameters at hazy days.

#### *4.1.2.2. Total attenuation in rainy days*

Figure 19 shows the total attenuation versus rainfall rate. It can be seen obviously that the influence of attenuation on transmission of FSO systems is more prominent during heavy rainfall compared to moderate and light rainfall. Figure 20 indicates the total attenuation versus link range. The atmospheric attenuation is proportional to link range, which showed


**Table 8.** Results of total attenuation for design parameters at hazy days.

Figure 17 represents total attenuation versus link range. From this figure, we found that total attenuation directly proportions with link range. When link range is 0.5 km, total attenuation is 17.3, 16.9 and 14.6 dB at wavelengths of 780, 850 and 1550 nm, respectively. In addition, when link range is 5 km, total attenuation becomes 115.0, 111.8 and 88.4 dB at wavelengths of 780, 850 and 1550 nm, respectively. Therefore, to reduce the effect of total attenuation on FSO, the distance between the transmitter and receiver shall be small. Figure 18 shows the relation‐ ship between total attenuation and laser beam divergence for three wavelengths. With increasing the beam divergence, the total attenuations are increased for three cases as dem‐

That means when the beam divergence at 1 mrad the total attenuations 32, 31, and 26 for wavelengths 780, 850, and 1550 nm, respectively. While at beam divergence of 10 mrad, we noticed that the total attenuation was increased 51.1, 50.8, and 46 dB for three previously indicated wavelengths. Therefore, to reduce atmospheric attenuation, the beam divergence should be small in accordance with the previous results. Table 8 shows the results of total

Figure 19 shows the total attenuation versus rainfall rate. It can be seen obviously that the influence of attenuation on transmission of FSO systems is more prominent during heavy rainfall compared to moderate and light rainfall. Figure 20 indicates the total attenuation versus link range. The atmospheric attenuation is proportional to link range, which showed

onstrated in Fig. 18.

194 Contemporary Issues in Wireless Communications

**Figure 18.** Total attenuation (dB) versus laser beam divergence (mrad).

attenuation for design parameters at hazy days.

*4.1.2.2. Total attenuation in rainy days*

that when the link range increases, the total attenuation would increase as well. The results of total attenuation for design parameters at rainy days are presented in Table 9.

**Figure 19.** Total attenuation (dB) versus rainfall rate (mm/hr).

**Figure 20.** Total attenuation (dB) versus link range (km).


**Table 9.** Results of total attenuation for design parameters at rainy days parameters.
