**4. Simulation results and analysis**

FSO system used the laser beam to transfer data through atmosphere. The bad atmospheric conditions have harmful effects on the transmission performance of FSO. These effects could result in a transmission with insufficient quality and failure in communication. So, the implementation of the FSO requires the study of the local weather conditions patterns. Studying of the local weather conditions patterns help us to determine the atmospheric attenuation effects on FSO communication that occurs to laser beam at this area. In this part of this work, we shall discuss the effects of atmospheric attenuation, scattering coefficient during rainy and hazy days and atmospheric turbulence during clear days on the FSO system performance. Finally, we will calculate the atmospheric turbulence.

Effect of Clear Atmospheric Turbulence on

respectively.

wavelengths 780 nm, 850 nm, and 1550 nm respectively.

Fig. 5. Scattering Coefficient (km-1) versus Low Visibility (km).

Quality of Free Space Optical Communications in Western Asia 59

Figure (5) represents the performance of scattering coefficient versus low visibility at wavelengths 780 nm, 850 nm and 1550 nm. This figure shows that the scattering coefficient inversely proportions with visibility. Scattering coefficient at low visibility 0.8 km is 4 km-1, 3.9 km-1 and 2.8 km-1 for wavelengths 780 nm, 850 nm and 1550 nm respectively. The scattering coefficient of 5 km low visibility is 0.55 km-1, 0.51 km-1 and 0.28 km-1 for

Figure (6) illustrates the scattering coefficient performance versus average visibility. When visibility is 6.4 km, scattering coefficient obtained was 0.39 km-1, 0.35 km-1 and 0.16 km-1 for wavelengths 780 nm, 850 nm and 1550 nm respectively while at 9.7 km visibility it was 0.26 km-1, 0.23 km-1 and 0.11 km-1 for wavelengths 780 nm, 850 nm, and 1550 nm

In this part we refer to the discussion and analysis of the effects of atmospheric attenuation, scattering coefficient and atmospheric turbulence on FSO system at weather conditions in the Republic of Yemen.

Results analysis is based on weather conditions data in Yemen which has been obtained from the (CAMA) for visibility are listed in Table (4) for the year 2008 and for wind velocity are listed in Table (10) for year 2003. The data of rainfall rate obtained from the (PAWR) are listed in Table (4) for the year 2008. Real data has been included in this analysis. Data has been collected and classified into two types:


Based on the above classification we have calculated the following:



Table 4. The Data of Visibility (km) obtained from CAMA for Year 2008.

#### **4.1 Scattering coefficient in hazy days**

In this part we shall discuss and analyze the effects of scattering coefficient on the FSO system performance during hazy days for Yemen and cities of Sana'a, Aden and Taiz. We will discuss and analyze the scattering coefficient during hazy days at low and average visibility. We will calculate the values of scattering coefficient using the Eq. 4.9 assuming that the size of distribution of scattering particles for low visibility is i = 0.585V1/3 and for average visibility is i = 1.3. The range of low visibility extends from 0.8 km to 5 km and the range of average visibility extends from 6.4 km to 5 km as shown in the Table (8).

In this part we refer to the discussion and analysis of the effects of atmospheric attenuation, scattering coefficient and atmospheric turbulence on FSO system at weather conditions in

Results analysis is based on weather conditions data in Yemen which has been obtained from the (CAMA) for visibility are listed in Table (4) for the year 2008 and for wind velocity are listed in Table (10) for year 2003. The data of rainfall rate obtained from the (PAWR) are listed in Table (4) for the year 2008. Real data has been included in this analysis. Data has

2. Rainy days' data: has been classified based on the rainfall rate at heavy, moderate and

Month Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Av. 9.9 8 9.1 9.1 9.5 7.3 5.6 8.6 9.8 9.8 10 10

Low 5 0.3 1 2 4 0.7 0.5 2 3 2 7 6

Av. 9.7 8.3 9.1 9.2 9.4 7 5.7 8 9.1 9.2 9.8 9.7

Low 6 2 5 7 3 3 1 5 3 0.05 8 7

Av. 9 8.4 9.7 9.7 9.9 8.9 8 8.8 9.9 9.5 9.3 8.9

Low 0.05 0.05 4 4 0.05 2 3 1.5 1 1.5 0.1 0.1

1. Hazy days' data: which has been classified based on low and average visibility.

2. Atmospheric and total attenuation at hazy and rainy days and link range.

Table 4. The Data of Visibility (km) obtained from CAMA for Year 2008.

In this part we shall discuss and analyze the effects of scattering coefficient on the FSO system performance during hazy days for Yemen and cities of Sana'a, Aden and Taiz. We will discuss and analyze the scattering coefficient during hazy days at low and average visibility. We will calculate the values of scattering coefficient using the Eq. 4.9 assuming that the size of distribution of scattering particles for low visibility is i = 0.585V1/3 and for average visibility is i = 1.3. The range of low visibility extends from 0.8 km to 5 km and the

range of average visibility extends from 6.4 km to 5 km as shown in the Table (8).

**4.1 Scattering coefficient in hazy days** 

3. Clear days' data: has been classified based on wind velocity. Based on the above classification we have calculated the following:

the Republic of Yemen.

light rainfall.

Visibility in km

Sana'a

Aden

Taiz

been collected and classified into two types:

1. Scattering coefficient at hazy and rainy days.

3. Effects of turbulence based on the wind velocity.

Figure (5) represents the performance of scattering coefficient versus low visibility at wavelengths 780 nm, 850 nm and 1550 nm. This figure shows that the scattering coefficient inversely proportions with visibility. Scattering coefficient at low visibility 0.8 km is 4 km-1, 3.9 km-1 and 2.8 km-1 for wavelengths 780 nm, 850 nm and 1550 nm respectively. The scattering coefficient of 5 km low visibility is 0.55 km-1, 0.51 km-1 and 0.28 km-1 for wavelengths 780 nm, 850 nm, and 1550 nm respectively.

Figure (6) illustrates the scattering coefficient performance versus average visibility. When visibility is 6.4 km, scattering coefficient obtained was 0.39 km-1, 0.35 km-1 and 0.16 km-1 for wavelengths 780 nm, 850 nm and 1550 nm respectively while at 9.7 km visibility it was 0.26 km-1, 0.23 km-1 and 0.11 km-1 for wavelengths 780 nm, 850 nm, and 1550 nm respectively.

Fig. 5. Scattering Coefficient (km-1) versus Low Visibility (km).

Effect of Clear Atmospheric Turbulence on

respectively.

Quality of Free Space Optical Communications in Western Asia 61

Figure (7) shows that the atmospheric attenuation inversely proportions with visibility. When the visibility is higher the effect of atmospheric attenuation is higher too. At low visibility of 0.8 km, atmospheric attenuation is 17.6 dB, 16.8 dB and 12.1 dB for wavelengths 780 nm, 850 nm and 1550 nm respectively. For 5 km low visibility, the atmospheric attenuation is about2.4 dB, 2.2 dB and 1.2 dB for wavelengths 780 nm, 850 nm and 1550 nm

Figure (8) represents the atmospheric attenuation versus average visibility. When visibility is 6.4 km, atmospheric attenuation is 1.7 dB, 1.5 dB and 0.69 dB for wavelengths 780 nm, 850 nm and 1550 nm respectively. For a 9.7 km visibility, the atmospheric attenuation is 1.1 dB,

0.99 dB and 0.46 dB for wavelengths 780 nm, 850 nm and 1550 nm respectively.

Fig. 7. Atmospheric Attenuation (dB) versus Low Visibility (km).

Fig. 6. Scattering Coefficient (km-1) versus Average Visibility (km).

These results show that the wavelength 1550 nm is scattered less than wavelengths 850 nm and 780 nm. The scattering affects are less in average visibility compared with low visibility. This is because the distribution of particles density at low visibility is higher than the density of particles at average visibility. The results of scattering coefficient due to hazy days are given in the Table (5).


Table 5. The Results of Scattering Coefficient due to Hazy Days.

#### **4.2 Atmospheric attenuation in hazy days**

In this part we will discuss and analyze the effects of atmospheric attenuation on the FSO system performance during hazy days. We have obtained the value of atmospheric attenuation from Eq. (12) assuming that the distance between transmitter and receiver is 1 km.

Fig. 6. Scattering Coefficient (km-1) versus Average Visibility (km).

Visibility Wavelength From To

Table 5. The Results of Scattering Coefficient due to Hazy Days.

**4.2 Atmospheric attenuation in hazy days** 

days are given in the Table (5).

Low

Average

These results show that the wavelength 1550 nm is scattered less than wavelengths 850 nm and 780 nm. The scattering affects are less in average visibility compared with low visibility. This is because the distribution of particles density at low visibility is higher than the density of particles at average visibility. The results of scattering coefficient due to hazy

> 780 nm 4 0.55 850 nm 3.9 0.51 1550 nm 2.8 0.28

> 780 nm 0.39 0.26 850 nm 0.35 0.23 1550 nm 0.16 0.11

In this part we will discuss and analyze the effects of atmospheric attenuation on the FSO system performance during hazy days. We have obtained the value of atmospheric attenuation from Eq. (12) assuming that the distance between transmitter and receiver is 1 km.

Scattering (km-1) Scattering (km-1)

Figure (7) shows that the atmospheric attenuation inversely proportions with visibility. When the visibility is higher the effect of atmospheric attenuation is higher too. At low visibility of 0.8 km, atmospheric attenuation is 17.6 dB, 16.8 dB and 12.1 dB for wavelengths 780 nm, 850 nm and 1550 nm respectively. For 5 km low visibility, the atmospheric attenuation is about2.4 dB, 2.2 dB and 1.2 dB for wavelengths 780 nm, 850 nm and 1550 nm respectively.

Figure (8) represents the atmospheric attenuation versus average visibility. When visibility is 6.4 km, atmospheric attenuation is 1.7 dB, 1.5 dB and 0.69 dB for wavelengths 780 nm, 850 nm and 1550 nm respectively. For a 9.7 km visibility, the atmospheric attenuation is 1.1 dB, 0.99 dB and 0.46 dB for wavelengths 780 nm, 850 nm and 1550 nm respectively.

Fig. 7. Atmospheric Attenuation (dB) versus Low Visibility (km).

Effect of Clear Atmospheric Turbulence on

and 1550 nm respectively.

Visibility Wavelength

**4.3 Scattering coefficeint in rainy days** 

Low

Average

Rainfall rate (mm/hr)

Quality of Free Space Optical Communications in Western Asia 63

Figure (9) indicates to the atmospheric attenuation versus link range that extends from 0.5 km to 5 km. Here we assume that the visibility is 1.2 km and i = 0.585 \* v1/3. The more the distance between the transmitter and the receiver, the more the atmospheric attenuation is. This means that when the distance between the transmitter and the receiver increases, it is able to reduce the quality of transmission and effectiveness of FSO system. Atmospheric attenuation for link range 0.5 km at low visibility is 5.7 dB, 4.5 dB, 3.7 dB for wavelengths 780 nm, 850 nm and 1550 nm respectively. When the link range was about 5 km, atmospheric attenuation was 56.9 dB, 54 dB and 37.2 dB, for wavelengths 780 nm, 850 nm

These results show that the attenuation at low visibility is higher than attenuation at average visibility. In addition, these readings have proved that the wavelength 1550 nm is capable to reduce the effect of atmospheric attenuation on FSO system. The distance between transmitter and receiver at low visibility should be reduced to avoid the effect of atmospheric attenuation on FSO system and improve its performance. The results of

> 780 nm 17.6 2.4 850 nm 16.8 2.2 1550 nm 12.1 1.2

> 780 nm 1.7 1.1 850 nm 1.5 0.99 1550 nm 0.69 0.46

Figures below were plotted based on Eq. (14) assuming the water density (ρ = 0.001 g/ mm�), gravitational constant (g = 127008 ∗ 10�mm/hr�), viscosity of air (η = 0.0648 g/ mm. hr) and scattering efficiency(Q = 2). The data of rainfall rate which listed in the Table

Month Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.

Sana'a 2.7 2.6 2.1 1 0 0 2.3 3 0 0 1 0.6

Aden 0 0 0 1 0.5 0 0 1.2 0 0 0 0

Taiz 2.4 0.5 3.75 5.77 5.41 3.1 5 4.72 4.02 2.4 1.5 0.5

Table 7. The Data of Rainfall Rate (mm/hr) obtained from (PAWR) for Year 2008.

From To Attenuation (dB) Attenuation (dB)

atmospheric attenuation due to hazy days are given in the Table (6).

Table 6. The Results of Atmospheric Attenuation due to Hazy Days.

(7) are divided into three states: light, moderate and heavy rain.

Fig. 8. Atmospheric Attenuation (dB) versus Average Visibility (km).

Fig. 9. Atmospheric Attenuation (dB) versus Link Range (km).

Fig. 8. Atmospheric Attenuation (dB) versus Average Visibility (km).

Fig. 9. Atmospheric Attenuation (dB) versus Link Range (km).

Figure (9) indicates to the atmospheric attenuation versus link range that extends from 0.5 km to 5 km. Here we assume that the visibility is 1.2 km and i = 0.585 \* v1/3. The more the distance between the transmitter and the receiver, the more the atmospheric attenuation is. This means that when the distance between the transmitter and the receiver increases, it is able to reduce the quality of transmission and effectiveness of FSO system. Atmospheric attenuation for link range 0.5 km at low visibility is 5.7 dB, 4.5 dB, 3.7 dB for wavelengths 780 nm, 850 nm and 1550 nm respectively. When the link range was about 5 km, atmospheric attenuation was 56.9 dB, 54 dB and 37.2 dB, for wavelengths 780 nm, 850 nm and 1550 nm respectively.

These results show that the attenuation at low visibility is higher than attenuation at average visibility. In addition, these readings have proved that the wavelength 1550 nm is capable to reduce the effect of atmospheric attenuation on FSO system. The distance between transmitter and receiver at low visibility should be reduced to avoid the effect of atmospheric attenuation on FSO system and improve its performance. The results of atmospheric attenuation due to hazy days are given in the Table (6).


Table 6. The Results of Atmospheric Attenuation due to Hazy Days.
