**4.3 Scattering coefficeint in rainy days**

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


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

Effect of Clear Atmospheric Turbulence on

Quality of Free Space Optical Communications in Western Asia 65

Fig. 11. Scattering Coefficient (km-1) versus Raindrop Radius (mm)**.**

Table 8. The Results of Scattering Coefficient due to Rainy Days.

rainy days depended on rainfall rate intensity and raindrop radius.

The highest attenuation is about 0.69 dB in heavy rain.

**4.4 Atmospheric attenuation in rainy days** 

The results of scattering coefficient due to rainy days are given in the Table (8).

Rainfall rate From To

Light rain 0.0083 0.041 Moderate rain 0.055 0.086 Heavy rain 0.10 0.16

In this part we will discuss the effects of atmospheric attenuation on the performance of FSO system during rainy days. The effects of atmospheric attenuation on FSO systems during

Figure (12) shows the atmospheric attenuation versus rainfall rate. The curves plotted were based on Eq. 17 at light, moderate and heavy rain, assuming the radius of rain a = 0. 5 mm and transmission range L = 1 km. When the rainfall rate increases the effect of atmospheric attenuation on the FSO system increases too. Therefore influence of attenuation on transmission of FSO systems is more prominent during heavy rainfall compared to moderate and light rainfall. The atmospheric attenuation is about 0.036 dB to 0.18 dB for light rain, bout 0.24 dB to 0.37 dB for moderate rain and 0.45 dB to 0.69 dB for heavy rain.

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

Figure (10) illustrates the performance of scattering coefficients versus rainfall rate at light, moderate and heavy rain. The curves plotted were based on Eq. (14) assuming the radius of raindropa = 0.5 mm. The scattering coefficient is proportional with rainfall rate*,* which showed that when the rainfall rate increases, the scattering coefficient increases too. For light rain the scattering coefficient is about 0.008 km-1 to 0.04 km-1, about 0.055 km-1 to 0.086 km-1 for moderate rain and 0.10 km-1 to 0.16 km-1 for heavy rain. The highest scattering coefficient is about 0.16 km-1 in heavy rain. The impact of scattering on transmission of FSO system is more pronounced during heavy rainfall compared to moderate and light rainfall.

Figure (11) shows that the scattering coefficient versus raindrop radius. This figure illustrated that the radius of raindrop was important in evaluating the scattering effect. The radii of raindrop fall in the range of 0.1 mm to 0.8 mm. The scattering coefficient of the rain is independent of wavelength because the radii of rain particles are much bigger than laser wavelengths.

Fig. 10. Scattering Coefficient (km-1) versus Rainfall Rate (mm/hr).

Figure (10) illustrates the performance of scattering coefficients versus rainfall rate at light, moderate and heavy rain. The curves plotted were based on Eq. (14) assuming the radius of raindropa = 0.5 mm. The scattering coefficient is proportional with rainfall rate*,* which showed that when the rainfall rate increases, the scattering coefficient increases too. For light rain the scattering coefficient is about 0.008 km-1 to 0.04 km-1, about 0.055 km-1 to 0.086 km-1 for moderate rain and 0.10 km-1 to 0.16 km-1 for heavy rain. The highest scattering coefficient is about 0.16 km-1 in heavy rain. The impact of scattering on transmission of FSO system is more pronounced during heavy rainfall compared to moderate and light rainfall. Figure (11) shows that the scattering coefficient versus raindrop radius. This figure illustrated that the radius of raindrop was important in evaluating the scattering effect. The radii of raindrop fall in the range of 0.1 mm to 0.8 mm. The scattering coefficient of the rain is independent of wavelength because the radii of rain particles are much bigger than laser

Fig. 10. Scattering Coefficient (km-1) versus Rainfall Rate (mm/hr).

wavelengths.

Fig. 11. Scattering Coefficient (km-1) versus Raindrop Radius (mm)**.**


The results of scattering coefficient due to rainy days are given in the Table (8).

Table 8. The Results of Scattering Coefficient due to Rainy Days.

#### **4.4 Atmospheric attenuation in rainy days**

In this part we will discuss the effects of atmospheric attenuation on the performance of FSO system during rainy days. The effects of atmospheric attenuation on FSO systems during rainy days depended on rainfall rate intensity and raindrop radius.

Figure (12) shows the atmospheric attenuation versus rainfall rate. The curves plotted were based on Eq. 17 at light, moderate and heavy rain, assuming the radius of rain a = 0. 5 mm and transmission range L = 1 km. When the rainfall rate increases the effect of atmospheric attenuation on the FSO system increases too. Therefore influence of attenuation on transmission of FSO systems is more prominent during heavy rainfall compared to moderate and light rainfall. The atmospheric attenuation is about 0.036 dB to 0.18 dB for light rain, bout 0.24 dB to 0.37 dB for moderate rain and 0.45 dB to 0.69 dB for heavy rain. The highest attenuation is about 0.69 dB in heavy rain.

Effect of Clear Atmospheric Turbulence on

attenuation decreases `when the radius of raindrop increases.

Fig. 14. Atmospheric Attenuation (dB) versus Link Range (km).

Light rain 0.036 0.18

Moderate rain 0.24 0.37

Heavy rain 0.45 0.69

Table 9. The Results of Atmospheric Attenuation due to Rainy Days.

due to rainy days are given in the Table (9).

Rainfall rate

Figure (14) indicates the atmospheric attenuation versus link range. This figure was plotted based on Eq. (17) assuming the raindrop radius is 0.5 mm. For 0.5 km link range the atmospheric attenuation is about 0.18 dB for light rain, 0.37 dB for moderate rain and 0.69 dB for heavy rain. For 10 km link range the atmospheric attenuation is about 1.8 dB for light rain, 3.7 dB for moderate rain and 6.9 dB for heavy rain. The atmospheric attenuation results

Atmospheric Attenuation (dB)

From To

Quality of Free Space Optical Communications in Western Asia 67

Figure (13) illustrates that the atmospheric attenuation versus raindrop radius. The radius of rain particles falls in the range of 0.1 mm to 0.8 mm. This figure shows that the atmospheric

Fig. 12. Atmospheric Attenuation (dB) versus Rainfall Rate (mm/hr).

Fig. 13. Atmospheric Attenuation (dB) versus Raindrop Radius (mm).

Fig. 12. Atmospheric Attenuation (dB) versus Rainfall Rate (mm/hr).

Fig. 13. Atmospheric Attenuation (dB) versus Raindrop Radius (mm).

Figure (13) illustrates that the atmospheric attenuation versus raindrop radius. The radius of rain particles falls in the range of 0.1 mm to 0.8 mm. This figure shows that the atmospheric attenuation decreases `when the radius of raindrop increases.

Fig. 14. Atmospheric Attenuation (dB) versus Link Range (km).

Figure (14) indicates the atmospheric attenuation versus link range. This figure was plotted based on Eq. (17) assuming the raindrop radius is 0.5 mm. For 0.5 km link range the atmospheric attenuation is about 0.18 dB for light rain, 0.37 dB for moderate rain and 0.69 dB for heavy rain. For 10 km link range the atmospheric attenuation is about 1.8 dB for light rain, 3.7 dB for moderate rain and 6.9 dB for heavy rain. The atmospheric attenuation results due to rainy days are given in the Table (9).


Table 9. The Results of Atmospheric Attenuation due to Rainy Days.

Effect of Clear Atmospheric Turbulence on

Quality of Free Space Optical Communications in Western Asia 69

Fig. 15. Log Irradiance Variance Scintillation versus Link Range (km).

Fig. 16. Beam Spreading (m) versus the Link Range (km).

## **4.5 Atmospheric turbulence**

The purpose here is to discuss the relationship for calculating irradiance variance, beam spreading and loss beam center for a range of parameters. We used the wavelengths of 780 nm, 850 nm & 1550 nm.


Table 10. The Data of Wind Velocity (km/hr) obtained from (CAMA) for Year 2003.

Figure (15) illustrates the log irradiance variance versus the link range for 780 nm, 850 nm and 1550 nm wavelengths. This figure was plotted based on Eq. 21. As the link range increases the variance (scintillation) increases too. For a 0.5 km link range, the variance is about 0.087, 0.079and 0.039 for wavelengths 780 nm, 850 nm and 1550 nm respectively. For a 5 kmlink range, the variance is about 5.9, 5.4 and 2.7 for 780 nm, 850nm and 1550 nm respectively. These results show that the use of a wavelength of 1550 nm is able to reduce the variance "atmospheric turbulence" effect on the FSO systems.

Figure (16) indicates the comparison between the beam spreading on a distance (L) from the transmitter, in case the atmospheric turbulences and its absence. Figure (15) was plotted based on Eq. 23 assuming the spot size of the beam at the transmitter (with the distance L = 0) equals 8 mm. At the distance 0.5 km from transmitter, the spot size of the beam is �(�� = 0.032 m in case of absence turbulence and ����(�� = 0.032 m in case of turbulences. At the distance 5 km, the �(��= 0.31 m and ����(�� = 0.33m*.* From the above results, we conclude the expansion of the spot size of the beam depends on the distance between transmitter and receiver and on the atmospheric turbulence on the along of transmission range.

The loss beam at center (dB) depends on transmission range and wavelength as shown Fig. (17). The loss beam at the center increases, corresponding to the increase of Link range. At the distance 0.5 km, the loss beam at center = 0.0454 dB, 0.0383 dB and 0.0116 dB for wavelengths 780 nm, 850 nm & 1550 nm respectively. At the distance 5 km, the loss beam at center is 2.4 dB, 2.1 dB and 0.72 dB for wavelengths 780 nm, 850 nm& 1550nm respectively.

The purpose here is to discuss the relationship for calculating irradiance variance, beam spreading and loss beam center for a range of parameters. We used the wavelengths of 780

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

Sana'a 15. 6 17.4 15.2 15.2 14.8 16.1 17.6 16.5 18.9 16.9 16.3 14.8

Aden 20.6 18.5 20.9 20 13.3 18.3 21.3 20.6 14.6 16. 7 20 19.1

Taiz 12.4 14.6 16.1 17.2 17 18.3 21. 7 17 17 14.8 14.4 13

Figure (15) illustrates the log irradiance variance versus the link range for 780 nm, 850 nm and 1550 nm wavelengths. This figure was plotted based on Eq. 21. As the link range increases the variance (scintillation) increases too. For a 0.5 km link range, the variance is about 0.087, 0.079and 0.039 for wavelengths 780 nm, 850 nm and 1550 nm respectively. For a 5 kmlink range, the variance is about 5.9, 5.4 and 2.7 for 780 nm, 850nm and 1550 nm respectively. These results show that the use of a wavelength of 1550 nm is able to reduce

Figure (16) indicates the comparison between the beam spreading on a distance (L) from the transmitter, in case the atmospheric turbulences and its absence. Figure (15) was plotted based on Eq. 23 assuming the spot size of the beam at the transmitter (with the distance L = 0) equals 8 mm. At the distance 0.5 km from transmitter, the spot size of the beam is �(�� = 0.032 m in case of absence turbulence and ����(�� = 0.032 m in case of turbulences. At the distance 5 km, the �(��= 0.31 m and ����(�� = 0.33m*.* From the above results, we conclude the expansion of the spot size of the beam depends on the distance between transmitter and receiver and on the atmospheric turbulence on the along of

The loss beam at center (dB) depends on transmission range and wavelength as shown Fig. (17). The loss beam at the center increases, corresponding to the increase of Link range. At the distance 0.5 km, the loss beam at center = 0.0454 dB, 0.0383 dB and 0.0116 dB for wavelengths 780 nm, 850 nm & 1550 nm respectively. At the distance 5 km, the loss beam at center is 2.4 dB, 2.1 dB and 0.72 dB for wavelengths 780 nm, 850 nm& 1550nm

Table 10. The Data of Wind Velocity (km/hr) obtained from (CAMA) for Year 2003.

the variance "atmospheric turbulence" effect on the FSO systems.

**4.5 Atmospheric turbulence**

nm, 850 nm & 1550 nm.

transmission range.

respectively.

Wind Velocity(km\hr)

Fig. 15. Log Irradiance Variance Scintillation versus Link Range (km).

Fig. 16. Beam Spreading (m) versus the Link Range (km).

Effect of Clear Atmospheric Turbulence on

(km) Wavelength Scintillation

(m-3/2)

Table 11. The Results of Atmospheric Turbulence due to Clear Days.

the transmission range and use wavelength 1550 nm.

780 nm 0.087 0.045 0.0

1550 nm 0.039 0.012 0.0

780 nm 5.92 2.35 0.0037

1550 nm 2.66 0.73 0.0033

In this chapter, we focused on haze, rain and turbulence effects on FSO systems. Mie scattering occurs in hazy days and it depends on wavelength. The scattering coefficient on hazy days is determined by using Beer's Law. From the results analysis and data in the Table 5.1 the fog and haze represent the most important atmospheric scatters. Their attenuation, which can reach about 17.6 dB at 1.8 km low visibility in Yemen and 163.5 dB (corresponding to very thick fog), at 0.05 km low visibility is in Taiz city. This attenuation value affects the performance of a FSO link for distances as small. Wavelength 1550 nm is less scattered from the wavelengths 850 nm & 780 nm and it is not harmful to the human

Rain does not introduce a significant attenuation in FSO systems links in Yemen. This is due to the rainfall affect mainly radio and microwave systems that use a longer wavelengths and attenuation at heavy rain 5.77 mm/hr in Yemen about 0.69 dB, is very small compared with attenuation due to fog. Therefore the effect of rain is neglected in Yemen. Atmospheric turbulence will change in refractive index structure of air from one area to another. Atmospheric turbulence fluctuates intensity of the laser beam. Scintillation is wavelength and distance dependent. We can reduce the effect of the turbulence by enlarging the diameter of the receiver's aperture or setting tracking system at the receiver. The results indicate that the attenuation depends on weather conditions which are uncontrollable and transmission range which can be controlled; hence, it is considered an important element in the design of FSO system. So, to improve the performance of FSO system, we must reduce

Table (11).

0.5 km

5 km

eyes.

**4.6 Conclusion** 

Link Range

Quality of Free Space Optical Communications in Western Asia 71

Figure (18) indicates to the beam wander versus link range. The beam wanders increases corresponding to increasing in the link range. At 0.5 km transmission range, the beam wander is 0 m for 780 nm, 850 nm and 1550nm wavelengths respectively and at 5 kmlink range the beam wander is 0.0037 m, 0.0037 m, and 0.0033 m for 780 nm, 850nm and 1550 nm wavelengths respectively. From the above results, we conclude that the loss beam at center for 780 nm and 850 nm wavelengths is more than the loss at 1550nm wavelength. So to reduce the loss beam at center we suggest to reduce the link range and 1550 nm wavelength must be used. The results of atmospheric turbulence effect due to clear days are given in the

> Loss at Beam center (dB)

850 nm 0.076 0.038 0.0 0.032 0.032

850 nm 5.35 2.050 0.0037 0.31 0.33

Beam wander (m) W (L) (m)

Weff (L) (m)

Fig. 17. Loss at Beam Center (dB) versus the Link Range (km).

Fig. 18. Beam Wander (m) versus the Link Range (km).

Figure (18) indicates to the beam wander versus link range. The beam wanders increases corresponding to increasing in the link range. At 0.5 km transmission range, the beam wander is 0 m for 780 nm, 850 nm and 1550nm wavelengths respectively and at 5 kmlink range the beam wander is 0.0037 m, 0.0037 m, and 0.0033 m for 780 nm, 850nm and 1550 nm wavelengths respectively. From the above results, we conclude that the loss beam at center for 780 nm and 850 nm wavelengths is more than the loss at 1550nm wavelength. So to reduce the loss beam at center we suggest to reduce the link range and 1550 nm wavelength must be used. The results of atmospheric turbulence effect due to clear days are given in the Table (11).


Table 11. The Results of Atmospheric Turbulence due to Clear Days.
