**1. Introduction**

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40 Optical Communications Systems

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The Free Space Optical (FSO) communication is also known as Wireless Optical Communication (WOC), Fibreless, or Laser Communication (Lasercom). FSO communication is one of the various types of wireless communication which witnesses a vast development nowadays. FSO provides a wide service and requires point-to-point connection between transmitter and receiver at clear atmospheric conditions. FSO is basically the same as fiber optic transmission. The difference is that the laser beam is collimated and sent through atmosphere from the transmitter, rather than guided through optical fiber [1]. The FSO technique uses modulated laser beam to transfer carrying data from a transmitter to a receiver. FSO is affected by attenuation of the atmosphere due to the instable weather conditions. Since the atmosphere channel, through which light propagates is not ideal.

In this study we will take republic of Yemen as case study. In some mountainous areas in Yemen, it is difficult to install the technique of fiber optics. But FSO technique will solve this problem with same proficiency and quality provided by fiber optics. FSO systems are sensitive to bad weather conditions such as *fog, haze, dust, rain and turbulence*. All of these conditions act to attenuate light and could block the light path in the atmosphere. As a result of these challenges, we have to study weather conditions in detail before installing FSO systems. This is to reduce effects of the atmosphere also to ensure that the transmitted power is sufficient and minimal losses during bad weather.

This chapter aims to study and analyze the atmosphere effects on the FSO system propagation in the Republic of Yemen weather environment. The study is focused more on the effects of haze, rain and turbulence on the FSO systems.

The analysis conducted depends basically on statistical data of the weather conditions in Yemen obtained from the Civil Aviation and Meteorology Authority (CAMA) for visibility and wind velocity and from the Public Authority for Water Resources (PAWR) for the rainfall rate intensity. So, the prominent objectives of this work are:

1. Calculating the scattering coefficient, atmospheric attenuation, total attenuation in the hazy and rainy days and scintillation at the clear days.

Effect of Clear Atmospheric Turbulence on

**2.1 FSO communication subsystem** 

Fig. 2. Block Diagram of a Terrestrial FSO System.

channel and a receiver.

Fig. 1. Schematic showing FSO Transmitter and Receiver LOS.

Quality of Free Space Optical Communications in Western Asia 43

FSO communication is a line of sight technology that uses laser beam for sending the very high bandwidth digital data from one point to another through atmosphere. This can be achieved by using a modulated narrow laser beam lunched from a transmission station to transmit it through atmosphere and subsequently received at the receiver station. The generalized FSO system is illustrated in Fig. (2), it is typically consists of transmitter, FSO

2. Studying the performance of FSO system at wavelengths 780 nm, 850 nm and 1550 nm, beam divergence angle, transmitter and receiver diameter apertures and transmission range.

The scope of this chapter focuses on studying and analyzing of FSO propagation under weather conditions in Yemen environment for outdoor system. The atmospheric effects divided into two kinds: atmospheric attenuation and atmospheric turbulence. Atmospheric attenuation due to Mie scattering is related to the haze and it is a function of the visibility, and attenuation due to rainfall independent on wavelength. The atmospheric turbulence is due to variance of refractive index structure.

There are three factors which enable us to test the FSO performance as: design, uncontrollable and performance. Design factors are relating to FSO design such as light power, wavelength, receiver and transmitter aperture diameter, link range and detector sensitivity. Uncontrollable elements such as rainfall elements include rainfall rate and raindrop radius, haze element include visibility and turbulence element include refractive index structure. Performance of system was tested during the rainy days and hazy days which can be calculated from the effect of scattering coefficient, atmospheric attenuation and total attenuation. However, the system performance in the clear days can be calculated from the effect of variance.

The remainder of this chapter is organized as follow: Discuses briefly the background of FSO. The concept of and the several stages of FSO transceiver are explained briefly. Illustrate the losses of FSO system due to atmosphere channel and geometric losses. Defining the aerosols and visibility and how they effect on FSO system were introduced. Beer's law which describes attenuation of atmospheric channel due to absorption and scattering coefficient was introduced. The atmospheric attenuation types are explained. Stroke's law which describes scattering coefficient due to rainfall was introduced. Geometrical loss and total attenuation are discussed. Represent the analytical study of Yemeni environment. Scattering coefficient and attenuation in hazy days at average and low visibility at three wavelengths (780 nm, 850 nm and 1550 nm) were introduced. The atmospheric attenuation at the lowest visibility conditions was plotted with the link range. Scattering coefficient and atmospheric attenuation in rainy days were plotted once versus rainfall rate and once again versus raindrop radius. Scattering coefficient and atmospheric attenuation to haze in Sana'a, Aden and Taiz cities were calculated. The geometric loss for two commonly used designs of transceivers was evaluated. The conclusion is done based on the overall findings of this work.
