**2.1 FSO communication subsystem**

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 channel and a receiver.

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

Effect of Clear Atmospheric Turbulence on

receiver telescope at the other end of the channel.

**Transmitter telescope** 

as haze, fog, dust, and soil [6].

detector, amplifier and demodulator.

illumination can be significantly minimized [6].

c. Receiver

**Receiver telescope** 

**Optical filter** 

**Detector** 

b. FSO channel

Quality of Free Space Optical Communications in Western Asia 45

The transmitter telescope collects, collimates and directs the optical radiation towards the

For FSO links, the propagation medium is the atmosphere. The atmosphere may be regarded as series of concentric gas layers around the earth. Three principal atmospheric layers are defined in the homosphere [5], the troposphere, stratosphere and mesosphere. These layers are differentiated by their temperature gradient with respect to the altitude. In FSO communication, we are especially interested in the troposphere because this is where most weather phenomena occur and FSO links operate at the lower part of this layer [5].

The atmosphere is primarily composed of nitrogen (N2, 78%), oxygen (O2, 21%), and argon (Ar, 1%), but there are also a number of other elements, such as water (H2O, 0 to 7%) and carbon dioxide (CO2, 0.01 to 0.1%), present in smaller amounts. There are also small particles that contribute to the composition of the atmosphere; these include particles (aerosols) such

Propagation characteristics of FSO through atmosphere drastically change due to communication environment, especially, the effect of weather condition is strong. The received signal power fluctuates and attenuates by the atmospheric obstacles such as rain, fog, haze and turbulence in the propagation channel. The atmospheric attenuation results from the interaction of the laser beam with air molecules and aerosols along the propagation. The main effects on optical wireless communication are absorption, scattering, and scintillation [7].

The receiver optics consists of five parts as shown in Fig. 2: receiver telescope, optical filter,

The receiver telescope collects and focuses the incoming optical radiation on to the photo detector. It should be noted that a large receiver telescope aperture is desirable because it collects multiple uncorrelated radiation and focuses their average on the photo detector [8].

By introducing optical filters that allow mainly energy at the wavelength of interest to impinge on the detector and reject energy at unwanted wavelengths, the effect of solar

The detector also called photodiode (PD) is a semiconductor devices which converts the photon energy of light into an electrical signal by releasing and accelerating current conducting carriers within the semiconductors. Photodiodes operate based on photoconductivity principals, which is an enhancement of the conductivity of p-n semiconductor junctions due to the absorption of electromagnetic radiation. The diodes are generally reverse-biased and capacitive charged [9]. The two most commonly used photodiodes are the pin photodiode and the avalanche
