Isfahan University of Technology

Department of Electrical and Computer Engineering

Issue date: Januarey , 2016

Degree: MSc

Language: Farsi

Contributor: Zahra Nazari

Supervisor: Dr. Asghar Gholami

Counselor: Dr. Masood Omoomi

Abstract

In the recent years, free space optical (FSO) communication has been more attracted owing to the advantages of FSO communication with respect to other communication links. The benefits of FSO communications include high bandwidth, high security, no electromagnetic interference, quick and easy installation and low price. This technology has the problems similar to other technologies. The problems of FSO communication comprise the difficulty of alignment of the transmitter and the receiver (so that should be in line of sight), signal attenuation due to absorption and scattering phenomena, various sources of noise and air turbulence.

In this thesis after introducing the free space optical communication systems, the comparison between these systems with other communication systems is introduced. Afterwards, the applications, the equipment features and limiting factors of FSO communication system are presented. In order to investigate performance of FSO systems under different air channel conditions and optical equipment, a complete FSO link is simulated and all effective factors on system have been considered. Some of these factors are the misalignment of transmitters and receivers, broadening of laser beam, raining and snowing, fog, air turbulence, wavelength dependency and link distance. After then, the performance of the FSO system under different turbulence condition for different received powers is investigated. Furthermore, the effects of‎ the communication link distance and the wavelength dependency under constant turbulence condition are investigated and the results are presented.

In order to investigate air turbulence, a FSO communication link was installed in the university campus. Experimental results show that turbulence after sunrise is increased and at noon reaches the maximum of its amount. After that it decreases gradually, so that the minimum amount occurs at night. Although in most papers, temperature is known as an effective factor on turbulence, by investigating the turbulence changes versus temperature, we found that it is also dependent on the time of the day. Therefore elevation angle, which is zero at sunrise and sunset and reaches the maximum of its amount at solar noon, was introduced as an affective factor on the system. ‎

‎In order to compensate the effects of turbulence on FSO communication systems, it could be important to know the frequency range of fluctuations due to turbulence. In this thesis, the maximum non-negligible frequency of fluctuations that are due to turbulence was calculated by using the signal spectrum. After Analyzing effective frequency band of scintillation, temperature and sun declination angle and the sun elevation angle were introduced as effective factors.