Performance evaluation of spatial diversity based free space optical communication links over atmospheric turbulence channels

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The ever increasing bandwidth demand on wireless communication systems has led to the development of several wireless technologies. Free Space Optical (FSO) Communication promises high speed data transmission, license-free operation, highly secured transmission and immunity to electromagnetic interferences. However index-of-refraction turbulence (IRT) has limited the performance of FSO communication systems by causing fast fluctuations in the received optical intensity and increasing the Bit-Error Rate (BER). As a result, several mitigation techniques have been proposed to reduce the effects of turbulence. This thesis evaluates the BER performance of spatial diversity based FSO links. We have characterized IRT using the PDF of gamma-gamma distribution. The Scintillation index (SI), statistical measure of IRT, has been shown to depend on the gamma-gamma parameters. Lower values of GG parameters lead to higher SI and therefore increasing IRT along the propagation path and average BER. We have derived closed-form expressions for the average BER by employing multiple receive and/or transmit apertures. Numerical results show significant reduction in the average BER as the number of transmit apertures or receive apertures are increased. Thus, offering substantial improvement in the performance of FSO links over IRT channels.
A thesis submitted to the Department of Electrical/Electronic Engineering, College of Engineering In partial fulfillment of the requirements for the degree of MSc Telecommunication Engineering.