Performance evaluation of FSO communication systems over weak atmospheric turbulence channel for eastern coast of South Africa.

Abstract

Free space optical (FSO) communication, otherwise known as optical wireless communication (OWC), is an established line-of-sight telecommunication technique which utilises an optical signal carrier to propagate modulated signals in the form of a light wave (visible or infrared) over the atmospheric medium. It has numerous advantages, including ease of deployment, large bandwidth, cost effective, full duplex high data rate throughput, protocol independence, highly secured data rate transmission, unregulated frequency spectrum, limited electromagnetic interference, and minimum amount of power consumption. With all the inherent advantages in FSO systems, the technology is impaired by atmospheric turbulence. Atmospheric turbulence occurs due to the persistent random changes of the refractive index as a result of variations in atmospheric temperature and pressure. This results in fluctuations in the irradiance of the laser (simply referred to as scintillation), which may lead to attenuation of optical signals in the FSO communication system. Thus, atmospheric attenuation and turbulent conditions have negative effects on the performance and ease of deployment of FSO communication systems. In this dissertation, we examine the performance of FSO systems over weak atmospheric turbulence channel for the eastern coast of South Africa. We evaluate the feasibility of the FSO link and how to improve the reliability by estimating the link margin, probability of attenuation exceedance, power scintillation index, overall power loss due to attenuation and turbulence, link budget estimate for different link lengths and wavelengths. The FSO system availability estimated for the eastern coast of South Africa is above 99% for link distances ranging from 1 km-4 km at 850 nm, 950 nm and 1550 nm. It is also observed that the FSO link availability increases with corresponding increase in wavelengths. Adopting the Kim model to estimate the atmospheric attenuation at 850 nm wavelength, the attenuation due to scattering contributes 9.47% to the absolute atmospheric losses while the atmospheric turbulence loss contributes 90.53% to the overall power loss at a link range of 4 km. Using the Ferdinandov model for a link range of 4 km at 950 nm wavelength, the attenuation due to scattering contributes 8.81% to the total power loss while the atmospheric turbulence loss contributes 91.19% to the overall power loss. It is observed that the attainable link distance increases with increase in atmospheric visibility status. The FSO system availability reduces with increase in the propagation link distance. Furthermore, it is found that the fading loss from scintillation effects strongly depends on the power scintillation index. An increase in the power scintillation index, causes an increase in the fading loss. Thus, the power scintillation index also increases per unit increase in transmission link length and refractive index. The compensation margin for such atmospheric fading loss increases with decrease in accessible FSO system bound probability. Therefore, for a highly reliable FSO system link, extra margin must be incorporated to compensate for fading loss caused by scintillation.