Analysis of Free Space Optics Using Wavelength Diversity

Abstract

Free space optical (FSO) communication systems has gained significant importance and commercial attention in recent years, due to their great advantages like cost effectiveness, security, licence free access, high bandwidth and ease of deployment. The free space optics (FSO) technology uses free space as its transmission media so the performance of FSO systems strongly affected by atmospheric conditions like fog, rain, snow , absorption ,scattering and atmospheric turbulences . The atmospheric turbulence effect is one of the biggest problems in FSO which degrades the overall system’s performance. Many techniques can be used to mitigate this problem, diversity is one of the method which can be used to mitigate the turbulence induced fading. The information signal-carrying laser beam is transmitted on more than one wavelength at same time by using wavelength diversity technique. In this thesis work the use of wavelength diversity over different channel models like lognormal channel model, gammagamma channel model and exponentiated weibull channel model has been considered also mathematical analysis has been performed and numerical simulation has been done on MATLAB. In lognormal channel model and gamma-gamma channel model, avg. BER vs. SNR graphs has been plotted in MATLAB for different link lengths (1 to 3 km) and keeping SNR constant at 30 dB different values of BER has been calculated to analyse the BER performance of the channel model. In Exponentiated weibull channel model avg. BER vs. SNR graphs has been plotted in MATLAB for different link lengths (1 to 3 km) and keeping SNR constant at 30 dB different values of BER has been calculated to analyse the BER performance of the channel model along with that considering diversity order w = 6 graphs has been plotted. EW channel model is compared with the lognormal and gamma-gamma channel model with the help of avg. BER vs. SNR graphs, and keeping the SNR constant at 30 dB, BER values has been calculated and compared to show that which is better in all turbulence condition. Effect of increasing link length L and aperture diameter D has been shown with the help of origin plots for all of these channel models. In order to utilize wavelength diversity we have used three wavelengths 1550 nm, 1310 nm, and 850 nm. The results obtained shows that the use of wavelength diversity results in considerable performance improvement at a moderate increase of the overall system’s complexity and cost.