Investigation of Free Space Optical Link Performance with Wavelength Diversity under Different Turbulence Conditions

Abstract

Free Space Optical (FSO) communication is rapidly growing technology in the field of wireless communication. Higher bandwidth, licence free spectrum, high security and quick deployment are the key advantages of this technology. However, the atmospheric losses due to bad weather and atmospheric turbulence are still the challenges which prevent the growth of FSO communication at a higher rate. Wavelength diversity technique has shown the capability to overcome these challenges. In this technique, the information signal is transmitted simultaneously on a different wavelength which increases the availability of FSO link and overcomes the problem of link blockage. Apart from this, each wavelength is immune against certain atmospheric elements so the use of multiple wavelengths for transmission helps to combat the atmospheric losses. Further, different wavelengths are affected differently by same atmospheric condition which makes FSO communication robust under different atmospheric turbulence conditions. In this thesis, wavelength diversity technique is applied to enhance the performance of FSO system under different turbulence conditions. Three different wavelengths of 1550, 1310 and 850 nm are chosen for this technique. Different turbulence conditions are realized by adopting well-defined channel model for a particular turbulence condition. K channel model has been considered to categorize strong turbulence condition and Exponentiated Weibull channel has been used to represent all turbulence scenarios. Outage probability and average Bit Error Rate (BER) are considered as a performance metrics. Simulations results are obtained using Matlab software. The performance of FSO system under strong turbulence with wavelength diversity technique is investigated with three different combining methods: optimal combining, equal gain combining and selection combining. Mathematical expressions are derived to evaluate average BER and outage probability. The results exhibit that wavelength diversity with optimal combining method achieves better improvement compared to equal gain combining and selection combining methods. The obtained BER results are also compared with the published article in which spatial diversity technique is used to mitigate the effect of strong turbulence using same channel model. It is observed that wavelength diversity archives 2–3 dB higher improvement. An effort has been made to identify an appropriate diversity order to improve the BER and outage probability of the system under all turbulence conditions. The effect of receiver aperture size on the results is also analyzed. 10 mm and 60 mm aperture size is considered to represent an ideal and practical FSO implementation. Different turbulence condition is characterized using Exponentiated Weibull channel and optimal combining method is considered at receiver. Numerical results achieved from the derived expression of average BER and outage probability show that increasing diversity order improves the results with both aperture size. It is observed that increasing receiver aperture size decreases the performance improvement. But, the BER requirement for modern communication is easily fulfilled with the diversity order of 3 even at 60 mm receiver aperture under all turbulence conditions. A comparative analysis of BER results of FSO system with wavelength diversity using under different turbulence conditions is carried out. The results obtained considering Exponentiated Weibull channel for all turbulence conditions are compared with the published articles in the literature in which different turbulence conditions are represented by appropriate classical channel models. It is found that deployment of wavelength diversity archives a maximum gain when different turbulence conditions are characterized with Exponentiated Weibull channel.