Investigation of Free Space Optical Link Performance with Wavelength Diversity under Different Turbulence Conditions by Dhaval Gopalchandra Shah

Guided by: Dr. D. K. Kothari With Synopsis and CD 11EXTPHDE72

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.