Abstract:
Free-space optical (FSO) communication system is an emerging technology for future broadband optical data communication network due to its lucrative properties like required no license, cost effective, simple hardware architecture, high data rate, full duplex communication, easy to deployment, etc.. During the last two decades, the user demand for data rate increased tremendously. To meet up the current data rate demand, FSO technology can play as a key parallel technology to enhance the capacity of the existing data communication network. However, there are some adverse weather effects like molecular absorption, scattering, beam divergence, background radiation, rain, snow, fog etc. severely limits the performance of FSO communication links. Beside these, pointing error due to earth quake, building sway and building vibration due to heavy mechanical load may occurs link failure which drastically degrades the performance of FSO communication links. Huge numbers of research works have already been published regarding how to mitigate the limitations of FSO communication system. Various methods of mitigation techniques like aperture averaging, diversity, adaptive optics, coding, modulation, relay transmission, background noise rejection, jitter isolation and rejection, hybrid RF/FSO link etc. are already employed by many of the researchers. In this dissertation, several analytical models are developed for a non-Hermitian OFDM FSO system followed by differential quadrature phase shift keying (DQPSK) modulation with polarization diversity over atmospheric turbulent channel.
Firstly, an analytical model is developed to evaluate the signal to noise ratio (SNR) and bit error rate (BER) of a non-Hermitian OFDM FSO system followed by DQPSK modulation in presence of atmospheric turbulence. To perform non-Hermitian OFDM, two different laser sources are required for transmitting real and imaginary part separately. Secondly, analysis is also carried out for the same system model with polarization diversity to overcome the requirement of two laser sources and evaluated the cross polarization induced crosstalk, signal to crosstalk plus noise ratio (SCNR) and BER performance of the system in presence of all atmospheric turbulent conditions. The effect of pointing error on system BER performance is also carried out. Further analysis is also carried out to mitigate the fading due to atmospheric turbulence by applying aperture averaging technique. Thirdly, a novel analytical model is developed to evaluate the output SCNR and average BER for Space Frequency Block Coded non-Hermitian coherent-optical OFDM DQPSK FSO system with polarization diversity under turbulent atmospheric conditions. For a given BER, the SFBC coding gain is also numerically determined. Fourthly, the analysis is extended to compare the coding gain of SFBC with STBC for the same system parameters. Results are compared in terms of SCNR, BER, power penalty and receiver sensitivity. Finally, the numerically evaluated performance results for different system and channel parameters are validated by numerical simulations. Optimum system design parameters are also determined. The results of this dissertation may be used for the future development of FSO communication systems.
