| Summary: | Free-space optical (FSO) communication has recently gained a lot of interest as an attractive solution for high-rate last-mile terrestrial applications. FSO has many attractive features including the use of unlicensed parts of the electromagnetic spectrum, ease of deployment, cost efficiency, high security, and high data rates. There are however challenges in the design of FSO communication systems. Specifically, the weather-dependent optical wireless channel introduces attenuation and intensity variations known as turbulence-induced fading, which impose severe challenges for reliable data transmission. Meanwhile, the limitation in transmit power due to eye-safety regulations adds yet another design constraint.
In this thesis, we first consider the performance analysis of FSO systems subject to Gamma-Gamma fading. The Gamma-Gamma probability density function (pdf) includes a modified Bessel function that precludes simple closed-form expressions. We employ a series representation of the modified Bessel function and derive closed-form expressions for the pairwise error probability (PEP) of FSO systems.
We then study the performance of multiple-input multiple-output (MIMO) FSO systems for general space-time codes (STCs) for both direct and coherent/differential detection. We develop comprehensive models for both detection schemes and also use the derived models for a fair comparison between different detection schemes. For performance analysis only, we limit the number of transmit apertures to two and derive the asymptotic PEP in closed form. Moreover, design criteria are established which are used to prove the quasi-optimality of repetition and Alamouti STCs for direct and coherent/differential detection in Gamma-Gamma fading, respectively.
Finally, we investigate dual-hop and multi-hop FSO systems employing electrical and all-optical relays. Erbium-doped fiber amplifiers (EDFAs) are assumed for all-optical relaying and comprehensive signal and noise models are then derived. We show that all-optical relays outperform electrical relays unless the number of relays is very large. Furthermore, we conclude that, for a fixed source-destination distance, performance improves as the number of hops increases up to a certain point. Adding more relays will then result in performance degradation for both amplification schemes.
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