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UBC Theses and Dissertations

Subcarrier optical wireless communications in atmospheric turbulence Song, Xuegui


In this thesis, we focus on performance study of subcarrier optical wireless communications (OWC) systems in atmospheric turbulence. Laser beam propagating through turbulence channel induces scintillation and phase aberration, which can result in significant performance degradation. The error rate performance of subcarrier OWC systems with lower order phase-shift keying (PSK) over the Gamma-Gamma turbulence channels is investigated using a direct integration method. Such analysis is generalized to M-ary PSK modulated OWC systems over strong atmospheric turbulence channels through a moment generating function approach. Since noncoherent or differentially coherent modulation schemes do not require carrier phase information at the receiver, such modulation schemes can be employed in scenarios where the carrier phase information cannot be tracked accurately. Error rate expressions of such OWC systems with noncoherent or differentially coherent modulation schemes over the Gamma-Gamma turbulence channels are also developed. Asymptotic relative performance of subcarrier OWC systems with noncoherent or differentially coherent modulation schemes with respect to those using coherent modulation schemes is quantified analytically. Besides atmospheric turbulence, other sources can also introduce performance degradation to an outdoor OWC system. Through our analysis, it is shown that pointing errors can also degrade the performance of an OWC system significantly. The effect of pointing errors together with the Gamma-Gamma turbulence channel can be evaluated using the analytical tools we developed. As another performance impairment source, the effect of carrier phase synchronization error on performance of subcarrier binary PSK and quadrature PSK systems over atmospheric turbulence channels is also studied. In order to quantify the asymptotic noise reference loss for OWC systems over the lognormal channels, a novel auxiliary random variable technique is introduced. Since the performance of outdoor subcarrier OWC systems is found to be impaired severely under strong turbulence conditions, spatial diversity techniques are introduced to mitigate the effects of turbulence induced fading. Multiple-input multiple-output OWC systems with repetition code and the Alamouti type orthogonal space-time block code are considered for the Gamma-Gamma turbulence channels. The performance analysis confirms that repetition code outperforms orthogonal space-time block code although both schemes achieve full diversity.

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