Free-Space Optical Communication Systems with a Partially Coherent Gaussian Beam and Media Diversity

Lee, It Ee (2014) Free-Space Optical Communication Systems with a Partially Coherent Gaussian Beam and Media Diversity. Doctoral thesis, Northumbria University.

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Abstract

Terrestrial free-space optical (FSO) communications is an emerging low-license-free and high-bandwidth access solution, albeit hampered by the combined effects from atmospheric loss, turbulence and pointing errors (PEs). Partially coherent beams (PCBs) are capable of mitigating the turbulence-induced signal fading and PEs, while causing reduction in the mean received signal intensity. This implies the necessity of PCB optimization technique, such that the trade-off between the reduction in the scintillation index and the decrease in the mean received irradiance can be achieved conveniently.

This thesis investigates the performance of partially coherent FSO communication links from the information theory perspective. The important link design criteria are considered, and the Gaussian-Schell beam model is adopted to characterize the optical beam propagation through random turbulent medium. Numerical results show that beam width optimization presents a feasible approach in promoting capacity enhancement for long-distance terrestrial FSO systems, since the optimum beam width is susceptible to the deterring effects of atmospheric turbulence and PEs. Next, joint investigation of the effects of a PCB and aperture averaging is presented, which confirms the distinctive advantages of introducing an enlarged receiver aperture and the interest of optimizing the beam width to maximize the FSO channel capacity. A theoretical beam width optimization model is proposed to determine the optimum beam width. Subsequent investigation studies on the characteristics of PCB propagating through the turbulent channel reveal the relationship between the beam width and spatial coherence length to optimize the PCB. Therefore, a joint beam width and spatial coherence length optimization technique is proposed to maximize the average capacity in partially coherent FSO links. An optimization metric is developed to enable a feasible translation of the joint optimal transmitter beam parameters into an analogous level of divergence of the received optical beam. It is demonstrated that the PCBs are desirable in the weak-to-moderate turbulence regimes, whereas coherent laser beams with high transmit power exhibit greater resilience to strong turbulences.

An experimental study is carried out to demonstrate the effects of aperture averaging and beam width on the FSO link under laboratory-controlled atmospheric turbulence conditions. The aperture-averaging effect is characterized through the signal density distributions, showing good agreement with the theoretical models. It is demonstrated that the relationship between the aperture averaging factor and point-received scintillation index can be described by a first-order linear regression model, whereby the coefficients of the model are provided and compared. Measurements of the Q-factor for an apertureaveraged
optical receiver and its corresponding finite point receiver reveals that manifold gain in the link performance can be achieved with increasing scaled aperture size, thus concluding that the introduction of an enlarged receiver aperture enhances the effective collection of the optical signal and potentially mitigates the scintillation effect.

Atmospheric loss resulting from visibility-limiting weather conditions significantly attenuates the intensity of a propagating laser beam, which imposes degrading impacts on the link range and availability. Hybrid FSO and radio frequency (RF) systems present the most prominent alternative to enable these technologies in complementing one another’s weaknesses, since fog and rain drastically affect the FSO and RF links, respectively, but only insignificantly vice versa. The viability of deploying the media diversity technique in the FSO system is investigated through a case study, in which a new hybrid-base transceiver station (H-BTS) system architecture is proposed for the green Metrozones. The hybrid FSO/RF system is integrated at the macro-cellular tier, to enable high-capacity, power-efficient wireless backhauling. A resource prioritization mechanism is designed, to maintain good control and optimal on-demand resource allocation, and to establish sustainable backhaul link availability. Next, a basic access signalling (BAS) scheme is introduced, to necessitate the discovery, registration and monitoring of active metro access points (M-APs). The proposed BAS scheme enables the sleep-wake-on-demand (SWoD) mechanism and the cooperative inter-cell support. Findings from this work suggest that adaptation and optimization at the link- and system-level are vital for Metrozones deployment, due to the occurrence of numerous time-varying factors in real networks.

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