Dissertation committee:
Τόμπρας Γεώργιος (Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ)
Νισταζάκης 'Εκτορας-Εμμανουήλ (Αναπληρωτής Καθηγητής, Τμήμα Φυσικής, ΕΚΠΑ)
Τζανακάκη Άννα (Επίκουρη Καθηγήτρια, Τμήμα Φυσικής, ΕΚΠΑ)
Ροδίτη Ευγενία (Επίκουρη Καθηγήτρια, Τμήμα Φυσικής, ΕΚΠΑ)
Καραγιαννίδης Γιώργος (Καθηγητής, Τμήμα Ηλεκτρολόγων Μηχανικών & Μηχανικών H/Y, ΑΠΘ)
Τσούλος Γεώργιος (Καθηγητής, Τμήμα Πληροφορικής & Τηλεπικοινωνιών, Πανεπιστήμιο Πελοποννήσου)
Σανδαλίδης Χαρίλαος (Αν. Καθηγητής, Τμήμα Πληροφορικής με Εφαρμογές στη Βιοϊατρική (Παν. Θεσσαλίας)
Summary:
In the current thesis, terrestrial optical wireless communication (OWC) systems, commonly known as terrestrial free space optical (FSO) systems in the international scientific literature, are investigated. In such systems, the information data is both rapidly and safely conveyed from the transmitter to the receiver, by a propagating laser beam through the troposphere. In recent years, the growing demand for transferring a constantly increasing, wide variety of information data in a both rapid and reliable manner, the enormous potential that offers the use of optical electromagnetic spectrum (visible and infrared wavelengths) as information bit carriers, along with the great scientific inventions concerning LED (blue LED, Nobel prize in Physics 2014), and the impressive evolution of Lasers, (Nobel prize in Physics 2018, "for the optical tweezers and their application to biological systems" and "for the Chirped Pulse Amplification- CPA method of generating high-intensity, ultra-short optical pulses."), which has additionally reduced their economical cost, have led particular international scientific attention to technologies that operate to the both more effective and free of applications optical spectrum, such as the wireless technology of FSO systems. Despite the concrete advantages of this kind of technology, mainly concerning the achievement of huge data rates, the reliability and robustness of data transfer, the relative low economic and energy cost, and the flexibility due to easy installation and re-installation of optical wireless links, such systems suffer from the relative short propagation distance of the optical signal that they can support, the atmospheric attenuation, the random weather conditions (especially fog), the scintillation effect that arise from the unavoidable atmospheric turbulence effect, the pointing errors effect and, for specific circumstances, the group velocity dispersion (GVD) effect due to atmosphere and the phase noise effect. To be more specific, scintillation effect along with pointing errors induce undesirable, rapid irradiance fluctuations of the optical information signal arriving at the receiver’s side, the different weather effects cause various amounts of scattering effects of the propagating signal, attenuation effect due to atmosphere obviously attenuates the amplitude, and thus, the power of the propagating optical pulses, the group velocity dispersion effect alters the propagating pulse’s shape, while the presence of phase noise effects undermines the process of the information signal’s detection at the receiver terminal. In this context, the initial goal of this thesis is the study, the evaluation and the estimation of all these above mentioned factors that may affect the propagating optical signal, in terms of the individual contribution of each of them, as a first step, and then, in terms of their joint contribution, alike. Therefore, in order to investigate the impact of the effects described above, proper numerical or statistical simulation models are used so as to extract the appropriate accurate mathematical equations which can estimate either the individual or the combined influence of such effects. The next goal of this thesis is the investigation of appropriate methods, configurations and architectures that could significantly mitigate the negative side-effects of the above mentioned factors that contribute to significant degradations of the efficient operation of FSO systems. Consequently, the proposed techniques aim to improve the performance of the investigated FSO systems, mainly in terms of availability reliability, and extension of their beneficial coverage area. More specifically, in order to meet these objectives, several signal processing and modulation techniques are investigated along with various configurations of diversity schemes and DF relaying. The proposed methodology and the analysis that we follow in the current thesis, so as to fulfill its intentions, are validated by numerical results, simulations, and/ or several implementations for practical FSO links along with experimental, real measured results. Indeed, through the appropriate processing of experimental data, measured for a real FSO link, it is proved, for the first time ever, the accuracy of a theoretical closed-form mathematical equation for the weak turbulence regime, which undoubtedly consists a very useful tool for the design and the investigation of any FSO link.
Keywords:
Free Space Optical Systems (Optical Wireless Systems), Atmospheric Turbulence, Scintillation, Time Dispersion, Phase Noise, Pointing Errors, Diversity, Relays
