Dissertation committee:
Έκτορας Ε. Νισταζάκης, Αναπληρωτής Καθηγητής, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών.
Γεώργιος Σ. Τόμπρας, Καθηγητής, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών.
Χαρίλαος Γ. Σανδαλίδης, Τμήμα Πληροφορικής με Εφαρμογές στη Βιοιατρική, Πανεπιστήμιο Θεσσαλίας.
Άννα Τζανακάκη, Επίκουρη Καθηγήτρια, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών.
Ευγενία Ροδίτη, Επίκουρη Καθηγήτρια, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών.
Μανώλης Χ. Τσίλης, Επίκουρος Καθηγητής, Τμήμα Φυσικής, Εθνικό και Καποδιστριακό Πανεπιστήμιο Αθηνών.
Νίκος Κονοφάος, Αναπληρωτής Καθηγητής, Τμήμα Πληροφορικής, Αριστοτέλειο Πανεπιστήμιο Θεσσαλονίκης.
Summary:
Free-Space Optical (FSO) communication systems have been gaining significant commercial and research interest in the last few years as a result of their various advantageous features. They are capable of meeting the fast-paced growing needs for transferring huge amounts of data in the existing and future telecommunications networks. FSO systems operate in the unlicensed band of frequencies between 300 GHz – 430 THz, offering a significant economic advantage compared to their radio frequency (RF) counterparts. They are immune to electromagnetic interference and exhibit high-security level due to their narrow optical laser beams. Also, they are considered as environmental-friendly due to their low electrical energy consumption.
Unlike their beneficial characteristics, the terrestrial FSO links are vulnerable to atmospheric effects. The atmospheric turbulence phenomenon is one of the main degradation factors for the electromagnetic optical-wave propagation in the atmospheric medium. Atmospheric turbulence arises as a result of inhomogeneities in the refractive index between air masses in the atmosphere, leading to intensity and phase fluctuations and eventually to amplitude loss on the receiver side. Due to the rapid fluctuations induced to the optical signal, the atmospheric turbulence effect is studied in a statistical manner through probability density functions for the characterization of irradiance fluctuations or the commonly referred to as scintillations.
FSO systems are usually installed at the rooftops of tall buildings or at high altitudes above the ground. Thus, these systems are susceptible to gusts of wind, potential sway of the buildings e.g. due to small earthquakes and thermal contraction and expansion. In a similar vein, additional irradiance fluctuations can be provoked to the optical signal. This phenomenon is well-known in the technical literature as pointing errors and is studied statistically in conjunction with the atmospheric turbulence effect. It is worth noting that an approximation of the Beckmann’s distribution model is employed in most cases, which takes account of the potential fixed non-zero deviation of the optical beam centre from the receiver centre and different standard deviations for the radial displacement for the vertical axes at the receiver.
Apart from the foregoing statistical phenomena, FSO links suffer from various deterministic effects such as background noise, optical power losses due to various atmospheric constituents and weather conditions such as fog, haze, rain, hail etc and free-space loss. All the mathematical models that describe the behavior of the aforementioned effects, with very high accuracy, are included in the thesis and their impact is studied to the final performance evaluation of the wireless optical links.
Spatial diversity techniques have been proved very effective in combating fading in RF wireless communication systems. In the present thesis, the application of spatial diversity to the FSO systems is studied. Specifically, the spatial diversity of the receivers is investigated along with the optimum case of using the maximum ratio combiner (MRC). The spatial diversity of the receivers is studied for a single-input multiple-output (SIMO) FSO link employing some of the most widely used modulation schemes in optical communications, such as on-off keying (OOK), pulse amplitude modulation (PAM) and pulse position modulation (PPM). The performance of the link is assessed in terms of the average bit error rate (BER) metric estimation, under the influence of the atmospheric turbulence effect modeled either by the gamma-gamma (GG) or the negative exponential (NE) distribution with pointing errors.
The block error rate (BLER) constitutes an essential performance measure for every communication link operating at relatively high throughput conditions. It’s a metric which has been investigated mostly in RF communications. In the context of the current thesis, the average BLER performance of an OOK FSO link is investigated over atmospheric turbulent conditions modeled by the GG and NE distributions with non-zero boresight pointing errors.
Analogue intensity modulation (AIM) technique has been extensively researched in optical fibre communications through the fields of microwave photonics (MWP) and radio over fibre (RoF). However, its application to the wireless optical links is at an immature stage. In this thesis, extensive research is conducted for the application of AIM techniques to the FSO links and especially of the RF signal transport scheme over FSO links, a technique known as Radio-over-FSO (RoFSO). Thus, RoFSO links are examined for transmission of OFDM and CDMA RF signals over atmospheric turbulence channels with pointing errors. It is worth mentioning that for the case of the CDMA RoFSO link, the performance is investigated for the first time for both directions of the forward and the reverse link over atmospheric turbulent conditions modeled by the M(alaga) distribution.
One of the most promising solutions, in order to enhance the performance, overcome the harmful atmospheric effects and eventually extend the distance coverage of FSO systems, is the use of relay architectures. The application of multi-hop architecture with serial decode-and-forward (DF) relay nodes to an OFDM RoFSO system is investigated. The specific DF relay nodes act as regenerators for the information signal and thus an optimum performance is achieved. The performance improvement for the multi-hop system is evaluated through the average bit error rate and the outage probability estimation.
Finally, the spatial diversity of the receivers is studied for OFDM and CDMA RoFSO links, where the specific spatially diverse links employ multiple laser sources. In this scheme, each one of the laser sources is linked to a specific receiver, through the use of very narrow optical beams. The derived results reveal the effectiveness of this configuration, rendering the links very reliable even in the most adverse operating conditions under the strong influence of the atmospheric turbulence and the pointing errors and the enhanced impact of the nonlinear distortion effects related to the RoFSO systems.
Keywords:
Optical Wireless Communications, Free-Space Optical Communications, Atmospheric Turbulence, Multiplexing, Spatial Diversity
