Τομέας Φυσικής Στερεάς ΚατάστασηςLibrary of the School of Science
Νισταζάκης Έ (κύριος επιβλέπων), Τόμπρας Γ., Τίγκελης Ι.
Επίγειες ασύρματες οπτικές τηλεπικοινωνιακές ζεύξεις και μελέτη τεχνικών για την βελτίωση της απόδοσής τους
Free space optical communications systems and a study on their efficiency
Free Space Optical systems (FSO) are investigated in this thesis. In such
systems, the transmitter and the receiver communicate using a laser beam that
propagates through the atmosphere. FSO systems attract research and commercial
interest due to the plenty advantages they offer, such as the high bit rate.
But, FSO systems are vulnerable to atmospheric and natural phenomena, like
weather conditions, atmospheric turbulence and pointing errors. All these
phenomena are responsible for irradiance attenuation and fluctuations at the
receiver. Especially atmospheric turbulence and pointing errors create fast
fluctuations at the receiver, a phenomenon also known as scintillation effect.
In order to study such fast fluctuations we employ statistical methods and the
channel is modeled with various statistical distributions depending on the
turbulence strength and the presence of pointing errors. First, we investigate
the effect of the above factors on the efficiency of an FSO system. More
precisely the metrics of outage probability, such as the bit error rate, the
average capacity and the outage capacity are considered by calculating
closed-form mathematical expressions that describe them. Furthermore, we apply
OFDM multiplexing to a FSO system and examine how the nonlinear clipping
affects the link and it is the first time that this nonlinear factor is studied
in a FSO system. The most important goal of the thesis is the investigation and
proposal of various methods to increase the efficiency of a FSO system.
Firstly, the technique of multi-hop is presented. This technique is used to
increase the total effective link length by using serially relay nodes. Then,
we examine the technique of diversity. In this technique, multiple copies of
the signal are sent to the receiver. Diversity can be applied in space, time
and wavelength. For both techniques of multi-hop and diversity, closed-form
mathematical expressions are derived and numerical results are presented for
the behavior of the system. Finally, we experimentally evaluate the theoretical
model in the case of weak turbulence. This evaluation is implemented by the
process of experimental data that are obtained from an experimental FSO system.
Optical communications, Wireless communications, Electromagnetic wave propagation, Channel capacity, System efficiency
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