Performance of sub-carrier modulated Free-Space Optical ...

Performance of sub-carrier modulated FSO communication link 343
Z. Ghassemlooy received his BSc in Electrical and Electronics Engineering
from the Manchester Metropolitan University in 1981; an MSc and a PhD from
the University of Manchester Institute of Science and Technology in 1984 and
1987, respectively. From 1987 to 1988, he worked as a Post-Doctoral Research
Fellow on optical sensors at the City University, London. He joined the
Sheffield Hallam University as a Lecturer in 1988 and became a Professor in
1997. In 2004, he moved to the Northumbria University as an Associate Dean
for research in the School of Computing, Engineering and Information
Sciences. His research interests include: photonics networks, modulation
techniques, high-speed optical systems, optical wireless communications and
optical fibre sensors. He is a Chartered Engineer, a Fellow of IET and a Senior
Member of IEEE.
V. Ahmadi received his BSc in Electrical Engineering from the Sharif
University of Technology, Tehran, Iran in 1985, and his MSc from the Tarbiat
Modares University, Tehran, Iran in 1989, and his PhD from the Kyoto
University, Kyoto, Japan in 1994 in Electronic Engineering–Optoelectronics.
He joined the Tarbiat Modares University as an Assistant Professor in 1994 and
became a Professor in 2006. He was the Head of the Semiconductor
Department of Laser Research Centre, Tehran, Iran from 1994 to 2006. He
became the Research Vice President of the Iran National Scientific Research
Centre (1996–1997). His research interests include: quantum photonics devices,
optical modulator and amplifiers, high-speed optical sources and detectors,
optical microresonator active and passive devices, optical wireless
communications and all optical switches.
1 Introduction
FSO communication systems as an alternative means of providing high bandwidth over a
short to medium range links has seen a growing increase in research and development
activities over the past few years. Increasing commercial deployment of the FSO could be
said to be partly responsible for this surge in research activities (Willebrand and Ghuman,
2002; Wilson et al., 2005). FSO is used in a number of applications including the cellular
communication backhaul, optical fibre communications back-up links, exhibition halls
and disaster recovery among other emerging applications (Willebrand and Ghuman,
2002; Uysal, Li and Yu, 2006). However, of major concern in FSO systems is the
dependence of its channel – the atmosphere – on the unpredictable weather conditions.
Effects of fog, rain, atmospheric gases and aerosols result in beam attenuation due to
photon absorption and scattering (Gagliardi and Karp, 1995).
In dense fog conditions, laser radiations suffer prohibitive amount of attenuation
limiting the FSO range to < 500 m in such conditions (Isaac, Kim and Korevaar, 2001).
However, in clear atmosphere with low extinction coefficients, a longer range is easily
achievable. In clear weather conditions, a primary factor that impairs the performance of
a FSO link is the random changes in atmospheric temperature. The temperature
fluctuations depend on the speed of wind and the height above the ground of the
traversing laser radiation (Osche, 2002).The resulting eddies/cells of varying sizes (from
~0.1 to ~10 m) act like refractive prisms of varying index of refraction (Killinger, 2002).
Consequently, a laser radiation traversing a turbulence atmosphere experiences random
variation (fading) in its irradiance and phase. A familiar effect of turbulence is the