الفهرس 
OFDM Block diagramOnly 14 pages are availabe for public view
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Abstract
The main challenges of today’s telecommunication networks are to provide
more and cheaper bandwidth in data communication motivates the development of
high-speed and low-cost optical links. Dense wavelength division multiplexing
(DWDM) was originally designed for long-haul transmission emerges as a technology
to alleviate this problem. Only DWDM technology is able to cope with terabit levels
of demand at reasonable costs. DWDM, providing routing at the wavelength level and
increasing the capacity of the current fiber, these abilities allow accommodating more
and more users at lower costs, as opposed to legacy technologies for which costs
saturate at some minimum levels.
Sub-Carrier Multiplexing (SCM) provides high capacity transmissions at lower
costs and enables fiber based wireless access. It uses optical components for high
performance but limitations of electronic components cause electronic bottleneck
which leads to limit the bit rate, number of users and the overall system performance.
New type of laser called Vertical-cavity surface-emitting laser (VCSEL) over
conventional lasers like lower threshold currents, better Wavelength stability, lower
manufacturing cost and high temperature operation up to 80°C. Using various
dispersion compensation techniques namely FBG (Fiber Bragg Grating), OPC
(Optical Phase Conjugation) and DCF (Dispersion compensating Fiber) lead to
compensate the dispersion and enhance bit rate up to 20Gbps through SMF. EDFA
(Erbium-doped fiber amplifier) used to amplify signals.
The Return to Zero (RZ) pulse just occupies a part of the bit slot therefore it has
wide spectrum and a duty cycle smaller than 1. A relatively wide optical spectrum is a
major characteristic of RZ modulated signals so it has a minimized the tolerance of
dispersion and reduced a spectral efficiency. NRZ modulation format has many
advantages as it requires a relatively low electrical bandwidth for transmitters and
receivers compared with RZ, it is not sensitive to laser phase noise, the cost is low and
the configuration of transmitter and receiver is simple so it is used in most commercial
systems. Unfortunately, for high bit rate and long distances, NRZ modulation format
is not appropriate. In case of the channels number is large, NRZ modulation is the
best.
Abstract
vii
For future wireless networks, radio over free space optics (Ro-FSO) is a
promising technology. FSO communication is an optical communication technology
where optical data wirelessly transmitted from one place to another. The major
advantages of FSO are free spectrum license, high security and faster installation. The
performance of FSO technique is affected by the conditions of the atmosphere such as
etc. To minimize the impacts of the atmospheric disturbances by utilizing some
techniques such as spatial diversity and Coherent Optical Orthogonal Frequency
Division Multiplexing (CO-OFDM) with dual polarization, the performance
enhancement of FSO system using DWDM OFDM technique under haze, snow and
fog conditions.