الفهرس 
Abastract
Orthogonal Frequency Division Multiplexing system [1]Only 14 pages are availabe for public view
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Abstract
The Orthogonal Frequency Division Multiplexing (OFDM) is a special form of multicarrier
modulation (MCM), where a single data stream is transmitted over a number of lower rate
subcarriers. It is worth mentioning here that OFDM can be seen as either a modulation
technique or a multiplexing technique. One of the main reasons to use OFDM is to increase
the robustness against frequency selective fading and narrowband interference. In a single
carrier system, a single fade or interferer can cause the entire link to fail, but in a multicarrier
system, only a small percentage of subcarriers will be affected. Error correction coding can
then be used to correct the few erroneous sub-carriers. In OFDM systems, where the data
symbols are transmitted in parallel on N different carriers, the length N of a symbol is
extended with a factor ν. This extension of the symbol length causes the OFDM system to be
less sensitive to channel dispersion than a single carrier system transmitting data symbols at
the same data rate. However, at the edges of the OFDM symbol, the channel dispersion still
causes distortion, and hence introduces interference between successive symbols (i.e. InterSymbol Interference, ISI) and interference between different carriers within the same symbol
(i.e. Inter-Carrier Interference, ICI). To reduce the effect of the ISI, each symbol is extended
with a guard interval. When the length of the guard interval is longer than the duration of the
channel impulse response, ISI can completely be removed. However, as the transmission
efficiency reduces with the insertion of the guard interval (during the guard interval, no new
information can be transmitted); the guard interval must be chosen sufficiently small. The
most commonly used guard interval is the cyclic prefix. Different guard interval techniques
for the (OFDM) transmission are suggested to reduce the interference between successive
symbols (i.e. ISI) and interference between different carriers within the same symbol (i.e.
Inter-Carrier Interference, ICI).
In this thesis, the codes have been written for data transmission using an OFDM technique
over two types of channels. The effect of fading channel and additive white Gaussian noise
(AWGN) channel is studied and suggested approaches are used to enhance the performance
of the OFDM system. In this thesis, the impact of replacing the Cyclic Prefix (CP) by zero
insertion (ZI) before the Inverse Fast Fourier Transform (IFFT) process on OFDM is studied.
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Associated with each OFDM symbol, the zeros will be added in the transmitter before the
IFFT process. For symmetry of the OFDM symbol, the zeros will be inserted with the same
length among the OFDM frames. The ZI acts as a buffer region where delayed information
from the previous symbols can get stored. The ZI acts as a safe region of the high frequency
component (details) of the signal where delayed information from the previous symbols
freely corrupts the low frequency components, and the high frequency components are
protected by giving it a high immunity (higher SNR) against the additive noise. The receiver
has to exclude samples from the ZI which got corrupted by the previous symbol when
choosing the samples for an OFDM symbol. The motivation of using the ZI instead of the
cyclic prefix is the reduction in the transmission rate and the high performance in reducing of
the channel distortion. The performance comparison among the proposed ZI technique, the
CP, the Zero Padding (ZP), and the Known Symbol Padding (KSP) is introduced in this
thesis. Simulation results show that the KSP has a slightly worse performance; however, the
proposed ZI approach provides better performance than the other techniques and achieves a
20 % reduction in the rate of the transmitted signal relative to the CP-OFDM system.
Another type of channels is the AWGN channel. The AWGN is a noise that affects the
transmitted signal when it passes through the channel. It contains a uniform continuous
frequency spectrum over a particular frequency band. The OFDM system has been built to
reduce the effect of fading channel without enhancement the effect of the AWGN channel. In
this work the approach of the wavelet thresholding denoising at the beginning of the receive r
is used to denoise the received radio frequency (RF) signal. The simulation results prove that
the signal power is enhanced by a 30 dBs relative to the SNR of the received signal.
Finally, the proposed ZI approach in conjunction with the wavelet thresholding approach is
used to modify the OFDM system and the overall system performance is tested.