الفهرس 
Problems Associated with OFDMOnly 14 pages are availabe for public view
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Abstract
With the rapid growth of digital communication in recent years, the need for
high speed data transmission is increased. Moreover, future wireless systems
are expected to support a wide range of services which includes video, data and
voice. Orthogonal Frequency Division Multiplexing (OFDM) is a promising candidate
for achieving high data rates in mobile environment, due to its resistance
to Inter-Symbol Interference (ISI), which is a common problem found in high
speed data communication. Moreover, the main problem in the design of a communication
system over a wireless link is to deal with multipath fading (specially
deep fading channel), which causes a significant degradation in terms of both the
reliability of the link and the data rate even though those systems exhibit efficient
bandwidth utilization. Since the errors caused by the mobile wireless channels
are bursty in nature, interleaving is a must in mobile communication systems.
Several interleaver schemes have been proposed. The simplest and most popular
of such schemes is the block interleaver scheme. In spite of the success of
this scheme to achieve a good performance in wireless communication systems
,it cannot deal with two dimensions (2-D) errors, there is a need for a much powerful
scheme for severe channel degradation cases to overcome 2-D errors.
In OFDM, the modulation may be coherent or non-coherent. When using
non-coherent data detection (such as differential modulation) which performs
data detection by processing multiple symbols without the knowledge of the
Channel Impulse Response (CIR) but its performance is inferior than the coherent
system. In the contrast, the coherent modulation requires the channel estimation
which gives better performance. Our objective in this thesis is to work on
channel estimation which can extract the CIR using just one block of the received
OFDM data.
This thesis presents a new interleaving scheme for efficient data transmission
with OFDM over fading channels. Chaotic interleaving generate permuted versions
from the sample sequences to be transmitted, with low correlation among
their samples, and hence a better performance can be achieved. Moreover, a
comparison between the proposed chaotic interleaving and the conventional block interleaving is performed. The proposed chaotic interleaving approach
adds a degree of encryption to the transmitted data. The performance of the
proposed approach is tested on the conventional Fast Fourier Transform OFDM
(FFT-OFDM), the DiscreteWavelet Transform OFDM (DWT-OFDM) and the Discrete
Cosine Transform OFDM (DCT-OFDM) with and without chaotic interleaving.
The proposed system comprises Frequency-Domain Equalization (FDE) to
obtain high diversity gains over frequency selective multipath fading channels.
The simulation results show that the FFT-OFDM, DWT-OFDM and DCT-OFDM
systems with the new interleaving scheme provide better Symbol Error Rate
(SER) performance on different types of channels. Additionally, the CIR must
be known to coherently detect the transmitted data. Expectation Maximization
(EM) algorithm is also proposed to efficiently estimate the CIR of such a system
operating on a channel with multipath fading. Starting from the Maximum Likelihood
(ML) principle, we derive an iterative estimation algorithm based on the
EM algorithm. This algorithms are capable of improving the channel estimate.
In the initialization phase of this iterative algorithm, the initial channel estimate
is based on the observation of the pilot carriers only. The soft information resulting
from the detector can be iteratively exploited to improve the estimation
process. To reduce the complexity of the proposed algorithm, a sub-optimal estimation
scheme is also introduced. The resulting SER essentially coincides with
the case of the perfectly known channel. By simulations, the efficiency of these
algorithms can be investigated with simulation and the results of estimation will
come to a comparison.
This thesis also presents, a Peak to Average Power Ration (PAPR) mitigation
technique, which improves system performance. This technique is based on
Constant envelope OFDM (CE-OFDM). The fundamental aspects of CE-OFDM
modulation are studied. The constant envelope signal can be efficiently amplified
with nonlinear power amplifiers thus achieving greater power efficiency.
We proposed Continuous Phase Modulation (CPM) based (FFT/DWT/DCT)-
OFDM systems with and without chaotic interleaver. Performance is evaluated
over AWGN and multipath fading channels with different modulation indices.
It is shown that CPM-OFDM outperforms conventional OFDM.