07-01-2014, 04:11 PM
New Preamble Structures for Synchronization and Cell searching in OFDM systems
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Abstract
This thesis proposes two new preamble structures for synchroniza-
tion and cell searching in cellular based Orthogonal Frequency Division
Multiplexing (OFDM) systems. The proposed preamble structures are
to impose Pseudo Noise sequences (PN-sequences) on the OFDM trans-
mitted signal in time-domain for time and frequency synchronization.
This scheme make it possible to achieve a good performance at a low
SNR, therefore, it can guarantee a reliable communication to a mobile
located at the cell edge. Herein, the PN-sequences occupies the whole
or part of the preamble according to the two different preamble struc-
tures. The proposed preamble structures are compared each other in
terms of frame and frequency synchronization, cell searching and mean
acquisition time.
Introduction
Orthogonal frequency-division multiplexing (OFDM) systems have re-
cently gained increased interest. OFDM is used in the European digital
broadcast radio system and is being investigated for other wireless appli-
cations such as digital broadcast television and mobile communication
systems, as well as for broadband digital communication on existing
copper network. OFDM systems have several advantages, firstly, its
robustness against inter-symbol interference(ISI) and mitigating the ef-
fect of multi-path delay spread in wireless radio channel make OFDM
systems to be one of the prospecting technique for the next generation.
Secondly, OFDM systems divide the available spectrum into several sub-
channels, so the data loaded at each sub-channel can be transmitted over
flat fading channel. Thirdly all sub-channels are narrow-band and they
experience flat fading, which makes equalization very simple. Finally
the spectrum of the sub-channels are overlapping under interference is
significantly reduced. It enhances the bandwidth efficiency.
OFDM systems
Orthogonal frequency division multiplexing (OFDM) is a parallel trans-
mission scheme, where a high rate serial data stream is split up into
a set of low rate substreams, each of which is modulated on a sep-
arate subcarrier. The concept of using parallel data transmission by
means of frequency division multiplexing (FDM) was published in mid
60s. The idea was to use parallel data streams and FDM with over-
lapping subchannels to avoid the use of high speed equalization and
to combat impulsive noise, and multipath distortion as well as to fully
use the available bandwidth. Weinstein and Ebert applied the discrete
Fourier transform (DFT) to parallel data transmission system as part of
the modulation and demodulation process. In addition to eliminating
the banks of subcarrier oscillators and coherent demodulators required
by FDM.
Organization
This thesis is organized as follows. Chapter II describes the system
model and in chapter III, we propose two preamble structures. Chapter
IV introduces frame and frequency synchronization process, cell iden-
tification method and mean acquisition of cell identification to each
preamble structure. In chapter V, we presents the performance of each
proposed preamble structure obtained by simulation. Conclusions and
further work are given in chapter VI.