الفهرس 
mimo system
conclusion and futuer workOnly 14 pages are availabe for public view
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Abstract
A major technical challenge in wireless communications is to
improve the error performance of the system while maintaining low
implementation complexity without increasing the bandwidth. Wireless
communication channel generally offers multiple sources of diversity
such as space, frequency, time, channel propagation state, polarization.
etc. However, most existing diversity techniques exploit only one of the
various diversity resources of wireless channel.
Multiple antenna communication systems provide high data rates
and improved performance without increasing the bandwidth or
transmitted power, Multiple Input Multiple Output (MIMO) has much
larger capacity in fading channels than standard wireless systems.
Practically, for frequency selective channels, a combination of MIMOOFDM
is promising. With the advent of next generation (40) broadband
wireless communications, the combination of MIMO wireless technology
with OFDM has been recognized as one of the most promising techniques
to support high data rate and high performance. In particular, the
appropriate use of space time codes (STC) allows achieving large
capacities , coding across the space, time, and frequency domains
provided by MIMO-OFDM will enable a much more reliable and robust
transmission over the harsh wireless environment. Maximum Likelihood
Decoding (MLD) is the optimum decoding algorithm that is used for
MIMO systems.
Space time coding is a powerful scheme that combines coding with
transmit diversity to achieve high diversity performance in wireless
systems. Such a coding scheme can in general be classified into two
major classes: Space Time Block Codes (STBC) which is diversity
oriented scheme and Space Time Trellis Codes (SITC) which is
multiplexing oriented scheme, a central issue in all these schemes is the
exploitation of multi path effects in order to achieve high spectral
efficiencies and performance gains.
STBCs constructed from orthogonal designs achieve full diversity
and have fast MLD at the receiver. The transmitted symbols can be
decoded separately, not jointly. Thus, the decoding complexity increases
linearly, not exponentially, with the code size. With the quasi-orthogonal
structure, the MLD at the receiver can be done by searching groups of
symbols. However, these codes do not achieve the full diversity. But has
better performance at low SNR. This is due to the fact that the slope of
the performance curve depends on the diversity order. Quasi-Orthogonal
STBC (QOSTBC) structures for quasi-static channels provide rate-one
and pair-wise MLD but fail to achieve full-diversity. Later on, improved
QOSTBC was proposed through constellation rotation. A rotated
QOSTBC provides full diversity, rate one and better performance
compared to OSTBC.
In this dissertation, a Space Time Frequency Block Code (STFBC)
based on quasi-orthogonal designs is proposed over a frequency selective
Rayleigh fading channel. The proposed code achieves rate-one and
exploits all of the spatial, multipath and temporal diversity gains offered
by the MIMO-OFDM channel. The designed block code over MIMO
wireless systems is capable of jointly extracting multiple sources of
diversity therefore improving the error performance. For quasi-static
channel over adjacent OFDM symbol durations, as numerical results
demonstrate, the proposed code introduces a significant performance gain
over the existing STFBC codes in terms of bit error rate performance. By
coding across the three dimensions of space, time and frequency with
four frequency diversity branches, the proposed code has a four-wise
maximum likelihood decoding. The system performance will be
investigated under different delay spread of the channel. In addition, a
performance comparison for different FFT size will be presented.
Finally, some open topics are highlighted as the possible future
research directions.