الفهرس 
Applications of MIMOOnly 14 pages are availabe for public view
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Abstract
One of the main challenging issues in wireless system implementation is the im-
balance between the In-phase (I) and Quadrature-phase (Q) branches, which can
be present at both the transmitter (Tx) and receiver (Rx). These imbalance is as-
sociated with analog processing due to component imperfections, which are nei-
ther predictable nor controllable, and tend to increase as the fabrication technolo-
gies scale down. In particular, IQ imbalance can be categorized into frequency
flat and frequency selective. The former is generally induced by an imperfectly
balanced local oscillator (LO) which cannot produce equal amplitudes and an
exact 90 phase shift between the I and Q branches. The latter is primarily caused
by imperfections in other analog components, such as analog filters, amplifiers,
and digital-to-analog (D/A) or analog-to-digital (A/D) converters.
Orthogonal frequency division multiplexing (OFDM) has received intense in-
terest from the research community during the past few decades. Its robustness
to frequency selective channels has made it one of the main candidates for high
data rate transmission for current and next-generation wireless applications. In
order to make OFDM more reliable, several transmitter and receiver diversity
techniques utilizing space-time or space-frequency codes can be used. Alamouti-
based space time coding is one of the most effective transmitter diversity tech-
niques and when combined with OFDM, it enhances the system performance.
However, OFDM, like any other digital communication systems, requires reli-
able IQ estimation and compensation schemes. Unfortunately, due to the narrow
spacing and spectral overlap between the subcarriers, OFDM systems are much
more sensitive to the IQ mismatch than single carrier systems. It leads to a loss
of subcarrier orthogonality. This loss introduces intercarrier interference (ICI) which results in a degradation of the global system performance. Furthermore, higher order modulation is more vulnerable to IQ imbalance than lower order modulation. Additionally, the channel impulse response (CIR) must be known to coherently detect the transmitted data. Several IQ estimation and compensation algorithms considering either only a receiver IQ imbalance or a transmitter IQ imbalance individually have been developed.
In this thesis, we study two sources I/Q imbalance in the transmitter and
the receiver. In addition, combine the effects of IQ coefficients for both the trans-
mitter and receiver with the CIR into one parameter refereed as the overall CIR.
Based on few pilots, the Maximum Likelihood (ML) principle is then used to
estimate the overall CIR. By using the expectation maximization (EM) algorithm,
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. Furthermore,
the problem of IQ imbalance is investigated and treated for Alamouti Coded
OFDM Systems, and introduces an expectation maximization (EM) algorithm
for jointly estimating the channel impulse response and frequency selective
IQ imbalance in transmitter and receiver. Computer simulations confirm that
the proposed estimation schemes are able to mitigate the effects of IQ, the
multipath channel, and achieve additional diversity gain. In order to make the
EM algorithm suitable for more practical cases, we also obtain a sub-optimal
algorithm to reduce the complexity and keeping almost the same performance.