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Performance Study of Nakagami-m Fading Channels with Correlated Unbalanced Branches
MS Thesis
In the design of wireless communication systems for use over wireless multipath fading channels, the designer needs to have accurate and time non-consuming tools to figure out in advance the potential performance and efficiency of the to be deployed designs. Analytic methods relying on the use of tested statistical channel models offer an insight into performance limits and performance dependences on system parameters of interest. It also significantly speeds up the attaining of usable results relative to computer simulations or field tests and experiments, which often don’t represent all possible conditions. In addition, simulation and field experiments only offer a glimpse of the whole picture through samples of reality depending on the specific field circumstances at experiment time and location, or on the choice of simulation parameters.
On the other hand, using Mathematical Analytic tools for measuring system performance, we could quantify the tradeoff between performance and complexity for all possible fading conditions and system implementations. And these tools would prove indispensable for the study for accurate system design, improvement, and optimization.
Thus, the derivation of analytic closed form expressions, or numerical computable expressions, would provide insightful design tools for the wireless communication designers.
In this thesis such expressions are derived to study the performance of implementing the Generalized Selection Combining (GSC) scheme in the design of Rake receivers for Wideband Code Division Multiple Access (WCDMA) systems. WCDMA is a core constitute of the standards proposed for the next generation mobile systems. It promises to offer high data exchange rates, with support for multimedia communications, roaming, user privacy and security, and other many features and services depending on market acceptance and user demand, which only future could reveal. The WCDMA system is characterized by high transmission bandwidth that often exceeds the fading channel’s coherence bandwidth, thus the fading channel appears frequency selective and time resolution of many paths is available at the receiver. The Rake receiver is a special receiver architecture that benefits from this inherent diversity. The combining methodology for such diversity system could be either maximal ratio combining MRC, selection combining SC, or generalized selection combining GSC.
GSC is a hybrid technique that is less complex than MRC, and less performance costly than SC. When used along with WCDMA, it would offer fixed complexity, better channel tractability, and robustness with affordable performance loss with respect to MRC.
In this thesis, three methods will be derived for the performance evaluation of GSC with generic fading statistics, and a wide variety of modulation techniques. The three methods complement each others limitations, and would offer a package of analytic tools for the evaluation of different performance metrics for WCDMA in generalized fading channels.
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Channel Matched Iterative Decoding for Magnetic Recording Systems
PhD Thesis
The perpendicular magnetic recording channel (PMRC) is corrupted by sever intersymbol
interference and data-dependent media noise, in addition to a variety of other bursty
impairments. Thus far, the hard decodable symbol correcting Reed-Solomon (RS) code has
been the industry standard for outer error control coding (ECC). This thesis proposes two
novel ECC schemes in the migration toward next generation high density recording. The
first scheme is a two-level concatenation of channel-matched turbo equalization (TE) and
outer RS, replacing current inner parity correction codes. Conventional TE is matched
to the channel via the incorporation of the error pattern correction code (EPCC), which
works iteratively with the other constituent code in TE, whether block or convolutional,
to suppress the occurrence of low-Euclidean-distance errors at the output of the channel
detector. To understand this mechanism, and with no loss of generality, we derive the
error Euclidean distance distribution of TE-EPCC for the Dicode channel, and show that
EPCC substantially increases the interleaver gain exponent of low Euclidean weight errors.
Furthermore, we derive an upper bound on the BER of TE-EPCC, and employ it to show
that TE-EPCC delivers significant gains in the error floor and cliff regions compared to
conventional precoded and unprecoded TE for a variety of channel conditions and code
rates. The second proposed ECC system is a tensor product concatenation of EPCC and
Q-ary LDPC (T-EPCC-QLDPC). This concatenation scheme enables the use of byte-long
component EPCC without jeopardizing the overall code rate. Hence, the multiple error
correction capability of EPCC is maintained at very low signal-to-noise ratios, while the
component non-binary LDPC insures correct syndromes are available for the decoding of
tensor symbols (EPCC code-blocks). We introduce a low complexity iterative soft decoder
of T-EPCC-QLDPC, in which the component EPCC and QLDPC exchange multi-level loglikelihood
ratios (mlLLR) that represent their beliefs on the reliability of error-syndromes.
Moreover, we show that the two-level decoder provides a better performance-complexity
tradeoff compared to single-level binary and Q-ary LDPC.
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