By employing spatial multiplexing, Multiple-Input Multiple-Output (MIMO) wireless antenna systems provide increases in capacity without the need for additional spectrum or power. Zero-Forcing (ZF) detection is a simple and effective technique for retrieving multiple transmitted data streams at the receiver. However the detection requires knowledge of the channel state information (CSI) and in practice accurate CSI may not be available. In this letter, we investigate the effect of channel estimation error on the performance of MIMO ZF receivers in uncorrelated Rayleigh flat fading channels. By modeling the estimation error as independent complex Gaussian random variables, tight approximations for both the post-processing SNR distribution and bit error rate (BER) for MIMO ZF receivers with M-QAM and M-PSK modulated signals are derived in closed-form. Numerical results demonstrate the tightness of our analysis. Index Terms-MIMO, MIMO ZF receiver, channel estimation error.

Due to the use of commodity software and hardware, crash-stop and Byzantine failures are likely to be more prevalent in today's large-scale distributed storage systems. Regenerating codes have been shown to be a more efficient way to disperse information across multiple nodes and recover crash-stop failures in the literature. In this paper, we present the design of regeneration codes in conjunction with integrity check that allows exact regeneration of failed nodes and data reconstruction in presence of Byzantine failures. A progressive decoding mechanism is incorporated in both procedures to leverage computation performed thus far. The fault-tolerance and security properties of the schemes are also analyzed.

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SummaryThe idea of formulating the detection of a lattice-type modulation, such as M-PAM and M-QAM, transmitted over a linear channel as the so-called universal lattice decoding problem dates back to at least the early 1990s.The applications of such lattice decoders have proliferated in the last few years because of the growing importance of some linear channel models, such as multipleantenna fading channels and multi-user CDMA channels. The principle of universal lattice decoding can trace its roots back to the theory and algorithms developed for solving the shortest/closest lattice vector problem for integer programming and cryptoanalysis applications. In this semi-tutorial paper, such a principle as well as some related recent advances will be reviewed and extended. It will be shown that the lattice basis reduction algorithm of Lenstra, Lenstra and Lovász can significantly improve the performance of suboptimal lattice decoders, such as the zero-forcing and VBLAST detectors. In addition, new implementation of the optimal lattice decoder that is particularly efficient at moderate to high signal-to-noise ratios will also be presented.

Dedicated to my mother and the memory of my father ii Acknowledgement I would like to acknowledge my supervisor, Dr. C. P. Kwong, who introduced me the sequence estimation problem and encouraged me to look at it from new viewpoints. It is also him who taught me the right attitude, which I believe every true researcher ought to have, towards a difficult problem.iii Abstract It is well-known that the use of the Viterbi algorithm to implement a sequence estimator is an optimal way to remove the effect of intersymbol interference for digital transmission systems. However, such an implementation usually results in a very complicated receiver. In this thesis, we transform the problem of maximum likelihood sequence estimation into the problem of finding the closest lattice point. Some related lattice algorithms such as the basis reduction algorithms and the enumeration algorithms are analyzed and some improved versions are suggested. Then efficient algorithms finding the nearest lattice point are derived. Based on these lattice algorithms, simple but effective sequence estimators are proposed for the PAM systems and their complexities are analyzed. Under some mild assumptions, our algorithms have both polynomial space and time complexities, and are therefore much superior to the conventional Viterbi detectors. Simulation results on three different channels show that the performance of the new sequence estimators depend on the distance spectrum of the channel. But, general speaking, the performance approaches optimal as the size of the signal set and the signal-to-noise ratio increase. Finally, the extensions to other lattice-type modulation schemes and the impacts of the lattice viewpoint on the design of bandlimited transmission systems are discussed.iv

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