Multipath fading has been long viewed as an impairment in wireless communication systems that limits the reliability and data rate of the communication link. By deploying multiple antennas at the transmitter and receiver, multipath fading can be turned into an advantage, allowing for greater reliability and higher data rates than would otherwise be possible. Furthermore, the rate and reliability benefits can be achieved without extra cost of bandwidth, making multiple antenna technology a cornerstone of current and future wireless systems.;The potential benefits of multiple antenna systems can be harnessed through the use of space-time coding. In space-time coding, information symbols are coded across two dimensions, the spatial dimension, which corresponds to the multiple antennas at the transmitter, and time dimension, which corresponds to the multiple signaling intervals. In this thesis, we focus on linear space-time block codes, in which the information symbols are linearly combined to form a two-dimensional code matrix, wherein the rows of the matrix correspond to transmission across multiple intervals, and the columns of the matrix correspond to transmission from different antennas.;In this thesis, we consider the problem of designing space-time block codes that have low maximum-likelihood (ML) decoding complexity. We first present a unified framework for determining the worst-case ML decoding complexity of space-time block codes. We use this framework to not only determine the worst-case ML decoding complexity of our own constructions, but also to show that some popular constructions of space-time block codes have lower ML decoding complexity than was previously known. Specifically, we show that the golden code, which harnesses both the reliability and rate benefits of the two-input two-output channel, has a worst-case ML decoding complexity that is substantially lower than that of an exhaustive-search decoder.;Recognizing the practical importance of the two transmit and two receive antenna system, we propose the asymmetric golden code, which is designed specifically for low ML decoding complexity. Unlike some previous constructions, which lose their reduced complexity decoding when the channel varies during the transmission period of the code matrix, the asymmetric golden code maintains its low decoding complexity regardless of channel variability. The asymmetric golden code has the lowest decoding complexity compared to previous constructions of space-time codes, regardless of whether the channel varies with time.;Space-time codes that layer or multiplex rate-one space-time codes to achieve arbitrary rates ranging from one to maximal rate have been proposed in literature. Two of the most important constructions are the threaded algebraic and perfect space-time codes. These codes, however, suffer from high decoding complexity and worse bit-error-rate performance when compared to other space-time codes that have been proposed for a particular rate and a particular number of transmit and receive antennas. In this research, we propose the embedded orthogonal space-time codes, which is a family of codes for an arbitrary number of antennas, and for any rate up to half the number of antennas. The family of embedded orthogonal space-time codes is the first general framework for the construction of space-time codes with low-complexity decoding, not only for rate one, but for any rate up to half the number of transmit antennas. Simulation results for up to six transmit antennas show that the embedded orthogonal space-time block codes are simultaneously lower in complexity and lower in error probability when compared to some of the most important constructions of space-time block codes with the same number of antennas and the same rate larger than one.;Having considered the design of space-time block codes with low ML decoding complexity on the transmitter side, we also develop efficient algorithms for ML decoding for the golden code, the asymmetric golden code and the embedded orthogonal space-time block codes on the receiver side. Simulations of the bit-error rate performance and decoding complexity of the asymmetric golden code and embedded orthogonal codes will be used to demonstrate their attractive performance-complexity tradeoff.
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