Theses: Performance Limits of Spatial Multiplexing MIMO Systems
L. García Ordóñez

Abstract

Multiple-input multiple-output (MIMO) channels are an abstract and general way to model many different communication systems of diverse physical nature. In particular, wireless MIMO channels have been attracting a great interest in the last decade, since they provide significant improvements in terms of spectral efficiency and reliability with respect to single-input single-output (SISO) channels.

In this thesis we concentrate on spatial multiplexing MIMO systems with perfect channel state information (CSI) at both sides of the link. Spatial multiplexing is a simple MIMO transmit technique that does not require CSI at the transmitter and allows a high spectral efficiency by dividing the incoming data into multiple independent substreams and transmitting each substream on a different antenna. When perfect CSI is available at the transmitter, channel-dependent linear precoding of the data substreams can further improve performance by adapting the transmitted signal to the instantaneous channel eigen-structure. An example of practical relevance of this concept is given by linear MIMO transceivers, composed of a linear precoder at the transmitter and a linear equalizer at the receiver.

The design of linear MIMO transceivers has been extensively studied in the literature for the past three decades under a variety of optimization criteria. However, the performance of these schemes has not been analytically investigated and key performance measures such as the average bit error rate (BER) or the outage probability have been obtained through time-comsuming Monte Carlo simulations. In contrast to numerical simulations, which do not provide any insight on the system behavior, analytical performance expressions help the system designer to identify the degrees of freedom and better understand their influence on the system performance. This thesis attempts to fill this gap by providing analytical average and outage performance characterizations in some common MIMO channel models. More exactly, we derive exact expressions or bounds (depending on the case) for the average BER and the outage probability of linear MIMO transceivers designed under a variety of design criteria. Special attention is given to the high signal-to-noise ratio (SNR) regime, where the system performance is investigated under two different perspectives. First, from a more practical point-of-view, we characterize the average BER and outage probability versus SNR curves in terms of two key parameters: the diversity gain and the array gain. Then, we focus on the diversity and multiplexing tradeoff framework in order to take into consideration the capability of the system to deal with the fading nature of the channel, but also its ability to accommodate higher data rates as the SNR increases.

The performance of linear MIMO transceivers is simultaneously analyzed for the most common wireless MIMO channel models such as the uncorrelated and semicorrelated Rayleigh, and the uncorrelated Rician MIMO fading channels. For this purpose, we have obtained a general formulation that unifies the probabilistic characterisation of the eigenvalues of Hermitian random matrices with a specific structure, which includes the previous channel distributions as particular cases, i.e., the uncorrelated and semicorrelated central Wishart, the uncorrelated noncentral Wishart, and the semicorrelated central Pseudo-Wishart distributions. Indeed, the proposed formulation and derived results provide a solid framework for the analytical performance evaluation of MIMO systems, but it could also find numerous applications in other fields of statistical signal processing and communications.

Finally, and as a consequence of our performance analysis, limitations inherent to all practical linear MIMO transceiver designs have been enlightened. Accordingly, new schemes have been proposed which achieve considerable performance enhancements with respect to classical linear MIMO transceivers.


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