TABLE OF CONTENTS Wireless system designers are facing a number of challenges. These include the limited availability of the radio frequency spectrum and a complex space-time varying wireless environment. In addition, there is an increasing demand for higher data rates, better quality of service, and higher network capacity. In recent years, Multiple-Input Multiple-Output (MIMO) systems have emerged as a most promising technology in these measures. MIMO communication systems can be defined intuitively [GSS+03, PNG03] by considering that multiple antennas are used at the transmitting end as well as at the receiving end. The core idea behind MIMO is that signals sampled in the spatial domain at both ends are combined is such a way that they either create effective multiple parallel spatial data pipes (therefore increasing the data rate), and/or add diversity to improve the quality (bit-error rate or BER) of the communication.
Clearly, the benefits from multiple antennas arise from the use of a new dimension space. Hence, because the spatial dimension comes as a complement to time (the natural dimension of digital communication data), MIMO technology is also known as 'space-time' wireless or 'smart' antennas. Until the 1990s, the use of antenna arrays at one end of the link was mainly oriented to the estimation of directions of arrival as well as diversity, leading to beamforming and spatial diversity. Beamforming is a powerful technique which increases the link signal-to-noise ratio (SNR) through focusing the energy into desired directions. The concept of spatial diversity is...
