In recent years, with an ever increasing number of wireless services to be allocated within a limited frequency resource, much research has focused on the development of spectrally efficient transmission technologies at the physical layer. The most promising among these technologies is that of MIMO systems, which prescribe radio links with multiple antennas at both ends. A critical factor in determining the performance of a MIMO system is the availability of some information about the propagation channel (Channel State Information, CSI) at the receiver and, possibly, at the transmitter. This...