Feedback methods for multiple-input multiple-output wireless systems
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The availability and performance of wireless communication systems have grown at unprecedented rates over the last ﬁfteen years. In order to maintain these growth rates, next generation wireless systems must supply both reliability and high data rates using a ﬁxed amount of spectrum and limited transmit power. Multiple-input multiple-output (MIMO) wireless communication systems, which use multiple antennas at both the transmitter and receiver, are expected to be one of the enabling technologies for next generation wireless systems. MIMO wireless systems provide both data rate and reliability improvements by designing signals over space as well as time and/or frequency. One of the key features of wireless links is the phenomenon known as fading. In a narrowband wireless link, fading can be modeled as a multiplicative channel gain. Many of the beneﬁts promised by MIMO wireless systems will not be realized without the availability of channel gain information at the transmitter. Current research focuses on the usage of MIMO signaling where the transmitter does not have any kind of channel information. When some form of channel information is available, however, the MIMO signal can be adapted to the current channel conditions to improve performance. Unfortunately, many wireless systems will not have any form of a priori channel knowledge without feedback from the receiver to transmitter. Because feedback will occupy a percentage of the data-rate in the reverse wireless link (i.e. the wireless link where the receiver terminal serves as the transmitter), feedback must be kept to a limited number of bits. This dissertation describes new, practical methods for limited feedback space-time signaling. It develops space-time techniques that provide improved performance when channel information, in the form of a ﬁxed number of bits, is conveyed from the receiver to the transmitter. Limited feedback techniques are developed for linearly precoded orthogonal space-time block codes and linearly precoded spatial multiplexing. A new adaptive modulation technique for linearly precoded spatial multiplexing called multi-mode precoding is presented. Past research in the area of space-time signaling is also overviewed.