TABLE OF CONTENTS Multiuser systems with multiple antennas at the transmitter(s) and/or receiver(s) are called MIMO multiuser systems. These multiple antennas can significantly enhance performance in many ways. The antennas can be used to provide diversity gain to improve BER performance. The capacity region of the multiuser channel is increased with multiple transmit and receive antennas, providing multiplexing gain. Finally, multiple antennas can provide directivity gain to spatially separate users, which reduces interference. There typically are trade-offs among these three types of performance gains in MIMO multiuser systems [68].
The multiplexing gain of a MIMO multiuser system characterizes the increase in the uplink or downlink capacity region associated with adding multiple antennas. The capacity regions of MIMO multiuser channels have been extensively studied, motivated by the large capacity gains associated with single-user systems. For AWGN channels the MIMO capacity region for both the uplink and the downlink is described (respectively) in Section 14.6.3 and Section 14.5.5. These results can be extended to find the MIMO capacity region in fading with perfect CSI at all transmitters and receivers. Capacity results and open problems related to MIMO multiuser fading channels under other assumptions about channel CSI are described in [69].
Beamforming was discussed in Section 10.4 as a technique to achieve full diversity in single-user systems at the expense of some capacity loss. In multiuser systems, beamforming has less of a capacity penalty because of the multiuser diversity effect, and in fact beamforming can achieve the sum-rate capacity of the MIMO...
