IEEE Signal Processing Magazine, Vol. 24, No. 5, pp. 36-36, Oct., 2007
Multiuser information theory reveals the limitations associated with the conventional application of MIMO communication as a physical layer modulation enhancement. In multiuser MIMO networks, the spatial degrees of freedom offered by the extra antennas can be advantageously exploited to enhance the multiple access, by scheduling multiple users to simultaneously share the spatial channel. In the cellular downlink, realizing the gains of MIMO antennas in the form of spatial division multiple access however entails a departure from single user toward multiuser communications. A fundamental paradigm shift from single user communications lies in that multiuser systems can experience substantial benefit from channel state information at the transmitter and, at the same time, require more complex scheduling strategies and transceiver methodologies. This paper reviews multiuser MIMO communication from an algorithmic perspective, discussing performance gains, tradeoffs, and practical considerations. Several approaches including channel-aware nonlinear and linear precoding are reviewed, along with more practical limited feedback schemes that require only partial channel state information. The interaction between precoding and scheduling is discussed. Several promising strategies for partial multiuser feedback design are looked at, some of which are inspired from the single user MIMO precoding scenario while others are fully specific to the multiuser setting.