Through the use of spatial multiplexing, multi-input, multi-output (MIMO) antenna technologies allow the transmission of multiple parallel data streams over the same time-frequency resources. Multi-user MIMO (MU-MIMO) offers the benefits of MIMO to multiple users, and is of particular interest to cellular systems and wireless local-area networks. While MU-MIMO deployments to date have involved a small number of base station antennas (for instance, LTE-Advanced, as standardized by 3GPP Release 10, uses 8 antenna ports per cell sector), recent research has shown that it may be possible and indeed desirable to deploy a very large number of antennas at the base station. Such technologies are called massive MIMO, and are characterized by having many more antennas at the base station than active mobiles within the cell. Although it can be applied to frequency-division duplexing (FDD) based systems, most massive MIMO systems use time-division duplexing (TDD), where, thanks to channel reciprocity, the number of pilots needed for channel estimation scales in the number of users rather than in the number of base station antennas. Typical design targets for massive MIMO are a few hundred antennas at the base station serving dozens of mobiles. Having so many antennas allows energy to be sharply focused into a particular point in space, providing a dramatic improvement in energy efficiency, while the excess of antennas provides additional degrees of freedom that provide opportunities for interference suppression and the use of reduced complexity hardware.
While massive MIMO is still in its early stages of development, it is such a radical departure for current technologies that it stands to revolutionize the way that future cellular networks are designed, standardized, and implemented. In this Best Readings, we introduce several archival papers and special issues on the topic of massive MIMO that are available on IEEE Xplore.
Issued December 2014 and Updated July 2017
Erik G. Larsson
Dept. of Electrical Engineering
Matthew C. Valenti
Lane Dept. of Computer Science and Electrical Engineering
West Virginia University
Morgantown, WV, USA
Cullen Trust Endowed Professor
Dept. of Electrical and Computer Engineering
The University of Texas at Austin
Austin, Texas, USA