Massive MIMO fundamentals

Massive multiple‐input and multiple‐output (MIMO) is a scalable version of multiuser MIMO where the base stations are equipped with many antennas. It provides very high array gain and multiplexing gain and, hence, offers huge spectral and energy efficiency. In addition, by exploiting favorable propagation and channel hardening properties, simple linear processing can obtain nearly optimal performance that makes massive MIMO easy to implement in practice. Therefore, massive MIMO has gained significant research attention over the past 10 years and became one of the key technologies for fifth‐generation (5G) networks and beyond. Very recently, a massive MIMO system with 64‐antenna base station has been commercially deployed for 5G. In this article, we present a comprehensive overview of state‐of‐the‐art research on massive MIMO. Fundamental aspects of this system will be discussed in detail. The discussion will serve as a good roadmap for start‐up and consequently motivate researchers from both industry and academia to work in this area. First, we provide the basic transmission protocol of massive MIMO system under time‐division duplex operation. Then, the derivations of spectral efficiency using the well‐known “use‐and‐then‐forget bounding” technique in massive MIMO are reviewed. Duplexing operations, favorable propagation, channel hardening, and pilot contamination are also detailed. Finally, we conclude with a range of important topics and future directions.