Massive MIMO is a Reality -- What is Next? Five Promising Research Directions for Antenna Arrays

TL;DR

Massive MIMO has become a standard technology in 5G networks, with ongoing research exploring advanced antenna array designs, positioning, radar, and intelligent systems to expand its applications beyond traditional communication.

Contribution

The paper reviews the current state of Massive MIMO and introduces five promising research directions for future exploration.

Findings

Massive MIMO is now a commercial and standard technology in 5G.

Development of fully digital Massive MIMO arrays for mmWave is progressing.

Five new research directions are proposed for expanding Massive MIMO applications.

Abstract

Massive MIMO (multiple-input multiple-output) is no longer a "wild" or "promising" concept for future cellular networks - in 2018 it became a reality. Base stations (BSs) with 64 fully digital transceiver chains were commercially deployed in several countries, the key ingredients of Massive MIMO have made it into the 5G standard, the signal processing methods required to achieve unprecedented spectral efficiency have been developed, and the limitation due to pilot contamination has been resolved. Even the development of fully digital Massive MIMO arrays for mmWave frequencies - once viewed prohibitively complicated and costly - is well underway. In a few years, Massive MIMO with fully digital transceivers will be a mainstream feature at both sub-6 GHz and mmWave frequencies. In this paper, we explain how the first chapter of the Massive MIMO research saga has come to an end, while the story has just begun. The coming wide-scale deployment of BSs with massive antenna arrays opens the door to a brand new world where spatial processing capabilities are omnipresent. In addition to mobile broadband services, the antennas can be used for other communication applications, such as low-power machine-type or ultra-reliable communications, as well as non-communication applications such as radar, sensing and positioning. We outline five new Massive MIMO related research directions: Extremely large aperture arrays, Holographic Massive MIMO, Six-dimensional positioning, Large-scale MIMO radar, and Intelligent Massive MIMO.