Massive MIMO is the new wireless access technology in 5G, in both sub-6 GHz and mmWave bands. Since its inception about a decade ago, it has evolved from a wild “academic” idea to become the core technology that likely will be utilized in all future wireless technologies.

Massive MIMO is a multi-user MIMO (multiple-input multiple-output) technology that can provide uniformly good service to wireless terminals in high-mobility environments. The key concept is to equip base stations with arrays of many antennas, which are used to serve many terminals simultaneously, in the same time-frequency resource. The word “massive” refer to the number of antennas and not the physical size. The antenna arrays have attractive form factors: in the 2 GHz band, a half-wavelength-spaced rectangular array with 200 dual-polarized elements is about 1.5 x 0.75 meters large. Massive MIMO operates in TDD mode and the downlink beamforming exploits the uplink-downlink reciprocity of radio propagation. Specifically, the base station array uses channel estimates obtained from uplink pilots transmitted by the terminals to learn the channel in both directions. This makes Massive MIMO entirely scalable with respect to the number of base station antennas. Base stations in Massive MIMO operate autonomously, with no sharing of payload data or channel state information with other cells.

Useful links:

• Textbook: Fundamentals of Massive MIMO
• Textbook: Massive MIMO Networks: Spectral, Energy, and Hardware Efficiency
• YouTube channel “Wireless Future” with many videos on Massive MIMO
• Massive MIMO for next generation wireless (magazine article)
• Massive MIMO: ten myths and one critical question (magazine article)