It is not hard to predict that use of massive multiple input-multiple output radio technologies is going to grow, as advanced 4G and 5G networks are built. Massive MIMO is required to make use of vast new spectrum resources to be released in the millimeter wave region to support 5G.
In fact, massive MIMO is intrinsically related to use of small cells, ultra-dense cell networks and millimeter wave frequencies.
Massive MIMO trials or limited deployments in 2017 were undertaken by Sprint, Deutsche Telekom, China Mobile, China Telecom, China Unicom, Singtel, T-Mobile Netherlands, Vodafone Australia, Optus, and Telefónica. Massive MIMO also is being developed by Telecom Infra, the open source telecom infrastructure effort.
The spectrum bands at which many of these trials have taken place include 2.5 GHz, 2.6 GHz, 3.5 GHz, 1.8 GHz, and 2.3 GHz. Except for 3.5 GHz, the remaining frequencies are also allocated for LTE in many countries. Telecom Infra is testing much-higher frequencies (60 GHz), designed in the U.S. market to use unlicensed spectrum.
MIMO antenna technology has been in use since the launch of 802.11n WiFi systems, but was first ratified for use in cellular systems in 3GPP’s Release 7 in 2008.
Deployments below 1 GHz are most likely to support eight or 16 antenna elements at most. Very-high frequencies above 30 GHz can have hundreds of antenna elements with some research citing below 500 antennas as an upper limit.