Tom Marzetta of Bell Labs has been called the "Father of Massive MIMO." His 2010 paper, Noncooperative Cellular Wireless with Unlimited Numbers of Base Station Antennas, has been cited over 1700 times. He and Bell Labs colleague Gerry Foschini have been working on MIMO since the 1990's and made many contributions. For those who want to understand in depth, I've included the abstract of that paper and others below.
In 2014, Tom told me he thought it would take several more years for practical systems. Masayoshi Son of Softbank was unwilling to wait and launched the first commercial deployment in September, 2016. Softbank is installing 100 systems across 43 cities in Japan. Softbank's early results, from five cities, show a 5X to 10X improvement in the same spectrum. They use 128 antennas. Some of the antennas are used for "beamforming," which is proving crucial for deployments. Many top engineers expect a 50X improvement from MIMO in the coming years. Remarkably, the increased performance does not require significantly more power.
Marzetta writes, "Massive MIMO is the most promising technology available to address the ever increasing demand for wireless throughput:
• Orders of magnitude spectral efficiency gains over LTE - large numbers of users communicate simultaneously over entire allotted spectrum through elementary multiplexing signal processing
• Uniformly excellent service throughout the cell - regardless of location relative to base station
• Drastically reduced radiated power
• Simple and scalable design - employs measured channel characteristics rather than assumed channel characteristics
• Naturally green technology - superior energy efficiency