Image courtesy IEEE
Guide to Massive MIMO

Much ado about MIMO?  Not really. In its extended form -- Massive MIMO -- the technology has the ability to expand the role of wireless communication  to diverse areas  from infotainment to healthcare, from  smart homes and cities to  manufacturing.
Early experiments in Massive MIMO have already begun -- at  Test beds in IIT Delhi and Madras and IIIT Hyderabad and  by service providers like Airtel.
As a services to readers challenged by the buzzwords being bandied about, We have compiled the guide  below to MIMO and Multiple MIMO from  two sources. April 12 2018
What is MIMO?
MIMO stands for Multiple-input multiple-output. While it involves multiple technologies, MIMO can essentially be boiled down to this single principle: a wireless network that allows the transmitting and receiving of more than one data signal simultaneously over the same radio channel.  
MIMO was first proposed in 1992 by India-born  Professor (Emeritus) Arogyaswami Paulraj  of Stanford University (US) who holds the key patent in this technology . The concept is the key to today’s 4G cellular and WiFi wireless networks  and is likely to see widespread realization in the upcoming 5G networks in a year or two.
What is  massive MIMO?
Standard MIMO networks tend to use two or four antennas Massive MIMO, on the other hand, is a MIMO system with an especially high number of antennas.
There’s no set figure for what constitutes a Massive MIMO set-up, but the description tends to be applied to systems with tens or even hundreds of antennas. For example, Huawei, ZTE, and Facebook have demonstrated Massive MIMO systems with as many as 96 to 128 antennas.
Because MIMO systems need to physically pack more antennas into a small area, they require the use of higher frequencies (and hence shorter wavelengths) than current mobile network standards.
What are the advantages of Massive MIMO?
The advantage of a MIMO network over a regular one is that it can multiply the capacity of a wireless connection without requiring more spectrum. Early reports point to considerable capacity improvements, and could potentially yield as much as a 50-fold increase in future.
The more antennas the transmitter/receiver is equipped with, the more the possible signal paths and the better the performance in terms of data rate and link reliability.
The greater number of antennas in a Massive MIMO network will also make it far more resistant to interference and intentional jamming than current systems that only utilise a handful of antennas.
Massive MIMO networks will utilise beam forming technology, enabling the targeted use of spectrum. Current mobile networks share a single pool of spectrum with  all users in the vicinity, which  could result in a performance bottleneck in densely populated area. With Massive MIMO and beam forming this is  handled more efficiently, so  that data speeds  are uniform ( almost!) across the network.
For a fuller explanation of the above, see  the article What is Massive MIMO by John Mundy  at the UK 5G site  here 
What is Massive MIMO not!
Various large antenna systems are investigated and deployed in the quest for higher capacity and robust wireless access solutions. "Full dimension", "3D beam forming", or even hybrid beam forming, are sometimes marketed as Massive MIMO. Such solutions are less scalable with respect to the number of antennas, and importantly, they rely on specific properties of propagation or user location that limit their application to a substantial extent.
Point-to-point MIMO solutions with large arrays at the transmitter and/or receiver also are not Massive MIMO: Massive MIMO truly entails multiuser MIMO, relying on channel state information obtained through measurements on uplink pilots.
Why is Massive MIMO becoming a reality Right now? 
Since its inception about a decade ago, the Massive MIMO concept has evolved from a wild "academic" idea to one of the hottest research topics in the wireless communications community, as well as a main work item in 5G standardization.
The time for Massive MIMO has come at this moment for two reasons: First, conventional technology has proven unable to deliver the spectral efficiencies that 5G applications are calling for. Second, the confidence in the exceptional value of the technology has spread rapidly since impressive real-life prototypes showed record spectral efficiencies, and the robust operation with low-complexity RF and baseband circuits has been substantiated
What are the Operational Limits of Massive MIMO? 
Massive MIMO is scalable with respect to antennas; additional antennas always help. The ultimate limit is dictated by mobility: The higher the  mobility, the less the  channel coherence. In high-mobility macro-cellular environments (highway), this limits the multiplexing gain to some dozen of terminals but in environments with less or no mobility, hundreds or even thousands of terminals could in principle be multiplexed. The number of antennas that ultimately prove useful scales similarly, and are likely to be limited to one or a few hundred in macro-cellular, but potentially thousands in low-mobility applications.
Will Massive MIMO work in practice? 
Massive MIMO does work in practice! It has been demonstrated to achieve a record spectral efficiency of 145 bps/Hz in a real-life test bed experiment with 100 antennas at the base station. This demonstration does show an improvement of more than 20 times over 4G systems, yet was achieved in a controlled lab environment. Recently the proof was extended to true mobile access: communication with pedestrians and cars can be maintained. These experiments show that channels can be tracked sufficiently fast to spatially multiplex moving users.
In parallel, hardware implementations have been progressed very favorably. It has been technically proved that Massive MIMO systems can be built with very low complexity hardware, both for the digital baseband and the analog RF chains. Moreover, innovative architectures and circuits that exploit the specific nature of Massive MIMO systems are being designed. These will further reduce power and cost and promote an attractive deployment of Massive MIMO systems.
What could Massive MIMO offer in the future? 
Massive MIMO can offer enhanced broadband services in the future, and more. 5G networks are expected to support a great variety of wireless services in areas ranging from infotainment to healthcare, smart homes and cities, manufacturing, and many others. Massive MIMO technology can be tailored to support a massive number of Massive Machine Type Communication (MTC) devices. Also, it is an excellent candidate to realize Ultra Reliable Communication as it can establish very robust physical links.
This treatment can be seen in detail in the IEEE 5G Tech Focus article by  Erik G. Larsson, Linköping University, Linköping, Sweden and Liesbet Van der Perre, KU Leuven, Leuven, Belgium