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Massive MIMO - What is it ?
As the name implies, Massive MIMO is an antenna array system using Massive Amount of Antenna. It is also called Large Scale MIMO.
Then you may ask "How many Antenna is required to be called 'Massive (Large Scale) MIMO' ?".
The answer may be different depending on how you design the reciever algorithm (equalizer design), but assuming that we use the simplest (the most primitive) reciever design, we may need over 300 antenna. If you think this is too big number, you may decrease the number by applying a little bit smarter reciever (equalizer) algorithm.
In some of the prototype system that can be called as Massive MIMO, I see the number to be 64, 256 but you would see different numbers as more prototypes are implemented. According to R1-163132 section 2.5, it is said that the number of antenna will be up to 256 in DL and 32 in UL in 5G (Ref )
In conventional LTE using a normal MIMO, the maximum number of antenna in MIMO as of now (Mar 2015) is 8 x 2 or 4 x 4 and recently (Apr 2016) even 8 x 8 is mentioned.
For further details of the Massive MIMO, refer to following pages.
 GFDM Interference Cancellation for Flexible Cognitive Radio PHY Design
R. Datta, N. Michailow, M. Lentmaier and G. Fettweis
Vodafone Chair Mobile Communications Systems,
Dresden University of Technology,
01069 Dresden, Germany
Email:[rohit.datta, nicola.michailow, michael.lentmaier, fettweis]@ifn.et.tu-dresden.de
 5G NOW. D3.1 5G Waveform Candidate Selection
 Massive MIMO and Small Cells : Improving Energy Efficiency by Optimal Soft-Cell Coordination
Emil Bjornson, Marios Kountouris and Merouane Debbah
Alcatel-Lucent Chair on Flexible Radio, SUPELEC, Gif-sur-Yvette, France
Department of Telecommunications, SUPELEC, Gif-sur-Yvette, France
ACCESS Linnaeus Center, Signal Processing Lab, KTH Royal Institue of Technology, Stockholm, Sweden
 Massive MIMO for Next Generation Wireless Systems
Erik G. Larson, ISY, Linkoping University, Sweden
Ove Edfors, Lund University, Sweden
Fredrik Tufvesson, Lund University, Sweden
Thomas L. Marzetta, Bell Labs, Alcatel-Lucent, USA
 Scaling up MIMO : Opportunities and Challenges with Very Large Arrays
Fredrik Rusek, Dept. of Electrical and Information Technology, Lund University, Lund, Sweden
Daniel Persson, Dept. of Electrical Engineering (ISY), Linkoping University, Sweden
Buon Kiong Lau, Dept. of Electrical and Information Technology, Lund University, Lund, Sweden
Erik G. Larsson, Dept. of Electrical Engineering (ISY), Linkoping University, Sweden
Thomas L. Marzetta, Bell Laboratories, Alcatel-Lucent, Murray Hill, NJ
Ove Edfors, Dept. of Electrical and Information Technology, Lund University, Lund, Sweden
Fredrik Tufvesson, Dept. of Electrical and Information Technology, Lund University, Lund, Sweden
 3GPP R1-163132. 3GPP TSG RAN WG1 Meeting #84bis - Discussion on the frame structure design for NR