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Since the seminal paper by Marzetta from 2010, Massive MIMO has changed frombeing a theoretical concept with an infinite number of antennas to a practicaltechnology. The key concepts are adopted in 5G and base stations (BSs) with$M=64$ full-digital transceivers have been commercially deployed in sub-6\,GHzbands. The fast progress was enabled by many solid research contributions ofwhich the vast majority assume spatially uncorrelated channels and signalprocessing schemes developed for single-cell operation. These assumptions makethe performance analysis and optimization of Massive MIMO tractable but havethree major caveats: 1) practical channels are spatially correlated; 2) largeperformance gains can be obtained by multicell processing, without BScooperation; 3) the interference caused by pilot contamination creates a finitecapacity limit, as $M\to\infty$. There is a thin line of papers that avoidedthese caveats, but the results are easily missed. Hence, this tutorial articleexplains the importance of considering spatial channel correlation and usingsignal processing schemes designed for multicell networks. We present recentresults on the fundamental limits of Massive MIMO, which are not determined bypilot contamination but the ability to acquire channel statistics. Theseresults will guide the journey towards the next level of Massive MIMO, which wecall ``Massive MIMO 2.0''.

Towards Massive MIMO 2.0: Understanding spatial correlation, interference suppression, and pilot contamination