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But also the ideal condition of 4 aligned lamba/2 antennas on each side only as good as there is a propagation difference between all the antenna paths. If the difference is smaller than the path the S/N goes down.
Another view of MIMO ist that it could improve the link budget over the worst case by providing robustness against frequency selective fading. So the practical use for less scatter rich enviroments is to avoid the worts case fading situation. So instead boosting marketing data rate help to be connected for bad links.
Nevertheless MIMO is a natural trade of spectrum conservation over hardware cost and energy.