Professor Emil Björnson (KTH, Sweden) answers the question “How many beams can you send from a MIMO array?”, which he has received repeatedly in emails, on his blog, and in comments on this YouTube channel.
The answer is not what you think it is, unless we reformulate the question.
Emil illustrates and explains the basic concepts in an accessible manner.
Watch the Video on YouTube: 
https://www.youtube.com/watch?v=tXDN4BFSSnI