MIMO rank -- please define/describe and provide online references defining "rank"?

Answer 1

This is a poorly worded question.

MIMO in controls in Multi-Input, Multi-Output and in a linear system can be described by a matrix. In linear algebra the rank of a matrix is the number of independent rows or columns. Being rank deficient in a MIMO system can cause problems in a control system. In most cases you want full rank. So if you are talking controls then...

Now in wireless communication the number of transmit streams is also known as MIMO rank (see http://www.wirelessnetdesignline.com/showArticle.jhtml?articleID=173401515 ), so if you are talking wireless communication there is your answer.