MIMO signal processing problem, how can I understand E[s*s']=I(unity matrix) ?

right now I am trying to simulate a scenario of MIMO: the base station transmitter has M antennas, and its beam former matrix V is M x d, its transmit signal s is d x 1 vector, then

(1) if s is assumed to be satisfied with E[s*s']=I(E is expectation), what does that stand for? can I say s is a unity signal and there is Irrelevance among different elements? but how can I generate such a signal that satisfy the assumption? (2) in the paper the author assumes the column of beam former matrix is d, so can I choose the value of d randomly? is there any material of relative knowledge of MIMO which I can read firstly?

thank you so much in advance for the answer and explanation!

Answer 1

[Jie]

anybody please help me with this! thanks!