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

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Jie on 22 Dec 2014

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https://www.mathworks.com/matlabcentral/answers/167512-mimo-signal-processing-problem-how-can-i-understand-e-s-s-i-unity-matrix

Answered: Jie on 23 Dec 2014

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!