explanation on the mimo technique In MIMO systems we have:
$y=Hx+n$ with $H$ is a matrix $N_r \times N_t$.

But the previous equation is only true for non-frequency selective channels.

What is the formula for frequency-selective channels?



Normally, $y=h \star x +n$. But if H is a matrix, we can not the convolution. How to solve this problem
The input-otput relation for frequency selective channel:

$y_i=\sqrt{\frac{P}{M_T}}H_is_i+n_i$

More information can be found here.