snr = 20;
x = [7.0 + 0.7i; -0.7 - 0.7i ; 0.7 + 0.7i; -0.7 + 0.7i];
h = (randn(4,4) + 1j.*randn(4,4))/sqrt(2);
y = h * x;
yi = awgn(y,snr,'measured');