The noise PSD is constant with frequency and equals to the variance of Gaussian R,V which is conventionaly No/2, we usually consider No to be the noise power for convinience
Hi
when u use y=awgn(x,snr,'measured'), u have the SNR value since u urself set it in the formula, u shold now calculate the power of x and convert it in dB, then using this formula:
power(x)-N0/2=snr.