power spectral density
PSD gives you the power of a random signal as a function of frequency ie with it, you can find how much power the signal has a given frequency.
Variance is a measure of the average power of a signal.
For white noise, the power is the same at all frequencies, thus you can simply say the PSD is No, because it is No at all frequencies.
variance of a WSS signal = 2 * PSD integrated from 0 Hz to ∞ hertz.
for WGN bandlimited to B Hz
variance = 2 * B * No
-b