In communications, noise spectral density N0 is the noise power per unit of bandwidth; that is, it is the power spectral density of the noise. It has dimension of power/frequency (see dimensional analysis), whose SI coherent unit is watts per hertz, which is equivalent to watt-seconds. If the noise is white noise, i.e., constant with frequency, then the total noise power N in a bandwidth B is BN0. This is utilized in signal-to-noise ratio calculations.
The thermal noise density is given by N0 = kT, where k is Boltzmann's constant in joules per kelvin, and T is the receiver system noise temperature in kelvins.
N0 is commonly used in link budgets as the denominator of the important figure-of-merit ratios Eb/N0 and Es/N0.