In communications, noise spectral density (NSD), noise power density, noise power spectral density, or simply noise density (N0) is the power spectral densityofnoise or the noise power per unit of bandwidth. It has dimensionofpower over frequency, whose SI unit is watt per hertz (equivalent to watt-secondorjoule). It is commonly used in link budgets as the denominator of the important figure-of-merit ratios, such as carrier-to-noise-density ratio as well as Eb/N0 and Es/N0.
If the noise is one-sided white noise, i.e., constant with frequency, then the total noise power N integrated over a bandwidth BisN = BN0 (for double-sided white noise, the bandwidth is doubled, so NisBN0/2). This is utilized in signal-to-noise ratio calculations.
For thermal noise, its spectral density is given by N0 = kT, where k is the Boltzmann constant in joules per kelvin, and T is the receiver system noise temperatureinkelvins.
The noise amplitude spectral density is the square root of the noise power spectral density, and is given in units such as 
Amplitude spectral density is computed as … The units are then in Vrms/√Hz or V/√Hz
The Amplitude Spectral Density is also used to analyze noise signals. It has units of V/√ Hz in the analog domain and FS/√ Hz in the digital domain.
