Noise spectral density

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In communications, noise spectral density N_o is the noise power per unit of bandwidth; that is, it is the power spectral density of the noise. It has units of watts/hertz, which is equivalent to watt-seconds or joules. If the noise is white, i.e., constant with frequency, then the total noise power N in a bandwidth B is BN_o. This is utilized in Signal-to-noise ratio calculations.

The thermal noise density is given by N_o = kT, where k is Boltzmann's constant in joules per kelvin, and T is the receiver system noise temperature in kelvin.

N_o is commonly used in link budgets as the denominator of the important figure-of-merit ratios E_b/N₀ and E_s/N₀.