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Reference.org
Noise spectral density
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<p>In communications, noise spectral density (NSD), noise power density, noise power spectral density, or simply noise density (<i>N</i>0) is the <a href="/facts/Power_spectral_density/q3Rfs7b0">power spectral density</a> of <a href="/facts/Electronic_noise/Izbv38Rr">noise</a> or the <a href="/facts/Noise_power/orFSKhF4">noise power</a> per unit of <a href="/facts/Bandwidth_(signal_processing)/7eMDzfJI">bandwidth</a>. It has <a href="/facts/Dimensional_analysis/t6UskoCD">dimension</a> of <a href="/facts/Power_(physics)/mkaEyRnn">power</a> over <a href="/facts/Frequency/QUSbPaW8">frequency</a>, whose <a href="/facts/SI_unit/S7YLglNz">SI unit</a> is watt per hertz (W/Hz), equivalent to <a href="/facts/Watt-second/9ndq9jD2">watt-second</a> (W ⋅ s) or <a href="/facts/Joule_(unit)/9ndq9jD2">joule</a> (J). It is commonly used in <a href="/facts/Link_budget/MjBDlWAh">link budgets</a> as the denominator of the important figure-of-merit ratios, such as <a href="/facts/Carrier-to-noise-density_ratio/DP42mbjT">carrier-to-noise-density ratio</a> as well as <a href="/facts/Eb/N0/DU5sMhd6"><i>E</i><i>b</i>/<i>N</i>0</a> and <a href="/facts/Es/N0/DU5sMhd6"><i>E</i><i>s</i>/<i>N</i>0</a>. </p><p>If the noise is one-sided <a href="/facts/White_noise/wJ0SR21n">white noise</a>, i.e., constant with frequency, then the total noise power <i>N</i> integrated over a bandwidth <i>B</i> is <i>N</i> = <i>BN</i>0 (for double-sided white noise, the bandwidth is doubled, so <i>N</i> is <i>BN</i>0/2). This is utilized in <a href="/facts/Signal-to-noise_ratio/qohClhyG">signal-to-noise ratio</a> calculations. </p><p>For <a href="/facts/Thermal_noise/ur238TgX">thermal noise</a>, its spectral density is given by <i>N</i>0 = <i>kT</i>, where <i>k</i> is the <a href="/facts/Boltzmann_constant/5DIejyR3">Boltzmann constant</a> in joules per kelvin (J/K), and <i>T</i> is the receiver <a href="/facts/System/hmqQ4ASX">system</a> <a href="/facts/Noise_temperature/t3Ecsmg5">noise temperature</a> in <a href="/facts/Kelvin/E3trzC4C">kelvins</a>. </p><p>The noise amplitude spectral density is the square root of the noise power spectral density, and is given in units such as volts per square root of hertz, V / H z {\displaystyle \mathrm {V} /{\sqrt {\mathrm {Hz} }}} . </p>