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DSP: Noise The most common form (distribution) of noise is additive (Noise eventually is combined with the desired signal) white (flat power spectral density over frequency) Gaussian Noise (AWGN). ENR (excess noise ratio) = 10 log [(Th–290)/290] Normalized measure of how much the noise source is above thermal in its power ????? Noise powers add as incoherent signals which means that their powers must be added. For example if your inject a noise source into a spectrum analyzer and see that the noise floor increases 3 dB, then the actual noise source power is at the original noise floor level. This relationship allows you to calculate the noise power of signals below the measurement noise floor: 10 log [{Inverse log (diff/10)} – 1)] Where diff is the dB difference in measured powers. Of course, small changes in power occur as the unknown noise is far below the known and this results in increasing inaccuracy as the power goes much lower. Measuring BER at high power levels eliminates the affects of uncharacterised low noise sources and increases the accuracy of the BER
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