Discussion
Why is white noise an idealisation rather than a physical signal?
Answer: B
Because of the power it would imply. Flat density multiplied by infinite bandwidth gives infinite power, so the model must always be qualified as flat over the band of interest - which is a perfectly good approximation, since thermal noise is essentially flat up to frequencies far above any communication band. Real noise does have a flat region, so the second option is too strong. The quantum correction eventually rolls the density off, but not in a way that matters at radio frequencies. And random signals are described by spectral density precisely through the Wiener-Khinchin relation, which is what makes the whole framework work.
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