<P> While noise is generally unwanted, it can serve a useful purpose in some applications, such as random number generation or dither . </P> <P> Different types of noise are generated by different devices and different processes . Thermal noise is unavoidable at non-zero temperature (see fluctuation - dissipation theorem), while other types depend mostly on device type (such as shot noise, which needs a steep potential barrier) or manufacturing quality and semiconductor defects, such as conductance fluctuations, including 1 / f noise . </P> <P> Johnson--Nyquist noise (sometimes thermal, Johnson or Nyquist noise) is unavoidable, and generated by the random thermal motion of charge carriers (usually electrons), inside an electrical conductor, which happens regardless of any applied voltage . </P> <P> Thermal noise is approximately white, meaning that its power spectral density is nearly equal throughout the frequency spectrum . The amplitude of the signal has very nearly a Gaussian probability density function . A communication system affected by thermal noise is often modeled as an additive white Gaussian noise (AWGN) channel . </P>

Which of the following is an example of noise in communication