Random signals X (t), referred to as "background noise," are stationary random signals of order 2, i.e., they have at least the first two moments independent of time, namely E {X } and E {X2} (the mathematical expectations).
Remember that variance is defined as the mean square of the centered variable X (t ) – E {X }.
Another consequence of stationarity is that the statistical mean of the product X(t)X(t + τ) depends only on the variable τ; this moment is the autocorrelation function, i.e.:
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