The Wiener–Khinchin theorem (also known as the Wiener–Khintchine theorem and sometimes as the Khinchin–Kolmogorov theorem) states that the power spectral density of a wide-sense-stationary random process is the Fourier Transform of the corresponding autocorrelation function.

Continuous case:

$$

S_{xx}(f)=\int_{-\infty}^\infty r_{xx}(\tau)e^{-j2\pi f\tau} d\tau where $r\_\{xx\}(\backslash tau)\; =\; E\backslash left*$ is the autocorrelation function defined in terms of statistical expectation, and where $S\_\{xx\}(f)$ is the power spectral density of the function $x(t)$. Note that the autocorrelation function is defined in terms of the expected value of a product, and that the Fourier transform of $x(t)$ does not exist, in general, because stationary random functions are not square integrable.

The asterisk denotes complex conjugate, and can be omitted if the random process is real-valued.

Discrete case:

$$

S_{xx}(f)=\sum_{k=-\infty}^\infty r_{xx}(k)e^{-j2\pi k f} where $r\_\{xx\}(k)\; =\; E\backslash left*$ and where $S\_\{xx\}(f)$ is the power spectral density of the function with impulse values $x(n)$. This spectral density of the discrete sequence is periodic in the frequency domain.

(See Chapter 6 of Digital and Analog Communications Systems by Leon W. Couch II, Sixth Edition, Prentice Hall, New Jersey, 2001, pp. 406-409)

Application

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The theorem is useful for analyzing linear time-invariant systems when the inputs and outputs are not square integrable, so their Fourier transforms do not exist. A corollary is that the Fourier transform of the autocorrelation function of the output of an LTI system is equal to the product of the Fourier transform of the autocorrelation function of the input of the system times the squared magnitude of the Fourier transform of the system impulse response. This works even when the Fourier transforms of the input and output signals do not exist because these signals are not square integrable, so the system inputs and outputs can not be directly related by the Fourier transform of the impulse response.

Since the Fourier transform of the autocorrelation function of a signal is the power spectrum of the signal, this corollary is equivalent to saying that the power spectrum of the output is equal to the power spectrum of the input times the power transfer function.

Discrepancy of definition

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By the definitions involving infinite integrals in the articles on spectral density and autocorrelation, the Wiener-Khintchine theorem is a simple Fourier transform pair, trivially provable for any square integrable function, i.e. for functions whose Fourier transforms exist. More usefully, and historically, the theorem applies to wide-sense-stationary random processes, signals whose Fourier transforms do not exist, using the definition of autocorrelation function in terms of expected value rather than an infinite integral. This trivialization of the Wiener-Khintchine theorem is commonplace in modern technical literature, and obscures the contributions of Aleksandr Yakovlevich Khinchin, Norbert Wiener, and Andrey Kolmogorov.

Fourier analysis | Signal processing