Power Spectral Density

The Fourier transform of the autocorrelation function $r_x(l)$ is called the power spectral density (PSD), or power spectrum, and may be denoted

$$S_x(\omega) \isdef \hbox{\sc DTFT}_\omega(r_x).$$

When the signal $x$ is real, its PSD is real and even, like its autocorrelation function.