Energy spectral density

The quantity $S_{xx}(w)\ge 0$ is the energy density spectrum of $x[n]$ While the quantity $S_{xy}(w)$ is the cross-energy density spectrum

Where

$$\mathcal{F}(r_{xx}[l])=\mathcal{F}(x[l]\ast x[-l])=S_{xx}(w)=X(\omega )X^{\ast }(\omega )=\mid X(\omega ){\mid }^2$$