In this section it will be interesting to note the influence on the spectral density of the temporal spacing between the r.v. For this reason we are now about to consider momentarily a WSS process where θ is a constant and where jθ has the significance of duration.
DEFINITION.– We say that the process Xθ possesses a spectral density if its covariance can be written in the form: and SXX (u) is then called the spectral density of the process Xθ.
Under the hypothesis :
1) the process Xθ admits a spectral density SXX;
2) SXX is continuous, periodic of period, real and even.