I am trying to understand a paper " Dynamic Model for Generating Synthetic ECG signal". The link is: http://web.mit.edu/~gari/www/papers/ieeetbe50p289.pdf. In that paper I came across a equation, which I am not able to understand . I have attached the same. The authors claim that S(f) is Power Spectral Density. So far, I understood that S(f) (omitting sigma1 2 and sigma2 2) is joint density function.  But, I am not able to connect the two (Joint Density Function with Power Spectral Density).

In short, I require a clarity on how S(f) is Power Spectral Density. Can you please throw some light