Re: autocorrelation
ok i am assuming that you are totally new to dsp and as such those two answers that you have already received probably mean nothing to you. Anyway the only relation that there is between autocorrelation and the fourier transfrom is actually the similarity between the DFT (discrete fourier transform) and auto correlation i.e. if you are learning DSP in your book it will give you a hand method of calculating the DFT, cross correlation (between two signals) and the autocorrelation. the calculation of these three items by hand is quite similar (and no i won't get into the technical details for now). the only relation between autocorrelation and psd is the fact that if you take a signal and get the autocorrelation of that signal, then if you apply the fourier transform to the result you will get the psd.
hope this helps,
sincere regards,
t