# How to get PSD from frequency domain representation of time-series data [duplicate]

Let's say we have a realization (time-series data) of a random process. I take fft() of that to get its frequency domain representation. Then how can I estimate the PSD of this random process by having this frequency domain representation?

Note 1: It is known that the PSD is the fft() of auto-correlation function. But here we don't have access to the auto correlation function, we only have one realization.

Note 2: I want the PSD estimation approach from the 'frequency domain' representation, not from the time-domain data.

Note 3: My question is not about the relationship between system transfer function and power spectral density (i.e. it is not about the PSD of filter input and output).

My proposed answer (not sure if it is right):

Calculate the signal power in a 1 Hz bandwidth as a function of frequency. The PSD can be formed by taking the ratio of the signal power in a 1 Hz bandwidth at a specified frequency offset, $$f_m$$, to the signal peak amplitude at frequency $$f_0$$, something like that.

But what if there is no peak or multiple peaks in the the frequency domain representation?