Transforming FFT to Detect Baud Rate I've seen in practice taking an FFT of a signal, then transforming it by the 2nd, 4th, etc. power (depending on signal type) to determine the baud rate. I'm trying to implement this in my code, but am lost as to what a transform of the 2nd power is. It appears to be some sort of non linear transform and not simply squaring of the FFT values.
As noted in the following link, baud rate is estimated from "the spectrum of the signal after nonlinear processing."  In their case, as noted in the first three sentences of the section entitled "Proposed Spectral Approach for Bit Time Estimation" - p. 916), the nonlinear processing is the first derivative followed by an absolute value operation:

http://www.iasj.net/iasj?func=fulltext&amp;aId=51991

Others have proposed/analyzed estimation using different nonlinear operations, such as squaring of the signal or using higher derivatives.  Obtaining the references for the above paper and doing a thorough search would be a good place to start to learn more.