Received signal envelope PDF to power PDF [Wireless Communications]

I am deriving the probability density function (pdf) of the received wireless signal envelope in multi-path fading channels. I wish to transform this envelop PDF to the power PDF of that signal.

I know that there exist a straightforward transform such as the one given in as in eqn $(2.3)$ in: http://samples.sainsburysebooks.co.uk/9780471715238_sample_385247.pdf

however, it seems that this transform does not always work (unless I have made the same mistake more than 10 times). I am sure that my envelop PDF is a true PDF as it integrates to 1 from $-\infty$ to $+\infty$, but there is some issue with the transform to a power PDF (or so I believe). Any comments that might be helpful?

• What is the problem that you say it does not work? – msm Oct 8 '16 at 10:10
• what @msm said: what did you calculate, and where was the problem? How should we know where you got into trouble if you don't share your calculations? – Marcus Müller Oct 8 '16 at 10:36