Extracting true delay in multipath UWB OOK-ED receiver

Guru(s), In Energy Detector (ED), I choose a window of observation, and collect how much energy is there (=integrate for that interval)

How do I figure out "start time" for each observation? In other words, in multipath environment, how do I calculate initial delay (tau0)

I have a ray tracer and it gives me some signal (CIR) at particular delays, but the first arriving signal may not be the fist sent signal (pulse, since I am using UWB), so I need to know which one of the received pulses give me the true delay.

(BTW, is it correct? I mean does my question make sense?)

Could someone give me some guidance? Some matlab code would be a superb, but some pointing to a (hopefully right) direction would be more than enough.

Any books/articles that you recommend for me to understand this?

Thank you.