Thank you for posting the doubt.
I am also searching the detail.
What I have done so far is given in one of my papers,
attached for your convenience .
You can go though gradulally.
After the publication I am working out what it explains
Take a sin wave of say 50 c/s for some duration and magnitude .
Take another 50 c/s sin wave of same duration but of higher magnitude .
The coherence is 1.
Take another sin wave of higher frequency say 100 c/s or non harmonic frequency of 80c/s .
the coherence is 1.
Take two sin wave of same frequency , same lenth ,same magnitude but displaced with refence to time for a fraction.The coherence is not uniformly 1.
It is stated that it represents Spectral Pwer density.
I am trying to understand myself .Once I understand I will get back to you
In the meanwhile if you find any thing further please let me know.
Dr. B P Singh