Coherence for order spectra
The topic is to the best of my knowledge an area of open research. The research contribution which helped me most in solving my problem was: The spectral analysis of cyclo-non-stationary signals. Specifically, the key seems to be:

&quot;In this paper, it is shown that the optimal way of describing
cyclo-non-stationary signals is jointly in the time and the angular
domains.&quot;