Given the required Probability of False Alarm(Pfa) and the number of training cells(N) of SO-CFAR, wondered what would be the correct method for threshold factor derivation based on the equation below:

Please define the terms of the equation.  Also, what kind of mimo?  radar staring at a noise-like background or radar staring at clutter.  In other words, what dominates the threshold, noise or clutter.