SO-CFAR MIMO Radar - threshold factor derivation.

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:
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Reply by ●December 8, 2018

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.