When is Clarke's approximation valid?

For several years since the publication of Clarke's model on small-scale fading in 1968, the well-known approximation relating the square of the magnitude of the complex correlation to being approximately equal to envelope correlation has been applied ; to several aspects of modern-day radio. There are cases in the literature where a negative envelope correlation has been reported, which is in contradiction to the approximation applied by Clarke. This therefore raises the question of why the envelope correlation is negative, and, also, does the Clarke assumption always hold? The authors are unaware of any literature that addresses this subject. As such, some experimentation with synthetic data as well as measured data is used to investigate the conditions under which the approximation is valid. Channel conditions are also analyzed, to investigate how the system is valid