I would like to understand how Eq. (36) in  was derived:
The rationale behind the definition of circular sample mean in Eq. (37) is clear, but there is no motiviation for the CMSE definition in Eq. (36).
 Lovell, Brian C., and Robert C. Williamson. "The statistical performance of some instantaneous frequency estimators." IEEE Transactions on Signal Processing 40.7 (1992): 1708-1723.