Direct computation of digital communication system degradations

Expressions are derived for the degradation in symbol error probability caused by small distortions that occur in digital communication systems. The degradation, which is defined as the required increase in signal power that just compensates for the distortion effects, is often a more practical measure of system performance than the increase in symbol error probability itself. It is a function of the undistorted signal-to-noise ratio and the moments of certain easily determined 'distortion parameters'. Calculation of only the low-order moments is accurate enough for most applications.