I. Introduction
Coherent optical receivers with electronic dispersion compensation have an inescapable impairment called equalization-enhanced phase noise (EEPN). EEPN has been analytically derived, and its impact has been studied in various simulations [1], [2]. Its probability distribution function (PDF) has also been studied [3], [4]. From the constellation and PDF of EEPN, EEPN causes phase distortion that is about twice as large as amplitude distortion. It is, however, only a preliminary estimate based on simulation. Further, the phase noise compensation algorithms in the receiver can partially compensate for the EEPN [5], but there is no closed-form forecast of the EEPN decrease after phase noise mitigation.