  An analytical model is presented for assessing the performance of the
bit-error-rate (BER) in the differential m-level phase shift keying (m-PSK)
transmission systems, where the influence of equalization enhanced phase noise
(EEPN) has been considered. Theoretical analysis has been carried out in
differential quadrature phase shift keying (DQPSK), differential 8-PSK (D8PSK),
and differential 16-PSK (D16PSK) coherent optical transmission systems. The
influence of EEPN on the BER performance, in term of signal-to-noise ratio
(SNR), are investigated for different fiber dispersion, LO laser linewidths,
symbol rates, and modulation formats. Our analytical model achieves a good
agreement with previously reported EEPN induced BER floors, and can give an
accurate prediction for the DQPSK system, and a leading-order approximation for
the D8PSK and the D16PSK systems.
