  A semi-analytical method evaluates the error probability of DPSK signals with
intrachannel four-wave-mixing (IFWM) in a highly dispersive fiber link with
strong pulse overlap. Depending on initial pulse width, the mean nonlinear
phase shift of the system can be from 1 to 2 rad for signal-to-noise ratio
(SNR) penalty less than 1 dB. An approximated empirical formula, valid for
penalty less than 2 dB, uses the variance of the differential phase of the
ghost pulses to estimate the penalty.
