The fiber nonlinearities of self-phase modulation(SPM) and cross-phase modulation(XPM) in 40 Gb/s and 10 Gb/s return to zero differential phase shift keying(RZ-DPSK) electrical pre-distortion (EPD) systems are studied by simulations.Nonlinearity in EPD system is more severe than that in optical dispersion compensation(ODC) system,but the degree differs depending on the bit rate.Simulation results of 800 km transmission over a standard single mode fiber(SSMF) show that in the single channel transmission infl...