Abslracl-We demonstrate by simulation a parallel decision feedback equalization (DFE) technique at multi-GSa/s sampling rate employing recursive least square (RLS) and mean square error (MSE) adaptive algorithms to mitigate the chromatic dispersion (CD) and nonlinear impairments in 100 Gbps 1000 km long-haul optically amplified fiber transmission systems under RZ-DQPSK modulation formats. The digital equalizer is embedded in the coherent lightwave receiver in cascade with the electronic preamplifier and an ADC. RLS algorithm offers a CD tolerance of nearly 300 kms of G652 Standard Single Mode Fiber; while the MSE algorithms can tolerate only about 240 kms. Our simulation for the long haul optical transmission system is developed on the Matlab Simulink platform.I.