How are you using DSP component for PS-QPSK, for increase fiber length. Can you attach the screen shot of the same. which version are you using? The new version of OptiSystem 13.0 has a new Universal DSP component with a complete suite of DSP algorithms (including a new nonlinear compensation model) for analysing various modulation formats.
The transmission length is increased due to chromatic, PMD compensation as well as mitigation of nonlinear effects. If I am not mistaken, the main techniques for mentioned compensations are forward error correction (FEC), up-sampling and Nyquist prefiltering and equalization of a transmitter frequency response. I agree with Rk Sethi that OptiSystem 13.0 has more advanced DSP than previous versions do.