When I made back to back DP_QPSK coherent optical system with and without DSP I got different results in both the cases in terms of BER. For the circuit having DSP component I got BER 0 but without DSP I got BER 0.5. The screen shot of the circuits is attached.Kindly check the circuit and suggest me which circuit should I use to study the effect of nonlinearities in DP-QPSK system.