Estimation of frequency offset prior to adaptive equalization for improved performance of DP-QPSK DWDM system

Abstract The processing of the signal in a digital coherent receiver helps to compensate for different linear and non-linear fiber impairments. In existing digital coherent receivers, adaptive equalization (AE), a slow varying phase correction process is employed prior to frequency offset estimation (FOE), which is a fast varying phase correction process. In this work, it is proposed that the fast varying correction process should be applied first because FOE can also partially remove the phase errors thereby facilitating the AE to perform better. The proposed technique is evaluated through exhaustive simulations for an eight-channel differentially encoded dual-polarization quadrature phase-shift keying (DP-QPSK) DWDM system with channel spacing of 50 GHz and spectral efficiency (SE) of 2.24 b/s/Hz. The results confirm that the proposed technique can enhance the capability of AE thereby improving the performance of the system in terms of BER. The proposed technique exhibits a characteristic under which the considered system performs with minimum BER over a wide range of launch power, which can further be traded to increase the transmission distance. The proposed technique also exhibits a lesser dependence on laser linewidth of the local oscillator (LO), to allow the use of less expensive laser sources. The proposed technique is also helpful in mitigating the effects of in-band crosstalk due to optical switches.