Mitigation of linear inter-channel interference for sub-Nyquist spacing in optical multicarrier systems

Nyquist optical multicarrier transmission based on gridless wavelength division multiplexing (WDM) technology suffers by inter-channel interference (ICI) due to carrier overlapping. Even though the unpredictable effect of the ICI has been treated as a non-linear noise, temporal correlations of data symbols may partially allow ICI mitigation by linear equalization techniques. In this research, we systematically evaluate and mitigate the impact of the ICI for sub-Nyquist carrier spacing in a multicarrier system by applying a linear equalization technique. System characterization is presented in terms of bit-error-rate (BER) estimation as a function of carrier frequency spacing variations, the roll-off factor of the digital Nyquist pulse-shaping filter, and transmission distance scenarios. The mitigation of the ICI effects is carried out by the linear constant modulus algorithm (CMA) equalizer after compensating fiber linear impairments and before the sampling process at the digital coherent receiver, in a 3 × 32 Gbaud single-polarization Nyquist quadrature phase shift keying (QPSK) multicarier system. The CMA updates the coefficients of an adaptive digital filter based on a blind estimation technique to minimize the error function defined by the constant modulus of QPSK signals. System performance is improved after the linear equalization, reaching optical transmission distance without amplification of up to 130 km assuming a 15% FEC limit. Furthermore, the linear equalization allows a sub-Nyquist spacing of 30.5 GHz assuming a 15% FEC limit. Moreover, the adopted equalization technique relaxes the computational cost of the digital filtering, enabling roll-off factor values of up to 0.3 for 120 km transmission assuming 9% FEC limit.