Frequency-Domain Blind Equalization for Long-Haul Coherent Pol-Mux 16-QAM System With CD Prediction and Dual-Mode Adaptive Algorithm

The combination of polarization-multiplexing (pol-mux) high-order quadrature-amplitude-modulation (QAM) format and non-data-aided (NDA) channel equalization largely enhances the spectral efficiency of the coherent optical systems. In this paper, we present a two-stage frequency-domain equalization method for long-haul coherent pol-mux NDA system with 16-QAM modulation format. Chromatic dispersion (CD) is blindly estimated and compensated in the first stage of the equalizer, while multimodulus-algorithm-assisted decision-directed multimodulus algorithm (MMA-DDMA) is designed for the second stage to further enhance the equalization performance. The proposed algorithms are experimentally demonstrated in a 80-Gb/s coherent 16-QAM system over 800-km fiber transmission and 30-ps mean differential group delay to achieve wide-range precise CD estimation and better equalization performance compared with conventional adaptive algorithms.