Pilot-Aided Channel Equalization in RGI-PDM-CO-OFDM Systems

A pilot-aided channel equalizer (PACE) is proposed to mitigate the impairments caused by the polarization mode dispersion and laser phase noise simultaneously in reduced-guard-interval polarization-division-multiplexing coherent-optical orthogonal-frequency-division-multiplexing (RGI-PDM-CO-OFDM) transmission systems. Since PACE updates the channel state information symbol-by-symbol, it enables us to track the drifts in the optical channel. Adaptive PACE (APACE) and boosted PACE (BPACE) are then proposed to further improve the performance of PACE. Numerical simulations are carried out to compare the performance of APACE and BPACE with a conventional training symbols aided channel equalizer (TSACE) and adaptive decision-direct channel equalizer for a 108-Gb/s (56-GS/s) RGI-PDM-CO-OFDM system. It is revealed that both APACE and BPACE offer superior performances over the other two equalizers in the presence of laser phase noise, and they can tolerate lasers with a line width around $\beta=2000~{\rm k}$ ($2\pi\beta T_{s}=2.24\times 10$ when it is normalized to the symbol rate $1/T_{s}$). We also show that only small additional computation efforts are required for APACE and BPACE when compared with TSACE.