PMD Estimation and Its Enabled Feedforward Adaptive Equalization Based on Superimposed FrFT Training Sequences

Fractional Fourier transform (FrFT) training sequences (TSs) have been proposed to superimpose with signals to achieve time synchronization and polarization mode dispersion (PMD) estimation simultaneously before equalization. The feasibility of the proposed scheme is verified by experiment of a 30 GBaud/s PDM quadrature phase shift keying (QPSK) system. Without sacrificing spectrum efficiency, the proposed PMD estimation is robust against amplifier spontaneous emission (ASE) noise and has steady performance in a wide PMD range from 0 ps to 108 ps. The root mean square error (RMSE) of PMD is 5.6 ps by experimental verification. Further, the PMD estimated before equalization is utilized to adjust the numbers of taps in a subsequently tap-varied equalizer, which achieves feedforward adaptive equalization (FFD-AEQ). The number of the AEQ taps is adjusted to an optimal value with the prior-PMD information estimated by FrFT TSs. The experimental results show that the tap-varied AEQ has 52.6% power consumption decrease compared to the tap-fixed AEQ with 13 taps.