A double-DD blind equalizer for PolMux QAM optical coherent systems

In polarization-multiplexed (PolMux) coherent system, adaptive blind equalization is efficient in demultiplexing and mitigating inter-symbol interference (ISI). We propose a double decision-directed (double-DD) blind algorithm which combines the output of soft decision-directed (SDD) algorithm with that of the equalizer in variable step size decision-directed (VSS-DD) mode. In our scheme, two equalizers can softly switch when the mean-square error (MSE) is sufficiently low and it can achieve fast convergence rate and small steady MSE in steady state. By simulation in PolMux-16QAM/64QAM coherent systems, we show that the double-DD algorithm outperforms existing blind equalizers.