Anti-Singularity Clock Recovery and Rx/Tx IQ Skew Compensation Scheme

To deal with the failure of clock recovery and detect Rx IQ skew at the singularity in which the azimuth is 45 degrees and different group delay (DGD) is odd multiple number of half symbol period, an anti-singularity clock recovery and receiver (Rx)/transmitter (Tx) IQ skew compensation scheme is proposed in this letter. The sensitivity of clock error detector at the singularity is restored by introducing polarization transformation. By judging the changes of Rx IQ skew, the effect of the Rx IQ skew detection error on the compensation at the singularity is eliminated. Besides, Tx IQ skew is detected by phase discriminators in frequency domain and compensated by interpolation filters in time domain. Compared with MIMO equalizers scheme, the proposed scheme has lower computational complexity. The simulation results of PM-16QAM Nyquist coherent system demonstrate that the scheme can work stably when DGD is odd multiple number of half symbol period and the azimuth changes rapidly. The performance of the scheme is also verified through the off-line experiment. The scheme can realize clock recovery plus Rx/Tx IQ skew detection and compensation, and the detection accuracy and compensation range could reach the requirement of practical application.