In-service Blind Transceiver IQ Imbalance and Skew Monitoring in Long-haul Non-dispersion Managed Coherent Optical Systems

Transceiver imbalance can rapidly degrade system performance, especially when high symbol rate, high modulation order and low roll-off pulse shape are used. Here, a widely linear transmission model is derived to analyze the interaction of transceiver IQ gain imbalance, IQ phase imbalance, skew and fiber link for long-haul transmission. In-service blind transceiver imbalance measurement method is designed to calibrate and monitor the transceiver status. Two $4\times 2$ complex-valued equalizers operated at twice symbol rates are designed before and after carrier phase recovery (CPR) to compensate receiver and transmitter imbalances, where the transceiver imbalance could be derived from the taps of two converged equalizers. The impact of transceiver imbalance and fiber nonlinearity on the measurement accuracy are numerically evaluated based on a 42 GBaud 16-QAM system through 1500-km standard single fiber (SSMF) transmission.