Low-Complexity Monitoring and Compensation of Transceiver IQ Imbalance by Multi-dimensional Architecture for Dual-Polarization 16 Quadrature Amplitude Modulation

In this paper, a low-complexity IQ imbalance compensation architecture is proposed, which reduces the effects of in-phase (I) and quadrature (Q) imbalance. The architecture consists of transceiver IQ skew estimation methods and a low-complexity MIMO equalizer structure. Before the IQ skew estimation, the chromatic dispersion(CD) is pre-compensated in the transmitter(TX) by chirp filtering. The receiver(RX) IQ skew is estimated by Gardner's phase detector, and the TX skew is estimated by finding the value that yields the lowest equalizer error. The low-complexity MIMO equalizer consists of a complex-valued MIMO(CV-MIMO) and a real-valued DD-LMS MIMO(RV-MIMO), which employ a butterfly and a non-butterfly structure, respectively. The CV-MIMO is used to perform polarization demultiplexing. The RV-MIMO equalizes each of the two polarisations and simultaneously compensates for the TX IQ imbalance. The architecture first compensates for the IQ skew at low-complexity, and the other imperfections are compensated by the low-complexity MIMO equalizer. Therefore, this architecture can equalize signals impaired by the transceiver IQ imbalance with low complexity. A 100 km transmission simulation and experiment with 36 Gbaud dual-polarization quadrature amplitude modulation(DP-QAM) signals and offline DSP showed that, with the RX IQ skew estimation, the number of real multiplications is reduced by more than 70% compared with conventional cases. With the low-complexity MIMO equalizer, the number of real multiplications is reduced by 51% compared with 4x4 MIMO