Blind Transmitter IQ Imbalance Compensation in M-QAM Optical Coherent Systems

Abstract

We investigate transmitter IQ imbalance compensation based on the blind adaptive source separation (BASS) method in a dual-polarization M-QAM optical coherent system. The robustness of the BASS method against the residual carrier frequency offset (CFO) is numerically investigated and compared to that of the Gram–Schmidt orthogonalization procedure (GSOP). We further validate experimentally the proposed method with 10 Gbaud optical 4-QAM and 16-QAM signals at 30° and 10° phase imbalance, respectively, with simulated impairments. More specifically, in the presence of 5×10−6 residual CFO (normalized to the sample rate), the optical signal-to-noise ratio penalty reduction of the BASS method compared to the GSOP method is 1 dB for 4-QAM at a bit error ratio (BER) of 2×10−3 and 2 dB for 16-QAM at a BER of 10−3. In contrast to the GSOP, which requires an independent block, the BASS method can be integrated into an equalizer, simplifying the operation and allowing parallel implementation.