Blind adaptive source separation (BASS) based compensation for transmitter (Tx) IQ imbalance is presented for the first time in an MQAM optical coherent system. The proposed method is numerically investigated with 4QAM and 16QAM signals in the presence of Tx IQ imbalance up to 30. The robustness of the BASS method is studied after 200-km optical fiber transmission, in which the effects of chromatic dispersion (CD) and carrier frequency offset (CFO) are assumed to be dominant. It is also found that CFO, inherent to frequency difference between the transmitter and receiver lasers in optical coherent transmission, should be compensated before IQ imbalance compensation to achieve a better performance. The proposed method outperforms the Gram-Schmidt orthogonalization procedure (GSOP) in the presence of CD and CFO. We further validate experimentally the proposed method with 10-Gbaud optical 4QAM and 16QAM signals at 30 and 10 phase imbalance, respectively, with an emulated 200-km optical fiber transmission and 200-MHz CFO. More specifically, the optical signal-to-noise ratio (OSNR) penalty reduction of the BASS method compared to the GSOP method is 1 dB for 4QAM at a bit-error-ratio (BER) of 210 3 and 2 dB for 16QAM at a BER of 10 3. Moreover, instead of being a fully independent block and requiring statistical estimation as in GSOP, the BASS method can be integrated into an equalizer and operated at the sample rate, simplifying the operation and allowing parallel implementation.
This paper addresses the problem of Inphase/Quadrature (I/Q) imbalance sensitivity of communication systems when it occurs at both Transmitter (TX) and Receiver (RX) sides of an optical coherent system. A novel blind technique is proposed based on the pseudo-rotation of the M-QAM constellation. The pseudo-rotation based compensator is a generic approach, because it does not depend on the modulation order and the level of imbalance. To implement the proposed compensation in practical systems, two algorithms are proposed: Recursive Pseudo-Rotation (RPR) that achieves the performance of the ideal compensator and LRPR, a Low complexity version of RPR. Monte Carlo simulation results show that the proposed compensator outperforms state-of-the-art algorithms for an Additive White Gaussian Noise (AWGN) and the complexity/performance trade-off is also discussed. The efficiency of the proposed method is experimentally validated with a 10 Gbaud QPSK optical system showing its operation in the presence of Inter-Symbol Interference (ISI).
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