Hardware-efficient and robust DSP scheme for coherent DSCM system in presence of transmitter IQ impairments

In this paper, a hardware-efficient and robust DSP scheme for the coherent digital subcarrier multiplexing (DSCM) system in presence of transmitter (Tx) in-phase/quadrature (IQ) impairments is introduced and demonstrated. An analytic complex model is given to describe the effects of IQ skew, IQ amplitude and phase imbalance (referred to as IQ mixing) on the DSCM signal. Based on the model, residual frequency offset, polarization and carrier phase noise are jointly recovered using frequency pilot tones (FPT) in presence of IQ mixing. This process avoids the coupling effect of phase noise and Tx IQ mixing in conventional schemes, which allows the Tx IQ mixing to be compensated for by the following joint equalization of conjugate symmetric subcarriers. Compared to the conventional scheme based on widely linear equalizer, the polarization and carrier phase tracking capability of the proposed scheme is robust to IQ mixing. Meanwhile, the proposed scheme can tolerate a wide range of Tx IQ mixing in the presence of residual frequency offset and carrier phase noise. Since polarization crosstalk is eliminated, the signals of the two polarizations can be equalized separately in the proposed scheme, thus reducing the computational complexity by half. For a 4-subcarrier DSCM system with baud rate of 32 Gbaud, experimental results show that the Q-factor degradation of the proposed scheme is less than 0.5dB in the presence of Tx IQ time skew within \pm15 ps, IQ phase imbalance within \pm20 degrees or IQ amplitude imbalance within \pm3 dB. With 1dB Q-factor degradation, the proposed scheme can track polarization variation up to 10 Mrad/s, residual frequency offset of 10 MHz or carrier phase noise with 3dB linewidth of 200 kHz in the presence of Tx IQ mixing. Therefore, the proposed scheme provides a hardware-efficient and robust DSP solution for coherent DSCM targeting access network applications.