Decoupled equalization of polarization and transmitter IQ skew in digital subcarrier multiplexing systems.

Transmitter (Tx) IQ skew induces aliasing of symmetric subcarriers in digital subcarrier multiplexing (DSCM) systems. This impairment further couples with channel effects and laser phase noise (LPN), thus degrading the performance of traditional polarization demultiplexing and carrier phase recovery. In this paper, we propose a decoupled equalization scheme based on a designed training sequence to mitigate the interference of transmitter IQ skew on polarization rotation and LPN when these impairments are compensated independently. We start out by designing a strictly linear (SL) 2 × 2 MIMO equalizer for polarization demultiplexing based on the designed training sequence to achieve more accurate channel estimation, which is attributed to the insensitivity of the designed training sequence to aliasing induced by Tx IQ skew. Then, we use a PLL-embedded widely linear 2 × 2 (PLLE WL 2 × 2) MIMO to compensate for the LPN and Tx IQ skew simultaneously. This can mitigate the coupled effect of LPN and Tx IQ skew. Through experiments on 100-GBaud PM 16-QAM DSCM coherent optical fiber transmission consisting of eight 12.5-Gbaud subcarriers, we find that the proposed improved equalization scheme can achieve a 0.53-dB Q² factor gain with 3-ps Tx IQ skew compared with the traditional equalization configuration. Besides, it also achieves performance comparable to that of a 4 × 4 WL filter with a 26.9% reduction in DSP complexity in terms of required real-valued multiplications.