Low-complexity coherent DSCM system with precise transceiver IQ skew calibration and simplified equalization embedding phase tracking

The digital subcarrier multiplexing (DSCM) technique has emerged as an appealing solution for coherent systems. However, it is more sensitive to the transceiver IQ skew, which introduces conjugate interference to the symmetric frequency subcarriers (SC). In this Letter, a low-complexity transceiver IQ skew calibration method is adopted for coherent DSCM systems based on a specially designed training signal, avoiding the power-hungry multiple-input multiple-output (MIMO) operation between two symmetric SCs. To further reduce the computational complexity for each SC, a simplified equalizer with embedded phase tracking (SEQPT) is proposed, achieving polarization de-multiplexing, channel equalization, and phase noise compensation simultaneously. The proposed methods are validated experimentally in a 16-channel wavelength division multiplexing system with 50 GBd 4SC-16QAM DSCM signals. The results indicate that after skew calibration by implementing the proposed method, performance is comparable to that of applying the 8 × 8 real-valued MIMO equalizer. Besides, using the proposed SEQPT incurs almost no performance penalty compared to the <i>N</i>-tap 2 × 2 CMMA equalizer followed by blind phase search while operating in a low-complexity manner.