Low-Complexity Compensation for IQ Skew-Induced Mirror-Subcarrier Crosstalk in High-Speed Coherent DSCM Systems

Digital subcarrier multiplexing (DSCM) has emerged as a promising candidate for future ultra-high-baud-rate transmissions. However, DSCM signals are susceptible to transmitter IQ skew. Traditionally, this issue has been mitigated using 4× 4 real-valued multiple-input multiple-output (RV-MIMO) equalizers. Nevertheless, such equalizers suffer from high computational complexity, making them unsuitable for low-power applications. In this paper, we propose and experimentally validate a low-complexity scheme for compensating transmitter IQ skew in coherent DSCM systems. Specifically, for each subcarrier pair, the mirror subcarrier is first processed through an adaptive FIR filter bank to estimate the image crosstalk induced by IQ skew. This estimated crosstalk is then subtracted from the subcarrier to be compensated, effectively mitigating IQ skew. Compared to the conventional 4× 4 RV-MIMO scheme, which requires 16 FIR filters, the proposed scheme utilizes only 8 FIR filters to compensate a subcarrier pair, thereby reducing the number of multipliers by 50%. Experimental results on a 4-subcarrier DSCM system, in which each subcarrier operates at 12 Gbaud, show that the proposed scheme performs comparably to the conventional 4× 4 RV-MIMO scheme over an IQ-skew range of$\pm$6 ps (7.2% of the subcarrier symbol period). Specifically, under an IQ skew of 2 ps, the received-optical-power (ROP) penalty of the proposed scheme relative to the conventional 4× 4 RV-MIMO scheme is only 0.19 dB on the high-frequency subcarrier at a BER of 3.8e-3. These findings validate the effectiveness of the proposed scheme as a promising low-power solution for future high-capacity coherent optical interconnects.