Low-complexity symbol-rate equalization schemes for short-reach coherent-lite optical interconnects

In this paper, we propose two low-complexity multiple-input multiple-output (MIMO) equalization schemes for short-reach coherent-lite optical interconnects: dynamic pruning MIMO (DP-MIMO) and dynamic clustering MIMO (DC-MIMO). Based on the conventional 4 × 4 real-valued (RV) MIMO architecture, both schemes aim to reduce the computational complexity of each channel filter according to real-time channel characteristics. DP-MIMO sparsifies filter coefficient matrices and dynamically adjusts the sparsity level of each channel filter based on the energy distribution across all channels, achieving adaptive tap optimization. DC-MIMO reduces redundant computations by clustering filter coefficients and further introduces an energy-adaptive mechanism that dynamically adjusts the number of clusters, balancing complexity and system performance. To further evaluate the benefits of the energy-adaptive mechanism, the static pruning MIMO (SP-MIMO) and static clustering MIMO (SC-MIMO) are also introduced as their static counterparts. The proposed schemes are experimentally validated in a C-band 80-GBaud dual-polarization 16QAM (DP-16QAM) system over 1-km, 2-km, and 5-km standard single-mode fiber (SSMF) links. Experimental results show that, compared with conventional 4 × 4 RV MIMO, DP-MIMO can achieve around 40% reduction in real-valued multiplications (RMs) per transmitted symbol within a 0.20-dB receiver optical power (ROP) penalty. Compared with SP-MIMO at a similar complexity level, DP-MIMO introduces a smaller ROP penalty. DC-MIMO achieves around 60% RMs reduction within a 0.15-dB ROP penalty compared with 4 × 4 RV MIMO, and compared with SC-MIMO at similar performance, DC-MIMO achieves lower complexity. To verify the robustness of the proposed schemes against in-phase/quadrature (IQ) skew impairments, an additional 2-ps IQ skew was introduced. Both DP-MIMO and DC-MIMO maintain stable performance under this condition, with DP-MIMO reducing around 40% of complexity within a 0.2-dB ROP penalty and DC-MIMO reducing around 60% of complexity within a 0.1-dB ROP penalty.