Upstream receiver IQ impairment analysis and compensation in coherent digital subcarrier multiplexing passive optical networks

The growing demand for high-capacity and flexible access has made digital subcarrier multiplexing (DSCM)-based coherent passive optical networks (PONs) increasingly attractive. However, in upstream transmission, amplitude, phase, and skew mismatches between the in-phase (I) and quadrature (Q) branches at the receiver of the optical line terminal may lead to image interference between symmetric subcarriers, thus degrading the system performance. To address this issue, this work theoretically analyzes the impairment mechanism and proposes a data-aided $8 \times 4$ real-valued multi-input multi-output (MIMO) equalization scheme for joint compensation. To our knowledge, this is the first experimental demonstration of MIMO-based subcarrier joint equalization for receiver IQ impairment compensation in the burst-mode upstream reception. Experimental results show that the proposed MIMO achieves stable compensation for IQ skew up to $\pm {40}\;{\rm ps}$ and IQ gain imbalance from ${-}{8}$ to ${+}{7}\;{\rm dB}$ in dual-subcarrier systems, while maintaining a Q-penalty within 0.5 dB under frequency offsets up to 500 MHz. A 400G TFDM-PON upstream system is further demonstrated, achieving ${-}{29.1}\;{\rm dBm}$ receiver sensitivity and a 32.1 dB power budget. These results provide a practical solution for IQ impairment mitigation and support robust, scalable deployment of coherent DSCM-based access networks.