Transmitter IQ impairment estimation and calibration with a time-interleaved preamble for 256 Gb/s coherent TFDM PON downstream

Coherent passive optical networks (PONs) enabled by digital subcarrier multiplexing (DSCM) provide a highly flexible and low-latency access solution for optical network units (ONUs). However, in-phase and quadrature (IQ) impairments induced by transceiver imperfections (e.g., IQ skew, IQ amplitude imbalance, and IQ phase imbalance) cause severe image crosstalk on symmetric subcarriers. In the downstream of time- and frequency-division multiplexing (TFDM) PON, the estimation of IQ impairments becomes particularly challenging because partial reception results in the loss of symmetric subcarrier information. In this paper, we propose a time-domain-interleaved preamble (TDIP) scheme to estimate and pre-calibrate transmitter (Tx) IQ impairments. In addition, receiver (Rx) IQ skew is estimated using a modified frequency-domain phase detector (FDPD), while Rx IQ amplitude and phase imbalance are estimated using an improved Gram–Schmidt orthogonalization procedure (GSOP). Numerical simulations and experimental demonstrations are conducted to validate the excellent robustness of the proposed scheme under challenging conditions, including edge-subcarrier transmission, varying degrees of impairments, multiple coexisting impairments, and low received optical power (ROP). Specifically, for preset transceiver IQ skew ranging from −15 to 15 ps and IQ amplitude imbalance from −3 to 3 dB, the proposed scheme achieves estimation errors within ±0.6ps and ±0.4dB, respectively. Consequently, BER performance is significantly improved after the calibration of transceiver IQ impairments, particularly for the edge subcarrier. For 256 Gb/s downstream coherent TFDM PON over 20 km standard single-mode fiber (SSMF) transmission, the compensated receiver sensitivity reaches −22.2dBm, corresponding to a 30.2 dB power budget. This work offers a promising solution for precise transceiver IQ impairment monitoring in future flexible coherent TFDM PON.