Preamble Design for Online IQ-Skew Estimation in Upstream 400G Coherent TFDM-PON

An online calibration method based on burst-mode digital signal processing (DSP) is proposed to solve the transceiver In-phase and quadrature (IQ) skew problem in the upstream system of 400G time frequency division multiplexing passive optical network (TFDM-PON). In traditional coherent PON, IQ skew can cause image frequency crosstalk, which seriously affects the system performance, especially in the scenario of multi-user subcarrier power imbalance. By designing a training sequence that simultaneously enables frame detection, one-tap SOP estimation, timing recovery, and frequency offset estimation (FOE), the proposed scheme also achieves online estimation and compensation of transceiver IQ skew. The experiment verifies the effectiveness of this method in three typical subcarrier allocation scenarios (single ONU four subcarriers, dual ONU two subcarriers each, and asymmetric subcarrier allocation). The results show that the transceiver IQ skew estimation error can be controlled within ±0.3 ps, and the sensitivity of 400G TFDM-PON system with 16-QAM signal after compensation reaches -28 dBm after 50-km fiber transmission. In addition, after calibrating the transceiver according to the estimated value of the proposed scheme, the sensitivity penalty of transceiver IQ skew under subcarrier power imbalance in 400G TFDM PON is explored. This work provides a feasible solution for online calibration of future high-speed coherent access networks.