IQ Skew and Imbalance Estimation for Coherent Point-to-Multi-Point Optical Networks

TL;DR

This work tackles IQ impairments in DSCM-based coherent PtMP optical networks, where estimating far-end IQ skew and power imbalance at a leaf is challenging due to access to only one side of the subcarrier. It introduces time-frequency interleaving tones (TFITs) with a four-slot structure and two tones to enable simultaneous estimation of hub Tx and leaf Rx impairments without requiring symmetrical subcarriers. The authors develop Rx-first estimation methods and validate them experimentally on an 8Gbaud/SC×4SC DSCM network, achieving estimation errors on the order of ±0.5 ps for skew and ±0.2 dB for imbalance, with BER improvements after compensation, especially on high-frequency subcarriers. The results indicate that TFITs-based IQ-impairments estimation is a promising approach for DSCM PtMP networks and can significantly enhance robustness, with future work addressing frequency-dependent impairments via extended TFITs and FFT lengths.

Abstract

Coherent point-to-multi-point (PtMP) optical network based on digital subcarrier multiplexing (DSCM) has been a promising technology for metro and access networks to achieve cost savings, low latency, and high flexibility. In-phase and quadrature (IQ) impairments of the coherent transceiver (e.g. IQ skew and power imbalance) cause severe performance degradation. In the DSCM-based coherent PtMP optical networks, it is hard to realize far-end IQ-impairments estimation for the hub transmitter because the leaf on one subcarrier cannot acquire the signal on the symmetrical subcarrier. In this paper, we propose a far-end IQ-impairments estimation based on the specially designed time-and-frequency interleaving tones (TFITs), which can simultaneously estimate IQ skews and power imbalances of the hub transmitter and leaf receiver at an individual leaf. The feasibility of the TFITs-based IQ-impairments estimation has been experimentally verified by setting up $8$Gbaud/SC $\times$ $4$SCs DSCM-based coherent PtMP optical network. The experimental results depict that the absolute errors in the estimated IQ skew and power imbalance are within $\pm 0.5$ps and $\pm 0.2$dB, respectively. In conclusion, TFITs-based IQ-impairments estimation has great potential for DSCM-based coherent PtMP optical networks.

Figures (10)

• Figure 1: Schematic diagram of DSCM-based coherent PtMP optical networks with Tx IQ impairments.
• Figure 2: Structure of the specially designed TFITs for one polarization.
• Figure 3: The specially designed TFITs including four time slots $t_1$, $t_2$, $t_3$, $t_4$ and two tone frequencies $f_1$, $f_2$ for (a) X polarization and (b) Y polarization.
• Figure 4: Schematic diagram of IQ-impairments estimation based on the specially designed TFITs including Rx IQ (a) skew and (b) power imbalance.
• Figure 5: Schematic diagram of far-end IQ-impairments estimation based on the specially designed TFITs including Tx IQ (a) skew and (b) power imbalance.