Coherent optical transmitter IQ imbalance mitigation with embedded carrier phase and frequency offset estimation

Area of Science:

• Optical Communications
• Signal Processing

Background:

• Coherent optical communication systems are susceptible to in-phase/quadrature (IQ) impairments originating from the transmitter.
• These impairments, including timing skew, gain imbalance, and phase errors, degrade signal quality and necessitate robust compensation techniques.

Purpose of the Study:

• To develop and validate a 4x4 real-valued channel equalizer with an embedded phase estimator for mitigating transmitter IQ impairments.
• To improve carrier phase and frequency offset estimation and compensation in coherent optical systems.

Main Methods:

• Implementation of a 4x4 real-valued equalizer architecture.
• Utilization of the decision-directed least mean square (DD-LMS) algorithm for adaptive filter coefficient control, enabling faster adaptation.
• Integration of a frequency offset estimation (FOE) module within the equalizer.

Main Results:

• Simulations show the proposed equalizer outperforms existing 2x2 and 4x4 real-valued equalizers in the presence of IQ impairments for 60 GBaud PM-16QAM signals.
• Experimental results demonstrate minimal SNR penalties (<0.41 dB) for various IQ impairments (7 ps skew, 3.5 dB gain imbalance, 0.5 V bias shift) on 60 GBaud PM-16QAM signals.
• Effective carrier phase estimation and frequency offset compensation were experimentally confirmed.

Conclusions:

• The proposed 4x4 real-valued equalizer with embedded FOE effectively compensates for transmitter IQ impairments in coherent optical communications.
• The DD-LMS algorithm facilitates rapid adaptation of equalizer coefficients, enhancing performance.
• The integrated approach offers a significant improvement over existing methods, particularly under challenging transmission conditions.