Training-Assisted Carrier Frequency and Phase Recovery in Optical Systems
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Solution Overview
Problem
Conventional methods for carrier frequency and phase recovery in digital coherent optical communication systems, especially for high-order QAM formats, face challenges due to high implementation complexity and the need for large FFT sizes, which reduces spectral efficiency and increases computational time, while phase-locked loop-based algorithms perform poorly in high-speed optical systems due to feedback delay and parallel processing requirements.
Innovation Solution
A method that uses a starting training sequence to estimate the frequency offset, tracks frequency offset variations using recovered carrier phase in a feedback configuration, and employs a two-stage phase recovery technique with sparsely inserted training symbols and a blind phase search algorithm to refine phase estimates over a small phase-varying range, reducing implementation complexity and eliminating the need for differential coding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional FFT-based carrier frequency recovery methods are used for high-order QAM systems, then frequency recovery accuracy is improved, but implementation complexity and required FFT size increase significantly
Solution Approach 1:
The patent segments the frequency recovery process into two distinct stages: a training-based stage for initial frequency offset estimation, and a blind tracking stage for continuous frequency offset tracking. This segmentation allows each stage to use optimized algorithms suitable for its specific function, reducing overall complexity while maintaining accuracy.
Solution Approach 2:
The patent performs preliminary frequency offset estimation using training symbols before the main data transmission begins. This preliminary action removes the bulk of the frequency offset early in the process, allowing subsequent blind tracking algorithms to operate with reduced complexity and smaller FFT sizes.
2Reliability
If large FFT sizes are used for reliable frequency recovery in high-order QAM, then frequency recovery reliability is improved, but computational time and spectral efficiency deteriorate
Solution Approach 1:
The training-based initial frequency offset estimation performs the computationally intensive frequency measurement in advance, before main data transmission. This preliminary action achieves reliable frequency offset estimation without requiring large FFT sizes during the critical data transmission phase, thereby reducing computational time and improving spectral efficiency.
Solution Approach 2:
The patent implements continuous frequency offset tracking during data transmission using simplified blind tracking algorithms. This continuous tracking maintains frequency recovery reliability throughout the transmission without requiring repeated large FFT operations, thus reducing overall computational time.
3Reliability
If conventional training-based carrier recovery algorithms are used, then frequency and phase tracking robustness is improved, but spectral efficiency decreases due to overhead
Solution Approach 1:
The patent uses a hybrid approach where training symbols are used partially for initial frequency offset estimation, and then blind tracking methods take over for continuous phase and frequency tracking. This partial use of training-based methods provides sufficient tracking robustness while minimizing the overhead required, thereby improving spectral efficiency compared to fully training-based approaches.
4Productivity
If PLL-based blind carrier recovery algorithms are used for high-speed optical systems, then parallel processing capability is improved, but performance deteriorates due to feedback delay
Solution Approach 1:
The patent performs preliminary frequency offset estimation using training symbols before main data transmission. This preliminary action removes the need for feedback-based frequency correction during high-speed transmission, eliminating feedback delay issues while maintaining the ability to perform parallel processing during the blind tracking phase.
Data Source
AI summary
A method of recovering frequency and phase associated with an optical carrier signal in an optical communication system includes determining an estimated frequency offset based on a starting training sequence, determining a current frequency offset based on the estimated frequency offset and a current phase during steady-state operation of the optical communication system, determining a current frequency based on the current frequency offset, determining an estimated phase using training symbols inserted into the optical carrier signal, and determining the current phase associated with the optical carrier signal based on the estimated phase and a blind phase search algorithm. A corresponding systems and computer-readable device are also disclosed.