Coherent Optical Receiver Adaptive Equalizer Initialization
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Solution Overview
Problem
Coherent optical receivers with adaptive equalizers face challenges in initializing a blind equalizer, leading to slow convergence or failure to converge, which hampers the rapid start-up or re-start of high-speed fiber-optic links.
Innovation Solution
An initialization system for a coherent optical receiver that uses a short, repetitive known data sequence to rapidly converge the adaptive equalizer by estimating initial frequency-domain taps, including a fast Fourier transform unit and peak detector, allowing for quick compensation of channel impairments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a blind adaptive equalizer is used without initialization, then the system can operate autonomously without prior channel knowledge, but the equalizer converges slowly or fails to converge
Solution Approach 1:
The patent applies preliminary action by transmitting a known initialization sequence before normal data transmission. This known sequence allows the equalizer to pre-compute initial tap values based on the expected correlation between transmitted and received sequences, providing a head start for convergence without requiring manual channel characterization.
Solution Approach 2:
The patent employs feedback mechanisms where the receiver correlates the received initialization sequence with the known transmitted sequence to generate error signals. These error signals are fed back to adjust the equalizer taps iteratively, enabling rapid convergence by continuously refining the channel estimate based on the known reference sequence.
2Device complexity
If a blind adaptive equalizer is used without initialization, then the system structure remains simple, but the equalizer may get hung up on singularities and fail to converge
Solution Approach 1:
By performing preliminary equalization using the known initialization sequence, the system establishes reliable initial tap values before processing actual data. This preliminary action prevents the equalizer from encountering singularities during normal operation, as the initial estimates are already positioned in favorable regions of the solution space.
Solution Approach 2:
The known initialization sequence acts as an intermediary that bridges the gap between system startup and normal operation. This intermediary sequence provides a controlled reference that guides the equalizer through the challenging initialization phase, preventing singularity issues without adding permanent complexity to the system architecture.
3Loss of time
If a known initialization sequence is transmitted, then rapid convergence is achieved, but additional overhead is introduced to the communication system
Solution Approach 1:
The patent uses partial action by transmitting the known initialization sequence only during system startup or re-initialization events, rather than continuously. This approach provides sufficient information for rapid convergence while minimizing overhead to only what is strictly necessary for the initialization phase.
Solution Approach 2:
The initialization sequence serves its purpose during the convergence phase and is then discarded once the equalizer is operational. The system recovers by switching to normal data transmission mode, where the equalizer operates autonomously using the established tap values, eliminating the need for continued overhead from the initialization sequence.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables rapid convergence of the adaptive equalizer, ensuring quick and reliable re-start of high-speed fiber-optic links by compensating for channel impairments such as chromatic dispersion and polarization mode dispersion, maintaining high symbol fidelity.
Implementation Method 1
a fast Fourier transform unit and peak detector, allowing for quick compensation of channel impairments
Data Source
AI summary
An adaptive-equalizer initialization system performs three functions: frequency offset estimation, taps estimation for chromatic dispersion filters, and taps initialization for an adaptive equalizer. The system contains hardware FFT and peak detector units that sense a pure tone that marks the beginning of a known, short data sequence.