Optical Clock Recovery Using Channel-Delay-Removed Equalizer Coefficients
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Solution Overview
Problem
In high-speed optical communications networks, severe inter-symbol interference (ISI) due to chromatic dispersion (CD) and polarization mode dispersion (PMD) can cause clock phase sensitivity loss, leading to random walk behavior of compensation vectors and performance penalties or system breakdown, especially when the equalizer adaptation loop interacts with the clock recovery Phase Locked Loop (PLL).
Innovation Solution
A method is introduced to separate the equalizer adaptation loop from the clock recovery PLL by estimating and subtracting channel delay from the compensation vectors, thereby preventing undesirable interactions and allowing the equalizer to continue compensating ISI while the clock recovery operates on signals compensated for ISI other than channel delay.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the equalizer adaptation loop is used for both signal equalization and clock recovery functions, then device complexity is reduced, but the clock phase sensitivity is lost and random walk behavior occurs
Solution Approach 1:
The patent segments the compensation vectors into two distinct sets: one set for equalization that includes channel delay compensation, and another set for clock recovery that excludes channel delay compensation. This segmentation allows each loop to operate independently with its own optimized parameters, preventing the random walk behavior that occurs when both functions share the same adaptation loop.
Solution Approach 2:
The patent extracts the channel delay compensation function from the clock recovery process. By removing the channel delay component from the compensation vectors used in clock recovery, the system eliminates the source of random walk behavior while maintaining separate optimization paths for equalization and clock recovery functions.
2Manufacturing precision
If the equalizer adaptation loop compensates for channel delay, then ISI compensation is improved, but the clock recovery PLL interacts undesirably causing performance penalties
Solution Approach 1:
The patent creates separate compensation vector sets with distinct functions: equalization vectors that include channel delay compensation for optimal ISI compensation, and clock recovery vectors that exclude channel delay to maintain PLL stability. This functional segmentation resolves the contradiction between achieving precise ISI compensation and maintaining system stability.
Solution Approach 2:
The patent applies different quality characteristics to different parts of the compensation process. The equalization path uses full compensation vectors with channel delay for maximum ISI mitigation, while the clock recovery path uses modified vectors without channel delay for stable phase locking. Each path is optimized for its specific local requirement.
Data Source
AI summary
In a coherent optical receiver of an optical communications network, a method of recovering a clock signal from a high speed optical signal received through an optical link. A set of compensation vectors are adaptively computed for compensating Inter-symbol Interference (ISI) due to at least polarization impairments of the optical signal. A channel delay is estimated based on the computed compensation vectors. The estimated channel delay is subtracted from the computed compensation vectors to generate corresponding modified compensation vectors. Finally, the modified compensation vectors are used to derive a recovered clock signal.