Fractional FFE for Higher-Level CDR in Passive Optical Networks

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Solution Overview

Problem

High-speed PONs face significant challenges with inter-symbol interference (ISI) and convergence time in clock and data recovery (CDR) due to increasing bandwidth demands, which affects data transmission quality and system complexity.

Innovation Solution

Implementing a fractional feed-forward equalizer (FFE) that performs both phase adjustment and ISI compensation, enabling higher-level CDR such as three-level for NRZ signals and seven-level for PAM4 signals, which reduces ADC and DSP complexity and shortens convergence time.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If higher bit rates per wavelength channel are implemented to meet increasing bandwidth demands, then network capacity is improved, but inter-symbol interference (ISI) worsens due to bandwidth restrictions

Engineering Contradiction:

Improvenetwork capacityVSAvoidinter-symbol interference

Core Design Contradiction:

ProductivityVSObject-affected harmful factors

Solution Approach 1:

The patent changes the equalization parameter from traditional 2-level to higher-level (3-level or 4-level) equalization, which fundamentally alters how the system handles signal levels and reduces ISI effects at higher bit rates

Inventive Principle:

Principle #35Parameter changes

2Device complexity

If conventional 2-level CDR is used for clock and data recovery, then system complexity is maintained at acceptable levels, but convergence time becomes excessively long affecting data transmission quality

Engineering Contradiction:

Improvesystem complexityVSAvoidconvergence time

Core Design Contradiction:

Device complexityVSLoss of time

Solution Approach 1:

The patent changes the CDR equalization parameter from 2-level to higher-level (3-level for NRZ, 4-level for PAM4) equalization, which dramatically reduces convergence time while maintaining manageable system complexity through a unified FFE structure

Inventive Principle:

Principle #35Parameter changes

3Adaptability or versatility

If separate CDR and equalization functions are implemented, then functional separation provides design flexibility, but device complexity increases due to multiple components

Engineering Contradiction:

Improvedesign flexibilityVSAvoiddevice complexity

Core Design Contradiction:

Adaptability or versatilityVSDevice complexity

Solution Approach 1:

The patent merges the CDR and equalization functions into a single fractional FFE structure, eliminating the need for separate components while maintaining both functions through a unified architecture that processes signals more efficiently

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The fractional FFE is designed to perform multiple functions simultaneously - it acts as both a CDR device and an equalizer, providing universal functionality that reduces overall system complexity while maintaining design flexibility

Inventive Principle:

Principle #6Universality (Multi-functionality)

Data Source

PatentEP3539228B1Higher-level clock and data recovery (CDR) in passive optical networks (PONS)

Publication Date: 2023.10.25 HUAWEI TECH CO LTD

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AI summary

An apparatus comprises : a CDR sub-system comprising : an FFE; a decision component coupled to the FFE; a subtractor coupled to the FFE and the decision component; and a tap weight updater coupled to the subtractor and the FFE; and a PR-MLSE component coupled to the CDR sub-system. A method comprises : converting an optical signal with a first modulation format to an analog electrical signal; converting the analog electrical signal to a first digital signal; equalizing the first digital signal into a second digital signal with a second modulation format, wherein the second modulation format has more levels than the first modulation format; and performing CDR on the second digital signal.