Optical Receiver IIR Decision Feedback Equalization

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Solution Overview

Problem

Optical receivers face challenges in achieving high bandwidth and signal-to-noise ratio while minimizing intersymbol interference (ISI) due to the tradeoffs between gain, bandwidth, and noise performance, particularly with large load resistors leading to increased ISI and reduced gain.

Innovation Solution

The implementation of an infinite impulse response (IIR) decision feedback equalizer in optical receivers, which uses a switching circuit to connect a current source to the internal capacitance of a photo detector, allowing for the subtraction of the decaying exponential tail of the voltage signal to cancel ISI, thereby enhancing signal quality and achieving higher sensitivity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a large load resistor is used in the optical receiver, then the gain is improved, but the bandwidth is reduced and intersymbol interference increases

Engineering Contradiction:

Improvesignal-to-noise ratioVSAvoidbandwidth

Core Design Contradiction:

Measurement precisionVSSpeed

Solution Approach 1:

The patent segments the equalization function into two distinct parts: a feedforward equalizer (FFE) that processes the current symbol and a decision feedback equalizer (DFE) that processes previous symbols. This segmentation allows the system to use a large load resistor for high gain while the DFE separately compensates for the ISI caused by the resulting slow bandwidth, resolving the contradiction between gain and bandwidth.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent implements a decision feedback equalizer that uses feedback from previously decided data symbols to generate a compensation signal. This feedback mechanism allows the system to maintain high gain with a large load resistor while actively canceling the intersymbol interference that would otherwise limit bandwidth, thus resolving the gain-bandwidth tradeoff.

Inventive Principle:

Principle #23Feedback

2Measurement precision

If a large load resistor is used in the optical receiver, then the gain is improved, but intersymbol interference increases

Engineering Contradiction:

Improvesignal-to-noise ratioVSAvoidintersymbol interference

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The decision feedback equalizer uses feedback from previously decided symbols to generate a compensation signal that is subtracted from the current symbol decision. This feedback mechanism directly cancels the intersymbol interference generated by the large load resistor, allowing the system to achieve high gain while eliminating the harmful ISI effect.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The patent converts the harmful intersymbol interference caused by the large load resistor into a useful feedback signal. By capturing the ISI through the feedback path and using it to generate a compensation signal, the system transforms the harmful effect into a beneficial cancellation mechanism, allowing high gain operation without ISI degradation.

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Reliability

If bandwidth is increased to reduce intersymbol interference, then signal quality is improved, but power consumption increases

Engineering Contradiction:

Improvesignal qualityVSAvoidpower consumption

Core Design Contradiction:

ReliabilityVSUse of energy by moving object

Solution Approach 1:

The patent segments the equalization task between a simple feedforward path and a feedback path, allowing the use of a large load resistor (which consumes less power than high-bandwidth amplifiers) while still achieving high signal quality through the DFE compensation mechanism.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent changes the system parameter from high bandwidth to high gain with active ISI cancellation. By using a large load resistor for high gain and adding the DFE for ISI cancellation, the system achieves high signal quality through parameter optimization rather than increasing bandwidth, thereby reducing power consumption.

Inventive Principle:

Principle #35Parameter changes

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

This approach allows for increased main cursor gain while noiselessly eliminating ISI, resulting in improved signal-to-noise ratio and sensitivity, enabling higher data rates with reduced power consumption and compact design.

Implementation Method 1

generating a voltage by a photo detector connected to a load resistor

Methodology Applied

Scientific EffectPhotoelectric effect: Photoelectric Effect

Data Source

PatentUS8977138B2Optical receiver using infinite impulse response decision feedback equalization

Publication Date: 2015.03.10 GLOBALFOUNDRIES US INC

AI summary

A technique is provided for configuring an optical receiver. A photo detector is connected to a load resistor, and the photo detector includes an internal capacitance. A current source is connected through a switching circuit to the load resistor and to the photo detector. The current source is configured to discharge the internal capacitance of the photo detector. The switching circuit is configured to connect the current source to the internal capacitance based on a previous data bit.