LUT-Based MAP Detector for Nonlinear ISI Symbol Detection

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Solution Overview

Problem

Channel inter-symbol interference (ISI) due to narrow filtering or nonlinear distortion in a transmitter cannot be easily compensated by a linear equalizer, leading to enhanced noise and distortion in optical communication systems.

Innovation Solution

A receiver system incorporating a hard decision decoder, look-up table (LUT), and soft metric generator to calculate bit log likelihood ratio (LLR) values based on neighboring symbols, reducing the complexity of the maximum a posteriori (MAP) detector by using a simplified LUT for the center symbol and neighboring transmitted symbols as an address index.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a linear equalizer is used to compensate for channel ISI, then the mean square error can be minimized, but enhanced noise is generated at high frequency

Engineering Contradiction:

Improvemean square errorVSAvoidenhanced noise

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The receiver is divided into multiple functional blocks: linear equalizer, post filter, hard decision decoder, LUT, and soft metric generator. Each block performs a specific function to progressively improve signal quality while managing noise separately, rather than trying to handle all issues in a single equalizer stage.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A post filter is introduced as an intermediary component between the linear equalizer and the hard decision decoder. This post filter acts as a mediator that suppresses the enhanced high-frequency noise generated by the equalizer before the signal undergoes hard decision decoding, thereby preventing noise propagation to subsequent stages.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Object-affected harmful factors

If a low-pass post filter is added to suppress enhanced noise, then noise is reduced, but ISI is introduced

Engineering Contradiction:

Improveenhanced noiseVSAvoidISI

Core Design Contradiction:

Object-affected harmful factorsVSMeasurement precision

Solution Approach 1:

The patent replaces traditional mechanical filtering approaches with a lookup table (LUT)-based system. Instead of using a low-pass filter that would introduce ISI, the invention uses a LUT to store pre-calculated channel impulse response values, allowing the system to compensate for ISI through table lookup and calculation without the frequency-selective filtering that causes distortion.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Measurement precision

If a MAP detector is used to handle both linear and nonlinear ISI, then detection accuracy is improved, but device complexity increases

Engineering Contradiction:

Improvesymbol detection accuracyVSAvoiddetector complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The system performs preliminary hard decision decoding on the received signal before feeding it to the LUT. This preliminary action reduces the complexity of subsequent processing by providing a simplified input to the soft metric generator, which then uses the LUT to calculate LLR values without needing to perform exhaustive maximum a posteriori calculations for all possible symbol combinations.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The LUT stores pre-calculated values representing channel impulse responses for various neighboring symbol combinations. Instead of performing complex calculations in real-time for each received symbol, the system copies appropriate pre-computed values from the LUT based on the hard-decoded neighboring symbols, significantly reducing computational complexity while maintaining MAP detection accuracy.

Inventive Principle:

Principle #26Copying

4Measurement precision

If LUT is trained with all possible center symbol candidates, then detection accuracy is improved, but memory requirements and processing time increase

Engineering Contradiction:

Improvecenter symbol detection accuracyVSAvoidLUT training time

Core Design Contradiction:

Measurement precisionVSLoss of time

Solution Approach 1:

The LUT is trained with a selected subset of center symbol candidates rather than all possible candidates. This partial action approach focuses training resources on the most probable or most critical symbol combinations, achieving sufficient detection accuracy without the excessive time and computational resources required for complete training of all possible symbols.

Inventive Principle:

Principle #16Partial or excessive action

Data Source

PatentUS12413245B2Maximum a posteriori detector for linear and nonlinear channel impairment

Publication Date: 2025.09.09 CISCO TECHNOLOGY INC

AI summary

A receiver configured to receive a plurality of symbols is disclosed. The receiver includes a hard decision decoder, a look-up table (LUT) coupled to the hard decision decoder, and a soft metric generator coupled to the LUT. The hard decision decoder is to receive a first set of symbols from the plurality of symbols and provide a set of hard coded neighboring symbols to the LUT. The first set of symbols comprises a center symbol with neighboring symbols. The LUT is to store a value representative of the center symbol that is addressable by the set of hard coded neighboring symbols. The soft metric generator is to calculate bit log likelihood ratio (LLR) values based on the center symbol and the value representative of the center symbol stored in the LUT.