Coherent Receiver DSP Segmentation for ISI Differentiation

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Solution Overview

Problem

Conventional digital signal processing units in coherent optical receivers face challenges in distinguishing between inter-symbol interference (ISI) due to fiber propagation impairments and ISI intentionally introduced by time-frequency packing (TFP), leading to suboptimal performance in compensating for phase and frequency offsets without prior knowledge of transmitter and receiver components.

Innovation Solution

A digital signal processing unit comprising an adaptive equalizer and a processing block that performs simultaneous channel response coefficients and equalizer coefficients estimation using a stochastic gradient algorithm, allowing the unit to differentiate between ISI from fiber impairments and ISI from TFP, ensuring proper task distribution between the adaptive equalizer and symbol sequence estimator.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a 2D-FFE is used to minimize MSE between equalized samples and transmitted symbols, then the contribution of noise and ISI is minimized, but the strategy becomes suboptimal for TFP systems where ISI is intentionally introduced

Engineering Contradiction:

ImproveMSE minimization accuracyVSAvoidAdaptability to TFP systems

Core Design Contradiction:

Measurement precisionVSAdaptability or versatility

Solution Approach 1:

The patent segments the equalization task into two distinct components: a 2D-FFE for linear equalization and a symbol-by-symbol maximum likelihood sequence estimator (MLSE) for dealing with intentional ISI. This segmentation allows each component to specialize - the 2D-FFE handles noise and linear distortions while the MLSE handles the intentional ISI introduced by TFP, resolving the contradiction between MSE minimization and TFP adaptability

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent introduces an intermediary processing stage between the 2D-FFE and the final detection. This intermediary MLSE component acts as a mediator that takes the equalized output and further processes it to account for intentional ISI, enabling the system to maintain optimal MSE minimization while also being adaptable to TFP systems

Inventive Principle:

Principle #24Intermediary (Mediator)

2Ease of operation

If the equalizer is adaptively controlled by stochastic gradient algorithm, then prior knowledge of transmitter and receiver components is not required, but the control strategy cannot properly distinguish between different sources of ISI

Engineering Contradiction:

ImproveAdaptive control without prior knowledgeVSAvoidISI source differentiation accuracy

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The patent segments the ISI handling into two distinct processing paths: one for linear ISI handled by the adaptively controlled 2D-FFE and another for intentional ISI handled by the MLSE. This segmentation enables the system to maintain ease of adaptive operation while achieving precise differentiation between ISI sources through the specialized MLSE component

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The MLSE acts as an intermediary that receives the adaptively equalized signal and further processes it to distinguish and handle intentional ISI. This intermediary layer preserves the adaptive control benefits while adding the capability to precisely differentiate ISI sources

Inventive Principle:

Principle #24Intermediary (Mediator)

3Measurement precision

If the 2D-FFE tries to remove ISI intentionally introduced by TFP, then MSE is minimized, but the intentional ISI cannot be properly accounted for

Engineering Contradiction:

ImproveMSE minimizationVSAvoidCorrect reception of TFP signals

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent segments the equalization function so that the 2D-FFE focuses on minimizing MSE from noise and linear distortions, while a separate MLSE component specifically handles the intentional ISI. This segmentation ensures that intentional ISI is not erroneously removed by the 2D-FFE, maintaining both MSE minimization and reliable TFP signal reception

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The MLSE serves as an intermediary that preserves intentional ISI while the 2D-FFE minimizes other sources of error. This intermediary structure ensures that the intentional ISI introduced by TFP is not destroyed by aggressive equalization, maintaining signal reliability

Inventive Principle:

Principle #24Intermediary (Mediator)

Data Source

PatentEP3219062B1Digital signal processing of an optical communications signal in a coherent optical receiver

Publication Date: 2021.06.02 TELEFONAKTIEBOLAGET LM ERICSSON (PUBL)

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AI summary

A digital signal processing, DSP, unit (10) for use in a coherent optical receiver for an optical communications network. The DSP unit comprises an adaptive equaliser (12) and a processing block (22). The equaliser (12) comprises input ports for receiving electrical signals, each corresponding to a different state of polarization of an optical signal received by the coherent optical receiver,and output ports,each connected to a processing branch (14). A processing branch comprises a symbol sequence estimator, SSE, (16) and a carrier phase estimator, CPE, (18) comprising an input for receiving signal taped from an output of the processing branch. An output of the CPE is connected to a phase adjuster (20) interconnecting the respective output port of the equaliser and the SSE. The processing block (22) is connected to an output of the CPE,an output of the processing branch and at least one of the output of the phase adjuster and the outputs of the equalizer.