Adaptive Turbo Equalization for Nonlinearity Compensation in WDM Systems

TL;DR

This paper introduces an adaptive turbo equalization scheme using RLS channel estimation and SISO LMMSE equalization to effectively compensate for nonlinear interference in WDM systems, improving signal quality and data rates.

Contribution

It presents a novel turbo equalization approach with adaptive channel estimation for nonlinear compensation in WDM systems, enhancing performance beyond existing methods.

Findings

Achieves up to 0.7 dB SNR gain

Provides up to 0.2 bits/symbol/pol in GMI

Effective for high-order QAM modulation

Abstract

In this paper, the performance of adaptive turbo equalization for nonlinearity compensation (NLC) is investigated. A turbo equalization scheme is proposed where a recursive least-squares (RLS) algorithm is used as an adaptive channel estimator to track the time-varying intersymbol interference (ISI) coefficients associated with inter-channel nonlinear interference (NLI) model. The estimated channel coefficients are used by a MIMO 2x2 soft-input soft-output (SISO) linear minimum mean square error (LMMSE) equalizer to compensate for the time-varying ISI. The SISO LMMSE equalizer and the SISO forward error correction (FEC) decoder exchange extrinsic information in every turbo iteration, allowing the receiver to improve the performance of the channel estimation and the equalization, achieving lower bit-error-rate (BER) values. The proposed scheme is investigated for polarization multiplexed 64QAM and 256QAM, although it applies to any proper modulation format. Extensive numerical results are presented. It is shown that the scheme allows up to 0.7 dB extra gain in effectively received signal-to-noise ratio (SNR) and up to 0.2 bits/symbol/pol in generalized mutual information (GMI), on top of the gain provided by single-channel digital backpropagation.