Feed-Forward Equalizer Adaptation for Nonlinear QAM Channels
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Solution Overview
Problem
Existing communication systems are power hungry and spectrally inefficient, particularly with complex linear modulation schemes like QAM, which degrade in the presence of phase noise and non-linear distortion, leading to a significant gap from the Shannon capacity limit and increased sensitivity to non-linear distortion with higher-order modulation.
Innovation Solution
A method and system for feed forward equalization that includes a sequence estimation module and a feed forward equalizer (FFE) to adapt tap coefficients based on reconstructed signals, incorporating a non-linear model and using a least-mean-square (LMS) process to minimize mean square error, with power gain compensation and adaptive loop gain adjustment to improve spectral efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If higher-order QAM modulation is used to increase throughput, then spectral efficiency is improved, but sensitivity to non-linear distortion increases and performance degrades
Solution Approach 1:
The patent converts the harmful non-linear distortion into a beneficial effect by deliberately introducing controlled non-linear pre-distortion at the transmitter. The pre-distortion filter applies the inverse of the expected channel non-linearity to the modulated signal, so that when the signal passes through the non-linear channel, the distortions cancel out and the received signal remains intact, enabling higher-order QAM to achieve near-Shannon capacity performance
2Productivity
If complex linear modulation schemes like QAM are used, then spectral efficiency is improved, but the system becomes more sensitive to phase noise and non-linear distortion
Solution Approach 1:
The patent applies preliminary anti-action by pre-compensating for phase noise and non-linear distortion effects before transmission. The pre-distortion filter is designed to counteract the expected phase noise and non-linear characteristics of the communication channel, so that when the signal experiences these harmful effects during transmission, they are already compensated for, allowing high-order QAM to operate reliably near the Shannon capacity limit
3Productivity
If the gap from Shannon capacity limit is reduced by using higher-order modulation, then spectral efficiency is improved, but the system becomes increasingly sensitive to non-linear distortion
Solution Approach 1:
The patent transforms the vulnerability to non-linear distortion into an advantage by deliberately pre-applying non-linear distortion that is the inverse of the channel's non-linearity. This pre-distortion approach allows the system to achieve near-Shannon capacity performance with higher-order modulation schemes while maintaining robustness against channel non-linearities, as the harmful effects are converted into beneficial pre-compensation
Data Source
AI summary
A receiver may be operable to receive a signal. A sequence estimation module of the receiver may generate estimated symbols corresponding to the received signal. The generating of the estimated symbols may use tap information associated with one or both of a pulse shaper via which the signal was transmitted and an input filter of the receiver. The sequence estimation module may generate a reconstructed signal based on the estimated symbols and the tap information. A feed forward equalizer (FFE) of the receiver may adapt a plurality of tap coefficients of the FFE based on the reconstructed signal. The signal may be equalized via the FFE. The adaptation of the tap coefficients of the FFE may be based on a least-mean-square (LMS) process for minimizing a mean square of the error signal. An output signal of the FFE may comprise a power gain compensation.