Efficient LLR Calculation for Non-Binary Modulations over Fading Channels

TL;DR

This paper introduces a convex optimization approach for efficient LLR calculation in non-binary modulations over fading channels, achieving near-true LLR performance with simplified approximations.

Contribution

It proposes a new measure for LLR accuracy and proves convexity of the optimization problem for piecewise linear LLR approximations.

Findings

Negligible performance loss with optimized approximate LLRs

Convexity of the coefficient optimization problem established

Effective LLR approximation method for non-binary modulations

Abstract

Log-likelihood ratio (LLR) computation for non-binary modulations over fading channels is complicated. A measure of LLR accuracy on asymmetric binary channels is introduced to facilitate good LLR approximations for non-binary modulations. Considering piecewise linear LLR approximations, we prove convexity of optimizing the coefficients according to this measure. For the optimized approximate LLRs, we report negligible performance losses compared to true LLRs.