Soft-In Soft-Out DFE and Bi-directional DFE

TL;DR

This paper introduces a novel soft-in soft-out decision feedback equalizer (DFE) with extrinsic information formulation that outperforms linear equalizers in turbo equalizer settings, and proposes a bidirectional DFE strategy for improved performance.

Contribution

The paper presents a new SISO-DFE design that accounts for error propagation and introduces a bidirectional DFE approach combining outputs from two DFEs for enhanced turbo equalization.

Findings

SISO-DFE outperforms SISO linear equalizer in simulations.

Bidirectional DFE improves performance by exploiting error correlation.

Performance remains stable with time-invariant filter constraints.

Abstract

We design a soft-in soft-out (SISO) decision feedback equalizer (DFE) that performs better than its linear counterpart in turbo equalizer (TE) setting. Unlike previously developed SISO-DFEs, the present DFE scheme relies on extrinsic information formulation that directly takes into account the error propagation effect. With this new approach, both error rate simulation and the extrinsic information transfer (EXIT) chart analysis indicate that the proposed SISO-DFE is superior to the well-known SISO linear equalizer (LE). This result is in contrast with the general understanding today that the error propagation effect of the DFE degrades the overall TE performance below that of the TE based on a LE. We also describe a new extrinsic information combining strategy involving the outputs of two DFEs running in opposite directions, that explores error correlation between the two sets of DFE outputs. When this method is combined with the new DFE extrinsic information formulation, the resulting "bidirectional" turbo-DFE achieves excellent performance-complexity tradeoffs compared to the TE based on the BCJR algorithm or on the LE. Unlike turbo LE or turbo DFE, the turbo BiDFE's performance does not degrade significantly as the feedforward and feedback filter taps are constrained to be time-invariant.