Hard- and soft-output trellis-based conflict resolution for bidirectional decision feedback equalization

Abstract

The decision feedback equalizer (DFE) is widely used in practice for intersymbol interference (ISI) mitigation because of its low complexity. The performance of the DFE can be enhanced by arbitrating between the output of a standard DFE and a second DFE that operates on the time-reversed received sequence. The combination is called a bidirectional DFE (BDFE). Several previous papers have discussed how to arbitrate between the outputs of the forward and reverse equalizers to make hard decisions. We show that the arbitration algorithms proposed by other researchers are inefficient and propose a trellis-based conflict resolution (TBCR) algorithm that simultaneously provides better performance and lower complexity. We further show how the TBCR approach can be used to create soft-output BDFE algorithms for use with soft-input decoding of an error-control code. The soft-output BDFE algorithms offer better performance than previously proposed algorithms while requiring lower complexity than the maximum a posteriori equalizer. We also propose a novel method to reduce the computational complexity of BDFE algorithms by simultaneously computing the coefficients of the forward and reverse equalizers.