Iterative equalization with hard and soft decisions for ISI-constrained channels

Although iterative equalizers are well-known for mitigating inter-symbol interference (ISI), recent results have shown that the schemes perform poorly in the presence of severe ISI, particularly when coding rates are high. Other equalization schemes, such as the non-iterative canonical decision-feedback equalizer (CDFE), have been shown to be asymptotically optimal on any channel. However, the scheme exhibits very high latency. To trade off between performance and latency, we propose an equalizer structure that iterates with both hard- and soft-decisions. In prior works, hard-decision iterative equalizers were seen to perform poorly, chiefly due to error-propagation in the feedback loop. However, the scheme proposed in this paper outperforms the linear turbo equalizer in both strong and weak ISI. On channels with weak ISI, the equalizer outperforms both the CDFE as well as the ideal DFE with perfect feedback. Simulation results on well-known ISI channels support the findings.