Decision feedback equalizer (DFE) uses the past decisions in the feedback filter to mitigate the intersymbol interference (ISI). For the underwater acoustic channel, the impulse response often covers tens to hundreds of symbols, requiring at least tens of taps in the feedback filter. The error propagation easily occurs when the signal to noise ratio (SNR) decreases. In order to improve the DFE performance, a soft decision device is used in the feedback loop. Differently with the hard DFE, the soft DFE takes into account the reliability of the decisions, and the likelihood of these decisions is feedback to mitigate the ISI. The soft DFE is demonstrated to be useful from two aspects in the real sea test: 1). It can overcome the SNR loss brought by the timing synchronization error; 2). In the decision-directed mode, the hard DFE easily diverges facing the occasional powerful noises, while the soft DFE is robust to more likely to converge again without the help of any training sequence.
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