Performance comparison of hybrid partial response detectors over frequency-selective fading channels

Frequency-selective fading channels are encountered in many modern wireless communication systems. In order to combat the intersymbol interference (ISI) introduced by such fading, equalization is required for reliable symbol detection. The maximum-likelihood sequence detector is the optimal equalization scheme; however its implementation complexity increases exponentially with the channel length and thus can be prohibitively high. In this paper, we compare the performance of two practically implementable suboptimal symbol detectors, including the partial response maximum-likelihood (PRML) detector and the partial response belief propagation (PRBP) detector, under frequency-selective fading channels. Both detectors employ a hybrid two-stage scheme, and allow a tradeoff between performance and complexity. The first stage is a partial response equalizer implemented as a linear filter which transforms the original channel impulse response to a target impulse response with reduced ISI. The residual ISI is then cancelled in the second stage using a more sophisticated nonlinear detector. In simulations, we consider a slow fading environment and use the ITU-R 3G channel models. From the numerical results, it is shown that in frequency-selective fading wireless channels, the PRBP detector provides superior performance over both the traditional minimum mean squared error linear equalizer and the PRML detector. Due to the effect of colored noise, the PRML detector in fading wireless channels is not as effective as it is in magnetic recording applications.