Matched-filter- and MMSE-based iterative equalization with soft feedback for QPSK transmission

In this paper, two novel equalization algorithms applying soft-decision feedback, designed for quaternary phase-shift keying (QPSK) transmission are introduced. The first one uses a matched filter (MF) as a front end and is derived from Gerstacker et al. (2000) where it was originally developed for binary transmission. The second one is an improved version of the first, employing a minimum mean-squared error (MMSE) filter in each iteration in order to refine the data estimates. The rule for generating soft decisions is adapted continuously to the current state of the algorithm. In most cases, standard DFE methods are clearly outperformed. In contrast to the algorithm of Gerstacker, a performance close to that of maximum-likelihood sequence estimation can be obtained for the MMSE-based scheme for a broad class of channels.