MMSE-based iterative equalization with soft feedback for QAM transmission over sparse channels

An equalization algorithm, based on soft-decision feedback and designed for transmission with square QAM constellations, is introduced. The algorithm employs 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. It is shown that the algorithm is especially well suited for transmission over certain sparse frequency-selective fading channels where optimum schemes are far too complex. A minimum mean-squared error decision-feedback equalizer (MMSE-DFE) is clearly outperformed for this application.