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In this paper, an equalization algorithm employing soft-decision feedback, designed for transmission with general square QAM constellations is introduced like e.g. 16QAM or 64QAM allowing high data rates. It is derived from [J.F. Robler, W.H. Gerstacker, A. Lampe, and J.B.Huder, 2002] where it was originally developed for 4QAM transmission. The algorithm employs a minimum mean-square 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 can be shown by simulations for a 16QAM transmission, that a linear finite length MMSE channel equalizer according to [J.G. Proakis, 1995] is clearly outperformed. In addition to [J.F. Robler, W.H. Gerstacker, A. Lampe, and J.B. Huder, 2002], we compare our new generalized scheme for different MMSE filter lengths and show the achievable extra gain.