Low-Complexity Methods for Soft Estimation of QAM Symbols

In this paper, we develop low-complexity methods to compute the soft estimation of quadrature amplitude modulation (QAM) symbols with applications to iterative receivers. To reduce the complexity of the soft QAM estimation, we first consider squared QAM constellations and present a bit flipping based soft estimation scheme. With the gray mapping we derive an efficient approach which has a very low complexity of O(log N) for an N-QAM constellation. To further simplify the approach, we propose a method which completely removes the multiplication operations at a cost of a slight performance degradation. Finally, we extend the proposed method to the non-squared QAM.