Iterative MMSE-SIC receiver with low-complexity soft symbol and residual interference estimations

In this paper, we develop low-complexity methods to compute the soft symbol and variance estimations for QAM constellations with applications to iterative MMSE-SIC receivers. In particular, we propose a bit flipping based estimation scheme. For an N-QAM constellation with Gray mapping, we derive efficient approaches for both soft symbol and variance estimations with the complexities of O(log N) and O(log² N), respectively. We also present an improved approximation approach which completely removes the multiplications at a cost of a slight performance degradation. We later extend the proposed methods to the non-squared QAM.