Low Complexity Switched Soft Demapper for Rotated QAM Constellations.

In the digital video broadcasting-second generation terrestrial standard, the constellation rotation and cyclic Q-delay techniques have been adopted to improve the bit error rate performance in fading channels. However, they would lead to a substantially increased demapping complexity at the receiver. In order to reduce the demapping complexity, we propose a switched soft demapper, where different demapping algorithms with different level of complexities can be selected according to the in-phase (I) and the quadrature (Q) channel fading conditions. A decision block is applied to determine the demapping algorithm. Three kinds of decision criteria are discussed and compared to verify that the decision criterion has an impact on the performance of the soft demapper. In addition, the algorithm that determines the threshold in the decision block is presented. For the rotated 64-QAM, the performance loss of the proposed demapping scheme is proved to be negligible if a suitable threshold is applied, while the demapping complexity is reduced by up to 77% in Rayleigh fading channel without erasures. For the rotated 256-QAM, the complexity is reduced by up to 95% in Rayleigh fading channel without erasures.