√M-best candidates based soft-demapper for rotated M-QAM constellation

In the second generation terrestrial digital video broadcasting (DVB-T2) systems, one of the important features is the rotated quadrature amplitude modulation (QAM) with cyclic Q delay exploiting signal space diversity to improve detection performance. However, the complexity of soft-demapper is increased, because it needs to calculate the Euclidean distances between the received signal and all candidate points, and then to find the minimums for log-likelihood ratio (LLR) calculation. In this paper, we propose a reduced complexity soft-demapper that selects only √M-best candidates for LLR calculation of either even or odd bits. The candidate set is formed by selecting the closest points to the image made by projecting the received signal onto the constellation grid lines. In total, it only calculates the Euclidean distances for 2√M points, which hence can reduce the complexity significantly. In spite of the reduced complexity, the √M-best candidate set is optimal because it always includes the minimum-distance point in the original full set. Finally, our results show that the proposed soft-demapper achieves the same bit error rate (BER) performance as the Max-Log based full search one, but the complexity is reduced by 67.5% and 85.9%, respectively for 64-QAM and 256-QAM cases.