Enhanced QRD-M algorithm for soft-output MIMO detection

QR-decomposition with M algorithm is a promising technique for multi-input multi-output (MIMO) systems. When the number of candidates is insufficient, its performance is, however, degraded due to an inaccurate log likelihood ratio (LLR). To address this problem, an enhanced soft LLR calculation scheme is proposed for soft-output MIMO detection. Based on QR-decomposition, the proposed algorithm can efficiently obtain a soft information of minimum mean-squared error (MMSE) equalization. With this information, an optimal weighted combining method is derived in an MSE sense. In addition, we compute a more reliable clipping value from the soft information of MMSE equalization. Simulation results show that the proposed algorithm provides a considerable performance gain over conventional algorithms and its performance is close to optimal performance with insufficient candidates.