A New Demapper for BICM System with HARQ

In this paper, a new demapper generating log-likelihood ratio (LLR) is proposed by using a linear approximation for the nonlinear factor of an exact LLR. The exact LLR is sufficient for the probability based decoding, but its calculation requires a large number of computations in the case of high order modulations such as 16 quadrature amplitude modulation (QAM) and 64QAM. To alleviate the computational complexity a piecewise-linear demapper was proposed by neglecting the nonlinear factor of the exact LLR. However, it is observed that the throughput performance of the piecewise-linear demapper in hybrid automatic repeat request (HARQ) system is deficient in low signal-to-noise ratio region. Therefore, the new demapper is proposed which has an equivalent throughput performance to the exact LLR in HARQ system, and at the same time, only needs a few elementary operations for calculating the simplified LLRs. Experimental evidences which show the viability of the proposed demapper are also provided.