A New Demapper for BICM System with HARQ

Abstract

In this paper, a new demapper generating log- likelihood ratio (LLR) is proposed by using a linear approxima- tion 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. Index Terms—bit interleaved coded modulation (BICM), demapper, hybrid automatic repeat request (HARQ), log- likelihood ratio (LLR), quadrature amplitude modulation (QAM), throughput