Soft-Output Demapper with Approximated LLR for DVB-T2 Systems

Bit-interleaved coded modulation (BICM) is at the heart of the second generation digital terrestrial television systems (DVB-T2), which achieves a large spectral efficiency by means of high-order quadrature amplitude modulation (QAM) and low-density parity-check (LDPC) codes. This paper proposes a simple but effective approximation for the log-likelihood ratio (LLR) of the bits estimated by the soft-output QAM demapper. The proposed approximation, which keeps few dominant terms of the LLR expression, needs low additional complexity with respect to the conventional Max-Log approximation. Simulation results for DVB-T2 in multipath channels show a performance gain with respect to the Max-Log approach, especially for QAM with large modulation order.