Mapeamento de bits para adaptação rápida a variações de canal de sistemas QAM codificados com LDPC

Abstract

Low-Density parity-check (LDPC) codes are being adopted as the error correction strategy in di erent system standards, such as the G.hn (home networking standard) and the IEEE 802.11n (wireless local standard). In these LDPC-coded quadrature amplitude modulation (QAM) systems, mapping the LDPC coded bits properly to the di erent sub-channels considering the fact that sub-channels have di erent qualities ensures an improved overall system performance. Accordingly, this thesis presents a new bit mapping technique based on the assumption that bits transmitted in \good" sub-channels, help bits transmitted in \bad" sub-channels. This can be made possible through some restrictions to be imposed on the associated Tanner graph, akin to Root-LDPC codes. An optimization of the root-like bit mapping through extrinsic information transfer (EXIT) charts analysis is also presented. We show that this mapping has the advantage of a reduced optimization search space when applied to single-carrier based systems. Moreover, in situations where the search space is not só reduced, such as in orthogonal frequency division multiplexing (OFDM)-based applications, we arrive at a rule of thumb associated with the bit mapping constraints that practically eliminates the need for an optimization. Finally, a study of the impact of the level of reliability imbalance across the sub-channels on the performance of the root-like bit mapping is presented. Simulation results show that the new bit mapping strategy improves performance, and that in the presence of channel variations, the system can, adaptively, apply a new bit mapping without the need of a complex optimization, which can be very useful in practical systems.