The Analysis And Optimization Of LDPC-coded BICM System

Low-Density Parity-Check (LDPC) coded Bit-Interleaved Coded-Modulation (BICM) system has been widely used in modern communication systems, due to its smart structure, outstanding decoding performance, high bandwidth efficiency and low implementation complexity. And it is also considered as the key technology in the next generation wireless transmission. The current researches of LDPC-coded BICM system, which include the error control coding, mapping constellation design, interleaving scheme and iterative decoding algorithm, have been studied rigorously. However, there are still many problems need to be resolved, especially for the high-ary modulated BICM systems. This dissertation focus on the topics of improvement on decoding algorithm, the mapping constellation design, the optimization of evaluation tool, and the interleaving scheme design, to improve the performance of LDPC-coded BICM systems. And the main contributions are as follows:1. The efficient decoding algorithm which has low computational complexity is studied in this dissertation firstly. Based on the conventional suboptimal parallel model of BICM system, the defects of corresponding decoding algorithm are investigated, and the joint demodulation decoding algorithm is proposed for the high-ary modulated BICM system. The proposed decoding algorithm is also proved to be the equivalent form of maximum a posteriori probability (MAP) decoding algorithm when some rational assumption is satisfied. The simulation results confirm that, the joint demodulation decoding algorithm can achieve better error performance which approaches the Shannon limit more than the traditional decoding algorithms could do.2. The appropriate mapping constellation scheme is crucial to the iterative decoding systems. As the joint demodulation decoding algorithm is applied in the BICM system, the mapping diversity gain of coded-modulation is utilized, the optimal constellation design criterion suitable for the suboptimal parallel model is no longer effective. Based on Caire's theory of BICM system, the channel capacity and the bit error rate (BER) asymptotical limit of the joint demodulation BICM system is deduced in this dissertation, and the optimal mapping constellation criterion for the joint demodulation BICM system is also derived. The simulation results show that, the mapping constellation scheme selected by the new criterion outperforms the mapping schemes selected by the traditional design criterion.3. The efficient evaluation tool for high-ary modulated BICM system is another focus in this dissertation. It is well known that extrinsic information transfer (EXIT) chart is widely used for the LDPC codes design in BPSK modulated transmitting system, due to its accurate evaluation and low complexity. However, as the transmitted signals in high-ary modulated BICM system are modulated symbols, whose log-likelihood rates (LLR) can not satisfy the Gaussian-like distribution, the traditional EXIT evaluation result no longer reflects the real performance of high-ary modulated BICM system. What is more, it is beyond the capability of the traditional EXIT chart to provide efficient analysis for the joint demodulation and decoding BICM system. Based on these defects, a symbol-level EXIT chart, which can give efficient convergence analysis and optimizations to the high-ary modulated BICM systems, is proposed in this dissertation. The simulation results show that, the symbol-level EXIT chart can achieve accurate convergence analysis to the high-ary modulated BICM systems. And this method is also suitable for the joint demodulation and decoding BICM system. With the symbol-level EXIT chart, the optimal mapping constellation scheme and LDPC code can be designed without the time-consuming BER simulations for the entire iterative decoding system.4. In this dissertation we also emphasize on the research of factors which can affect the BICM performance. Based on the adaptation between the code structure and mapping schemes, the influence caused by multi-edge modulated symbols to the BICM system is studied. We firstly analyze the impacts on the reliability of signal constellation diversity and the efficiency of parity-check constraints, and then with the mutual information transfer theory, we prove our analysis. According to this new factor, it obtains some new directions which can improve the BER performance of LDPC-coded BICM system.5. In order to improve the BER performance of high-ary modulated BICM system, this dissertation proposes a multi-edge elimination-based (ME-based) interleaving scheme to diminish the impact caused by multi-edge modulated symbols in BICM system. Compared to the directly LDPC coding design, the ME-based interleaving scheme has very low complexity. The simulation results show that, the ME-based interleaving scheme can improve the LDPC-coded BICM system when different decoding algorithms, modulation schemes and transmitted channels are applied.