專為無線通訊應用之低密度奇偶校驗編碼正交分頻多工調變系統設計

Abstract

Trellis coded modulation (TCM) invented by G. Ungerboeck (1976) combined coding and modulation using constellation expansion and mapping by set partitioning to achieve substantial coding gains. Low-Density Parity-Check (LDPC) code invented by Gallager (1962) and rediscovered by Mackay and Neal recently (1996) are shown to have near-Shannon-limit performance. Combining with orthogonal frequency division multiplexing (OFDM) modulation technique using efficient coded modulation scheme, performance of LDPC coded OFDM modulation can be better than traditional concatenated Reed-Solomon with convoluational coded OFDM and commonly used LDPC coded OFDM bit-interleaved coded modulation (BICM) schemes. In this thesis, we investigate efficient LDPC coded OFDM modulation schemes for fixed broadband wireless access (FBWA) communication system. Based on the parameters of IEEE 802.16a-2003 standard OFDM-256 PHY layer specification, we combine LDPC codes with several multilevel quadrature amplitude (QAM) modulations. A set of efficient LDPC coded modulation schemes with different constellation sizes is investigated in this thesis. The bit error rate (BER) performance of these schemes over additive white Gaussian noise (AWGN) and multipath fading channels is simulated and discussed. We investigate several LDPC coded OFDM modulation schemes, including MultiLevel Coding (MLC), Bit-Interleaved Coded Modulation (BICM), LDPC Coded Modulation (LCM), and Reed-Solomon with LCM (RS-LCM) schemes. Among them, we choose LCM and RS-LCM schemes to evaluate their performance by simulations, and BICM scheme for comparison. RS-LCM scheme has coding gains by 0.3~0.8 dB at a BER (Bit Error Rate) of 10-5 over conventional LDPC coded modulation BICM scheme, while does not suffer from error floors like that in LCM scheme at low bit error rates when low rate component codes are applied. RS-LCM scheme with little extra complexity by the use of Reed-Solomon code can make the transmission of LDPC coded OFDM system more efficient and less vulnerable to multipath fading effect.