Abstract
Systematic Luby transform (LT) codes with grey-mapped QAM constellations over additive white Gaussian noise (AWGN) channels are investigated. For high-order modulations, it can be learnt that the traditional Gaussian approximation method developed for binary PSK (BPSK) is not accurate any more. To improve BER performance of systematic LT codes with grey-mapped QAM, therefore, presents a novel coded modulation scheme, which only extracts Gaussian alike log-likelihood ratios for decoding, based on the modified QAM constellation. More importantly, the proposed scheme allows to optimise degree distributions for QAM-modulated systematic LT codes as easily as for BPSK. Simulation results show that the degree distribution related to the proposed coded modulation scheme can provide outstanding BER performance for systematic LT codes over the AWGN channel.
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