Raptor Codes With Descending Order Degrees for AWGN Channels

We propose a novel construction of rateless codes for the additive white Gaussian noise (AWGN) channel with high order quadrature amplitude modulation (QAM). The proposed construction is structurally similar to a Raptor code consisting of an inner Luby transform (LT)-style code and an outer low density parity check (LDPC) code. The main novelty is that the output degree distribution for the LT part is time-varying and the degree of the parity bits decreases with time, parity bits with the same degree are clustered together rather than random, which is totally different from traditional fixed degree distributions. Simulation results with a joint iterative decoding algorithm demonstrate that both the goodput and the bit error rate (BER) performance of our proposed scheme are better than those of newly reported Raptor codes for the AWGN channel in most cases.