Two-Level LDPC-Coded Modulation With Probabilistic Shaping

In this letter, we propose a new coded modulation scheme, namely two-level coding with probabilistic shaping, for transmission with higher spectral efficiency in future 6G communications. At the transmitter, a distribution matcher for probabilistic shaping is concatenated with two-level coding, where the first level is designed based on bit-interleaved coded modulation while the second level is left uncoded. At the receiver, a two-stage decoding is performed prior to shaping decoding. A labeling strategy is optimized to get good performance for the first-level coding. Meanwhile, labels of signal subsets are sign-bit decomposable to accommodate uniform check bits. Numerical results show that, for 256-QAM, performances of the proposed scheme are $0.7\sim 1.4$ dB better than uniform BICM at various transmission rates, while the required LDPC code length is shorter by 50%.