Protograph-Based Raptor-Like LDPC Codes With an Add-On Structure for Reliable Communications

The 5G multicast and broadcast service (MBS) has been discussed since 3GPP Release 17, emphasizing resource-efficient transmission for multiple users. A primary focus of 5G MBS is enhancing reliability, even for the broadcast mode without retransmissions. In discussing 6G, the hyper reliable communication is also an important use case. In this context, the design of channel codes with low error floors is crucial to ensure robust communication for such demanding scenarios. Protograph-based raptor-like (PBRL) low-density parity-check (LDPC) codes have good error-correcting performance and rate-compatibility but the construction has focused on waterfall rather than error floor. In this paper, we propose an add-on structure for PBRL LDPC codes to have low error floors, which consists of edges added on the protographs of original PBRL LDPC codes. The added edges play a role of boosting up the reliability of weak variable nodes in the original PBRL LDPC codes. We propose two construction algorithms, one for use at a fixed rate and the other for rate-compatible use. It is shown via simulations that the proposed codes have lower error floors than the original PBRL LDPC codes for various rates. Since the edge addition does not change the existing edge connections in the protograph, an adaptive use with/without the add-on structure has an effect of implementing two PBRL LDPC codes for high-speed and reliable communications in an efficient way while keeping the system backward-compatible with the original PBRL LDPC code.