A low-complexity decoding scheme of low-density parity-check codes for the layered-division multiplexing (LDM) systems in ATSC 3.0 is proposed. Based on the unique characteristic of the LDM and the special structure of a parity-check matrix (PCM) for the core layer, the proposed scheme applies the sum-product algorithm (SPA) to only a part of the PCM. For this reason, it is referred to as the partial SPA, compared with the conventional scheme, denoted by the full SPA. Clearly, it has a significantly lower complexity than the full SPA. Furthermore, we propose hybrid decoders employing both the full SPA and the partial SPA to achieve a better performance, while maintaining their complexity low. Numerical results show that the proposed schemes...