Shortening for LDPC-Coded Multi-User Systems

This letter explores shortening technology for achieving near Gaussian multiple access channel (GMAC) capacity in low-density parity-check coded multi-user systems. To optimize the shortening patterns, a hybrid extrinsic information transfer (H-EXIT) tool, which integrates EXIT and protograph-based EXIT, is firstly developed. Based on this analysis, H-EXIT priority (HEP) algorithm is proposed to facilitate the optimization of shortening patterns. It can be observed that columns with larger degrees are prior to be selected, which differs from those for point-to-point scenarios. Inspired by this finding, we further propose largest-column-degree priority (LCDP) algorithm, which narrows the selection space to lower the complexity while maintains a comparable performance. Extensive simulation results demonstrate the superiority of proposed shortening schemes from two aspects: 1) Proposed shortening can bring nonnegligible gain over unshortening with consistent sum rate; 2) HEP and LCDP algorithms outperform benchmark algorithms.