Protograph-Based LDPC-Coded Orbital Angular Momentum Systems with Index Modulation

This paper investigates the joint design of protograph-based low-density parity-check (PLDPC) codes and orbital angular momentum (OAM) systems with index modulation (IM) over line-of-sight (LoS) channels. To begin with, we analyze the probability density function (PDF) of extrinsic log-likelihood-ratios (LLRs) for coded bits output from channel detector, and propose a customized Box-Cox transformation (CBCT) to make it achieve symmetry and Gaussian characteristics. We also develop a CBCT protograph-based extrinsic information transfer (CBCT-PEXIT) algorithm to predict the convergence performance of PLDPC-coded OAM-IM systems. Furthermore, with the aid of such an algorithm, we construct two new types of improved PLDPC codes tailored for OAM-IM systems, including unpunctured codes and rate-compatible punctured codes. Both theoretical analyses and simulation results demonstrate that our proposed PLDPC-coded OAM-IM systems can significantly outperform the state-of-the-art counterparts.