Improved Modulation Schemes for Lattice-Partition-Based Downlink Non-Orthogonal Multiple Access Systems

This letter proposes alternative modulation schemes for a two-user lattice-partition-based non-orthogonal multiple access (NOMA) system, where the NOMA transmission signal is composed of four elements from four amplitude-level groups, respectively, instead of two elements from two amplitude-level groups in the conventional method. Each user's message is split into most significant bits (MSBs) and least significant bits (LSBs), with the MSBs modulated at one of the higher-level groups and the LSBs modulated at one of the lower-level groups to meet the NOMA principle. Both analysis and simulation results show that the proposed schemes offer higher system capacity than the conventional method.