Fairness-Aware Non-Orthogonal Multi-User Access With Discrete Hierarchical Modulation for 5G Cellular Relay Networks

Non-orthogonal multiple access based on superposition coding (SC) for 5G cellular systems without relays is gaining increasing interest from both academia and industries. Since relay stations will be an integral part of future cellular networks, we propose evolved non-orthogonal multi-access schemes for both direct and relayed users. Our schedulers are built upon SC-relaying schemes with practical discrete hierarchical modulations (HMs), where the messages of two selected users are superposed into different HM layers, each layer being allocated an optimized amount of power and bearing a message flow to be decoded through either a direct or relayed link. As opposed to conventional schedulers that allocate orthogonal resources to each user in wireless relaying systems, our non-orthogonal schedulers allow a pair of selected users to simultaneously share their allocated resource unit. Moreover, unlike the SC-relaying schemes in the literature based on Gaussian codebooks, the proposed schemes are designed and analyzed under the practical constraints of discrete HMs. In spite of the complexity of the power optimization under discrete HMs, we provide a simple and near-optimal power allocation method for sum-rate maximization and proportional fairness. The simulation results show that the conventional orthogonal schedulers are outperformed by the proposed schedulers in terms of sum rate and fairness, even under the practical assumption of HMs.