Two-Tier NOMA-Based Wireless Powered Communication Networks

Abstract

In this article, we investigate two-tier nonorthogonal multiple access (NOMA)-based wireless powered communication networks. The considered network consists of a hybrid access point and two groups of users, namely near and far users. A near user and a far user are selected to form a NOMA pair, which harvest energy during the downlink phase and transmit information during the uplink phase. Specifically, we consider two cases of energy-transfer time and analyze the corresponding performance. We first analyze the performance of the case with fixed energy-transfer time and derive the exact expression for the ergodic rate. Due to the doubly near-far effect, the far user experiences poor performance. To address this issue, an adaptive energy-transfer time scheme is designed by maximizing the far user’s data rate, and the optimal solution is derived. To reduce the system complexity, a suboptimal solution is proposed, and the ergodic rate's exact expression is derived. Simulation results reveal that the suboptimal solution achieves near-optimal performance and has a lower computational complexity. Moreover, a one-tier NOMA network is set to be a benchmark, and numerical results show that the two-tier system outperforms the one-tier system significantly under the appropriate radius setting.