Time Minimization in Downlink Hybrid NOMA Wireless Powered Communication Networks

In this paper, we investigate the time minimization in the downlink (DL) wireless powered transmission network (WPCN) consisting of one base station (BS) and two energy harvesting (EH) users. The hybrid non orthogonal multiple access (H-NOMA) and hybrid EH scheme are adopted, in which each user can harvest energy from both the dedicated energy signal and the information signal for the other user. We aim to minimize the total time consumption under the data rate and energy consumption constraints. To solve the formulated non-convex problem, we propose a two-layer iterative algorithm. The problem in the inner layer is transformed to convex and the problem in the outer layer focused on the power allocation coefficient can be solved by binary search. Simulation results show that H-NOMA can achieve the best performance compared with time-division multiple access (TDMA) and NOMA, and NOMA also outperforms TDMA under the zero circuit power and little decoding power.

I. Introduction

With the development of the Fifth Generation (5G) wireless communications, billions of devices can be supported by year 2025 [1]. However, Internet-of-Things (IoT) networks, with a large number of devices, face the problem that the lifetime of networks is hard to sustain due to limited energy supply in IoT devices. It is worth noting that energy harvesting, which scavenges energy from ambient radio frequency (RF) signals [2], has been widely used in wireless powered communication networks (WPCNs). Furthermore, in order to utilize the harvested energy efficiently, “harvest then transmit” (HTT) protocol has been adopted in WPCNs [3], which divides the transmission into two stages, including wireless energy transmission (WET) firstly and then wireless information transmission (WIT).