*2.1. WPCN*

WPCN basically guarantees ENO because all of the devices in the cell use only the energy supplied by the HAP or power beacon. Thus, most studies on WPCNs have been focused on system optimization considering resource allocation [21], beamforming [22], cooperative communication [23], and full-duplex communication [24]. These topics have been investigated in a flat network structure where the wireless device directly transmits data to the HAP, while hierarchical networks like WSNs have not been considered.

A few studies have addressed hierarchical structures in WPCNs. For instance, cluster-based cooperation in a WPCN was proposed in [25], where an HAP with multiple antennas exploited energy beamforming to focus more transferred power to the CH considering the performance to be limited by the high energy consumption of the CH. Then, joint optimization of the energy beamforming design, transmit times of all of the nodes, and transmission power of the CH was performed to maximize the minimum data rate achievable among all of the nodes (i.e., max-min throughput). In [26], a WPSN consisting of one HAP, a near cluster, and the corresponding far cluster was considered and a cluster cooperation concept was proposed. If the sensors in the near cluster do not have their own information to transmit, acting as relays, they can help the sensors in the far cluster forward information to the HAP in an amplify-and-forward manner. In [27], a WPSN was divided into several layers and the exact border of each layer was obtained in order to alleviate the doubly near-far effect, and the energy broadcasted by the HAP and radius of each layer were optimized jointly. Furthermore, a multi-hop WPCN based on user-cooperative multi-hop transmission was considered in [28] to improve the throughput fairness, and the max-min throughput was optimized by resource allocation (i.e., transmission power and time). In a WPCN environment, these previous studies considered the WET from the HAP without using SWIPT technology.

## *2.2. SWIPT in WSN*

SWIPT has been applied to WSNs in various ways to overcome their energy limitations. Numerous surveys of SWIPT in WSNs have been published recently [29–31]. For instance, [29] summarized the current research on SWIPT-based cooperative sensor networks, in which SWIPT is applied to WSNs in terms of dual-hop and multi-hop relays. Meanwhile, [30] focused on the integral aspects of SWIPT in other prominent networks, such as device-to-device networks, vehicular ad hoc networks, wireless body area networks, WSNs, and so on, and presented open issues and challenges in SWIPT application. In [31], an overview of SWIPT/WPT-enabled WSNs was provided and future directions were suggested.

Meanwhile, [32] described a WSN consisting of multiple clusters and a sink node, in which the CH of each cluster performs SWIPT to give energy to relay nodes with low energies. In [33], a SWIPT-powered sensor network was considered in which each source node operates SWIPT as both an information transmitter and an energy transferrer, and the destination node works as an information receiver while the other nodes work as energy harvesters. Furthermore, [34] focused on the deployment of a WSN and its routing strategy when SWIPT is applied to the WSN. The basic idea is to reduce the total recharging cost to enhance the lifespan of the WSN. In addition, [35] proposed a dynamic routing algorithm for a renewable WSN with SWIPT. SWIPT has been adopted in many studies to overcome the energy limitations of WSNs, but SWIPT has not yet been applied in the WPCN

environment. Furthermore, these previous studies were focused on increasing the lifetimes of WSNs rather than guaranteeing ENO.
