*2.3. ENO Based on WPT*

ENO based on WPT has been investigated in some recent work. For example, the feasibility of an energy-neutral Internet of Things (IoT) network was studied in [36] by joint utilization of EH and WPT technologies, and design guidelines for energy-neutral wireless powered networks were presented. In [37], duty cycle operation for a self-powering dual mode SWIPT system in which a sensor node adaptively controls single-tone or multi-tone communication mode was introduced and an adaptive mode switching problem was solved to maximize the achievable rate under ENO to realize a battery-free IoT network. Meanwhile, an energy management scheme for a WPSN was proposed in [38], which maximizes the RF energy transfer efficiency while guaranteeing ENO, and the proposed energy management scheme was implemented in an actual testbed to verify its operation. In [39], an adaptive duty cycle control algorithm was proposed to prevent the energy storage of a sensor node from being depleted and its ENO was validated in a multi-antenna WPSN testbed that dynamically steered a microwave beam towards a sensor node. In addition, the energy neutral Internet of drones was conceptualized in [40] to overcome the energy limitations for continuous operation of drones and to enhance the connectivity between drones. Communication and networking architectures and protocols for drones energized by WPT were presented. Although ENO was considered in SWIPT-enabled networks or WPCNs in these works, to the best of our knowledge, ENO considering both SWIPT and WPCNs has not been investigated yet.
