# Current Trends on Green Wireless Sensor Networks
## 4 Wireless Power Transfer (WPT) and Simultaneous Wireless Information and Power Transfer (SWIPT)  
In the area of 5G networks, massive IoT and sensor networks the goal is to provide reliable communications under requirements of low complexity, cost and power. One promising piece of the solution to the latter requirement is energy harvesting from radio frequency signals. The original far-field Wireless Power Transfer (WPT) paradigm with dedicated frequencies to send recharging beams to devices is evolving towards harvesting the energy from radio frequency signals already there, either ambient signals or the ones used for communication as it happens with Simultaneous Wireless Information and Power Transfer (SWIPT). This evolution parallels the one from disjoint power and communication lines towards the integrated use of the former in Power Line Communications (PLC) as pointed out in [194]. In recent times several surveys have been published around WPT and SWIPT, some of them focused on some particular application. The survey by [PERSON] et al. [194] describes thoroughly WPT and SWIPT technologies. It covers Near-Field WPT used to recharge devices at short distances, Far-Field WPT used to transfer energy at longer distances, SWIPT architectures such as separate receivers for power and data communication, either time switching or power splitting in the shared receiver approach, and antenna switching. It also presents interference exploitation approaches in SWIPT. Finally, it comments upon several emerging scenarios amenable to a SWIPT approach such as WPT/SWIPT on cooperative relay, SWIPT-enabled cooperative non-orthogonal multiple access (NOMA) and secure WPT/SWIPT transmission. A brief survey by [PERSON] et al. [195] presents an overview of SWIPT, explores its application on several 5G scenarios and discuss the mmWave network case. The survey by [PERSON] et al. [196] focuses on the SWIPT technology as used mainly in cooperative relay networks. Allowing a base station to transmit the power needed by a device to altruistically relay data nullifies the cost in battery power that it would incur into otherwise. This approach guarantees the collaboration of all nodes in a more efficient quid pro quo network operation. A broader survey by [PERSON] et al. [197] centers on use, redistribution, trading and planning of energy harvested in future wireless networks interoperating with smart grids. SWIPT technology is the focus of its seventh Section. Its Section XI identifies SWIPT as a promising technology in Smart Grid-Powered Wireless Communications applied to 5G such as NOMA-based wireless powered sensor networks and Unmanned Aerial Vehicle (UAV) aided communication networks. In Subsection XII.E it identifies several lines of future research in SWIPT Systems. To complete the overview of SWIPT applications related to WSNs, several trending topics in recent research can be identified as shown in Figure 7.  
Several active lines of research deal with UAV assisted communication networks, some of them with higher potential for the WSNs case. [PERSON] et al. (2019) focus on WPT, computing the UAV trajectory that maximizes the minimum received energy among all ground nodes. This would be a feasible approach for recharging sensor nodes in case their ambient energy-harvesting capabilities would not suffice. [PERSON] et al. (2019) center on power allocation and trajectory design of UAVs in a SWIPT-enabled IoT network. [PERSON] et al. (2020) explore UAV deployment and clustering of nodes in case of disaster and unavailability of conventional networks. This design could be adapted to WSNs to make them more resilient. [PERSON] et al. (2021) propose a UAV-aided SWIPT-enabled NOMA scheme to guarantee secure transmission for ground passive receivers (PRs). Well-designed WSNs with single ownership should be less prone to eavesdropping and denial of service attacks though.