# Current Trends on Green Wireless Sensor Networks
## 4 Wireless Power Transfer (WPT) and Simultaneous Wireless Information and Power Transfer (SWIPT)
[PERSON] and [PERSON] (2020) explore the use of Intelligent Reflecting Surface (IRS) assisted SWIPT scenario with a multi-antenna Access Point (AP) and both multiple single-antenna either information or energy users. This approach would be able to save power capabilities in WSNs, especially those with some level of geographical clustering. NOMA where several devices simultaneously transmit using the same frequency at different power levels such that receivers can reconstruct the original signals through interference cancellation brings about a more efficient use of wireless communication resources. Its use in WSNs has also been proposed. [PERSON] et al. (2020) evaluate a SWIPT-enabled WSN deploying NOMA and compare the performance of both time switching (TS) and power splitting (PS) approaches in terms of outage probability. Several optimization studies deal with the general case of SWIPT-enabled NOMA systems. We comment upon several of them without being exhaustive. [PERSON] et al. (2020) propose a design aimed to maximize system wide energy efficiency of a full-duplex cell-center relaying a cell-edge user's data. [PERSON] et al. (2020) propose several SWIPT schemes for cooperative communication systems with multiple relays and multiple destination users communicating simultaneously. [PERSON] et al. (2020) propose a deep learning approach in order to determine an approximation to the optimal solution for the minimization of the total transmit power of a SWIPT-enabled NOMA system while satisfying the quality of service requirements. [PERSON] and [PERSON] (2020) propose sensor nodes with high harvesting energies recharging the cluster head through SWIPT to maximize the achievable rate of sensing data transmission while guaranteeing ENO. Figure 7: Trending topics in research about SWIPT/WPT in WSNs. [PERSON] et al. (2008) analyze the influence of interference on SWIPT, propose an interference-aware route metric and design an interference-aware SWIPT routing algorithm. Backscatter technology aims to use an ambient signal (like WiFi (209) or LTE (210)) for communication from a tag-device through passive radio using a low-power reflective RF switch and a passive antenna towards a receiver. [PERSON] et al. (2011) propose using backscatter to enhance a Wireless Powered Communication Network collaboration between two devices that first harvest wireless energy from a Hybrid Access Point (HAP) and afterwards transmit their information to the HAP. Hence, the weaker device backscatters the received energy signal to transmit its information to the relay device in a passive manner. [PERSON] and [PERSON] (2012) explore the outage probability of a relay harvesting energy through power splitting from both a primary source and a secondary backscattering one at the same time that forwards both their data. They study the performance of the hybrid system through analysis and simulation, and find a larger throughput gain than in a single primary link scenario at the cost of negligible degradation of the primary throughput.