# Current Trends on Green Wireless Sensor Networks
## 5 Discussion and Future Directions  
A wide range of applications as diverse as the one that underlies the concept of sensor networks, with such disparate geographical scenarios ranging from deployments with minimum distances between sensors and BS (e.g., WBAN) to those that rely on satellite transponders to receive data, with very demanding quality of service requirements in some cases (industrial environments, alarm, surveillance, e-health...), creates a scenario where multiple alternative solutions/mechanisms will coexist, not necessarily complementary/compatible, perhaps even designed ad hoc for a particular application. In addition, the possibility of energy-neutral operation (ENO) of network elements only further complicates the range of techniques that will have to be properly combined to achieve proper system operation. The fact that we have focused on natural energy sources (sun, wind, temperature gradient...) for recharging the batteries of the sensors, and the inherent unpredictability of these sources, means that the dynamic management of the remaining energy of the device, and of that which can be collected in the near future, is an important factor in the tasks to be carried out by the onboard processor. The incorporation into this research field of techniques for predicting energy that can be harvested is one of the hot points, and we can find from proposals based on complex automatic learning techniques to simpler solutions that try to avoid the processor's own energy consumption in this prediction. The combination of different harvesting mechanisms in the same sensor and the related problems is also an interesting topic for further research. In this sense, the possibility of exploiting the radio frequency signals that surround us ubiquitously as a source of energy is one of the most promising issues and where there is currently a lot of potential for improvement. When energy is harvested from radio signals, it can be done not only from ambient ones already there (WiFi, LTE) but from signals sent explicitly by dedicated equipment or some node of the network for either only power transfer (WPT) or for dual use, i.e., both information and power transfer (SWIPT). With respect to special equipment, research on unmanned aerial vehicles mainly assisting in energy needs of the network has been even focused on disaster and unavailability situation. This research may bring about more resilient WSNs. Special case of dedicated equipment is that of intelligent reflecting surfaces (IRS) assisting an access point in conquering higher power efficiency of its sensor set, especially with some geographical clustering of its nodes. As a mirror image scenario there is a proposal to power up the cluster head in a WSN with the surplus energy from its dependent nodes. Several lines of research focus on NOMA networks, such as power and time splitting comparison or minimization of total transmit power in the network while abiding to service requirements. Finally, backscatter technology can be a disrupting technology to WSNs. In mainly urban areas with high level of LTE signals they would even permit batteryless operation of sensor nodes. Other approaches consider a hybrid network with backscatter nodes and active transmitting ones collaborating towards a more efficient WSN. Finally, one of the most promising technologies in this area is that of the combination of backscattering with either an ambient EH or properly SWIPT framework, achieving dual radio signal reusing both in the power and the information domains. Only the first steps have been done towards its use in WSNs and future research may deal with different aspects such as multihop and hybrid nodes design implications. The rise of machine learning techniques can also have a beneficial impact on achieving an energy-autonomous application.