Leveraging RIS and QoS-Aware Transmission Rate for Energy-Efficient FSO Non-Terrestrial Networks

Future 6G networks envision non-terrestrial networks (NTNs) featuring various airborne devices, such as unmanned aerial vehicles (UAVs), to enable flexibility, rapid deployment, and ubiquitous coverage. Free-space optics (FSO), a promising communication technology, aims to connect network devices across the Earth, sky, and space. This paper focuses on the innovative concept of simultaneously transmitting data and energy to an operational UAV from a core network, with the assistance of reconfigurable intelligent surfaces (RIS) to navigate through cloudy regions. The FSO link employs the simultaneous lightwave information and power transfer (SLIPT) technique, incorporating a power-splitting strategy that allocates received power for data decoding and wireless energy charging. To enhance data transmission, we introduce an adaptive rate design with QoS awareness. The analyses in this study assess four key performance metrics: (1) data transmission, quantified by the transmission rate, (2) energy transmission, evaluated based on harvested power, (3) UAV lifespan, represented by the remaining operational time for different SLIPT-powered UAV types, and (4) energy efficiency, defined as the ratio of data transmitted to total energy expended and harvested. The proposed system, which integrates SLIPT, RIS, and adaptive rate design, demonstrates significant improvements in the lifespan and energy efficiency of the UAV, validating its effectiveness under challenging operational conditions. Finally, the theoretical results are verified with the Monte Carlo simulation.