Harvested Energy Evaluation of Free-Space Optics RIS-Assisted Ground-HAP-UAV System Over Composite Channels

Our future networks will contain three dimensions called 3D networks to support terrestrial and in-the-air devices. Unmanned aerial vehicles (UAVs) are vital network elements due to their flexibility of geographic installation and great line-of-sight (LOS) transmission performance. To extend UAV lifespans, an energy harvesting strategy, in which UAVs are charged wirelessly by optical signals, is introduced. Our system uses free space optics (FSO) to transfer optical signals carrying both data and energy from a ground station to a flying UAV. However, using FSO has advantages only if a LOS link is available. To mitigate this limitation, the proposed system includes a high-altitude platform (HAP) in between assisted by a reconfigurable intelligent surface (RIS) to relay all signals. The laser beam is transmitted to RIS at HAP and then reflected to the UAV. In this study, we develop a composite channel model that includes various channel impairments such as atmospheric attenuation due to Mie scattering, turbulence-induced beam scintillation and beam wander, and RIS vibration-induced fading. Based on the developed composite channel model, the harvested energy of the proposed system is evaluated by theoretical analysis and simulation.