Joint power allocation and splitting ratio for SWIPT-enabled UAV assisted time-shared cooperative NOMA network

Unmanned aerial vehicles (UAVs) can support higher data rates and enhance network coverage by leveraging air-to-ground (A2G) line-of-sight (LOS) channels, which makes them ideal to act as relays in wireless communications networks. However, the limited battery constraint and severe interference are the fundamental challenges that may degrade the energy efficiency (EE) of UAV-aided communications. Therefore, this work considers UAV-assisted cooperative communications networks with multiple users in a shared spectrum. Taking into account the aforementioned challenges, we propose radio frequency (RF) energy harvesting at the UAV and adopt non-orthogonal multiple access (NOMA) scheme with simultaneous wireless information and power transfer (SWIPT) protocol to improve the performance of the considered network. In particular, NOMA allows spectral reuse and separates the users in the power domain while mitigating interference through successive interference cancellation (SIC). With the aim to maximize the total EE of the network, we formulate joint power allocation and power splitting ratio control while considering user pairing and quality of service (QoS) constraints in time-shared full duplex mode. The resulting optimization is a linear fractional programming problem that is non-convex. We relax the problem and propose a low-complexity algorithm based on subgradient to obtain the near-optimal solution. The obtained results show that the proposed algorithm may improve the EE of the system by more than 300% for several cases of interest.