Resource Allocation for Cellular-Connected UAV-Enabled Relay Networks Based on C-NOMA

In this paper, we investigate a novel cooperative nonorthogonal multiple access (C-NOMA) strategy for unmanned aerial vehicle (UAV)-enabled wireless communications, where UAV simultaneously works as a user and a time-division-duplex (TDD) relay in two hops. In the first hop, the cell-edge terrestrial users (TUs) transmit signals to UAV. In the second hop, UAV re-transmit TU's signals to base station (BS) together with its own signals based on C-NOMA. We propose a closed-form nearoptimal elevation angle (NOEA) based UAV's height optimization approach, where the elevation angle is utilized to derive a closedform solution to the UAV height optimization, based on the proof that the channel gain in the second hop is concave with respect to the elevation angle between UAV and BS. The proposed NOEA approach achieves near-optimal performance, while requiring no exhaustive search and no iteration. We propose a joint optimization in the TDD relay mode, referred to as JOT, to maximize TU's data rate, while guaranteeing UAV's target data rate. The proposed JOT algorithm allows a low-complexity closed-form optimal power allocation between TU and UAV in two hops, and provide TU's achievable data rate and spectral efficiency higher than the state-of-the-art methods.