Analysis of Power-Domain NOMA for Uplink UAV-HAP Communications in Sub-6 GHz Band

This paper presents a theoretical model to estimate the coverage probability and a semi-analytical model to compute the throughput of a power-domain non-orthogonal multiple access (PD-NOMA) scheme applied to an aerial network operating in the sub-6 GHz band. The presented framework investigates an uplink scenario in which multiple unmanned aerial vehicles (UAVs) contend for channel access on a PD-NOMA basis to deliver information towards a common destination, represented by a high-altitude platform (HAP). The analysis is developed in a realistic propagation environment by accounting for path-loss attenuation, small-scale fading, airframe shadowing, random waypoint mobility, multi-lobe antenna patterns, polarization losses, and pointing errors. Closed-form formulas are derived for the cumulative distribution function (cdf) of the power received by the HAP from the single UAV, and simple integral expressions are obtained for the corresponding coverage probability by exploiting the properties of order statistics. The theoretically evaluated cdfs are then integrated into a numerical framework to explore the impact of the channel parameters, of the modulation order, and of the residual interference on the throughput of a PD-NOMA UAV-HAP network. The accuracy of the model and the impact of the introduced approximations are assessed through extensive Monte Carlo simulations.