# Non-Orthogonal Multiple Access for Air-to-Ground Communication

## TL;DR

This paper develops optimized UAV trajectory and association strategies for uplink NOMA cellular networks to minimize mission time, demonstrating significant improvements over traditional OMA schemes through graph theory and convex approximation techniques.

## Contribution

It introduces a novel joint optimization framework for UAV trajectory and association in uplink NOMA networks, revealing the optimal fly-hover-fly structure and proposing efficient solution algorithms.

## Key findings

- SCA-based design outperforms fly-hover-fly method.
- UAV mission time is significantly reduced with NOMA.
- Higher QoS demands increase UAV mission completion time.

## Abstract

This paper investigates ground-aerial uplink non-orthogonal multiple access (NOMA) cellular networks. A rotary-wing unmanned aerial vehicle (UAV) user and multiple ground users (GUEs) are served by ground base stations (GBSs) by utilizing the uplink NOMA protocol. The UAV is dispatched to upload specific information bits to each target GBSs. Specifically, our goal is to minimize the UAV mission completion time by jointly optimizing the UAV trajectory and UAV-GBS association order while taking into account the UAV's interference to non-associated GBSs. The formulated problem is a mixed integer non-convex problem and involves infinite variables. To tackle this problem, we efficiently check the feasibility of the formulated problem by utilizing graph theory and topology theory. Next, we prove that the optimal UAV trajectory needs to satisfy the \emph{fly-hover-fly} structure. With this insight, we first design an efficient solution with predefined hovering locations by leveraging graph theory techniques. Furthermore, we propose an iterative UAV trajectory design by applying successive convex approximation (SCA) technique, which is guaranteed to coverage to a locally optimal solution. We demonstrate that the two proposed designs exhibit polynomial time complexity. Finally, numerical results show that: 1) the SCA based design outperforms the fly-hover-fly based design; 2) the UAV mission completion time is significantly minimized with proposed NOMA schemes compared with the orthogonal multiple access (OMA) scheme; 3) the increase of GUEs' quality of service (QoS) requirements will increase the UAV mission completion time.