  Current network access infrastructures are characterized by heterogeneity,
low latency, high throughput, and high computational capability, enabling
massive concurrent connections and various services. Unfortunately, this design
does not pay significant attention to mobile services in underserved areas. In
this context, the use of aerial radio access networks (ARANs) is a promising
strategy to complement existing terrestrial communication systems. Involving
airborne components such as unmanned aerial vehicles, drones, and satellites,
ARANs can quickly establish a flexible access infrastructure on demand. ARANs
are expected to support the development of seamless mobile communication
systems toward a comprehensive sixth-generation (6G) global access
infrastructure. This paper provides an overview of recent studies regarding
ARANs in the literature. First, we investigate related work to identify areas
for further exploration in terms of recent knowledge advancements and analyses.
Second, we define the scope and methodology of this study. Then, we describe
ARAN architecture and its fundamental features for the development of 6G
networks. In particular, we analyze the system model from several perspectives,
including transmission propagation, energy consumption, communication latency,
and network mobility. Furthermore, we introduce technologies that enable the
success of ARAN implementations in terms of energy replenishment, operational
management, and data delivery. Subsequently, we discuss application scenarios
envisioned for these technologies. Finally, we highlight ongoing research
efforts and trends toward 6G ARANs.
