A Review of 6G Enabler: Vertical Heterogeneous Network (v-HetNet)

Unmanned aerial vehicles (UAVs), commonly referred to as drones, are the added element for the beyond 5G quality of service (QoS), capacity, and reliable connection enhancement. UAVs are an effective tool in bridging gaps between terrestrial base stations by increasing band capacity due to the free mobility features and their reliable line-of-sight communication. The widespread usage of UAV have potential but also new challenges such as the constantly changing network topology, and vertical height variations that caused changes in their geometry. UAVs' energy efficiency, 3D deployment, and security have shown to be bottlenecks in various applications together with the satellite, aerial and terrestrial layers that constitute the vertical Heterogeneous Network (v-HetNet). Scholars have studied the different aspects of UAV's aerial network as an extension of the terrestrial network, but few have developed the management of the various layers together. In this paper, we review works on the v-HetNet as an important enabler of 6G mobile networks. This study reviews works surrounding the concept, future opportunities as well as challenges in the coexistence of terrestrial and aerial layers. This survey aims to provide guidelines and motivations for additional studies for upcoming 6G communications system development and research in v-HetNet.