Role and Evolution of Non-Terrestrial Networks towards 6G systems

Today, the worldwide economy and society are already taking advantage of the first deployments of 5G networks. However, the challenges that will be posed by 2030 networks on the infrastructure, technologies, and performance requirements call for a further leap in mobile communications. To this aim, evolutions of the 5G concepts in 5G-Advanced, and rethinking of future Radio Access Technologies, i.e., 6G which is expected to be the next revolution in the mobile ecosystem, are already being addressed by the scientific and industrial communities. In this framework, as demonstrated by 3GPP Rel. 17 specifications, Non-Terrestrial Networks (NTN) are expected to play a vital role in near and long term mobile communications, based on the joint optimisation of the terrestrial and non-terrestrial components in a unified 3D architecture. In this paper, we extensively discuss the 3GPP normative activities for 5G (Rel. 17), 5G-Advanced (Rel. 18 onward), and 6G (Rel. 20+) in terms of enabling features, architectures and interfaces, and protocols. Then, we explore the evolution of the 5G services towards 5G-Advanced and 6G NTN applications. Finally, we discuss the evolution to 6G network infrastructures with respect to the main performance requirements.