Abstract
As the need for limitless connectivity surges, non-terrestrial networks (NTN) will play a central role in fifth generation (5G) and beyond communications. The 3rd Gener-ation Partnership Project (3GPP) defines NTN as networks, or segments of networks, using an airborne or space-borne vehicle as a relay node or base station. An NTN-enhanced cellular network supplements a conventional terrestrial cellular network. This article provides an overview of NTN-enhanced cellular networks with a particular focus on satellite-mobile direct communications. First, we review satellite system classifications such as Geostation-ary Orbit (GEO), Medium Earth Orbit (MEO), and Low Earth Orbit (LEO), spectrum usage, and key challenges of satellite communications. We then summarize recent 3GPP activities in NTN. In addition, we describe our recent proof-of-concept system involving a satellite channel emulator and modification of the 5G New Radio (NR) protocol stack to handle the challenge of long round-trip time - demonstrating the feasibility of NTN and the adoption of NTN-enhanced cellular networks in 5G and beyond communications. Finally, we highlight the main open issues and future research challenges of NTN-enhanced cellular networks.
