Hands-On Solutions for Testing Integrated Terrestrial and Non-Terrestrial Networks: A Comprehensive Survey

The integration of Terrestrial and Non-Terrestrial Networks (T/NTNs) represents one of the most disruptive innovations on the basis of the upcoming 6th Generation (6G) of mobile communication systems. By incorporating Unmanned Aerial Vehicles, High Altitude Platform, and multi-orbit satellite constellations, it ushers in a new concept of network deployment that intends to enrich the coverage of typical terrestrial wireless systems in congested scenarios, provide ubiquitous connectivity to unconnected areas, and offer concrete support to next-generation use cases and applications. At the time of this writing, the standardization process of integrated T/NTNs is going to reach its peak of activity and many technological issues (spanning from advanced physical layer transmission techniques to flexible network management operations) are still under investigation. Indeed, considering that experimental tests can significantly sustain the study of both academia and industry, the knowledge of hardware and software tools able to emulate specific aspects of the overall network architecture would significantly boost the research towards concrete and solid technical solutions moving from theory to practice. Based on these premises, the contribution presented herein provides a comprehensive survey of hands-on solutions through which experimental testbeds dedicated to integrated T/NTNs can be deployed. Specifically, it encompasses hardware (such as Software-Defined Radios, channel emulators, Commercial Off-The-Shelf User Equipment, etc.), software (including those emulating core and radio access network), remotely accessible platforms, and other key enabling technologies of 6G. After reviewing some significant scientific works that adopted a subset of these solutions in their investigations, this survey sheds light on their adoption in more complex and heterogeneous network deployments. Finally, it summarizes the lessons learned and provides a list of challenging research topics related to the experimental tests of integrated T/NTNs.