O-RAN based Non-Terrestrial Networks: Trends and Challenges.

While 5G networks are already being deployed for commercial applications, Academia and industry are focusing their effort on the development and standardization of the next generations of mobile networks, i.e., 5G-Advance and 6G. Beyond 5G networks will revolutionize communications systems providing seamless connectivity, both in time and in space, to a unique ecosystem consisting of the convergence of the digital, physical, and human domains. In this scenario, Non-Terrestrial Networks (NTN) will play a crucial role by providing ubiquitous, secure, and resilient infrastructure fully integrated into the overall system. The NTN nodes will be organized into a MultiLayer Multi-dimensional (ML-MD) architecture. This ML-MD network will rely on the interoperability of very different network elements, enabled by the disaggregation and virtualization of network components, their interconnection by open standard interfaces and orchestrated by data-driven network Artificial Intelligence. This paradigm, which has been standardized by the O-RAN Alliance, is now being implemented in Terrestrial Networks (TNs) but has not been fully addressed in NTN, yet. Therefore, this paper aims at exploring the possible implementation of an NTN infrastructure based on the O-RAN approach. By starting with the review of the State of the Art of O-RAN in TNs and flying platforms, we identify a possible architecture solution for an O-RAN-based NTN system and we foresee the ORAN implementation trends that will increase the NTN system efficiency.