On the Architectural Split and Radio Intelligence Controller Placement in Integrated O-RAN-Enabled Non-Terrestrial Networks

The integration of Terrestrial Networks (TNs) with Non-Terrestrial Networks (NTNs) poses unique architectural and functional challenges due to heterogeneous propagation conditions, dynamic topologies and limited onboard processing capabilities. This paper presents a taxonomy of architectural and functional split strategies for integrated Open Radio Access Network (O-RAN)-enabled TN-NTN systems. We analyze key trade-offs in performance, latency, and autonomy by evaluating configurations that distribute RAN and core functions between satellites and ground nodes, from onboard Distributed Unit (DU) deployments to full gNB and User Plane Function (UPF) integration. The placement of Near-Real Time (RT) and Non-RT RAN Intelligent Controllers (RICs) is also explored, with flexible strategies proposed to optimize control loop performance and scalability. We provide a comprehensive mapping between architectural splits and RIC placement options, highlighting implementation constraints and interoperability. Finally, we outline key challenges and future directions to facilitate efficient TN-NTN convergence within the O-RAN framework.