## 3 Mobile RAN Entity Integrated into 5G and Beyond
### Integration of Low-Altitude UAVs into 5G and Beyond  
The integration of low-altitude UAVs and Uncrewed Aerial System (UAS) traffic management into 5G systems has become a prominent research focus in both academic and global standardization communities [48; 49; 50; 51]. As illustrated in Figure 4, UAS Phase 1 defines the architecture and protocols for the UAS application layer, encompassing UAS application enablers within the 3 GPP standardization framework [52; 53; 54; 55; 56]. UAS Phase 2 is currently under standardization, with a specific focus on meeting regulatory requirements for broadcast remote ID and detect and avoid (DAA) solutions [57; 58]. Concurrently, UAS Phase 3 is in the process of standardization to develop additional service requirements. As depicted in Figure 5, within the context of 3 GPP standardization, UAS Phase 1 comprehensively outlined the functional architecture, procedures, and information flows that govern the integration of the UAS Application Enabler (UAE) layer with 5G/LTE systems. This integration effectively harnessed the capabilities offered by the Service Enabler Architecture Layer (SEAL). Figure 4: Studies and works for Uncrewed Aerial Systems (UAS) in 3 GPP. The following functionalities were specified within the UAS Phase 1 framework, contributing to the enhancement of precision and versatility in UAV monitoring within the network:  
* UAV remote identification--Incorporated into 5G/LTE systems, the civil aviation administration (CAA)-Level UAV ID serves as a globally unique and easily readable identifier for UAVs. This identification plays a crucial role in ensuring the accurate routing of requests to the designated UAV service supplier (USS) for the retrieval of UAV-related information. During the initial registration of a UAV by its owner with the USS, the CAA-Level UAV ID is assigned to the UAV. It encompasses essential aviation-level data, including the UAV's serial number, pilot information, and UAS operator details. Subsequently, this information is transmitted to the USS for meticulous record keeping and management;
* UAV USS authentication and authorization (UUAA)--After successful 3 GPP authentication of the UE using the credentials of a mobile network operator (MNO), a dedicated UUAA procedure is established to verify the UAV's registration with the USS. This procedure relies on the CAA-Level UAV ID for USS authentication. In 5G systems, it can occur during 3 GPP registration or packet data unit (PDU) session setup for  
Figure 5: System architecture and UAS application enabler layer for UAS over 5G/LTE system in UAS Phase 1. UAS services and in LTE systems, it is during packet data network (PDN) connection establishment. The UUAA procedure comprises a service-level-AA container, a Service-level-AA procedure for authentication/authorization, and an API-based method. Security material specifics are considered outside the scope of 3 GPP as they are application layer concerns;
* Command and Control (C2) communication over 5G/LTE--To facilitate C2 communication over 5G/LTE systems, specifically involving a UAV user plane connection with the UAV controller, it is imperative to obtain authorization from the USS. This authorization encompasses several key aspects, including permission for the UAV and UAV controller pairing and, optionally, flight authorization for the UAV, a process overseen by the USS. To support these vital functions, the existing non-access stratum PDU session establishment and modification procedures have been expanded. These enhancements enable the inclusion of critical elements such as the CAA-level UAV ID and other application-layer authorization information within the service-level-AA container, ensuring a robust framework for secure and controlled cellular communication in UAV operations;
* UAV location reporting and tracking--The specification of UAV location reporting and tracking extensively relies on the reuse of existing location procedures [(59)].