# Machine-Learning-Enabled Intrusion Detection System for Cellular Connected UAV Networks
## 2 Background and Related Works  
### Satellite Architecture  
A Public Land Mobile Network (PLMN) can have both terrestrial 3 GPP access and satellite 3 GPP access. However, the coverage of the satellite access network may span over the coverage of the terrestrial access network, as shown in Figure 1[30]. A satellite access network is shared between multiple core networks in a 5G Multi-Operator Core Network (MOCN) sharing architecture. In this case, the shared satellite Radio Access Network (RAN) broadcasts the system information for both PLMNs, whose core networks are available. According to Figure 2, these PLMNs might have different Mobile Country Codes (MCCs) [30]. A satellite back haul is used between the core and terrestrial access network, providing a backup transport for the N2/N3 reference points as demonstrated in Figure 3. The N2/N3 reference points are generally used for connecting standalone non-3 GPP accesses (e.g., WLAN access) to the 5G core network via control plane and user plane functionality, respectively. The User Plane Function (UPF) is one of the Network Functions (NFs) of a 5G core (5 GC) network. The 5 GC network consists of more than one Access and Mobility  
Figure 1: Satellite and terrestrial 3 GPP access networks within a PLMN. (**a**) architecture, (**b**) coverage. Figure 2: Multi Operator Core Network sharing architecture with satellite radio access network: (**a**) architecture, (**b**) coverage. Management Functions (AMFs) and UPFs. The 5 GC is linked with the distributed gNB through standard N2 and N3 interfaces [31]. The satellite system transparently carries the communication payload of the 3 GPP reference points [30]. In the case of Non-Geostationary Satellite Orbits (NGSO) such as Low-Earth Orbiting (LEO), Medium-Earth Orbiting (MEO), and Highly Eccentric Orbiting (HEO), the attached cells and tracking areas move with the corresponding gNBs. The NGSO with beam steering is capable of seamless handover from one satellite to another to guarantee the connectivity service for moving gNBs when proceeding on non-geostationary satellites. In satellite access, the one-way propagation delay between a User Equipment (UE) and a satellite communication payload may range between 2 ms and 140 ms according to the satellite altitude and the relative location of the UE. In our case, the UAVs are considered UEs. It is possible that in a constellation of non-geostationary satellites including Inter Satellite Links (ISLs), the delay between a UE and functional elements of the Core Network will increase depending on the actual location of the communication endpoints. The delay also depends on the function and mode of operation of the configuration of the NGSO Access Network. The Non-access Stratum (NAS) is a functional layer that provides communication between the mobile user nodes and the core network nodes. The impacts of delays in the satellite access on the 5G system in the NAS are as follows:  
* As mentioned above, the propagation delay between the UE and access node can change significantly, i.e., between 2 ms to 140 ms.
* The need for the 5G core network implies tackling different access capabilities such as propagation delays, coverage, etc., which can satisfy a terrestrial network. * UEs can utilize the concept of multi-connectivity, which is the capability of supporting simultaneous UEs by multiple sessions that can take advantage of various 3 GPP access networks (terrestrial and satellite in the forward and backward direction), as shown in Figure 4.  
While 4G is the most common network in the world, it will not be able to manage the huge number of connections that will be on the network in the future, at which point 5G will come into existence. Unique Radio Frequencies (RF) are used by 5G networks to gain what 4G networks were unable to obtain. Each radio spectrum includes several bands from low frequencies to high frequencies that uniquely have particular features.