## 3 SATis5 Testbed Architecture  
This paper proposes an network architecture to provide high bandwidth connectivity through base stations backhauled by geostationary satellites. The architecture for this testbed network aims to consider the latest 3 GPP standardization and extend it with satellite inter-working functionality. The testbed system includes an edge node and a central core network node using a satellite backhaul connection among other backhaul opportunities, as shown in Figure 4. The User Equipment (UE) connects to the edge node, which then connects to the central core network node through the satellite backhaul. The backhaul is seen as a transport layer for the messages between the edge node and the central core network site. Because of this, the backhaul is as transparent as possible, while at the same time assuring a guaranteed communication quality. The core network consists of the specific functions defined by 3 GPP for the 5G core network. In the architecture for the SATis5 testbed, the Fraunhofer FOKUS Open5 GCore toolkit [25] has been deployed as the implementation of the 3 GPP 5G core network. It provides a prototype of the 3 GPP Release 15 for the core network functionality and its integration with 5G New Radio. Note that that there are two major modes being developed by 3 GPP to deploy radio base stations in 5G: the Non-Standalone (NSA) mode and the Standalone (SA) mode. In NSA mode, the 5G radio base station is co-located with an Evolved Node B (eNB) and connects to the Evolved Packet Core (EPC). In this model, the next generation Node B (gNB) acts as a secondary cell to provide capacity and throughput. This does not require a new core network, the Next Generation Core (NGC), and is an appealing option to network operators. To deploy 5G in SA mode requires the NGC instead. The SA architecture has been considered as a future Phase 2 item (Release 16 and Release 17 of the 3 GPP 5G standardization process). In this testbed demonstration, we consider an NSA-like architecture, as illustrated in Figure 5 with the only difference that we use COTS eNBs for the RAN instead of gNB implementation because the latter are not yet commercially available. The background for this architecture was prepared with the LTE introduction of the concept of dual connectivity that allows the UE to connect simultaneously to two different base stations. These basestations do not need to be time-synchronized and therefore are not required to be collocated at one site. Figure 3: Satellite use cases [23]. * Option 3: Traffic is split across 4G and 5G at eNodeB. * Option 3a: Traffic is split across 4G and 5G at EPC (S-GW). * Option 3x: Traffic is split across 4G and 5G at 5G cell. In this testbed demonstration study, we considered an NSA Option-3-like architecture and the traffic is split across 4G and 5G at eNodeB. The main core network functions deployed in our testbed demonstration are listed below:  
* The Home Subscriber Server (HSS) is the master user database. * The Serving Gateway (SGW) is the interconnection point between the radio side of the network and the EPC. It forwards the uplink data traffic to the Packet Data Network Gateway (PGW). * The PGW provides connectivity to external packet networks. * The Mobility Management Entity (MME) is responsible for all the control plane functions related to session management and subscriber services. Furthermore, an additional network function has been added to the edge node that is able to determine which backhauls are available, and forwards the specific traffic according to a set of routing policies over the satellite backhaul or an available local MNO network. In addition, we use 4G eNB for the Radio Access Network (RAN) part instead of 5G gNB implementations for all of our base stations because the latter were not yet commercially available at the time of the over-the-air experiments. The final deployment (Figure 6) of the SATis5 testbed will include more edge node functionalities to deploy further core network elements at the edge of the network.