# A Flexible Infrastructure-Sharing 5G Network Architecture Based on Network Slicing and Roaming
## 3 Proposed Architecture
### Conceptual Architecture  
The proposed neutral host architecture aims to promote efficient sharing of 5G network resources and infrastructures among different clients. For this purpose, it provides service without the need for third-party network elements in the connection and routing of traffic while also allowing for any type of CSP, whether MNOs or not. To distinguish between different types of stakeholders throughout the paper, CSPs who are MNOs have been termed \"operator clients\", while those who are not MNOs are referred to as \"non-operator clients\". The NSP, which provides the shared infrastructure, is named \"neutral host\". Lastly, these clients may support various UEs corresponding to their service subscribers, who are referred to as \"users\". The proposed architecture, exemplified in Figure 2, leverages network slicing and RAN sharing with the LBO roaming architecture described in Section 2.3. This figure illustrates three distinct network slices, each represented by a different color and dedicated to a specific client. As represented, while all UEs access the network through the shared RAN and the access and mobility management function (AMF), indicated by solid lines, each client has its dedicated slice, responsible for managing the session, along with the user plane function (UPF), session management function (SMF), and policy control function (PCF). In this example, two operator clients (Slice 2 and Slice 3), with their respective PLMN IDs 2 and 3, are requesting service via roaming agreements, while the non-operator client (Slice 1), with PLMN ID 1, does not require roaming agreements and is authenticated directly in the neutral host's core. Figure 3 represents the procedures undertaken to enable a new client on the neutral host network. The steps are slightly different depending on whether the client is an operator or not. For an operator client, we use roaming agreements to provide user authentication and set up the neutral host SEPP; whereas for a non-operator client, it is necessary to obtain a user database and set UDM and AUSF to perform the user's authentication. Given those  
Figure 2: Conceptual neutral host architecture of the proposed solution. different users and authentication methods, the user's initial attach registration procedure also differs, i.e., from a roaming authentication to a direct NH authentication, if it is from an operator or non-operator client, respectively, as illustrated in Figure 4.  
Typically, the predominant models proposed for neutral hosts focus either on passive sharing or RAN sharing with the implementation of MORAN or MOCN functionalities. These approaches have been the most common among MNOs for cost savings to date. However, the advent of 5G and emerging technologies, such as NFV, SDN, and RAN centralization, broaden the operators' perspectives with respect to new types of infrastructure sharing, enabling MNOs to maintain network control even when operating on third-party infrastructures. Therefore, RAN sharing is a component of the proposed architecture, as it is necessary to share the same RAN among different clients and, consequently, different PLMN IDs. However, unlike the MORAN and MOCN models, this architecture does not route traffic to multiple cores, containing only the core of the neutral host and catering not just to operators but also to non-operator clients. Figure 3: Flowchart of a new client configuration in the neutral host. The proposed architecture aims for core sharing and leverages 5G-associated network slicing to provide an isolated and secure service for each client. This proposal involves implementing a core with all NFs but granting the client a network slice composed of three NFs, namely, SMF, PCF, and UPF, which directly impact the user experience and are crucial in controlling QoS and configuring user sessions. Thus, this architecture offers flexibility and dynamism to the network, allowing clients to configure QoS policies and maintain visibility over their network slice.