# A Flexible Infrastructure-Sharing 5G Network Architecture Based on Network Slicing and Roaming
## 2 Background and Related Work
### Neutral Hosts  
Neutral host models enable network sharing among multiple tenants under the management of MNOs or third-party business organizations, ensuring connectivity and coverage for devices across public and/or private mobile network operators.  
In the context of 5G, neutral host networks provide a wide array of applications and benefits. Many use-cases for these networks are concentrated in rural areas, university campuses, public events, and indoor spaces. In rural areas, the cost of implementing individual networks for each MNO is not justifiable due to low population density, making neutral host networks a suitable solution to minimize costs for all parties involved. In indoor areas, university campuses, and public events, the reduction in the space needed for infrastructure deployment, cost, and energy consumption makes infrastructure sharing an ideal solution [7].  
This approach also enables MNOs to expand their mobile network coverage, addressing the increasing difficulty in acquiring sites for antennae and base stations, facilitating a swifter evolution of 5G, and reducing the cost and development time of technologies to meet the functionalities and demands of 5G [3; 7].  
However, neutral host models also present disadvantages and pose challenges in their development. One major obstacle is the loss of end-to-end network visibility by operators, leading to reliance on third-party companies for network maintenance. This can result in diminished control over service quality and customer experience. As a result, service-level agreements (SLAs) would become more demanding and complex [7; 10]. Additionally, the lack of dedicated resources and software is a challenge as networks are not planned and designed with neutral host models in mind. Security and implementation are also concerns.  
Figure 1 illustrates different models for sharing 5G network infrastructures, categorizing them into two groups: passive sharing and active sharing, where A and B represent two different MNOs [11]. Passive sharing involves the sharing of non-electronic infrastructure components, such as sites and towers, power supplies, and the physical elements of transport in backhaul, e.g., optical fibers. Site sharing is a common and straightforward practice that allows operators to maintain their competitive strategies. It is operationally simple since the equipment is independent for each operator, yet it can still provide long-term cost reductions. Active sharing refers to the sharing of electronic components of the network, including elements of the RAN, such as base stations, antennae, controllers, etc. It can also encompass spectrum sharing and even sharing the CN, including servers and network functionalities.  
Regarding RAN sharing, there are two main models: the multiple-operator core network (MOCN) and the multiple-operator radio access network (MORAN). The MOCN involves sharing the RAN and spectrum by multiple operators, whereas in the MORAN, there is no spectrum sharing, so operators must use their dedicated spectrum range. CN sharing is much less common due to the complexity of operation and maintaining strategy differentiation among operators, making it a less attractive model despite being more economical than RAN sharing.  
Roaming is also a form of infrastructure sharing. In the case of a neutral host, when an operator lacks coverage in an area, it can use another operator's coverage through a roaming agreement [9; 11].  
Figure 1: Classification of infrastructure sharing models (adapted from [11]).