# How to Isolate Non-Public Networks in B5G: A Review
## 1 Introduction
Virtual networks are formed in 5G systems by network slicing, and spectrum isolation refers to virtual networks sharing wireless spectrum resources without deteriorating others' QoS performance metrics. Customized configuration means that virtual networks can be configured with differentiated parameters for network functions [17]. With regard to spectrum isolation, the performance is represented by the satisfaction of QoS performance metrics (such as resource utilization, the throughput, the mean bit rate, etc.) and focuses on the optimization of these performance metrics. For example, improving the QoS performance metrics [16], maximizing the throughputs [18], trading off the  
Figure 1: Existing work of NPN isolation. terminal throughputs and the signal-to-noise ratio of the slices [19], as well as minimizing the latency [20], are treated as the requirements of slice isolation [21]. Then, to meet the requirements, methods such as game theory and convex optimization, as well as some other methods, are used to allocate wireless resources among the slices. However, there is no quantitative requirement for or index of isolation. Thus, it is hard to judge whether the isolation requirements have been achieved or not. To evaluate the isolation performance, an isolation index, representing the isolation performance of the spectrum, can be defined and calculated with the QoS performance metrics or the spectrum resource assignment parameters [22]. QoS requirements are treated with or mapped onto the isolation requirement. By comparing the isolation index and the isolation requirement, whether the spectrum isolation of one virtual network has been achieved or not can be evaluated. The system tries its best to fulfill the isolation requirements when assigning spectrum resources among virtual networks, and the main ideas that these methods use to achieve spectrum isolation are given in Section 3. For the differentiated configuration, various control parameters are configured in protocol layers 3 and 2 of the radio access network (RAN) to achieve customized scheduling and access control functions of QoS guaranteed and non-guaranteed services [23]. Furthermore, various ratios of uplink and downlink subframes in protocol layer 1 can be configured to obtain different wireless throughputs for the virtual networks. More descriptions of the customized configuration are given in Section 4. Using slicing technology, researchers have attempted to achieve network isolation at full steam [24]. However, NPNs require guaranteed isolation rather than isolation as best as the system can achieve. One NPN can be deployed by the resource and the infrastructures, which are partly occupied by the NPN and partly by the others sharing the system. However, there has been no analysis or summary of the isolation required by various deployments and isolation technologies to achieve guaranteed isolation. This is important for the application of NPNs. Therefore, this paper provides a review of isolation aspects for B5G NPNs, and the contributions are as follows. Firstly, we present an overview of NPNs' deployments and compare the performance metrics of the deployments. We conclude that sharing parts of resources and infrastructures is a promising way to establish NPNs, but it will bring great challenges in form of three kinds of isolation. Subsequently, for the three kinds of isolation--i.e., spectrum isolation, radio access network isolation and core network isolation--the main idea necessary to achieve the guaranteed isolation is described using the present isolation technologies, and the advantages, disadvantages and limitations are summarized. Finally, the open research challenges of B5G NPN isolation are discussed. The isolation technologies for NPNs in the B5G era are discussed in this article. To achieve this, the article is organized as follows. In the following section, deployments of NPNs are described and the isolation demands are analyzed.