## 4 Network Architecture
### Centralized Access Architecture  
Cloud radio access network (C-RAN) is considered as a promising solution to provide high communication services in the high-speed mobile scenario as it is a clean, centralized processing, collaborative radio, and real-time cloud computing infrastructure wireless access architecture [38]. As such, we utilize the C-RAN architecture for the Hyperloop communication based on the 5G network.  
Generally, the C-RAN in 5G system consists of three functional entities, i.e., trackside ground AAUs, centralized units (CUs), and distributed units (DUs) [39]. The CU mainly processes the non-real-time wireless high-level protocol, i.e., radio resource management and dual connection. CUs can be deployed on a general hardware platform together with mobile edge computing. A DU mainly processes physical layer functions and the real-time hybrid automatic repeat request (HARQ) flow through a dedicated equipment platform or a general + dedicated hybrid platform. The AAU contains parts of physical layer function and all radio frequency (RF) functionality, such as the transmit and receive functions, filtering, and amplification. With the merits of easy-to-install, the compact AAUs are distributed along the line evenly to realize seamless wireless coverage. Considering the stringent uRLLC services demands, the roadside AAUs and DUs are integrated into one entity. The CUs together with mobile edge computing (MEC) platform and parts of the core network functions are located near the Hyperloop lines, which is called the edge cloud. Network functions virtualization (NFV) technology is used for the edge cloud by decoupling the network functions from proprietary hardware appliances [40]. Hence, the edge cloud runs in software on standardized computer servers. Usually, a handover involves the participation of the core network. By migrating parts of core network function to the edge cloud, it can reduce the latency evidently, which caters to the demands of uRLLC traffic. The AAUs connect with the DUs via the midhaul, whereas the DUs are connected to the CUs via the F1 interface, i.e., the midhaul.  
Figure 5 presents a diagram of the C-RAN for the Hyperloop. The nearby AAUs connect with the LWS via the cable and forward data to the edge cloud (or vice versa). The on-board terminals connect with the antennas embedded inside the train via WiFi technology, these antennas converge all terminals signal and forward them to the outside multi-input and multi-output (MIMO) antennas via cable or vice versa. This two-hop relay structure avoids a radio wave penetration attenuation of the vactrain body, guaranteeing a relatively stable received amplitude. The edge cloud can obtain a timely and accurate information of the train position based on the monitoring and detecting devices distributed along the tube line. As such, it can enable the approaching cell and disentangle the passing one as the train proceeds forward. This is the basic idea of the moving cell scheme, which integrates several adjacent AAUs connecting to the same edge cloud into a logical cell to achieve a free-handover effect [41].  
Despite the fact that this architecture can cope with the frequent handover efficiently, this benefit will disappear as the Hyperloop travels across two edge clouds. This challenging issue will be addressed in the following section.  
Figure 5: The diagram of the C-RAN architecture for Hyperloop.