## 6 Passengers Internet Resources Management
### Cache-Based Pre-Fetching Scheme  
In the conventional pre-fetching scheme, the load balancer will pre-store some popular Internet resources into the cache device preliminarily. As the passengers request for an Internet content, the load balancer will search it in the pre-stored cache first. If found, it will be transmitted to the user instead of being fetched from the Internet via the core network [22]. The conventional cache scheme seems attractive since it can reduce the transmission latency greatly. However, if a passenger requests an un-stored content, the load balancer can do nothing but fetch it from the Internet. Hence, we aim to propose a novel cache-based pre-fetching scheme to deal with such situation. Generally, the requested contents of small file size can be transferred to users within a short interval. As for those contents of large file size such as the high-definition video (HDV), they usually need a long transmission time, which reduces the communication QoE dramatically. Thanks to the breakpoint transmission technology, a video content of large file size can be chunked into a group of small segments to be independently downloaded and consumed by the Hyperloop on-board passengers. The basic idea of our proposal is to pre-store parts of the requested contents (un-stored in current cache) to the cache of next load balancer as shown in Figure 10. Then the passenger can fetch it directly from the cache as the Hyperloop enters next logical cell. The detailed scheme is formulated as follows:  
1. As a passenger \(u\) requests for an Internet content, it will arrive at the roadside load balancer first via the wireless link. 2. The load balancer searches the requested content in its cache database. If found, then transfer the content to the passengers via the wireless link directly. 3. If not found, fetch it from the Internet via the edge cloud. Simultaneously, estimate the transmission completion time \(\Delta t_{complete}\) based on the average allocated data rate mentioned in Equation (15), which can be expressed as \[\Delta t_{complete}=S_{file}/\overline{R}_{u}(t),\] (19) where \(S_{file}\) is the file size of the requested content. Calculate the remaining residence time \(\Delta t_{residence}\) based on Equation (19) inside current logical cell according to Hyperloop position. 4. If \(\Delta t_{complete}>\Delta t_{residence}\), i.e., the requested content will not be transferred to the passenger within the current logical cell completely, broadcast this content request to the nearby edge clouds. 5. Assess the burden status of the nearby edge clouds which connects to the load balancer that covers the next logical cell. Select an edge cloud with little communication burden and download the rest part of the requested content from the Internet and store them in the cache of the next load balancer. 6. As the Hyperloop enters the next logical cell, the passenger can fetch the remaining transmission-unfinished content from the cache via the wireless link directly. Figure 10: The diagram of the pre-fetching scheme. In conclusion, we propose a novel pre-fetching scheme by coordinating multiple edge clouds based on the proposed network architecture mentioned in Section 4. The merits of this proposal lie not only in reducing the content transmission delay but also making more margin for the transmission of mission-critical services data. Figure 11 presents the simulation results of the pre-downloading scheme. The red line denotes the maximum transmission rate of the midhaul, whereas three schemes will be compared, i.e., Scheme 1: without cache, Scheme 2: conventional cache-based strategy (store some popular Internet content in the cache in advance), and Scheme 2: our proposal. The vactrain starts from AAU1 and proceeds across AAU2. The actual throughput of three schemes stays the same in AAU1. Notably, Scheme 3 enables the pre-downloading procedure as the midhaul turns congested. Then, in AAU2, an abrupt throughput ascent of Scheme 3 appears since it pre-downloads some requested contents to the cache.