## 4 Algorithm Description
### The Network Operation Process
As for the improved routing discovery algorithm, due to the fixed locations of wayside nodes, on the one hand, it only sends RREQ packets to the determined direction and does not need to broadcast throughout the entire network. On the other hand, the improved routing discovery algorithm greatly utilizes existing paths, reducing routing discovery overhead. From Figure 5, it can be seen that the route discovery overhead of the AODV routing protocol increases as the distance from the source node to the destination node increases. This is because the farther the distance is, the larger the flooded area will be. This article sets two destination nodes at stations at both ends. When the train travels from the starting station to the destination station, due to the closer distance between the train and the destination node at the starting station, the train will communicate with the destination node at the starting station, causing the train to be further away from the destination node. When the train travels in the second half of the journey, it will be closer to the destination node of the terminal station, so it will communicate with the destination node of the terminal station, causing the train to become closer to the destination node. Therefore, there will be a trend of increasing and then decreasing routing discovery overhead. The route discovery overhead of the improved route discovery algorithm is significantly smaller than that of traditional AODV routing protocols, highlighting the advantages of the improved route discovery algorithm. #### 5.1.2 Packet Delivery Rate of Improved AODV

As shown in Figure 6, the simulation results of train location and packet delivery rate are described. The improved AODV route discovery algorithm has a weak advantage in improving the packet delivery rate. As the distance between the train location and the sink node increases, the gap between the two becomes larger because, during route discovery, the improved AODV route discovery algorithm utilizes the routing cost value to select paths with better link quality, and in the subsequent route discovery process, it will use paths with better link quality that have already been found. Compared with the AODV route discovery algorithm, it has been searching for the path with the minimum number of hops. When the source and destination nodes are close, the number of hops is small, and the path difference between the two points is not significant. Therefore, the difference in packet delivery rate is small. When the source and destination nodes are far away, the diversity of paths increases with more hops. The improved AODV routing discovery algorithm makes more use of existing paths with good link quality, while AODV continues \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Tunnel length & 2000 m \\ Tunnel width & 3 m \\ Number of nodes & 150 \\ Transmission range & 90 m \\ Transmit power & 0 dBm \\ Receiving sensitivity & \(-87\) dBm \\ Packet loss rate threshold & 1\% \\ Packet length & 256 \(\times\) 8 bytes \\ Train running speed & 20 m/s \\ Bandwidth & 2 Mbps \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameters of simulation. to search for the minimum-hop path. Therefore, the improved AODV routing discovery algorithm has advantages. ### Residual Link Lifetime

As shown in Figure 7, the simulation results of the train location's residual link lifetime are shown. As can be seen from Figure 7, for nodes selected without considering the residual link lifetime, the residual link lifetime between them and the train is smaller; that is, the link lifetime is shorter and does not have good stability. Figure 8 shows the relationship between the residual link lifetime and the train speed. It shows that the residual link lifetime between the train and the fixed node reduces when the train speed increases because, as the train speed increases, the distance between the fixed node and the train will change faster and faster until it exceeds the communication range.