# A Reliable Low-Latency Multipath Routing Algorithm for Urban Rail Transit Ad Hoc Networks
## 3 The Improved Multipath Routing Algorithm
### Local Routing Maintenance Scheme Based on Maintenance Nodes  
In the traditional AOMDV, the route repair time is long with the alternate path repair scheme [16], and the alternate path will be idle for a long time when no failure occurs, which has a large maintenance cost. Therefore, combined with the characteristics of an urban rail transit network, a static configuration scheme is used to improve the routing maintenance scheme in the traditional AOMDV routing protocol, and a local routing maintenance scheme based on maintenance nodes for local repair is proposed. In the network model shown in Figure 1, the network area is divided into several virtual cells based on the area size and node location information; one or more maintenance nodes are arranged in each cell and configured with the routing information of the maintenance nodes, which reaches each network node in the current cell and reaches the maintenance node in the next cell. According to the network geographic area analysis, each maintenance node should be located in the center of the cell, and its position should satisfy the Equations (15) and (16). \[dis1=D(m_{j}(i)-\frac{1}{N}{\sum_{N}m_{j}(i)}) \tag{15}\]  
\[dis2=D(\frac{1}{N}{\sum_{N}m_{j}(i)}-\frac{1}{N}{\sum_{N}m_{j}(i+1)}) \tag{16}\]  
in which _dis_1 and _dis_2 represent the Euclidean distances between the maintenance node and each other node in the current cell, the maintenance node in the current cell, and the maintenance node in the next cell, and \(m_{j}\) represents the node position information in the \(i\)-th cell. The structure of the vehicle-ground communication network is shown in Figure 2.  
It is seen from the figure that by configuring the routing information of the maintenance node and other nodes in advance, it is beneficial to find the routing information of the maintenance node quickly and complete the path replacement during the route repair process, which can avoid the problem of the long, time-consuming route repair scheme. Considering whether the destination node can successfully receive the packet is related to the reception power of the node. So, in the route maintenance scheme, through monitoring the strength of the control signals sent by the nodes (such as Hello messages and Hello-ACK messages), it is possible to build a path loss model between two nodes, as shown in Equation (17). \[P_{L}(d)=P_{L}(d_{0})+10n\log_{10}(\frac{d}{d_{0}}) \tag{17}\]  
\[P_{L}(d_{0})=10\log(\frac{P_{t}}{P_{r}})=-10\log[\frac{G_{t}G_{r}\lambda_{1}{} ^{2}}{(4\pi d_{0})^{2}}] \tag{18}\]  
where \(P_{t}\) is the transmit power, \(P_{r}\) is the received power, \(P_{L}(d_{0})\) is the path loss in free space, n is the attenuation factor, \(G_{t}\) and \(G_{r}\) represent the gain of transmitting and receiving antennas, \(\lambda_{1}\) is the wavelength, \(d_{0}\) is the reference distance, and \(d\) is the distance of two nodes. With the known transmit power, the received signal strength of the next hop can be calculated based on the path loss model to determine whether it is lower than the set reception threshold so that the occurrence of a failure can be predicted. If the signal strength of the next hop is less than the set threshold, a failure is considered to exist, transmitting data to the next hop will fail, and a route repair algorithm needs to be initiated to replace the path. There are three repair cases depending on the location of the fault. Figure 2: Vehicle–ground communication network architecture. 1. Case 1: the established path is broken due to the mobile node moving beyond the communication distance, and data transmission cannot be successful. For this case, the mobile node restarts the route discovery algorithm to find a newly available path. 2. Case 2: the data transmission fails due to the failure of the Intra-cell link. In this case, the node that discovers the failure sends an RRER message to the maintenance node in the area and establishes a temporary path with the help of the maintenance node, as shown in Figure 3.
3.