## 2 Materials and Methods
### Architecture Design
The routing process of the cross-layer pre-equalization method is shown in Figure 4. In the process of establishing and updating the route based on geographical location, the nodes were strictly limited to obtain the geographic location information of nodes that were not related to them, but the wiretap nodes were unable to use the channel state of the overall route for information restoration so that the safety of information transmission could be ensured. The route information stored from node A to node G was reduced in turn, and the information that the back-end node had nothing to do with the current node that was reduced, so that it was able to interfere with the illegal data during the information transmission and improve the transmission security of information at the physical layer [16; 17; 18]. Figure 3: Method overall architecture diagram. The information transmission process in VANET was divided into three stages: sending, relay routing, and receiving. We then discussed its safety performance. Figure 5 shows the information transmission process in which the transceiver was a single node. The source node transmitted the information to the receiving node through the relay route. The information received by the first relay routing node was \(X_{1}\), the information received by the malicious eavesdropping node was \(Z_{1}\), and the information received by the receiving node was \(Y_{1}\). According to the existing theories, the capacity of safe channel is:  
\[C_{1}=\max[I(S_{1};Y_{1})-I(S_{1};Z_{1})] \tag{12}\]  
When VANET adopted the regional cooperation mode, the source node cooperated with the surrounding nodes to complete the information joint sending and the selection of relay routers [19; 20]. The destination node cooperated with the surrounding nodes to complete the information receiving. The secure channel capacity under this model is:  
\[C_{2}=\max[I(S_{2};Y_{2})-I(S_{2};Z_{2})] \tag{13}\]  
where \(I(S_{2};Y_{2})\) is the mutual information between the sending node and the receiving node, indicating the amount of information received by the receiving node. According to the three processes of regional cooperation, such as the sending cooperation, relay routing, and receiving cooperation, the mutual information is \(I(S_{2};Y_{2})\), which can be further divided into the sending cooperation  
Figure 4: Route discovery process diagram. Figure 5: A schematic diagram of VANET single-node information transmission. mutual information \(I(S_{2};X_{2})\) and receiving cooperation mutual information \(I(S_{2};X_{2},Y_{2})\). The regional cooperation increased the amount of information. Meanwhile, the relay routing was better and channel capacity was larger under multi-node cooperation. Compared with the single node information transmission, the regional cooperative information transmission had the following inequality. \[I(S_{2};X_{2})\geq I(S_{1};X_{1}),\;I(S_{2};X_{2},Y_{2})\geq I(S_{1};X_{1},Y_{1}) \tag{14}\]  
According to Formula (3), \(I(S_{2};Y_{2})\geq I(S_{1};Y_{1})\). When the malicious nodes had the same ability to obtain information, that is to say, \(I(S_{2};Z_{2})=I(S_{1};Z_{1})\), \(I(S_{2};Y_{2})-I(S_{2};Z_{2})\geq I(S_{1};Y_{1})-I(S_{1};Z_{1})\). Therefore, \(\text{C}_{2}\geq\text{C}_{1}\). It can be concluded that the secure channel capacity of the regional joint mode was larger than that of the single node mode. Figure 6 shows the VANET regional collaborative information transmission.