## 2 Materials and Methods
### Neighbor RSU Discovery
In this simulation scenario, the maximum vehicle speed is set to 10 m/s. Due to the change of vehicle communication range, more nodes can be \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Simulation area & 3600 m \(\times\) 3000 m \\ Simulation time & 1200 s \\ Warm-up period & 160 s \\ Mac protocol & IEEE 802.11p \\ Number of intersections & 24 \\ Number of street segments & 38 \\ Signal transmission radius & 260\(\sim\) 320m \\ Packet sending rate & 1\(\sim\)12 pkt/s \\ Maximum vehicle speed & 6\(\sim\)22 m/s \\ Time decay constant \(\alpha\) & 0.0001 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. selected to relay data packets, which affects the transmission of ant packets to a certain extent, and then affects the ant pheromone value of the street segment, thus dynamically affecting the routing of data packets, so the packet delivery ratio will fluctuate slightly. Figure 9 presents the variation of the packet delivery ratio with the vehicle speed. With the increase of vehicle speed, the packet delivery ratio fluctuates to some extent, which indicates that the vehicle speed affects the transmission of data packets. The change of vehicle speed will affect the distribution of vehicle neighbor nodes at a certain time, thus affecting the selection of the next relay node of the packet, and then affecting the packet delivery ratio. Figure 10 depicts the packet delivery ratio for different packet rates. When the packet rate increases, that is, the number of data packets sent per second increases, the packet delivery ratio may increase in the initial stage, but when the packet rate keeps increasing,

Figure 8: Packet delivery ratio with the signal transmission radius. Confidence interval with \(\alpha=0.05\). Figure 9: Packet delivery ratio with the maximum speed. Confidence interval with \(\alpha=0.05\). the network communication is heavily loaded, which will lead to no increase or decrease in the packet delivery ratio. Figure 11 shows the average end-to-end delay against the signal transmission radius. In this simulation scenario, the maximum vehicle velocity is set to 10 m/s. The proposed routing scheme always selects the street segment with low delay, so it results in stable low delay. However, the delay time of the scheme presented in the Figure 11 is the delay time for the source vehicle node to send packets to the destination vehicle after obtaining the anchor path, excluding the route request time, the time for the SDN server to calculate the anchor path and the route response time. If the above time is included, the delay time is greater than that of the other two routing schemes. The IEEE 802.11p protocol uses the carrier sense multiple access/collision avoidance (CSMA/CA) mechanism for wireless terminal media access. In the dense traffic scenario, when a large number of nodes compete for channel resources at the same time, the conflict

Figure 11: Average delay with the signal transmission radius. Confidence interval with \(\alpha=0.05\). Figure 10: Packet delivery ratio with the data packet rate. Confidence interval with \(\alpha=0.05\). will increase, and the transmission performance of the network will decline. In Figure 11, the average end-to-end delay of the proposed scheme fluctuates when the signal transmission radius expands to about 310 m, which corresponds to the fluctuation of the packet delivery ratio of the proposed scheme when the signal transmission radius is also about 310 m in Figure 8. The possible reason is that under the current simulation configuration, the vehicle density in the signal coverage area increases when the transmission radius expands to about 310 m. A large number of vehicle nodes compete for channel resources, which causes the fluctuation of the network performance. ## 4 Conclusions

A intersection-based unicast routing protocol using ACO in SDN was proposed. In the proposed routing scheme, a single-hop RSU neighbor discovery protocol was proposed, through which each RSU can obtain all its neighboring RSUs.