## 5 Proposed Method
### Packet Delivery Rate (PDR)  
Packet delivery rate (PDR) is defined as the ratio of packets received by the destination nodes to all packets transferred by the source nodes. In the simulation process, two scenarios are intended to evaluate the packet delivery rate. The purpose of the first scenario is to calculate PDR based on the packet sending rate (PSR). As shown in Figure 10, when PSR is high, PDR decreases in all methods and vice versa because the packet sending rate indicates the number of packets produced in the network per time unit. Therefore, high PSR increases the network load. In this case, the buffer capacity of vehicles is completed quickly, and it is very likely that some data packets will be lost due to high network congestion. Thus, PDR is reduced in the network. The purpose of the second scenario is to evaluate the packet delivery rate according to the signal transmission radius (STR) of vehicles. As shown in Figure 11, when STR is large, PDR will also increase because the number of single-hop neighbors of each vehicle increases in this case, and a vehicle can cover a wider communication range. In this case, vehicles have a higher chance to find a next-hop node. This reduces the probability of the local optimum issue. According to Figures 10 and 11, QFHR has an optimal PDR in comparison with other approaches. In Figure 10, which shows the evaluation of PDR based on PSR, QFHR increases the packet delivery rate by 2.74%, 6.67%, 22.35%, and 29.98% compared to IRQ, IV2 XQ, QGrid, and GPSR, respectively. Additionally, in Figure 11, which shows PDR based on STR, QFHR has improved PDR by 3.45%, 8%, 23.29%, and 26.17% compared to IRQ, IV2 XQ, QGrid, and GPSR, respectively. This is mainly rooted in the fact that QFHR performs the Q-learning-based routing process using a distributed manner to discover various paths between network intersections. However, IRQ, IV2 XQ, and QGrid present centralized routing processes. The distributed routing method is more compatible with the dynamic environment of VANET and can find routes with less delay and high connection quality in the network. Thus, it improves the packet delivery rate. Moreover, if the network has high congestion, QFHR can quickly discover this issue and replace the new path. However, IRQ selects the best route based on traffic information from the central server. In this scheme, the central server should wait for receiving traffic messages from RSUs to detect the congestion on the network. However, this process causes a high delay and may lose some data packets on the network.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Network simulator & NS2 \\ \hline Network size & \(3\times 3\) km\({}^{2}\) \\ \hline Simulation time & 1000 s \\ \hline Vehicles & 450 \\ \hline Road sections & 38 \\ \hline Intersections & 24 \\ \hline Vehicle density & 0.005–0.02 (Vehicle/m) \\ \hline Vehicle velocity & 14 m/s \\ \hline Communication range of vehicles & 250–300 m \\ \hline Communication range of RSUs & 300 m \\ \hline Packet size & 512 byte \\ \hline Packet sending rate & 1–6 (Packet/s) \\ \hline Hello broadcast period & 1 s \\ \hline Learning rate (\(\alpha\)) & 0.1 \\ \hline \(\epsilon\) & 0.2 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Simulation parameters.  
Furthermore, in IV2 XQ, the central server chooses the best route based on historical traffic information stored in its memory, and the route selection process depends only on the vehicle density in the road sections. In addition, QGrid selects the best gird (i.e., the gird with maximum density) using a greedy routing process. However, the high-density grid is not always desirable and may cause congestion on the network and packet loss. GPSR is a greedy routing process and deals with the local minimum problem, which rises packet loss.