## 6 Numerical Results  
In this section, we evaluate the performance of RBCB scheme. The simulations were conducted with MATLAB and PTV VISSIM. PTV VISSIM is a well-known vehicle traffic simulator [42]. We used PTV VISSIM to simulate the mobility of vehicles in the simulation scenario. Here, we consider a highway scenario, where vehicles move on the road with the same mean speed. The speed of vehicles follows a normal distribution with mean 100 km distributed along the road following a Poisson distribution. Then, the mobility traces of vehicles were input into MATLAB. MATLAB was used to simulate the wireless channel and the operations of broadcasting schemes. We adopted the Rayleigh fading channel during the simulations, and the fading was assumed to be constant within each time slot (block fading) [41]. The parameters of simulations are shown in Table 1. The performance of RBCB scheme is compared with RBB scheme in terms of packet delivery probability for once broadcasting, overall packet delivery probability, and average packet delay. Figure 8 illustrates the PDPs of RBB and RBCB schemes for once broadcasting. In Figures 8 and 9, the number of helpers is set to one. In Figure 8a, the transmission range is set to 200 m, and the density of vehicles is changed from 0.02 vehicles/meter (v/m) to 0.04 v/m, while in Figure 8b, the density of vehicles is set to 0.02 v/m, and the transmission range is changed from 200 m to 300 m. According to Figure 8, the PDP of RBCB for once broadcasting is significantly improved compared to RBB scheme. Furthermore, the PDP of RBCB for once broadcasting is even greater when the density of vehicles is 0.04 v/m (Figure 8a) or the transmission range is 300 m (Figure 8b). This is because an increase in vehicle density or transmission range raises the number of potential helpers, which means the probability of cooperative broadcasting is also increased. In addition, the PDPs of RBCB and RBB schemes for once broadcasting are almost 0, when the signal noise ratio is 15 dB. This is because almost no nodes can receive the packet when the channel is very poor. Thus, no potential helpers exist, and the probability of cooperative rebroadcasting is very small. Figure 9 plots the overall PDPs of RBCB and RBB schemes. In order to meet the required PDP, a packet may need to be broadcasted more than one time. Here, the required PDP is set to 0.95, when the signal noise ratio is 30 dB. In order to meet the required PDP, the labeled packet is broadcasted three times for RBB scheme, while it is just broadcasted two times for RBCB scheme. Even so, the PDP of RBCB is still greater than that of RBB, because cooperative rebroadcasting significantly improves the PDP for once broadcasting. Figure 10 shows the comparison of the average packet delay of RBB and RBCB schemes. In Figure 10, the number of helpers is set to one. It is observed that the average packet delay of RBCB is shorter than that of RBB. This is because the cooperative broadcasting decreases the times of broadcasting it needs to meet the required packet delivery probability. For RBB, it needs three times of broadcasting to meet the required packet delivery probability, while for RBCB, it just needs two times of broadcasting to meet the required packet delivery probability. Thus, the delay that a packet waits in the buffer of the source node is significantly decreased. Furthermore, the improvement is more significant when the traffic load is medium. The reason is as follows. When the traffic load is low, both RBB and RBCB \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter** & **Value** \\ \hline Number of lanes & 2 \\ Width of a lane & 5 m \\ Length of road & 5 km \\ Density of vehicles (\(\rho\)) & 0.02 and 0.04 v/m \\ Transmission range (\(R_{T}\)) & 200, 300 m \\ Number of slots (\(F\)) & 50 \\ Length of a frame & 20 ms \\ Packet size & 1024 bit \\ Number of helpers & 1 and 2 \\ Number of rebroadcasting & 2 and 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters.