# A Cooperative Communication Protocol for QoS Provisioning in IEEE 802.11p/Wave Vehicular Networks
## 5 Performance Evaluation  
For evaluation purposes, we compare our proposed scheme with the WAVE standard and overhearing-driven counter-based delayed forwarding (OCBDF) scheme in WAVE standard [42]. Performance of the proposed scheme and other protocols are evaluated using the discrete event simulator Omnet++ [52]. The simulation parameters are summarized in Table 3.  
A radio module implements the 802.11p PHY and MAC model of OMNET++, and parameters are set according to the recommended values in [1]. We consider a Rayleigh fading model with the path-loss exponent of 2.5. RSUs are uniformly distributed along the road segment with a predefined distance. Simulations are run for a duration of 600 s, and we limit the communication path to 2-hop. 100 vehicles move with speed ranging from 35 to 100 km/h. In this simulation, we consider 10 km highway. RSUs are deployed uniformly in the highway. In this highway, the vehicles can move in two opposite directions, and there are three lanes in each direction of the highway. Figure 14 shows the throughput obtained when the distance between RSUs increases. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Value** \\ \hline Frequency band & 5.8 GHz \\ Physical/MAC Layer & IEEE 802.11p \\ Tx Power RSU & 50 mW (500 m radio range) \\ Tx Power OBU & 11 mW (250 m radio range) \\ Distance between neighboring RSUs & 500 m \(\sim\)2000 m \\ Average speed & 35 Km/h (default) \(\sim\)100 Km/h \\ Simulation Time & 600 s \\ Data Packet Length & 512 Byte \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Simulation Parameters. Figure 13: The timing diagram for the exchange of the control message for the resource release. As shown in Figure 14, the performance of the proposed scheme outperforms the legacy schemes, and it is also observed that the throughput drops for all schemes. This is because the shadow zone exists between RSUs. Although the distance between RSUs increases, the proposed scheme and OCBDF scheme is likely to find a two-hop connection to an RSU. However, the performance of the OCBDF scheme is getting worse since the OCBDF scheme does not consider the link status to select the relay node. Figure 15 shows the throughput as a function of the vehicle's speed. In this simulation, we fixed the distance between RSUs to 1000 m. In Figure 14, it is observed that the performance of the WAVE standard is worst. This is due to the existence of the shadow zone between the adjacent RSUs. The performance of the OCBDF scheme is superior to the WAVE standard since the OCBDF scheme can transmit data frames through the two-hop connection. However, when the speed of vehicles increases, link disconnection frequently occurs, and the throughput of the OBDF scheme decreases. The proposed scheme performs better than two existing schemes since it selects the relay node considering the speed of vehicle and link status. However, the proposed scheme also cannot avoid link disconnection resulting from the increase of vehicle speed, and the performance of the proposed scheme is also degraded. Figure 16 shows the throughput as a function of the number of vehicles. Figure 14: The throughput as a function of the distance between road-side units (RSUs). Figure 15: Throughput as a function of the vehicle’s speed. In Figure 16, the trends of throughput regarding the number of vehicles on the road can be observed. For both the WAVE standard and OCBDF scheme, the throughput decreases when the number of vehicles on the road increases. This is the result of an increase in congestion when there are more nodes in the vehicular network. In addition, when the number of vehicles increases, the LOS between vehicles is frequently lost, and the packet loss also increases. In the proposed scheme, because OBUs can transmit data frames without the contention in the allocated SCH interval, the proposed scheme can provide better throughput than the legacy protocols. Figure 17 shows the packet delivery ratio as a function of the number of vehicles.