# Cooperation-Assisted Spectrum Handover Mechanism in Vehicular Ad Hoc Networks
## 4 Performance Evaluation
### Performance Results  
(1) Spectrum utilization  
The spectrum usage is higher, the overall network performance is higher. Figures 13-15 show the performance result of spectrum utilization under different parameters. We can see that the cooperation-assisted mechanism can improve the spectrum utilization. Moreover, the partial scan scheme can reduce the scan time and increase the transmission time. Therefore, the performance of the partial scan scheme is better than the full scan scheme. In Figure 14, \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Simulation area & 2000 \(\times\) 2000 (m\({}^{2}\)) \\ Number of PU (PU\({}_{\text{number}}\)) & 2, 4, 6, 8, 10 \\ Probability of PU car access network (P\({}_{\text{PUaccess}}\)) & 0.3–0.8 \\ Number of SU (SU\({}_{\text{number}}\)) & 2, 4, 6, 8, 10 \\ Number of BS & 10 \\ Transmission range of BS & 1000 (m) \\ Mobility model & Random way point \\ Moving speed of vehicle & 5–20 (m/s) \\ Bandwidth & 100 (Mbps) \\ Simulation time & 600 (sec) \\ Number of antenna per vehicle & 2 \\ Transmission range of vehicle & 100 (m) \\ Packet size & 1500 bytes \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. when the moving speed of the vehicle increases, the SU must constantly switch BSs and then scan the spectrum, resulting in a significant drop in the overall spectrum utilization. In Figure 15, the higher the probability of the PU car accessing the network, it means that the PU itself is already using the spectrum, so the overall spectrum utilization rate is not too low. The SU can only access the spectrum through fragmented time, so the overall spectrum utilization rate performance improvement is limited. Interestingly, we can see the random access method. When the probability of PU cars accessing the network is low, the spectrum utilization rate will be higher than when the probability of PU cars accessing the network is high. This is because when the probability of the PU car accessing the network is high, the spectrum collisions of the SU will often occur, which will affect the performance of the PU. Therefore, when the probability of the PU car accessing the network is high, we suggest doing a spectrum scanning procedure so as not to cause performance inversion. Figure 14: Spectrum utilization ratio under different moving speeds (\(\text{PU}_{\text{number}}=10\), \(\text{P}_{\text{PUaccess}}=0.5\), \(\text{SU}_{\text{number}}=10\)). Figure 13: Spectrum utilization ratio under different number of SU cars (\(\text{PU}_{\text{number}}=10\), \(\text{P}_{\text{PUaccess}}=0.5\), \(\text{V}=10\)). (2) Average scan time  
In a fixed transmission cycle time, the spectrum scanning time is shorter, the interruption time of transmission is shorter and the increase of the time for transmitting data. From the results in Figures 16-18, the number of cars with different SUs has little effect on the average spectrum scanning time. However, as the moving speed increases and the probability of the PU vehicle access network increases, the average spectrum scanning time will increase proportionally. The scanning time of the random selection scheme is zero since there is no scanning behavior in this way. However, we can observe from other simulation results that although random access retains the most transmission time, the overall performance is not good. After excluding the randomly selection scheme, our proposed method has better results regardless of the number of SU cars, different moving speed, and the different probabilities of the PU cars accessing the network. This is because our method is to shorten the spectrum scanning time through partial scanning; then the SU obtains the spectrum usage of the new BS in advance through the cooperative assistance mechanism. Figure 16: Average scan time under different number of SU cars (PU\({}_{\text{number}}\) = 10, PPLaccess = 0.5, V = 10).