# Cooperation-Assisted Spectrum Handover Mechanism in Vehicular Ad Hoc Networks
## 3 Proposed Scheme
### Cooperation-Assisted Scheme
However, radio resources will suddenly become inadequate, or a PU may suddenly want to use this spectrum. If the SU performs the handover procedure too frequently, it may not be able to effectively use the spectrum resources. If the SI values of all spectrums are high, we recommend using passive scanning methods to reduce the ping-pong effect. Finally, Figure 9 shows a flow chart of the cooperative assistance spectrum handover mechanism. * Dynamic scan period  
In this section we will discuss dynamically adjusting the scan period due to the performance; the scan time is a trade-off. In Figure 10, we assume that a transmission period is M time slots and the sensing time takes m time slots. It means that the time that the SU can transmit is (M-m) time slots. If the sensing time is too short, it may not be able to scan all available spectrums; if the sensing time is too long, it may be compressed to  
Figure 8: An example of scanning three rounds. the transmission time, resulting in a decrease in overall spectrum performance. Therefore, we define Equation (2) to dynamically adjust the sensing time, so that the transmission performance can meet the system requirements. According to the 3 GPP specification [27], the scan period has an actual upper limit, such as the neighbor cell list (NCL), which can only record up to 32 data. Therefore, we preset a complete scan time upper limit as the time to scan 32 spectrums. If we use partial spectrum scanning, it means that the system can complete several rounds of partial scanning within a complete scanning time. T represents the total time allocated to SU, \(T_{tran}\) is the total transmission time, \(T_{scan}\) is the total scan time, \(P_{collision}\) is the probability of a SU collision during transmission, \(P_{fail}\) is the probability of transmission failure, and \(T_{round}\) is the time consumption of scanning a spectrum. In order to scan multiple spectrums, \(\alpha\) is the number of spectrums to be scanned partially, \(\alpha\) must be less than or equal to 32, \(\beta\) is the number of rounds to be scanned, and D(\(\alpha\), \(\beta\)) is the allocation ratio of the transmission time and the sensing time that the system wants. Figure 11 illustrates the effect of \(\beta\) on the probability of SU transmission failure in the full scan mode. From the figure, we can observe two points. First, the collision probability is higher, and the SU transmission failure probability is higher. The second point is that the more scan rounds \(\beta\), the probability of SU transmission failure is higher. The reason is that the more scans, the longer the sensing time, and the probability of the SU suffering a collision is higher. But the more rounds of scanning, SU can find a more stable spectrum to use. Figure 9: The flow chart of CASH. Figure 12 shows the effect of different scanning modes on the transmission magnification within a fixed sensing time. The total sensing time is 32 time slots. D(32,1) is a full scan mode, and the others are partial scan modes. In Figure 12, we can observe three things: First, when the transmission failure probability is higher, the transmission time multiplier of the system is smaller. Second, when using the full scan method, a higher transmission time magnification can be obtained, but the stability is not necessarily better. Third, if the system would like to obtain a specific transmission time magnification, it can select an appropriate scan mode according to the current transmission failure probability. For example, if we want to obtain an 8 times transmission magnification, when the transmission failure probability is 10%, use D(2,16) scan mode; when the transmission failure rate is 30%, the D(8,4) scan mode is adopted; and when the transmission failure rate is 50%, the D(16,2) scan mode is adopted. As a result, if we want higher performance, we need to reduce the probability of transmission failure (\(P_{Fail}\)) and reduce the scan time (\(T_{scan}\)).