# Cooperation-Assisted Spectrum Handover Mechanism in Vehicular Ad Hoc Networks
## 3 Proposed Scheme
### Cooperation-Assisted Scheme  
The cooperative assistance spectrum handover mechanism is divided into two parts, the first is the cooperation-assisted scheme combined with the fast scanning mechanism of the previous section, and the second is the discussion of spectrum stability. These two items will play an important role in the overall perception of the overall performance of the wireless system. The first part is mainly to shorten the time to scan the channel, and the second item is mainly to ensure the stability of the scan results. If the stability of the spectrum is too poor, it means that this spectrum is often occupied, but the length of the occupied time is uncertain. In this way, if the SU switches to this spectrum, it is very likely that some PU will use the spectrum soon, causing the SU to search for a new available  
Figure 5: An example of a fast spectrum scanning mechanism. spectrum again. Due to the high-speed mobile nature of in-vehicle networks, stability is more of a challenge. (1) Cooperate to assist with the rapid scanning mechanism  
The cooperative communication mode of this paper will adopt the relay method. Figure 6 shows the cooperation-assisted mechanism and cooperation trigger timing. In Figure 6a, SU 2 receives the current spectrum usage of the BS 2 broadcast, and then informs SU 1 through V2V communication to achieve a cooperative coordination mechanism. In this way, the SU 1 can obtain the usage status of the BS 2 spectrum before it has entered the range of the new BS, and then make early decision-making preparations. Figure 6b shows the circumstances under which SU 1 triggers the request for cooperation. Basically, if the RSSI of SU 1 to BS 1 is lower than the threshold, it will start to send out cooperation request messages. Once other SUs are in the signal range of different BSs, they will reply to SU 1, and provide their current BS spectrum usage status (e.g., available bandwidth or spectrum utilization rate). Figure 7 is a step-by-step diagram of cooperation-assisted mechanism combined with the fast scanning mechanism. (2) Spectrum stability judgment  
If the variability of the spectrum resources is too large, we think that the stability of the spectrum is not good. In principle, we hope that the spectrum that the SU will switch to can meet the resource requirements of the SU and use the spectrum as long as  
Figure 6: (a) Cooperation-assisted mechanism and (b) cooperation trigger timing. Figure 7: Cooperation-assisted with fast scan mechanism. possible. As mentioned earlier, in order to avoid the ping-pong effect causing the SU to always perform handover behavior, we will use Equation (1) to determine the stability of each spectrum (that is,, stable index, SI). This index represents the degree of change in the available resources of the spectrum. The greater the change in the surrounding environment, the frequent execution of scanning procedures by the PU or SU will increase the number of potential handovers. Therefore, the greater the value of the SI, the poorer the stability of the spectrum. The term \(\mu\) is the average of the available resource, \(y_{i}\) is the available resources for each scan of the spectrum, \(i\)=1,..., \(n\), \(N\) is the total of the scan times. Figure 8 shows an example of scanning three rounds. \[\text{SI}=\frac{(y_{1}-\mu)^{2}+(y_{2}-\mu)^{2}+\cdots+(y_{n}-\mu)^{2}}{N}= \frac{\sum_{i=1}^{n}(y_{i}-\mu)^{2}}{N} \tag{1}\]  
From the results presented in the Figure 8, we discuss three cases. * Best case: Spectrum 2 has the highest stability because the variability of the spectrum is zero. Although Spectrum 2 is the most stable, it does not have the most resources available. * General case: The SI values calculated for spectrum 4 and spectrum 5 are the same, which means that they have the same stability. If the stability is the same, we can choose a spectrum with more available resources to switch. * Worst case: Spectrum 1 has the lowest stability, which means that the SU switches to this spectrum.