# Seamless Handover Scheme for MEC/SDN-Based Vehicular Networks
## 4 Novel Seamless Handover Scheme for MEC/SDN Vehicular Networks
### Estimating Threshold Levels  
Figure 4 presents the three considered threshold levels deployed for the developed fast handover mechanism. The first threshold level, L\({}_{1}\), represents the starting point of the developed handover mechanism, i.e., the start of the inspection state. The second threshold level, L\({}_{2}\), is variable, and it represents the end of the inspection state and the start of the execution phase of the handover process. The handover process must be ended before the third level, L\({}_{3}\), at which the packet reception ratio from the current serving base station decreases below the required level. Thus, the handover process should be ended before this point to maintain communication with the required QoS.  
The main factor in deciding the handover process is the value of the received signal that results in an acceptable packet reception ratio (PRR). Once the value of the received packet ratio decreases below the critical PRR, the communication is interrupted and the quality of service (QoS) is decreased below the required level. Thus, the handover process must be performed before the received packet ratio decreases below the critical PRR. The probability of successful packet reception, P\({}_{r,sr}\), is calculated by Equation (1).  
\[P_{r,sr}=P(L\ (dB)\leq\ P_{Tx}(dB)-P_{Rx}(dB))=P(SINR_{Rx}\geq\ SINR_{Rx-th-3}), \tag{1}\]  
\[L(dB)=n_{0}B_{Wave}(dB)+PL(d)\ (dB), \tag{2}\]  
Figure 4: Signaling flow of the developed handover scheme.  
where L is the communication losses, \(\rm P_{Tx}\) and \(\rm P_{Rx}\) are the total transmitted and received power, \(\rm SINR_{Rx}\) is the signal-to-noise ratio of the received signal, and \(\rm SINR_{Rx-th-3}\) is the threshold of the received signal-to-noise ratio after which the packets cannot be received successfully. The communication losses can be calculated as the sum of the channel loss and the path loss (PL), which can be calculated as a function of the communication distance (d) using the street canyon model as introduced in [40]. The power spectral density of the considered 60 GHz spectrum is \(\rm n_{0}\), and the Bwave is the allocated bandwidth of the wireless access of the vehicular environment (wave). Thus, to guarantee the successful reception of packets, the received signal-to-noise ratio must be higher than the threshold level, \(\rm SINR_{Rx-th-3}\). This condition is included in the developed handover mechanism, as the handover process should be performed before \(\rm SINR_{Rx}\) decreases below the threshold level of \(\rm SINR_{Rx-th-3}\).  
During the inspection state, the initial MEC server calculates the time required to migrate the current services of the vehicle, \(\rm T_{m}\). Based on the received data about the vehicle's mobility, \(\rm V_{i}\), and the vehicle's current position, the MEC server calculates the distance corresponding to the \(\rm T_{m}\). This is the required distance a vehicle should travel during the migration process and is referred to as \(\rm dm\). If the distance between the first threshold level, \(\rm L_{1}\), to the third threshold level, \(\rm L_{3}\), is d, the threshold level \(\rm L_{2}\) is chosen to achieve \(\rm d_{2}>\rm d_{m}\).