# Seamless Handover Scheme for MEC/SDN-Based Vehicular Networks
## 4 Novel Seamless Handover Scheme for MEC/SDN Vehicular Networks  
When a vehicle comes to the intersection between two coverage regions of neighboring RSUs, the vehicle should perform a seamless, fast handover process. Setting up the handover process and selecting the optimum time to turn from one coverage region of an RSU to the other coverage region is a challenge, due to vehicles' mobility. This section presents the developed seamless, fast handover scheme for the considered vehicular network structure introduced in the previous section.  
The quality of the communication process depends on the received signal power, i.e., the received signal-to-interference noise ratio (SINR\({}_{\text{Rx}}\)), which should not be decreased lower than a threshold level. The main steps of the developed handover scheme are presented in Figure 3; Figure 4 shows the signaling flow of the developed handover scheme. The first step is the initialization phase, when the vehicle triggers the handover according to the gathered information. These data contain parameters gathered by the  
Figure 2: Structure of the OBU.  
different communication layers, e.g., the transport and network layers, such as the signaling parameters, the spectrum parameters, the mobility, and the throughput. Once the received signal strength decreases below an initial threshold level (SINR\({}_{\text{Rx-th-2}}\)), the vehicle turns from the steady state to the inspection state. In the inspection state, the vehicle calculates the expected time to reach the next threshold level of received power (SINR\({}_{\text{Rx-th-2}}\)), at which the handover process should start.  
In the inspection state, the OBU communicates the MEC server of the serving base station to set up a connection with the MEC server of the targeted RSU. The serving MEC server receives information on the vehicle's current mobility and position. The MEC server calculates the expected time to perform the handover process according to the measured data. It then sends a handover request to the targeted MEC server with the expected time. The targeted MEC server starts to set up a connection with the relevant vehicle through the targeted base station at the estimated time. When reaching the threshold level SINR\({}_{\text{Rx-th-2}}\), the connection with the targeted MEC server should be set up, and the third phase of the handover mechanism, i.e., the execution state, starts.  
In the execution state, the handover process should be performed, and the vehicle should establish a connection to the targeted coverage region, while the connection to the currently serving coverage region should be disabled. This process should be carried out seamlessly for the end-user, with no discontinuity of the communication process. This requires migrating the current tasks of the vehicle from the current MEC server to the targeted MEC server. This takes a length of time that should be calculated in the second phase, i.e., the inspection state, as the threshold power SINR\({}_{\text{Rx-th-2}}\) is allocated according to this length of time. During the migration process, the vehicle starts sending information to the targeted RSU. After the process ends, the connection with the old RSU is ended through the MEC server. This should be done before the received power reaches the third threshold level (SINR\({}_{\text{Rx-th-3}}\)), which represents the critical received signal, after which the packet reception will be affected.  
To perform a seamless handover process, we introduce SDN. The SDN controller maintains the connection at the transport layer unaltered, thus avoiding the limitations of the congestion window (cwnd). To perform a faster migration process and am efficient  
Figure 3: Handover stages at intersection of coverage regions.  
handover process, we introduce the facilities of SDN to the distributed edge computing servers. This can be achieved by our previously developed MEC platform, which deploys an MEC controller with an interface to the SDN controller and implements a lightweight version of the control scheme.