## 5 Proposed Hybrid V2X Technology Selection Algorithm
### Intelligent Hybrid V2X Technology Selection Algorithm  
In this subsection, an intelligent hybrid V2X wireless communication technology selection algorithm is presented for transmission of C-ITS messages. For this, a decision tree has been generated to facilitate the intelligent technology selection based on a set of selection criteria. The proposed algorithm assumes that all V2X nodes support C-ITS services and transmit periodic CAMs. These assumptions ensure real-time statistics-driven decision making on all V2X nodes. It is important to mention that short-range will only be considered for low throughput data transfer of certain C-ITS messages. Contrarily, the high throughput connections like infotainment and remote driving using video streaming use long-range Uu link, as described in Section 6.2.2. Considering the size of the proposed intelligent hybrid V2X technology selection decision tree, only a simplified version is shown in Figure 4. The detailed decision tree can be found in the Appendix A of the paper. In a V2V scenario, the execution of the algorithm starts as soon as the vehicle ignition is turned on. In the first phase, also known as the warm-up phase, the environment is scanned and different statistics (referred as selection criteria) are evaluated. During this phase, also denoted as the time sliding window, every vehicle unintelligibly transmits CAMs via all three technologies and the received CAMs from the nearly nodes, if any, are logged. In the second phase, based on the observed traffic situation in the vicinity, CAMs and/or DENMs are sent intelligently using hybrid V2X. It is checked if CAM(s) from other vehicles are being received or not. If CAMs are not received, statistics-driven decision cannot be made. In this scenario, CAMs are sent via DSRC, PC5 and 5G links in an attempt to maximize the chances of CAM reception in the vicinity of the V2X node by one of the technologies. If CAMs are received, it is checked if receptions are from DSRC and/or PC5 and/or 5G. Depending on the receptions, a specific branch is followed in the  
Figure 4: Simplified hybrid V2X technology selection decision tree. detailed decision tree (as shown in the Figures in Appendix A), whereas, for simplicity purposes, in Figure 4, the reported average one-way end-to-end CAM reception latency values are compared with the latency threshold. For this to be accurate, synchronized clocks are needed between all the V2X nodes e.g., using GPS synchronization. After this, the 95 th percentile of the distance for CAMs received via respective technologies is also calculated by using the GPS coordinates included in the CAMs. For latency, a threshold, \(L_{thr}\), is defined for all three technologies in our study, whereas, the 95 th percentile distance threshold values for DSRC and PC5 are denoted as \(D_{thr1}\) and \(D_{thr2}\), respectively. After taking latency and distance into consideration, the speed of the V2X node is checked in order to adapt the V2X technology selection. If the speed is below the \(S_{thr}\), and the 95 th percentile of the speed of other V2X nodes in the vicinity is also below \(S_{thr}\), it means that the traffic dynamics do not change a lot, meaning that it is not necessary to enforce CAM transmission via multiple technologies during the next technology selection decision. If speed is higher than \(S_{thr}\), implying relatively frequent variation in traffic dynamics, CAM is sent via multiple technologies to enhance the V2X communication reliability. Afterwards, a check has been placed on the network traffic density (received CAMs \(\geq\)\(Dense_{thr}\) in the time sensing window) so that the CAM transmission frequency can be optimized. \(Dense_{thr}\) is also a flexible parameter and can be adapted according to the QoS requirements. The frequency of technology selection decisions is contingent upon the dynamics of traffic.