# V2X Communications in Highway Environments: Scheduling Challenges and Solutions for 6G Networks
## 2 Scheduling Algorithms and V2X in Highway Environments
It evaluates and analyzes the performance of several prominent scheduling algorithms. Additionally, the authors offer insights into potential challenges and future research directions related to scheduling. However, they acknowledge that significant research is needed to design a fully functional URLLC scheduling algorithm for emerging 6G systems, based on their extensive analysis. Another review of the latest methods for several scheduling schemes can be found in [11]. The comprehensive survey examines the evolution of scheduling algorithms in the literature. It categorizes them into four main groups based on the QoS metrics used, the machine-learning approach adopted, the performance goals, and the suitability to achieve 5G performance requirements for various UCs. The paper analyzes the suitability of each scheduling algorithm for the three major operating scenarios of 5G: Enhanced Mobile Broadband (eMBB), URLLC, and massive Machine-Type Communications (mMTC). Notably, all scheduling mechanisms considered in this paper for URLLC (associated with safety for V2X communications) are QoS unaware except for the E-model Maximum Queue Size (E-MQS) scheduler presented in [12]. This is a QoS-aware scheduler that improves the quality of the experience for real-time users. However, it uses many parameters, increasing the computation time and making it hard to implement, which is the main difference from the proposed SOVANET+ scheduler. The scheduler presented in [13] provides a less complex scheduler by associating the log rule with the PF metric and the CQI of each user divided by the maximum CQI among all users. It enhances spectral efficiency and ensures equity among users. Nevertheless, it is a QoS-aware scheduler that only operates for the DL. This is why it may not be preferred for V2X communications, as many of the stringent requirements of V2X UCs are identified in the UL. Regarding the less explored topic of V2X communications in highway environments, the authors in [14] focus on the functioning of the high-density truck platooning UC, achieved with various wireless technologies for V2V communications. The authors map the performance of V2V communications, measured in terms of latency and reliability, and they also evaluate the performance of the platoon, based on the achievable inter-truck spacing. The families of radio technologies compared are 802.11p and C-V2X. The C-V2X technology discussed in this work is based on Release 14 of the LTE standard, which includes two modes for V2V communications: Mode 3 (base-station scheduled) and Mode 4 (autonomously scheduled). The results indicate that C-V2X in both modes enables shorter distances between trucks than 802.11p. This is due to its more reliable communication performance, even as congestion from surrounding vehicles increases on the wireless channel. However, in this work, modern wireless networks such as 5G or B5G are not considered, nor is a broader set of V2X UCs examined. Moreover, the authors in [15] provide a comprehensive and unbiased assessment of technologies tested in a real-world highway environment under consistent conditions. Tailored evaluation tools have been utilized to perform the tests and analyze the results for different well-specified test cases. The performance evaluation indicates that in short-range technologies, C-V2X PC5 generally has a longer range than ITS-G5. However, in low-density scenarios, ITS-G5 provides lower latency compared to C-V2X PC5. Additionally, long-range 4G C-V2X could be a viable alternative for specific UCs. The outcome of this experimentation study according to the authors can also be used as valuable information for the further development of 5G V2X UCs for connected and autonomous driving. The authors in work [16] present a performance comparison between C-V2X and IEEE 802.11p-based DSRC in dense highway scenarios. C-V2X and the DSRC implementations considered in the paper were compliant with the [17; 18] standards, respectively.