# V2X Communications in Highway Environments: Scheduling Challenges and Solutions for 6G Networks
## 2 Scheduling Algorithms and V2X in Highway Environments  
Scheduling data transmissions is a critical aspect of mobile networks, as the performance of a network's data throughput largely depends on the scheduling algorithms used. These algorithms are often proprietary and not detailed in any formal standards, but an effective scheduler is vital for the success of any network product. The data scheduler operates within the base station (gNB) at the MAC layer, coordinating data transmissions for both the UL and DL for all users within its coverage area. It allocates physical layer resources separately for the DL and UL. To implement an efficient scheduling algorithm, the gNB needs to collect measurement data from UEs. This includes the data buffer status for each UE, as well as their QoS requirements. For instance, delay-sensitive data may be prioritized over bulk download data to ensure optimal performance. In the literature, besides the well-known algorithms for wireless communications, several research efforts aim to improve the QoS for 5G communications by maximizing the throughput, minimizing the delay and jitter, and improving the allocation of resources. In the following paragraphs, we will describe some well-known existing scheduling algorithms and highlight proposed efficient scheduling techniques. Some of the well-known existing packet scheduling algorithms are the following:  
* The Maximum Rate (MR) algorithm aims to achieve high capacity and maximum throughput by continuously assessing changing channel conditions. It prioritizes users with better channel conditions, while users with poor channel conditions are not scheduled. However, this may result in an unfair distribution of resources among users. * The Round-Robin (RR) algorithm fairly distributes resources among users. Unlike the Maximum Rate algorithm, the RR algorithm allows users to take turns transmitting packets. This greatly improves fairness but may lead to reduced throughput since it does not consider the channel's quality. * The Proportional Fair (PF) algorithm was created to balance fairness and throughput by prioritizing UEs with a better instantaneous achievable data rate rather than mean throughput. However, the PF algorithm is not ideally suited for real-time services because it does not consider the buffer status of the UEs. The scheduling techniques mentioned above have their own set of advantages and deficiencies. In our work, we choose the PF scheduler as the standard scheme because it aims to provide essential service for all UEs. However, like the other schedulers, the PF follows the same standard, and resource allocation is applied per UE rather than per service. This is not ideal because different V2X services (even those related to the same family, e.g., safety) have different requirements. When scheduling occurs at the UE level, it may not meet the desired requirements (e.g., throughput, delay, or reliability), particularly during high-access network loads. Modern vehicular communications face significant challenges like UCs' stringent performance requirements, traffic congestion, high mobility, and rapidly changing topology. These factors significantly affect the expected requirements and QoS. Therefore, parameters like MCS should be well integrated into packet scheduling mechanisms to cope with these challenges. Scheduling for future cellular networks has been thoroughly researched. A survey of scheduling in 5G URLLC and emerging 6G Systems can be found in [10]. This paper surveys the potential works that address the packet scheduling algorithms for 5G and later systems in recent years. It provides a state-of-the-art review covering three main perspectives: decentralized, centralized, and joint scheduling techniques. Specific focus on centralized techniques was given to single and multi-connected network perspectives. The paper discusses joint scheduling algorithms in detail, aiming to provide a comprehensive understanding of key scheduling approaches.