# V2X Communication over Cellular Networks: Capabilities and Challenges
## 1 Introduction  
Autonomous driving is one of the hot topics for cellular telecommunications networks. The International Society of Automotive Engineers (SAE) has created a roadmap of six evolutionary levels [1]. The less advanced levels simply provide the transmission of warnings and momentary assistance messages (e.g., automatic emergency break, blind spot warning, etc. ), whereas the most advanced level addresses full driving automation under any environmental condition (i.e., local driverless taxi). Although autonomous vehicles can rely on their on-board systems, enabling them to communicate can improve their performance by enhancing their perception area and allowing the execution of collaborative maneuvers. Vehicle to Everything (V2X) communications is one of the enablers that can be employed to improve traffic safety and efficiency. For example, in [2, 3], the reader will discover a wide range of services that require the communication among vehicles. These descriptions have been provided by the 3 rd Generation Partnership Project (3 GPP). The data exchange among vehicles includes the transmission of simple Cooperative Awareness Messages (CAM) and Decentralized Environmental Notification Messages (DENM), as well as specific trajectory information and sensor data. The end-to-end (E2E) delay for this information exchange typically ranges from 100 to 10 ms and in some extreme cases, even down to 3 ms. The throughput ranges from a few kbps up to 1 Gbps, whereas the reliability varies from 90% to 99.999% [2]. The requirements described [3] were addressed with the provision of Long Term Evolution-Vehicle (LTE-V), Release 14 [4]. This Release offered two types of direct communication between vehicles: One coordinated by the network, representing so-called Mode 3, and one where the vehicles communicate autonomously, without network assistance, representing the so-called Mode 4. These use cases (UCs) and the related solutions were rather limited, and lacked sophistication, in terms of reliability and Quality of Service (QoS) guarantees for the end users. Following an evolution path, 3 GPP defined the first 5G specification (Release 15) with enhancements for Mode 3 and Mode 4 (e.g., 64 Quadrature amplitude modulation (QAM) and shorter Transmission Time Interval (TTI)), still aimed at initial deployments and fundamental UCs [4, 5]. In Release 16, 3 GPP considering inputsfrom verticals started addressing more advanced UCs and provided sophisticated solutions [6]. These solutions included more complex tools, such as mobility prediction, data analytics, and slicing support, as well as information received from the applications. Most of the abovementioned descriptions of UCs and requirements were mainly based on the input from telecommunication-related industries. Therefore, the need for a closer collaboration between the telecom industry and the automotive industry was apparent. In 2016, the 5G Automotive Association (5 GAA) ([https://5 gaa.org/](https://5 gaa.org/)) was established, with the aim of identifying realistic scenarios; studying architectural proposals; dealing with the defined scenarios; and analyzing business, deployment, and regulatory aspects. 5 GAA has produced a very thorough set of UCs and requirements and addressed methodological aspects that may have been ignored in the past. In the telecommunication research domain, EU-funded projects, such as METIS 2020 [7] and METIS 2020-II [8], have also provided descriptions of V2X safety and efficiency scenarios that closely match those discussed by telecommunication alliances [9]. More recently, dedicated V2X projects, such as 5 GCAR [10], 5 GCroCo [11], and 5G CARMEN [12], further elaborated on the definition of UCs for cooperative, connected, and automated mobility. Furthermore, they focused on the development of technologies, such as Mobile Edge Computing (MEC) and the evolution of the air-interface, targeting specific demonstrations and trials (e.g., tele-operated driving and see-through) in complex environments (such as cross-country border crossings).