## 1 Introduction  
The automotive industry and the information technology (IT) industry are joining forces towards connected and autonomous vehicles that offer a multitude of benefits, such as driving safety, traffic flow efficiency, driver comfort, and new infotainment experiences for passengers. Along with sensors and computing intelligence, a key component in the development is the vehicle-to-everything (V2X) communication, which allows a vehicle to communicate with not only other vehicles and road-side equipment, but also nearby pedestrians and the Internet. Among the prospective applications of connected and autonomous vehicles, the most crucial one that strongly drives the V2X technology evolution is the safety. Analyses by the U.S. Department of Transportation's National Highway Traffic Safety Administration (NHTSA) show that V2X communication can address up to 80 percent of the crash scenarios involving non-impaired drivers [1]. In vehicular safety communication, the periodic exchange of Basic Safety Messages (BSMs) containing such information as position, speed, acceleration, and heading among others [2], is indispensable for reducing the collision risk by facilitating the tracking and short-term prediction [2] of neighboring vehicles' kinematics. It immediately enables driving safety applications such as Forward Collision Warning (FCW) [3]. For long, the Dedicated Short-Range Communication (DSRC) based on the IEEE 802.11 Outside-the-Context-of-BSS (OCB) mode [4] has been the sole candidate lower layer technology for the vehicular communication. Both the IEEE WAVE framework in the U.S. [5; 6; 7] and the Intelligent Transport Systems (ITS) G5 framework in the Europe [8] build on the DSRC radio. Recently, however, the V2X communication using the cellular communication infrastructure is emerging as a strong contender of the DSRC-based communication. Although some argue that cellular V2X (C-V2X) is not as ready, has compatibility problems between different 3 GPP Releases, and has exaggerated coverage when measured in reality [9], proponents of the C-V2X argue that the cellular alternative has multiple advantages over DSRC [10; 11; 12; 13]. Although DSRC is a mature technology and does not require a coordinating network infrastructure or bulky signaling procedures, it has weaknesses in the spectral efficiency [14] and the capability to deal with congestion and hidden terminal problem at scale [15]. Typically, C-V2X is considered superior to DSRC in these aspects: longer range [16; 17] and enhanced reliability, more consistent performance under traffic congestions, clear evolution path towards 5G for emerging applications [14], and better coexistence with other technologies. On the other hand, the higher packet reception performance of C-V2X over DSRC is highly affected by the modulation and channel coding scheme (MCS) [18], and the capacity can be lower than DSRC under very high vehicle density and short distances [16]. Therefore, C-V2X must be less reliant on the cellular infrastructure for efficiency, by leveraging the sidelink communication in particular. Cellular V2X communications have already been standardized by the 3 rd Generation Partnership Project (3 GPP), in Release 14 [19]. It is focused on the communications for basic safety use cases. In particular, the sidelink (SL) device-to-device (D2D) communications through the PC5 interface as a key enabling technology. There are two modes, Mode 3 and Mode 4, that were designed for direct V2X communication. The difference between the two modes is in the radio resource allocation method. Resources are allocated by the cellular network under mode 3. Mode 4 does not require cellular coverage, and vehicular user equipment (UEs) autonomously select their radio resources using a distributed scheduling scheme with a message collision resolution mechanism. Mode 4 is considered the baseline mode and represents an alternative to DSRC. C-V2X lacks the contention resolution mechanism like that of the IEEE 802.11 in the DSRC communication.