## 2 System Overview
### V2X Communications  
There exists a range of wireless communication technologies that can be employed for CAV systems and intelligent transportation system (ITS) applications (e.g., 5.9 GHz DSRC, cellular communications, Wi-Fi, Bluetooth, satellite radio, and visible light communications) [20; 46]. The V2X communications based on 5.9 GHz DSRC is a mature technology that is the most widely tested and commercially available [46; 47; 48]. The DSRC-based V2X communication technology is based on the IEEE 802.11p and the IEEE 1609 series of standards, which are collectively known as wireless access in vehicular environments (WAVE) standards [46]. The 5.9 GHz frequency band is divided into seven 10-MHz channels, which include one control channel (CCH) and six service channels (SSHs) [49]. As an alternative to the DSRC-based V2X communications, the cellular V2X (C-V2X) communications based on the 3 GPP standards has attracted significant attention in recent years. The C-V2X communications utilizes the cellular network infrastructure (e.g., LTE and 5G networks) to enable V2X applications. For the upper layers, the C-V2X technology is expected to leverage existing standards (e.g., IEEE, SAE, ISO, and ETSI) and utilize common ITS message types, such as the basic safety message (BSM) of IEEE WAVE and the cooperative awareness message (CAM) and the decentralized environmental notification message (DENM) of ETSI ITS-G5. Some of the advantages of the C-V2X technology over 5.9 GHz DSRC include a much larger coverage area, higher throughput, lower latency, and more robust scalability [47; 48]. Despite the advantages, the standardization work of C-V2X communications is still in process and the availability of C-V2X commercial hardware equipment is limited at this stage. In this work, we utilized 5.9 GHz DSRC for V2X communications, and the test vehicles were equipped with Cohda MK5 OBUs for the exchange of information among vehicles. The characteristics of DSRC-based V2X communications are shown in Table 3.  
The vehicle state information is broadcast and shared among the vehicles equipped with DSRC devices by exchanging the BSM, which is defined in the SAE J2735 message set dictionary [29]. The BSM contains data obtained from the vehicle CAN bus and the  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Type** & **IEEE WAVE** \\ \hline Frequency & 5.850-5.925 GHz \\ Channel & 1 CCH, 6 SCH \\ Bandwidth & 10 MHz \\ Data rate & 3–27 Mbps \\ Maximum range & 1000 m \\ Modulation & OFDM \\ Media access control & CSMA/CA \\ \hline \hline \end{tabular}
\end{table}
Table 3: Dedicated short-range communications (DSRC)-based vehicle-to-everything (V2X) communications characteristics.  
GNSS receiver, which include safety critical state information such as the vehicle position, heading, speed, and yaw rate. The BSM is typically transmitted with a period of 100 ms on the dedicated safety channel (i.e., Channel 172 in the U.S.) [29; 46]. The BSM Part I data contain the BSM core data frame that shall be included when broadcasting a BSM, while the BSM Part II data contain optional additional information (e.g., event flags, path history, path prediction, and exterior lights). The contents of the BSM core data frame are described in Table 4.