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APPENDIX A. CELLULAR VEHICLE-TO-EVERYTHING (C-V2X)
In the past few decades, DSRC, a wireless communication technology, has traditionally carried the research, testing, and deployment of CV technologies in the ITS domain. Even though the technological advancement of DSRC reached its peak in the early 2010s, comparative adoption of CV technologies being deployed within the United States has been limited to testing and demonstration phases only. A list of deployments featuring DSRC-based CV technologies is given in the USDOT ITS JPO Connected Vehicle testbed website.
In the last couple of years, another wireless technology, called C-V2X, has moved up in the ranks to compete with DSRC. C-V2X technology is based on cellular network technology and is capable of using two transmission modes. C-V2X with mode 3 uses existing cellular infrastructure and mode 4 uses PC5 for direct V2V communication.
The wireless protocol used for C-V2X is significantly different from DSRC, due to different channelization schemes and timing requirements. The wireless front end design of the two technologies entails a significant variation in operation and deployment strategies. In terms of operation, the two wireless communication systems use the same frequency range—the 5.9 GHz, identified by the Federal Communication Commission (FCC) as a core wireless band for vehicular safety application.
In the case of DSRC, the 5.9 GHz is divided into seven independent 10 MHz channels using the total bandwidth of 75 MHz. However, the most recent NPRM (and subsequent ruling) released by the FCC identified devoting the first 45 MHz to unlicensed wireless operation, such as Wi-Fi, and allowing the remaining 30 MHz to be used for vehicle safety applications. With these recent policies, it is becoming more and more evident that DSRC could be phased out to make room for the newer and more advanced, yet superficially understood technology, in C-V2X. It is still an open research topic whether C-V2X will fulfill the promises that DSRC started with, but failed to showcase.
C-V2X is similarly based on the wireless architecture of cell phones. This technology is enabled by a new interface, called PC5, for vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communication using the side link in LTE. The vehicular application messages are broken into multiple radio layer frames, which are multiplexed into channel symbols transmitted in blocks of subcarriers within a fixed amount of consecutive timeslots, called subframes. In C-V2X, the smallest subframe is 1 millisecond, and the subcarrier is 15 KHz. Messages are collated into a group of transmission blocks, called resource blocks, which are 12 subcarriers that span ½ of a subframe (0.5 ms) in size. Contrary to DSRC, which uses a single carrier for transmission within the 10 MHz bandwidth in 5.9 GHz frequency, C-V2X uses multiple carriers, also known as subcarriers, in blocks of smaller sub-frames. This allows for robust resource utilization and scheduling techniques for high-density, high-speed use cases, featuring roadway safety applications.
In mode 4, a C-V2X transmitter is able to listen for usage in the subcarriers, and determine how many resources they would require based on the application message that needs to be supported. This process happens autonomously, with no assistance from the infrastructure. Semi-persistent scheduling (SPS) is a resource allocation and acquisition technique used by the C-V2X transmitters in mode 4 that allows the radios to independently identify the quantity of resources needed. This is in contrast to DSRC, which is based on the IEEE 802.11p protocol and is based on carrier sense multiple access. DSRC transmissions can happen virtually, as soon as needed, if the channel is open, compared to C-V2X, where the transmitter has to wait for the scheduled time slot and subframes to fill the resource blocks with data to be transmitted.
However, studies have shown that C-V2X responds better in terms of highly congested scenarios (i.e. high-density vehicular traffic) compared to DSRC. Studies have also found that C-V2X is capable of supporting the requirements placed upon it by safety applications, which affect network performance metrics such as latency, data rate, and packet loss. Such studies have been focused mostly on a simulated environment; more research is needed to understand real world implications for widespread usage of C-V2X.
Further study references:
Overview on C-V2X technology:
- F. Eckermann, M. Kahlert, and C. Wietfeld. “Performance analysis of C-V2X mode 4 communication introducing an open-source C-V2X simulator.” 2019 IEEE 90th Vehicular Technology Conference (VTC2019-Fall), pp. 1–5. doi: 10.1109/VTCFall.2019.8891534.
- R. Weber, J. Misener and V. Park, “C-V2X - A Communication Technology for Cooperative, Connected and Automated Mobility,” Mobile Communication - Technologies and Applications; 24. ITG-Symposium, Osnabrueck, Germany, 2019, pp. 1-6.
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