  With rising interest in autonomous vehicles, developing radio access
technologies (RATs) that enable reliable and low latency vehicular
communications has become of paramount importance. Dedicated Short Range
Communications (DSRC) and Cellular V2X (C-V2X) are two present-day technologies
that are capable of supporting day-1 vehicular applications. However, these
RATs fall short of supporting communication requirements of many advanced
vehicular applications, which are believed to be critical in enabling fully
autonomous vehicles. Both DSRC and C-V2X are undergoing extensive enhancements
in order to support advanced vehicular applications that are characterized by
high reliability, low latency and high throughput requirements. These RAT
evolutions---IEEE 802.11bd for DSRC and NR V2X for C-V2X---can supplement
today's vehicular sensors in enabling autonomous driving. In this paper, we
briefly describe the two present-day vehicular RATs. In doing so, we highlight
their inability to guarantee quality of service requirements of many advanced
vehicular applications. We then look at the two RAT evolutions, i.e., IEEE
802.11bd and NR V2X and outline their objectives, describe their salient
features and provide an in-depth description of key mechanisms that enable
these features. While both, IEEE 802.11bd and NR V2X, are in their initial
stages of development, we shed light on their preliminary performance
projections and compare and contrast the two evolutionary RATs with their
respective predecessors.
