  Vehicular communications have attracted more and more attention recently from
both industry and academia due to its strong potential to enhance road safety,
improve traffic efficiency, and provide rich on-board information and
entertainment services. In this paper, we discuss fundamental physical layer
issues that enable efficient vehicular communications and present a
comprehensive overview of the state-of-the-art research. We first introduce
vehicular channel characteristics and modeling, which are the key underlying
features differentiating vehicular communications from other types of wireless
systems. We then present schemes to estimate the time-varying vehicular
channels and various modulation techniques to deal with high-mobility channels.
After reviewing resource allocation for vehicular communications, we discuss
the potential to enable vehicular communications over the millimeter wave
bands. Finally, we identify the challenges and opportunities associated with
vehicular communications.
