  Vehicular communications essentially support automotive applications for
safety and infotainment. For this reason, industry leaders envision an enhanced
role of vehicular communications in the fifth generation of mobile
communications technology. Over the years, the number of vehicle-mounted
sensors has increased steadily, which potentially leads to more volume of
critical data communications in a short time. Also, emerging applications such
as remote/autonomous driving and infotainment such as high-definition movie
streaming require data-rates on the order of multiple Gbit/s. Such high
data-rates require a large system bandwidth, but very limited bandwidth is
available in the sub-6 GHz cellular bands. This has sparked research interest
in the millimeter wave (mmWave) band (10 GHz-300 GHz), where a large bandwidth
is available to support the high data-rate and low-latency communications
envisioned for emerging vehicular applications. However, leveraging mmWave
communications requires a thorough understanding of the relevant vehicular
propagation channels, which are significantly different from those investigated
below 6 GHz. Despite their significance, very few investigations of mmWave
vehicular channels are reported in the literature. This work highlights the key
attributes of mmWave vehicular communication channels and surveys the recent
literature on channel characterization efforts in order to provide a gap
analysis and propose possible directions for future research.
