  Millimeter wave wireless spectrum deployments will allow vehicular
communications to share high data rate vehicular sensor data in real-time. The
highly directional nature of wireless links in millimeter spectral bands will
require continuous channel measurements to ensure the transmitter (TX) and
receiver (RX) beams are aligned to provide the best channel. Using real-world
vehicular mmWave measurement data at 28 GHz, we determine the optimal beam
sweeping period, i.e. the frequency of the channel measurements, to align the
RX beams to the best channel directions for maximizing the
vehicle-to-infrastructure (V2I) throughput. We show that in a realistic
vehicular traffic environment in Austin, TX, for a vehicle traveling at an
average speed of 10.5 mph, a beam sweeping period of 300 ms in future V2I
communication standards would maximize the V2I throughput, using a system of
four RX phased arrays that scanned the channel 360 degrees in the azimuth and
30 degrees above and below the boresight. We also investigate the impact of the
number of active RX chains controlling the steerable phased arrays on V2I
throughput. Reducing the number of RX chains controlling the phased arrays
helps reduce the cost of the vehicular mmWave hardware while multiple RX
chains, although more expensive, provide more robustness to beam direction
changes at the vehicle, allowing near maximum throughput over a wide range of
beam sweep periods. We show that the overhead of utilizing one RX chain instead
of four leads to a 10% drop in mean V2I throughput over six non-line-of-sight
runs in real traffic conditions, with each run being 10 to 20 seconds long over
a distance of 40 to 90 meters.
