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Millimeter wave (mmWave) has great potential in realizing high data ratethanks to the large spectral channels. It is considered as a key technology forthe fifth generation wireless networks and is already used in wireless LAN(e.g., IEEE 802.11ad). Using mmWave for vehicular communications, however, isoften viewed with some skepticism due to a misconception that the Dopplerspread would become too large at these high frequencies. This is not true whendirectional beam is employed for communications. In this paper, closed formexpressions relating the channel coherence time and beamwidth are derived.Unlike prior work that assumed perfect beam pointing, the pointing error due tothe receiver motion is incorporated to show that there exists a non-zerooptimal beamwidth that maximizes the coherence time. To investigate themobility effect on the beam alignment which is an important feature in mmWavesystems, a novel concept of beam coherence time is defined. The beam coherencetime, which is an effective measure of beam alignment frequency, is shown to bemuch larger than the conventional channel coherence time and thus results inreduced beam alignment overhead. Using the derived correlation function, thechannel coherence time, and the beam coherence time, an overall performancemetric considering both the channel time-variation and the beam alignmentoverhead is derived. Using this metric, it is shown that beam alignment inevery beam coherence time performs better than the beam alignment in everychannel coherence time due to the large overhead for the latter case.

The Impact of Beamwidth on Temporal Channel Variation in Vehicular Channels and its Implications