Basic Relationship between Channel Coherence Time and Beamwidth in Vehicular Channels

The availability of large bandwidth at millimeter wave (mmWave) frequencies makes mmWave an ideal candidate to realize multi-Gbps data rates. MmWave has been applied to indoor applications (e.g. IEEE 802.11ad), and it is also being considered for cellular. There is not much work, however, in applying mmWave to vehicular channels. This is likely due to the misconception that the Doppler spread will be too high at small mmWave wavelengths, which is true only for omnidirectional communication. In this work, closed form approximate expressions relating the coherence time and beamwidth are derived taking directional communication into account. The result shows that the coherence time increases at least proportional to the inverse of the beamwidth. The derivation also takes the beam misalignment due to receiver movement into account and it can be shown that the optimal beamwidth is non-zero in contrast to models that assume perfect beam pointing.