  Due to high mobility in multipath propagation environments,
vehicle-to-vehicle (V2V) channels are generally time and frequency variant.
Therefore, the criteria for wide-sense stationarity (WSS) and uncorrelated
scattering (US) are just satisfied over very limited intervals in the time and
frequency domains, respectively. We test the validity of these criteria in
measured vehicular 60 GHz millimeter wave (mmWave) channels, by estimating the
local scattering functions (LSFs) from the measured data. Based on the
variation of the LSFs, we define time-frequency stationarity regions, over
which the WSSUS assumption is assumed to be fulfilled approximately. We analyze
and compare both line-of-sight (LOS) and non-LOS (NLOS) V2V communication
conditions. We observe large stationarity regions for channels with a dominant
LOS connection, without relative movement between the transmitting and
receiving vehicle. In the same measured urban driving scenario, modified by
eliminating the LOS component in the post-processing, the channel is dominated
by specular components reflected from an overpassing vehicle with a relative
velocity of 56 km/h. Here, we observe a stationarity bandwidth of 270 MHz.
Furthermore, the NLOS channel, dominated by a single strong specular component,
shows a relatively large average stationarity time of 16 ms, while the
stationarity time for the channel with a rich multipath profile is much
shorter, in the order of 5 ms.
