  Vehicular communication channels are characterized by a non-stationary time-
and frequency-selective fading process due to fast changes in the environment.
We characterize the distribution of the envelope of the first delay bin in
vehicle-to-vehicle channels by means of its Rician $K$-factor. We analyze the
time-frequency variability of this channel parameter using vehicular channel
measurements at 5.6 GHz with a bandwidth of 240 MHz for safety-relevant
scenarios in intelligent transportation systems (ITS). This data enables a
frequency-variability analysis from an IEEE 802.11p system point of view, which
uses 10 MHz channels. We show that the small-scale fading of the envelope of
the first delay bin is Ricean distributed with a varying $K$-factor. The later
delay bins are Rayleigh distributed. We demonstrate that the $K$-factor cannot
be assumed to be constant in time and frequency. The causes of these variations
are the frequency-varying antenna radiation patterns as well as the
time-varying number of active scatterers, and the effects of vegetation. We
also present a simple but accurate bi-modal Gaussian mixture model, that allows
to capture the $K$-factor variability in time for safety-relevant ITS
scenarios.
