  Millimeter wave (mmWave) vehicular channels are highly dynamic, and the
communication link needs to be reconfigured frequently. In this work, we
propose to use a passive radar receiver at the roadside unit to reduce the
training overhead of establishing an mmWave communication link. Specifically,
the passive radar will tap the transmissions from the automotive radars of the
vehicles on the road. The spatial covariance of the received radar signals will
be estimated and used to establish the communication link. We propose a
simplified radar receiver that does not require the transmitted waveform as a
reference. To leverage the radar information for beamforming, the radar azimuth
power spectrum (APS) and the communication APS should be similar. We outline a
radar covariance correction strategy to increase the similarity between the
radar and communication APS. We also propose a metric to compare the similarity
of the radar and communication APS that has a connection with the achievable
rate. We present simulation results based on ray-tracing data. The results show
that: (i) covariance correction improves the similarity of radar and
communication APS, and (ii) the radar-assisted strategy significantly reduces
the training overhead, being particularly useful in non-line-of-sight
scenarios.
