  Integrated sensing and communication (ISAC) has become a promising technology
for future communication system. In this paper, we consider a millimeter wave
system over high mobility scenario, and propose a novel simultaneous
transmission and reflection reconfigurable intelligent surface (STAR-RIS) aided
ISAC scheme. To improve the communication service of the in-vehicle user
equipment (UE) and simultaneously track and sense the vehicle with the help of
nearby roadside units (RSUs), a STAR-RIS is equipped on the outside surface of
the vehicle. Firstly, an efficient transmission structure is developed, where a
number of training sequences with orthogonal precoders and combiners are
respectively utilized at BS and RSUs for channel parameter extraction. Then,
the near-field static channel model between the STAR-RIS and in-vehicle UE as
well as the far-field time-frequency selective BS-RIS-RSUs channel model are
characterized. By utilizing the multidimensional orthogonal matching pursuit
(MOMP) algorithm, the cascaded channel parameters of the BS-RIS-RSUs links can
be obtained at the RSUs. Thus, the vehicle localization and its velocity
measurement can be acquired by jointly utilizing these extracted cascaded
channel parameters of all RSUs. Note that the MOMP algorithm can be further
utilized to extract the channel parameters of the BS-RIS-UE link for
communication. With the help of sensing results, the phase shifts of the
STAR-RIS are delicately designed, which can significantly improve the received
signal strength for both the RSUs and the in-vehicle UE, and can finally
enhance the sensing and communication performance. Moreover, the trade-off for
sensing and communication is designed by optimizing the energy splitting
factors of the STAR-RIS. Finally, simulation results are provided to validate
the feasibility and effectiveness of our proposed STAR-RIS aided ISAC scheme.
