  To deal with the explosive growth of mobile traffic, millimeter-wave (mmWave)
communications with abundant bandwidth resources have been applied to vehicular
networks. As mmWave signal is sensitive to blockage, we introduce the unmanned
aerial vehicle (UAV)-aided two-way relaying system for vehicular connection
enhancement and coverage expansion. How to improve transmission efficiency and
to reduce latency time in such a dynamic scenario is a challenging problem. In
this paper, we formulate the joint optimization problem of relay selection and
transmission scheduling, aiming to reduce transmission time while meeting the
throughput requirements. To solve this problem, two schemes are proposed. The
first one is the random relay selection with concurrent scheduling (RCS), a
low-complexity algorithm implemented in two steps. The second one is the joint
relay selection with dynamic scheduling (JRDS), which fully avoids relay
contentions and exploits potential concurrent ability, to obtain further
performance enhancement over RCS. Through extensive simulations under different
environments with various flow numbers and vehicle speeds, we demonstrate that
both RCS and JRDS schemes outperform the existing schemes significantly in
terms of transmission time and network throughput. We also analyze the impact
of threshold selection on achievable performance.
