  Real-time status update in future vehicular networks is vital to enable
control-level cooperative autonomous driving. Cellular Vehicle-to-Everything
(C-V2X), as one of the most promising vehicular wireless technologies, adopts a
Semi-Persistent Scheduling (SPS) based Medium-Access-Control (MAC) layer
protocol for its sidelink communications. Despite the recent and ongoing
efforts to optimize SPS, very few work has considered the status update
performance of SPS. In this paper, Age of Information (AoI) is first leveraged
to evaluate the MAC layer performance of C-V2X sidelink. Critical issues of
SPS, i.e., persistent packet collisions and Half-Duplex (HD) effects, are
identified to hinder its AoI performance. Therefore, a piggyback-based
collaboration method is proposed accordingly, whereby vehicles collaborate to
inform each other of potential collisions and collectively afford HD errors,
while entailing only a small signaling overhead. Closed-form AoI performance is
derived for the proposed scheme, optimal configurations for key parameters are
hence calculated, and the convergence property is proved for decentralized
implementation. Simulation results show that compared with the standardized SPS
and its state-of-the-art enhancement schemes, the proposed scheme shows
significantly better performance, not only in terms of AoI, but also of
conventional metrics such as transmission reliability.
