  In this paper, we investigate the performance of ambient backscatter
communication non-orthogonal multiple access (AmBC-NOMA)-assisted short packet
communication for high-mobility vehicle-to-everything transmissions. In the
proposed system, a roadside unit (RSU) transmits a superimposed signal to a
typical NOMA user pair. Simultaneously, the backscatter device (BD) transmits
its own signal towards the user pair by reflecting and modulating the RSU's
superimposed signals. Due to vehicles' mobility, we consider realistic
assumptions of time-selective fading and channel estimation errors. Theoretical
expressions for the average block error rates (BLERs) of both users are
derived. Furthermore, analysis and insights on transmit signal-to-noise ratio,
vehicles' mobility, imperfect channel estimation, the reflection efficiency at
the BD, and blocklength are provided. Numerical results validate the
theoretical findings and reveal that the AmBC-NOMA system outperforms its
orthogonal multiple access counterpart in terms of BLER performance.
