  To overcome the limitations of DSRC with short range, non-supportability of
high density networks, unreliable broadcast services, signal congestion and
connectivity disruptions, cellular vehicle-to-everything (C-V2X) communication
networks, standardized in 3rd Generation Partnership Project (3GPP) Release 14,
have been recently introduced to cover broader vehicular communication
scenarios including vehicle-to-vehicle (V2V), vehicle-to-pedestrian (V2P) and
vehicle-to-infrastructure/network (V2I/N). In C-V2X, vehicles can directly
communicate over PC5 based dedicated sidelinks called direct mode or V2V
communication. However, high vehicle densities may require reuse of cellular
spectrum for V2V. Moreover, infrastructure mode communication through V2I/N
links can augment V2V communication by enhancing communication range and
reliability for enhanced safety along with consistent performance under traffic
congestions. Motivated by the stringent connection reliability, spectral
efficiency, and coverage requirements in C-V2X, this paper presents the first
comprehensive and tractable analytical framework for performance of C-V2X
networks over shared V2V and cellular uplink channels, where the transmitting
vehicles can deliver their information via infrastructure or direct mode, based
on their distances, propagation environments and the bias factor. By
practically modeling the vehicles on the roads using the doubly stochastic Cox
process and the base-stations, we derive new association probabilities, new
success probabilities of infrastructure and direct mode, and overall success
probability of the C-V2X communication over shared channels, which are
validated by the simulations results. Our results reveal the benefits of our
proposed model (possibility of selecting both direct and infrastructure modes
over shared channels) compared to V2V network in terms of success probability.
