  Recent advancements in V2X communications have greatly increased the
flexibility of the physical and medium access control (MAC) layers. This
increases the complexity when investigating the system from a network
perspective to evaluate the performance of the supported applications. Such
flexibility needs in fact to be taken into account through a cross-layer
approach, which might lead to challenging evaluation processes. As an accurate
simulation of the signals appears unfeasible, a typical solution is to rely on
simple models for incorporating the physical layer of the supported
technologies, based on off-line measurements or accurate link-level
simulations. Such data is however limited to a subset of possible
configurations and extending them to others is costly when not even impossible.
The goal of this paper is to develop a new approach for modelling the physical
layer of vehicle-to-everything (V2X) communications that can be extended to a
wide range of configurations without leading to extensive measurement or
simulation campaign at the link layer. In particular, given a scenario and
starting from results in terms of packet error rate (PER) vs.
signal-to-interference-plus-noise ratio (SINR) related to a subset of possible
configurations, we derive one parameter, called implementation loss, that is
then used to evaluate the network performance under any configuration in the
same scenario. The proposed methodology, leading to a good trade-off among
complexity, generality, and accuracy of the performance evaluation process, has
been validated through extensive simulations with both IEEE 802.11p and LTE-V2X
sidelink technologies in various scenarios.
