Understanding the impact of persistence and propagation on the Age of Information of broadcast traffic in 5G NR-V2X sidelink communications

The current Cellular V2X (C-V2X) standard for direct communication between vehicles is based on the so called
Semi-Persistent Scheduling (SPS) algorithm. It is based on the multiple access structure of 5G New Radio (NR)
and exploits sensing and persistence to realize a randomized multiple access. We consider the application
of SPS to support periodic broadcast traffic where no acknowledgments are provided. Persistence is a key
feature, which consists of a node using the same resource for its transmissions for multiple frames. The specific
resource used is randomly selected from among those that are perceived to be idle, and reselected anew after
a randomized number of frames. We define a model to gain insight into the interplay between persistence and
key performance metrics for the type of traffic considered, namely probability of successful delivery and Age
of Information (AoI). A core version of the model is validated against simulations and used to show that there
exists an optimal level that minimizes AoI. The model is then extended to account for a distance-dependent
propagation model, allowing further insight into the effects of the interplay between sender–receiver distance
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and persistence. Finally, an even more detailed model is investigated, using ns-3-based simulations. This further
analysis confirms the qualitative behaviors revealed by the analytical model and provides more insight into the
complex interactions of system parameters and channel characteristics. The obtained results help to identify
the limits of SPS, opening the way to system-principled parameter optimization and design of more powerful
variants of the multiple access scheme.