MAC Trade-offs Between Age and Reachability of Information in Vehicular Safety Applications

Vehicular networking offers the promise of greatly improving transportation safety but has stringent requirements on information age as well as information reachability, where the later refers to the range over which information is propagated. We consider an idealized model of a one-dimensional vehicular networks and show that there is a basic trade-off between these two metrics: a smaller age can be obtained by reducing the reachability of information. We apply this to two current technologies: Cellular V2X (C-V2X) and Dedicated Short Range Communication (DSRC) and derive an equation that characterizes the trade-off between these two metrics for both technologies. In the case of exponential path loss and negligible noise, this relationship becomes a fixed invariant ratio. Given this relationship, under high congestion, these two protocols tradeoff these metrics differently. C-V2X tends to achieve a smaller age while DSRC tends to maintain a larger reachability. The idealized model is also applied to analyze the steady state of rate control and power control mechanisms such as those in the SAE standard J2945/1. We show that the ratio of age and reachability is still governed by the same trade-off curve: rate control tries to maintain a large reachability, while power control helps improve the age.