On the Influence of Obstacle Shadowing in Evaluating VANET Safety Performance

Wireless communication between vehicles enables both safety applications, such as accident avoidance, and nonsafety applications, such as traffic congestion alerts [1] with the intent of improving safety in driving conditions. Because the high costs of implementing limited test-bed environments constrain prototype testing, VANET researchers often turn instead to simulation toolsets from which a rich set of environmental scenarios are modeled. However, despite the availability of such tools, results are inconsistent. While VANET simulations allow investigators to select from a wide range of models for radio wave propagation, fading, and shadowing, they often fail to consider realistic road topologies and the presence of obstacles [2]. Safety performance assessments improve when models accurately reflect environmental conditions such as the fading effects of radio wave propagation through buildings and other obstacles. The goal of this research is to show quantitatively how accurate, deterministic obstacle fading models impact the performance assessment of VANET safety applications. An obstacle model and a fading model that uses it, along with a VANET simulation script, have been implemented and contributed to the open source ns-3 network simulation community. Results from simulation experiments that use these models show that deterministic obstacle fading greatly degrades Packet Delivery Ratio (PDR) and compares differently than stochastic fading models, such as Nakagami-m. Such networklevel results can have dramatic impacts to safety performance assessment, such as awareness range. By considering the number of packets and time tolerance window, awareness probability and communications awareness range [3] can be used to assess application reliability, thereby providing comparative insight among safety application performance. Failing to account for the effects of obstacles in safety assessment can therefore inaccurately or even greatly overstate the performance of VANET safety applications. Including realistic obstacle fading in VANET simulation modeling improves VANET assessment and strengthens safety, thus supporting one of the primary goals of connected vehicle systems. Keywords— VANET; obstacles; propagation loss; fading; shadowing; simulation; ns-3