Interoperability between Deterministic and Non-Deterministic Vehicular Communications over DSRC/802.11p

In recent works, a priority-aware deterministic access protocol that is based on 802.11p/DSRC was introduced to allow vehicles to access the shared medium in collision-free periods. The VANET Deterministic Access (VDA) protocol as introduced in [8] has no mechanism that prevents a non VDA-enabled vehicle from accessing the channel in a scheduled VDA opportunity (VDAOP). A non VDA-enabled vehicle, i.e. a vehicle not configured with the optional VDA capability over 802.11p, may start transmitting on the shared channel just before or during the VDAOPs reserved for vehicles with VDA capabilities. Also, non VDA-enabled vehicles may be prevented from accessing the shared channel due to the transmission of VDA-enabled vehicles during their respective VDAOPs with a higher priority (shorter AIFS). In this work, we propose a new enhanced VDA scheme, called EVDA that avoids the above issues and prevents interfering transmissions from VDA-enabled vehicles and non VDA-enabled vehicles. We also analyzed the impact of several design tradeoffs between the Contention Free Period (CFP)/Contention Period (CP) Dwell-time ratios on the performance of safety applications with different priorities with EVDA. Simulations show that the proposed scheme clearly outperforms the VDA scheme in high communications density conditions while bounding the transmission delay of safety messages and increasing the packet reception rate.