EAPRAD: A MAC protocol for Enhancing Access Probability and Reducing Access Delay in VANETs

Vehicular Ad Hoc Networks (VANETs) present challenges in terms of access collision and merge collision, leading to increased access delay in end-to-end vehicle communication. How to address these issues remains an unsolved problem. In this paper, we propose EAPRAD (Enhancing Access Probability and Reducing Access Delay), a novel MAC protocol based on a Markov model. Our protocol defines the number of vehicle accessing time slot failures, with different failure states forming a partial Markov chain. The highest failure status vehicle is prioritized to access the time slot, effectively resolving the problem of indefinite time slot reservations during the back-off phase. Furthermore, after acquiring the time slot, vehicles may still experience package collision, particularly when vehicles traveling in opposite directions share the same frequency band. To address this, our protocol introduces a new MAC frame structure that partitions available frequency band resources based on distinct driving directions, eliminating merge collision in opposite directions. For vehicles traveling in the same direction, intermediate nodes are utilized to proactively anticipate and prevent package collision. To model the state transition of access collision probability and merge, we develop a Continuous Time Markov Chain (CTMC) analytical model. Simulation results demonstrate that EAPRAD reduces end-to-end delay and beacon lost rate (BLR), thereby improving overall network performance in VANETs.