MIRA-EDCA: Multi-Indicator Responsive Adaptation of the Contention Window for Priority-Aware Safety Broadcasting in IEEE 802.11p Vehicular Networks

Reliable vehicular safety broadcasting requires channel access that limits collisions without imposing excessive delay on urgent messages. However, fixed contention windows and single-indicator adaptation do not fully represent the combined effects of contender population, channel occupancy, and repeated backoff interruption. This study proposes Multi-Indicator Responsive Adaptation integrated with Enhanced Distributed Channel Access (MIRA-EDCA), a deterministic contention-control mechanism that separates potential contention through a normalized contender indicator, realized channel occupancy through the Channel Busy Ratio (CBR), and node-level backoff interruption through the Backoff Freeze Ratio (BFR), and combines them through a bounded contention-pressure function. The measure drives class-specific contention-window updates through discrete levels and a one-level-per-epoch transition rule while preserving priority differentiation between event-driven and periodic safety traffic. MIRA-EDCA was evaluated in a controlled slot-level IEEE 802.11p simulation over 30 density-traffic operating points formed by ten vehicle-population levels and three event-load levels, with 30 Monte Carlo realizations per operating point. Relative to Standard EDCA, MIRA-EDCA reduced collision probability by 31.92% while increasing conditional mean event-message MAC access delay by 0.66%; for vehicle populations of 100 or more, collision reduction reached 33.40% with a delay increase below 1%. Under the implemented common-environment proxy comparisons, MIRA-EDCA exhibited an 88.6% lower conditional mean event-message MAC access delay than the Dynamic Contention Window common-environment proxy (DYCW-2023-CE) and a 31.7% lower conditional mean MAC access delay than MA-DRL-2026-CE-PS. The corresponding transmitter-side SBR difference relative to DYCW-2023-CE was +0.899 percentage points. These comparisons refer only to the proxy controllers implemented under the matched assumptions of this study and not to the complete source protocols. These results support the proposed MAC-layer contention-control logic within the evaluated IEEE 802.11p common-contention-domain abstraction; they characterize collision-access-delay behavior at the modeled MAC layer and do not constitute receiver-side or end-to-end reliability validation.