A novel reliable broadcast protocol for VANET's safety applications

In IEEE1609.4/IEEE 802.11p for vehicular ad hoc network (VANET), periodic broadcasting of status messages on the control channel (CCH) can realize collision avoidance, accident early warning and many other safety applications, which has attracted a great deal of attention in recent years. However, because IEEE 802.11p broadcast mode for VANETS has no acknowledgement mechanism and its contention window (CW) cannot be adjusted with vehicles density, collision probability is high and broadcast success rate is low. For this reason, a novel reliable broadcast protocol is designed, which consists of three kinds of mechanisms: Geographical-Location-Based Asynchronous Time Division Multiplexing (GLATDM), Chanel-State-Feedback-Based Back-off (CSFB) and Adaptive Dynamic Contention Window (ADCW). GLATDM is proposed to avoid hidden nodes problem. CSFB mechanism can assure vehicles know whether their broadcast packets have been set successfully or not and when to retransmit. Following by ADCW scheme, vehicles can adjust their CW size based on the vehicles number. An analytical model has also been built. Simulation results validated the accuracy of the model and effectiveness of the protocol. The proposal can assure vehicles have higher broadcast success rate, lower collision rate and average access delay. Besides, the CCH occupancy rate is lower, which can save more channel resource for other services.