A Tone-Based Time-Slotted Protocol for Multi-Hop Emergency Message Dissemination in VANETs

To notify all vehicles within a geographical area of an urgent situation, multi-hop communication is employed in a vehicular ad hoc network (VANET) to propagate the emergency warning messages. While being interfered by the existing periodic single-hop safety messages, the transmission of warning messages can also be strongly affected by the broadcast storm problem and the collisions due to hidden nodes. This paper develops an efficient tone-based time-slotted multi-hop protocol that significantly improves the performance of the emergency warning message transmissions. Specifically, we assign the responsibility of message forwarding to only a subset of vehicles on the road, known as ``segment leader," as part of our broadcast storm suppression strategy. To avoid the potential interference caused by the safety messages, we employ separate multi-hop time slots for the warning messages, which are reserved by a newly-proposed out-of-band long range CLEAR tone. Such a tone-based reservation mechanism allows the continuous transmission of the safety messages in the absence of a CLEAR tone, giving rise to a much higher network utilization. Our extensive simulation results confirm the timely and reliable delivery of the emergency warning messages while the required number of multi-hop transmissions is significantly reduced. At the same time, a high packet reception rate and a low end-to-end delay are maintained for the existing single-hop safety messages.