Medium Access Control Protocol Design for Vehicle–Vehicle Safety Messages

We propose a medium access control protocol design for a vehicle to send safety messages to other vehicles. We develop a Quality-of-Service (QoS) model for safety messages that are consistent with the active vehicle safety systems literature. Each message has a range and useful lifetime. The QoS target has each message received with high probability within its specified lifetime by each vehicle within its specified range. The protocol design is based on rapidly rebroadcasting each message multiple times within its lifetime in combination with the 802.11 Distributed Coordination Function. This makes the design compatible with emerging standards for Dedicated Short-Range Communication. Six different design variations are proposed. We derive equations and develop a simulation tool to assess the performance of the designs. Using these, we identify the best and most easily implemented designs. Design performance depends on the number of rebroadcasts, power, modulation, coding, and vehicular traffic volumes. We show that under certain assumptions on the loss probability tolerated by safety applications, the design is able to transport safety messages in vehicular ad hoc networks.