Modified R-ALOHA: Broadcast MAC Protocol for Vehicular Ad hoc Networks

The primary objective of a Medium Access Control (MAC) layer in Vehicular Ad hoc Networks (VANETs) is reliable and rapid broadcast of safety messages. Fast mobility of vehicles, hidden terminal problem, reliability, and congestion control of broadcast channel are challenging issues in VANETs. This paper proposes an R-ALOHA-based channel access mechanism that renders a broadcast channel capable of broadcast failure detection while at the same time maintaining congestion control. The proposal employs a combination of CSMA and TDMA techniques to mitigate the hidden terminal problem and combat fast mobility of vehicles in VANETs. Performance comparison of preexisting TDMA protocols and the proposed protocol-both analytical and in simulation-is also provided. Simulation results validate analytical models and demonstrate significantly improved reliability and increased channel assignment speed to manage the fast mobility of vehicles.