Direction matters: A decentralized direction-based TDMA scheduling strategy for VANET

The upcoming vehicular communication standard IEEE 802.11p adopts a Carrier Sense Multiple Access (CSMA) based MAC protocol, in which the mobile nodes listen to the wireless channel before transmission. In this paper, we show the disadvantages of the CSMA for Vehicular Ad Hoc Network (VANET): 1) lack of the assurance of the delivery delay for the emergent safety messages; 2) poor performance on periodically broadcasting routine safety messages. A real-time MAC protocol is demanded for VANET. Some recent studies suggest designing the protocol based on Time Division Multiple Access (TDMA). The major challenge of designing a TDMA based MAC protocol for VANET with high mobility is to provide efficiently decentralized scheduling. For TDMA, if a node occupies a slot, it will keep the slot until detecting a collision. Thus, by reducing the chance of the encountered nodes occupying the same slot, the efficiency of TDMA can be significantly improved. The vehicular traffic trace shows that in VANET, the majority of the encounters happen in the opposite direction rather than the same direction. According to our theoretical analysis and simulations, we demonstrate that the ratio of the number of encounters from the opposite direction to the total number of encounters exceeds 80%. We further propose an innovative direction-based TDMA scheduling strategy by exploiting this property to cope with fast-changing vehicular topology. Through extensive simulations, we show that the proposed strategy can reduce over 80% of collisions with an acceptable overhead.