The TDMA-based Medium Access Control (MAC) protocol has been recently proposed as a promising solution to well support delay-sensitive safety applications in vehicular ad hoc networks (VANETs), since a time-slotted access scheme ensures the transmission within the ultra-low delays. However, due to the varying vehicle mobilities, existing TDMA-based MAC protocols result in collisions of slot assignment when multiple sets of vehicles move together. To avoid slot-assignment collisions, in this paper, we propose a mobility-aware TDMA MAC, named as MoMAC, which can assign every vehicle a time slot according to the underlying road topology and lane distribution on roads with the consideration of vehicles' mobilities. In MoMAC, different lanes on the same road segment and different road segments at intersections are associated with disjoint time slot sets. In addition, each vehicle broadcasts safety messages together with the time slot occupying information of neighboring vehicles; by updating time slot occupying information of two-hop neighbors (obtained indirectly from one-hop neighbors), vehicles can detect time slot collisions and access a vacant time slot in a fully distributed way. We demonstrate the efficiency of MoMAC through theoretical analysis and extensive simulations; compared with state-of-the-art TDMA MACs, the transmission collisions can be reduced by 59.2%, and the rate of safety message transmissions/receptions can be greatly enhanced.

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