PTMAC: A Prediction-Based TDMA MAC Protocol for Reducing Packet Collisions in VANET

With the rapid development of wireless communications, vehicular ad hoc networks ( VANETs) have recently attracted great attention. Although IEEE 802.11p has been approved as the standard medium access control ( MAC) protocol for vehicle-to-vehicle communications, its contention-based nature and inability to handle hidden-terminal problems may incur high packet collision probability under high-traffic-density situations. To overcome the shortcoming of IEEE 802.11p, time-division multiple-access ( TDMA)-based protocols are proposed. However, packet collisions can still occur due to contention or multiple vehicles using the same slot while approaching each other, i.e., encounter collisions, particularly in two-way traffic roads. Some proposed remedying the encounter collisions for two-way traffic by partitioning a frame into two sets: one for the traffic in each direction. However, these proposed protocols are harder to adapt to the uneven traffic loads on both directions and cannot solve the problem of four-way intersections. In this paper, we propose a new TDMA protocol called prediction-based TDMA MAC ( PTMAC) based on a novel way of predicting encounter collisions and effectively reducing the number of collisions. To the best of our knowledge, PTMAC is the first protocol that is designed for both two-way traffic and four-way intersections. It has shown that, based on this predictability, the encounter collisions can be greatly reduced in both two-way traffic and four-way intersections, regardless of the traffic loads on different road segments.