Adaptive Contention Window Design to Minimize Synchronous Collisions in 802.11p Networks

The vehicular ad hoc network (VANET) capable of wireless communication will enhance traffic safety and efficiency. The IEEE 802.11p standards for wireless communication in the US and Europe use a single shared channel for the periodic broadcast of safety messages. Coupled with the short contention window and inflexibility in window size adaptation, the synchronous collisions of periodic messages are inevitable in a large scale intelligent transportation system (ITS). To this end, we propose an adaptive contention window design to reduce synchronous collisions of periodic messages. The proposed design replaces the aggressive window selection behaviour in the post transmit phase of IEEE 802.11p with a weighted window selection approach after a successful transmission. The design relies on the local channel state information to vary contention window size. Moreover, in high density networks, the design gives prioritized channel access to vehicles experiencing dropped beacons. The proposed design can be readily incorporated into the IEEE 802.11p standard. The discrete-event simulations show that synchronous collisions can be reduced significantly to achieve higher message reception rates as compared to the IEEE 802.11p standard.