Research On IEEE802.11p Protocol Back-off Algorithm

VANET or Mobile AdHoc, as a new way to solve various traffic problems, Internet of Vehicles,has got deep development in recent years. As a basic application, safety application is mainly usedto solve the traffic accidents, provide traffic warning, ITS problems and so on. However, safetyapplication calls for high broadcast reception rate and low average end-to-end delay, traditionalbinary exponential back off algorithm in IEEE802.11p protocol has its limitation, its performancecan’t satisfy the demand of safety application. Therefore, it is meaningful to deeply study the backoff algorithm during the process of medium access in VANET.Firstly, aiming at the problem that the broadcast reception rates in traditional back off algorithmare too low, the relationship between collision probability and minimum contention window size,expiration probability and minimum contention window size of beacons are deduced throughbuilding a Markov model. The conclusion is: optimum contention window size should guaranteethat the two probability values are equal, so that the best broadcast reception rate can be guaranteed.Secondly, aiming at the problems that in traditional back off algorithms of VANET, the reasoncalls for retreat is only a single factor, and the magnitude is too large in every retreat. A new backoff algorithm called Density-Speed-Priority Collision Expiration Back off (DSPCEB) is proposedon the basis of probability predication. The algorithm is divided into two parts:(1) Consider asingle vehicle, a single kind of message, the contention window size is adjusted according to therelative value between collision probability and expiration probability;(2) Consider real scenariosof VANET, density, speed and messages’ categories are taken into consideration to optimize retreattimes and equalization of channel access.Thirdly, the algorithm proposed in this paper is simulated by using VanetMobiSim and NS2. Thesimulation results show that the performance of DSPCEB algorithm is better than existed back offalgorithms, and the fairness of the channel access can be guaranteed, emergency broadcastingmessages can be sent out immediately on the basis that the beacon messages’ performance isguaranteed.