A new adapted back-off scheme for broadcasting on IEEE 1609.4 control channel in VANET

In this paper, we propose a novel MAC scheme for broadcasting in IEEE 802.11p/WAVE standard during the CCH periods. We address two problems which lead to the same undesired result: the degradation in control channel performance. The first problem is related to the congestion control and the challenge is viewed in the random backoff mechanism to be adapted for broadcasting messages. It has been evident that the exponential back-off is not efficient in broadcast and where there are beacons. The problem becomes more challenging when these beacons have an expiry time and they are dropped once this time is reached before transmission. When the vehicle density is high, the probability to have two or more vehicles having the same random back-off increases and results in an increase of collision probability. One solution is in incrementing the Contention Window (CW) but if the CW is increased, the waiting time before transmitting increases and it results in the rise of the packet drop rate due to expiry time. The absence of acknowledgement for broadcast messages makes the CW adaptation very difficult. Our proposed scheme addresses this problem. This paper also covers the issue of channel congestion phenomenon following a channel switch (from SCH to CCH) mentioned in Annex B of the IEEE Std 1609.4-2010. Just after a channel switch, a large number of WAVE devices are found in a period of above-average channel congestion resulting in an unexpectedly high collision rate. Our scheme presents a MAC layer approach used by the devices to avoid the “start-of-interval contention”, especially in a high vehicle density. Our simulation results show that using a modified backoff procedure after spreading the density of transmissions on the CCH interval is an effective solution to decrease the collision probability and increases the packet reception rate. The spread is based on geographical positions. Moreover, the modified backoff, instead of freezing the counter when th- channel is occupied, it still decreases it.