Adaptive Sliding Contention Window Design to Minimize Safe Message Collision Rates with Different Priority Levels in VANET

In Vehicular Ad Hoc Networks (VANETs), the broadcast is an important form of communication and occupies the main traffic in the network. However, since there is no recovery for broadcast frames in the VANET network, the collision rate between safe message traffic can become very high, especially in vehicle-dense network conditions. This paper proposes a new coordination mechanism that adaptively controls the Contention Window (CW) size for broadcast to reduce the safe message collision rate with different priority levels. In our mechanism, each vehicle in the VANET can automatically adjust the CW based on the perception of the current network condition by analyzing the percentage of successfully received frames. The algorithm controls the CW size by sliding the window with a dynamic persistence factor according to each type of safe message traffic. Each data traffic chooses a backoff timer dynamically varying in the range [0, CW[AC[i]]], which can overlap the CW range with other data traffic to improve efficient bandwidth depending on the network conditions. Simulation results prove that the proposed mechanism significantly reduces the collision rate for both safe message traffic in high-priority and low-priority in vehicle-dense network conditions.