A Clustering-Based Contention-Free MAC Protocol for Vehicular Ad Hoc Networks

Contention-based medium access control (MAC) protocols for vehicular ad hoc networks (VANETs), such as IEEE 802.11p, have been revealed inefficient performance due to merging collisions, hidden terminal problems, poor resource utilization, and inconsistency in various traffic scenarios. This paper proposes a novel clustering-based contention-free MAC (CC-MAC) protocol to handle intra-cluster and inter-cluster transmission collision problems in VANETs. A cluster head (CH) assigns the time slots to the cluster members depending on their traffic demands. Inter-cluster interference among vehicles is minimized by comparing the relative offset time and reducing access delay during the slot allocation. Furthermore, a TDMA-based predetermined scheme is proposed to improve resource utilization for both control channel and service channels, and merging collisions are resolved by disjointed slot assignment. Hidden terminal problem is resolved by detecting slot assignment identification on gateway vehicles regarding the direction of the vehicle's mobility. Simulation results show that the proposed protocol outperforms the IEEE 802.11p, STCM, and optimized CB-MAC protocols in both highway and urban traffic scenarios in terms of normalized throughput, packet delivery ratio and average end-to-end delay.