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

Time Division Multiple Access (TDMA)-based Medium Access Control (MAC) protocols are widely regarded as a pivotal solution for enhancing road safety through reliable broadcast services in Vehicular Ad Hoc Networks (VANETs). Within centralized architectures, ensuring cluster stability remains a paramount challenge for protocol efficacy. While a recent Cluster-based Contention-free MAC (CC-MAC) protocol was proposed to mitigate intra- and inter-cluster transmission collisions, its performance is inherently sensitive to the underlying assumptions of radio propagation and vehicle mobility. This paper addresses these limitations by proposing an Optimized Clustering-based Contention-free MAC (OCC-MAC) protocol. The primary contribution is the development of a robust and well-balanced clustering framework designed for highly dynamic vehicular environments. First, we integrate realistic vehicular mobility and Nakagami-m fading channel models to accurately reflect the physical layer dynamics of VANET communications. Second, we introduce a novel metaheuristic clustering optimization algorithm, which employs an encirclement marching strategy for Cluster Head (CH) selection. This approach incorporates efficient CH dissolution and cluster merging mechanisms to minimize unnecessary re-affiliations and significantly enhance overall cluster stability. The protocol is rigorously evaluated through simulations conducted on real-world road maps, incorporating pragmatic vehicular traffic scenarios and channel characteristics. Comparative simulation results for both highway and urban traffic scenarios demonstrate that the proposed OCC-MAC protocol outperforms existing benchmarks, achieving superior normalized throughput, packet delivery ratio, access delay, end-to-end delay, and collision rate.