Optimised CSMA/CA protocol for safety messages in vehicular ad-hoc networks

Vehicular ad-hoc networks (VANETs) that enable communication among vehicles have recently attracted significant interest from researchers, due to the range of practical applications they can facilitate, particularly related to road safety. Despite the stringent performance requirements for such applications, the IEEE 802.11p standard still uses the carrier sensing medium access/collision avoidance (CSMA/CA) protocol. The latter when used in broadcast fashion employs a randomly selected backoff period from a fixed contention window (CW) range, which can cause performance degradation as a result of vehicular density changes. Concerns regarding the robustness and adaptiveness of protocols to support time-critical applications have been raised, which motivate this work. This paper investigates how the maximum CW size can be optimised to enhance performance based on vehicular density. A stochastic model is developed to obtain the optimal maximum CW that can be integrated in an amended CSMA/CA protocol to maximise the single-hop throughput among adjacent vehicles. Simulations confirm our optimised protocol can greatly improve the channel throughput and transmission delay performance, when compared to the standardised CSMA/CA, to support safety application in VANETs.