The Throughput-Reliability Tradeoff in 802.11-Based Vehicular Safety Communications

There is a promising development in wireless communications technology which indicates that road accidents could be significantly reduced if vehicles were allowed to communicate with each other. By regularly exchanging their current position, velocity, etc., vehicles could predict an upcoming accident and alert the human drivers in time or proactively take precautionary actions to avoid the accident. The realization of this vision would require the design of wireless channel access schemes that can guarantee a high level of message reliability as well as a minimum throughput for vehicles to communicate frequently enough. In this paper, we propose a Markov chain model to analytically derive the throughput and reliability of the popular 802.11 protocol which has been adopted for vehicular communications. The model reveals the existence of a throughput-reliability tradeoff influenced by the contention window, the parameter which coordinates the access to the underlying wireless channel. By adjusting this window, it is possible to gain an extra level of reliability at the expense of some throughput. Through numerical experiments, we show that, for low to medium vehicular traffic density, it is possible to achieve a very high message reliability by trading off the "excess" throughput not required for vehicular communications. Our study also reveals that, the tradeoff capability of 802.11 diminishes with increasing traffic density. For extremely dense traffic, it may not be possible to meet the requirements of vehicular safety communications by simply adjusting the 802.11 parameters.