Enhancing Transmission Collision Detection for Distributed TDMA in Vehicular Networks

Abstract

The increasing number of road accidents has led to the evolution of vehicular ad hoc networks (VANETs), which allow vehicles and roadside infrastructure to continuously broadcast safety messages, including necessary information to avoid undesired events on the road. To support reliable broadcast of safety messages, distributed time division multiple access (D-TDMA) protocols are proposed for medium access control in VANETs. Existing D-TDMA protocols react to a transmission failure without distinguishing whether the failure comes from a transmission collision or from a poor radio channel condition, resulting in degraded performance. In this article, we present the importance of transmission failure differentiation due to a poor channel or due to a transmission collision for D-TDMA protocols in vehicular networks. We study the effects of such a transmission failure differentiation on the performance of a node when reserving a time slot to access the transmission channel. Furthermore, we propose a method for transmission failure differentiation, employing the concept of deep-learning techniques, for a node to decide whether to release or continue using its acquired time slot. The proposed method is based on the application of a Markov chain model to estimate the channel state when a transmission failure occurs. The Markov model parameters are dynamically updated by each node (i.e., vehicle or roadside unit) based on information included in the safety messages that are periodically received from neighboring nodes. In addition, from the D-TDMA protocol headers of received messages, a node approximately determines the error in estimating the channel state based on the proposed Markov model and then uses this channel estimation error to further improve subsequent channel state estimations. Through mathematical analysis, we show that transmission failure differentiation, or transmission collision detection, helps a node to efficiently reserve a time slot even with a large number of nodes contending for time slots. Furthermore, through extensive simulations in a highway scenario, we demonstrate that the proposed solution significantly improves the performance of D-TDMA protocols by reducing unnecessary contention on the available time slots, thus increasing the number of nodes having unique time slots for successful broadcast of safety messages.