Improving Safety Messages Dissemination in IEEE 802.11e Based VANETs Using Controlled Repetition Technique Ghazal Farrokhi, Saadan Zokaei and Ebrahim Ghazisaeedi Electrical and Computer Engineering Department K. N. Toosi University of Technology Tehran, Iran ghfarrokhi@ee.kntu.ac.ir, szokaei@eetd.kntu.ac.ir, eghazisaeedi@ieee.org Mehdi Khabazian INRS-EMT University of Quebec Montreal, QC, Canada khaba@emt.inrs.ca Abstract— Vehicular ad hoc networks (VANETs) have been proposed to provide safety and non-safety services for the passengers on board. Reliability in broadcasting of safety messages is one of the fundamental challenges in the design of such networks. In this paper, we study the impact of employment of repetition and piggybacking techniques on the performance of safety messages dissemination in an IEEE 802.11e-based VANET. The adequacy of the proposed techniques has been evaluated by a simulation. The results of this simulation show that the performance of VANET improves by setting proper parameters and the number of repetitions per priority class. Further, controlling the number of repetitions by piggyback technique can highly improve the performance of VANET for broadcasting of safety messages. Keywords- VANETs; Safety messages; IEEE 802.11e; Repetition Technique; Piggyback Technique (Controlled Repetition Communication). I. INTRODUCTION Vehicular ad hoc Networks are forms of mobile ad hoc network, which provide communications among vehicles or between vehicles and fixed roadside equipments. VANETs are equipped with short-range to medium-range communication systems and their prospective applications include safety and comfort applications which share the wireless channel with mobile applications from a large, decentralized array of service providers. Examples of vehicular safety applications include collision avoidance, lane changing assistant and other safety warning services. Non-safety applications include traffic congestion avoidance, alternative route proposal, high-speed tolling and mobile infotainment. Due to the VANETs unique characteristics, such as scalability, high robustness expectation, strict delay requirements and security issues, the design of such a technology becomes an extraordinary challenge for the wireless research community. As a standardization approach, Dedicated short range communication (DSRC) standard licensed 75 MHz of frequency spectrum at 5.9 GHz band to support low-latency wireless data communications among vehicles and from vehicle to roadside units in USA [1-3]. Essentially, DSRC radio technology is based on IEEE802.11a which is adjusted for low overhead operations in DSRC spectrum and is being standardized as IEEE802.11p [2-4]. Due to the highly dynamic topology and the stringent delay requirements, safety applications usually need a one hop ad hoc communication [5]. In the following, we review some of the available studies on MAC layer which tackle the reliability issues in the message disseminations in VANET. In [6], the authors propose several single-hop broadcast protocols to improve reception reliability and channel throughput. Torrent-Moreno et al. [7] proposes a priority access scheme for IEEE 802.11-based vehicular ad hoc networks and show that the broadcast reception probability can become very low under saturation conditions. Jiang et al. [8] raises channel congestion control issues for vehicular safety communication, and introduce feedback information to enhance system performance and reliability. ElBatt et al. [9] discusses the suitability of DSRC periodic broadcast message for cooperative collision warning application. None of the above studies consider a message priority scheme. [10- 13] are among those few studies which consider message priorities in their analysis. In [10], the authors proposed priority Carrier Sense Multiple Access (P-CSMA) and polling P-CSMA (PP-CSMA). However, the protocol is not compatible with the IEEE 802.11 and IEEE 802.11e standards. In some recent papers, such as [11], repetition techniques are proposed for priority-based scenarios; however no optimized value is derived for the number of transmission repetitions. In fact, a high number of repetitions increase the collisions in the network, and as a result, the performance of the network degrades. In [12], the authors propose the piggyback cooperative idea for controlling the collision rates. In [13], we considered a controlled repetition technique for car-to-car and car-to-infrastructure communications in a network with a fixed number of vehicles. In that study it has been shown that using this framework we can provide better reliability for important safety messages in VANETs. This work extends the results of [13] in several directions. First of all, we enhance the repetition-based broadcasting mechanism by applying a piggyback technique 2010 Sixth International Conference on Wireless and Mobile Communications 978-0-7695-4182-2/10 $26.00 © 2010 IEEE DOI 10.1109/ICWMC.2010.51 395 2010 Sixth International Conference on Wireless and Mobile Communications 978-0-7695-4182-2/10 $26.00 © 2010 IEEE DOI 10.1109/ICWMC.2010.51 395