## 1 Introduction  
The rapid growth in the number of vehicles has caused some serious problems in urban roads and highways, such as traffic congestions and road crashes. As part of an intelligent transportation system (ITS), vehicular ad hoc networks (VANETs) emerge as a promising technology to solve the abovementioned problems. VANETs enable information sharing and interactions among vehicles, infrastructures, and the Internet. In VANETs, every vehicle is equipped with a communication device called an on-board unit (OBU), and a set of stationary units, referred to as roadside units (RSUs), are deployed on transportation infrastructures such as traffic lights, street lights, and road signs. Through OBUs and RSUs, vehicles can communicate with each other as well as with the Internet [1]. Therefore, VANETs support two modes of communication: vehicle to vehicle (V2V) and vehicle to infrastructure (V2I). VANETs are expected to support a large spectrum of applications that range from safety-related applications (e.g., collision detection, lane change warning, and cooperative merging) and transportation management applications (e.g., traffic signal control, intelligent traffic scheduling, and fleet management) to infotainment applications (e.g., email, videos, and advertisements) [2]. These applications have very different demands of quality of service (QoS). Generally, safety messages have a very strict delay demand and require a reliable and timely delivery, while non-safety messages require high throughput and fair access to the channel but can tolerate a higher transmission delay. Furthermore, the delivery of safety messages mainly depends on the broadcasting service, especially the one-hop broadcasting service, because most safety applications require their messages to be disseminated to all neighboring nodes of the source node [3]. Usually, the lifetime of safety messages is less than 100 ms, and 99% of nodes in the one-hop range must receive the messages [4; 5]. Hence, the one-hop broadcasting plays a very important role in VANETs. However, the special characteristics of VANETs such as the high mobility of vehicles, the dynamic network topology, and the frequent link breakages make it very challenging to design a reliable and timely broadcasting scheme to meet the QoS demands of safety messages. Several media access control (MAC) schemes for vehicular communications have been proposed in the literature, among which the IEEE 802.11p [6] is a standardized protocol. However, it still has some shortcomings. As a contention-based MAC, it is very difficult to guarantee the delay of safety messages. Furthermore, no request-to-send/clear-to-send (RTS/CTS) packets are exchanged before the transmission of broadcast messages, and no acknowledgement (ACK) message is used to acknowledge the reception. Thus, broadcasting using the IEEE 802.11p suffers from the hidden terminal problem. In order to provide a delay-guaranteed broadcasting service for safety messages, several distributed time division multiple access (TDMA)-based schemes such as ADHOC MAC [7] and VeMAC [8] have been proposed in the literature. However, it is still difficult for these TDMA-based MACs to provide a reliable broadcasting service for safety messages. In recent years, repetition-based techniques have been proposed to meet such requirements as timeliness and reliability for broadcasting [9; 10]. These schemes allow the source vehicle to repeatedly transmit a message during the lifetime of the safety message so that its neighbors can have more opportunities to receive the message. The great advantage of repetition-based strategies is that they can improve the broadcasting reliability without dependence on any collision avoidance or acknowledgement mechanisms. Thus, the delay of safety messages can be easily guaranteed.