## 1 Introduction  
Recently, the development of Vehicular Ad Hoc Networks (VANETs) has been accelerated in order to support accident prevention and the safety of vehicle drivers. The number of vehicles continues to increase worldwide, and millions of people are killed or injured by fatal accidents every year [1]. In spite of speed regulations and safety sign boards installed around the roads, technological developments in intelligent vehicle communication networks are essential for accident prevention and the safety of drivers. VANET is a form of Mobile Ad Hoc Network (MANET), but it differs from MANET in that the network topology of VANET dynamically changes, link quality is usually unstable due to the high mobility of a vehicle, there are various types of road patterns, sufficient energy and storage are supported for mobile devices installed in a vehicle, and various quality of service (QoS) are required for VANET services [2]. The applications for VANET can be divided into the following three services: safety services, traffic management, and user-oriented services. Safety services require special requirements of QoS, such as bounded transmission delays and low access delays, both of which are mandatory to offer the highest level of user safety. On the other hand, traffic management services are required to be provided according to vehicle density and data traffic load, and user-oriented services need a high bandwidth. As such, from the perspective of the various requirements of VANET services,medium access control (MAC) plays an important role in VANET. In particular, it is noticed that, from the perspective of safety services, MAC should be able to periodically broadcast a safety-related message that includes useful information such as vehicle accident alerts and road conditions [3]. Wireless Access in Vehicular Environment (WAVE) based on IEEE 802.11p and IEEE 1609 standards is a representative standardization study of Intelligent Transport Systems (ITSs) using VANET technologies, which is designed to be suited for the characteristics of vehicles such as high mobility and short communications time. WAVE is based on competition-based resource allocation that provides priority-based access using both EDCA (Enhanced Distributed Channel Access) and Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The contention-based resource allocation scheme can guarantee low data transmission delay and high user QoS when traffic density is low. However, if offered traffic load and the mobility of the terminal is high, collision probability increases. Accordingly, the data transmission delay increases and the user QoS deteriorates, so that it is not possible to effectively prevent the accident in the VANET environment. To overcome the shortcomings of IEEE 802.11p and facilitate efficient communication in VANET, Time Division Multiple Access (TDMA)-based contention-free MAC protocols have been proposed. There are two types of TDMA-based MAC protocols for VANET systems; distributed TDMA and centralized TDMA. In distributed TDMA, each vehicle allocates and manages its own resources and communication with 1-hop neighbors [4]. In contrast, in centralized TDMA, the resource allocation/management of each vehicle is managed by a roadside unit (RSU) or a central coordinator [5; 6]. However, vehicles traveling at high speed have short dwelling times to stay within the coverage of an RSU, which results in frequent resource collisions. In addition, to support seamless communication for fast moving vehicles, the number of RSUs installed per unit-area increases, and consequently, there are disadvantages of mutual interference between RSUs and collision of resource allocation by hidden nodes in the cluster. In a TDMA-based VANET system, a frame is usually divided into a control channel and a service channel. Each vehicle generates and transmits a safety message including vehicle information, and this message is periodically delivered to 1-hop neighbor vehicles via the control channel.