## 2 Related Work  
To the best of our knowledge, [PERSON] et al. [9] first proposed a repetition-based broadcasting scheme to improve the transmission reliability of periodic beacon messages in VANETs. In the proposed scheme, each message is transmitted \(k\) times during \(n\) time slots. A node can pick any \(k\) slots to form its repetition pattern. If the repetition patterns of two nodes have the same slot, a collision will happen in this slot. To minimize collisions, [PERSON] et al. [10] designed a protocol that uses the optical orthogonal codes (OOC) to select the retransmission patterns with the least possible correlation. However, an efficient repetition pattern distributing technique is needed to maximize the benefit brought by the OOC-based scheme. Therefore, [PERSON] et al. [15] presented an adaptive broadcasting strategy for VANETs, which enables a node to decide the probability of rebroadcasting based on the number of its neighbors. [PERSON] et al. [16] proposed a dynamic receiver-oriented packet repetition scheme to enhance the broadcast reliability of emergency messages. Furthermore, [PERSON] et al. [17] proposed a cross-layer broadcasting scheme for one-hop emergency messages, which includes preemptive priority in safety services and dynamic receiver-oriented packet. To overcome the broadcast storm problem, [PERSON] et al. [18] established a direction oriented and stochastic controlled repletion scheme in IEEE 802.11e-based VANETs. [PERSON] et al. [19] introduced network coding into the design of rebroadcasting pattern and proposed a rebroadcasting frame of network coded safety messages. In recent years, cooperative retransmission has been employed to enhance the transmission reliability of both unicast and broadcast modes in VANETs. While most of the existing works focus on the unicast mode, just a few works consider the broadcast mode [11]. Generally, cooperative MAC protocols proposed for the unicast mode of communication can be classified into two categories: contention-free and contention-based. [PERSON] et al. [20; 21] designed a cooperative ad hoc MAC (CAH-MAC) for VANETs. In CAH-MAC, idle slots of a frame are used to perform the cooperative retransmission. A relay node is enabled to help retransmit the packet in an idle slot when the direct transmission fails. However, collisions will happen when cooperative retransmission of the relay node and the channel access of a new node occur in the same idle slot. To deal with the issue of slot reservation collision, the authors proposed an enhanced version of CAH-MAC, named eCAH-MAC [22]. In eCAH-MAC, a relay node suspends the cooperative retransmission if it detects any other node attempts to access the same slot. The abovementioned protocols utilize idle slots to conduct cooperative retransmission. However, idle slots may not be available when the density of vehicles is high. [PERSON] et al. [23] presented a vehicular cooperative TDMA-based (VC-TDMA) MAC protocol that opportunistically exploits the slot of the relay node to improve the throughput. In VC-TDMA, a relay node with the empty buffer is selected to forward the packet. However, VC-TDMA protocol does not take channel conditions into consideration. Its performance changes with channel conditions among the source, the relay, and the destination nodes. Different from the abovementioned protocols that are based on TDMA, [PERSON] et al. [24] proposed an IEEE 802.11-based adaptive distributed cooperative MAC (ADC-MAC) for VANETs. In ADC-MAC, each node periodically broadcasts heartbeat packets so that the source node can get the instant channel information of each potential helper. Nevertheless, heartbeat packets introduce a large signaling overhead and increase packet collisions. [PERSON] et al. [25] proposed a concurrent transmission enabled cooperative MAC protocol (referred as Mizar) for VANETs. Although Mizar significantly increases the throughput, the cooperation also incurs a significant overhead. Limited work has been done for the broadcast mode of communication. [PERSON] et al.