# Adjacent Vehicle Number-Triggered Adaptive Transmission for V2V Communications
## 1 Introduction  
Recently, vehicular communication has been attracted as a crucial use case for 5G communication. The vehicles can communicate directly with other vehicles or adjacent infrastructures, such as access points (APs), or fixed equipment beside the road referred to as roadside units (RSU) [1]. The vehicular ad-hoc network (VANET), a variation of the mobile ad-hoc network (MANET), is a technology used for an intelligent transport system (ITS) where the moving vehicles are wirelessly connected [2]. The initial motivation and major goal for the VANET are to improve road safety, increase transit efficiency, and scale down the impact of shipping on the environment [3]. In the meantime, other kinds of applications rapidly grow due to the passengers' desire for IP-based Internet services such as internet surfing, email, file download, multimedia entertainment, parking, and tourist guide information. These require the introduction of wireless local area network (WLAN) technology to the VANET [4], such as IEEE 802.11p or short-range communication (DSRC) since 2010 [5]. The concept of drive-thru systems was suggested in [6], where each RSU AP connects to the internet cloud server and the vehicles moving in the AP's coverage can connect to the AP. Then, through the RSU AP, the vehicles can acquire IP-based internet connection and a variety of infotainment services. However, due to the high mobility of vehicles, the sparse deployment of RSU APs, and the limited capacity of the AP, it is hard to guarantee the quality and stability of communication between the vehicle and the AP. On the other hand, vehicle-to-vehicle (V2V) communication can take Wi-Fi direct technology. The Wi-Fi direct is a standard defined by the Wi-Fi Alliance to enable devices to directly communicate with others [7]. Therefore, in [8], an easily installed inter-vehicle communication system utilizing the Wi-Fi direct was proposed and verified for the VANET scenario. Additionally, since the IEEE 802.11p has been the sole standard technology realizing the V2V until now [9], the Wi-Fi direct can easily be incorporated with the IEEE 802.11 standards [10]. The V2V has potential to be an alternative method to enhance the on-road internet service in real-life situations, such as platoons or traffic jams. Platooning refers to vehicles moving in a convoy with a short distance, which can be a promising technology with advantages of higher traffic throughput and higher energy efficiency [11]. For the V2V communication, the platoon or the traffic has an identical channel characteristic, which has a specular component and low Doppler shift, since the vehicles are closely spaced and relatively static. Since it is a relatively optimistic channel situation, V2V communication can be considered as an assistant method to bridge the service to the vehicles. More detailed cases are necessary to be investigated for the vehicular communication [12]. Here, are two challenges in platooning and traffic jam scenarios. * The vehicles in the gap area between the APs' coverage  
Due to the sparse AP deployment, this may happen due to the limited coverage of the APs. In this case, it is difficult for the users in vehicles to achieve reliable and stable internet service. In the Figure 1, the vehicles are located in the gap area between the coverages of the AP1 and the AP2, where the vehicles are hard to connect to the APs when, for example, the drivers want to download the tourist information through the electric map. The feasible solution is to acquire the data from the adjacent vehicle in the AP coverage. This backup solution can be approached as a V2V relay, where the AP coverage can be extended and, thus, the area spectral efficiency be improved [13]. In [13], an approach using channel state information was proposed to increase the reliability of the V2V relay. In [4; 14], two efficient opportunistic relay protocols were proposed.