# Adjacent Vehicle Number-Triggered Adaptive Transmission for V2V Communications
## 2 Related Works  
According to the performance analysis in [18], the stability and reliability of IEEE 802.11p based vehicle communication are difficult to be guaranteed. The primary reason is that IEEE 802.11p uses the same physical layer as the IEEE 802.11a standard, which was originally designed for a relatively stable environment like an indoor environment [18]. In the 802.11p standard, with the combination of four types modulation (BPSK, QPSK, 16 QAM, and 64 QAM) and three kinds of coding rate (1/2, 2/3, and 3/4), the data rate varies from 3 Mbps to 27 Mbps. Although various data rate choices are available, a conventional rate adaption (RA) algorithm is used, in which only one type of data rate will be selected by transmitter vehicle according to the channel condition estimation before transmission starts [19]. Some early research regarding rate selection also fixed the data rate to the highest option [20] or the lowest [21]. In order to overcome the challenges resulting from the mobility of vehicles and the variation in transmission situation, the scalability of the physical layer (PHY) must be improved, as discussed in [22]. Adaptive modulation and coding (AMC) is a technology to be introduced into a long term evolution (LTE) system to maximize the throughput of the system by changing the modulation and coding method adaptively, according to the current channel situation [23]. Following the same idea, some researchers have already made efforts in improving the transmission scheme of the PHY. In [24], a novel approach called automatic doppler shift adaptation (ADSA) is proposed to compensate for the doppler effect by extensive simulation. Also, a loss differentiation  
Figure 2: Data traffic jam in traffic jams or platoons.  
rate adaption scheme, which can choose the optimal data rate by taking interference losses and fading losses to enhance the reliability of V2V communication, is designed in [19].  
Conventional RA schemes, such as AMC and the literatures mentioned above, all focus most of their attention on the channel condition but do not take variations of network topology into consideration when designing scalable transmission schemes. In order to acquire high throughput and high-reliability V2V transmission, the time-varying topological structure of the vehicular network is also a serious issue needing to be addressed even if the channel situation is good enough. Specifically, consider a situation in which vehicles are moving as a platoon on the road. In this platoon, one vehicle may decide to depart the platoon for some reason, then the connection will be broken if some other vehicle is communicating with it. Although the transmitter vehicle can choose to temporally store the data received from APs into a buffer and transmit it later when receiver vehicles appear, the transmission speed may not be high enough to compensate for the drop of throughput caused by the interruption, especially when the transmission speed is fixed to a low data rate. On the other hand, always fixing the data rate to quite a high level is arbitrary because, in V2V scenario channels, quality is not always optimal.  
For the purpose of addressing the issues described above, in this work, we propose a physical topology adapting transmission scheme in which a vehicle can adjust the transmission data rate by changing the modulation type according to the number of the nearby connectable vehicles. The innovation of this scheme is using a scalable physical layer to handle the degradation of system performance caused by unpredictable changes in physical topology. As far as we know, there is no other research or physical device which is similar to the proposed physical topology triggered adaptive data transmission scheme in the field of transport or other areas of application. Through the simulation results, this adaptive scheme is verified to be able to improve the performance of the V2V communication transmission.