# Adjacent Vehicle Number-Triggered Adaptive Transmission for V2V Communications
## 3 Adaptive Transmission Scheme for V2V Based on Physical Topology  
In this section, the proposed adaptive transmission scheme is described. When the number of available connected vehicles is high, the V2V communication may be resumed with another neighboring vehicle, even if the current connection is broken. Therefore, the data transmission can be kept more continuous and reliable since the data stored in the buffer can be transmitted even with a low data rate. On the other hand, if the number of connectable vehicles is low, it is hard for the vehicle to resume the transmission since there are few chances to link to another vehicle. Therefore, it is necessary to transmit the information stored in the buffer in a more aggressive manner, with a higher data rate. Here, we utilize the beacon signal between the vehicles as an auxiliary means to achieve the dynamic adjustment of the data rate according to the random topology changes. In the vehicular communication systems, the beacon signal plays a special role in providing rich information such as location, heading and other status information [25]. We assume the vehicles are equipped with the periodic broadcast beacons to announce its presence to the surrounding vehicles. For simplicity and clear demonstration of the proposed algorithm, the following assumptions are made: the channel and position information are integrated in the beacon signal, its exchange between vehicles is frequent enough, the vehicle can detect the number of adjacent connectable vehicles when receiving the beacons, and the algorithm is operated for the vehicle (called a data donor) which transmits the data to other vehicles. Figure 3 shows the flow chart of the proposed algorithm. Once the vehicle starts, it keeps sensing the beacon signal broadcasted by adjacent vehicles and determines the number of connectable vehicles denoted as N. When N equals to zero, the vehicle stores the data in the buffer without transmission, since there are no adjacent vehicles. When N is equal to a number larger than zero, the communication can be established. However, if the connection is broken due to a vehicle leaving the platoon, it seems difficult to connect to another vehicle again. In order to minimize the possible data loss when no adjacent vehicle exists, a more aggressive data rate selection can be considered. We try to reason why the possible rates are chosen and describe the details as follows. To guarantee the reliability of the data transmission, i.e., to keep the bit error rate (BER) lower than the threshold, the received power at the vehicle \(P_{R}\) must be larger or equal to the receiver sensitivity \(S_{R}\)[26]. The \(S_{R}\) is usually defined for a different threshold value for different modulation and coding types, as seen in Table 1[27]. As the table shows, a higher level modulation requires a higher \(S_{R}\) and, thus, asks for a higher received power. \(P_{R}\) can be calculated as  
\[P_{R}=P_{T}+G_{T}+G_{R}-L-I-F \tag{1}\]  
where \(P_{T}\) is the transmitter power of the transmitter, \(G_{T}\) is the transmitter antenna gain, \(G_{R}\) is the receiver antenna gain, \(L\) is the path loss, \(I\) is the interference, and \(F\) is the channel fading. We assume that the \(P_{T}\), \(G_{T}\), and \(G_{R}\) values are fixed. When there are fewer vehicles around, the interference and the channel fading is reduced, since less scattering causes better channel conditions due to less adjacent vehicles [28]. Therefore, a sufficient \(P_{R}\) can allow for the achievement of values higher or equal to \(S_{R}\) (such as 64 QAM). Meanwhile, when the number of vehicles is large, the interference and the channel fading become higher. Therefore, a lower \(P_{R}\) cannot allow for a value higher or equal to \(S_{R}\) to be chosen. Therefore, a lower order modulation is chosen. Therefore, a higher order modulation such as 64 QAM is selected because the number of connectable vehicles is low. Meanwhile, as N increases, there are more and more connectable vehicles which can be backup options.