# Adjacent Vehicle Number-Triggered Adaptive Transmission for V2V Communications
## 3 Adaptive Transmission Scheme for V2V Based on Physical Topology
Therefore, the data rate can be adjusted to be lower (16 QAM) so as to ensure the reliable communication in the harsh channel condition. With the proposed algorithm, more reliable and continuous data transmission for the V2V communication can be expected with the adaptive modulation, shown in Table 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Modulation and Coding Types** & **Receiver Sensitivity \(S_{R}\) (dBm)** \\ \hline BPSK 1/2 & \(-99\) \\ QPSK 1/2 & \(-96\) \\ QPSK 3/4 & \(-94\) \\
16 QAM 1/2 & \(-90\) \\
16 QAM 3/4 & \(-87\) \\
64 QAM 2/3 & \(-82\) \\
64 QAM 3/4 & \(-80\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Receiver sensitivities for different modulation and coding types [27]. Figure 3: Flow chart for the proposed algorithm. To investigate the performance of the proposed algorithm, we performed the computer simulations and compared these with the conventional static data transmission. We assume for a fixed time interval for how long the vehicle keeps trying to transmit the data to other vehicles. Also, the success rate for the connection is assumed to be 100% if there is a connectable vehicle. When there is no connectable vehicle, the data is assumed to be queued in the buffer. For the physical topology of the vehicle network, we assume that the maximum number of adjacent vehicles is 4 and the number of connectable vehicles is randomly generated from 0 to 4. Depending on the different N values, the different modulation order is determined. When N equals zero, the data is stored in the buffer without transmission in both conventional and proposed schemes. When N is not equal to zero, the conventional schemes assume the fixed modulation, such as QPSK, 16 QAM, or 64 QAM. In the proposed method, the different modulation type is chosen. More specifically, 64 QAM is selected when N equals to 1 or 2. 16 QAM is selected for N = 3 or N = 4. When N is larger than 4 in a realistic environment, the modulation order can be kept as 16 QAM, as N = 4. This is because there are many possibilities that vehicles can be connected when N is larger than 4. Note that in the conventional method, 64 QAM is continuously utilized during the entire time period for the ideal situation. This is because, in an 802.11p standard, 64-QAM can be reached when the channel is optimal with a line-of-sight (LOS) component and a low doppler shift [24]. Rician fading channel is assumed in the simulation. The Rician K-factor is defined as the ratio of the signal power in the dominant component over the scattered power. The K-factor is normally set to a constant value in the simulations [28]. However, reference [28] investigated the variation of the K-factors in variable vehicular channels and found that the K-factor became lower when more vehicles existed. Furthermore, reference [29] estimated the K-factor for the V2V communication and concluded that the K-factor ranged from \(-10\) dB to \(10\) dB in rural, suburban, and urban environments. Therefore, we consider two scenarios, where one is scenario I with the fixed K-factor of 10 dB and another is scenario II with the different K-factors from 10, 5, \(-5\), and \(-10\) dB depending on the number of connectable vehicles. The turbo code with \(1/2\) code rate is chosen as the channel coding scheme for all conventional methods and the proposed method. Detailed simulation parameters for two simulations are summarized in Table 3. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Number of Connectable Vehicles, N** & **0** & **1** & **2** & **3** & **4** \\ \hline Conventional Method 1 & Store data & QPSK & QPSK & QPSK & QPSK \\ Conventional Method 2 & Store data & 16 QAM & 16 QAM & 16 QAM & 16 QAM \\ Conventional Method 3 & Store data & 64 QAM & 64 QAM & 64 QAM & 64 QAM \\ Proposed Method & Store data & 64 QAM & 64 QAM & 16 QAM & 16 QAM \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Modulation types.