## 2 Background  
### The IEEE 802.11p Standard  
IEEE 802.11p is an amendment of the IEEE 802.11a standard [7]. Its purpose is to adapt existing indoor wireless communication systems to vehicular environments, where it is important to address the high mobility of the users. IEEE 802.11p uses OFDM as modulation format in the Physical Layer (PHY) and it operates at the 5.9 GHz radio frequency band. The transmitter sends several parallel data streams through orthogonal subcarriers, by improving the spectral efficiency and mitigating the severity of multi-path fading [7]. Table 1 shows the most important parameters defined on the IEEE 802.11p amendment. In general, the time domain parameters of this amendment are two times larger with respect to the IEEE 802.11a standard. For instance, the bandwidth is reduced from 20 MHz to 10 MHz, and the subcarrier frequency spacing is reduced by a factor of 2. As mentioned, these changes are made in order to increase the reliability of the transmission by increasing the duration of the OFDM symbols. Figures 1 and 2 depict a general IEEE 802.11p transceiver. The transceiver is based on traditional OFDM architectures. For simplicity, the block diagrams of the transceiver are depicted in its base-band representation. At the transmitter side, the data are modulated and then passed through an Inverse Fast Fourier Transform (IFFT) block to get orthogonal signals. Afterwards, a Cyclic Prefix (CP) is added to diminish inter-symbol interference, and finally a temporal preamble is inserted [7]. The purpose of this preamble is to synchronize the signal (short training symbols) and estimate the channel (long training symbols). Nevertheless, the structure of the preamble is detailed at the end of this Subsection. Figure 2 shows that the transmitted signal is distorted by the wireless channel, which is time-varying and frequency-selective for V2V communications (refer to the next Section), and by Gaussian additive noise, which is typical to all communication system. At the receiver side, the first signal processing task consists of removing the CP, and then applying a Fast Fourier Transform (FFT) to the signal. Then, the channel is estimated, and the signal is equalized by exploiting the subcarriers used as pilot signals. Finally, the signal is demodulated, and the data can be recovered. The signal received after the FFT operation is given as follows [6]  
\[Y_{i}(k)=H_{i}(k)X_{i}(k)+N_{i}(k),i=\{1,...,N_{symbol}\},k=\{1,...,M\}, \tag{1}\] \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Number of data subcarriers (\(N_{SD}\)) & 48 \\ Number of pilot subcarriers (\(N_{SP}\)) & 4 \\ Number of subcarriers total (\(N_{ST}\)) & 52 \\ Subcarrier frequency spacing (\(\Delta f\)) & 0.15625 MHz \\ IFFT/FFT periods (\(T_{FFT}\)) & 6.4 \(\upmu\)s (1/\(\Delta f\)) \\ PHY preamble duration (\(T_{PREAMBLE}\)) & 32 \(\upmu\)s \\ Duration of the & 8 \(\upmu\)s \\ Signal BPSK-OFDM symbol (\(T_{SIGNAL}\)) & 3.2 \(\upmu\)s \\ Training symbol guard interval duration (\(T_{GI}\)) & 8 \(\upmu\)s \\ Symbol interval (\(T_{sym}\)) & 16 \(\upmu\)s \\ Short training sequence duration (\(T_{SHORT}\)) & 16 \(\upmu\)s \\ Long training sequence duration (\(T_{LONG}\)) & 16 \(\upmu\)s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: IEEE 802.11p system parameters for a 10 MHz channel. Figure 1: Block diagram of the OFDM transmitter. Figure 2: Block diagram of the OFDM receiver. where \(i\) corresponds to the respective OFDM symbol, \(N_{symbol}\) denotes the total number of OFDM symbols, \(k\) is the corresponding subcarrier, and \(M\) represents the total number of subcarriers. Furthermore, \(X_{i}(k)\) represents the transmitted signal that is obtained after the modulation in the transmitter block diagram, \(Y_{i}(k)\) is the received signal after the FFT module at the receiver side, \(H_{i}(k)\) is the Channel Frequency Response (CFR), and \(N_{i}(k)\) is the Additive White Gaussian Noise (AWGN). Based on [6], Figure 3 depicts the subcarrier distribution of the IEEE 802.11p standard.