# An Improved Channel Estimation Technique for IEEE 802.11p Standard in Vehicular Communications
## 4 The Proposed Scheme  
The block diagram of the receiver is shown in Figure 3. The pink shaded box depicts the proposed channel estimator. It is placed after the 64 point FFT block. Table 4 presents the timing parameters used in the simulation of the proposed scheme, and they are in accordance with Section 18 of the IEEE standard 802.11p [1]. The timing synchronization and fine frequency correction are performed based on the output of packet detection and coarse frequency correction block. The detail about this block can be found in [42]. **Definition 2**: _Packet Detection: The process of detecting a packet based on the information in the received time domain signal \(X_{t}\), such as channel bandwidth. The output of this process is the offset, from the start of the received input waveform to the start of the detected preamble using auto-correlation._  
Figure 4 shows a single IEEE 802.11p packet waveform, where the upper part is the transmitted time domain signal and the lower part is the received time domain signal ready for channel estimation block. It should be noted that the transmitted time domain waveform \(T_{x}(t)\) is a vector composed of complex numbers, therefore we have taken the absolute of the transmitted waveform i.e., \(\sqrt{T_{x}(t)^{2}}\) and plotted in the upper part of Figure 4.  
**Definition 3**: _Coarse Frequency correction: It is the correction in terms of frequency error which is estimated by utilizing the pre-known STS training field. The estimate contains the carrier frequency offset in Hertz. The short length of the periodic sequence in the STS allows coarse frequency offset estimation._ \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Scheme** & **SM Requirement** & **CSI Requirement** & **CCL** & **PL** & **Compatibility** \\ \hline STA & No & Yes & 0.3 & 0.2 & Invariant \\ MBCE & Yes & No & 0.2 & 0.1–0.3 & Reduced \\ TDLSE & Yes & No & 0.2 & 0.2 & Reduced \\ LS & No & No & 0.1 & 0.5 & Invariant \\ WF & No & Yes & 0.3 & 0.3 & Invariant \\ GDPS & No & Yes & 0.5 & 0.3 & Invariant \\ Improved GDPS & Yes & Yes & 0.4 & 0.3 & Invariant \\ DD & No & Yes & 0.5 & 0.3 & Invariant \\ DD Lossless reduction & No & Yes & 0.4 & 0.3 & Invariant \\ DD Lossy reduction & No & Yes & 0.5 & 0.3–0.4 & Invariant \\ Pseudo-pilot & No & Yes & 0.5 & 0.3–0.4 & Invariant \\ CDP & No & Yes & 0.2 & 0.1–0.2 & Invariant \\ Proposed & No & Yes & 0.3 & 0.1 & Invariant \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Comparison of the proposed scheme with existing schemes for the Vehicle-to-Vehicle (V2V) channel Estimation. Figure 3: IEEE 802.11p receiver block diagram. Similarly, the received waveform signal \(R_{x}(t)\) that is modulated by channel [28] and at \(SNR~{}=~{}35\) dB is drawn in the lower part of Figure 4 as \(\sqrt{R_{x}(t)^{2}}\). For the purpose of clarity, we have plotted only 5 data symbols. Figure 4 also gives the detailed timing information in terms of parameters that are described in Table 4. The output of the FFT block is \(S_{R,i}(k)\), which is the frequency domain representation of the received symbol at \(k\)th subcarrier. In contrast to the CDP scheme, the proposed scheme also includes the 4 phase tracking pilots and therefore \(k\) represents 52 subcarriers. The proposed estimator updates the \(i\)th symbol's channel estimate by using the channel response \(H_{CDP,i-1}(k)\), obtained from the previous \((i-1)th\) symbol. The equalization process is given by Equation (6). \[\hat{S}_{T,i}(k)=\frac{S_{R,i}(k)}{H_{CDP,i-1}(k)} \tag{6}\]  
Since \(\hat{S}_{T,i}(k)\) contains a total of 52 subcarriers. In order to construct 52 pilots, the \(\hat{S}_{T,i}(k)\) is divided into two groups of subcarriers, i.e., the data subcarriers group \(\hat{S}_{T,i}(k_{D})\) and the phase tracking pilots subgroup \(\hat{S}_{T,i}(k_{P})\). These groups are then demodulated separately and then demapped to the respective constellation points to generate the two groups of pilots, i.e., one for the data subcarriers and the other for the phase tracking pilots.