# Preamble-Based Adaptive Channel Estimation for IEEE 802.11p +
## 2 IEEE 802.11p Standard and Channel Model  
### IEEE 802.11p PHY Layer  
Orthogonal frequency division multiplexing (OFDM) is employed in the IEEE 802.11p PHY layer. The OFDM transmitter sends several parallel data streams through several orthogonal subcarriers, thereby improving the spectral efficiency and mitigating the severity of multipath fading. The transmitter and receiver in the OFDM system are shown in Figure 1. In the transmitter, a convolutional encoder is first utilized for forward error correction. Puncturing can be used to obtain higher code rate, e.g., 2/3 or 3/4. The coded data is then interleaved to avoid burst errors. Subsequently, the signal is modulated according to the modulation scheme. After that, a 64-point inverse fast Fourier transform (IFFT) is performed with 48 data subcarriers, four pilot subcarriers, and 12 null subcarriers. Finally, the cyclic prefix (CP) and preamble are added. The CP is employed to prevent inter-symbol and inter-carrier interference, and the preamble is primarily employed for synchronization and channel estimation.  
In the receiver, assuming perfect timing and frequency synchronization, the CP is first eliminated from the received packet. After the signal-to-parallel (S/P) and fast Fourier transform (FFT) blocks, the preamble and frequency guard band are removed in turn and the channel coefficients are estimated by two long training symbols. IEEE 802.11p uses a 1-tap equalizer, which compensates for the channel fading effect by using estimated channel coefficients. The equalized data symbols are then demodulated according to the modulation scheme. After the decoder and deinterleaver, the demodulated data bits are converted to sink bits. Table 1 represents the specific system parameters defined in IEEE 802.11p.  
The 802.11p PHY layer also defines two types of pilot arrangements, i.e., comb and block types, for channel estimation, as shown in Figure 2. The comb-type arrangement dedicates four uniformly distributed pilot subcarriers in each OFDM symbol whereas the block type dedicates all subcarriers as the pilot except for those dedicated to the null subcarriers. A single OFDM symbol is composed of a total of 64 subcarriers, numbered \(-32\) to \(32\); \(48\) data subcarriers are assigned numbers from \(-26\) to \(26\), not including \(-21\), \(-7\), \(0\), \(7\), and \(21\). The pilot subcarriers are allocated to subcarriers \(-21\), \(-7\), \(7\), and \(21\). The remaining indexes, i.e., \(-32\) to \(-27\), \(0\), and \(28\) to \(31\), are used as null subcarriers. IEEE 802.11p uses a block-comb pilot pattern that combines these two types. As shown in Figure 2, two OFDM symbols of block type represent long training symbols in the preamble and the remaining OFDM symbols utilize the comb-type.  
In these pilot patterns, the block pilot and comb pilot are suitable for frequency-selective fading and fast-fading, respectively. However, in vehicular environments, the coherence time is typically shorter than the packet duration, and the coherence bandwidth is smaller than the four pilot-subcarrier  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Bandwidth & 10 MHz \\ Bit rate & 3, 4.5, 6, 9, 12, 18, 24, 27 Mbps \\ Modulation schemes & BPSK, QPSK, 16 QAM, 64 QAM \\ Code rate & 1/2, 2/3, 3/4 \\ Data subcarriers & 48 \\ Pilot subcarriers & 4 \\ Total subcarriers & 64 \\ FFT period & 6.4 μs \\ CP duration & 1.6 μs \\ Symbol duration & 8.0 μs \\ Subcarrier spacing & 0.15625 MHz \\ \hline \hline \end{tabular}
\end{table}
Table 1: IEEE 802.11p system parameters.  
Figure 1: Block diagram of transmitter and receiver in OFDM system.  
spacings [12; 13]. Therefore, accurate channel estimation is difficult when only the pilots defined in the IEEE 802.11p are employed, which degrades performance.