## 2 Improvements and Comparative Analysis of IEEE 802.11 bd
### Key Improvements in Data Transmission Reliability  
The high-speed movement of vehicles introduces challenges such as high dynamics and spatiotemporal complexity, resulting in a significant Doppler shift and a rapidly varying wireless propagation environment [29; 30]. Other factors, such as high density, resource collisions, proximity effects, and intricate interference conditions, further impact the reliability of data transmission. Therefore, data transmission reliability has become a critical performance metric in evaluating V2X standards. To address these challenges, the IEEE 802.11 bd standard adopts a variety of new technologies and services to provide gains in SNR, including LDPC coding, midambles, and improved beamforming technology. IEEE 802.11 bd implements LDPC coding to replace the Binary Convolutional Coding (BCC) used in 802.11p. Compared to BCC, LDPC coding presents lower encoding and decoding complexity and a more uniform check code distribution. This quality enables LDPC coding to resist various channel interferences better and provides superior error correction, particularly in high-SNR situations. According to [31], substituting BCC with LDPC coding could lead to a 1-4 dB SNR gain in multiple scenarios. Moreover, ref. [24] estimates that this SNR gain can be translated into a Packet Reception Rate (PRR) gain of approximately 20%. Regarding the data frame format, 802.11 bd maintains the 40 \(\upmu\)s preamble from 802.11p and introduces an NGV preamble of at least 36.8 \(\upmu\)s, as shown in Figure 1. Notably, the NGV preamble includes a flexible NGV-LTF field strategically designed to enhance the data transmission reliability in real-world communication scenarios. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & \multicolumn{4}{c}{**Data Transfer Rate/Mbps**} \\ \cline{3-5}
**MCS** & **R** & **IEEE 802.11p** & **IEEE 802.11 bd** \\ \cline{3-5}  & & **10 MHz** & **10 MHz** & **20 MHz** \\ \hline BPSK & 1/2 & 3.0 & 3.3 & 6.8 \\ QPSK & 1/2 & 6.0 & 6.5 & 13.5 \\ QPSK & 3/4 & 9.0 & 9.8 & 20.3 \\
16-QAM & 1/2 & 12.0 & 13.0 & 27.0 \\
16-QAM & 3/4 & 18.0 & 19.5 & 40.5 \\
64-QAM & 2/3 & 24.0 & 26.0 & 54.0 \\
64-QAM & 3/4 & 27.0 & 29.3 & 60.8 \\
64-QAM & 5/6 & N/A & 32.5 & 67.5 \\
256-QAM & 3/4 & N/A & 39.0 & 81.0 \\
256-QAM & 5/6 & N/A & N/A & 90.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Available data transmission modes and rates of IEEE 802.11 bd and IEEE 802.11p. Figure 1: IEEE 802.11p (**a**) and 802.11 bd (**b**) PPDU format comparison. The IEEE 802.11 bd standard introduces a midamble to the data frame format, selectively placed after a specified number of OFDM symbols (e.g., 4, 8, or 16, per upper layer instructions) in the NGV PPDU data field. The content of the midamble should be the same as that of the NGV-LTF field in the preamble. To avoid spectral line generation in the NGV PPDU spectrum, randomization is applied to both the midamble and subsequent data field OFDM symbols. Midambles convey vital information about the data transfer rate, MCS, and transmission parameters, aiding the receiver in precisely determining suitable transmission parameters. This enhances the data transmission efficiency and reliability, particularly in high-density communication environments, mitigating errors and losses due to incorrect parameter settings. In addition to new technologies, 802.11 bd introduces advanced IoV services to improve the data transmission reliability, such as NON_NGV_10 repetition transmission and non-NGV duplication operation. In the NON_NGV_10 repetition transmission mode, the physical layer transmits (1+N_PPDU_REP) NON_NGV_10 PPDUs in sequence. The time interval between every two PPDU repetition transmissions is indicated by the aSIFSTime parameter, while the higher layer indicates the N_PPDU_REP parameter.