# An Effective Wide-Bandwidth Channel Access in Next-Generation WLAN-Based V2X Communications
## 5 Results
Since all back-off AIFS has good performance in terms of the medium access delay of 10-MHz transmission, it will be beneficial when delay sensitive data are transmitted using 10-MHz bandwidth in V2X applications. Thus, the proposed scheme provides the maximum fairness with 10-MHz frame transmissions in the secondary channel. In addition, the all back-off AIFS scheme shows good delay performance, in terms of mean and standard deviation of the medium access delay of 20-MHz frames. If there is a design limitation with one receiving antenna and the decoder, the proposed scheme cannot be used, and the alternative channel access schemes described in Section 3.2 should be used. When the station includes only one receiving antenna and decoder, trade-off occurs between the medium access delay of 10-MHz frames in the secondary channel and the medium access delay of 20-MHz frames depending on the channel access scheme with ED sensing of the secondary channel. Specifically, when the conventional AIFS scheme is applied to the channel access of a 20-MHz frame, fair contention with 10-MHz frame transmissions including stations using IEEE 802.11p is not possible. On the other hand, the start AIFS + end AIFS scheme and all back-off EIFS scheme ensure fairness and low impact on 10-MHz frame transmissions in the secondary channel. However, the EIFS sensing procedure in such channel access schemes (start AIFS + end AIFS scheme and all back-off EIFS scheme) causes overprotection of 10-MHz frames transmitted in the secondary channel. This results in a large medium access delays in the transmission of 20-MHz frames. In order to measure the performance of the proposed channel access method clearly, channel load in each 10-MHz channel needs to be considered. Medium access delay is highly affected by channel load in each 10-MHz channel. Because two 10-MHz channels are considered and high channel load clearly shows the impact on the performance, two cases for the channel load are considered: high channel load in the primary channel and high channel load in the secondary channel. The simulation results of Figure 9 show how high channel load in the secondary channel affects the performance of the channel access schemes. In order to show the impact on the channel access schemes in case of high \begin{table}
\begin{tabular}{c c} \hline \hline
**Name** & **Value** \\ \hline Frame Length (10-MHz frame) & 500 Bytes \\ Frame Length (20-MHz frame) & 2000 Bytes \\ AC & AC\_BE \\ MCS & 2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Simulation Parameters. channel load in a primary channel, the mean access delay of 10-MHz frames in the secondary channel is measured as the number of stations transmitting 20-MHz frames increases from 10 to 200 under the high load condition of the primary channel. As shown in Figure 10, all channel access schemes show similar performance since 20-MHz channel access is not easily initiated due to a high channel load condition in the primary channel. In order to clearly show how well the proposed scheme performs in terms of channel access delay, the mean access delay of 20-MHz frames is compared with the mean access delay of 10-MHz frames under the same channel load level. For 20-MHz frame transmission, the number of stations transmitting 20-MHz frames increases from 10 to 200 and for 10-MHz frame transmission, the number of stations transmitting 20-MHz frames increases from 10 to 200 to provide the same channel load level for both cases. The result in Figure 11 shows the outstanding delay performance of the proposed scheme in terms of fair contention with 10 MHz transmissions. Figure 9: Comparison between channel access schemes in terms of effects on transmissions in secondary channel and mean access delay of 20-MHz frame: (**a**) mean access delay of 10-MHz frame sent in the secondary channel; (**b**) mean access delay of 20-MHz frame; (**c**) standard deviation of 10-MHz frame sent in the secondary channel; (**d**) standard deviation of 20-MHz frame.