## 5 Performance Analysis
The only difference between Figures 11a and 11b is that Figure 11b depicts the V2X-UORA channel access this paper proposes. Because OCW sizes can affect delay and PLR performance, we set the OCW size to one-tenth the number of users for adaptive OCW control. \begin{table}
\begin{tabular}{c c c} \hline  & **Description** & **Value** \\ \hline N\({}_{\text{V}}\) & Number of V2X Vehicles & X-axis \\  & & N\({}_{\text{V}}\)/10 (1-band) \\ OCW & Contention window size of UORA & N\({}_{\text{V}}\)/20 (2-band) \\  & & N\({}_{\text{V}}\)/40 (4-band) \\ T\({}_{\text{V2X}}\) & Transmission Opportunity (TXOP) duration of NG V2X transmission & 2880 us \\ T\({}_{\text{S-V2X}}\) & Transmission Opportunity (TXOP) duration of S-NG V2X transmission & 2700 us \\  & & 720 us (1-band) \\ T\({}_{\text{Legacy}}\) & Transmission Opportunity (TXOP) duration of NG-V2X transmission & 405 us (2-band) \\  & & 225 us (4-band) \\ M\({}_{\text{Queue}}\) & Size of frame queue in number of frames & 1000 frames \\ \(\lambda\)v2X & Frame arrival rate of each V2X vehicle. & 10 Hz \\ N\({}_{\text{sensor}}\) & Number of ITS sensor & 0 or 30 \\ \hline \end{tabular}
\end{table} 
Table 1: Simulation parameters. Figure 10: Delay and packet loss rate (PLR) of a single V2X STA for conventional IEEE 802.11p channel access in a single band transmission: (**a**) 60 V2X STAs scenario; (**b**) 80 V2X STAs scenario. OCW control can be performed by IEEE 802.11 beacon frames transmitted by the V2X infrastructure STA. Because an IEEE 802.11p V2X STA needs to receive the beacon frame before transmitting its frame to the V2X infrastructure STA, receiving the OCW control value included in the beacon frame is reasonable and easy to implement in NG-V2X systems. After receiving the OCW control value, each V2X can generate its own UORA back-off value for UORA channel access based on the received OCW control value. We will examine the effects of this OCW size in the last part of this section. In this paper, we will call the largest number of users that have convergence value of delay with 0 PLR as the user capacity threshold. Therefore, we will need to examine the user capacity threshold in various scenarios. As can be observed from Figure 11, the proposed V2X UORA enhances its user capacity threshold performance slightly. However, the specific value of performance gain could vary, depending on the specific implementation method. In some bad implementation methods, the overhead of V2X-UORA may degrade the performance gain. In some bad implementation scenarios, it is possible that the V2X-UORA channel access has a smaller user capacity than the conventional channel access case. An important thing we can observe from Figure 11 is that the delay value in the converged delay range (dashed circles) has a relatively averaged delay value rather than having fluctuating values of delay, unlike the conventional IEEE 802.11p case. Figure 12 shows the average delay and the PLR in a two-band transmission scenario. Figure 12a shows a conventional IEEE 802.11p channel access scenario simulated and the proposed V2X-UORA channel access applied is shown in Figure 12b. Because a larger band allows small transmission opportunity (TXOP) duration in the conventional IEEE 802.11 case, the user capacity threshold is improved compared with the single band case. Unfortunately, V2X-UORA channel access cannot take advantage of the reduced TXOP duration from the expanded bandwidth. Instead of the reduced TXOP, V2X-UORA channel access can allocate more RUs, owing to wideband operation. According to the latest version of the IEEE 802.11 ax standard, the trigger frame can allocate 18 UL OFDMA RUs at most. For a realistic simulation, 18 RUs are used in the two-band transmission case of the V2X-UORA channel access. From Figure 12a,b, we can observe that the V2X-UORA channel access is much more scalable although it requires a slightly larger delay in the case of a small number of STAs.