## 5 Performance Analysis  
To support a real-world V2X system, a reasonable data arrival rate of V2X vehicular STAs must be considered. In this work, a 10-Hz V2X uplink data frame arrival rate is assumed to analyze V2X channel access performance. In other words, each V2X vehicle generates its data frame to a V2X infrastructure STA every 100 ms. As the V2I uplink delay is the key delay factor of the NG-V2X system, we focus on the channel access delay and packet loss rate (PLR) performance of the proposed V2X-UORA channel access. To demonstrate specific performance enhancement of the proposed V2X-UORA channel access, legacy IEEE 802.11p-based channel access simulation is also performed. The simulation is a system-level simulation and communication range is determined depending on its received signal power instead of its actual distance. Wideband operation can also be performed with a various number of channels and the wideband operation procedure follows IEEE 802.11 wideband operation rules. Conventional IEEE 802.11p system supports 2-band wideband operation by utilizing 2 contiguous 10 MHz channels and we assumed at most 4-band wideband operation can be supported in the next-generation V2X system. Each simulation was performed in a limited traffic generation time and a limited simulation time. Because we set unlimited retransmission tries, packet loss that affects the value of PLR means that the data packet has not been delivered in simulation time after it is generated. Therefore, the PLR is not the main target of our performance comparison but it is a constraint parameter which needs to be 0 value if the number user value is under user capacity threshold. The simulation time was set to five times the traffic generation time. This means that each V2X STA generates uplink data traffic during traffic generation time and then stops generating data frames for four times the traffic generation time. Conventional channel access procedures for transmitting data frames were performed during the whole simulation time in a conventional IEEE 802.11p scenario, and a V2X infrastructure STA transmits an UORA trigger frame every trigger frame interval over the whole simulation time. Simulation parameters we used are listed in Table 1. Transmission Opportunity (TXOP) duration and frame arrival rate parameters on the list are calculated based on the corresponding specifications and OCW value for each case are determined by experimental results. In Figure 10, we can observe the delay and the PLR of the conventional IEEE 802.11p-based channel access procedure with 80 STAs using a single 10-MHz channel. In Figure 10a, the delay and the PLR of a single STA in a scenario with 60 V2X STAs are shown, whereas in Figure 10b, the delay and the PLR of a single STA in a scenario with 80 V2X STAs case are shown. It can be observed that conventional IEEE 802.11p works well in the 60 STAs case but not in the 80 STAs case. Almost all the packets are delivered in 10 ms and no packet loss is observed. Unlike in the 60 STAs case, we can observe that the delay does not have a convergence value. This means that conventional IEEE 802.11p cannot support 80 V2X STAs with a single V2X infrastructure STA in the environment we assumed. In other words, the user capacity threshold is a value between 60 and 80 for the case of the Figure 10.  
In Figure 11a, the average delay and the PLR performance over the number of users in a single-channel conventional IEEE 802.11 scenario are shown. The X-axis represents the number of users and the delay values are log scaled. We can observe that the case of 80 users makes catastrophic delay and it is not recommended to accommodate over 80 users in the system. V2X-UORA is also simulated in a similar scenario. In a single channel, the UORA trigger frame can allocate nine RUs for uplink OFDMA transmission at most. Therefore, we use UORA trigger frames with nine RUs in the V2X-UORA channel access scenario. Similar to Figure 11a, Figure 11b shows the average delay and the PLR performance of STAs using a single-band transmission.