## 3 Proposed Novel V2X Channel Access Schemes  
The basic model of UORA channel access we considered in previous sections can be applied to a novel V2X channel access scheme. Because UORA channel access can be applied to non-AID STAs, IEEE 802.11p-based V2X STAs that do not have an association procedure to obtain an AID are good targets for applying the UORA channel access procedure.  
In addition to conventional IEEE 802.11p and UORA channel access that we have considered, we consider a limited association procedure for enhancing the V2X system. Although the legacy V2X system considers communications between vehicles and other STAs, including V2X infrastructure STAs, other vehicles, etc., we can assume that some extra sensor STAs could be installed near a V2X infrastructure STA communicating with the V2X infrastructure STA in the NG-V2X scenario.  
Sensor STAs may have extremely low or no mobility in contrast to V2X vehicular STAs. This means that the association procedure, which the conventional V2X system does not support due to the high mobility and short link lifetime of V2X STAs, can be useful for these V2X sensor STAs of NG-V2X. The sensor STAs deployed for accommodating traffic information could transmit the information to a V2X infrastructure STA. In many cases, the sensor STAs intend to report the information to certain V2X infrastructure STAs periodically. In this case, using random-access channel access mechanisms including DCF and UORA is very inefficient. To minimize the inefficiency, an association procedure between the V2X infrastructure STA and the sensor STAs for scheduled transmission opportunity needs to be considered. If the V2X system allows association procedures for some special STAs including the sensor STAs, V2X infrastructure STAs can allocate AIDs to the STAs. In this case, the V2X infrastructure STAs can trigger OFDMA transmissions of the special STAs by using a trigger frame with dedicated AIDs the V2X infrastructure STAs have allocated. Figure 7 shows the UORA channel access procedure with dedicated AIDs. RUs with dedicated AIDs cannot be accessed by V2X-UORA STAs, and only the special STA having a unique dedicated AID can access the matched RU and transmit its own information. Unless dedicated RUs are used, the special STAs also need to participate in the contention procedure, causing longer system delay and higher packet loss rate. Therefore, to optimize V2X-UORA channel access, dedicated RU-based channel access with the association procedure needs to be considered to be applied.  
For further optimization, we consider the scheduled NG-V2X system that we propose. The UORA trigger frame we use for NG-V2X is especially useful for soliciting uplink transmissions from multiple STAs. However, if a channel is determined to be used for UORA transmission and time sync is calibrated by other frames transmitted by an ITS AP, a beacon frame or a new indication frame is sufficient and the trigger frame is not required for the UORA channel access procedure. This means that the overhead caused by transmitting trigger frames can be eliminated so that the scheduled NG-V2X could achieve a higher user capacity threshold than the NG-V2X we considered.  
Figure 6: Basic concept and operation of uplink OFDMA random access (UORA) procedure.