## 2 Background Technology
As we mentioned earlier, the delay this paper focuses on is a V2I delay in an SCH. More specifically, this paper considers the V2I uplink delay in the SCH channel. Because each vehicle transmits its own frames to a V2X infrastructure STA without any centralized control, the delay and collision they cause could be a great threat against contention-based V2X systems. Although the IEEE 802.11p standard is not a very recent standard, it is verified and being deployed for novel V2X systems. Although the latest research groups, including 3 GPP and IEEE, are considering NG-V2X network standards, they are still considering IEEE 802.11p as the baseline technology for NG-V2X systems. Therefore, we consider IEEE 802.11p-based V2X channel access schemes for the NG-V2X systems in this work. The NG-V2X system that we consider shall support backward compatibility to IEEE 802.11p. This means that if the SCH or CCH is used for both conventional IEEE 802.11p and NG-V2X, the existence of conventional IEEE 802.11p STAs needs to be considered and conventional IEEE 802.11p communication should not be adversely affected. This can be interpreted that the channel access scheme we propose needs to be examined under the scenarios of legacy IEEE 802.11 coexistence. In this study, we propose a specific uplink channel access mechanism based on IEEE 802.11p. We consider highly dense scenarios of V2X vehicles in a traffic jam to measure how reliable and scalable the V2X system is in extreme scenarios. To handle the dense scenario, we have chosen to use the trigger frame that is defined in the IEEE 802.11 ax standard but not in IEEE 802.11p. Basically, the trigger frame is used for soliciting uplink multiuser transmission by an STA whose address is indicated in the trigger frame. Because the trigger frame needs the target uplink STAs' addresses, using the trigger frame for soliciting uplink multiuser transmissions in the V2X system that does not have an association procedure for exchanging STA information is not appropriate. Therefore, performing uplink multiuser transmission in the V2X system requires a random-access-based channel access procedure and not dedicated scheduling using the basic trigger frame. Fortunately, a resource unit (RU)-based random-access procedure called UORA procedure is defined in the IEEE 802.11 ax standard as an optional feature. The UORA is a unique channel access scheme for allowing STAs to perform RU-based random access. Figure 6 shows the basic concept and operation of the UORA procedure. In this procedure, the AP allows STAs to access some RUs indicated as UORA RUs, which are indicated by a special value of association ID (AID). If the AID of some RUs is written in the special AID value, any STA receiving the UORA trigger frame counts the number of UORA RUs. The number is used for decreasing the STAs' back-off counter as in the conventional IEEE 802.11 distributed coordinating function back-off procedure. Unlike in the conventional procedure, the back-off counter decreases based on the number of RUs and not based on the number of time slots the STAs have waited. Although the UORA procedure is not defined for IEEE 802.11p and hence cannot be used in the existing IEEE 802.11p-based V2X system, we propose to use the UORA procedure for the NG-V2X system by demonstrating its performance gain by proposing novel enhancing algorithms in this work.