## 1 Introduction
In another word, the NG-V2X must provide V2X communications. Thus, the NG-V2X system needs to consider communications among various and numerous stations (STAs) than ever before. Usually, 1000 m is considered as the ideal coverage range of an IEEE 802.11p-based system under the assumption of Line of Sight (LOS) communications environment like highway [23; 24; 25; 26; 27] without considering obstacles and severe fading and shadowing, which is not a realistic scenario [28]. It means that the IEEE 802.11p STAs can deliver data frames without any problem if there are not so many vehicles, as in a rural area or an uncrowded road unless there are shadowing and fading which can make the IEEE 802.11p coverage range smaller [29; 30]. However, even though the actual coverage range is smaller than ideal coverage range of IEEE 802.11p-based system, the coverage range is sufficient to accommodate a large number of connected cars that could cause channel access problems in traffic jam scenarios because every car will be required to equip a V2X system in the near future [31; 32; 33]. Furthermore, the NG-V2X network we consider may include various STAs including pedestrian STAs and ITS sensor STAs as shown in Figure 1; this means that the NG-V2X network may suffer severe channel access problems without novel and efficient channel access schemes. In other words, supporting numerous V2X STAs including all cars required to be equipped with the V2X system, ITS pedestrians, and ITS sensors will be difficult without novel and efficient NG-V2X channel access schemes. These kinds of scalability problems are the most well-known problems of wireless networks. However, different from other cases, downlink ITS information has a broadcast property and uplink ITS information may not require a large size of frames. Furthermore, the presence of a large number of connected cars in the area of an AP means that their mobility might be reduced than in usual cases, and hence the data frames become more delay tolerable. In other words, if there is no traffic jam, a connected car will pass through a coverage range of an ITS AP installed on the roadside in a short time. However, if there is a heavy traffic jam, the car cannot leave the coverage range soon. Figure 2 shows the concept of these two scenarios. An ITS AP can be installed on the roadside and ITS sensors can be installed on the roadside or buried under the road for these two scenarios. Figure 2a shows a sparse case, and Figure 2b shows a dense case. In the Figure 2a scenario, the initial channel access delay is important, not a collision. On the other hand, in the Figure 2b scenario, a collision may occur as shown in Figure 3; after collision, an additional channel access delay needs to be considered because of the enhanced distributed channel access (EDCA) procedure of the IEEE 802.11 standard. It is worth noting that the delay after channel collision could be much longer than the data collision time. This means that the time loss due to collision, which is inevitable because random access is the only solution for a newcomer STA unless there were prior procedures such as an association or a dedicated resource scheduling, is not so long relatively, but the delay generated by random back-off after collision has a large value relatively. Hence, the random back-off delay needs to be minimized or eliminated for an effective channel access procedure. Figure 1: A sample scenario of the next-generation vehicle-to-anything (V2X) system components. ITS, intelligent transport system; AP, access point. Figure 2: Two different scenarios of V2X system: (**a**) Sparse scenario of V2X; (**b**) Dense scenario of V2X. In the design of the proposed scheme, two essential aspects, namely, backward compatibility and massive V2X support, are carefully considered. Since the IEEE 802.11p-based V2X network is already being deployed and used for ITSs, NG-V2X networks shall be designed under the consideration of the IEEE 802.11p-based V2X network to provide backward compatibility.