## 1 Introduction
Since all future cars are expected to be equipped with V2X communication devices, dense V2X communication scenarios will be common and massive V2X communication support will be required. As part of the efforts to solve two challenging issues of V2X network, the IEEE 802.11 working group has started to study the NG-V2X system. A study group for NG-V2X (NGV-SG) has been formed to consider various scenarios for the V2X communication system. It is expected that the study group will produce an NG-V2X standard based on the conventional IEEE 802.11p standard to ensure backward compatibility. It is considering a V2X system that will be more reliable, have a higher capacity, and support higher mobility, though specific details of the requirements are being discussed. In the proposed scheme, an IEEE 802.11-based extension is employed to provide backward compatibility, ability to operate in a mode in which the devices can interoperate with conventional IEEE 802.11p devices [34], and the emerging IEEE 802.11 ax standard-based orthogonal frequency-division multiple access (OFDMA) is adopted and extended to provide massive V2X support. This paper proposes a novel NG-V2X channel access scheme based on uplink OFDMA. Our main contributions in this paper consist of two parts: (1) a scalable and efficient channel access method using modified IEEE 802.11 ax uplink OFDMA random access (UORA) appropriate for massive V2X support and (2) an accurate mathematical modeling and analysis on the adopted IEEE 802.11 ax uplink OFDMA random access (UORA). The adopted IEEE 802.11 ax-based OFDMA schemes including UORA for the proposed NG-V2X channel access is modified to work in 10 MHz channel bandwidth by doubling the symbol length of IEEE 802.11 ax symbols. The operation frequency band of the proposed NG-V2X channel access scheme is the 5.9 GHz ITS band which is the same operation band as the conventional IEEE 802.11p based ITS systems. The proposed scheme includes a scalable channel access method appropriate for V2X applications, and it also enables effective communication between ITS sensors and ITS APs without impacting V2X system performance. Furthermore, a scheduled NG-V2X channel access scheme is also proposed for the extra optimization. To show how the proposed NG-V2X channel  
Figure 3: Channel collision in dense scenario of V2X. access scheme can enhance network scalability, mathematical modeling, and analysis, more accurate and closer to real standards than ever to the best of our knowledge on uplink OFDMA random access (UORA) are provided. In the mathematical modeling of UORA, some errors of the previous works are fixed and the actual UORA procedure of the IEEE 802.11 ax standard is considered. Then, the delay and user capacity performance of the proposed NG-V2X channel access scheme, including some further optimization methods, are evaluated in various simulation scenarios to demonstrate how the proposed schemes enhance user capacity performance of the NG-V2X network. The rest of paper is organized as follows. In Section 2, we introduce some background technologies including the conventional IEEE 802.11p standard and UORA, which are used as a basic channel access concept in this work. In Section 3, the proposed NG-V2X channel access scheme using the UORA procedure is explained. In Section 4, mathematical models of UORA and novel and efficient V2X enhancing methods including scheduled NG-V2X channel access are explained. Since the proposed mathematical modeling of the UORA procedure considers the latest updates on UORA procedure of IEEE 802.11 ax, the proposed modeling is the most up-to-date and accurate modeling to the best of our knowledge. The proposed NG-V2X scheme is specifically designed to transport ITS information in dense scenarios for the next-generation V2X networks. In Section 4, the system performance of the proposed uplink V2X channel access is investigated through extensive simulation.