## 1 Introduction  
Connected cars are attracting attention increasingly and several commercial products for the connected car are expected in the near future. After a dedicated spectrum at 5.9 GHz was allocated for intelligent transport systems (ITSs), both in the US and in Europe in 1999 and 2008, respectively, different families of standards have been developed: the IEEE/SAE dedicated short-range communication (DSRC) in 2010 in the US, Release 1 of the ETSI/CEN Cooperative-ITS (C-ITS) in 2013 in Europe, and 3 GPP Cellular-V2X (C-V2X) in early 2017 as a feature of Release 14 of the LTE standard [1; 2; 3; 4; 5; 6; 7]. DSRC and C-ITS both use the IEEE 802.11p standard [4; 6; 7], which is a short-range technology for connected car communications, for the physical and data link layers [4]. Based on the IEEE 802.11p standard, the SARTRE project, which ran from 2009 to 2012, deployed a platoon of multiple connected cars driven autonomously in close formation [8]. In Japan, the fully automated truck platooning with a connected car system was tested on an express way in the Energy ITS project in 2012 [9]. The European Truck Platooning Challenge demonstrated the automated trucks of six major truck vendors that were driven in platoons on public roads [10]. Since currently wireless communication networks for connected cars are based on the IEEE 802.11p and wireless access in vehicular environments (WAVE) standards, the future wireless communication network for connected cars should provide a backward compatibility with IEEE 802.11p- and WAVE-based communication networks [11; 12; 13]. To enhance the safety and user experience of next-generation autonomous vehicles, not only enhancements in signal processing but also wireless network-based ITSs have been developed [14; 15]. To provide a wireless network-based ITS, vehicle-to-anything, denoted as V2X, communication systems are expected to be used in autonomous driving vehicles [14; 15; 16; 17; 18]. V2X is a wireless communication technology used to exchange ITS information among cars, between a car and infrastructure network access points (APs), or among user devices in the car. Therefore, in the future V2X communication, from the V2X communication point of view, connected cars will include autonomous cars. To effectively provide users with a comfortable and safe driving environment, the ITS AP shall be considered as an ITS sensor or an ITS beacon for every navigating connected car. In other words, the ITS AP shall accommodate ITS information from connected cars, ITS pedestrians, and some local sensors. Then, it shall transmit the information gathered and processed by the ITS AP or crucial ITS information received from a remote node, such as ITS service provider servers, remote vehicles to numerous ITS nodes including connected cars and ITS pedestrians by using its wired and wireless connections. Therefore, the numerous communication links between ITS nodes and an AP must be reliable and need to have low delay properties for the AP to perform the role of a reliable ITS beacon. Unfortunately, if the number of ITS nodes that an AP needs to support is too large, the ITS AP cannot establish those wireless connections rapidly in a random-access-based V2X network. Because connected cars have large mobility, the connection establishment delay caused by many ITS nodes in next-generation V2X networks is much more important than in other wireless network scenarios. In this paper, the next-generation V2X (NG-V2X) scenario described in Figure 1 is considered. ITS sensors are sensors collecting information related to ITS system. They can collect weather information, pavement condition information (road surface temperature, water film thickness, residual salt, etc. ), and traffic information including vehicle classification [19; 20]. They are anticipated to be used more widely for safety in various ITS scenarios in the future. The NG-V2X system must provide communications between a vehicle and anything including pedestrians, ITS sensors, vehicles, and ITS APs [21; 22].