## 1 Introduction
### Our Contribution  
The recent works have shown great potential of V2X communications with challenges such as energy efficiency, spectrum efficiency, dense vehicles, outage performance, and multiple services. Firstly, together with other techniques, NOMA is a key solution for the optimization of operation efficiency in the circumstance of crowded vehicles. Note that most of the papers focused on supporting low latency and high reliable services. For example, the authors in [35] considered the secure energy-efficient transmission in the context of SWIPT and vehicle-to-infrastructure (V2I) communication system with the imperfect channel state information (CSI). However, they did not apply the advantage of CR and NOMA. The coexistence of satellite and vehicular networks is further studied together with efficient resource utilization; however, the authors mainly investigated power allocation for the cognitive satellite-vehicular network in [36]. In fact, the inherent mobility of V2X-NOMA communications can be further considered to improve these metrics including outage performance, multiple services, network energy efficiency, and reliable transmission. Furthermore, a large number of vehicles need an infrastructure that is associated with a larger number of RSUs. With the rapid growth of infrastructure development, the energy consumption of RSU located everywhere is also a challenging issue. Meanwhile, most of the portable RSU only has a limited battery capacity due to size and space constraints, which directly affects users' quality of experience (QoE). Thus, energy efficiency has become a key performance indicator for V2X-NOMA wireless networks. In particular, we derive an exact closed-form of the outage probability for each secondary destination under the context of CR. Such CR aims to further improve spectrum efficiency. In general, three folds would be the main benefits: (1) The SWIPT is proposed as a popular technique to prolong the operation duration of power-supply-limited devices in wireless networks. (2) CR-based V2X is considered as a special form of V2X, wherein the requirements of secondary and primary devices are strictly met so that performance improvement can be achieved. (3) The intermediate device can be selected to enhance the performance of related transmission. Those reasons motivate us to implement wireless power transfer to RSU to serve better transmission to vehicles and to overcome energy limitations at these low-cost RSUs.  
The main contributions of this work are summarized as follows:  
* It is the first work that formulates a situation in which many RSUs serve vehicles under operation of a CR-based V2X network. Such a V2X and NOMA scheme get benefits from the ability of energy harvesting to RSUs, which are devices with limited power source. The harvesting energy of each individual RSU then is selected to serve vehicles to achieve a minimum transmission rate and interference constraint. The link quality from the base station to vehicles is further benefitted by the RSU selection scheme.
* The outage probability metric is measured in exact expressions. The fixed power allocation factor is assigned to two vehicles. The power allocation factors and detection priority are considered as the main factors of different performance of two vehicles in terms of outage probability.
* Simulation results show that the outage performance achieved under the energy harvesting model may be affected by target rates, amount of harvested energy, and interference constraint. Moreover, simulation results show that there is a trade-off between the system performance and the power allocation factors assigned to vehicles.  
The rest of this paper is organized as follows. In Section 2, the system model is presented. Section 3 presents the outage performance analysis. Section 4 presents simulation results. Finally, the paper is concluded in Section 5.