## 1 Introduction
### Related Work  
To cope with the increasing number of connected vehicles, one promising key candidate is allowing two vehicles to employ the same communication channel without increasing the spectrum requirement. In particular, non-orthogonal multiple access (NOMA) techniques are introduced as promising candidates for the future wireless communication vehicular networks [10; 11]. In recent years, it has become popular to adopt NOMA in vehicular networks. In [12], the authors studied the NOMA in the V2X scenario in terms of capacity, and the difficulties in future generation communications are considered. By exploiting the principle of NOMA, two vehicles communicate with another vehicle through a vehicle-to-vehicle (V2V) link. In other way, a vehicle communicates with an RSU via a V2I and dedicated vehicles are known to have the ability of signal decoding adopted by successive interference cancellation (SIC). On the other hand, NOMA provides a new dimension for V2X services due to the capability of achieving high overloading transmission over limited resources. Therefore, implementing NOMA in V2X addresses the congestion situation, i.e., reducing the latency as transmission in a dense environment. In [13; 14; 15], NOMA-enabled 5G vehicular communication systems were studied to consider the key problems of interference management and handover. To enhance the spectral efficiency of the NOMA-enabled 5G vehicular communication system, the authors proposed a layered power control scheme to deploy joint optimization of power control and cell association, and then the traffic load conditions and the high mobility can be adapted. The authors in [16] studied a full duplex NOMA (FD-NOMA)-based decentralized V2X system model. In [17], the authors considered NOMA-based heterogeneous vehicular networks and proposed robust resource allocation to enhance both the throughput performance and reliability of such network. In the scenario reported in [18], the base station distributes time-frequency resources and employs semi-persistent scheduling in a non-orthogonal manner while a distributed power control scheme is adopted at the vehicles autonomously. It is confirmed that NOMA in a dense network can reduce the access latency and then the reliability can be improved. Regarding other transmission techniques, namely cooperative relaying is introduced since it mitigates the effects of path-loss, shadowing, and multi-path fading [19]. Relay selection techniques in relaying network benefit to better signal are selected to serve destinations. Among those techniques, an opportunistic relay selection (ORS) is studied in [19]. The performance of near and far users can be improved when the partial relay selection (PRS) scheme is applied by selecting the best node among multiple intermediate relays [20; 21]. In [20; 21], the main performance metric is considered, such as the closed-form expressions of the outage probability. Through analyses and simulations, these relay-assisted cellular networks are considered as reasonable schemes to improve the coverage of 5G networks. In addition, the reduced outage probability can be achieved in an ORS-based network at the mode of delay-tolerant. Both the NOMA and orthogonal multiple access (OMA) transmission in the relay network are explored in [22]. This work introduced a buffer-aided relay selection scheme based on novel prioritization. The authors in [23] presented the two optimal relay selection (RS) schemes, namely max-weighted-harmonic-mean (MWHM) schemes and the two-stage weighted-max-min (WMM). Two RS schemes are implemented with cooperative NOMA with two modes of power allocations (fixed and adaptive) for the relays, and hence higher diversity transmission can be achieved. It is hard to replace the battery and/or there is no power line for the secondary network containing the limited energy supply for the secondary users. Therefore, such situation limits performance of the secondary network.