# Two-Way Transmission for Low-Latency and High-Reliability 5G Cellular V2X Communications
NOMA was adopted in V2X services to reduce resource collision, by which the SE was improved and the latency was reduced [21]. It introduced a new paradigm to obtain high overloading transmission over limited resources [21]. Vehicular networks have opportunities as well as challenges, where solutions for challenges have been proposed in various forms in recent works [22; 23]. In order to achieve low latency and high reliability in vehicular networks, NOMA was employed in various scenarios. A V2X broadcasting system utilizing NOMA was studied to provide a mixed, centralized/distributed architecture and to distribute power control to participating vehicles [23]. [PERSON] and [PERSON] pointed out that the V2X network can achieve a rate close to the capacity bound by exploiting new graph-based practical encoding and joint belief propagation decoding techniques [24]. In [25], NOMA-spatial modulation (NOMA-SM), which benefits from the robustness of SM against channel correlation, was proposed to improve bandwidth efficiency and to deal with the harmful effects of wireless V2V environments. 5G V2X communications using two relay-assisted NOMA transmission schemes were studied in [26], in which optimal power allocations for full-duplex relay-assisted NOMA (FDR-NOMA) and half-duplex relay-assisted NOMA (HDR-NOMA) broadcasting and multicasting systems. A NOMA system employing power allocation algorithm with opportunistic constraints was proposed to improve the throughput of V2X networks [27]. A hierarchical power control method via dynamic power allocation from BSs to vehicles was introduced to optimize the energy efficiency of the vehicular networks [28]. Inspired by the above observations, we apply NOMA technology to 5G V2X communications and investigate the performance at each vehicle. We consider a system model conveying the problem of half-duplex relay-assisted broadcasting/multicasting for V2I. Different from [26], we consider the relay-assisted NOMA-based broadcasting/multicasting schemes for the links from BS to vehicles. We propose a resource management algorithm which is central and thus needs to be implemented at the BS. We add FDR-NOMA scheme to the result of [20] and compare it with HDR-NOMA. The proposed transmission schemes can be adopted in communications from infrastructure to vehicles. By integrating NOMA and relay-assisted broadcasting/multicasting communications, we can improve spectrum efficiency, increase the number of connections, and achieve lower latency and high reliability. The main contributions of this paper are listed below:  
* We consider multi-antennas at vehicles. NOMA scheme is suitable for communications between two vehicles driving around the roadside units (RSU). In order to share information more efficiently, the communication mode of V2X will mainly be two-way transmission, rather than one-way connection. * For the sake of expanding the network coverage, we use RSU to assist vehicles to communicate with BS. In the broadcasting scenario, each RSU transmits information to vehicles in different directions. Different from the traditional OMA scenario, RSU serves multiple vehicles using NOMA in both uplink and downlink scenarios, since vehicles in the same group need to receive different information from the RSU. * We derive the exact outage probability expressions for two vehicles in a pair in terms of perfect SIC (pSIC) and imperfect SIC (ipSIC). The obtained closed-form expression for outage probability is quite accurate. The system throughput is also discussed in the mode of delay-limited transmission. * Impacts of SIC operation at receivers in the context of NOMA are carefully considered. The system performance in terms of outage probability can be enhanced by increasing the number of transmit antennas equipped at vehicles. The rest of this paper is organized as follows. In Section 2, the model of V2X communication employing NOMA and multiple antennas is introduced.