# Two-Way Transmission for Low-Latency and High-Reliability 5G Cellular V2X Communications
## 4 Numerical Results  
For performance evaluation of NOMA-V2X, we investigate a typical pair of vehicles with random pairing in NOMA networks. We perform Monte Carlo simulations to obtain outage probabilities of vehicles with the following parameters. Note that chosen parameters are similar to those used in [30]. Target data rates for vehicles \(\mathrm{D}_{1}\) and \(\mathrm{D}_{2}\) are set as \(R_{1}=3\) [BPCU] and \(R_{2}=1\) [BPCU], respectively. Without loss of generality, the power allocation ratio for transmitting \(x_{1}\) and \(x_{2}\) at RSU is set as \(a_{1}=0.8\) and \(a_{2}=0.2\). We suppose Nakagami-m distributed channel gains \(h_{n^{*},1}\) and \(g_{1,n^{*}}\) with \(\lambda_{h_{1}}=\lambda_{g_{1}}=d^{-\alpha}\), and \(h_{k^{*},2}\) and \(g_{2,k^{*}}\) with \(\lambda_{h_{2}}=\lambda_{g_{2}}=(1-d)^{-\alpha}\), where \(\alpha=2\) and \(d=0.3\) are considered. We also consider \(\lambda_{f_{1}}=\lambda_{f_{2}}=-10\) [dB]. We let all channels have the same fading parameters, i.e., \(m=m_{h_{1}}=m_{h_{2}}=m_{g_{1}}=m_{g_{2}}=m_{f_{1}}=m_{f_{2}}\). Note that other remaining key parameters are specified in simulation results. Figure 2 shows how the increase of transmit power at the RSU influences the performance of NOMA-V2X system in term of outage probability, where various numbers of antennas at vehicles are considered. We use the same number of antennas for D\({}_{1}\) and D\({}_{2}\), i.e., \(N=K\), where \(N=K=1,3,5\) are considered. Since a better channel can be selected more probably with a higher number of antennas at vehicles, the lowest outage probability can be observed for the second vehicle D\({}_{2}\) with \(N=K=5\). It is observed that the outage probability of two vehicles are considerably different at low transmit power, i.e., \(\rho<15\) [dB], while the outage performance of two vehicles are similar at high transmit power regime. It is also observed that, in general, D\({}_{2}\) has lower outage probability than D\({}_{1}\). Note that we obtain lower outage probability over all ranges of transmit SNR by using pSIC at D\({}_{1}\). In Figure 3, we plot outage probabilities obtained theoretically versus transmit SNR \(\rho\) with the fixed number of antennas at two vehicles. The lowest outage probabilities of two vehicles are obtained with pSIC condition and a fading parameter \(m=5\), where different fading parameters result in different outage performance. In case of using pSIC at the first vehicle D\({}_{1}\), the cooperative NOMA system under consideration results in the same outage probability for both vehicles at high SNR, i.e., \(\rho>12\) [dB]. In Figure 4, we plot outage probabilities of NOMA-V2X and OMA-V2X systems obtained analytically and by simulations with pSIC and ipSIC at the first vehicle. The outage performance comparison is presented to illustrate the outage alleviation occurring at specific range of \(\rho\). Note that theoretical evaluations match well with simulation results. All other trends of the outage probabilities of two vehicles are similar to previous results. Figure 2: Outage probabilities of two vehicles versus transmit signal-to-noise ratio (SNR) with a fading parameter \(m=1\). Figure 5 shows the outage probability with respect to the number of antennas at each vehicle with perfect and imperfect SIC, where two fading parameters \(m=1\) and 2 are considered. With the pSIC at \(\text{D}_{1}\), the outage performance can be improved tremendously by increasing the number of antennas. The gap between the outage performances of \(\text{D}_{1}\) obtained with pSIC and ipSIC gets more considerable as the number of antennas grows. In general, the outage performance of NOMA-V2X can be improved by using a higher number of antennas because using more antennas results in higher possibility of  
Figure 4: Comparison between NOMA-V2X and orthogonal multiple access (OMA)-V2X regarding outage performance of two vehicles with \(N=K=3\) and \(m=3\). Figure 3: Outage probability obtained for various fading parameters \(m\) with \(N=K=3\).