# Survey and Perspectives of Vehicular Wi-Fi versus Sidelink Cellular-V2X in the 5G Era
## 4 Results  
Results are shown focusing on the Highway in Figure 3, on Bologna in Figure 4, and on Cologne in Figure 5. Each of these figures is divided into four subfigures and all results are shown varying the adopted MCS. The upper two subfigures show the maximum distance that allows a PRR higher than 0.9. The lower two subfigures show the 99.99 percentile of the update delay. On the left, results refer to IEEE 802.11p, legacy or with LTE PHY layer (_11p w/LTE-PHY_ in the figures), while on the right, results correspond to LTE-V2X. From Figures 3-5, we can derive the following considerations:  
* **Varying the MCS in IEEE 802.11p:** Observing the impact of the MCS on PRR in IEEE 802.11p, it can be noted that the optimal value strictly depends on the considered scenario. The optimal MCS is in fact subject to a clear trade-off: a higher MCS allows for shortening the transmission duration and to decrease the congestion of the channel, but, at the same time, it reduces the reliability of the communication. For example, considering the standard version, the best MCS moves from 4 in Highway to 6 in Bologna, where there is a more congested situation, and to 3 in Cologne, where the propagation plays a more relevant role than the congestion. In addition, the performance in terms of UD is not monotonic and the best MCS is not coincident with the one that maximizes the PRR. In the case of UD, what is relevant is not only the error rate, but also the correlation among errors, which increases when the channel is more congested. Anyway, except for high MCSs, the variability of the UD is limited. * **Varying the MCS in LTE-V2X:** In LTE, the trade-off between congestions and reliability is also influenced by the way packets are allocated in the frequency domain. For example, moving from MCS 3 to MCS 5, the number of available resources is always 1 per TTI (see Table 3), thus the reduced reliability does not reflect a less congested channel. Due to this reason, in all scenarios and settings, the best MCS in terms of PRR is either MCS 3 (the lowest with one packet per TTI), MCS 6 (the lowest with two packets per TTI), or MCS 12 (the lowest with three packets per TTI). In particular, MCS 3 provides the best performance in Cologne and MCS 6 in Highway and Bologna (except for Highway, Mode 4 with \(p_{\text{k}}=0\), where MCS 12 is slightly preferable). In addition, in this case, with few exceptions, the variability of UD is not remarkable and the best MCS in terms of UD does not exactly coincide with the one obtained looking at the PRR. It can also be noted that, in most cases, the best MCS in terms of UD is higher than the one in terms of PRR, since, in LTE-V2X, a higher MCS means more orthogonal possible allocations for the packets and thus less probability to choose the same resource for long intervals. * **Standard IEEE 802.11p vs. LTE-V2X Mode 3:** Comparing the PRR of the standard IEEE 802.11p and that of LTE-V2X with Mode 3, the difference is always remarkable and, considering the best cases, the latter provides from 1.4x (in Bologna) to 2.9x (in Highway) times more distance. In terms of UD, LTE-V2X is again preferable in Highway, where neighbours are considered within 200 m, whereas IEEE 802.11p provides better results in the urban scenarios, when neighbours are considered within 100 m. The lower UD achieved in some cases by IEEE 802.11p does not appear anyway sufficient to balance the results in terms of PRR. * **Standard IEEE 802.11p vs. LTE-V2X Mode 4:** When Mode 4 of LTE-V2X is adopted, the comparison with IEEE 802.11p leads to less definite results. Indeed, the conclusion changes depending on \(p_{\rm k}\): if \(p_{\rm k}=0\) is assumed, the PRR of IEEE 802.11p is higher than that of LTE-V2X in Bologna, similar in Highway, and lower in Cologne, whereas the UD is lower in Bologna and Cologne, and similar in Highway; if \(p_{\rm k}=0.8\) is set, then the PRR of LTE-V2X is higher, but at the cost of a UD that is also higher.