# Cellular-V2X Communications for Platooning: Design and Evaluation
## 6 Performance Evaluation
### Results
#### 6.4.2 PM-to-PM Phase  
We now focus on the PM-to-PM phase in scenarios with multiple platoons, and assess the performance of different configurations and parameters settings. **Sequential vs. simultaneous transmissions. ** We first compare sequential and simultaneous transmission schemes in terms of the average time required to complete a CAM cycle. Considering a scenario with \(N=12\) platoons, Figure 7 shows the average time required to complete a CAM cycle using the two modes. Since the platoon size affects the cycle duration, we group the platoons by the number of PMs and report a different measurement on the x-axis for each group. As expected, the completion time with sequential transmissions increases linearly with the platoon size. This is obvious, since each PM sends its CAM only after it has received the information from the preceding vehicle. In particular, the time required to obtain the frequency resources (i.e., sending RAC request and reporting the BSR) has to be paid at each hop, hence extending the time to reach the last member of the platoon. When considering platoons with more than six members, the completion time is longer than \(T=100\) ms. On the other hand, the completion time is almost insensitive to the platoon size in case of simultaneous transmissions and it stays at about 30 ms. Such results are in line with the theoretical bounds reported in Section 5. This is because the latency incurred to obtain frequency resources is paid off almost simultaneously for each PM. **Impact of CQI reporting. ** We now consider simultaneous PM-to-PM transmissions and evaluate how the MCS selection for P2P V2V communications affects the performance. The proposal has been evaluated under two different settings: (i) the MCS to be used is decided a priori (curves labeled as _CQI Reporting Disabled_ in Figure 8); and (ii) the MCS is set according to the CQI reports from the VUEs (curves labeled as _CQI Reporting Enabled_ in Figure 8). In the former case, the eNodeB schedules PM-to-PM transmissions with the same modulation as PL-to-PM ones, which we assumed to be the one corresponding to CQI = 7. In the latter case, each PM periodically reports the _wideband_ CQI measured on the P2P V2V link to the eNodeB, i.e., they report a CQI value that corresponds to the average channel quality perceived over _all_ the RBs. The reporting period is set to 6 ms (the tuning of the reporting period is outside the scope of the present work). Given the proximity of the endpoints in a PM-to-PM communication, the CQI reported by the PMs will generally be higher than the one used when reporting is disabled. This results in fewer RBs to be allocated every TTI for a single PM-to-PM transmission. Figure 7(a) shows the average number of RBs allocated by the eNodeB in a TTI with increasing network load, where we can observe that enabling CQI reporting allows the eNodeB to save RBs. With reporting disabled, the eNodeB starts to approach the saturation at 20 platoons, whereas using CQI reporting the subframe capacity saturates when there  
Figure 7: Average completion time of one CAM cycle (\(N=12\)). are 28 platoons under the eNodeB's control. When the maximum capacity is approached, the network is unable to serve rapidly all VUEs, hence some update cycles experience large delays or they even fail to complete at all, due to shortage of radio resources. Figure 8b,c show, respectively, the ratio of successfully completed cycles and their corresponding completion time. In order to perform a fair measurement, we consider only platoons with the same length \(L=10\). We observe that before going into a congestion situation, enabling CQI reporting guarantees higher reliability and significantly lower latency. We also note that with CQI reporting enabled, confidence intervals are quite large at intermediate traffic loads. We ascribe this behavior to different effects of wideband CQI reporting in different simulations runs.