## 6 Performance Comparison of IEEE 802.11p and LTE-V2V  
In this section, we present some example results to compare the performance of the two main candidates, IEEE 802.11p and LTE-V2V. To provide example results in realistic conditions, we consider the road network layout of downtown Bologna; specifically, a portion of 1.8 \(\times\) 1.6 km\({}^{2}\) has been simulated, corresponding to an area of 2.88 km\({}^{2}\), as shown in Figure 3. Traffic traces have been generated offline with a VISSIM micro-mobility simulator and given as an input to LTEV2 Vsim [58]. An average number of 455 vehicles travel in the scenario and periodically exchange beacon messages of 300 bytes with a frequency of 10 Hz. As a performance indicator, we use the packet reception ratio (PRR), which is the ratio between the total number of successfully received messages and the total number of neighbors of each vehicle, where we consider neighbors of all those vehicles that have an Euclidean distance from the source below the given awareness range (i.e., the given distance). Concerning IEEE 802.11p, we evaluated the performance of MCS 3, which corresponds to the use of a quadrature phase shift keying (QPSK) modulation and a coding rate of 1/2 (i.e., 6 Mbps), suggested as the best compromise between range and duty cycle by [59] and, for comparison, MCSs 1 and 8, since MCS 1 corresponds to BPSK modulation and a coding rate of 1/2, providing the lower raw data rate (i.e., 3 Mbps), but the higher reliability and MCS 8 corresponds to 64-quadrature amplitude modulation (QAM), coding rate 3/4, providing the higher raw data rate (i.e., 27 Mbps), but the lowest coverage. MCS 3 is a somehow \"average\" best choice, but, if we assume a sparse network, the best choice would be to use MCS 1, which allows higher range, since the channel occupation would not be a problem; in the case of a road congestion, instead, maybe using MCS 8 could be a better trade-off for the range (which would not be a problem, given that cars move slowly and close to each other) with shorter transmissions over the wireless channel. Hence, we consider various MCS options. For the LTE-V2V system, we assume Sidelink Mode 4 (i.e., autonomous resource allocation) and different MCSs: MCS 4 and 7, corresponding to QPSK modulation and MCS 12 and 20, corresponding to 16 QAM modulation. Similar considerations as those for IEEE 802.11p could also be applied for the choice of MCSs in LTE (further details on MSC and beacon resource (BR) allocation for LTE-V2V Mode 4 can be found in [5].). For the channel model, we refer, for both IEEE 802.11p and LTE-V2V, to the one proposed by 3 GPP, the Winner+ model B1, with shadowing log-normally distributed and having standard deviation of 3 dB in line of sight (LOS) and 4 dB in non line of sight (NLOS) [60]. The main settings are summarized in Table 3.  
Figure 3: Simulated scenario: an area of 2.88 km\({}^{2}\) of Bologna (Italy) downtown with 455 vehicles on average. In Figure 4, we plotted the PRR as a function of the transmitter-receiver distance for IEEE 802.11p and LTE-V2V varying the MCS. As expected, the curves are monotonically decreasing as the distance increases. It can be observed that the performance is highly affected by the adopted MCS and it is not easy a priori to predict which technology outperforms the other. For distances lower than 50 m, both of the technologies provide a PRR close to 1 and then LTE-V2V outperforms IEEE 802.11p when adopted with a MCS lower than 20. By adopting LTE-V2V with MCS = 20, the PRR is very similar to that of IEEE 802.11p with MCS 1 and 3 up to 100 m; then, IEEE 802.11p outperforms LTE-V2V. The worst performance, in this urban scenario, is provided by IEEE 802.11p with MCS = 8 since, as is known, it has the lower reliability in terms of covered distance; in fact, its PRR falls rapidly after 50 m of distance between the transmitter and the receiver.