# Experimental Demonstration of MmWave Vehicle-to-Vehicle Communications Using IEEE 802.11 ad
## 5 Experimental Result
### Campus Experiment  
Figure 10 shows the measurement results while driving through the campus and on the city roads. The MCS level was configured statically as level 1 (maximum 385 Mbps), with which a receiver can receive approximately 2500 packets every 100 ms by MAC aggregation. Considering the vehicle length and inter-vehicle distance on the roads, we concluded that the higher MCS level of 3 or 4, which was achievable only when the distance between vehicles was within approximately 5 m, was not achievable considering the driving testbed. The throughput shown in Figure 10 was instantaneously derived in a link layer that included error detection and retransmission mechanism. A receiver calculated the throughput of correctly received packets every 100 ms.  
In the measurement results of the drive through campus, which is illustrated in Figure 10a, the instantaneous throughput was widely scattered between 0 and 300 Mbps. Certain parts of measurement time demonstrated notable throughput. We observed that the throughput improved after 300 s while disconnections were observed clearly between 200 and 300 s. The corresponding SNR is plotted in Figure 10c. The average SNR increased marginally with time. Consequently, the required SNR for MCS 1 (i.e., measured value, \(-1\) dB in Table 4) was satisfied within approximately 200-300 s.  
\begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline SNR (dB) & \(-1\) & 2.5 & 4.5 & 6 & 7.5 & 8.5 & 9.5 \\ Distance (m) & 10 & 8 & 6 & 5 & 4 & 3.5 & 2.5 \\ Pathloss exponent (\(\alpha\)) & 2.1 & 2.07 & 2.09 & 2.0 & 2.08 & 2.1 & 2.2 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Pathloss exponent calculation based on SNR measurements.  
Figure 9: Received SNR of a slowly moving vehicle with beam angle \(\pm\)30\({}^{\circ}\).  
To investigate the relation between inter-vehicle distance and received signal quality during the driving test, we attempted to estimate the inter-vehicle distance using a computer vision technique (i.e., detecting a vehicle object using a marker on the front vehicle, as shown in Figure 4, and estimating the distance from bird's eye view) while measuring the signal quality of the received packets; however, it was difficult to couple a received signal sample from the IEEE 802.11 ad module and calculated distance based on the dash-cam video because of the processing delay in the real-time video and the distance estimation error, although the disconnectivity pattern presented in the following sections demonstrated an approximate correlation with the estimated inter-vehicle distance. However, the inter-vehicle distance during driving could be inferred from the measurement results shown in Figures 8 and 9.  
Figure 11 shows the disconnection interval and its probability. A disconnection interval is a period when a receiver cannot receive packets, i.e., the interval of samples with non-zero values. Our sampling interval could impose an error deviation of up to 100 ms. Most of the intervals existed for less than 2-3 s, as shown in Figure 11a. The cumulative distribution function (CDF) of the interval in Figure 11c shows that almost 90% of disconnection periods were less than 5 s.  
Figure 10: Campus and city road measurement results.  
Observation 5: The SNR and connectivity in IEEE 802.11 ad communications of slow-moving vehicles were more stable when compared to the fast-moving vehicles.