# Hybrid Multi-Antenna Techniques for V2X Communications--Prototyping and Experimentation
## 4 Performance Evaluation, Field Tests, and Results  
The performance of the diversity engine has been evaluated in field measurements using various key performance indicators (KPIs). The engine collects, measures, and/or calculates the following KPIs:  
* Output SNR;
* SNR diversity gain, as compared to a single-input-single output omni (SISO-OMNI);
* Bit error rate (BER);
* Packet error rate (PER);
* Coverage probability;
* Channel capacity/Achievable throughput;
* Latency.  
The field tests were performed in the context of the H2020-ROADART project, with the integration of a real-world T2T communication testbed. The measurements were conducted in a test truck (from MAN Trucks and Busses) as well as in public highways. Tests involved two trucks moving as a convoy for various inter-vehicle distances.  
Next, a few representative performance results, which have been obtained using the data gathered during these tests, are presented and discussed. In Figure 8, a plot of the time variation of SNR values as the two trucks were driving along the test highway route is presented. The superiority of the implemented diversity engine is clearly depicted as compared to a SISO omni system. Moreover, in Figure 9, the empirical probability density function (PDF) is shown for the diversity gain of the proposed hybrid diversity scheme as compared to SISO-OMNI. From this plot, it is observed that the diversity gain is 12 dB on average. In the same figure, the empirical PDF of the received SNRs for the proposed hybrid scheme vs. one employing conventional MRC diversity reception, that is, without ESPAR antennas and pattern selection, is also presented. It is shown that the reconfigurability of the antennas considerably improves the output SNR and thus the overall system's performance.  
In Figure 10, the empirical PDFs of the achievable throughput for the hybrid diversity vs. SISO-OMNI schemes are plotted, assuming 10 MHz signal bandwidth at 5.9 GHz. It is observed that the diversity engine increases by 2.5 times the channel capacity confirming the performance improvement achieved by the proposed scheme.  
Figures 11-15 depict screenshots from the conducted real-world experiments, as captured from the Grafana visualization tool that was used for real-time system monitoring and demonstration during the trials. In the specific set of experiments, inter-vehicle distance was initially quite large (more than 1 km) and it was gradually reduced. At the end of the experiments, the inter-vehicle distance was below 50 m. Figure 11 represents the significant SNR gain offered by the use of the diversity engine compared with the SISO-OMNI system. Consequently, the following figures present the comparison of the aforementioned KPIs. Figure 12 presents the coverage probability of the hybrid diversity engine vs. the SISO-OMNI system. It is clear that in the SISO-OMNI, coverage is quite limited emphasizing the impact of the truck trailer as a shadowing component. On the other hand, connectivity and availability are significantly increased using the proposed scheme. The same conclusion is extracted from Figures 13 and 14, where it becomes clear that for the specific scenarios the SISO-OMNI system has very high bit and packet error values, while the diversity engine provides reliable links for all measured distances. Finally, in Figure 15 the difference in physical layer latency is presented caused by the need for retransmissions due to the errors for the SISO-OMNI case.  
Figure 8: Signal-to-noise ratio (SNR) vs time for diversity and SISO-OMNI.  
Figure 9: Histograms of SNR for communications links (diversity vs. SISO-OMNI).  
Figure 11: SNR gain of the diversity engine vs. SISO-OMNI (visualization with Grafana tool).  
Figure 12: Coverage probability of the diversity engine vs. SISO-OMNI (visualization with Grafana tool).  
Figure 10: Achievable throughput at 10 MHz bandwidth (diversity vs. SISO-OMNI).