## 6 System Performance and Test Case Analysis
### FCW Test Case
#### 6.2.2 Testing Procedure and Results' Analysis  
Based on the research report DOT HS 811 501 by the National Highway Traffic Safety Administration (NHTSA) [40], we designed an FCW test scheme and conducted case tests.  
Next, we will introduce the FCW test requirements, test procedures, and test results' analysis.  
* Test Requirements  
The test requires that the DUTs have a communication range greater than 300 m, a data update frequency (i.e., transmission frequency) of less than or equal to 10 Hz, a system latency of less than or equal to 100 ms, and a positioning error of less than or equal to 1.5 m.  
The test expects the HV to issue a forward collision warning between the distance points marked in Figure 13, which are \(\text{TTC\_max}\times\text{V\_HV}\) (\(\approx\)55.56 m) and \(\text{TTC\_min}\times\text{V\_HV}\) (\(\approx\)29.17 m). Here, \(\text{TTC\_max}\) is 4 s, and \(\text{TTC\_min}\) is 2.1 s, as defined in DOT HS 811 501. TTC refers to Time To Collision.  
* Test Procedures  
The FCW test procedure is as follows:  
Figure 12: The administrator login interface of the GUI.  
Figure 13: FCW testing procedure.  
The HV is parked at the center of the lane, 600 m from the RV. Its longitudinal direction is parallel to the road edge, facing in the same direction as the RV. * The HV accelerates straight ahead until it reaches a constant speed of 50 km/h before reaching a distance of 150 m from the RV. * Test recording begins when the HV is 150 m away from the RV. * The test ends when one of the following conditions occurs: a. The FCW alarm is triggered by the HV between the distance points of TTC_max \(\times\) V_HV (\(\approx\)55.56 m) and 90% \(\times\) TTC_min \(\times\) V_HV (\(\approx\)29.17 m); b. the HV fails to trigger the FCW alarm before reaching the distance point of 90% \(\times\) TTC_max \(\times\) V_HV; c. the HV triggers the FCW alarm before reaching the distance point of TTC_max \(\times\) V_HV; d. a false alarm is triggered. * After the test, the HV should turn (recommended) or brake to avoid colliding with the RV. * The real-time speeds, travel trajectories of both the HV and RV, the FCW application warning time, and the FCW application warning distance are recorded.
* Test Result Analysis The application response feature is shown in Figure 14, while the communication performance is shown in Figure 15. The test was repeated ten times under each working condition.  
As shown in Figure 14, the FCW TTC value declines slightly with the increase in node density. Moreover, there were differences in response time TTC between products from different Tier 1 vendors. One possible reason for this is the difference between the application response algorithms. Nevertheless, the overall, TTC performance was relatively stable.  
As shown in Figure 15, the packet loss rate rises with the increase in node density. 3 GPP has specified quantitative performance requirements for advanced V2X applications in TS 22.186; the reliability range for these applications is 90% to 99.999% [41]. This implies that, for V2X applications, the maximum packet loss rate should not exceed 10%, as a higher rate would indicate a severely degraded communication environment.  
Comparing Figures 14 and 15, we can conclude that communication performance has a limited impact on response time TTC. Even when the packet loss rate reaches 15.6%, FCW response time TTC is also higher than the TTC_min value, which is 2.1 s. The above test results prove the effectiveness of C-V2X technologies from another perspective.