# VLC Network Design for High Mobility Users in Urban Tunnels
## 6 Results and Discussion
### Single Vehicle  
This evaluation contemplates a single vehicular node under a controlled environment. Figure 16 shows and compares the service availability, defined in Equation (7), as a function of the APs separation under the three different scenario configurations described in Section 5. We observe that configurations 2 and 3 keep a high availability when the separation between the APs is less than 14 m. For a higher separation, the service availability begins to decay. With configuration 1, this service declines below 85% at a distance of 8 m and gets worse for larger distances. It is clear that the proposed 2.5 layer topology yields a significant improvement in the link reliability as compared with the conventional topology. The main reason for this behavior is that in a conventional topology the link can be broken before the handover starts, which causes to stop gathering information to execute the handover.  
Figure 17 provides the service time ratio using a logarithmic scale. The best overall performance is achieved with Configuration 3. We see that the interruptions in Configuration 2, using only the uplink information, are ten times longer than the interruptions in Configuration 3, where the service is interrupted averagely 1% of the time while it only uses the uplink information. Otherwise, this interruption time decreases until 0.1% in Configuration 3. It is not always possible to make the handover when the distance that separates the APs is more than 12 m, therefore, the performance decreases. We highlight that the 2.5 layer topology can support communication even when the link has been broken, as a result, it provides a supplementary communication time to detect the movement, and as consequence, there is seamless communication without severe interruptions. This improvement guarantees that the vehicle will stay connected almost all the time.  
The measured EFER of the VLC network with a single vehicular node is now shown in Figure 18. Here, we observe that Configuration 1 delivers EFER rates of about \(10^{-1}\) which are considered too high for vehicle safety messages. Configuration 2 achieves a performance of EFER rates lower than \(3\times 10^{-3}\) when the separation between APs is under 13 m, then it increases up to \(7\times 10^{-2}\) for separation of 15 m. Finally, Configuration 3 shows the most favorable performance with an EFER of \(5\times 10^{-4}\), almost one order of magnitude higher than Configuration 2. From a separation of 12 m between APs, the performance begins to decline, and eventually, Configurations 3 and Configuration 2 obtain the same results.  
Figure 16: Service availability comparison with single user.  
Similarly, in Figure 19 the compared PFER is presented concerning the position of the AP. From these results, the maximum errors for Configuration 2 and 3 do not exceed 15% in the case where the separation between the APs is below 14 m. Configuration 3 shows the minimum PFER of about 12% up to a distance of 13 m. On the contrary, the successful transmissions with the conventional topology reach a PFER of about 0.65 for distances below 12 m, and for longer distances between APs, the received frames are reduced to half of them.  
Finally, we plot the behavior of the data rate service in the Figure 20. The network is set to transmit a steady data rate of 450 kbps per vehicular node that proves to be enough to hold message-based safety applications as suggested in [63]. These results reflect that there is not a substantial throughput difference between Configurations 2 and 3. The system can handle the maximum requirements for both configurations thanks to the redundancy of the  
Figure 17: The outage ratio comparison (logarithmic scale).  
Figure 18: End-to-end frame error rate considering the backoff mechanism (logarithmic scale).  
uplink channel. On the other hand, Configuration 1 suffers from throughput degradation problems because of the lack of redundancy.