# VLC Network Design for High Mobility Users in Urban Tunnels
## 5 Evaluation Methodology
### Experimental Setup  
Our proposed VLC network topology and handover protocol for high mobility is evaluated by simulations in two different scenarios. The first scenario measures and compares the performance of the three configurations considering a single user. While the second scenario selects the configuration with the best performance. Then, it evaluates the protocol's execution when the network increases its user number. Figure 13 shows the graphical representation of the first scenario. It considers a single-vehicle moving at 80 km/hr through a two-lane tunnel with a lighting system that contains a set of luminaries (APs). The enclosure has the maximum regulatory dimensions with a width of 13.2 m and 4.9 height [61]. The luminaries usually are mounted on the wall at 4.2 m above ground level with an elevation angle of 20\({}^{\circ}\). We remark that the lamp positions follow the existing norm of tunnel-lighting in most countries [62], which recommends 8 to 13 m between two luminaries to avoid generating flickering effects to the drivers. Therefore, our configuration proves feasible in real scenarios by reusing the hardware of the existing illumination  
Figure 12: Physical layer submodule. systems. On each AP, the VLC receiver is 13 m forward of the luminary mounted on the wall and 50 cm above the road, opposite to the traffic direction with a 90\({}^{\circ}\) elevation, to ensure a good signal reception over an adequate period. On the other hand, the vehicular node has a built-in VLC system, which uses its headlamps as the transmitter and adds a receiver near to the left headlamp (on the hood), pointing to the tunnel ceiling with an elevation of 180\({}^{\circ}\). The vehicular node is assumed to be transiting on the lane closest to the tunnel wall with the transmitter (headlamps) at 1 m above the ground level and its receiver at 2.4 m from the tunnel wall. This is the major distance between the lane edge and the closest wall in a two-lane tunnel. Consequently, the critical scenario for the uplink. Since the natural driving of a person makes the car moves from side to side in its lane, it loses its alignment with the road lane and consequently with the tunnel wall. Therefore, the handover process may be affected by these displacements, but also the narrow radiation pattern of the headlamps. For these reasons, we include an error margin of 0.225 m to the vehicle's position against the tunnel wall to simulate these types of behaviors. For the simulations, the initial position of the vehicular node starts from 10 to 13 m behind the first tunnel luminary. This configuration evaluates the vehicle's communication with a receiver placed in the closest wall. Additionally, this simulation setup assumes that the vehicles keep a safe separation distance, providing Line-of-Sight communications at any time. The simulation process is performed as follows. First, the vehicular node moves straight forward with a steady speed while the APs are transmitting beacon frames. When the vehicular node receives one of these frames, it starts the discovery process to find the best possible connection. Then, it waits for a random period and then sends an association request. After the association process is resolved, the AP transmits data frames with a constant throughput. When the vehicle is leaving the AP coverage area, the network needs to detect its departure using the handover decision algorithm described before. If the estimation is correct, the handover protocol is executed to start a new session in the next AP and continue the communication. In other cases, the handover process fails, the AP will continue transmitting the data frames until the disconnection time expired. At the same time, the vehicular node detects a failure if it stops receiving the beacons. Then, the vehicular node needs to keep sensing the channel looking for an available AP to start the association process again.