# VLC Network Design for High Mobility Users in Urban Tunnels
## 5 Evaluation Methodology
### Experimental Setup
The second scenario considers vehicles in a convoy that commute in a tunnel, see Figure 14. For this scenario, we extend the experimental evaluation of the handover process  
Figure 13: Scenario 1: Graphical representation of the VLC network with one vehicular node. which can be depleted by vehicles sharing the same channel. In this setup, there are \(m\) vehicles moving at the same speed as a caravan. The infrastructure configuration is the same as the single-vehicle evaluation. And the leader vehicles are placed in the same position too. The distance between subsequent vehicles is defined as a random variable uniformly distributed over the range of 7 to 10 m. This scenario does not accomplish the suggested safety separation distance between vehicles, but it is done to evaluate the possible case when a vehicle ingresses an occupied cell. Under this situation, the vehicular nodes need to share resources. Therefore, there is a possibility that one of the handover messages may collide with another vehicle's communication. Other relevant changes in the experimental configuration include the vehicular nodes' position, the network's conditions, the number of nodes connected to a particular AP, etc. The simulation process of this second scenario follows the same process as described in the single vehicular node case. Table 2 summarizes the more relevant simulation parameters for the experimental scenarios. Each experiment was evaluated 100 times under the same parameters. The parameters considered has been 8 different distances between AP, 3 system configuration (1 start topology, and 2 proposed topology), and 2 scenarios (single-user and multiples users). There is a total of 2400 simulations for the first scenario, and 3200 simulations for the second scenario. Each simulation recreates 18 s of communication where the vehicle needs to perform between 27 to 50 handovers to have seamless communication. The decision algorithm is executed when a vehicle sends a pilot every 5 ms, it implies that the detection algorithm is executed around 3600 times in a single simulation. Additionally, the nodes update their channel every 1 ms to have smooth changes and have an accurate model. The communication system follows the IEEE 802.15.7 specs using a 60 MHz optical clock with an OOK modulation in the PHY II mode. Moreover, the separation range between the APs is 8 to 15 m, the VLC transmitter powers of the tunnel luminary and the vehicular nodes' headlamp are 50 W and 15 W respectively, and an active surface area of the VLC receiver of 1 mm\({}^{2}\) and Field-of-View of 60\({}^{\circ}\). For this initial evaluation, there is not considered the integration of a lent in the receiver. Additionally, some MAC parameters were set up. Firstly, BI was set at 2 ms. This time is long enough to allow two or more users to share the same medium but not so prolonged in case they lose synchronization. Such time could be fixed by adjusting B0, SO, and \(aBaseSuperframeDuration\) values, as can be seen on the parameters table. Also, the channel access mechanism was set to execute up to 5 additional transmissions tries before it discards the information. In each new attempt, the distribution time for the random access increases exponentially using a \(aBackofUnit\) base of 200 in its calculation. The simulation considers a frame data structure that comprises a payload size of 1000 bits with a header of 207 bits. The header size is an approximation considering the bits in each one of its fields. The payload for beacon and signaling frames was fixed to 500 bits. The  
Figure 14: Scenario 2: Graphical representation of the VLC network with multiple vehicular nodes. frame is encoded with Run-length limiting (RLL) which increases the frame size with a ratio of 10 bits per 8 input bits (8b10b), these aspects determine the transmission period.