# VLC Network Design for High Mobility Users in Urban Tunnels
## 4 Proposed VLC Network for High Mobility
### VLC Network Topology  
The network communication layout comprises a group of APs built-in the illumination system along the entire tunnel, which acts as RSUs. Each vehicular node has a VLC module that uses the headlamps as a transmitter and a photodiode-based receiver on the vehicle's hood. A full-duplex VLC system is assumed between a vehicular node and the APs. Figure 4 depicts the deployed vehicular VLC network scenario in which the tunnel roadside infrastructure provides coverage for the vehicular nodes. The AP's receiver is at 13 meters (m) forward its transmitter and a half meter from the ground level. This receiver's position is determined based on our previous analysis presented in [55; 56], where the channel propagation is studied for both uplink and downlink. The results show that the AP benefits to separate RX from the rest of its hardware to get a LOS contribution from the vehicular node. Due to the AP's receiver location, the uplink's power signal increases gradually. But suddenly, it suffers a drastic power reduction. Under this situation, an untimely handover initialization can break the uplink easily. So, it is necessary to increase the redundancy in this link to avoid abrupt interruption in the communication. As a partial solution, a new topology was presented in [57]. The topology extends the uplink range by integrating all the infrastructure's receivers in a new sublayer named \"2.5\". For this case, the receivers work as standalone devices connected by fiber to a common switch, which acts as an intermediary between receivers and the APs. Each receiver performs the optical signal demodulation and decodification to recover the binary information. Then, they send the information to the switch which checks the OWPAN-ID address to know the destination and redirect it. Finally, the AP receives the frame information and manages the channel access and other processes in the communication. This topology aims to extend the uplink range by increasing the time to resolve a handover. It allows the AP to receive a VLC frame \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Parameter** & **Value** & **Parameter** & **Value** \\ \hline \(T_{k}\) Temperature absolute & 298 & \(\Gamma\) FET channel noise factor & 1.5 \\ \hline \(I_{3}\) Noise Bandwidth Factor & 0.0868 & \(g_{m}\) FET transconductance & 30 mS \\ \hline \(I_{2}\) Noise Bandwidth Factor & 0.562 & \(k\) Boltzmann’s Constant & 1.3806e\({}^{-23}\) J/K \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._  
Figure 4: Graphical representation of the vehicular VLC communication system in V2I mode. when the vehicular node has passed its nearest receiver. It is because the switch redirects the frame using its header information allowing the collection of frames from any receiver. There are some cases when two or more receivers can receive successfully the same frame. If this situation happens, the switch will eliminate the redundant frames of the network. It allows the system to operate when a handover has not been initialized opportunely without disrupting the service. Figure 5 displays the diagram of this 2.5 layer topology. Furthermore, the APs are managed and coordinated by using an Aggregation Agent (AA). This topology follows the structure of IoT Device Management protocol (IDMP) which includes self-configurable devices [58]. Also, AA has to deal with the main actions during the mobility process. Internally, it has two virtual modules: Mobility Anchor Point (MAP) and Handover Manage Entity (HME). MAP keeps a database of the current user location in the network. So, the users are tracked using a local ID address following the HMIPv6 scheme. It also helps the user to retain its IP Address so that the mobility process will take less time. On the other hand, HME collects the information from other network entities to decide when starting a handover. The decision algorithm and the gathered information process are described in Section 4.2. Another system design consideration is the distribution of the cells.