# VLC Network Design for High Mobility Users in Urban Tunnels
## 2 Related Works
### Handover Process
Sometimes, the modules require adding links to exchange messages such as X2 in LTE technology. The mobility mechanism can have a hard handover event when the transmission is cut off and immediately a new link is established. It requires the least processing by the network to be imperceptible to the users' applications. On the other hand, a soft handover event happens when the current and target APs provide service coordinately during the transition phase until the handover process has been resolved. An example of these schemes is Coordinated MultiPoint transmission (CoMP) which has been studied for VLC applications [28]. In this scheme, the current and target APs share a time window to transmit coordinately the exactly optical signal simultaneously. It helps to avoid a communication interruption when the user commute from one AP to the other. This technique requires precise coordination to prevent symbol interference. Other soft handover schemes use unsynchronized transmissions, the packets are sent by both AP, but not at the same time/medium [36]. A significant part of the time spent during the handover resolution is applied in updating external agents about the user location. The network needs to give a new address to the user, so it is necessary to check Duplicate-Address-Detection (DAD) and later upload at Home Address entity the Care-of-Adress (CoA); both actions take significant time. Indoor protocols take a long time, even the fastest version like Fast Handover Mobile IPv6 (FMIPv6) takes over 30 ms [37]. However, in high mobility networks such as vehicular networks, they can not tolerate a long delay in the resolution. Thus, they need to opt for the fastest protocol possible. An approach for this is to redirect the traffic to the new target AP during the handover execution. The user is made aware of the current AP about mobility. Then, the current AP exchanges parameters with the target AP and the mobile user to the start session between them. When the packets are delivered to the previous AP, it buffers them using a tunneling session [38; 39; 40; 41; 42]. One of the most common tunneling protocols is MultiProtocol Label Switching (MPLS). The process continues until the handover has been resolved. Proxy Mobile IPv6(PMIPv6) is a protocol based on this technique [43]. Another existing approach is to avoid updating the network configuration to external agents. In these cases, the user identifies its link with an internal address. When the node needs to exchange AP, the resolution is locally known as handover Layer 2 (L2). L2 reduces many processes regarding a typical handover (L3). It only requires the scanning process to determine when it is necessary to move the node, an authentication that involves a local network coordinator, and the re-association signaling message. This scheme avoids updating the node localization to other networks. One protocol that can provide tools to do this action is Hierarchical Mobile IPv6 (HMIPv6) [44]. This model will be detailed further in Section 4.1. The protocols are not mutually exclusive and they can be combined [45].