# VLC Network Design for High Mobility Users in Urban Tunnels
## 2 Related Works
### Handover Process  
The handover process in the literature is commonly divided into three phases regardless of the communication technology and the handover strategy employed. These phases are information gathering, handover decision, and handover execution. In general, the research works in this area cover only one of these sub-processes. Information gathering is the sub-process of collecting the input signal to determine when the handover starts. This signal definition depends on the problem to study. For the vertical handover, it requires something that can be measured no matter the propagation medium. So, it uses upper-layer information that reflects the Quality-of-Service (QoS). This can be determined by many aspects such as the total latency, the bandwidth provided, or how reliable is the communication in each path. For example, protocols like Multipath MTCP senses each link's round-trip latency to determine the best option [23; 24]. Additionally, multiple inputs can be computed to get more accurate results analyzing them through a cost function [25; 26; 27]. It is modeled to see the spent resources considering the number of signaling messages necessary to complete a handover. When the network increases in size, it affects its performance and cost. On the other hand, horizontal handover requires collecting information that reflects the spatial position of the user. It compares the information of multiple APs to determine the closest. One common Physical indicator is the Received Signal Strength (RSS), which considers the proximity to an AP [28; 29]. In a complementary way, the Signal-to-Noise Ratio (SNR) or Signal-to-Interference ratio (SIR) can execute the same function. Furthermore, there are some link-layer indicators, such as Bit-Error-Rate and Frame-error-Rate, which can be used to estimate the time to proceed with a handshake process [30; 31; 32]. This collected information is treated in further stages of the handover process. The second phase is the application of the signal detection strategy to trigger the handover execution. This phase can use algorithms with fixed or dynamic thresholds, depending on the system's nature. Additionally, the handover criteria use multiple input and memory statements to improve efficiency. One of the main issues to deal with is the temporal anomalies of the tested signal. For example, optical systems suffer from shadowing effects. These phenomena can incorrectly trigger the handover process. For this reason, the handover-based RSS resolves after a second dwell measure corroborates the failure [33]. In some scenarios, a fixed delay presents a poor performance in the handshake estimation. For example, if a user often moves, the waiting time needs to be shorter. A dynamically dwell can deal better with an outage in these cases. It can consider the frequency that the user requires to do a handover during the last days or how much the received signal changes to adjust the tolerance time [34]. These algorithms require considering the communication signal from neighbors to decide to initialize a horizontal handover. When the signal difference between the APs is relatively small, the system may perform an incorrect estimation. To avoid these fluctuations in the handover decision, the target AP's signal value needs to surpass the current value with an additional tolerance, which is known as Handover Margin (HOM). When the user has changed the service provider, it cannot return to the previous AP until more robust return conditions have been accomplished. It shapes a hysteresis loop, which prevents the user hops from AP to AP when the link fluctuates [30; 35]. There is also a time restriction to avoid multiple handovers in a short period, called Time-to-Trigger (TTT). The last phase is the handover execution that performs the steps to resolve the mobility. These steps include a set of messages to request system resources and inform all the network elements about the new user localization. It is also necessary to establish the network topology with upper layer modules.