# Visible Light Communication: A System Perspective--Overview and Challenges
## 4 Hybrid VLC
### WiFi-VLC
This system involves, among other components, a custom network protocol with physical, link, network and transport layers, to support the developed system. While other layers are similar to traditional networking layers, the networking layer, however, is modified significantly to provide seamless mobility in addition to routing packets. The authors have performed experiments and have shown that the optical femtocells in the WiFO could achieve a data rate of up to 50 Mbps at a distance of 3 m.  
In [70], a hybrid VLC and RF system is considered that exploits the longer range of RF to provide control functions and increase per user rate coverage of the VLC cells. It quantifies the minimum spectrum and power requirements which can achieve a certain per user outage performance. The hybrid system consists of central unit (CU) which assign users to either VLC or RF network based on the user's channel conditions (users with the low data rate in the VLC network are migrated to RF network). The proposed system is evaluated for two scenarios, D1 and D2. In D1 power and spectrum are kept constant while in D2, CU dynamically adjusts power and spectrum according to the traffic demands. Results have shown that that D2 outperforms D1. In [71] a hybrid VLC and RF system has been proposed which aims to support diverse quality of service (QoS) needs of users. The QoS needs are studied for limits on the buffer overflow and delay violation probabilities. Results show that RF performs better for arrival rate in some settings, while VLC has better delay performance. In [72] a hybrid WiFi-VLC system has been implemented which consists of two duplex links. One is an aggregate of VLC and WiFi, and the other is WiFi duplex link. WiFi is used for the uplink transmission. The Linux bonding technique is used to achieve link layer aggregation using the Linuxbonding driver to build a logical interface (LI) on the client. Throughput and round-trip time (RTT) of the hybrid systems are evaluated under different contention scenario and VLC operating distances. The proposed system was benchmarked against the conventional WiFi and a hybrid WiFi-VLC system (non-aggregate system with asymmetric VLC downlink and WiFi uplink). Experiments have shown that the throughput of the aggregate system outperforms the standalone WiFi and a conventional WiFi-VLC hybrid system. The RTT of the aggregate system shows considerable improvement over the benchmark system particularly when the WiFi is facing congestions. Most of the existing work in VLC-WiFi hybrid system is based on a single VAP in a confined area. WiFi is used for uplink in close proximity similar to WiFi direct. As shown in Figure 3, one WiFi AP can cover multiple VAP's. However, WiFi signals attenuate over the distance, which would increase the errors in the uplink and thus feedbacking in the downlink to decrease the data rate. As the VAP is deployed at multiple locations to cover a large indoor area, then the collaboration between different VAP and one or more WiFi is not evaluated. This creates a bottleneck towards the scalability of VLC-WiFi network. Furthermore, there has been less reported work on the cost, fault tolerance, security and overall throughput of the systems.