# Visible Light Communication: A System Perspective--Overview and Challenges
## 4 Hybrid VLC
### WiFi-VLC  
WiFi (wireless fidelity) is a trademark of WiFi Alliance, based on the 802.11x family of Ethernet standard for wireless local area networks (WLANs). Devices can connect to the internet via a WLAN access points (APs). WiFi is primarily used for internet access in the indoor environment connecting via wire to the backbone network [60] as shown in Figure 3. WiFi direct is another variant of WiFi, providing direct connectivity between user devices without any intermediate device [61]. The main application of VLC is to provide high-speed connectivity in an indoor environment. Light fidelity (LiFi) is one such indoor application of VLC, proposed by [PERSON] in 2011 [62]. LiFi promises a fully networked indoor system with bidirectional support for point-to-point (P2P) and multipoint-to-point communication (MP2P). It also provides the capability of multi-access points and the formation of wireless networks between them resulting in LiFi atto cells (comparable to femtocells). These atto cells provide seamless handover and promise to provide full user mobility. Different researchers have proposed different system models for LiFi [63; 64; 65; 66]. One of the proposed models is to  
Figure 3: WiFi VLC communication network. augment LiFi with WiFi, where the latter works as an overlay network over on the former. In another approach, a mixed WiFi and LiFi is used to provide an aggregated system providing aggregated channels for communication. Light radio (LiRa) is another application developed by augmenting VLC with legacy WiFi [64]. LiRa does not require VLC for the uplink transmission by the mobile client and instead employs a WiFi uplink. The system achieves VLC automatic repeat request (ARQ) feedback via WiFi without excessive delay with a newly introduced method, AP-spoofed multi-client ARQ (ASMA). This method provides the mechanism to minimise the impact of VLC control frames on legacy WiFi traffic while providing enough feedback for the visible light downlink. [PERSON] et al. [66] proposed a VLC-WiFi hybrid system for energy efficiency. An analytical framework for energy efficiency is proposed that is based on two requirements; (a) selection of the appropriate number of APs to provide the lighting, (b) to satisfy the user's request for real-time communication. The results showed that the VLC-WiFi hybrid system is 75% more efficient than a standalone WiFi network. [PERSON] et al. carried out a similar investigation for energy efficiency and LOS availability of a VLC-RF hybrid system [67]. The system parameters that affect the energy footprint of the communication system are analysed. Results showed that energy efficiency improved at the cost of less LOS availability. [PERSON] et al. have proposed two variants of hybrid systems as proof of concept for the coexistence of WiFi and VLC [68]. In the first phase, a WiFi-VLC hybrid system is proposed where VLC is used for the downlink and WiFi for the uplink transmission. Results have shown that higher data rates can be achieved compared to the conventional WiFi systems. In order to take advantage of the higher bandwidth of VLC on the downlink and exploit the higher availability and range of WiFi, the hybrid systems are further enhanced by bundling together WiFi and VLC channels using bonding techniques based on the concept used in the Linux operating system. Practical experiments are performed in the crowded environments for comparing the hybrid system with WiFi only system. Robustness and throughput are used as performance metrics. Results have shown that hybrid systems are robust and provide higher throughput compared to WiFi systems. In [69], a hybrid system has been developed which consists of optical femtocells overlaid over WiFi cells, named as WiFO. The WiFO exploits the high bandwidth of optical femtocells and high mobility of WiFi to provide high throughput along with seamless handover and good bit error rate (BER).