# Visible Light Communication: A System Perspective--Overview and Challenges
## 4 Hybrid VLC  
A VLC network arrangement is shown in Figure 2a. The downlink consists of a LED driver which drives the LED illuminator to work as a VLC transmitter and a power line modem to connect the VLC transmitter to the network. For simplification, this downlink VLC transmitter is called a VLC access point (VAP). The power line communication (PLC) technology is typically utilised to connect all the VAPs to the rest of the network both internally and externally. In the user devices, the downlink VLC transmission can be received either through a photodiode or an image sensor. The uplink from the user device could be through either (a) VLC transmitter, (b) a wireless fidelity (WiFi) link, (c) an infrared (IR) link, (d) a combination of any two. Figure 2b shows another arrangement of the VLC network. An IR transmitter is used on the uplink instead of a WiFi. In this scenario, VAP consists of PLC modem, VLC transmitter, and an infrared receiver (IR). The user device consists of a photodiode to receive the downlink VLC transmission and an IR transmitter to communicate on the uplink. Both IR and VLC require line of sight for communication. Instead of an IR transmitter, we could use a LED transmitter for the uplink to make a bidirectional VLC link. However, as light would be continuously emitting in an upward direction, which is aesthetically not pleasing from a user perspective. In any case, the VLC would occur through a heterogeneous network that would be using more than one networking standard, i.e., a combination of PLC-VLC, PLC-VLC-WiFi, PLC-VLC-IR. There are limitations of each existing technology to support VAP. For example, PLC is used to connect the VAP with the rest of the network. PLC can achieve a data rate of 1 Gbit/s [56; 57]. Thus creating a bottleneck to use VLC for high data rates although theoretically, VLC can provide high-speed data up to 15 Gbit/s [30]. Some researchers have considered fibre optic as an alternative to PLC [58]. However, it will increase infrastructure cost. Figure 3 shows a typical deployment of VLC network in a house. It consists of a heterogeneous network of PLC-VLC-WiFi. One WiFi AP is used to cover the entire house whereas VAP's are  
Figure 2: VLC downlink and infrared is used for uplink in VLC Hetnet. (**a**) VLC downlink and WiFi for uplink in VLC Hetnet. (**b**) VLC–VLC communication. distributed throughout the house and connected via PLC to the main gateway. This arrangement is cost effective and realistic as typically the links are asymmetric considering that uplink can have low data rates as compared to downlink transmission. VLC users at the boundary of WiFi would have high signal attenuation, which could deteriorate the overall throughput. A second arrangement is to deploy low power WiFi transmitters (WiFi Direct) in VAPs. However, it would increase the complexity of hardware and software in VAP. Furthermore, VAP's are prone to interference as it cannot penetrate objects due to characteristics of its frequency. However, WiFi operates in 2.4 and 5 GHz range, and it can penetrate in walls. Therefore fine tuning WiFi transmitter to a very small area would be practically unachievable. This will increase the interference among wifi transmitters in close proximity. A third alternative is to deploy multiple WiFi to provide good coverage and throughput, but it would require excellent RF planning to mitigate interference issues. We will discuss this in detail later in the next subsection. In most of the cases, photodetectors are employed as a receiver for the VLC. However CMOS sensor-based camera are also experimented with and demonstrated for reception of visible light and extracting information from it. [PERSON] et al. have developed a proof of concept system which shows that rolling shutter feature of CMOS sensors can be used to achieve higher data rates [59]. The overall system consists of an LED transmitter, a mobile camera-based receiver, and an Android application equipped with a decoder.