# Visible Light Communication: A System Perspective--Overview and Challenges  
1 Department of Electrical and Electronic Engineering, Auckland University of Technology, Auckland 1010, New Zealand; [EMAIL_ADDRESS]  
2 Department of Information Technology and Software Engineering, Auckland University of Technology, Auckland 1010, New Zealand; [EMAIL_ADDRESS]  
3 Department of Telecommunications, Brno University of Technology, Technicka 12, 601 90 Brno, Czech Republic; komosy@teec.  
Visible light communication (VLC) is a new paradigm that could revolutionise the future of wireless communication. In VLC, information is transmitted by modulating the visible light spectrum (400-700 nm) that is used for illumination. The information signal is superimposed on the LED light without introducing any flickering to the end user. Thus, it would be \"green\" as compared to providing two separate sources for illumination and communication network connectivity. On the other hand, the exhaustion of low-frequency bands to cope with the exponential growth for the highspeed wireless access is another reason for exploring new technologies. The visible light spectrum is unlicensed and hardware readily available, which can be used for data transmission. Furthermore, the exponential improvement in the high power light emitting diodes is an enabler for high data rate VLC Network. It has the potential to provide high-speed data communication with improved energy efficiency along with security/privacy. Standardisation efforts such as visible light communications association (VLCA) standards and IEEE 802.15.7 shows that VLC would augment existing wireless networks in coming years. VLC can have applications in indoor wireless communication [10], intelligent transport system [11; 12], smart cities [13], localisation in warehouses/robotics [14; 15; 16], human sensing [17], safe and hazard-free data access in hospitals [18], toys and theme parks [19], indoor point to point (PPP) communication and vehicular communication [12].  
VLC in its basic form like any other communication system in the downlink consists of a LED as a transmitter, a free space optical communication channel and a photodetector or an image sensor as a receiver. The uplink could be a WiFi transmitter or an IR transmitter or LED based VLC transmitter. Much of the work is focused on the downlink transmission to increase the data rate and improve VLC performance under different environmental conditions such as shadowing, non-LOS scenario and mobility. However, the uplink is equally essential for seamless integration of VLC in the existing ICT infrastructure. Unlike other review articles, this paper provides an overview of the VLC from the uplink and system perspective. This review article critically analyses the existing solutions of the VLC network from the system as well as uplink perspective. This paper has following contributions  
* We have critically analysed the existing literature of the VLC regarding the uplink from the user device to VLC access point (VAP).
* We have discussed the open challenges associated with the use of existing RF spectrum for uplink connectivity.  
The rest of the paper is organised as follow: Section 2 discusses the history and standardisation efforts. Section 3 discusses applications of VLC. Section 4 gives an overview of the different technologies used in the uplink and discusses its limitation. Section 5 summarises open research challenges toward the integration of VLC in the existing systems. Section 6 concludes the paper.