# Visible Light Communication: A System Perspective--Overview and Challenges
## 2 Visible Light Communication System  
In the early 1990's, mobile phones were mainly used for voice conversation or text messaging. However, the introduction of the iPhone in 2007 has started a new era in wireless communication [20]. Nowadays smartphones are equipped with all kinds of sensors and applications to provide health monitoring, video chatting, online streaming along with bank transactions and using cloud services. A larger bandwidth should be allocated for wireless communication in order to provide seamless connectivity and higher data rates. However, frequency spectrum below 5 GHz is well utilised, which leave no room to relocate spectrum for mobile communication. This led scientists to seek new wireless technologies that can fulfil the needs of the higher data rate at a low cost. One such candidate is the use of the visible light spectrum. It has the advantage of its availability (LED lights), link level security, higher bandwidth, and frequency reuse. Furthermore, the demand for data is higher in an indoor environments because 80% of the time people stay in an indoor environments [21]. Thus using the VLC in indoor applications for high data rate applications would free up the precious RF spectrum for other future applications such as autonomous cars and smart cities. The history of VLC goes back to Romans when polish metallic plates were used to reflect sunlight and convey signals over a long distance. In 1794, [PERSON] developed a semaphore system consisting of a series of towers equipped with [PERSON] arms to transmit information. In the late 19 th and early 20 th Century, heliograph was used for long distance communication. In heliograph, the sunlight was reflected with a mirror to transmit Morse code. The British and Australian armies used it till 1960. [PERSON] is mostly known for his invention of the modern telephone, which uses electricity to transmit voice. However, [PERSON] described the photo-phone as one of his important inventions [22]. Photophone uses voice to vibrate the mirror, which in turn is used to modulate the sunlight. It was the idea of [PERSON], which led to fibre optic communication. The first commercial fibre optic communication system was deployed in 1975 and was capable of operating at a bit rate of 45 Mbit/s. VLC is a form of optical communication that instead of using a guided media (fibre optic) operates in the open air in close proximity of two to three meters. In 2003, the term VLC was coined first by Nakagawa Laboratory at Keio University, Japan [23]. The Nakagawa Laboratory demonstrates the first VLC system at Keio University in 2000. Light emitting diodes (LEDs) are used for transmitting the data. [PERSON] et al. have proposed a VLC system consisting of red-green-blue (RGB) LEDs as a transmitter and complimentary metal oxide semiconductor (CMOS) image sensor as a receiver [24]. The authors observe that RGB LEDs can increase the VLC transmission data rate with wavelength division multiplexing (WDM). For such systems, colour signals are isolated using colour filter array which relies on colour-sensitive sensing elements. However, the wide optical bandwidth of colour filters can increase spectral overlap between channels and can cause inter-channel interference (ICI). In order to solve this issue, the authors have applied CMOS image sensors with multiple input-multiple output (MIMO) to mitigate the ICI and demodulate the rolling shutter pattern. In [25; 26], [PERSON] et al. have proposed a VLC based communication system that consists of an LED panel working as a transmitters and CMOS based camera as a receiver. In [27], [PERSON] et al. have further improved their work using RGB LEDs to provide non-flickering for long-distance communication. The rolling shutter effect (RSE), and under-sampled modulation (USM) is analysed for VLC under different international standard organisation (ISO) values and distance. Results show that USM has better error performance over long distance compared to RSE.