VLC SYSTEM OVERVIEW
VLC system block diagram is shown in Figure 1.
A. VLC Transmitter
Precise dimming control can be done by LED. Driving circuit combines the input data of communication module and they drive a dimming control of LED array. LED arrays are generating multiple light signal.
For transmitting Light signal, the LED luminaire can be used. LED luminaire unit which consists of an LED lamp, ballast housing, and other components, and Driving circuit which controls the current flowing through LEDs to control its brightness.
White light is the most commonly used form of illumination in both indoor as well as outdoor applications. This is because colors of objects as seen under the white light closely resemble the colors of the same objects under the natural light.
White light is produced in two ways . First one is blue LED with the yellow phosphor coating on the outside. When the blue light traverses through the yellow coating, the combination produces a white light. Different variations of the white light are produced by modifying the thickness of the phosphor layer. The second one is RGB combinations in which white light can be produced by proper mixing of red, green and blue lights. In this method, three separate LEDs are used which increase the cost of LED luminaire compared to using the blue LED with phosphor. RGB combination is more preferable for communication because we use color shift keying to modulate the data using three different wavelengths.
B. VLC Receiver
The optical concentrator is used in the receiver, to set the receiver at a particular angle of light beam. An optical filter is also used to filter out data distorted by noise and multipath effect. Two types of receivers can be used to receive the signal transmitted by an LED . First one is Photodetector, also referred to as photodiode or non-imaging receiver which receives the précised light incident on it and then converts into a current. The second one is Imaging Sensor which is also called a camera sensor. Camera sensor consists of many photodetectors arranged in a matrix or array in an integrated circuit .
C. VLC Channel
In VLC channel, experimental arrangement or setup is made with LED as transmitter and photodetectors as a receiver in a Line of Sight(LOS) link. For calculating channel gain we have to analyze indoor link. For VLC, indoor link four types are possible . First is Line of Sight (LOS) link, in which receiver and transmitter are in-line. Second is Non-directed LOS link where at receiver side there is LOS path and multipath due to physical obstacles and the wall of a room. A third is a diffuse link with no LOS path but having only multipath signals on the receiver side Fourth is Quasi-diffuse link with multiple narrow beam transmitters and narrow Field of View (FOV) receiver, achieving lower multipath effects and lower path loss.