Pulse Modulation Schemes And Dimming Support For Visible Light Communication

The feature of a longer life and energy saving of the LEDs makes it the mainlighting source of the next generation. And we have seen a growing concern in visiblelight communication based on LEDs. We begin our study of the data modulation anddimming for visible light communication.First, there is a brief introduction of the composition and characteristics of theVLC. We analysis the link model from the transmitter to the receiver, and they can besummarized as the direct line of sight link and the diffuse links. Then there is adetailed presentation on the Mathematical model of the link model and signal to noiseratio of the VLC systems.Second, this paper analyses the scheme of the brightness control and datatransmission for VLC exhaustively. People usually employ pulse width modulationfor brightness control, and the other modulation scheme for data transmission, thenpeople can use LEDs for both illumination and communication. For this scheme needsPWM signaling for brightness control, therefore the complexity of the system isgreatly increased. A second approach is to use a sub-carrier pulse position modulation;by changing modulation depth the brightness can be controlled. However, it requires ahigh frequency sub-carrier for data transmission. Finally, the scheme who are differentfrom the above two methods is variable rate multi-pulse pulse position modulation(VR-MPPM), it is possible to realize data transmission and dimming simultaneouslyby itself. So MPPM has a natural advantage in the visible light communication.Finally, based on the unique advantages of MPPM in VLC, the key research isthe bit error rate, bandwidth and power requirements of the MPPM’s. Based on thesimulation we find the BER of the MPPM is much higher than PPM, so we haveproposed a interval MPPM, for VLC system based on LEDs, to reduce the BER andimprove the reliability of the MPPM’s. Two ways to add a guard interval also be designed, the minimum time slot method and minimum BER methods. After analysisand simulation the BER, bandwidth and power requirements of the two methods, theresults shows that both the BER of the two methods have reduced substantiallycompare to MPPM, and have a better ability to suppress inter-symbol interferencecompare to PPM. The minimum time slot method is merely more appropriate for thecase of when the bits of the source is smaller, and the minimum BER methods is moreflexible and has a wider range of applications.