A Hybrid Modulation Method for Dimming in Visible Light Communication

In visible light communication in which information is superimposed on light, its communication range is visible. On the other hand, when the modulation that brightness is changed according to information is used, it is not good for the human eye because human feels flicker for the changed brightness. Therefore, the pulse width modulation which is used in the infrared remote controller, is not suitable for the visible light communication. And the pulse position modulation that sends information at the position of a pulse is often used. In pulse position modulation, for example, when one symbol uses four slots, quad information can be sent by turning on only one slot among the four slots and turning off the other three slots. In this method, the brightness becomes 1⁄4 and feels not flicker but darker than when it is always on. On the other hand, when inverted pulse position modulation in which these slots of ON and OFF are reversed is used, since one slot is OFF, and three slots are ON, the brightness becomes 3/4, which does not become too dark. In this paper, we propose a hybrid modulation that can vary the brightness other than 1/4(25%) or 3/4(75%) by combining these pulse position modulation and inverted pulse position modulation. We have two experiments; one is measuring its communication performance and the other is the evaluation for visual flicker by the human eye and. From the view of the communication performance, each brightness is the same performance. So, the changing brightness does not affect to the performance. For the dimming, we interviewed 10 people how to feel the flicker in visible light communication. As the result, they do not feel the flickers. So, our proposed hybrid modulation method can be adapted for the dimming lighting equipment.