Joint Rate-Brightness Control using Variable Rate MPPM for LED Based Visible Light Communication Systems

LED based lighting systems provide an opportunity for data transmission in addition to their traditional use as source of illumination. Brightness control is required to achieve either desired level of illumination or to achieve energy conservation. Conventionally, simultaneous data transmission as well as brightness control is achieved using two different modulation schemes. Either pulse width modulation or pulse amplitude modulation is used for brightness control and some variants of pulse position modulation are employed for data transmission. The need for two different modulation schemes, to meet the dual objective, makes the system design complex. In this paper we propose variable-rate multi-pulse-position-modulation (VR-MPPM), for LED based visible light communication system, to achieve joint brightness control and data transmission. The proposed approach eradicates the need for either pulse width modulation or pulse amplitude modulation and still achieves the brightness control. Encoder and decoder algorithms for VR-MPPM realization are developed and are implemented on the hardware testbed. Experimental results revealing the effect of brightness level variation on symbol error rate are also provided. Existence of an underlying trade-off between achievable resolution for brightness control and the corresponding successful data transmission rate is recognized. To exploit this trade-off, an optimization problem is formulated.