Modulation techniques with lighting constraints

The physical layer design of visible light communication (VLC) systems is of substantially different characteristics from standard RF communications in that it involves a new constraint, namely a lighting constraint. This kind of constraint is imposed on the average intensity and flicker of the light emission. Since the flicker has little impact on human eye perception when light pulses blink at 200 Hz or higher frequency, the average intensity constraint is mainly addressed in this chapter. While this constraint is usually given as an inequality in optical wireless communication, it is represented as an equality in VLC. In addition, compared to radio-frequency communication, where the signal power, the squared value of signal level, is usually constrained, the intensity, the signal level itself, is constrained. In other words, the lighting constraint is defined with respect to the average (the first-order moment) of the signal, instead of the variance (the second-order moment). Therefore, this new constraint, which will be referred to as the dimming target, introduces a new domain of system design which has rarely been considered in existing communication media. In this chapter, several ways of communicating a message subject to the average constraint are addressed. This chapter is far from comprehensive but attempts to offer several promising ways of achieving such a goal. To satisfy the lighting constraint represented by the average constraint, several approaches have been addressed, and they can be categorized as to shift signal levels, to compensate in time, and to change level distribution. Some of those schemes are simply realized, and some provide improved throughput. First, the shift of the signal level is one of the simplest approaches. A typical non-return-to-zero (NRZ) on-off keying (OOK) has 50% average intensity for uniform probability of binary symbols. For the lighting constraint of 75% dimming, it offers a simple solution of moving the OFF symbol level from 0% intensity to 50%, which is referred to as analog dimming. Although this is conceptually simple, a non-linear characteristic of LEDs poses some technical difficulties, and the reduced level spacing degrades detection performance.