On the DC balance of multi-level PAM VLC systems

Abstract

Lighting requirements and their impact on visible light communication (VLC) systems based on multi-level pulse-amplitude modulation are discussed. We will show that the moving average of the signal must not fluctuate too much. Two solutions are presented. One is based on a simple extension of the well-known 5B6B line coding to codes with M-ary symbols. The other is based on the so-called Hadamard-coded modulation. Closely related to the topic of DC balance is the resistance of the received signal to high-pass filtering. The impact of such filtering on multi-level PAM systems with frequency domain equalization is investigated. We will show that the demand for flicker-free lighting can easily be met with line codes. Together with an equalization, which has to be used for multi-level PAM anyway, the AC coupling at the receiver is no problem either. The feasibility of an adaptive modulation is also discussed.