  Visible light communication (VLC) systems using the indoor lighting system to
also provide downlink communications require high average optical powers in
order to satisfy the illumination needs. This can cause high amplitude signals
common in higher-order modulation schemes to be clipped by the peak power
constraint of the light emitting diode (LED) and lead to high signal
distortion. In this paper we introduce Hadamard coded modulation (HCM) to
achieve low error probabilities in LED-based VLC systems needing high average
optical powers. This technique uses a fast Walsh-Hadamard transform (FWHT) to
modulate the data as an alternative modulation technique to orthogonal
frequency division multiplexing (OFDM). HCM achieves a better performance for
high illumination levels because of its small peak to average power ratio
(PAPR). The power efficiency of HCM can be improved by reducing the DC part of
the transmitted signals without losing any information. The resulting so-called
DC-reduced HCM (DCR-HCM) is well suited to environments requiring dimmer
lighting as it transmits signals with lower peak amplitudes compared to HCM,
which are thus subject to less nonlinear distortion. Interleaving can be
applied to HCM to make the resulting signals more resistant against
inter-symbol interference (ISI) in dispersive VLC links.
