Adaptive Modulation Schemes for Visible Light Communications

A major limitation of existing visible light communication (VLC) systems is the limited modulation bandwidth of light-emitting diodes used in such systems. Using adaptive modulation to improve the spectral efficiency for radio communications has been well studied. For VLC with various physical layer schemes, however, how adaptive modulation works is not well understood yet. The goal of this paper is to provide an in-depth analysis of the achievable spectral efficiency of adaptive modulation for three different schemes for high speed VLC: dc-biased optical orthogonal frequency division multiplexing (DCO-OFDM), asymmetrically clipped optical OFDM (ACO-OFDM), and single-carrier frequency-domain equalization (SC-FDE). We will show that in the low signal-to-noise ratio region, the ACO-OFDM-based adaptive modulation scheme outperforms the other two schemes. SC-FDE-based adaptive modulation achieves a better performance than the DCO-OFDM-based scheme, and it is much simpler than the other two schemes.