Enhanced Subband Index Carrierless Amplitude and Phase Modulation in Visible Light Communications

An enhanced subband index carrierless amplitude and phase modulation (eSI-CAP) scheme is developed to improve the spectral efficiency of the multiband CAP ( $m$ -CAP) in visible light communication (VLC) systems. Using indexing technique and a dual distinguishable constellations, the eSI-CAP carries information bits by modulating symbols on the subbands of $m$ -CAP. It also encodes additional information bits on the selection of the subband indices that is modulated with the dual constellation symbols. The performance advantage of eSI-CAP is demonstrated through theoretical analysis, computer simulations and VLC experimental validations. In addition, a novel maximum likelihood detector is developed for eSI-CAP with the same order of complexity as the conventional $m$ -CAP detector. On comparison with $m$ -CAP, eSI-CAP requires lower signal-to-noise ratio (SNR) per bit ( $\gamma _b$ ) to reach a fixed data rate ( $R_\text{b}$ ). Alternatively, when the $\gamma _b$ is fixed, it achieves a higher $R_\text{b}$ for a target BER. The results show that eSI-CAP is very attractive for VLC systems due to its flexible configuration and its improvement of the power efficiency of the conventional $m$ -CAP modulation scheme.