  A novel dimming control scheme, termed as generalized dimming control (GDC),
is proposed for visible light communication (VLC) systems. The proposed GDC
scheme achieves dimming control by simultaneously adjusting the intensity of
transmitted symbols and the number of active elements in a space-time matrix.
Both the indices of the active elements in each space-time matrix and the
modulated constellation symbols are used to carry information. Since
illumination is deemed as the prior task of VLC, an incremental algorithm for
index mapping is proposed for achieving target optical power and uniform
illumination. Next, GDC having the optimal activation pattern is investigated
to further improve the bit-error rate (BER) performance. In particular, the BER
performance of GDC is analyzed using the union bound technique. Based on the
analytical BER bound, the optimal activation pattern of GDC scheme with the
minimum BER criterion (GDC-MBER) is obtained by exhaustively searching all
conditional pairwise error probabilities. However, since GDC-MBER requires high
search complexity, two low-complexity GDC schemes having the maximum free
distance criterion (GDCMFD) are proposed. The first GDC-MFD scheme, coined as
GDC-MFD1, reduces the computational complexity by deriving a lower bound of the
free distance based on Rayleigh-Ritz theorem. Based on the time-invariance
characteristics of the VLC channel, GDC-MFD2 is proposed to further reduce the
required computation efforts. Simulation and numerical results show that
GDC-MBER, GDC-MFD1 and GDC-MFD2 have similar BER performance, and they can
achieve 2 dB performance gains over conventional hybrid dimming control scheme
and 7 dB performance gains over digital dimming control schemes.
