  We propose an enhanced spatial modulation (SM)-based scheme for indoor
visible light communication systems. This scheme enhances the achievable
throughput of conventional SM schemes by transmitting higher order complex
modulation symbol, which is decomposed into three different parts. These parts
carry the amplitude, phase, and quadrant components of the complex symbol,
which are then represented by unipolar pulse amplitude modulation (PAM)
symbols. Superposition coding is exploited to allocate a fraction of the total
power to each part before they are all multiplexed and transmitted
simultaneously, exploiting the entire available bandwidth. At the receiver, a
two-step decoding process is proposed to decode the active light emitting diode
index before the complex symbol is retrieved. It is shown that at higher
spectral efficiency values, the proposed modulation scheme outperforms
conventional SM schemes with PAM symbols in terms of average symbol error rate
(ASER), and hence, enhancing the system throughput. Furthermore, since the
performance of the proposed modulation scheme is sensitive to the power
allocation factors, we formulated an ASER optimization problem and propose a
sub-optimal solution using successive convex programming (SCP). Notably, the
proposed algorithm converges after only few iterations, whilst the performance
with the optimized power allocation coefficients outperforms both random and
fixed power allocation.
