  In this paper, we investigate the performance of generalized spatial
modulation (GSM) in indoor wireless visible light communication (VLC) systems.
GSM uses $N_t$ light emitting diodes (LED), but activates only $N_a$ of them at
a given time. Spatial modulation and spatial multiplexing are special cases of
GSM with $N_{a}=1$ and $N_{a}=N_t$, respectively. We first derive an analytical
upper bound on the bit error rate (BER) for maximum likelihood (ML) detection
of GSM in VLC systems. Analysis and simulation results show that the derived
upper bound is very tight at medium to high signal-to-noise ratios (SNR). The
channel gains and channel correlations influence the GSM performance such that
the best BER is achieved at an optimum LED spacing. Also, for a fixed
transmission efficiency, the performance of GSM in VLC improves as the
half-power semi-angle of the LEDs is decreased. We then compare the performance
of GSM in VLC systems with those of other MIMO schemes such as spatial
multiplexing (SMP), space shift keying (SSK), generalized space shift keying
(GSSK), and spatial modulation (SM). Analysis and simulation results show that
GSM in VLC outperforms the other considered MIMO schemes at moderate to high
SNRs; for example, for 8 bits per channel use, GSM outperforms SMP and GSSK by
about 21 dB, and SM by about 10 dB at $10^{-4}$ BER.
