  In this paper, we investigate the physical-layer security for a spatial
modulation (SM) based indoor visible light communication (VLC) system, which
includes multiple transmitters, a legitimate receiver, and a passive
eavesdropper (Eve). At the transmitters, the SM scheme is employed, i.e., only
one transmitter is active at each time instant. To choose the active
transmitter, a uniform selection (US) scheme is utilized. Two scenarios are
considered: one is with non-negativity and average optical intensity
constraints, the other is with non-negativity, average optical intensity and
peak optical intensity constraints. Then, lower and upper bounds on the secrecy
rate are derived for these two scenarios. Besides, the asymptotic behaviors for
the derived secrecy rate bounds at high signal-to-noise ratio (SNR) are
analyzed. To further improve the secrecy performance, a channel adaptive
selection (CAS) scheme and a greedy selection (GS) scheme are proposed to
select the active transmitter. Numerical results show that the lower and upper
bounds of the secrecy rate are tight. At high SNR, small asymptotic performance
gaps exist between the derived lower and upper bounds. Moreover, the proposed
GS scheme has the best performance, followed by the CAS scheme and the US
scheme.
