  This paper considers secrecy enhancement mechanisms in visible light
communication (VLC) systems with spatially distributed passive eavesdroppers
(EDs) under the assumption that there are multiple LED transmitters and one
legitimate receiver (UE). Based on certain amplitude constraints, we propose an
optimal beamforming scheme to optimize secrecy performance. Contrary to the
case where null-steering is made possible by using knowledge of the ED
locations, we show that the optimal solution when only statistical information
about ED locations is available directs the transmission along a particular
eigenmode related to the intensity of the ED process and the intended channel.
Then, a sub-optimal LED selection scheme is provided to reduce the secrecy
outage probability (SOP). An approximate closed-form for the SOP is derived by
using secrecy capacity bounds. All analysis is numerically verified by Monte
Carlo simulations. The analysis shows that the optimal beamformer yields
superior performance to LED selection. However, LED selection is still a highly
efficient suboptimal scheme due to the complexity associated with the use of
multiple transmitters in the full beamforming approach. These performance
trends and exact relations between system parameters can be used to develop a
secure VLC system in the presence of randomly distributed EDs.
