  In this paper, we consider the 2 X 2 multi-user multiple-input-single-output
(MU-MISO) broadcast visible light communication (VLC) channel with two light
emitting diodes (LEDs) at the transmitter and a single photo diode (PD) at each
of the two users. We propose an achievable rate region of the Zero-Forcing (ZF)
precoder in this 2 X 2 MU-MISO VLC channel under a per-LED peak and average
power constraint, where the average optical power emitted from each LED is
fixed for constant lighting, but is controllable (referred to as dimming
control in IEEE 802.15.7 standard on VLC). We analytically characterize the
proposed rate region boundary and show that it is Pareto-optimal. Further
analysis reveals that the largest rate region is achieved when the fixed
per-LED average optical power is half of the allowed per-LED peak optical
power. We also propose a novel transceiver architecture where the channel
encoder and dimming control are separated which greatly simplifies the
complexity of the transceiver. A case study of an indoor VLC channel with the
proposed transceiver reveals that the achievable information rates are
sensitive to the placement of the LEDs and the PDs. An interesting observation
is that for a given placement of LEDs in a 5 m X 5 m X 3 m room, even with a
substantial displacement of the users from their optimum placement, reduction
in the achievable rates is not significant. This observation could therefore be
used to define "coverage zones" within a room where the reduction in the
information rates to the two users is within an acceptable tolerance limit.
