  Visible light communications (VLC) is an emerging technology with a promise
of viable solution to spectrum crunch problem in conventional radio frequency
(RF) bands. In this work, we consider a downlink multiuser VLC network where
users randomly change their location and vertical orientation. In order to
increase the spectral efficiency, we consider the non-orthogonal multiple
access (NOMA) transmission to serve multiple users simultaneously. In
particular, we propose individual and group-based user ordering techniques for
NOMA with various user feedback schemes. In order to reduce the computational
complexity and link overhead, feedback on the channel quality is proposed to be
computed using mean value of the vertical angle (instead of the exact
instantaneous value), as well as the distance information. In addition, a
two-bit feedback scheme is proposed for the group-based user scheduling, which
relies on both the distance and vertical angle, and differs from the
conventional one-bit feedback of the distance only. The outage probability and
sum-rate expressions are derived analytically, which show a very good match
with the simulation data. Numerical results verify that the practical feedback
scheme with the mean vertical angle achieves a near-optimal sum-rate
performance, and the two-bit feedback significantly outperforms the one-bit
feedback.
