  We consider a downlink multiuser visible light communications (VLC) network
where users randomly change their location and vertical orientation. The
non-orthogonal multiple access (NOMA) strategy is adopted to serve multiple
users simultaneously, and, hence, to improve spectral efficiency. We propose
two novel user scheduling schemes for NOMA, which are referred to as individual
and group-based. In order to further reduce the computational complexity and
link overhead, novel limited-feedback schemes (on channel quality) are also
proposed, which basically involve mean vertical angle (instead of its
instantaneous value). Moreover, a two-bit feedback scheme is proposed for
group-based user scheduling, which relies on not only distance but also
vertical angle (in contrast to conventional one-bit feedback with 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.
