  Visible Light Communications (VLC) has been studied thoroughly in recent
years as an alternative or complementary technology to radio frequency
communications. The reliability of VLC channels highly depends on the
availability and alignment of line of sight links. In this work, we study the
effect of random receiver orientation for mobile users over VLC downlink
channels, which affects the existence of line of sight links and the receiver
field of view. Based on the statistics of vertical receiver orientation and
user mobility, we develop a unified analytical framework to characterize the
statistical distribution of VLC downlink channels, which is then utilized to
obtain the outage probability and the bit error rate. Our analysis is
generalized for arbitrary distributions of receiver orientation/location for a
single transmitter, and extended to multiple transmitter case for certain
scenarios. Extensive Monte Carlo simulations show a perfect match between the
analytical and the simulation data in terms of both the statistical channel
distribution and the resulting bit error rate. Our results also characterize
the channel attenuation due to random receiver orientation/location for various
scenarios of interest.
