  Light-fidelity (LiFi) is a networked optical wireless communication (OWC)
solution for high-speed indoor connectivity for fixed and mobile optical
communications. Unlike conventional radio frequency wireless systems, the OWC
channel is not isotropic, meaning that the device orientation affects the
channel gain significantly, particularly for mobile users. However, due to the
lack of a proper model for device orientation, many studies have assumed that
the receiver is vertically upward and fixed. In this paper, a novel model for
device orientation based on experimental measurements of forty participants has
been proposed. It is shown that the probability density function (PDF) of the
polar angle can be modeled either based on a Laplace (for static users) or a
Gaussian (for mobile users) distribution. In addition, a closed-form expression
is obtained for the PDF of the cosine of the incidence angle based on which
line-of-sight (LOS) channel gain is described in OWC channels. An approximation
of this PDF based on the truncated Laplace is proposed and the accuracy of this
approximation is confirmed by the Kolmogorov-Smirnov distance (KSD). Moreover,
the statistics of the LOS channel gain are calculated and the random
orientation of a user equipment (UE) is modeled as a random process. The
influence of the random orientation on signal-to-noise-ratio (SNR) performance
of OWC systems has been evaluated. Finally, an orientation-based random
waypoint (ORWP) mobility model is proposed by considering the random
orientation of the UE during the user's movement. The performance of ORWP is
assessed on the handover rate and it is shown that it is important to take the
random orientation into account.
