  The evolving explosion in high data rate services and applications will soon
require the use of untapped, abundant unregulated spectrum of the visible light
for communications to adequately meet the demands of the fifth-generation (5G)
mobile technologies. Radio-frequency (RF) networks are proving to be scarce to
cover the escalation in data rate services. Visible light communication (VLC)
has emerged as a great potential solution, either in replacement of, or
complement to, existing RF networks, to support the projected traffic demands.
Despite of the prolific advantages of VLC networks, VLC faces many challenges
that must be resolved in the near future to achieve a full standardization and
to be integrated to future wireless systems. Here, we review the new, emerging
research in the field of VLC networks and lay out the challenges, technological
solutions, and future work predictions. Specifically, we first review the VLC
channel capacity derivation, discuss the performance metrics and the associated
variables; the optimization of VLC networks are also discussed, including
resources and power allocation techniques, user-to-access point (AP)
association and APs-to-clustered-users-association, APs coordination
techniques, non-orthogonal multiple access (NOMA) VLC networks, simultaneous
energy harvesting and information transmission using the visible light, and the
security issue in VLC networks. Finally, we propose several open research
problems to optimize the various VLC networks by maximizing either the sum
rate, fairness, energy efficiency, secrecy rate, or harvested energy.
