  Integrating visible light communication (VLC) and radio-frequency (RF)
networks can improve the performance of communication systems in terms of
coverage and data rates. However, adding RF links to VLC networks weakens the
secrecy performance due to the broadcast and ubiquitous nature of RF links.
This paper studies the physical layer security (PLS) in cooperative
non-orthogonal multiple access (CoNOMA) hybrid VLC/RF systems. Consider a VLC
system, where two entrusted users close to a VLC access point (AP) help an
out-of-coverage legitimate user using RF signals in the presence of an
eavesdropper. The AP transmits data to both entrusted users and the legitimate
user using the principle of NOMA, where the entrusted users harvest energy from
the received light intensity, decode the legitimate user's message, forward it
using a RF link, and then decode their messages. It is required to maximize the
secrecy rate at the legitimate user under quality-of-service (QoS) constraints
using beamforming and DC-bias and power allocation. Different solutions are
proposed for both active and passive eavesdropper cases, using semidefinite
relaxation, zero-forcing, beamforming, and jamming. Numerical results compare
between the different proposed approaches and show how the proposed approaches
contribute in improving the secrecy performance of the proposed model.
