Secure Cooperative Hybrid VLC-RF Systems

Abstract

In this paper, a cooperative hybrid visible light communication (VLC)-radio frequency (RF) system with spatially random terminals is considered. Especially, a source node (<inline-formula> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula>) with a group of light-emitting diode (LED) lamps transmits information bits to a destination (<inline-formula> <tex-math notation="LaTeX">$D$ </tex-math></inline-formula>), which is located out of <inline-formula> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula>&#x2019;s coverage area, via a relay node (<inline-formula> <tex-math notation="LaTeX">$R$ </tex-math></inline-formula>). <inline-formula> <tex-math notation="LaTeX">$R$ </tex-math></inline-formula> is equipped with a photodetector and an antenna to set up a VLC link between <inline-formula> <tex-math notation="LaTeX">$S$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$R$ </tex-math></inline-formula>, and a radio frequency (RF) link between <inline-formula> <tex-math notation="LaTeX">$R$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$D$ </tex-math></inline-formula>, respectively. Meanwhile, an eavesdropper (<inline-formula> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula>) equipped with a photodetector and an antenna tries to overhear the information delivery over VLC and RF links. Also, diversity receiving scheme is considered at <inline-formula> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula>, while the same modulation scheme is considered over both VLC and RF links. Furthermore, decode-and-forward scheme is adopted at <inline-formula> <tex-math notation="LaTeX">$R$ </tex-math></inline-formula> to process and forward the received VLC signals. By employing stochastic geometry, we first characterize the probability density function and cumulative distribution function of the received signal-to-noise-ratio over both VLC and RF links, while considering the randomness of the locations of both <inline-formula> <tex-math notation="LaTeX">$R$ </tex-math></inline-formula>, <inline-formula> <tex-math notation="LaTeX">$E$ </tex-math></inline-formula>, and <inline-formula> <tex-math notation="LaTeX">$D$ </tex-math></inline-formula>. Then, the secrecy performance of the target system is studied by deriving the approximated expressions for the secrecy outage probability under various cases. Finally, the proposed analytical models are verified via Monte-Carlo simulations.