A visible light communication (VLC) broadcast channel is considered, in which a transmitter communicates with two receivers in the presence of an external eavesdropper. Trusted cooperative half-duplex relays are deployed to aid with securing the transmitted data. Transmission is amplitude-constrained to maintain operation within the light emitting diodes (LEDs) dynamic range. Achievable secrecy rate regions are derived under such amplitude constraints for this multi receiver wiretap channel, first for direct transmission without the relays, and then for cooperative jamming, decode-and-forward, and amplify-and-forward relaying schemes. Superposition coding with uniform signaling is used for transmission, along with secure beamforming at the relays. Superiority of the secure relaying schemes over direct transmission is shown, with performance depending on how far the eavesdropper is from the transmitter and the relays.
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