This paper considers improving the confidentiality of visible light communication (VLC) links within the framework of physical-layer security. We study a VLC scenario with one transmitter, one legitimate receiver, and one eavesdropper. The transmitter has multiple light sources, while the legitimate and unauthorized receivers have a single photodetector, each. We characterize secrecy rates achievable via transmit beamforming over the multiple-input, single-output (MISO) VLC wiretap channel. For VLC systems, intensity modulation (IM) via lightemitting diodes (LEDs) is the most practical transmission scheme. Because of the limited dynamic range of typical LEDs, the modulating signal must satisfy certain amplitude constraints. Hence, we begin with deriving lower and upper bounds on the secrecy capacity of the scalar Gaussian wiretap channel subject to amplitude constraints. Then, we utilize beamforming to obtain a closed-form secrecy rate expression for the MISO wiretap channel. Finally, we propose a robust beamforming scheme to consider the scenario wherein information about the eavesdropper's channel is imperfect due to location uncertainty. A typical application of the proposed scheme is to secure the communication link when the eavesdropper is expected to exist within a specified area. The performance is measured in terms of the worst-case secrecy rate guaranteed under all admissible realizations of the eavesdropper's channel.

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Free-space optical (FSO) communication links provide high data rates; however, their reliability is heavily dependent on weather conditions. This paper presents our experimental urban 1.87 km FSO link based on a customized commercial system and develops a library of channel measurements in clear and light rain weather conditions. A channel model for the link is proposed and experimentally quantified. Channel measurements are obtained by modulating a 60 mW laser source. At the receiver, a 2 GSa/s data converter is used and 16 fast-Fourier transform cores are implemented in the hardware to improve noise immunity. The resulting signal-to-noise ratio of the channel samples is around 40 dB under clear weather conditions. Fittings with log-normal, gamma-gamma, and Erlang distributions are presented, and the scintillation index and coherence time are measured. A computationally efficient finite-state Markov chain is derived for the channel to model both the distribution and the autocorrelation of the fading and is verified by the measurements. The Markov models and channel measurements in a variety of atmospheric conditions are available for download to permit easy verification of communication algorithms on this urban FSO channel.