  This paper studies the channel capacity of intensity-modulation
direct-detection (IM/DD) visible light communication (VLC) systems under both
optical and electrical power constraints. Specifically, it derives the
asymptotic capacities in the high and low signal-to-noise ratio (SNR) regimes
under peak, first-moment, and second-moment constraints. The results show that
first- and second-moment constraints are never simultaneously active in the
asymptotic low-SNR regime, and only in few cases in the asymptotic high-SNR
regime. Moreover, the second-moment constraint is more stringent in the
asymptotic low-SNR regime than in the high-SNR regime.
