Optical wireless communications: System model, capacity and coding

This work is motivated by the need to understand the ultimate capacity of the outdoor long-distance optical wireless communication with intensity modulation and direct detection. The channel under weak atmospheric turbulence is modeled as a stationary ergodic channel with log-normal intensity fading, where signals experience asymmetric statistics due to on-off keying signaling. Ergodic channel capacity with several different values of channel parameter sigma(z) and outage probabilities are computed to provide an information-theoretic view of the channel. Turbo codes matched to the channel are also investigated to shed insight into how much can be achieved with the state-of-the-art coding schemes. It is shown that fixed rate turbo codes can perform close to the capacity when turbulence is weak, and that variable rate adaptive coding is necessary to bridge the gap when turbulence gets strong.