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Achievable information rate for outdoor free space optical communication with intensity modulation and direct detection

This work investigates the achievable information rate with the state-of-the-art turbo coding and intensity modulation/direct detection for outdoor long-distance free-space optic (FSO) communications. The channel under weak atmospheric turbulence is modeled as a log-normal intensity fading channel where on-off keying makes it look asymmetric. While no effort is made to spectrally match the code to the asymmetry of the channel, the decoding strategy is optimally adjusted to match to the channel response. In addition to fixed rate turbo coding, a family of variable rate turbo codes are constructed and discussed. Shannon capacity is also briefly visited to denote the theoretic limit. It is shown that under low turbulence a single long turbo code is sufficient to get within 1 dB from the capacity, but when the turbulence gets strong, adaptive coding is necessary to close the gap. We expect these results to be useful for current and immediate future systems.