Coding for the turbulent atmospheric optical channel

The turbulent atmospheric optical channel can be characterized, to a first approximation, as a log-normal fading channel having a large coherence bandwidth, a long coherence time, negligible multipath time dispersion, and a significant dispersion in space and spatial frequency. Turbulence-induced scintillations can degrade communication performance significantly through deep signal fades. The present investigation considers a turbulent atmospheric channel with the described properties. The signaling scheme used is pulse position modulation (PPM), and channel coding is employed to provide a form of time diversity to combat fading. It is shown that under most conditions coding improves communication performance substantially. Attention is given to free-space channel performance, the effect of log-normal fading on channel symbol error probability, and details of channel coding.