Pilot-Symbol Assisted Modulation for Correlated Turbulent Free-Space Optical Channels

In free-space optical links using intensity modulation and direct detection, atmospheric turbulence can cause signal fading. Pilot-symbol (PS) assisted modulation (PSAM) can help to mitigate this fading, improving system performance. We periodically insert an On-state PS in front of M-1 information bits and form a M-bit frame. We derive the PS assisted maximum-likelihood (PSA-ML) decision rule under the assumption that the temporal coherence of fading is known, but the instantaneous fading state is not known. We also propose a simpler PS assisted detection scheme with variable threshold (PSA-VT). Although these two techniques will introduce some delay to the detection system, they can help to mitigate the atmospheric turbulence induced fading. We have performed numerical simulations to show this improvement. We also describe how to choose the frame size M to optimize the performance of PSAM systems.