Direct detection free-space optical communications through atmospheric turbulence

Free-space optical (FSO) communication systems suffer from both average optical signal power loss and random power fading due to the atmospheric turbulence. For focused laser beams propagating along the horizontal path, the average received optical signal power and the statistics of the power fading as functions of the communication distance and receiver aperture size are studied via large scale numerical simulations. The avalanche photodiode (APD) is chosen for optical power detection, and the accurate McIntyre-Conradi APD model is adopted for system evaluation. The on-off keying (OOK) method is used for laser beam modulation. The FSO communication channel is assumed to be shot-noise limited and experiences slowly time-varying power fading. The optimal detection threshold is determined and used for demodulation at each fading level. The average uncoded average bit-error-rate (BER) curves of the FSO communication system are obtained. The optimal receiver aperture sizes to achieve the lowest average BER are determined.