In this study, the authors theoretically study the performance of direct-detection free-space optical communication systems using binary phase-shift keying subcarrier-intensity modulation and avalanche photodiode (APD). The system biterror rate and channel capacity are theoretically derived in cases of log-normal and gamma-gamma channel models for weakto-moderate and moderate-to-strong atmospheric turbulence conditions, respectively. The authors quantitatively discuss the optimal values of the

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... PD average gain, required transmitted optical power, and operating bit-rate considering various turbulence conditions, APD shot noise and thermal noise. It is seen that, although the impact of turbulence is severe, a proper selection of APD average gain could significantly improve the system performance in both cases of turbulence channels. The optimal value of APD average gain remains almost the same for different levels of turbulence; nevertheless it varies significantly in accordance to the change of receiver noise temperature.