Effect of APD and thermal noises on the performance of SC-BPSK/FSO systems over turbulence channels

In this paper, we comprehensively study the performance of direct-detection free-space optical (FSO) communication systems using subcarrier binary phase-shift keying (SC-BPSK) modulation and avalanche photodiode (APD). The system bit-error rate is theoretically derived in case of two atmospheric turbulence channel models: the log-normal and gamma-gamma ones for the weak-to-moderate and moderate-to-strong turbulence conditions, respectively. We quantitatively discuss the optimal values of the average APD´s gain, required received optical powers and operating bit-rates considering various turbulence conditions, APD shot noise and thermal noise. It is seen that the effect of temperature on the value of optimal gain is stronger than that of the turbulence. We also find that the impact of turbulence is severe; however using APD receiver with optimal average gain can significantly improve the system performance in both cases of turbulence channels.