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In this paper, we present simulation results for the bit error rate (BER) performance and the fading penalty of a BPSK subcarrier intensity modulation (BPSK-SIM) free-space optical (FSO) communication link in a log-normal atmospheric turbulence model. The results obtained are based on the Monte-Carlo simulation. Multiple subcarrier modulation schemes offer increased system throughput and require no knowledge of the channel fading in deciding what symbol has been received. In an atmospheric channel with a fading strength σl2 of 0.1 obtaining a BER of 10-6 using a 2-subcarrier system will require a signal-tonoise (SNR) of 23.1 dB. The required SNR increases with the fading strength and at a BER of 10-9 the fading penalty due to the atmospheric turbulence is ~ 41 dB for σl2 = 0.9. The comparative studies of the OOK and BPSK-SIM schemes showed that for similar electrical SNR, BPSK-SIM offered improved performance across all range of turbulence variance.