In order to improve the bit-error-rate (BER) performance of free-space optical (FSO) communication systems employing binary phase-shift keying subcarrier intensity modulation (BPSK-SIM), an innovative hybrid modulation scheme called PPM-MSK-SIM is proposed, which is based on pulse position modulation (PPM) and minimum shift keying (MSK) subcarrier intensity modulation. Subsequently, the BER performance of PPM-MSK-SIM is studied in detail for an FSO system over log-normal turbulence channels

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... avalanche photodiode detection. The results of the numerical simulation show that PPM-MSK-SIM has the advantages of improving the BER performance compared with BPSK-SIM and PPM. For example, at the same received irradiance of À2.1 dBm and the same strength of turbulence C 2 n ¼ 7:5 Â 10 À15 m À2=3 , the BER performance of 2-PPM-MSK-SIM can decrease to 1:02 Â 10 À9 , whereas those of 2-PPM and BPSK-SIM are just 1:19 Â 10 À7 and 2:29 Â 10 À6 , respectively. This makes PPM-MSK-SIM a favorable candidate for the modulation technique in FSO communication systems. Index Terms: Free-space optical communication, subcarrier intensity modulation, atmospheric turbulence, atmospheric attenuation, bit-error rate (BER).