BER Analysis of a Hybrid Modulation Scheme Based on PPM and MSK Subcarrier Intensity Modulation

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 PPMMSK-SIM is studied in detail for an FSO system over log-normal turbulence channels with 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 -2.1 dBm and the same strength of turbulence C²_n 1/4 7.5 × 10^-15 m^-2/3, the BER performance of 2-PPMMSK-SIM can decrease to 1.02 × 10^-9, whereas those of 2-PPM and BPSK-SIM are just 1.19 × 10^-7 and 2.29 × 10^-6, respectively. This makes PPM-MSK-SIM a favorable candidate for the modulation technique in FSO communication systems.