In this paper a hybrid modulation scheme based on pulse position modulation (PPM) and binary phase shift keying subcarrier intensity modulation (BPSK-SIM) schemes for free-space optical communications is proposed. The analytical bit error rate (BER) performance is investigated in weak and saturated turbulence channels and results are verified with the simulation data. Results show that performance of PPM-BPSK-SIM is superior to BPSK-SIM in all turbulence regimes; however, it outperforms 2-PPM for the turbulence variance σ(1)(2)>0.2. PPM-BPSK-SIM offers a signal-to-noise ratio (SNR) gain of 50 dB in the saturation regime compared to BPSK at a BER of 10(-6). The SNR gain in comparison to PPM improves as the strength of the turbulence level increases.

In this paper a hybrid PPM-BPSK modulation scheme is proposed. The numerical simulation results for the bit error rate (BER) performance of the PPM-BPSK modulation scheme in the lognormal atmospheric channel for a range of turbulence variance is presented. Results are compared with the BPSK-subcarrier intensity modulation (SIM) and 4-PPM, which shows the performance of the hybrid PPM-BPSK scheme, is superior to the BPSK while having the same bandwidth requirement. However, for all level of turbulence the performance of the proposed hybrid PPM-BPSK is deficient to the PPM scheme. Moreover the paper also presents the experimental results for 4PPM and BPSK (SIM).

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