Abstract:  In order to reduce the bit error rate caused by atmospheric turbulence in FSO(Free Space Optical)
communication system, a coded multilevel modulation scheme based on LDPC (Low-Density Parity-Check)
codes is proposed. This scheme employs the combination of BPSK(Binary Phase-Shift Keying) and PPM (Pulse
Position Modulation) with LDPC codes to improve the performance of the FSO system. The transmitter and
receiver models are designed based on PPM-BPSK-LDPC coded multilevel modulation scheme. The numerical
results indicate that the bandwidth utilization and the bit error rate performance of the proposed PPM-BPSK-LDPC
coded multilevel modulation scheme are always better than ordinary PPM-LDPC coded modulation scheme under
weak turbulence channel with average power constraint.

Key words: free space optical communication,  coded multilevel modulation, low-density parity-check codes