Analysis the performance of a LDPC coded FSO system with Q-ary pulse-position modulation

Free-space optical communication is an attractive and cost-effective solution for high-rate image, voice, and data transmission. However, optical wave propagation through the air experiences fluctuation in amplitude and phase due to atmospheric turbulence. In this paper, an analytical approach is presented to evaluate the bit error rate performance of a free space optical link using LDPC coded Q-ary optical PPM through atmospheric turbulence channel. Performance results are evaluated for multiple-laser and multiple photo-detector combination without and with LDPC code to combat the effect of atmospheric turbulence. The performance results are evaluated in terms of bit error rate (BER) and coding gain. It is found that LDPC coded system provides significant coding gain of 10 to 20 dB over uncoded system can be evaluated at BER 10^-12 for multiple source and photo-detector combinations. It also provides better performance under strong turbulence rather than weak turbulence conditions.