Free Space Optical (FSO) Communications are able to deliver us to an age of unprecedented bandwidth, low signal attenuation, small space requirements and ultimately low cost. In FSO communications the influence of atmospheric effects can be specified by the attenuation and fluctuations of the transmitted optical power, caused by the atmospheric turbulence. This paper investigate the performance of FSO communication systems employing on-off keying (OOK) , subcarrier binary phase-shift keying (BPSK) modulation and Q-ary pulse position modulation (QPPM) in turbulence regime. The performance results are evaluated in terms of bit error rate (BER) employing subcarrier OOK, BPSK, and QPPM as modulation technique. It is found that the BER performance under BPSK modulation is better compared to other modulation techniques but in Q-ary PPM if we increase the order of Q then the performance will improve and it provide maximum 4dB improvement.

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 20dB 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. Keywords-Low density parity check (LDPC) code, free space optics (FSO), bit interleaved coded modulation (BICM), multiple input/multiple output(MIMO) processing, pulse position modulation (PPM), multi laser multi detector (MLMD).

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