Comparison of hard-decision and soft-decision channel coded M-ary PPM performance over free space optical links

Error control coded M-ary pulse position modulation (PPM) can be used over free space optical links to mitigate the atmospheric attenuations and the turbulence induced fading. In this paper, we present performance analysis for coded free space optics (FSO) communication systems operating over various atmospheric conditions. We justify the selection of higher state PPM as an appropriate modulation scheme for the links and provide a comparative analysis between hard decision decoded (Reed-Solomon, RS) and soft decision decoded (Turbo) channel coded PPM over varying link conditions. Simulation results strongly indicate RS-coded PPM as a robust and well-performing coded-modulation scheme for the FSO links