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9 June 2014 Diversity effects in modulating retro-reflector links
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Optical modulating retro-reflectors (MRRs) can allow free space optical (FSO) links to platforms too small to carry a conventional FSO terminals. However, retro-reflectors suffer from very high optical scintillation due to the double-pass nature of the link. For links near the ground, scintillation indices greater than 10 are often observed during daytime hours. This strong scintillation causes deep and frequent fades. Spatial and temporal diversity can be powerful tools to reduce the effects of scintillation on MRR links. Experiments were conducted in which several retro-reflectors were simultaneously illuminated to measure the fade statistics of retroreflector links. The dependence of scintillation on the number and spacing of these retro-reflectors was measured. Under optimal conditions the scintillation index was shown to decrease approximately linearly with the number of retroreflectors. The recorded intensity waveforms were analyzed to determine the frequency, duration and depth of fades. The effects of using temporal diversity, in which duplicate data packets are sent multiple times, was analyzed to determine the reduction in required margin as a function of the number of times a packet was resent, and the spacing in time between packets. The analysis showed that spatial and temporal diversity, used in combination, can reduce the required margin for MRR links in strong turbulence by more than 20 dB.
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William S. Rabinovich, Rita Mahon, Mike S. Ferraro, Peter G. Goetz, and James Murphy "Diversity effects in modulating retro-reflector links", Proc. SPIE 9080, Laser Radar Technology and Applications XIX; and Atmospheric Propagation XI, 90801B (9 June 2014); doi: 10.1117/12.2052944;

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