The use of multiple laser transmitters combined with multiple photodetectors (PDs) is studied for terrestrial , line-of-sight optical communication. The resulting multiple -input/multiple-output channel has the potential for combatting fading effects on turbulent optical channels. In this paper, the modulation format is repetition-ary PPM across lasers, with intensity modulation. Ideal PDs are assumed, with and without background radiation. Both Rayleigh and log-normal fading models are treated. The focus is upon both symbol-/bit-error probability for uncoded transmission, and on constrained channel capacity. Index Terms-multiple-input/multiple-output (MIMO) processing , optical communication, optical modulation/demodulation.

Abstract-We study the use of multiple laser transmitters combined with multiple photodetectors for atmospheric, line-of-sight optical communication, and focus upon the use of multiple-pulseposition-modulation as a power-efficient transmission format, with signal repetition across the laser array. Ideal (photon counting) photodetectors are assumed, with and without background radiation. The resulting multiple-input/multiple-output channel has the potential for combating fading effects on turbulent optical channels, for which both log-normal and Rayleigh-fading models are treated. Our focus is upon symbol error probability for uncoded transmission, and on capacity for coded transmission. Full spatial diversity is obtained naturally in this application.Index Terms-Multiple-input/multiple-output (MIMO) systems, optical communications, pulse-position-modulation (PPM).

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