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-The performance and design of turbo codes using coherent BPSK signaling on the Rayleigh fading channel is considered. In low signal-to-noise regions, performance analysis uses simulations of typical turbo coding systems. For higher signalto-noise regions beyond simulation capabilities, an average upper bound is used in which the average is over all possible interleaving schemes. Fully interleaved and exponentially correlated Rayleigh channels are explored. Furthermore, the design issues relevant to turbo codes are examined for the correlated fading channel. Turbo interleaver design criteria are developed and architectural modifications are proposed for improved performance.

Iterative decoding of turbo codes, as well as other concatenated coding schemes of similar nature, requires accurate knowledge of the signal-to-noise ratio of the channel so that proper blending of the a posteriori information of the separate decoders is achieved. In this paper, we study the sensitivity of decoder performance to mis-estimation of the SNR, and propose a simple on-line scheme that estimates the unknown SNR from each code block, prior to decoding. We show this scheme is su ciently adequate in accuracy to not appreciably degrade performance.

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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