Absrrucf-A bound on the probability of error is obtained for an Wary direct-detection optical communications system consisting of an amplitude-stabilized source, a lognormal atmospheric channel, and a photocounting detector array. Equal-energy, e&probable, orthogonal signaling, and flat independent fading at all detectors is assumed. The result reduces to that obtained previously in the absence of fading. A comparison is made with the analogous solution for the heterodyne array receiver.

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Abstract. Spatially localized bursts of intense lower hybrid wave Description of Experiment radiation are often encountered in ionospheric observations. These electric field enhancements have been correlated with depletions in plasma density. This laboratory investigation was undertaken to provide the kind of insight into the nature of the interaction which is difficult to obtain from ionospheric observations alone. The experiments were performed in the Large Plasma Device (L^PD) at UCLA. The dependence of the interaction on wave frequency, depletion shape and size and density gradient is investigated in three spatial dimensions and in time. This letter concentrates on the dependence of the phenomenon on the density gradient. The main result is that the lower hybrid waves are observed to have the largest electric field in the regions of steepest density gradient.

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The structure of the optimum direct detection array receiver is obtained for a system consisting of an amplitude-stabilized optical source, a lognormal channel, and a bank of photocounting detectors. Additive independent background radiation and detector dark current are taken into account. Both orthogonal and nonorthogonal M-ary signaling formats are considered. Attention is given to detection intervals small in comparison with the correlation time of the atmospherically induced fluctuations. A saddlepoint integration provides an excellent approximation to the optimum processor, resulting in a considerably simplified structure. Suboptimum aperture integration and maximum a posteriori (MAP) receivers are also considered. The performance in related papers (Parts 2 and 3).