Approximate expressions are derived for the probability density functions of the i.f. signal magnitudes from optical heterodyne detection systems operating in the presence of clear air turbulence. The effects of lognormal amplitude fluctuations and Gaussian phase perturbations, in addition to local oscillator shot noise, are considered for both passive receivers and those employing active tilt-tracking systems to eliminate angle-of-arrival fluctuations. In Part 2, experimental results are presented that verify the density functions developed here.
Experimental results are presented for the probability density functions of the i.f. signal magnitude from a static optical heterodyne receiver operating in the presence of clear air turbulence. These results are compared with the theory developed in Part 1 and are shown to give excellent agreement. The experimental system includes a He-Ne laser transmitter operating at 632.8 nm, a 1.6-km propagation path through the open atmosphere, and an optical heterodyne receiver.
An averaged threshold receiver is developed for an optical communication system consisting of a symmetric binary, pulse-code modulated transmitter, a lognormal channel, and an array of independent photocounting detectors. When compared to previously described receivers, it is shown to be a much simpler structure to implement and to provide generally lower bit error rates. Probability of error curves demonstrating this improved performance are presented for various combinations of turbulence strength, background radiation level, SNR, number of diversity channels, and, in the newly developed processor, number of bits used for threshold averaging.
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