Mathematics – Probability
Scientific paper
Dec 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt.........5x&link_type=abstract
Thesis (PhD). UNIVERSITY OF CENTRAL FLORIDA, Source DAI-B 59/11, p. 6009, May 1999, 132 pages.
Mathematics
Probability
Scientific paper
Since the atmosphere is the transmission channel for both laser communications and laser radar systems, both systems suffer the same detection problems-turbulence-induced signal fluctuations and fade. The objective of this dissertation is to demonstrate theoretically and experimentally a coherent array detection system which can mitigate effectively the signal fluctuations and fade due to the atmospheric turbulence for both applications. A model of laser scintillation through the atmosphere and reflected from the target with Lambert surface or point target was developed and verified in the field tests of the system. The carrier-to-noise ratio (CNR) gain of the coherent array detection was compared to monolithic aperture coherent detection both theoretically and experimentally. It was shown that the mean CNR gain of the coherent array detection increases linearly with the number of apertures/receivers. The gain rate depends on the statistics of laser scintillation. The signal fade statistics of coherent array and monolithic aperture coherent detection was also analyzed theoretically and experimentally. It was shown that the probability density function (PDF) of IF intensity from the monolithic aperture coherent detection approaches the negative exponential distribution if the aperture size is larger than the transverse coherence length of the received signal field. Hence, there is no improvement in probability of fade of the IF signal by increasing the size of receiving aperture or the emitting power of laser source for monolithic aperture coherent detection. The PDF of the IF intensity from the coherent array detection approaches a gamma distribution. The probability of fade of IF signal tends to zero by increasing the number of apertures. Hence, the advantages of the coherent array detection system over the monolithic aperture coherent detection system in both CNR performance and signal fade statistics have shown that the coherent array detection is a potential solution for laser communications and radar to overcome the turbulence-induced signal fluctuations and fade. Finally, a new scheme of the coherent array detection system was discovered which could automatically cancel out the turbulence-induced random phase and target movement-induced Doppler frequency shift in IF signal. This will make the system more practical.
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