Atmospheric Turbulence in Free-Space Optical Communication

It is well known that when an optical signal is transmitted over kilometer distances, atmospheric turbulence causes fluctuations in the amplitude and phase of the received signal. This "turbulence-induced fading" impairs free-space optical links in much the same way that flat multipath fading impairs radio-frequency wireless links. We begin with joint temporal and spatial distributions of turbulence-induced fading that are derived from physical models. We have evaluated various temporal- and spatial-domain detection techniques to mitigate turbulence-induced fading. In the temporal domain, we have derived performance bounds for various error-correction coding techniques. We have developed an optimal maximum-likelihood sequence detector (MLSD) that jointly detects the data and estimates the instantaneous fading state, and have derived a reduced-complexity implementation of MLSD based on a single-step Markov model of fading correlation. We have also studied pilot-symbol assisted modulation as a low-complexity means to estimate the instantaneous fading state, improving detection efficiency. We have experimentally demonstrated various temporal-domain detection techniques on outdoor free-space links. In the spatial domain, we have evaluated the use of spatial diversity reception, in which signals obtained at multiple receivers are processed using various optimal or suboptimal techniques.

Modeling of Turbulence and Transmission Techniques to Mitigate Turbulence

X. Zhu and J. M. Kahn, "Free-Space Optical Communication through Atmospheric Turbulence Channels", IEEE Trans. on Commun., vol. 50, no. 8, pp. 1293-1300, August 2002. PDF

X. Zhu and J. M. Kahn, "Markov Chain Model in Maximum-Likelihood Sequence Detection for Free-Space Optical Communication through Atmospheric Turbulence Channels", IEEE Trans. on Commun., vol. 51, no. 3, pp. 509-516, March 2003. PDF

X. Zhu and J. M. Kahn, "Performance Bounds for Coded Free-Space Optical Communications through Atmospheric Turbulence Channels", IEEE Trans. on Commun., vol. 51, no. 8, pp. 1233-1239, August 2003. PDF

X. Zhu and J. M. Kahn, "Pilot Symbol-Assisted Modulation for Correlated Turbulent Free-Space Optical Channels", Proc. of SPIE Intl. Symp. on Optical Science and Technol., San Diego, CA, July 29-August 3, 2001. The version published in the conference proceedings contains some minor typographical errors that are corrected in this version. PDF

Experimental Demonstration of Transmission Techniques to Mitigate Turbulence

X. Zhu, J. M. Kahn and J. Wang, "Mitigation of Turbulence-Induced Scintillation Noise in Free-Space Optical Links using Temporal-Domain Detection Techniques", IEEE Photon. Technol. Lett., vol. 15, no. 4, pp. 623-625, April 2003. PDF

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Last modified: June 20, 2003.