By Arun K. Majumdar

This is a complete instructional at the rising know-how of free-space laser communications (FSLC). The ebook bargains an all-inclusive resource of knowledge at the fundamentals of FSLC, and a evaluation of state of the art applied sciences. assurance contains atmospheric results for laser propagation and FSLC platforms functionality and layout. Free-Space Laser Communications is a necessary source for engineers, scientists and scholars attracted to laser verbal exchange structures designed for the atmospheric optical channel.

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Extra info for Free-Space Laser Communications: Principles and Advances (Optical and Fiber Communications Reports)

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Bit Error Rate Determination for a Direct-Detection Binary Optical Communication Link The most important question for a laser communication system operated in the presence of turbulence is, how does optical turbulence affect the bit error rate (BER) of the system? Here we review a model for a direct-detection binary optical communication link consisting of a laser transmitter, atmospheric channel containing optical turbulence, and a maximum likelihood receiver [67]. The effects of the atmospheric channel are described by the aperture-averaged scintillation index O";~ s - The maximum likelihood receiver consists of an avalanche photodiode (APD) photodetector, a matched filter implemented in the form of a moving integrator, and a clocked comparator.

The effects of aperture averaging on the scintillation index can be seen from Figs. 15 (a, b). The scintillation index for a point corresponds to a lens diameter of zero. As receiver aperture size increases, the scintillation index decreases significantly. 55 /lm , aperture sizes larger than about 6 ern do not lead to any significant addition in aperture averaging. 785 /lm roughly halves the aperture size required to "average" the majority of scintillations. It is clear that aperture averaging can provide a powerful advantage in reducing atmospheric turbulence-induced signal fades.

38 X 10- 23 J/K is Boltzmann's constant, T is effective noise temperature in K, and RL is load resistor input to the amplifier in ohms. For simplicity, background light and photodetector dark current are neglected. dS S' (84) where S is a dimensionless random variable describing optical turbulence-induced fading in the received signal, and the average of In S is (In S) = ar~ sFor on-off signaling, the optical power levels at the receiver can be represented under the two signaling hypotheses as either a digital I (ON state) given by _1/2 P~Pt = S(Popt), (85) or a digital 0 (OFF state) given by P~Pt = cS(Popt) , (86) where e represents residual laser light in the OFF state due to the source modulator having an imperfect extinction ratio.

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