Minimization of acquisition time in short-range free-space optical communication.

We consider short-range (1-3-km) free-space optical communication between moving parties when covertness is the overriding system performance requirement. To maximize covertness, it is critical to minimize the time required for the acquisition phase, during which the party initiating contact must conduct a broad-field scan and so risks revealing his position. Assuming an elliptical Gaussian beam profile, we show how to optimize the beam divergence angles, scan speed, and design of the raster scan pattern so as to minimize acquisition time. In this optimization, several constraints are considered, including signal-to-noise ratio, required for accurate bearing detection and reliable decoding; limited receiver bandwidth; limited scanner speed; and beam divergence as limited by the scanner mirror dimensions. The effects of atmospheric turbulence are also discussed.