We report on the demonstration of a bidirectional free-space link over 18 km as part of the European-Union project "VERTIGO" that investigates technologies for the optical GEO-satellite feeder link. Two different terminals were deployed: a single-aperture "satellite" terminal and a 4-aperture "ground" terminal. Using SFP+ transceivers with OOK modulation at 10 Gbit/s, real-time bit error rates (BER) were measured for each aperture in both directions using an FPGA platform. In both directions, diversity signals at the receiver were processed digitally for combining. We report on the achieved performance improvement compared to a single aperture.