Real-Time 10 Gbit/s Digital Combining Over a 3 km Free-Space Optical Link With Multi-Aperture Receiver

In this article, we demonstrate a real-time implementation of the digital combining of 10 Gbit/s signals received by a multi-aperture terminal over a 3 km free-space optical link. The optical signal is sent by a single aperture and received by an array of four apertures. The diversity signals, which experience different impairments over the distinct optical paths, are separately photo-detected and digitized. The digital processing performed on an FPGA platform consists of synchronization and combining of the received bit streams. The combining method uses weighting factors which are determined from continuous measurements of the coupled optical power at each aperture. A five-channel FPGA-based bit error rate tester is used to simultaneously measure the transmission performance of the individual channels and the combined signal. We report on the benefit of the multi-aperture combining technique under atmospheric turbulence with the analysis of the received optical power and achieved bit error rate distributions.