Real-time demonstration of two-aperture coherent digital combining free-space optical transmission with a real-valued MIMO adaptive equalizer

Compared with the single-aperture system, the multi-aperture coherent digital combining system has the technical advantage of the effective mitigation of deep fading under strong turbulence, ease of scalability, and potential higher collected optical power. However, the tricky problem of a multi-aperture system is to efficiently combine multiple branch signals with a static skew mismatch and with time-varying characteristics of received power scintillation. In this Letter, a real-valued massive array multiple-input multiple-output (MIMO) adaptive equalizer is proposed for the first time to our knowledge to realize multi-aperture channel equalization and combining, simultaneously. In the proof-of-principle system, the feasibility of the combining technique is verified based on a MIMO 4 × 2 equalizer in a 2.5-GBaud data rate QPSK modulation FPGA-based two-aperture coherent receiver with a dynamic turbulence simulator. The results show that no reduction in combining efficiency is observed under static turbulence conditions at the hard-decision forward error correction (HD-FEC) limit of 3.8 × 0-3, and combining efficiencies of 95% and 88% are obtained for the dynamic moderate and strong turbulence.