Real-time demonstration of a MIMO adaptive equalizer with non-circular symmetry CMA strategy in a two-aperture coherent digital combining FSO system

Severe time-dependent fading caused by atmospheric turbulence remains a major bottleneck in spatial coherent communications. To address this, we propose and implement an FPGA-based two-aperture real-time MIMO adaptive equalizer for coherent digital combining. By innovatively adopting the non-circular symmetric constant modulus algorithm (NCS-CMA), the design eliminates traditional reconfiguration delays. It adaptively supports BPSK, QPSK, and 16QAM formats without requiring tap coefficient switching. To handle massive data throughput in multi-aperture systems, we detail the first systematic large-scale parallel DSP architecture, which seamlessly integrates I/Q imbalance compensation, clock recovery, MIMO equalization, and frequency/phase offset compensation. Experiments show that compared to a MIMO 2 × 1 setup, the proposed MIMO 4 × 2 equalizer achieves OSNR gains of ∼2.8 dB, ∼2.7 dB, and ∼2.3 dB for BPSK, QPSK, and 16QAM, respectively. Additionally, it demonstrates strong robustness against inter-aperture delay skew and effectively mitigates spatial I/Q imbalance.