Multi-Aperture Coherent Digital Combining Based on Complex-Valued MIMO 2N x 2 Adaptive Equalizer for FSO Communication

Collecting light using multiple small apertures offers a lot of advantages over doing so using a single large aperture, including the effective mitigation of scintillation under strong turbulence, low implementation cost, and ease of scalability. However, the tricky of a multi-aperture system is to combine the multiple branches signals with static skew mismatch and with time-varying characteristics of gain, phase and state of polarization, especially for coherent digital combining of polarization multiplexing (PM) signals. In this article, a complex-valued multiple-input multiple-output (MIMO) 2N x 2 adaptive equalizer is designed to combine multi-aperture signals, and the tap coefficient of each branch is calculated based on the constant modulus algorithm (CMA). This technique relaxes hardware constraints on the time alignment of multi-aperture branch since the skew can be compensated electronically thanks to the proposed blind digital combining algorithm. In addition, laser linewidth within evaluation range smaller than 10 MHz has no effect on the performance of 2N x 2 adaptive equalizer. Moreover, the feasibility of the proposed adaptive equalizer for moderate and strong turbulence suppression is verified by a 10-Gbps data rate PM-QPSK modulation offline two- and four-aperture simulated turbulence experiment.