Nonsingular Constant Modulus Equalizer for PDM-QPSK Coherent Optical Receivers

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Adaptive electronic equalizers using the Constant Modulus Algorithm (CMA) algorithm often converge to a singular coefficient matrix that produces the same signal at multiple outputs. The authors address this issue in the context of optical communications systems with polarization-division multiplexing and coherent receivers. They study, by computer simulation, the performance of multiuser CMA equalizer, an enhanced CMA equalizer initially proposed for use in wireless multiuser and later multiple-input/multiple-output communications systems. They show that the proposed adaptive electronic equalizer does not exhibit singularities and, therefore, is superior to the commonly used CMA equalizer.