Receiver IQ Imbalance and Skew Compensation for High Order Modulation Formats by Frequency Domain 4 x 2 MIMO

A typical coherent front-end may encounter imperfections, such as amplitude imbalance, phase imbalance and skew, between four sampling channels. These receiver (Rx) imperfections, together with traditional chromatic dispersion (CD) compensation, would induce in-phase (I) and quadrature (Q) mixing, which greatly degrades the system performance. In this work, we propose a modulation format independent frequency domain (FD) 4 x 2 multi-inputs and multi-outputs (MIMO) based on decision directed least mean square (DD-LMS) algorithm to solve this problem. Compared with traditional 2 x 2 MIMO, it enables to compensate for all of Rx imperfections. Compared with 4 x 4 MIMO, it can eliminate performance penalty induced by the IQ mixing after optical fiber transmission. Compared with traditional time domain 4 x 2 MIMO, the proposed FD 4 x 2 MIMO has advantages in terms of complexity and parallel processing. We also evaluate the Rx imperfections compensation capability, noise tolerance tolerance, polarization tracking capability, residual CD and transmitter imperfections tolerance of the proposed equalizers. The proposed FD 4 x 2 MIMO algorithms are verified by numerical simulation with different modulation formats and experimentally validated with 40 Gbaud polarization division multiplexed 16-ary quadrature amplitude modulation signal.