Partial Freezing for MIMO Adaptive Equalizers: Precise Demodulation in Dynamic Environments

We propose partially frozen (PF) multiple-input multiple-output (MIMO) adaptive equalizers (AEQs). By increasing the degrees of freedom (DOFs) of widely used complex 2 × 2 MIMO AEQs, complex 4 × 2 and 8 × 2 MIMO AEQs can cope with various signal impairments that occur within a transmitter and receiver, such as in-phase/quadrature (IQ) skew and IQ imbalance. Although precise signal demodulation with such MIMO AEQs are useful for enhancing the information rate of a transmission even to reach beyond 1 Tb/s, the increase in internal DOFs ultimately comes with a decrease in the speed of tracking polarization fluctuations and an increase in computational complexity, which are problematic for efficient demodulation in a dynamic environment. To solve this, as the speed of fluctuation in the state of polarization (SOP) of the signal is much faster than those of impairments within the transmitter and receiver, PF MIMO AEQs reduce effective DOFs by separating each DOF corresponding to the SOP, transmitter, and receiver, then freezing unnecessary updates of transmitter and receiver DOFs. In this article, we describe the principle and show the results of numerical simulations and experiments with 128-Gbaud quadrature phase shift keying (QPSK) signals under IQ impairments. PF MIMO AEQs increased the tracking speed of the 8 × 2 MIMO AEQ, enabling the tracking of polarization fluctuations of over 11 Mrad/s (faster than fluctuation induced by lightning strikes) with less computational complexity than the conventional configuration. This method is a promising solution for high-speed data transmissions in dynamic environments.