Digital Post-Distortion for Joint Mitigation of Transmitter I/Q Imbalance and Power Amplifier Nonlinearities in Multiuser MIMO Networks

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In this paper, we propose a novel digital post-distortion (DPoD) technique designed to improve the received signal quality in orthogonal frequency-division multiple (OFDM)-based multi-user multiple-input multiple-output (MU-MIMO) systems suffering from severe coexisting transmitter impairments in the form of power amplifier (PA) nonlinearities and in-phase/quadrature (I/Q) imbalance. The proposed one-shot or non-iterative processing approach combines augmented memory polynomial-based joint impairment modeling and cancellation with practical parameter estimation using 3GPP 5G NR standard-compatible demodulation reference signal (DMRS) structures. Numerical evaluations conducted in extensive 5G NR MU-MIMO uplink scenarios encompassing both 2.1 GHz and 28 GHz frequency bands as well as LOS and NLOS channel conditions reveal significant gains in receiver performance, measured through effective error vector magnitudes (EVMs), while similar EVM gains are shown also with measured 6G mid-band PA responses at 7 GHz. The results show that the proposed method maintains its effectiveness even under aggressive transmitter impairments and high-order modulation schemes including 64-QAM, 256-QAM, and 1024-QAM, thus offering a promising path for improved power-efficiency and coverage or link-distances in future wireless networks toward the 6G era. The numerical results also show and highlight that the proposed Joint-DPoI approach achieves lower receiver EVM values compared to prior-art methods where coexisting impairments are neglected. Finally, it is also demonstrated that the proposed DPoD approach is robust against residual carrier frequency offsets, reasonable user mobilities, frequency-selective impairments, and different types of PA nonlinearities.