Zero-Forcing MU-MIMO Precoding under Power Amplifier Non-Linearities
Juan Vidal Alegría, Ashkan Sheikhi, Ove Edfors
TL;DR
This work addresses interference cancellation in MU-MIMO downlinks when power amplifier non-linearities distort linear precoding. By applying Bussgang theory, the authors formulate Non-Linearity-Aware Zero-Forcing (NLA-ZF), deriving iterative 2x2 solutions and extending to larger, even numbers of BS antennas via a block-wise approach. The proposed methods aim to restore interference cancellation without costly digital pre-distortion, showing performance gains in interference-limited regimes and under realistic PA models. The results suggest that NLA-ZF can reduce distortion floors and improve SINDR as the number of BS antennas increases, highlighting its practical relevance as a low-power alternative to perfect DPD in multi-user systems.
Abstract
In multi-user multiple-input multiple-output (MU-MIMO) systems, the non-linear behavior of the power amplifiers (PAs) may cause degradation of the linear precoding schemes dealing with interference between user equipments (UEs), e.g., the zero-forcing (ZF) precoder. One way to minimize this effect is to use digital-pre-distortion (DPD) modules to linearize the PAs. However, using perfect DPD modules is costly and it may incur significant power consumption. As an alternative, we consider the problem of characterizing non-linearity-aware ZF (NLA-ZF) precoding schemes, hereby defined as linear precoders that achieve perfect interference cancellation in the presence of PA non-linearity by exploiting knowledge of this non-linear response. We provide initial iterative solutions that allow achieving NLA-ZF (up to adjustable tolerance) in a two-UE downlink MU-MIMO scenario where the base station (BS) has an even number of antennas, and each antenna is connected to a PA exhibiting third-order memory-less non-linear behavior. The proposed approach allows for performance gains in scenarios with significant residual interference.