Distributed Distortion Aware Beamforming Designs for Cell Free mMIMO Systems

ABSTRACT

In massive multiple-input multiple-output (MIMO) downlink systems, implementing base stations (BSs) often requires low-cost, power-efficient power amplifiers (PAs) to limit hardware expenses and energy consumption. However, such PAs generally have a limited linear amplification range, and operating beyond this range introduces nonlinear distortions that can significantly degrade system performance. Traditional methods for mitigating these distortions, such as digital predistortion (DPD), typically rely on precise knowledge or measurement of the PA transfer function. In this paper, we propose a novel approach to mitigate PA distortions. Assuming a uniform linear array (ULA) at the BS, we employ a Sigma-Delta (ΣΔ) modulator to spatially shape the PA distortions toward high-angle regions. By operating the system in the low-angle region, the received signals are less affected by these distortions. To demonstrate the potential of this spatial shaping approach, we investigate its application in a multi-user MIMO downlink scenario using orthogonal frequency-division multiplexing (OFDM). We develop symbol-level precoding (SLP) and zero-forcing (ZF) precoding schemes that incorporate the new design requirements imposed by the spatial shaping method. Numerical simulations are conducted to validate the effectiveness of the proposed precoding schemes in mitigating PA-induced distortions.

KEYWORDS: Massive MIMO, MIMO-OFDM, nonlinear distortion, power amplifier, sigma-delta modulation, symbol-level precoding.