Impact of Phase Noise and Power Amplifier Non-Linearities on Downlink Cell-Free Massive MIMO-OFDM Systems

TL;DR

This paper analyzes how phase noise and power amplifier nonlinearities affect the spectral efficiency of cell-free massive MIMO-OFDM systems, revealing phase noise as the more detrimental impairment especially in distributed setups.

Contribution

It provides a comprehensive analysis of hardware impairments on downlink spectral efficiency using Bussgang decomposition, including both centralized and distributed precoding strategies.

Findings

Phase noise causes more severe degradation than PA nonlinearities.

Distributed operation is more sensitive to hardware impairments.

Future precoding should consider hardware distortion effects.

Abstract

Cell-free massive MIMO (multiple-input multiple-output) is a key enabler for the sixth generation (6G) of mobile networks, offering significant spectral and energy efficiency gains through user-centric operation of distributed access points (APs). However, its reliance on low-cost APs introduces inevitable hardware impairments, whose combined impact on wideband downlink systems remains unexplored when analyzed using behavioral models. This paper presents a comprehensive analysis of the downlink spectral efficiency (SE) in cell-free massive MIMO-OFDM systems under practical hardware impairments, including phase noise and third-order power amplifier nonlinearities. Both centralized and distributed precoding strategies are examined. By leveraging the Bussgang decomposition, we derive an SE expression and quantify the relative impact of impairments through simulations. Our results reveal that phase noise causes more severe degradation than power amplifier distortions, especially in distributed operation, highlighting the need for future distortion-aware precoding designs.