Cell-Free Massive MIMO with Hardware-Impaired Wireless Fronthaul

TL;DR

This paper investigates the impact of hardware impairments on wireless fronthaul links in cell-free massive MIMO systems, proposing a novel amplify-and-forward scheme and distortion-aware processing to improve spectral efficiency.

Contribution

It introduces a new AF-based wireless fronthauling scheme for cell-free massive MIMO that accounts for hardware impairments at both access points and fronthaul transceivers.

Findings

Distortion-aware processing significantly improves spectral efficiency.

Hardware impairments at fronthaul links impact overall system performance.

The proposed scheme offers a cost-effective solution for future cell-free networks.

Abstract

Cell-free massive MIMO (multiple-input multiple-output) enhances spectral and energy efficiency compared to conventional cellular networks by enabling joint transmission and reception across a large number of distributed access points (APs). Since these APs are envisioned to be low-cost and densely deployed, hardware impairments, stemming from non-ideal radio-frequency (RF) chains, are unavoidable. While existing studies primarily address hardware impairments on the access side, the impact of hardware impairments on the wireless fronthaul link has remained largely unexplored. In this work, we fill this important gap by introducing a novel amplify-and-forward (AF) based wireless fronthauling scheme tailored for cell-free massive MIMO. Focusing on the uplink, we develop an analytical framework that jointly models the hardware impairments at both the APs and the fronthaul transceivers, derives the resulting end-to-end distorted signal expression, and quantifies the individual contribution of each impairment to the spectral efficiency. Furthermore, we design distortion-aware linear combiners that optimally mitigate these effects. Numerical results demonstrate significant performance gains from distortion-aware processing and illustrate the potential of the proposed AF fronthauling scheme as a cost-effective enabler for future cell-free architectures.