Cell-Free Massive MIMO with Hardware-Impaired Wireless Fronthaul

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.

💡 Research Summary

This paper addresses a critical gap in the study of cell‑free massive MIMO systems: the joint impact of hardware impairments on both the access link (UE‑AP) and the wireless fronthaul (AP‑CPU). While most prior work assumes ideal, wired fronthaul, the authors propose an amplify‑and‑forward (AF) wireless fronthaul architecture that is more realistic for large‑scale deployments of low‑cost access points (APs).

System model – The network consists of L APs, each equipped with N antennas, serving K single‑antenna UEs. A central processing unit (CPU) with M antennas receives the forwarded signals from all APs. The UE‑AP channels follow correlated Rayleigh fading, whereas the AP‑CPU channels follow correlated Rician fading with a 10 dB K‑factor. Uplink transmission is divided into two time slots: (i) UEs transmit to APs, and (ii) each AP linearly precodes its received signal and forwards it to the CPU over a wireless link.

Hardware impairment modeling – Both hops are modeled with a quality factor κ∈(0,1] (κ_ac for the access side, κ_frt for the fronthaul side). The distortion introduced by non‑ideal RF chains is approximated as additive Gaussian noise whose covariance is proportional to the instantaneous signal power. For the access side, the distortion covariance is D_ac,l = diag