Spectral and Energy Efficiency of Cell-Free Massive MIMO–OFDM Over Measured Urban Channels

Abstract

Cell-free massive multiple-input multiple-output (CF-mMIMO) is a promising architecture for future sixth-generation (6G) wireless networks because cooperation among geographically distributed access points enables seamless connectivity, more uniform quality of service, and improved spectral and energy efficiency. However, most existing performance evaluations rely on idealized or simulated channel models. This paper presents a measurement-driven system-level evaluation of downlink CF-mMIMO orthogonal frequency-division multiplexing (OFDM) using urban channel measurements acquired at $5.89{\,}\mathrm {GHz}$ . The considered framework combines user-centric clustering, distributed partial zero-forcing (PZF) precoding, and local peak-to-average power ratio (PAPR)-aware precoding under nonlinear power-amplifier constraints. Using the measured channels, we investigate the spectral-efficiency and energy-efficiency tradeoffs associated with access-point density, the number of antennas per access point, and the power-amplifier input back-off. Increasing the number of antennas per access point significantly improves spectral efficiency, although it progressively shifts the deployment away from the cell-free paradigm by reducing the number of distributed access points. In the considered fixed antenna budget, the configuration with 32 antennas per access point and 13 access points achieves the highest energy efficiency among the evaluated configurations. PAPR-aware techniques, with power amplifiers operating at an input back-off of $3{\,}\mathrm {dB}$ , provide up to a 2.5 times improvement in spectral efficiency and a 120 percent increase in energy efficiency, indicating the potential of CF-mMIMO under realistic propagation and hardware constraints for 6G wireless deployments under real-world propagation and hardware conditions.