Uplink Performance of Cell-Free Massive MIMO Over Spatially Correlated Rician Fading Channels

TL;DR

This paper analyzes uplink spectral efficiency in cell-free massive MIMO systems with spatially correlated Rician fading, considering phase shifts due to user movement, and derives new closed-form expressions validated by numerical results.

Contribution

It introduces new closed-form spectral efficiency expressions for cell-free massive MIMO over Rician channels with phase-aware estimators and compares different combining schemes.

Findings

SE benefits from spatial correlation

Performance gap increases with number of antennas

LMMSE estimator has lower SE without phase-shift knowledge

Abstract

We consider a practical cell-free massive multiple-input-multiple-output (MIMO) system with multi-antenna access points (APs) and spatially correlated Rician fading channels. The significant phase-shift of the line-of-sight component induced by the user equipment movement is modeled randomly. Furthermore, we investigate the uplink spectral efficiency (SE) with maximum ratio (MR)/local minimum mean squared error (L-MMSE) combining and optimal large-scale fading decoding based on the phase-aware MMSE, phase-aware element-wise MMSE and linear MMSE (LMMSE) estimators. Then new closed-form SE expressions with MR combining are derived. Numerical results validate our derived expressions and show that the SE benefits from the spatial correlation. It is important to observe that the performance gap between L-MMSE and MR combining increases with the number of antennas per AP and the SE of the LMMSE estimator is lower than that of other estimators due to the lack of phase-shifts knowledge.