🤖 AI Summary
Cell-free massive MIMO systems under spatially correlated Rician fading suffer from pilot contamination, limiting spectral efficiency (SE).
Method: This paper proposes a low-complexity optimal bilinear equalizer (OBE) design framework to overcome the SE bottleneck. We first derive closed-form achievable SE expressions for both centralized and distributed bilinear equalization structures. Based on these, we propose three OBE schemes tailored to varying availability of channel statistics: C-OBE (centralized), DG-OBE (distributed group-wise), and DL-OBE (distributed local).
Contribution/Results: The proposed schemes are theoretically rigorous and practically implementable, enabling flexible deployment. Simulation results demonstrate that, under severe pilot contamination, all three OBE schemes achieve significantly higher SE than conventional linear equalizers—validating the effectiveness and superiority of the bilinear structure in capturing nonlinear channel correlations.
📝 Abstract
In this paper, we explore the low-complexity optimal bilinear equalizer (OBE) combining scheme design for cell-free massive multiple-input multiple-output networks with spatially correlated Rician fading channels. We provide a spectral efficiency (SE) performance analysis framework for both the centralized and distributed processing schemes with bilinear equalizer (BE)-structure combining schemes applied. The BE-structured combining is a set of schemes that are constructed by the multiplications of channel statistics-based BE matrices and instantaneous channel estimates. Notably, we derive closed-form achievable SE expressions for centralized and distributed BE-structured combining schemes. We propose one centralized and two distributed OBE schemes: Centralized OBE (C-OBE), Distributed OBE based on Global channel statistics (DG-OBE), and Distributed OBE based on Local channel statistics (DL-OBE), which maximize their respective SE expressions. OBE matrices in these schemes are tailored based on varying levels of channel statistics. Notably, we obtain new and insightful closed-form results for the C-OBE, DG-OBE, and DL-OBE combining schemes. Numerical results demonstrate that the proposed OBE schemes can achieve excellent SE, even in scenarios with severe pilot contamination.