Near-Field Cell-Free Massive MIMO With Channel Aging: A Modified LoS Model

In this letter, we propose a novel and modified beam-steering model based on asynchronous reception under a near-field cell-free massive multiple-input-multiple-output (CF mMIMO) system. Our model reveals that CF mMIMO establishes a near-field environment via multiple far-field planar waves from distributed access points. We further prove that the proposed model has the potential to reduce multi-user interference. Accordingly, we analyze the downlink spectral efficiency (SE) of the near-field CF mMIMO system with channel aging. We employ the near-field steering model to characterize line-of-sight components and spatial correlation matrices. Subsequently, we derive the closed-form SE expression of maximum-ratio transmission. Numerical results show that the modified model outperforms the common model due to much lower correlation between two adjacent locations, and CF mMIMO can benefit from its inherent near-field characteristic.