Problem
Research questions and friction points this paper is trying to address.
asynchronous reception
cell-free massive MIMO
multi-user interference
spectral efficiency
fluid antenna system
Fluid Antenna System-Enabled Mitigation of Asynchronous Reception in Cell-Free Massive MIMO Systems
🤖 AI Summary
This study addresses the performance degradation and multiuser interference in distributed cell-free massive MIMO systems caused by asynchronous signal arrivals. To mitigate these issues, the work introduces fluid antenna systems (FAS) for the first time, leveraging their reconfigurable spatial positions to provide additional spatial degrees of freedom. A downlink transmission model incorporating delay-induced phase shifts is formulated, and a non-monotonic accelerated projected gradient ascent algorithm is proposed to jointly optimize antenna placements and power allocation. The approach significantly enhances system robustness and spectral efficiency under both coherent and non-coherent transmission regimes: in coherent scenarios, it effectively compensates for performance loss due to unknown delay-induced phase shifts, while in non-coherent scenarios, it further boosts signal strength, consistently outperforming conventional fixed-antenna systems.
📝 Abstract
Practical distributed deployments inherently suffer from asynchronous signal arrivals, which exacerbate multi-user interference and degrade system performance, especially for coherent transmission. To natively mitigate the asynchronous reception effect, this paper proposes integrating fluid antenna systems (FASs) into distributed cell-free massive MIMO systems, exploiting their reconfigurable spatial positions to release additional spatial degrees of freedom (DoFs). We establish the FAS-enabled data transmission model with asynchronous reception, i.e., delay phases. We also derive the analytical downlink spectral efficiency (SE) performance of the proposed system under coherent and non-coherent transmissions, using low-complexity Maximum Ratio (MR) precoding to provide fundamental theoretical bounds. Specifically, we propose a novel nonmonotone accelerated projected gradient ascent algorithm to jointly optimize FAS positions and power control coefficients, maximizing the downlink sum SE. Numerical results demonstrate that while asynchronous reception severely degrades system performance for coherent transmission, the spatial DoFs unlocked by optimized FAS positions, along with efficient power control, can significantly counteract the effects of unknown delay phases and outperform traditional fixed-position antennas. For non-coherent transmission, which inherently bypasses asynchronous reception, the application of FAS leverages spatial reconfigurability to natively maximize signal strength and achieve more pronounced SE gains. Ultimately, our proposed FAS-enabled system, coupled with efficient power control, mitigates performance degradation due to asynchronous reception and outperforms traditional fixed-position antennas, paving the way for the practical deployment of FASs in robust, highly efficient 6G cell-free massive MIMO systems.
Innovation
Methods, ideas, or system contributions that make the work stand out.
Fluid Antenna System
Cell-Free Massive MIMO
Asynchronous Reception
Spatial Degrees of Freedom
Noncoherent Transmission
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