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.

翻译：实际分布式部署本质上存在异步信号到达问题，这会加剧多用户干扰并降低系统性能，特别是在相干传输场景中。为从根源上缓解异步接收效应，本文提出将流体天线系统（FAS）集成到分布式无小区大规模MIMO系统中，利用其可重构空间位置释放额外空间自由度（DoFs）。我们建立了考虑异步接收（即延迟相位）的FAS赋能数据传输模型，并在采用低复杂度最大比（MR）预编码的条件下，推导了相干与非相干传输模式下所提系统的下行频谱效率（SE）解析表达式，以提供基础理论界。具体而言，我们提出了一种新颖的非单调加速投影梯度上升算法，用以联合优化FAS位置与功率控制系数，最大化下行和速率。数值结果表明：尽管异步接收会严重恶化相干传输的系统性能，但经优化的FAS位置所释放的空间自由度，配合高效的功率控制，能够显著抵消未知延迟相位的影响，且性能优于传统固定位置天线。对于天然规避异步接收的非相干传输，FAS的应用则利用空间可重构性固有地最大化信号强度，实现更显著的SE增益。最终，本文提出的FAS赋能系统结合高效功率控制，能够缓解异步接收导致的性能退化，并超越传统固定位置天线，为在稳健高效的6G无小区大规模MIMO系统中实际部署FAS铺平道路。