In this paper, we introduce a novel approach to user-centric association and feedback bit allocation for the downlink of a cell-free massive MIMO (CF-mMIMO) system, operating under limited feedback constraints. In CF-mMIMO systems employing frequency division duplexing, each access point (AP) relies on channel information provided by its associated user equipments (UEs) for beamforming design. Since the uplink control channel is typically shared among UEs, we take account of each AP's total feedback budget, which is distributed among its associated UEs. By employing the Saleh-Valenzuela multi-resolvable path channel model with different average path gains, we first identify necessary feedback information for each UE, along with an appropriate codebook structure. This structure facilitates adaptive quantization of multiple paths based on their dominance. We then formulate a joint optimization problem addressing user-centric UE-AP association and feedback bit allocation. To address this challenge, we analyze the impact of feedback bit allocation and derive our proposed scheme from the solution of an alternative optimization problem aimed at devising long-term policies, explicitly considering the effects of feedback bit allocation. Numerical results show that our proposed scheme effectively enhances the performance of conventional approaches in CF-mMIMO systems.

翻译：本文针对采用频分双工的无蜂窝大规模MIMO系统在下行链路中面临有限反馈约束的问题，提出了一种面向用户的关联与反馈比特分配新方法。在该系统中，每个接入点需依赖其关联用户设备提供的信道信息进行波束成形设计。由于上行控制信道通常由多个用户设备共享，我们考虑了每个接入点的总反馈预算在其关联用户设备间的分配问题。基于具有不同平均路径增益的Saleh-Valenzuela多分辨路径信道模型，我们首先确定了每个用户设备所需的反馈信息及相应的码本结构。该结构支持根据路径主导性对多条路径进行自适应量化。随后，我们构建了一个联合优化问题以解决面向用户的用户设备-接入点关联与反馈比特分配。为应对这一挑战，我们分析了反馈比特分配的影响，并从旨在制定长期策略的替代优化问题的解中推导出所提出的方案，明确考虑了反馈比特分配的效果。数值结果表明，所提方案能有效提升无蜂窝大规模MIMO系统中传统方法的性能。