User-Centric Association and Feedback Bit Allocation for FDD Cell-Free Massive MIMO

2024-05-19

arXiv.org

AI Summary

This paper addresses the joint user-centric access point (AP)–user equipment (UE) association and uplink feedback bit allocation in frequency-division duplex (FDD) massive MIMO cellular systems under limited feedback constraints. Method: We propose a joint optimization framework that introduces an adaptive multipath quantization codebook based on path-gain disparity, explicitly modeling the impact of feedback quantization accuracy on downlink beamforming performance. A non-convex optimization problem is formulated to maximize long-term sum rate, and a low-complexity statistical solution strategy is developed leveraging the Saleh–Valenzuela channel model. Contribution/Results: To the best of our knowledge, this is the first work to jointly optimize UE–AP association and dynamic feedback bit allocation. The proposed framework significantly improves both sum rate and edge-user performance. Under strict total feedback-bit budgets, it demonstrates superior effectiveness and robustness compared to conventional approaches.

Technology Category

• Search and Optimization: Mixed Discrete/Continuous Search
• Multiagent Systems: Mechanism Design
• Machine Learning: Calibration & Uncertainty Quantification

Application Category

• User Modeling, Personalization and Recommendation: Federated recommendation systems and personalization
• Graph Algorithms and Modeling for the Web: Efficient manipulation of static and dynamic Web-related graphs
• Systems and Infrastructure for Web, Mobile and WoT: Energy management for devices in mobile Web and WoT environments

Abstract

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.

Problem

• Optimize user-centric association
• Allocate feedback bits efficiently
• Enhance CF-mMIMO system performance

Innovation

• User-centric UE-AP association
• Feedback bit allocation optimization
• Adaptive path quantization codebook