Hierarchical Coordination in 6DMA-Enhanced Cell-Free Networks: Joint Association, Position, and Orientation Optimization

We investigate six-dimensional movable antenna (6DMA)-enhanced cell-free networks, where each access point (AP) hosts multiple multi-antenna surfaces capable of dynamic position and orientation adjustments. To maximize uplink performance, we propose a hierarchical coordination framework that jointly optimizes AP-surface-user association, constrained position adaptation based on orientation-aware channel gains and cluster-weighted centroids, and orientation optimization via Riemannian manifold techniques. A two-stage orientation algorithm, combining global grid search with local manifold refinement, ensures convergence to high-quality solutions under nonconvex Stiefel constraints. The proposed architecture achieves per-surface complexity bounded independently of the total number of users. Numerical results demonstrate that the proposed framework simultaneously enhances spectral efficiency (SE), system sum rate, and fairness by preferentially benefiting users with weaker initial channels. These findings highlight the potential of 6DMA-based cell-free networks for adaptive, high-capacity, and equitable wireless systems.