Cell-Free MIMO with Rotatable Antennas: When Macro-Diversity Meets Antenna Directivity

TL;DR

This paper introduces rotatable antennas in cell-free MIMO networks to improve macro-diversity and fairness, optimizing both beamforming and antenna orientation for better worst-user rates.

Contribution

It develops an alternating-optimization algorithm and a two-stage scheme to jointly optimize beamforming and antenna orientations, enhancing network performance.

Findings

Orientation-aware designs significantly improve worst-user rates.

Larger antenna directivity boosts fairness with proper orientation.

The proposed methods outperform conventional beamforming benchmarks.

Abstract

Cell-free networks leverage distributed access points (APs) to achieve macro-diversity, yet their performance is often constrained by large disparities in channel quality arising from user geometry and blockages. To address this, rotatable antennas (RAs) add a lightweight hardware degree of freedom by steering the antenna boresight toward dominant propagation directions to strengthen unfavorable links, thereby enabling the network to better exploit macro-diversity for higher and more uniform performance. This paper investigates an RA-enabled cell-free downlink network and formulates a max-min rate problem that jointly optimizes transmit beamforming and antenna orientations. To tackle this challenging problem, we develop an alternating-optimization-based algorithm that iteratively updates the beamformers via a second-order cone program (SOCP) and optimizes the antenna orientations using successive convex approximation. To reduce complexity, we further propose an efficient two-stage scheme that first designs orientations by maximizing a proportional-fair log-utility using manifold-aware Frank-Wolfe updates, and then computes the beamformers using an SOCP-based design. Simulation results demonstrate that the proposed orientation-aware designs achieve a substantially higher worst-user rate than conventional beamforming-only benchmarks. Furthermore, larger antenna directivity enhances fairness with proper orientation but can degrade the worst-user performance otherwise.