Two-Timescale Design for Movable Antenna-Enabled Multiuser MIMO Systems

TL;DR

This paper introduces a two-timescale framework for movable antenna-enabled multiuser MIMO systems, optimizing antenna positions based on statistical CSI and beamforming based on instantaneous CSI, leading to significant performance improvements.

Contribution

It proposes a novel two-timescale design for MA-enabled MU-MIMO systems, combining long-term antenna positioning with short-term beamforming optimization, and develops algorithms for ergodic sum rate maximization.

Findings

Significant ergodic sum rate gains over fixed-position antennas.

MA with ZF beamforming outperforms MRT in most scenarios.

MA with MRT offers a simplified alternative in strong LoS conditions.

Abstract

Movable antennas (MAs), which can be swiftly repositioned within a defined region, offer a promising solution to the limitations of fixed-position antennas (FPAs) in adapting to spatial variations in wireless channels, thereby improving channel conditions and communication between transceivers. However, frequent MA position adjustments based on instantaneous channel state information (CSI) incur high operational complexity, making real-time CSI acquisition impractical, especially in fast-fading channels. To address these challenges, we propose a two-timescale transmission framework for MA-enabled multiuser multiple-input-multiple-output (MU-MIMO) systems. In the large timescale, statistical CSI is exploited to optimize MA positions for long-term ergodic performance, whereas, in the small timescale, beamforming vectors are designed using instantaneous CSI to handle short-term channel fluctuations. Within this new framework, we analyze the ergodic sum rate and develop efficient MA position optimization algorithms for both maximum-ratio-transmission (MRT) and zero-forcing (ZF) beamforming schemes. These algorithms employ alternating optimization (AO), successive convex approximation (SCA), and majorization-minimization (MM) techniques, iteratively optimizing antenna positions and refining surrogate functions that approximate the ergodic sum rate. Numerical results show significant ergodic sum rate gains with the proposed two-timescale MA design over conventional FPA systems, particularly under moderate to strong line-of-sight (LoS) conditions. Notably, MA with ZF beamforming consistently outperforms MA with MRT, highlighting the synergy between beamforming and MAs for superior interference management in environments with moderate Rician factors and high user density, while MA with MRT can offer a simplified alternative to complex beamforming designs in strong LoS conditions.