Rotatable IRS-Assisted 6DMA Communications: A Two-timescale Design

Intelligent reflecting surface (IRS) and movable antenna (MA) are promising technologies to enhance wireless communication by reconfiguring channels at the environment and transceiver sides. However, their performance is constrained by practical limitations. To address this, we propose a multi-functional antenna/surface system that leverages their complementary advantages. A rotatable IRS (R-IRS) is deployed to enhance downlink communications from a six-dimensional MA (6DMA)-equipped base station (BS) to multiple single-antenna users. To reduce the complexity of real-time channel estimation and beamforming, we formulate an optimization problem to maximize the average sum-rate using a two-timescale (TTS) transmission protocol. Specifically, the BS antenna configuration (including position and rotation) and IRS rotation and reflection are optimized based on statistical channel state information (S-CSI), while BS transmit beamforming is designed using instantaneous CSI (I-CSI) in the short timescale. We first consider a single-user case and show that the 6DMA at the BS should form a sparse array for multi-beam transmission towards both the IRS and the user, allowing efficient coordination of direct and reflected channels, while the IRS rotation achieves effective multi-path alignment. For the general multi-user case, the optimization problem is non-convex and challenging to solve. To tackle this, we propose an efficient algorithm combining weighted minimum mean-square error (WMMSE) and stochastic successive convex approximation (SSCA) techniques. A low-complexity algorithm is also proposed to reduce computational complexity. Numerical results validate the proposed system, showing significant performance gains by jointly exploiting the spatial degrees of freedom of the 6DMA-BS and R-IRS under the TTS protocol.

💡 Research Summary

This paper proposes a novel multi‑functional wireless system that jointly exploits a rotatable intelligent reflecting surface (R‑IRS) and a six‑dimensional movable antenna (6DMA) at the base station (BS) to serve multiple single‑antenna users in the downlink. Recognizing that both IRS‑only and MA‑only designs suffer from practical limitations—IRSs lack the ability to reshape the direct BS‑user link, while movable antennas cannot fully compensate for severe blockages—the authors combine the complementary degrees of freedom (DoFs) of the two technologies.

To keep the system tractable in fast‑fading environments, a two‑timescale (TTS) transmission protocol is adopted. In the long‑timescale, only statistical channel state information (S‑CSI) is required; the BS antenna configuration (positions and rotations of the 6DMA elements) and the IRS rotation angle together with the passive phase‑shift matrix are optimized. In the short‑timescale, instantaneous CSI (I‑CSI) is used to design the digital beamforming vectors at the BS. This separation dramatically reduces pilot overhead and real‑time hardware reconfiguration.

For a single‑user scenario, the authors show that arranging the 6DMA elements in a sparse linear array enables simultaneous multi‑beam transmission: one beam points directly to the user, another toward the IRS. By rotating the IRS, the phases of the direct and reflected paths can be aligned, achieving effective multi‑path alignment. The average‑rate maximization problem is reformulated as an expected channel‑gain maximization and split into two independent sub‑problems. The 6DMA placement/rotation is solved by a differential evolution (DE) algorithm, while the IRS rotation and phase‑shift design are tackled via semi‑definite relaxation (SDR).

In the general multi‑user case, the problem becomes non‑convex due to coupled beamforming, IRS coefficients, and antenna positions. The authors first apply the weighted minimum mean‑square error (WMMSE) method to obtain the short‑timescale beamformers. The long‑timescale variables are then optimized through a two‑layer approach: the inner layer updates the IRS reflection coefficients using stochastic successive convex approximation (SSCA), and the outer layer optimizes the 6DMA positions/rotations and IRS rotation angle via an extended DE algorithm. To further reduce complexity, a low‑complexity scheme is proposed that directly maximizes the average sum‑channel gain, bypassing the iterative SSCA steps.

Extensive numerical results validate the proposed designs. In the single‑user case, IRS rotation provides an additional DoF that improves the reflected‑path gain, while the sparse 6DMA configuration enables constructive interference between direct and reflected links, yielding up to a 30 % sum‑rate gain over conventional fixed‑array/F‑IRS setups. In multi‑user simulations, the joint 6DMA‑R‑IRS system mitigates inter‑user interference more effectively than IRS‑only or MA‑only baselines, achieving a 20–25 % increase in average sum‑rate. The low‑complexity algorithm reduces computational load by more than 40 % with negligible performance loss.

Overall, the work demonstrates that integrating rotatable IRSs with six‑dimensional movable antennas, together with a two‑timescale optimization framework, can substantially enhance spectral efficiency while keeping channel estimation and hardware reconfiguration overhead manageable. The paper opens avenues for future research on multi‑dimensional movable/rotatable arrays, experimental prototyping, and joint design with other emerging technologies such as cell‑free massive MIMO and terahertz communications.