A rotatable antenna can steer its radiation pattern toward a stronger direction, but physical rotation changes more than pointing. It also changes the antenna’s local polarization basis. A direction that improves cascade gain can therefore introduce polarization mismatch and give back part of the benefit.
A new public preprint studies this coupling in a symbiotic radio system assisted by a dual-polarized reconfigurable intelligent surface (RIS). Its central design principle is to optimize spatial direction and polarization together.
Rotation helps the cascade but perturbs polarization
RIS-assisted links can suffer double fading because a signal traverses two propagation segments through the surface. Reorienting an antenna can strengthen directional illumination and reception across this cascade.
The same rotation transforms the local polarization coordinates. If the transmitter, receiver, and dual-polarized RIS are configured as though those coordinates remained fixed, polarization mismatch can offset the directional gain. The optimization must therefore follow both spatial and polarization changes.
One problem couples four control surfaces
The paper minimizes transmit power while jointly choosing digital beamforming, antenna rotations, transmitter and receiver polarization states, and dual-polarized RIS phase shifts. Constraints protect the primary and secondary rates and limit interference temperature for users outside the symbiotic-radio relationship.
The resulting problem is non-convex. The proposed alternating procedure combines semidefinite programming, difference-of-convex programming, and Riemannian conjugate-gradient updates for different variable blocks.
Lower-complexity rotation makes the hardware question explicit
The authors also propose two simplified antenna controls: one shares a rotation across a subarray, and the other restricts rotation to a discrete codebook. The public abstract reports that both retain performance comparable to the full design in the evaluated simulations while reducing rotation complexity. The polarization-aware designs also require less transmit power than fixed-orientation and polarization-unaware benchmarks.
The evidence does not establish motor precision, rotation latency, calibration stability, dual-polarized RIS loss, or control overhead in hardware. Those effects will decide whether the numerical power advantage survives a physical system.
Research notes
Polarization-Aware Rotatable Antennas for RIS-Empowered Symbiotic Radios
Authors: Chao Zhang, Ruizhe Long, Boon-Hee Soong, and Ying-Chang Liang.
Status: Public arXiv record dated 3 September 2026.
What the public evidence establishes: The work jointly optimizes digital beamforming, antenna orientation, polarization states, and dual-polarized RIS phases and proposes lower-complexity shared-rotation and discrete-codebook variants.
Limits: The results are simulation-based and do not establish mechanical, calibration, control, or dual-polarized-surface performance in measured hardware.