Grouped Annulus-Modulated Transceiver Is Almost Full DoF-Achieving for RIS-Assisted Symbiotic Radios Over Spatial-Correlated Channels

2026-05-13

AI Summary

This work addresses the limited degrees of freedom in conventional reconfigurable intelligent surface (RIS)-assisted symbiotic radio systems, which rely solely on single-element phase modulation. To overcome this constraint, the authors propose a structured matrix factorization algorithm that effectively suppresses channel decomposition residuals. Building upon this, they design a grouped annular modulation (GAM) transceiver architecture incorporating hexagonal lattice constellations. This approach represents the first integration of GAM and hexagonal lattice constellations into RIS-assisted symbiotic radio, fully exploiting the spatial multiplexing gain offered by the RIS. Compared to conventional phase-only modulation schemes, the proposed system achieves significantly enhanced spectral efficiency and data rates while maintaining comparable bit error rate performance, thereby approaching the theoretical full-degrees-of-freedom limit.

Abstract

This paper considers a RIS-assisted symbiotic communication system, where additional information is conveyed by the passive reconfigurable intelligent surface (RIS). In existing schemes, individual phase modulation is usually adopted at the RIS elements, which severely limits exploiting all extra multiplexing gains brought by the RIS. To address the issue, we propose a novel matrix decomposition algorithm that transforms the equivalent channel into a structured form while effectively suppressing the decomposition residual. Based on this, a novel transceiver architecture employing grouped annulus modulation (GAM) with a hexagonal-lattice-based constellation is developed, which is capable of achieving the full degrees of freedom (DoFs) when the decomposition algorithm performs as expected. Numerical results demonstrate that the proposed transceiver achieves much higher communication rates, thereby leading to higher spectral efficiency, compared to the conventional phase-only modulation scheme, while maintaining comparable error performance.

Problem

• RIS-assisted symbiotic radios
• spatially correlated channels
• degrees of freedom
• multiplexing gains
• phase modulation

Innovation

• Grouped Annulus Modulation
• Reconfigurable Intelligent Surface
• Full Degrees of Freedom
• Matrix Decomposition
• Symbiotic Radio