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Beyond Diagonal RIS for 6G Non-Terrestrial Networks: Potentials and Challenges

Wali Ullah Khan, Asad Mahmood, Muhammad Ali Jamshed, Eva Lagunas, Manzoor Ahmed, Symeon Chatzinotas

TL;DR

BD-RIS extends traditional RIS by incorporating interconnected phase response elements, enabling unitary or non-diagonal reflection matrices for enhanced controllability in non-terrestrial networks. The paper surveys BD-RIS fundamentals, hardware architectures (single-, fully-, and group-connected) and operating modes (reflective, hybrid, multi-sector), then links them to NTN-specific channel modeling and applications. A case study on BD-RIS aided NOMA in a LEO downlink demonstrates significant spectral-efficiency gains over CD-RIS, validating the potential of BD-RIS to improve NTN coverage and reliability. The work also identifies key challenges—adaptive channel realization, RF circuitry constraints, and receiver sensitivity—and prescribes research directions in AI/ML, physical-layer security, and joint sensing and communications to unlock practical BD-RIS deployments in 6G NTNs.

Abstract

Reconfigurable intelligent surface (RIS) has emerged as a promising technology in both terrestrial and non-terrestrial networks (NTNs) due to its ability to manipulate wireless environments for better connectivity. Significant studies have been focused on conventional RIS with diagonal phase response matrices. This simple RIS architecture, though less expensive, has limited flexibility in engineering the wireless channels. As the latest member of RIS technology, beyond diagonal RIS (BD-RIS) has recently been proposed in terrestrial setups. Due to the interconnected phase response elements (PREs), BD-RIS significantly enhances the control over the wireless environment. This work proposes the potential and challenges of BD-RIS in NTNs. We begin with the motivation and recent advances in BD-RIS. Subsequently, we discuss the fundamentals of BD-RIS and NTNs. We then outline the application of BD-RIS in NTNs, followed by a case study on BD-RIS enabled non-orthogonal multiple access low earth orbit satellite communication. Finally, we highlight challenges and research directions with concluding remarks.

Beyond Diagonal RIS for 6G Non-Terrestrial Networks: Potentials and Challenges

TL;DR

BD-RIS extends traditional RIS by incorporating interconnected phase response elements, enabling unitary or non-diagonal reflection matrices for enhanced controllability in non-terrestrial networks. The paper surveys BD-RIS fundamentals, hardware architectures (single-, fully-, and group-connected) and operating modes (reflective, hybrid, multi-sector), then links them to NTN-specific channel modeling and applications. A case study on BD-RIS aided NOMA in a LEO downlink demonstrates significant spectral-efficiency gains over CD-RIS, validating the potential of BD-RIS to improve NTN coverage and reliability. The work also identifies key challenges—adaptive channel realization, RF circuitry constraints, and receiver sensitivity—and prescribes research directions in AI/ML, physical-layer security, and joint sensing and communications to unlock practical BD-RIS deployments in 6G NTNs.

Abstract

Reconfigurable intelligent surface (RIS) has emerged as a promising technology in both terrestrial and non-terrestrial networks (NTNs) due to its ability to manipulate wireless environments for better connectivity. Significant studies have been focused on conventional RIS with diagonal phase response matrices. This simple RIS architecture, though less expensive, has limited flexibility in engineering the wireless channels. As the latest member of RIS technology, beyond diagonal RIS (BD-RIS) has recently been proposed in terrestrial setups. Due to the interconnected phase response elements (PREs), BD-RIS significantly enhances the control over the wireless environment. This work proposes the potential and challenges of BD-RIS in NTNs. We begin with the motivation and recent advances in BD-RIS. Subsequently, we discuss the fundamentals of BD-RIS and NTNs. We then outline the application of BD-RIS in NTNs, followed by a case study on BD-RIS enabled non-orthogonal multiple access low earth orbit satellite communication. Finally, we highlight challenges and research directions with concluding remarks.
Paper Structure (37 sections, 4 figures, 2 tables)

This paper contains 37 sections, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Related Works
  3. Non-Terrestrial Networks
  4. Atmospheric Layer
  5. Space Layer
  6. Channel Modeling in NTNs and Integration with BD-RIS
  7. Preliminaries of BD-RIS
  8. Hardware Architectures of BD-RIS
  9. Single-connected BD-RIS
  10. Fully-connected BD-RIS
  11. Group-connected BD-RIS
  12. Operating Modes of BD-RIS in NTNs
  13. Reflective BD-RIS
  14. Hybrid BD-RIS
  15. Multi-Sector BD-RIS
  16. ...and 22 more sections

Figures (4)

  • Figure 1: BD-RIS enabled 6G NTNs which consist of three layers, i.e., space layer, atmospheric layer, and earth surface.
  • Figure 2: Different architecture of BD-RIS: (a) Cell-wise single-connected BD-RIS architecture with 2 cells; (b) Cell-wise fully-connected BD-RIS architecture with 4 cells and 2 groups; (c) Element-wise group-connected BD-RIS with 2 groups.
  • Figure 3: Different modes BD-RIS supported by the same circuitry of reconfigurable impedance network: (a) Reflective BD-RIS, (b) Element wise reflective BD-RIS with group-connected architecture with 2 groups, where the group size is 4, (c) Transmissive BD-RIS, (d) Hybrid BD-RIS; (e) Cell wise group-connected BD-RIS architecture with 2 groups and 4 cells, where the size of each cell is 2; (f) Multi-sector BD-RIS; and (g) Cell wise single-connected multi-sector BD-RIS architecture with 3 sectors and 2 cells, where the cell size is 3.
  • Figure 4: (a) System model of NOMA BD-RIS aided LEO communications, (b) Achievable spectral efficiency of the system versus varying transmit power of LEO, (c) Achievable spectral efficiency of the system versus varying phase response elements.