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Investigation of RIS Integration in Satellite Systems Guided by ITU-R VSAT Radiation Diagram

Symeon Vaidanis, Athanasios D. Panagopoulos

arXiv:2610.16699v1eess.SPcs.ITmath.IT

TL;DR

The paper addresses the limited attention to RIS-assisted satellite-VSAT downlink design, particularly antenna radiation patterns and topology. It proposes optimizing RIS position and orientation and reports that RIS-primary placement can improve the link power margin under near-field and pointing conditions.

  • Problem

    The introduction identifies a satellite-VSAT downlink gap: prior RIS orientation work did not account for RIS topology or ground-terminal radiation patterns.

    Source: §I · p. 2

  • Method

    The paper optimizes RIS position and orientation using antenna patterns and system topology, and refines the channel model for tropospheric attenuation and polarization.

    Source: §I · pp. 2–3, B. Coordination systems · p. 6, C. Path loss modeling · p. 8

  • Results

    RIS-primary topology yields greater enhancement than RIS-assisted topology, while the conclusion conditions sufficient power-margin improvement on optimum RIS positioning, near-field placement, and VSAT pointing toward the RIS.

    Source: §IV · p. 15, CONCLUSION · p. 15

  • Takeaways & Limitations

    A notable power-margin improvement requires RIS size on the order of the VSAT diameter, and fixed RIS element counts yield less improvement as frequency increases.

    Source: CONCLUSION · p. 15

    The study leaves interference from nearby RIS–VSAT pairs linked to different satellites as a future research topic.

    Source: CONCLUSION · p. 16

Abstract

from arXiv · show

The introduction of Reconfigurable Intelligent Surfaces (RIS) technology marks a transformative era in modern communication systems, enhancing coverage and facilitating higher data transmission rates. While recent research has delved into the integration of RIS into satellite networks, a critical and practical gap persists concerning its implementation in the satellite-Very Small Aperture Terminal (VSAT) downlink, specifically amplifying contributions from the VSAT side. This paper addresses this gap by proposing a simple and engineering methodology to optimize RIS position and orientation, incorporating considerations for the antennas' radiation patterns and the system's topology to streamline optimization variables. The primary objective of this paper is to alleviate the double fading constraint in the reflect link without the need for active elements, achieved by leveraging the Fresnel zone of the RIS. The improvement power ratio depends on the frequency and the VSAT 's diameter. The results show that for a VSAT with diameter 1m the gain is up to 9dB at 2 GHz (S band) and up to 14dB at 4 GHz (C band). Additionally, the RIS channel equation is refined, encompassing the tropospheric attenuation and accounting for electromagnetic wave polarization.

I. INTRODUCTION

Prior satellite-RIS work spans multiple network settings, but the cited orientation-and-beamforming study omits RIS topology and ground-terminal radiation patterns. This paper addresses the VSAT downlink by optimizing RIS placement and orientation, examining beam pointing, and refining the channel model.

I. INTRODUCTION

  • The paper targets satellite-VSAT downlink design, where prior satellite RIS orientation work ignored RIS topology and ground-terminal radiation patterns.The introduction also surveys RIS uses in satellite networks, including LEO, THz, NOMA, and interference-mitigation settings.Source: §I · p. 2
  • RIS placement and orientation are optimized in relation to the ground-terminal VSAT, with both VSAT and RIS-element radiation patterns shaping achievable improvement and its maximizing topology.The paper compares directing the VSAT beam toward the satellite versus toward the RIS.Source: §I · pp. 2–3
  • The proposed RIS-primary arrangement directs the VSAT toward the RIS and uses the RIS Fresnel zone to address reflective-link double fading without active elements.The paper also refines the RIS channel equation to include tropospheric attenuation and electromagnetic-wave polarization, guided by ITU-R.Source: §I · p. 3

A. Antennas theory preliminaries and radiation zones

This section establishes the antenna-pattern and radiation-zone assumptions used to select channel models and describe the VSAT–RIS system geometry.

A. Antennas theory preliminaries and radiation zones

  • The VSAT directional-gain mask follows ITU-R guidance for small fixed-satellite ground terminals across 2–31 GHz and is cylindrically symmetric about its beam focus.The section also introduces radiation diagrams for VSAT diameters of 1 m and 2 m and for a RIS element.Source: A. Antennas theory preliminaries and… · p. 3, Figure 1 · p. 4, Figure 2 · p. 4
  • The parabolic-antenna far-field distance depends on antenna diameter D and wavelength λ, while the RIS patch pattern is frequency-independent and depends on patch size.The RIS element pattern is defined only for the half space; the section also gives far-field and radiating-near-field formulas for antenna arrays.Source: A. Antennas theory preliminaries and… · pp. 3–4
  • Channel-model selection depends on system topology and engineering restrictions, with far-field approximation or analytical near-field equations used according to the radiation-zone bounds.The rectangular-RIS case further relates the far-field bounds to VSAT and RIS dimensions under stated equal-size and element-spacing conditions.Source: A. Antennas theory preliminaries and… · p. 5

B. Coordination systems

The system topology is represented with a global reference frame and separate local frames for the VSAT and RIS, which encode position and antenna orientation.

B. Coordination systems

  • A Global Coordination System (GCS) serves as the reference for the system topology.The VSAT and RIS each have their own Local Coordination System (LCS).Source: B. Coordination systems · p. 6
  • Each local frame is described by its center position vector relative to the GCS and a rotation vector for the antenna radiation-pattern orientation.The rotation vector contains bearing, down-tilt, and slant angles.Source: B. Coordination systems · p. 6
  • Bearing α, down tilt β, and slant γ are anti-clockwise rotations about the z, y, and x axes, respectively, under the right-hand rule.Source: B. Coordination systems · p. 6

C. Path loss modeling

The channel model treats RIS links as a superposition of per-element transmitter–receiver links, with separate reflected and direct paths. It accounts for path loss, tropospheric attenuation, directional gains, and polarization alignment.

C. Path loss modeling

  • The cited RIS channel model is formulated for distances beyond the RIS Fresnel-distance bound, with a simpler approximation also described.The paper's channel coefficients are presented for a beamforming case involving one VSAT.Source: C. Path loss modeling · pp. 6–7
  • The model represents the beamforming channel as per-element RIS links for one VSAT, distinguishing reflected and direct paths.The multiple-user perspective is left for future work.Source: C. Path loss modeling · p. 7
  • Each RIS element's contribution includes directional gains along the satellite–RIS–VSAT path and polarization alignment between the communicating antennas.The model also includes the VSAT-to-satellite directional gain for the direct path.Source: C. Path loss modeling · pp. 7–8
  • The channel formulation includes free-space path loss and tropospheric attenuation for the reflected and direct paths.It treats small-scale propagation fading as handled by perfect CSI; space-time variations in path loss and interference are future work.Source: C. Path loss modeling · p. 8

III. TOPOLOGY AND OPTIMIZATION OF RIS POSITION AND ORIENTATION

The paper compares RIS-assisted and RIS-primary topologies and optimizes RIS position and orientation relative to the VSAT. It reduces the geometric search and solves the resulting nonconvex optimization numerically using successive convex approximation.

III. TOPOLOGY AND OPTIMIZATION OF RIS POSITION AND ORIENTATION

  • RIS-primary is proposed after the RIS-assisted topology is found not to provide sufficient improvement; it uses coordinated RIS and VSAT directivity with looser angular restrictions.In RIS-assisted operation, the VSAT remains satellite-pointed and RIS adds a reflection path; in RIS-primary operation, it points toward the RIS and transmission uses only the reflected path.Source: §III · pp. 8–9
  • The optimization compares RIS-assisted and RIS-primary received power against a satellite-pointed, RIS-absent topology.The improvement factor is expressed as a percentage relative to that baseline.Source: §III · p. 9
  • In RIS-primary, the VSAT position stays fixed while its orientation focuses on the RIS center, and RIS position and orientation are optimization variables.The RIS beam points along its local x-axis.Source: §III · p. 10
  • Cylindrical symmetry reduces the search from three dimensions to a plane, and the RIS and VSAT directional axes are aligned through their orientation parameters.The stated variables are βris, γris, βvsat, and γvsat; the γ rotations account for polarization.Source: §III · pp. 10–11
  • Because the optimization is nonconvex, the paper solves it numerically with successive convex approximation and a Newton-based surrogate.The surrogate uses a positive-definite identity matrix choice for H, and the diminishing step size is stated to yield stationary limit points.Source: §III · pp. 11–12

IV. SIMULATION RESULTS

The simulations compare RIS-assisted and RIS-primary configurations across two VSAT diameters and downlink bands, with the VSAT placed in the RIS radiating near field. RIS-primary produces greater enhancements, while frequency trends and required RIS size vary with topology and VSAT diameter.

IV. SIMULATION RESULTS

  • The simulations use 1 m and 2 m VSATs at 2 GHz and 4 GHz, with passive RIS elements and a square RIS sized to keep the VSAT in its radiating near field.The scenario is a hypothetical GEO system; improvement is evaluated against VSAT diameter, carrier frequency, and RIS elements per row.Source: §IV · pp. 12, 14, 15
  • RIS-primary shows greater enhancement than RIS-assisted across the simulated cases, with the latter receiving the RIS signal through lower VSAT side lobes.The RIS-primary setup uses VSAT directivity to address dual fading when the VSAT is in the RIS radiating near field.Source: Figure 7 · p. 13, Figure 8 · p. 13, Figure 9 · p. 14
  • As frequency rises at fixed VSAT diameter, improvement decreases for RIS-assisted and increases for RIS-primary.This trend is reported alongside comparisons in Figs. 7–10.Source: §IV · p. 15
  • For a 1 m VSAT in the RIS-primary case, 3 dB improvement requires 25 elements per dimension at 2 GHz, 49 at 4 GHz, and 97 at 8 GHz.Each case corresponds to an RIS length of approximately 1.85 m.Source: §IV · p. 15

CONCLUSION

The conclusion states that RIS can improve FSS power margin when optimally positioned, with the VSAT in the RIS near field and its antenna focused on the RIS. It also identifies RIS size and nearby-system interference as practical considerations.

CONCLUSION

  • RIS can provide sufficient FSS power-margin improvement when optimally positioned, with the VSAT in the RIS near field and its antenna focused on the RIS.The RIS size should be of the same order of magnitude as the VSAT diameter for notable improvement, such as 3 dB.Source: CONCLUSION · pp. 15–16
  • With a fixed number of RIS elements, improvement decreases as frequency increases.The conclusion also states that the RIS must be on the order of the VSAT diameter for notable power-margin improvement.Source: CONCLUSION · pp. 15–16
  • Interference from nearby VSAT–RIS pairs, potentially linked to different satellites, is identified as a future research topic.The paper does not report an interference analysis in this conclusion.Source: CONCLUSION · p. 16

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