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Movable-Element STAR-RIS for 6G Non-Terrestrial Networks: Architectures, Enabling Technologies, and Open Challenges

Wali Ullah Khan, Muhammad Adil

arXiv:2610.00143v1eess.SP

TL;DR

NTNs face changing geometry, propagation loss, Doppler, blockage, and payload-energy constraints, while prior movable-element STAR research is predominantly terrestrial. This article develops ME-STAR-RIS for NTN settings and compares it with fixed STAR-RIS, finding a consistent WSR advantage in the considered power range.

  • Problem

    Prior movable-element STAR and movable-antenna/STAR studies are predominantly terrestrial, leaving open how to combine full-space control and local geometry reconfiguration under NTN mobility, Doppler, multi-tier deployment, and actuation constraints.

    Source: §I · p. 1

  • Method

    The article presents ME-STAR-RIS, combining STAR transmission/reflection control with element-position reconfiguration, and discusses NTN architectures and enabling mechanisms.

    Source: §I · p. 1

  • Results

    At 50 dBm, ME-STAR-RIS achieves 1.943 bit/s/Hz versus 1.491 bit/s/Hz for fixed STAR-RIS, a 30.3% improvement, and it outperforms the fixed scheme across the considered transmit-power range.

    Source: B. Results and Design Insights · pp. 6–7

  • Takeaways & Limitations

    Element mobility provides an additional geometry-control degree of freedom, with gains that diminish as movement range grows and relative benefit that falls as surface size increases.

    Source: B. Results and Design Insights · p. 7, §VII · p. 8

Abstract

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Non-terrestrial networks (NTNs) are emerging as a key component of 6G systems by extending ubiquitous and resilient connectivity across satellite, aerial, and terrestrial layers. However, severe propagation loss, rapidly varying geometry, Doppler, blockage, and stringent payload and energy constraints make efficient beam and propagation control particularly challenging. Movable-element STAR-RIS (ME-STAR-RIS) offers a new approach by combining full-space transmission/reflection control with reconfigurable surface geometry, thereby introducing an additional spatial degree of freedom for geometry-aware NTN operation. In this article, we first describe the operating principle, movement constraints, and potential hardware realizations of ME-STAR-RIS, and then present representative satellite-borne, aerial-relay, and cross-tier NTN architectures. We discuss the key enabling mechanisms, including joint geometry--electromagnetic optimization, mobility-aware channel acquisition, multi-timescale control, and predictive configuration. A LEO-centered numerical case study compares ME-STAR-RIS with an otherwise identical fixed STAR-RIS and demonstrates the benefit of element mobility across different transmit powers, movement ranges, and surface sizes, while revealing diminishing returns from increasingly large displacements. Finally, we discuss the main hardware, CSI, scalability, near-field, sensing, security, reliability, and standardization challenges, and highlight promising directions toward practical ME-STAR-RIS-enabled NTNs

I. INTRODUCTION

NTNs combine rapidly changing satellite, aerial, and terrestrial links with propagation and payload constraints, while existing STAR-RIS and movable-surface research leaves their combination under NTN conditions unresolved. The article introduces ME-STAR-RIS, representative NTN architectures, and a LEO-centered comparison with fixed STAR-RIS.

I. INTRODUCTION

  • NTNs face propagation loss, Doppler, blockage, handover, synchronization, and payload-energy challenges amid rapidly moving orbital and aerial nodes.Their heterogeneous links span satellite, aerial, and terrestrial infrastructure.Source: §I · p. 1
  • Existing studies largely leave open how to combine full-space STAR control and local aperture-geometry reconfiguration under orbital mobility, Doppler, multi-tier deployment, and actuation constraints.Prior movable-element STAR and movable-antenna/STAR work is predominantly terrestrial, while satellite-RIS and aerial STAR studies assume fixed element locations.Source: §I · p. 1
  • ME-STAR-RIS combines STAR transmission/reflection control with physical element-position changes, adding aperture geometry as a design degree of freedom.The article uses ME-STAR-RIS for this concept; recent literature also uses ME-STARS.Source: §I · p. 1, Figure 1 · p. 2
  • The article presents satellite, aerial, and cross-tier NTN architectures and identifies position-dependent channel acquisition, Doppler-aware control, and multi-timescale optimization as enabling mechanisms.It also examines practical movement tradeoffs and compares ME-STAR-RIS with an otherwise identical fixed STAR-RIS in a LEO-centered numerical case study.Source: §I · pp. 1–2

II. FROM STATIC STAR-RIS TO GEOMETRY-RECONFIGURABLE SURFACES

ME-STAR-RIS combines the full-space electromagnetic control of STAR-RIS with movable element positions. Its channel and array response depend on both control layers, while feasible movement is constrained by spacing and practical hardware considerations.

A. Evolution of the Programmable Aperture

  • ME-STAR-RIS combines position control with STAR transmission/reflection control, unlike fixed-geometry STAR-RIS or reflection-oriented ME-RIS.Its design space is jointly geometric and electromagnetic rather than electromagnetic alone.Source: A. Evolution of the Programmable… · p. 2

B. Operating Principle and Movement Constraints

  • Elements move within feasible local regions while maintaining minimum spacing, and their positions change phase progression, spatial correlation, and effective array response.Movement regions may be tracks, two-dimensional cells, or group-wise displacement regions.Source: B. Operating Principle and Movement… · p. 2
  • The cascaded channel depends on both element locations and STAR states, with electronic STAR control operating on the geometry-dependent channel.In energy-splitting operation, elements divide incident power between transmission and reflection, though practical hardware can couple their phases and amplitudes.Source: B. Operating Principle and Movement… · p. 2
  • Payload, structural, thermal, vibration, actuator, calibration, and wiring constraints make limited-stroke or group-wise motion more realistic than frequent arbitrary movement, particularly in space.Source: B. Operating Principle and Movement… · pp. 2–3

C. Candidate Hardware Realizations

The article treats hardware implementations as candidates and describes satellite-borne ME-STAR-RIS as a slow geometry-reconfiguration mechanism paired with faster electronic STAR updates.

C. Candidate Hardware Realizations

  • Candidate implementations include motorized positioning, mechanical tracks, MEMS-scale actuation, and electronically reconfigurable structures that emulate spatial repositioning.For spacecraft, limited-stroke, group-wise, or quasi-static movement may be more realistic than frequent per-element motion; these remain research candidates, not established space-qualified solutions.Source: C. Candidate Hardware Realizations · p. 3

III. ME-STAR-RIS-EMPOWERED NTN ARCHITECTURES

  • The representative NTN architectures span satellite-borne, aerial-relay, and cross-tier integrated deployments.They differ in roles, movement benefits, control hierarchies, coupled design variables, and practical limitations.Source: §III · p. 3, Figure 2 · p. 3

A. Satellite-Borne ME-STAR-RIS

  • A satellite-borne surface can be part of the beamforming front end, using STAR coefficients for fast adaptation and limited element repositioning to reshape the aperture more slowly.Frequent satellite movement risks energy use, vibration, thermal-design difficulty, and reduced reliability, making hardware realism the principal limitation.Source: A. Satellite-Borne ME-STAR-RIS · p. 3

B. Aerial-Relay ME-STAR-RIS

Aerial-relay and cross-tier deployments place local element movement alongside platform or network-tier mobility. Their control and optimization must account for different timescales, energy costs, and coupled design variables.

B. Aerial-Relay ME-STAR-RIS

  • On a UAV or HAPS, platform movement changes global position while element movement refines local aperture response and STAR coefficients manage transmitted and reflected fields.The platform can move toward a favorable macro-location before element positioning refines the response.Source: B. Aerial-Relay ME-STAR-RIS · p. 4
  • Aerial-relay applications include temporary coverage, disaster recovery, hotspot offloading, and blockage bypass, but continuous movement of both platform and elements may waste energy.Platform trajectory can respond to large-scale geometry while element movement addresses residual channel structure, interference, or user clustering.Source: B. Aerial-Relay ME-STAR-RIS · p. 4

C. Cross-Tier Integrated ME-STAR-RIS

  • Cross-tier surfaces at gateways, facades, BSs, HAPS, or relay nodes can support alternate paths around blockage, cell-edge users, handover, or aperture sharing across NTN and terrestrial services.Source: C. Cross-Tier Integrated ME-STAR-RIS · p. 4
  • Cross-tier multi-connectivity can assist satellite and terrestrial paths simultaneously, but optimization spans association, scheduling, terrestrial beamforming, STAR coefficients, and element locations.The coupled design variables make scalable decomposition essential.Source: C. Cross-Tier Integrated ME-STAR-RIS · p. 4

IV. KEY ENABLING MECHANISMS

ME-STAR-RIS design jointly optimizes geometry-dependent channels and electromagnetic controls, while NTN latency motivates hierarchical rather than monolithic optimization. Mobility also requires channel models that cover candidate element positions and evolve with satellite motion.

A. Joint Geometry–Electromagnetic Optimization

  • Element positions change the channel that STAR coefficients act on, so the design jointly considers geometry, beamforming, power, association, and transmission/reflection coefficients.Terrestrial approaches include alternating optimization, gradient-based updates, successive convex approximation, and heuristic methods.Source: A. Joint Geometry–Electromagnetic… · p. 4
  • A hierarchical strategy assigns satellite selection, operating mode, and geometry to slow updates, while short-term channel information supports faster phase, power, and beamforming updates.Fig. 3 also places element actuation at an intermediate timescale and electronic control at a fast timescale.Source: A. Joint Geometry–Electromagnetic… · p. 4, Figure 3 · p. 4

B. Mobility- and Doppler-Aware Channel Acquisition

  • Channel acquisition must map the allowed movement regions because a channel estimated at only one surface geometry is insufficient after elements move.The proposed ingredients include field-response modeling, compressed measurements, geometric information, and statistical CSI.Source: B. Mobility- and Doppler-Aware Channel… · p. 4
  • LEO motion makes the geometry-aware channel model time-indexed, with ephemerides predicting the dominant line-of-sight direction and pilots or sensing refining local multipath.This motivates predictive rather than purely reactive control.Source: B. Mobility- and Doppler-Aware Channel… · p. 4

C. Multi-Timescale Control

The proposed control framework separates slow geometry decisions from faster electronic adaptation and uses predictable satellite motion to inform configuration. The numerical study isolates the added geometry freedom by comparing ME-STAR-RIS with an otherwise identical fixed STAR-RIS.

C. Multi-Timescale Control

  • Element positions update at an intermediate timescale only when expected gain exceeds actuation cost, while fast STAR coefficients and beamforming track short-term channel variation without mechanical movement.Slow planning uses orbital geometry, service demand, and long-term channel statistics.Source: C. Multi-Timescale Control · p. 5

D. Predictive and Learning-Assisted Configuration

  • Predictable satellite geometry can support repositioning before link quality deteriorates, while learning can reduce online search through movement-benefit decisions, solver warm starts, or geometry-state selection.A digital twin can evaluate candidate movements against expected gain, mechanical wear, or energy expenditure.Source: D. Predictive and Learning-Assisted… · p. 5

V. NUMERICAL CASE STUDY

  • The LEO-centered case study compares ME-STAR-RIS with an otherwise identical fixed STAR-RIS, making element repositioning their only additional design freedom.Both schemes share the link budget, user locations, propagation realizations, STAR protocol, and electronic control.Source: §V · p. 5

A. System, Channel, and Optimization Setup

The case study benchmarks element mobility under a controlled LEO gateway-mounted setup, keeping the satellite beam and electronic STAR capability aligned across the movable and fixed surfaces. Its channel and optimization assumptions isolate geometry reconfiguration as the source of any performance difference.

A. System, Channel, and Optimization Setup

  • The study models a 550 km LEO satellite serving two users through a gateway-mounted STAR surface, with a 2 GHz carrier, 1 MHz bandwidth, and 200 m surface-to-user distances.One user is on each side of the surface; Fig. 4(a) depicts the gateway-mounted full-space geometry.Source: A. System, Channel, and Optimization… · p. 5, Figure 4 · p. 6
  • The satellite-to-surface link is far-field LoS, while surface-to-user links use a position-dependent Rician field-response model with a 2 dB factor and four diffuse components.Doppler is assumed compensated at the satellite/receiver level, focusing the experiment on geometry reconfiguration.Source: A. System, Channel, and Optimization… · p. 5
  • Direct satellite-to-user links are omitted to represent shadowed or locally blocked service and isolate surface-assisted geometry gains, which are not universal end-to-end gains for every configuration.The omission also avoids direct/cascaded coherent-combining effects.Source: A. System, Channel, and Optimization… · p. 5
  • The default surface has 64 elements on an 8 × 8 grid, with one-dimensional local tracks and movement-cell centers separated by 1.5λ.For D ≤ λ, adjacent elements in a row remain at least 0.5λ apart; the fixed surface uses the same cell centers and aperture.Source: A. System, Channel, and Optimization… · p. 5
  • Both schemes use the same fixed satellite beam and ideal independent-phase STAR energy splitting, while ME-STAR-RIS additionally optimizes element positions using perfect cascaded-channel knowledge.A sequential coordinate search evaluates 11 candidate locations per element, accepts WSR-increasing positions, and performs two sweeps; evaluation count scales linearly with movable elements for fixed candidates and sweeps.Source: A. System, Channel, and Optimization… · pp. 5–6

B. Results and Design Insights

Across the tested power, movement-range, and surface-size settings, ME-STAR-RIS outperforms the fixed STAR-RIS. The relative advantage is strongest at lower-to-moderate power, grows with movement range but with diminishing returns, and declines as the fixed aperture grows.

B. Results and Design Insights

  • At 50 dBm, equal-weight WSR is 1.491 bit/s/Hz for fixed STAR-RIS and 1.943 bit/s/Hz for ME-STAR-RIS, a 30.3% improvement.ME-STAR-RIS remains ahead across the considered transmit-power range; the relative gain is 52.1% at 40 dBm and 14.5% at 60 dBm.Source: B. Results and Design Insights · pp. 6–7
  • At D = 0, both schemes achieve approximately 1.491 bit/s/Hz; the mobility gain increases from 14.7% at D = 0.25λ to 34.7% at D = λ.The reported gain is 30.3% at D = 0.75λ, while increasing track length yields diminishing returns.Source: B. Results and Design Insights · p. 7
  • With 144 elements, WSR rises from 3.330 bit/s/Hz for fixed STAR-RIS to 3.927 bit/s/Hz for ME-STAR-RIS, an absolute improvement of approximately 0.597 bit/s/Hz.The relative mobility gain decreases as surface size grows, although the absolute rate difference remains significant.Source: B. Results and Design Insights · p. 7
  • Element mobility is especially valuable when power, aperture size, or deployment flexibility is constrained, rather than serving simply as a substitute for more transmit power or surface elements.A larger fixed aperture reduces the relative importance of the added geometry degree of freedom.Source: B. Results and Design Insights · p. 7

VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS

Practical ME-STAR-RIS deployments must address hardware, channel-acquisition, control, and reliability issues. Geometry reconfiguration also creates opportunities in near-field operation, sensing, security, and cooperative networking.

VI. CHALLENGES AND FUTURE RESEARCH DIRECTIONS

  • Practical deployment requires resolving hardware, channel-acquisition, control, and reliability issues.The section frames these as remaining challenges to practical ME-STAR-RIS-assisted NTNs.Source: §VI · p. 7
  • Geometry reconfiguration creates opportunities for near-field operation, sensing, and security.The section also identifies cooperative multi-platform networking as a research direction.Source: §VI · p. 7

A. Hardware and Electromagnetic Constraints

Physical movement adds energy, latency, wear, and calibration costs, while changing electromagnetic behavior. High mobility and coupled design variables also make channel acquisition and scalable control challenging.

A. Hardware and Electromagnetic Constraints

  • Movement adds actuation energy, repositioning latency, mechanical wear, and calibration requirements; satellite motion may also induce micro-vibration and interact with thermal and attitude-control constraints.Limited-stroke, group-wise, or event-triggered movement is suggested instead of continuous repositioning.Source: A. Hardware and Electromagnetic… · p. 7
  • Element movement changes spacing, coupling, radiation, and transmission/reflection response, so models should jointly represent geometry, orientation, hardware imperfections, and position uncertainty.The authors call for hardware prototypes and space-relevant measurements to establish retained geometry gain.Source: A. Hardware and Electromagnetic… · p. 7

B. CSI and Scalable Control

  • Estimating channels for every possible element configuration is impractical in rapidly varying NTN environments; statistical and geometric information, compressed measurements, maps, sensing, and codebooks offer alternatives.Group-wise movement and geometry codebooks can reduce signaling overhead.Source: B. CSI and Scalable Control · pp. 7–8
  • Adding element coordinates to coupled STAR coefficients, beamforming, power, and scheduling creates a highly nonconvex problem that network-scale NTN control must address through decomposition, distributed optimization, or learning-assisted initialization.Prediction and learning may help decide when geometry reconfiguration is worthwhile rather than replace physical models.Source: B. CSI and Scalable Control · p. 8

C. Near-Field, Sensing, and Security

The conclusion reports that movable-element operation outperforms an otherwise identical fixed STAR-RIS, with diminishing returns from larger movement ranges and relatively smaller benefits for larger apertures. The section also outlines near-field, sensing, security, cooperation, reliability, and standardization directions.

C. Near-Field, Sensing, and Security

  • Electrically large deployments may operate partly in the near field, where element positions affect both angle and range focusing and could support user separation, localized energy delivery, and adaptive focusing.These capabilities also bring calibration and channel-estimation requirements.Source: C. Near-Field, Sensing, and Security · p. 8
  • In NTNs, element movement could reshape sensing beampatterns, improve localization geometry, suppress clutter, or create spatial nulls toward eavesdroppers and jammers.Because the geometry-control link is a potential attack surface, the authors motivate authenticated commands, robust feedback, and secure fallback configurations.Source: C. Near-Field, Sensing, and Security · p. 8

D. Networks Cooperation, Reliability, and Standardization

  • Multiple satellites, HAPS, gateways, and STAR surfaces could coordinate geometry and electronic states, with only nodes offering sufficient system-level benefit physically reconfigured.Sparse movement could retain much of the geometry gain while reducing actuation energy and control overhead.Source: D. Networks Cooperation, Reliability,… · p. 8
  • Reliability planning should accommodate stuck elements, actuator saturation, drift, calibration loss, and control-link failures, with affected elements falling back to fixed STAR-RIS operation.Digital twins could track actuator health and thermal state; deployment also needs standard signaling, calibration, failure-reporting, and NTN coordination mechanisms.Source: D. Networks Cooperation, Reliability,… · p. 8

VII. CONCLUSION

  • Movable-element operation consistently outperforms an otherwise identical fixed STAR-RIS, but its benefit has diminishing returns as movement range grows and is relatively smaller for larger apertures.The conclusion favors selective, slower or event-triggered repositioning while electronic STAR reconfiguration handles faster channel variations.Source: §VII · p. 8
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