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Transmissive RIS-Assisted Vehicular Direct-to-Satellite Communications: Opportunities, Limitations, and Comparison with Phased Arrays
Wali Ullah Khan, Abdullah Abdullah, Juan Andres Vazquez-Peralvo, Eva Lagunas
TL;DR
Vehicular D2S terminals must balance link performance, tracking, power, profile, and implementation complexity, motivating a comparison of transmissive RIS and phased arrays. The study finds that passive RIS suits low-power aperture shaping, active RIS offers partial signal enhancement, and phased arrays retain advantages in high-capability beam control.
Problem
Vehicular D2S terminals must sustain directional links under mobility and changing satellite geometry while meeting strict size, weight, power, thermal, and aerodynamic constraints.
Method
The paper compares transmissive RIS and analog, hybrid, and digital phased-array architectures across gain, power, complexity, scalability, tracking, profile, and prototype maturity.
Results
Passive RIS offers low-power operation and aperture scalability, active RIS partially improves signal strength with added noise and thermal costs, and phased arrays provide stronger beam control and multi-beam capability.
Takeaways & Limitations
RIS is best treated as a low-power aperture or front-end layer in selected regimes, while phased arrays remain preferable for demanding tracking, link performance, and multi-beam operation.
Abstract
from arXiv · showhide
This article studies transmissive reconfigurable intelligent surface (RIS)-assisted architectures. It compares them with electronically steered phased arrays for the deployment of vehicular direct-to-satellite (D2S) communications in future satellite networks. Rather than treating RIS as a direct replacement for phased arrays, we clarify the operating regimes in which RIS can serve as a low-power wavefront-shaping aperture and those in which phased arrays remain preferable because of their high gain and mature beam-tracking capability. Moreover, phased arrays can support multi-beam operation, which is particularly beneficial for dual connectivity and seamless handover. We distinguish analog, digital, and hybrid phased arrays, discuss the relationship between transmissive RIS and reconfigurable transmitarrays, and highlight practical profile, tracking, and link-budget constraints for mobile terminals. The comparison shows that passive RIS offers attractive power efficiency and aperture scalability, active RIS can partially improve the link budget, and phased arrays remain preferable for high-throughput.
I. INTRODUCTION
Vehicular D2S terminals must maintain directional links under mobility and changing satellite geometry while meeting strict SWaP, thermal, and aerodynamic constraints. The paper therefore compares transmissive RIS and phased arrays by their operating regimes and engineering trade-offs.
- Vehicular D2S links must withstand mobility, blockage, and changing satellite geometry within strict size, weight, power, thermal, and aerodynamic constraints.
- Phased arrays remain a reference architecture because they offer high aperture gain, fast electronic steering, mature tracking, and flexible multi-beam operation.Analog, hybrid, and fully digital implementations impose different power and complexity burdens.
- Transmissive RIS shapes outgoing wavefronts with fewer RF chains and is closely related to a reconfigurable transmitarray, but inherits feed, illumination, loss, and profile constraints.
- Passive RIS provides low control power and scalable apertures but cannot amplify incident signals, limiting performance in severe satellite link budgets.Active RIS can amplify the incident signal but introduces local noise and still lacks the full functionality of digital phased arrays.
- The study provides a balanced comparison across gain, power, complexity, scalability, tracking, profile, and prototype maturity to identify suitable RIS operating regimes.
II. BEAMFORMING ARCHITECTURES FOR VEHICULAR DIRECT-TO-SATELLITE COMMUNICATIONS
The paper frames passive RIS, active RIS, and phased arrays as distinct vehicular D2S architecture families. Transmissive RIS can shape or enhance fields with fewer RF chains, but transmitarray-related profile and loss constraints remain important.
- LEO vehicular links emphasize continuous tracking and rapid handover, while MEO and GEO links place greater weight on propagation range and received-power margin.
- Figure 1 contrasts passive transmissive RIS for low-power wavefront shaping, active transmissive RIS for limited aperture amplification, and fully digital phased arrays.
- A transmissive RIS uses programmable unit cells to control incident-field phase, amplitude, or polarization and can form directive or shaped beams with fewer RF chains than a fully active digital array.
- Transmissive RIS, transmitarrays, and profile constraints: Transmissive RIS is closely related to a reconfigurable transmitarray and inherits feed spacing, illumination taper, spillover, insertion loss, and support-blockage constraints.
- Transmissive RIS, transmitarrays, and profile constraints: Large feed-to-aperture distance can make a transmissive RIS terminal deeper than a commercial low-profile phased-array terminal.Compact feeds, folded paths, conformal metasurfaces, or hybrid RIS–array configurations can reduce profile.
- Passive and active RIS implementations: Passive RIS obtains aperture gain through coherent wavefront transformation without RF amplification, whereas active RIS adds amplification or gain control within the aperture.
- Diagonal and beyond-diagonal RIS control: Beyond-diagonal RIS introduces controllable inter-element coupling for joint phase and amplitude control, enabling finer wavefront manipulation and multi-lobe synthesis.
B. Electronically Steered Phased-Array Architectures
Phased arrays span analog, hybrid, and fully digital architectures with increasing RF-chain and processing demands. They provide mature, high-performance beam control, but implementation burden constrains vehicular deployment.
- Electronically steered phased arrays are classified as analog, hybrid, or fully digital according to where and how much signal processing is performed.
- Analog phased arrays: Analog arrays provide comparatively simple, low-power steering with few RF chains but limited simultaneous beams, beam-squint constraints, and reduced processing flexibility.
- Fully digital phased arrays: Fully digital arrays connect each element to an independent RF chain and converter, enabling maximum beamforming flexibility, multi-beam operation, interference suppression, and advanced processing.These capabilities require substantially higher power, calibration, thermal load, hardware complexity, and cost.
- Hybrid beamforming: Hybrid beamforming combines RF-domain analog steering within subarrays with baseband digital processing across subarray outputs using fewer RF chains.
- Directly comparing passive transmissive RIS with fully digital arrays is misleading because digital arrays support independent data streams and digital multi-beam processing.
- Vehicular phased-array deployment is constrained by RF chains, converters, amplifiers, calibration, cooling, rooftop area, power, aerodynamic profile, and cost.Despite these burdens, phased arrays currently provide the highest technological maturity and link-level capability.
- Prototype evidence: Reported prototypes demonstrate high antenna gain and broad electronic scanning, while also exposing implementation burdens from beamforming circuits, RF front ends, calibration, and power supplies.
C. Key Design Insight
Passive RIS, active RIS, phased arrays, and hybrid RIS–array solutions occupy different points in the performance, power, and hardware-complexity trade space. Future vehicular D2S systems are therefore unlikely to use one universal architecture.
- Passive RIS offers the best energy efficiency and scalability but limited link-budget improvement, while active RIS improves received signal strength with added power, thermal, and noise costs.
- Architecture selection depends on link budget, mobility, frequency, terminal size, and power constraints, allowing multiple antenna solutions to coexist.
III. PROTOTYPE-LEVEL COMPARISON OF RIS AND PHASED ARRAYS
The prototype comparison emphasizes engineering trade-offs among RIS and phased arrays rather than a single performance metric. Passive RIS prototypes generally use low DC power, while active RIS improves received signal strength through amplification but adds power and thermal burden.
- A fair prototype comparison must account for architecture because analog, hybrid, and digital phased arrays impose different RF-chain and processing burdens.Analog arrays can use one or a few RF chains, while highly parallel hybrid and fully digital arrays require substantially more converters, amplifiers, control electronics, and baseband processing.
- Prototype selection emphasizes aperture size, gain, power consumption, phase resolution, and scanning performance.
- Passive RIS prototypes generally consume only a few watts, typically remaining below 10 W even for apertures with several hundred unit cells.One reported passive RIS prototype consumes approximately 8.7 W with 1024 PIN diodes, while a Ka-band transmitarray requires approximately 4.7 W.
- Active RIS increases received signal strength through distributed amplification but adds hardware complexity, thermal burden, and power demand.Reported active-RIS DC consumption ranges from less than 1 W to several tens of watts, depending on architecture.
- RIS gain values are not directly comparable to phased-array transmitter EIRP because passive RIS improves the effective channel response without RF amplification.Active RIS provides limited amplification, whereas phased arrays increase EIRP through dedicated RF power amplifiers.
B. Implications for Vehicular Direct-to-Satellite Links
Vehicular D2S terminals must balance received signal strength, beam steering, and low power under mobility and satellite path loss. Passive RIS is power-efficient but link-limited, active RIS offers moderate amplification, and phased arrays retain mature high-gain tracking.
- Analog phased arrays report approximately 26–30 dBi gain, elevation steering from about −45° to +50°, and power consumption from approximately 10 W to several tens of watts.
- Passive RIS cannot compensate for severe satellite path loss, limiting its link improvement despite its low-power operation.
- Active RIS provides moderate amplification and stronger received signals with lower complexity than phased arrays, but offers more limited steering and requires additional thermal management.
IV. SYSTEM-LEVEL DESIGN INSIGHTS FOR VEHICULAR D2S
The system-level comparison is a qualitative technology map rather than a like-for-like numerical ranking. It positions passive RIS, active RIS, and phased arrays according to differing link-budget, power, complexity, and beamforming trade-offs.
- Direct numerical comparison is difficult because reported prototypes differ in frequency, aperture, feed architecture, active elements, measurement configuration, and link assumptions.A thorough quantitative comparison would require common assumptions for orbit, frequency, aperture, bandwidth, EIRP, receiver G/T, pointing loss, propagation loss, and implementation losses.
- The design map should be interpreted as an engineering technology map, not proof that one architecture always performs better.
- Passive RIS occupies the low-power end and is attractive when power and heat are constrained, but weak incident signals restrict received-power enhancement.
- Active RIS provides more link improvement than passive RIS at the expense of higher power and hardware overhead, making it an intermediate option.
- Phased arrays remain preferable when maximum beamforming gain and connection dependability dominate terminal requirements.
B. Beam Steering, Tracking, and Pointing Robustness
Beam steering must be evaluated together with power, tracking speed, latency, pointing accuracy, aperture scalability, and hardware complexity. Passive RIS supports energy-efficient coverage control, whereas phased arrays provide more robust rapid tracking; active RIS and hybrid RIS–array designs occupy intermediate trade-off regions.
- Passive RIS offers wide angular control with low RF power consumption, supporting coverage shaping and tolerance to pointing variations.
- Phased arrays provide wide-angle steering, but their key advantage is mature and fast electronic beam tracking through independent RF control.This capability comes with significantly greater power and hardware expense.
- Vehicular D2S assessment should jointly consider beamwidth, tracking speed, control latency, and pointing accuracy rather than steering range alone.
- Active RIS offers an intermediate option, while hybrid RIS–array architectures can balance steering capability, tracking performance, and terminal power consumption.
- Passive RIS occupies the high-scalability, low-complexity region because aperture growth mainly adds controllable unit cells instead of complete RF transceiver chains.
- Phased-array hardware complexity increases with additional antenna elements, phase shifters, amplifiers, converters, and processing channels, especially in hybrid and digital architectures.
D. Overall Engineering Interpretation
The comparison places passive RIS, active RIS, and phased arrays in a qualitative trade-off among link improvement, power demand, steering capability, aperture scalability, and hardware complexity. Phased arrays favor high-performance terminals, while RIS options favor lower-power or intermediate operating regimes.
- Phased arrays provide high gain, rapid tracking, and precise beam control, but require more power and hardware complexity.
- Passive RIS offers low-power operation and scalable apertures, but cannot amplify weak satellite signals.
- Active RIS combines wavefront control with signal enhancement while introducing additional power, noise, biasing, and thermal constraints.
- Hybrid RIS–array architectures may balance tracking reliability, link improvement, aperture scalability, and hardware complexity.
V. OPPORTUNITIES, CHALLENGES, AND FUTURE RESEARCH DIRECTIONS
The paper identifies opportunities for RIS-enabled lightweight terminals, hybrid architectures, adaptive beam management, integrated-network coordination, and shared sensing, while emphasizing deployment, training, control, and evaluation challenges.
- Opportunities: Passive RIS supports low-power coverage shaping, active RIS improves link budgets, and phased-array variants trade RF-chain cost for tracking or multi-beam flexibility.
- Opportunities: Hybrid RIS–array terminals can combine a compact active feed or phased-array module with a larger aperture for beam steering and coverage extension.
- Challenges: RIS beam training and channel estimation remain difficult because passive elements lack full RF chains under vehicle mobility and satellite motion.
- Challenges: Transmissive RIS deployment must address feed-to-aperture depth, vibration, weather, temperature, radiation, degradation, control latency, and system integration.
- Future Research Directions: Future work should jointly design hybrid architectures, active-element placement, feed configuration, aperture control, thermal management, and terminal packaging.
- Future Research Directions: AI-enabled beam management may use trajectory, sensor, blockage, ephemeris, weather, and historical-link data to reduce beam search and accelerate handovers.
- Future Research Directions: Integrated terrestrial–non-terrestrial use cases require careful evaluation of control overhead, synchronization, scheduling, and simultaneous satellite access.
- Future Research Directions: Fair comparisons require common link-budget assumptions and field trials under realistic mobility, vibration, weather, thermal, and control-latency conditions.
VI. CONCLUSION
The conclusion rejects a universal architecture for vehicular D2S communications: passive RIS emphasizes efficiency, active RIS offers intermediate enhancement, phased arrays provide tracking and multi-beam capability, and hybrid RIS–array terminals appear most practical.
- Passive RIS is low-power and energy-efficient but cannot amplify signals, limiting link-budget improvement especially for satellite uplinks.
- Active RIS improves received signal strength with fewer RF-chain complexities, but adds power consumption, noise, and thermal constraints.
- Analog arrays provide efficient directive tracking, whereas digital and hybrid phased arrays offer stronger multi-beam flexibility at higher RF-chain and processing cost.
- Hybrid RIS–array terminals have the potential to provide a practical balance between phased-array reliability and RIS energy efficiency and scalability.