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STAR: Simultaneous Transmission And Reflection for 360° Coverage by Intelligent Surfaces
Yuanwei Liu, Xidong Mu, Jiaqi Xu, Robert Schober, Yang Hao, H. Vincent Poor, Lajos Hanzo
TL;DR
Conventional reflecting-only RISs constrain source and destination to the same half-space, motivating STAR-RISs for full-space coverage. The paper develops their hardware and signal-model foundations, proposes three operating protocols, surveys applications, and evaluates them against baselines. It reports that STAR-RISs outperform conventional RISs and the omni-surface in unicast numerical studies, while the research area remains at an early stage with open problems.
Problem
Conventional RISs mainly reflect signals, forcing source and destination into the same half-space and limiting deployment flexibility.
Method
The paper compares STAR-RISs with reflecting-only RISs, introduces a signal model, proposes ES, MS, and TS protocols, and studies applications and downlink MISO performance.
Results
STAR-RISs outperform conventional RISs and the omni-surface in unicast numerical case studies across the considered operating protocols.
Takeaways & Limitations
STAR-RISs provide full-space coverage and expanded propagation degrees of freedom for wireless communication design.
Abstract
from arXiv · showhide
A novel simultaneously transmitting and reflecting (STAR) system design relying on reconfigurable intelligent surfaces (RISs) is conceived. First, an existing prototype is reviewed and the potential benefits of STAR-RISs are discussed. Then, the key differences between conventional reflecting-only RISs and STAR-RISs are identified from the perspectives of hardware design, physics principles, and communication system design. Furthermore, the basic signal model of STAR-RISs is introduced, and three practical protocols are proposed for their operation, namely energy splitting, mode switching, and time switching. Based on the proposed protocols, a range of promising application scenarios are put forward for integrating STAR-RISs into next-generation wireless networks. By considering the downlink of a typical RIS-aided multiple-input single-output (MISO) system, numerical case studies are provided for revealing the superiority of STAR-RISs over other baselines, when employing the proposed protocols. Finally, several open research problems are discussed.
I. INTRODUCTION
Conventional RISs primarily reflect incident signals, forcing source and destination into the same half-space. STAR-RISs divide incident signals between reflection and transmission spaces, enabling full-space manipulation.
- RISs use controllable reconfigurable elements to adjust signal phase and potentially amplitude without requiring RF chains.
- Conventional RISs mainly reflect incident signals, so source and destination must lie on the same side of the surface.
- STAR-RISs reflect signals within one half-space while transmitting them into the other, creating a full-space SRE.
- A prototype realizes full reflection, full transmission, and simultaneous transmission and reflection by modifying the metasurface–substrate distance.
- In simultaneous operation, the surface divides incident signals between reflection and transmission spaces for full-space manipulation.
B. Key Advantages and Motivations for Employing STAR-RISs in Wireless Communication Systems
STAR-RISs extend signal manipulation across both half-spaces and provide additional propagation degrees of freedom. The paper motivates this design, distinguishes it from related surfaces, and evaluates protocols and applications.
- STAR-RISs extend coverage to the entire space and can serve both half-spaces using a single RIS.
- Their enhanced propagation degrees of freedom increase design flexibility for stringent communication requirements.
- STAR-RISs independently reconfigure transmitted and reflected signals through separate transmission and reflection coefficients.
- STAR-RIS communication design remains in its infancy, motivating a systematic treatment of differences, models, protocols, applications, and evaluation.
- The paper introduces a signal model and three operating protocols: energy splitting, mode switching, and time switching.
- Applications are proposed for outdoor and indoor wireless environments, while protocol performance is compared with baselines in downlink MISO systems.
II. KEY DIFFERENCES BETWEEN REFLECTING-ONLY RISS AND STAR-RISS
STAR-RISs differ from reflecting-only RISs in substrate, element, and electromagnetic requirements. Transparent substrates and elements supporting electric and magnetic currents enable independent transmission and reflection control.
- Hardware Design Differences: STAR-RIS substrates must be transparent at the operating frequency, unlike the opaque substrates of reflecting-only RISs.
- Hardware Design Differences: STAR-RIS elements support both electric and magnetic currents, making them more complex and thicker than conventional elements.
- Hardware Design Differences: These structural properties allow STAR-RISs to independently control transmission and reflection coefficients.
- Hardware Design Differences: The conceptual comparison contrasts reflecting-only elements on an opaque metal-plate-like substrate with STAR-RIS elements in a transparent substrate.
B. Physics Principles Differences
STAR-RISs differ physically from reflecting-only RISs by supporting both electric and magnetic responses, enabling simultaneous and more independent control of transmitted and reflected fields. These capabilities provide adjustable energy ratios and independent beamforming, but require more complex elements.
- Hardware and design trade-off: Compared with reflecting-only RISs, STAR-RISs require more complex elements to support simultaneous and independent transmission and reflection control.This added hardware capability underpins the additional communication-system design options.
- Production and radiation: STAR-RIS operation comprises induction, production, and radiation of transmitted and reflected fields.Time-varying polarization and magnetization currents radiate both fields into free space and impose phase differences relative to the incident field.
- Induction: STAR-RIS elements respond to both electric and magnetic incident-field components, inducing polarization and magnetization densities.The tunable electric and magnetic susceptibilities adjust these densities within a quantization error.
- Physical distinction: Supporting magnetic currents breaks the symmetry of non-magnetic reflecting-only RISs and enables simultaneous control of transmitted and reflected signals.The magnetization density introduces extra degrees of freedom for independently adjusting phase shifts.
- Communication-system implications: STAR-RISs provide adjustable transmitted-to-reflected energy ratios and independent beamforming for the two half-spaces.Element amplitudes control the energy ratio, while magnetic currents enable separate transmission and reflection phase-shift control.
III. BASIC SIGNAL MODEL AND PRACTICAL OPERATING PROTOCOLS
The paper formulates a STAR-RIS signal model based on independently reconfigurable transmission and reflection coefficients, subject to energy conservation. It then introduces energy splitting, mode switching, and time switching as practical operating protocols.
- Operating protocols: The paper proposes three practical protocols: energy splitting, mode switching, and time switching.These protocols are introduced for integrating STAR-RISs into wireless communication systems and have distinct advantages and disadvantages.
- Signal model: The STAR-RIS signal model maps the incident signal at each element to transmitted and reflected signals through reconfigurable coefficients.Transmission and reflection phase shifts can generally be selected independently, while amplitude coefficients obey energy conservation.
- Signal constraints: Energy conservation requires the transmitted and reflected signal energies to sum to the incident signal energy.Adjusting the amplitude coefficients supports full transmission, full reflection, or simultaneous transmission and reflection modes.
B. Energy Splitting
The three protocols distribute STAR-RIS transmission and reflection across elements or time. Energy splitting jointly optimizes both paths per element, mode switching partitions elements, and time switching alternates complete modes.
- Energy Splitting: Energy splitting operates every STAR-RIS element in simultaneous transmission-and-reflection mode, splitting each incident signal into transmitted and reflected components.Transmission and reflection amplitudes and phase shifts can be jointly optimized for diverse wireless-network objectives.
- Mode Switching: Mode switching partitions STAR-RIS elements into transmission and reflection groups, creating reduced-size transmitting-only and reflecting-only RISs.Element-wise mode selection and phase shifts are jointly optimized, making the on-off protocol easy to implement.
- Mode Switching: Mode switching generally cannot match energy splitting’s transmission and reflection gain because only subsets of elements serve each function.This gain trade-off follows from assigning different element groups to transmission and reflection.
- Time Switching: Time switching alternates all elements between full transmission and full reflection in orthogonal time slots.The fractions of time allocated to the two modes can be optimized to balance communication quality on the front and back sides.
- Time Switching: Time switching decouples transmission and reflection coefficients for independent optimization, but requires stringent synchronization and increases implementation complexity.Its coefficient advantage is conditioned on the chosen time allocation.
- Protocol comparison: The paper summarizes the optimization variables and the respective advantages and disadvantages of energy splitting, mode switching, and time switching.The protocols are illustrated as three practical operating choices for STAR-RISs.
IV. PROMISING APPLICATIONS OF STAR-RISS IN 6G
STAR-RISs are proposed for outdoor, outdoor-to-indoor, and indoor coverage extension where obstacles or building penetration loss constrain conventional links. Their transmission and reflection capabilities can create additional paths and reduce propagation distance.
- Coverage scenarios: STAR-RIS coverage extension is organized into outdoor, outdoor-to-indoor, and indoor communication scenarios.The applications target coverage area and quality for base stations and satellites.
- Outdoor communications: Outdoor links can use STAR-RISs on building facades, roadside billboards, or vehicle windows to create additional communication links.Vehicle-mounted surfaces can exploit transmission to enhance signal strength received inside cars, aircraft, and cruise ships.
- Outdoor-to-indoor communications: Outdoor-to-indoor communication is constrained by severe building-wall penetration loss, especially at mmWave and THz frequencies.STAR-RISs are presented as an outdoor-to-indoor bridge for this setting.
- Indoor communications: In indoor environments, reflecting-only RISs may require multi-hop bounces because they provide only half-space coverage.STAR-RIS transmission and reflection provide full-space coverage that may reduce propagation distance and increase received signal power.
- Coverage comparison: An illustrated comparison uses two hops for a conventional reflecting-only RIS and one hop for a STAR-RIS.The paper states that STAR-RISs substantially outperform conventional reflecting-only RISs in the discussed scenarios and add transmission-based design options.
B. Transmission-Reflection NOMA
STAR-RISs enable transmission-reflection NOMA by creating sufficiently different channel conditions for users on opposite sides, while supporting broader full-space applications such as CoMP and secure communications.
- Conventional reflecting-only RISs may not fully realize NOMA because users in the local reflected space generally have similar channel conditions.
- Transmission-reflection NOMA groups users on the transmission- and reflection-oriented sides of a STAR-RIS to facilitate NOMA.
- By optimizing element-based energy splitting or mode selection, STAR-RIS-aided transmission-reflection NOMA achieves higher gain than conventional reflecting-only RIS-aided NOMA.
- STAR-RIS-aided CoMP supports a cell-edge user in the transmission half-space and a cell-center user in the reflection half-space, enhancing the CoMP-user's received SINR through cooperative transmission.
- Full-space STAR-RIS propagation can enhance physical-layer security regardless of the eavesdropper's location, unlike same-side assumptions in conventional reflecting-only RIS studies.
E. Indoor Localization and Sensing
STAR-RISs extend full-space coverage in indoor environments, improving localization and sensing capabilities and supporting applications such as smart-factory robotics and control links.
- By overcoming signal blockages and providing full-space coverage, STAR-RISs can improve wireless-network localization and sensing, especially indoors.
- In smart factories, STAR-RIS deployment improves mobile-robot positioning and the data rate of control links.
- Additional proposed applications include STAR-RIS-aided SWIPT, visible-light communications, mmWave/THz communications, and robotic communications.
- These applications constitute future research directions for STAR-RISs in 6G networks.
V. NUMERICAL CASE STUDIES
Numerical studies compare STAR-RIS operating protocols with conventional RIS and omni-surface baselines in blocked-link downlink MISO unicast and multicast scenarios. Performance depends on the communication mode: TS is best for unicast, whereas ES is preferable for multicast, and STAR-RISs generally exploit more degrees of freedom than the baselines.
- The study evaluates minimum AP transmit power versus STAR-RIS element count for blocked-link downlink MISO unicast and multicast scenarios.Users occupy the transmission and reflection half-spaces, and the channels follow Rician fading with path-loss exponent 2.2.
- The baselines use a reflecting-only RIS plus a transmitting-only RIS, or an omni-surface with identical transmission and reflection coefficients.Each baseline provides full-space coverage for comparison with a STAR-RIS having M elements.
- Increasing M decreases the minimum required transmit power for STAR-RIS protocols and baselines because higher transmission/reflection gain is achieved.
- TS achieves the best performance in unicast, whereas ES is preferable in multicast because TS provides interference-free unicast communication while ES uses the entire communication time without multicast inter-user interference.
- ES always outperforms MS, since MS is theoretically a special case of ES with binary amplitude coefficients for each element.
- STAR-RISs outperform conventional RISs and the omni-surface in unicast regardless of operating protocol by exploiting element degrees of freedom for desired-signal enhancement and interference mitigation.
- In multicast, conventional RISs can outperform TS for small M, while the omni-surface can outperform TS and MS; ES remains superior because it retains more degrees of freedom.
VI. CONCLUDING REMARKS AND FUTURE RESEARCH
The paper establishes STAR-RIS fundamentals, protocols, applications, and baseline comparisons, while emphasizing that the field remains early-stage. Future work must address spatial analysis, channel estimation, and deployment under full-space operation.
- The paper identifies STAR-RIS differences from conventional reflecting-only RISs across hardware design, physics principles, and communication-system design.
- It proposes energy splitting, mode switching, and time switching protocols, discusses their tradeoffs, and identifies outdoor and indoor wireless applications.
- Numerical case studies evaluate STAR-RIS protocols against baseline schemes in downlink MISO unicast and multicast scenarios.
- STAR-RIS research remains at a very early stage, with numerous open research problems.
- Spatial Analysis of STAR-RIS Aided Networks using Stochastic Geometry: Spatial analysis requires new point processes because transmission and reflection depend on node locations and orientations.
- Channel Estimation for STAR-RISs: Channel estimation must balance TS's high accuracy and considerable pilot overhead against ES's reduced overhead from simultaneous link estimation.
- Deployment Strategies for STAR-RISs: Deployment strategies must carefully balance users across transmission and reflection half-spaces in realistic multi-user scenarios.