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Reconfigurable Intelligent Surfaces vs. Relaying: Differences, Similarities, and Performance Comparison
M. Di Renzo, K. Ntontin, J. Song, F. H. Danufane, X. Qian, F. Lazarakis, J. de Rosny, D. -T. Phan-Huy, O. Simeone, R. Zhang, M. Debbah, G. Lerosey, M. Fink, S. Tretyakov, S. Shamai
TL;DR
The paper compares RISs configured as anomalous reflectors with relays in high-frequency wireless networks using scaling laws and numerical simulations. It finds that sufficiently large RISs can outperform relay-aided systems in data rate while reducing implementation complexity, but accurate modeling, experimental validation, and low-overhead control remain open challenges.
Problem
The paper examines how RISs configured as anomalous reflectors differ from and resemble relays in high-frequency wireless networks, where RIS benefits depend on their size and propagation-distance scaling.
Method
The paper compares RISs and relays using simple scaling laws, numerical simulations, and case studies across RIS size and transmission distance.
Results
Sufficiently large RISs can outperform relay-aided systems in data rate, with a 1.5 m RIS providing a rate similar to an ideal full-duplex relay without a power amplifier.
Takeaways & Limitations
RISs may offer data-rate and implementation-complexity advantages over relays when their effective size is sufficiently large relative to the wavelength.
Takeaways & Limitations
RIS research still relies on simplified models, has limited experimental validation of scaling laws, and faces stringent constraints from nearly passive implementation.
Abstract
from arXiv · showhide
Reconfigurable intelligent surfaces (RISs) have the potential of realizing the emerging concept of smart radio environments by leveraging the unique properties of meta-surfaces. In this article, we discuss the potential applications of RISs in wireless networks that operate at high-frequency bands, e.g., millimeter wave (30-100 GHz) and sub-millimeter wave (greater than 100 GHz) frequencies. When used in wireless networks, RISs may operate in a manner similar to relays. This paper elaborates on the key differences and similarities between RISs that are configured to operate as anomalous reflectors and relays. In particular, we illustrate numerical results that highlight the spectral efficiency gains of RISs when their size is sufficiently large as compared with the wavelength of the radio waves. In addition, we discuss key open issues that need to be addressed for unlocking the potential benefits of RISs.
INTRODUCTION
High-frequency wireless links offer capacity potential but are vulnerable to blockages and attenuation, motivating alternative propagation routes. RISs extend this idea by making wave manipulation programmable after deployment, unlike static reflectors.
- 77 exabytes of monthly mobile traffic were expected by 2022, motivating migration beyond sub-6 GHz bands toward millimeter- and sub-millimeter-wave frequencies.
- Millimeter- and sub-millimeter-wave signals are highly susceptible to blockage by large structures and severe attenuation from human bodies and foliage.
- Relays can create alternative propagation routes, but require dedicated power, reception, processing, and retransmission circuitry.
- Half-duplex relaying reduces resource availability, while full-duplex relaying introduces self-interference, destination interference, complexity, and power consumption.
- RISs are configurable after deployment and can support programmable smart radio environments, whereas non-reconfigurable reflectors cannot dynamically shape impinging waves.
- The article introduces RIS applications at high frequencies and focuses on differences between RIS anomalous reflectors and relay-aided systems.
RECONFIGURABLE INTELLIGENT SURFACES
RISs are thin electromagnetic surfaces whose sub-wavelength elements can be arranged and electronically programmed to shape incident radio waves. Their reconfigurability enables control after fabrication but requires limited control energy.
- An RIS is an artificial electromagnetic surface that customizes the propagation of incident radio waves.
- Meta-surfaces are thin, sub-wavelength sheets made from sub-wavelength metallic or dielectric scattering particles called meta-atoms or unit-cells.
- Non-reconfigurable meta-surfaces have fixed structural arrangements, whereas reconfigurable meta-surfaces can be modified and programmed using external stimuli.
- Electronic phase-changing components control switches or tunable elements, allowing the scattered wavefront to be manipulated and optimized.
- Reconfigurable surfaces are nearly passive: control circuitry consumes energy, but configured signal transmission requires no dedicated power supply.
C. Uses of RISs in Wireless Communications
RISs support multiple operational roles, from directional reflection and focusing to encoding and scattering engineering. The paper emphasizes anomalous reflectors as a basic mechanism for manipulating waves around environmental objects.
- RIS applications include anomalous reflection or transmission, beamforming or focusing, joint transmitter/RIS encoding, and single-RF multi-stream transmission.
- Anomalous reflection/transmission: Anomalous reflection steers waves toward specified directions independently of fading channels and receiver locations, but generally does not maximize signal-to-noise ratio.
- Beamforming/focusing: Beamforming or focusing maximizes signal-to-noise ratio at selected locations, but its optimization depends on fading channels and receiver locations.
- Joint transmitter/RIS encoding: Joint transmitter/RIS encoding uses meta-atom states to modulate additional data while requiring joint optimization of the transmitter and RIS.
- Single-RF multi-stream transmitter design: A feeder-based RIS can reflect multiple data-modulated signals and realize multi-stream transmission with a limited number of RF chains.
- Scattering engineering: RISs can increase the rank of multiple-antenna wireless channels by creating a rich-scattering environment, a function called scattering engineering.
- The paper primarily studies anomalous-reflector RISs as fundamental elements for manipulating waves around environmental objects and realizing smart radio environments.
WIRELESS 2.0: SMART RADIO ENVIRONMENTS
Smart radio environments treat propagation as a controllable design variable rather than an exogenous impairment. RISs apply this principle to redirect signals, suppress interference, improve security, and create richer scattering.
- Conventional communication design adapts transmitters, receivers, and protocols to a wireless environment modeled as uncontrollable.
- RISs can shape wavefronts across the network, making the environment an optimizable parameter for rate, latency, reliability, energy efficiency, privacy, and connectivity.
- Signal engineering: In signal engineering, an RIS redirects a blocked line-of-sight beam toward a mobile terminal to maximize received signal strength.
- Interference engineering: In interference engineering, an RIS shapes an interfering wave so it destructively combines with another signal at the target terminal.
- Security engineering: In security engineering, an RIS steers reflections toward the intended terminal and away from a malicious user while supporting diversity combining.
- Scattering engineering: In scattering engineering, an RIS creates a high-rank channel from a low-scattering environment to support high-data-rate multiple-input multiple-output transmission.
- Across these scenarios, RISs can engineer and optimize radio-wave propagation at low complexity in ways that benefit the network.
RECONFIGURABLE INTELLIGENT SURFACES VS. RELAYING
RIS anomalous reflectors and relays share some communication roles but differ in hardware, noise, duplexing, and signal-processing requirements. RISs avoid additive noise and certain relay constraints, while nearly-passive operation prevents amplification or regeneration.
- RISs and relays are compared qualitatively here, with quantitative performance comparisons presented in the following section.
- Relays use powered active components such as DACs, ADCs, mixers, power amplifiers, and low-noise amplifiers for reception and transmission.These components support decode-and-forward and amplify-and-forward relaying.
- RISs use metallic or dielectric patches with low-power switches or varactors, typically avoiding dedicated power amplifiers, mixers, and DACs/ADCs.The paper envisions lower complexity, particularly with inexpensive large-area electronics.
- Noise: Relay active components introduce additive noise; amplify-and-forward relays amplify noise, while decode-and-forward reduces it through regeneration at higher complexity and power consumption.Full-duplex relaying also suffers from residual loop-back self-interference.
- Noise: RIS anomalous reflectors avoid additive noise but may experience phase noise and, when nearly passive, cannot amplify or regenerate signals.
- RIS anomalous reflectors avoid half-duplex rate loss and loop-back self-interference, and their reflection coefficients can combine transmitter- and RIS-received signals.
D. Power Budget
Relays allocate RF power between the transmitter and relay and require independent power for transmission and electronics. RISs can operate nearly passively, while their received signal-to-noise ratio scales more favorably with the number of tunable elements.
- Power requirements: Relays require an independent power source for signal transmission and electronic components.
- Power requirements: RISs can use passive components with low-power switches or varactors, making energy harvesting suitable for close-to-passive implementations.
- RF power allocation: RISs place the total RF power at the transmitter, while reflected power depends on an optimizable surface transmittance and equals impinging power ideally.
- Scaling with N: With maximum ratio weighting and N relay antennas, average end-to-end signal-to-noise ratio increases linearly with N.
- Scaling with N: An RIS with N individually tunable antennas has average end-to-end signal-to-noise ratio increasing quadratically with N.The paper indicates N may reach a few thousands for inexpensive tunable antennas and ten thousands for meta-surfaces.
N reconfigurable meta-surfaces,
RIS distance behavior depends on whether its physical size is electrically large or small relative to wavelength and transmission distances. Electrically large RISs behave as anomalous mirrors, whereas electrically small RISs behave as diffusers and share AF-relay received-power scaling.
- Geometry and assumptions: The analysis assumes a two-dimensional source-relay/RIS-receiver geometry with equal-distance notation based on dSR and dRD.A relay is at the origin, and a one-dimensional RIS of length 2L is centered there.
- Relay scaling: AF-relay end-to-end received power scales with the reciprocal of the product of transmitter-to-relay and relay-to-receiver distances.
- Relay scaling: With noise, DF and AF relay signal-to-noise ratio scales with the reciprocal of the weaker of the two paths.
- RIS regimes: RIS distance scaling depends on its geometric size, wavelength, and relative transmitter-to-RIS and RIS-to-receiver distances.The paper analyzes electrically large and electrically small regimes.
- Electrically large RISs: Electrically large RISs behave asymptotically as anomalous mirrors when their size is large relative to wavelength and transmission distances.Received power and average end-to-end signal-to-noise ratio scale as (αkdSR + βkdRD)^-1.
- Electrically small RISs: Electrically small RISs behave asymptotically as diffusers and scale as 4L^2(dSRdRD)^-1, matching AF-relay received-power scaling.Their end-to-end signal-to-noise ratio also depends on RIS length 2L.
- Prototype scale: A 1 m^2 RIS prototype operating at 10.5 GHz was reported to operate in the far field beyond approximately 70 m analytically and 28 m experimentally.
G. Takeaway Messages from the Comparison
RIS and relay performance depends jointly on transmission distance and RIS size. Electrically large RISs can offer similar distance scaling with relay transmission, while sufficiently large RISs may outperform relays.
- Comparison setup: The comparison assumes equal transmitter-to-device and device-to-receiver distances, with RIS length determined by Mma meta-atoms spaced at λ/D.
- Short distances: Relay-aided transmission and electrically large RISs offer similar distance scaling for short distances d0.
- Short distances: RISs can potentially provide a better rate than relays at fixed RIS size when transmission distances are not too long.The stated comparison concerns anomalous-reflector RISs without half-duplex and loop-back self-interference constraints.
- Long distances: Electrically small RISs have less favorable distance scaling than relays for long distances d0.
- Long distances: A sufficiently large electrically small RIS can still potentially outperform relay-aided transmission because its average end-to-end signal-to-noise ratio scales quadratically with size.The paper concludes that RIS-aided transmission may outperform relaying when RIS size is sufficiently large.
NUMERICAL RESULTS
The numerical study compares a single RIS with a single relay under symmetric placement and a shared total-power constraint. It evaluates data-rate behavior versus transmission frequency and RIS size using analytical models.
- Evaluation setup: The study considers one relay and one RIS, located equidistantly from the transmitter and receiver.DF relaying is used for the relay results, while ideal full-duplex performance is also reported.
- Evaluated comparisons: Fig. 4 compares RIS and relay data rate as a function of transmission frequency.
- Evaluated comparisons: Fig. 5 compares RIS and relay data rate as a function of RIS size.
- Evaluation setup: A total power constraint splits the available power equally between the transmitter and the relay.RIS fields are computed using the analytical frameworks reported in the study.
A. RISs vs. Relays as a Function of the Transmission Distance
The distance study shows that a sufficiently large RIS can approach an ideal full-duplex relay without a power amplifier, while focusing operation can provide higher rates at additional channel-information cost. RIS behavior depends on distance and electrical size.
- Distance dependence: At 28 GHz with a 2L = 1.5 m RIS, anomalous-reflector operation provides a rate similar to an ideal full-duplex relay without a power amplifier.The RIS length corresponds to 140λ; the result can require approximately 280–700 meta-atoms for spacings between λ/2 and λ/5.
- Distance dependence: The RIS behaves as an anomalous mirror for distances d0 up to approximately 25–50 m and as a diffuse scatterer beyond that range.
- Anomalous mirrors vs. focusing lenses: A focusing-lens RIS generally outperforms an anomalous-reflector RIS and can achieve rates similar to the reflector’s long-distance approximation.The focusing lens requires estimating transmitter and receiver locations and adapting RIS phases to the wireless channels.
- RIS-size dependence: At d0 = 100 m, an RIS shorter than approximately L = 0.5–0.75 m is insufficient to provide rates similar to an ideal full-duplex relay at 28 GHz.
- Observed behavior: Short-distance results exhibit oscillations caused by coherent summation of secondary waves reflected by the RIS with different phases.
THE ROAD AHEAD
The paper identifies open issues spanning physics-based modeling, experimental validation, constrained system design, and information theory. These issues concern accurate models, empirical support, low-overhead control, and optimization of programmable channel states.
- Physics-Based Modeling: Current RIS models are simplified, motivating accurate but analytically tractable physics-based models grounded in electromagnetism.Existing local-structure models ignore spatial coupling among meta-atoms.
- Experimental Validation: Equivalent RIS models require validation through hardware testbeds and empirical measurements, but only a few experiments have validated received-power scaling laws.The relevant variables include RIS size, transmission distance, and the wave transformation applied.
- Constrained System Design: Nearly passive RIS implementation imposes stringent constraints on signal-processing algorithms and communication protocols.Without power amplifiers and channel-estimation units, RISs cannot perform channel estimation, creating a need for low-overhead acquisition protocols.
- Information and Communication Theory: RIS-based smart radio environments challenge information-theoretic models that treat channel transition probabilities as fixed and non-optimizable.Including channel states among encoding and modulation degrees of freedom opens new system-optimization directions.
- Conclusions: The paper concludes that sufficiently large anomalous-reflector RISs can outperform relay-aided systems in data rate while reducing implementation complexity.