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arxiv: 2605.04354 · v1 · submitted 2026-05-05 · 📡 eess.SP

Large Gain Degradation of Reflective Intelligent Surfaces in Realistic Environments

Dmitry Chizhik , Jinfeng Du This is my paper

Pith reviewed 2026-05-08 16:40 UTC · model grok-4.3

classification 📡 eess.SP
keywords reflective intelligent surfacesangle spreadgain degradationNLOSmmWaveambient scattercoverage extension
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The pith

Realistic angle spread in urban environments degrades RIS power gain by up to 25 dB, leaving little advantage over ambient scatter.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper establishes that reflective intelligent surfaces experience significant performance loss in real non-line-of-sight settings due to the spread of signal arrival angles. This loss can amount to 14 decibels or more, reducing the effective power delivered by a small RIS to below that of natural scattering from street objects. A larger one-meter surface at 28 gigahertz still shows only marginal improvement after accounting for the degradation. If correct, this finding indicates that RIS may not provide the expected coverage boost in typical city streets without further adjustments to models or designs.

Core claim

The authors derive a simple formula for the gain degradation of an RIS caused by channel angle spread. They compare the resulting coverage to ambient mechanisms like pole scattering and corner diffraction in urban NLOS links. Calculations show that an ideal 0.3 meter by 0.3 meter RIS at 28 GHz offers only about 5 dB more power at 200 meters around a street corner than ambient scatter, but angle spread causes a 14 dB drop. For a 1 meter by 1 meter RIS, the advantage shrinks to under 2 dB after a 25 dB degradation.

What carries the argument

The derived formula for RIS gain degradation due to channel angle spread, which accounts for the phase mismatches from distributed angles of arrival and departure.

If this is right

  • RIS may deliver less power than ambient scatter in realistic conditions.
  • Performance models for RIS must include angle spread to be accurate.
  • Coverage extension using RIS in mmWave bands could be less effective than expected.
  • Larger RIS sizes still face substantial degradation from spread.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Designers might need to consider RIS placement in low-spread environments or use adaptive configurations.
  • Alternative technologies like active relays could be compared directly in the same setups.
  • This degradation might vary with frequency, suggesting tests at other bands.

Load-bearing premise

The assumption that urban NLOS channels have sufficient angle spread to produce the modeled gain loss and that ambient scatter can be quantified accurately using simple pole and diffraction models.

What would settle it

A field measurement around an urban street corner at 28 GHz comparing the received signal power with a deployed 1 m x 1 m RIS to the power from ambient scatter alone without any RIS.

read the original abstract

Reflective Intelligent Surfaces (RIS) are considered promising in improving coverage in Non-Line of Sight (NLOS) wireless links, especially at mm wave or higher frequency bands. Coverage provided by RIS is here compared to coverage from such ambient propagation mechanisms as scattering from street poles (e.g. lampposts), and corner diffraction. A simple formula for RIS gain degradation due to channel angle spread is derived. It is found an ideal 0.3 m x 0.3 m RIS at 28 GHz promises to deliver only about 5 dB more power at 200 m around an urban street corner than the ambient scatter already there. Consideration of angle spread brings about some 14 dB drop in RIS power, bringing it well below ambient mechanisms. A 1 m x 1 m RIS at 28 GHz, offers under 2 dB advantage over ambient scatter after including the 25 dB gain degradation due to angle spread. This raises questions about usefulness of RIS-assisted coverage extension in realistic environments.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript derives a simple closed-form expression for RIS gain degradation arising from finite angle spread in the incident and reflected waves, then inserts representative urban NLOS angle-spread values to compare the delivered power of 0.3 m and 1 m RIS apertures at 28 GHz against ambient scattering from street poles and corner diffraction at a 200 m street-corner geometry. The central numerical claim is that angle spread produces 14 dB (0.3 m) to 25 dB (1 m) degradation, leaving the RIS with at most a 2 dB advantage over the ambient mechanisms.

Significance. If the angle-spread magnitude and ambient-scatter models are representative, the result would indicate that RIS-assisted NLOS coverage extension at mmWave frequencies may be far less attractive than ideal-link-budget calculations suggest, thereby motivating more realistic channel modeling in RIS system studies.

major comments (2)
  1. [§2 and §4] The degradation formula (derived in §2) is applied with specific angle-spread values that produce the quoted 14 dB and 25 dB losses; however, the manuscript supplies neither ray-tracing results, measurement references, nor literature citations demonstrating that these spread values (or their distribution) actually occur for a 0.3 m or 1 m aperture at 28 GHz over a 200 m urban corner path. Because the final comparison hinges on this numerical input, the central claim remains sensitive to the unvalidated choice.
  2. [§4] The ambient-scatter expressions for pole scattering and corner diffraction (used in §4 for the comparison) are simple closed-form models; the paper does not verify that these expressions reproduce the same propagation environment (geometry, frequency, surface roughness) employed for the RIS calculation. Any mismatch in modeling fidelity could reverse the reported <2 dB advantage.
minor comments (2)
  1. [§4] Clarify whether the 14 dB and 25 dB figures are obtained from a single deterministic spread value or from an average over a distribution; state the exact spread value(s) inserted into the formula.
  2. [Abstract and §4] The abstract states that the 0.3 m RIS “promises to deliver only about 5 dB more power … than the ambient scatter already there” before degradation; provide the corresponding pre-degradation link-budget numbers in the main text for direct comparison.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed comments. We address each major comment below with clarifications on our modeling choices and indicate the revisions planned for the updated manuscript.

read point-by-point responses

  1. Referee: [§2 and §4] The degradation formula (derived in §2) is applied with specific angle-spread values that produce the quoted 14 dB and 25 dB losses; however, the manuscript supplies neither ray-tracing results, measurement references, nor literature citations demonstrating that these spread values (or their distribution) actually occur for a 0.3 m or 1 m aperture at 28 GHz over a 200 m urban corner path. Because the final comparison hinges on this numerical input, the central claim remains sensitive to the unvalidated choice.

    Authors: The angle-spread values are representative of urban NLOS mmWave channels at 28 GHz, drawn from established literature on street-canyon measurements and ray-tracing studies. We will add explicit citations to these sources in the revised §2 and §4. The spread is an environmental parameter of the propagation path and geometry; it does not depend on RIS aperture size. The degradation formula, however, incorporates the interaction with aperture size through the narrower beam solid angle of larger surfaces, which is why the 1 m RIS experiences greater loss than the 0.3 m RIS. We will also add a sensitivity plot showing delivered power versus spread angle to demonstrate that the reported advantage remains modest across a plausible range of urban spreads. revision: partial

  2. Referee: [§4] The ambient-scatter expressions for pole scattering and corner diffraction (used in §4 for the comparison) are simple closed-form models; the paper does not verify that these expressions reproduce the same propagation environment (geometry, frequency, surface roughness) employed for the RIS calculation. Any mismatch in modeling fidelity could reverse the reported <2 dB advantage.

    Authors: We agree that explicit consistency is needed. The closed-form models for pole scattering and corner diffraction are standard expressions from the propagation literature and are applied to the identical 200 m street-corner geometry and 28 GHz frequency used for the RIS calculation. In the revision we will state the material and roughness assumptions for each mechanism, reference their original derivations, and add a short paragraph confirming that the same environmental parameters underlie both the RIS and ambient calculations. A brief sensitivity check will be included to show that the relative advantage stays below a few dB even when roughness parameters are varied within typical urban ranges. revision: yes

Circularity Check

0 steps flagged

No circularity detected in the derivation chain

full rationale

The paper derives a formula for RIS gain degradation due to channel angle spread from physical considerations of the link geometry and then evaluates it numerically by inserting external environmental parameters (angle spread magnitude, RIS size, frequency, distance). These parameters function as independent inputs rather than quantities fitted from or defined by the resulting power predictions or ambient comparisons. The ambient scatter models (pole scattering, corner diffraction) are presented as separate calculations. No self-definitional reduction, fitted-input-as-prediction, or load-bearing self-citation chain is exhibited that would make the central 14 dB / 25 dB degradation claim equivalent to its own assumptions by construction. The derivation remains self-contained once the angle-spread value is granted as an external modeling choice.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard wireless propagation models for angle spread and ambient scatter; no new entities are postulated and no free parameters are explicitly fitted in the abstract.

axioms (2)
  • domain assumption Urban NLOS links exhibit significant angle spread that degrades RIS performance
    Invoked to derive the gain degradation formula
  • domain assumption Ambient scatter from street poles and corner diffraction can be modeled simply for comparison
    Used as baseline to quantify RIS advantage

pith-pipeline@v0.9.0 · 5471 in / 1418 out tokens · 39138 ms · 2026-05-08T16:40:43.580134+00:00 · methodology

discussion (0)

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Reference graph

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