Indoor Channel Characterization with Extremely Large Reconfigurable Intelligent Surfaces at $300$ GHz

Fabio Cardoso; George C. Alexandropoulos; Lu\'is M. Pessoa; S\'ergio Matos

REVIEW 4 major objections 6 minor 26 references

Indoor Channel Characterization with Extremely Large Reconfigurable Intelligent Surfaces at $300$ GHz

T0 review · 4 major / 6 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read A 100x100, 2-bit RIS at 304 GHz can be represented in ray tracing as three equivalent rays rather than a full near-field aperture.

desk verdict Credible three-ray characterization of a specific large THz RIS under normal incidence, but the indoor-generalization claims outrun the simulations. read the letter →

arxiv 2501.12752 v1 pith:SWF4JOZ2 submitted 2025-01-22 cs.IT cs.ETmath.IT

Fabio Cardoso , Sérgio Matos , Luís M. Pessoa , George C. Alexandropoulos This is my paper · ORCID

classification cs.ITcs.ETmath.IT

keywords reconfigurableintelligentsurfaceTHzcommunicationsindoorchannelcharacterizationraytracingradarcrosssectionnear-fieldpropagationanomalousreflection2-bitunitcell

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

The paper seeks a cheap way to put extremely large reconfigurable intelligent surfaces into indoor channel simulations at sub-THz frequencies, where full-wave modeling of the surface is too heavy. It designs a 100x100, 2-bit RIS on a 5x5 cm aperture operating at 304 GHz, aimed at tilting a normally incident plane wave to +30 degrees. Full-wave radar cross section simulations show that the surface sends energy mainly into three directions: the intended +30 degree beam, a spurious symmetric beam at -30 degrees, and a leftover specular reflection at 0 degrees. The paper argues that these three rays can be inserted into existing ray tracing tools, and that the far-field approximation, despite a theoretical far-field distance of 10 m, is accurate to within 0.4 dB for RIS-to-receiver distances beyond 2 m. If true, indoor RIS-assisted THz channel characterization becomes far lighter while still capturing the main multipath behavior.

What carries the argument

The machinery is the three-ray decomposition of the full-wave RCS pattern of the entire metasurface. Full-wave RCS simulations replace the coupled near-field problem by an equivalent set of plane-wave rays: one for the designed anomalous beam, one for the spurious symmetric beam, and one for the specular leftover; each ray carries the amplitude and phase read from the simulated pattern. The 2-bit unit cell with four phase states, here $0^\circ$, $96^\circ$, $184^\circ$, and $273^\circ$, provides the discrete phase profile that creates the anomalous reflection, and the bistatic radar equation supplies the path-loss scaling for the dominant ray.

What would settle it

Simulate or measure the same 100x100 RIS under plane-wave illumination at oblique incidence angles such as $10^\circ$, $20^\circ$, or $30^\circ$. If a spurious lobe at $-\theta_{\mathrm{out}}$ disappears, a new strong lobe appears, or the relative amplitudes of the three dominant directions change by more than a few dB, then the three-ray model as stated fails for non-normal TX positions.

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Extended reading notes

Core claim

The paper claims that an extremely large RIS does not need to be modeled as a full near-field aperture in ray tracing; its scattering can be compressed to three equivalent rays. For the designed 100x100 element, 2-bit RIS at 304 GHz, the full-wave RCS pattern has a main lobe at $\theta_{\mathrm{out}}=30^\circ$ with RCS 15.6 dBm$^2$, a specular lobe at $0^\circ$ with 6.1 dBm$^2$, and a spurious symmetric lobe at $-30^\circ$ with 3.9 dBm$^2$, with a main-beam half-power width of only $1^\circ$. The phase of the main lobe is nearly constant over the central 12 dB angular range, so a single ray with the bistatic radar equation captures that lobe, and the two smaller lobes are added as interfering rays. In addition, the paper shows that the far-field approximation is adequate after 2 m even though the textbook far-field distance of the 5 cm aperture at 304 GHz is about 10 m.

Load-bearing premise

Everything rests on the RCS pattern measured under a normally incident plane wave staying representative when waves arrive at the RIS from the oblique angles that occur in real indoor links; the paper only simulates normal incidence.

Editorial extensions

If this is right

Ray tracing tools can model this 304 GHz RIS by launching three rays instead of embedding the full surface, which removes the need for near-field integration over 10,000 elements.
For indoor rooms with TX and RX distances above roughly 2 m, the far-field formula can be used for RIS-to-RX links, so coverage calculations stay simple.
A spurious symmetric beam and a specular leftover will appear in the channel as extra interference paths, so wideband indoor channel models should include these rays rather than assuming a single ideal reflection.
Because the beam direction shifts with frequency according to $\Delta\theta(f) = \theta_{\mathrm{out}} - \arcsin(f_0/f)$, simulations across a 10 GHz bandwidth need to update the ray directions; the RCS varies by about 3 dB over that band.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

If the three-ray compression holds for oblique incidence, something the paper does not simulate, the same shortcut would apply to many TX positions in a room, since real links rarely hit the RIS at exactly normal incidence.
The 65% efficiency relative to ideal specular reflection suggests the energy in the spurious and specular lobes is not negligible; a designer could deliberately reuse those rays for coverage diversity rather than treating them only as interference.
A natural extension is to recompute the three-ray parameters for other 2-bit phase distributions: the model structure may persist, but the lobe angles and relative amplitudes must be re-extracted for each configuration.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

The paper is worth your time, but with the same caveat the reader flagged: the three-ray model is derived and verified only for normal plane-wave illumination, and the title and abstract claim a lot more than that. The specific contributions are concrete and credible for what they simulate: a 100x100 two-bit RIS at 304 GHz with a designed +30° anomalous reflection, full-wave CST RCS patterns showing a main beam at +30°, a specular leftover at 0°, and a spurious symmetric lobe at -30°, and a claim that the far-field approximation holds within 0.4 dB beyond 2 m of the RIS. The phase analysis in Fig. 8 is a nice touch, because coherent multipath models need phase, not just amplitude.

What is genuinely new here is the complete chain from unit-cell design to a reduced-order three-ray description that a ray tracer could actually use. Most prior work either idealizes the RIS or stops at the RCS pattern. The paper also checks robustness to ±20 µm fabrication variations, which is practical. The bistatic radar equation application is standard, and the frequency-dispersion formula (Eq. 2) is correctly identified as something ray tracers often ignore.

Now the soft spots, in proportion. First, the model is self-referential: the three rays are read off the same full-wave simulation they are supposed to approximate, so the agreement is not an independent prediction. That is acceptable as a modeling step, but it should be stated more honestly. Second, and more importantly, only normal incidence is simulated. In the indoor scenario of Fig. 1, the TX can be anywhere, so waves hit the RIS at oblique angles and with spherical wavefronts. The generalized anomalous reflection law means the main beam, grating lobe, and specular component all shift with incidence angle; the paper does not show that the three-ray structure survives that change. Without oblique-incidence or finite-distance TX simulations, the "indoor channel characterization" claim overshoots the evidence. Third, the far-field validation in Fig. 5 is amplitude-only at one observation angle; phase errors could matter for coherent interference, though the phase-flatness finding partially mitigates this. No measurements or code/data are included, so the numbers are not independently checkable.

The citation pattern is fine; the authors cite their own prior work on spurious rays as background, which is appropriate. My overall take: the central result is credible for the condition it actually tests, and the paper would be a solid contribution if the claims were scaled to normal incidence or augmented with at least one oblique-incidence simulation. As is, it is a useful design-and-characterization study that a serious ray tracing or THz RIS researcher should read. I'd send it to peer review, with a recommendation to add oblique-incidence validation or temper the title/abstract.

Referee Report

4 major / 6 minor

Summary. The paper presents a 2-bit static RIS design at 304 GHz with a 100×100 element, 5×5 cm aperture intended to redirect normally incident plane waves to θout = 30°. Full-wave RCS simulations of this RIS show three dominant scattering directions: the main beam at +30°, a spurious symmetric beam at −30°, and a specular leftover at 0°. Based on these observations, the authors propose approximating the RIS behavior in ray tracing by a three-ray model and claim that the far-field approximation is valid at RIS-RX distances as small as 2 m. They also derive a single-ray received-power estimate via the bistatic radar equation and discuss the frequency dependence of the RIS beam direction.

Significance. If fully validated, the proposed three-ray approximation would be a useful, computationally efficient means of incorporating extremely large RISs into ray tracing for THz indoor channels. The paper's strengths include a full-wave simulation of an electrically large 100×100 RIS, a 2-bit unit-cell design with a 20 µm fabrication-tolerance check, RCS patterns showing narrow beams, use of the standard bistatic radar equation, and an amplitude-based far-field comparison at the main-beam angle. However, the central transfer from a single normal-incidence full-wave RCS characterization to arbitrary indoor TX/RX positions remains unverified, and the coherent (phase) behavior needed for ray interference is not validated. The significance of the result is therefore conditional on additional checks.

major comments (4)

[§IV-B and Figs. 4, 8] The three-ray model is extracted only for normal plane-wave illumination of the RIS. The indoor scenario in Fig. 1, however, involves a TX whose position implies generally oblique incidence on the RIS, and the phase profile in Section III is explicitly designed for normal incidence. For oblique incidence the main beam direction, the quantization-induced grating lobe, and the specular component all change in angle and relative power. Since no oblique-incidence or finite-distance source simulation is reported, the proposed ray-tracing shortcut is not yet supported. A concrete test would be to simulate at least two oblique incidence angles (e.g., θin = 20° and 40°) and show that three rays with the same structure and stable amplitudes/directions emerge, or to provide an incidence-angle-dependent parameterization of the rays.
[§IV-B and Fig. 8] The three-ray model parameters are read from the same full-wave RCS simulation that is used to demonstrate the approximation, so the 'agreement' is a measure of self-consistency rather than an external prediction. No validation is shown in an actual channel: there is no comparison of the three-ray model against full-wave fields for a TX-RX link with the geometry of Fig. 1, and no ray-tracing implementation is run. Please provide a quantitative error metric (e.g., integrated angular field error or a sample NLOS channel impulse response computed both with the full-wave RIS data and with the three-ray model).
[§III-C and Fig. 5] The far-field approximation is validated only for the amplitude of the E-field at the fixed observation angle of 30°. Ray tracing for interference requires the complex field, i.e., both amplitude and phase, because the relative phases of the three rays determine their coherent summation. Since Fig. 8 explicitly reports the relative phase between the rays, the claim that the far-field approximation is valid at short distances should be checked for the phase as well, for example by comparing complex E-field versus distance at several angles.
[§IV-A, Eq. (2)] The frequency-shift formula Δθ(f) = θout − arcsin(f0/f) is dimensionally inconsistent: at f = f0 it gives a nonzero angle shift (arcsin(1) = 90°), and for f < f0 the argument of arcsin exceeds unity. For a fixed phase gradient, the correct relation is sin θ(f) = (f0/f) sin θ0 (or, for small angles, Δθ ≈ (1 − f0/f) θ0). Please correct Eq. (2) and re-derive any frequency-dependence claims that rely on it.

minor comments (6)

[Abstract and §I] The phrase 'An 100×100' should be 'A 100×100'; the same grammatical issue appears at the beginning of Section II.
[§III-B] The acronym 'FTTD' in 'the Finite Difference Time Domain (FTTD) solver' should be 'FDTD'.
[§III-B] The sentence 'It can observed that' is missing 'be'; it should read 'It can be observed that'.
[§III-C] The phrase 'with and amplitude variation below 0.4 dB' should be 'with an amplitude variation below 0.4 dB'.
[Fig. 1 caption] The caption contains a duplicated definite article: 'to facilitate the the TX-RX link' should read 'to facilitate the TX-RX link'.
[§IV-A and Fig. 6] The text states that 'the RCS can vary by 3 dB' within a 10 GHz bandwidth, whereas Fig. 6 shows E-field values; please clarify whether the 3 dB variation refers to the RCS or to the received E-field, since the two differ by a square-root relation.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the three-ray model is a compact description of the same full-wave RCS simulation, and the paper does not present it as an independent prediction.

full rationale

The paper's central result is a characterization study: a 100x100 2-bit RIS is designed, its RCS is obtained via full-wave CST simulation, and the authors observe that the scattered energy is concentrated in three directions (the designed theta_out = 30 deg beam, a symmetric spurious lobe at -30 deg, and a specular leftover at 0 deg). Calling this a 'three-ray model' is a data reduction of the simulated pattern, not a prediction of a quantity that was not already used as input; there is no fitted parameter that is then renamed as a forecast, and no equation in which the claimed result is identical to an input by construction. The far-field check in Fig. 5 compares two evaluations within the same CST model (near-field vs far-field approximation), which is a legitimate numerical convergence test rather than a circular validation. The paper does rely on self-citations [17] and [20], but these are contextual (e.g., noting that spurious rays are an inherent RIS effect) and are corroborated by the paper's own simulated RCS pattern, so they are not load-bearing. The main limitation is generality: the three-ray structure is demonstrated only for normal plane-wave illumination, so transferring it to arbitrary indoor TX positions with oblique incidence is an untested extrapolation. That is a validity or scope concern, not a circularity of the derivation chain.

Assumptions & free parameters 2 free parameters · 4 assumptions · 0 invented entities

The central claims rest on one reduced model whose coefficients are read from the same full-wave simulation used for support, plus design choices for the unit cell. Four background assumptions are listed; the two most important are that CST results are physically accurate and that normal plane-wave illumination represents all indoor TX positions. No new physical entities are introduced.

free parameters (2)

Three-ray model coefficients (amplitudes, phases, directions) = Not reported; extracted from Fig. 8
The three dominant rays are selected from the same full-wave RCS simulation the model is claimed to approximate, making the model a compressed version of the simulation output.
Unit-cell state dimensions (win, wr) = win: 0.13 to 0.17 mm, wr: 0.08 mm, plus an all-metal state
Chosen by design to realize the four phase states in Table I; the RCS pattern and therefore the three-ray model depend on these choices, though they are not fitted to the target outcome.

assumptions (4)

domain assumption CST full-wave simulation accurately represents the physical RIS
All central claims rely on RCS and near/far-field fields computed in CST; no experimental validation is provided.
domain assumption Normal plane-wave illumination is representative of all indoor TX positions
The RIS is designed and simulated for incoming normal plane waves (Sections II and IV), while real indoor TX positions produce oblique incidence.
standard math The bistatic radar equation correctly links RCS to received power for RIS-assisted paths
Equation (1) is a standard result, but its application to a finite RIS in the near field is not separately justified.
ad hoc to paper The three dominant directions selected from the RCS pattern capture all relevant channel multipath
Direction selection is based on visual inspection of the RCS pattern in Fig. 8, with no quantitative comparison against full-wave fields in a full channel scenario.

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Cite this review

Pith. "Pith review of Indoor Channel Characterization with Extremely Large Reconfigurable Intelligent Surfaces at $300$ GHz." pith.science (2026). https://pith.science/paper/SWF4JOZ2

@misc{pith2026250112752,
  author       = {Pith},
  title        = {Pith review of: Indoor Channel Characterization with Extremely Large Reconfigurable Intelligent Surfaces at $300$ GHz},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/SWF4JOZ2}},
  note         = {Machine review of arXiv:2501.12752}
}

abstract

The technology of Reconfigurable Intelligent Surfaces (RISs) is lately being considered as a boosting component for various indoor wireless applications, enabling wave propagation control and coverage extension. However, the incorporation of extremely large RISs, as recently being considered for ultra-high capacity industrial environments at subTHz frequencies, imposes certain challenges for indoor channel characterization. In particular, such RISs contribute additional multipath components and their large sizes with respect to the signal wavelength lead to near-field propagation. To this end, ray tracing approaches become quite cumbersome and need to be rerun for different RIS unit cell designs. In this paper, we present a novel approach for the incorporation of RISs in indoor multipath environments towards their efficient channel characterization. An $100\times100$ RIS design with $2$-bit resolution unit cells realizing a fixed anomalous reflection at 300 GHz is presented, whose radar cross section patterns are obtained via full-wave simulations. It is showcased that the RIS behavior can be conveniently approximated by a three-ray model, which can be efficiently incorporated within available ray tracing tools, and that the far-field approximation is valid for even very small distances from the RIS.

Figures

Figures reproduced from arXiv: 2501.12752 by the authors.

**Figure 2.** The geometry of the designed single-layer patch-based unit cell at 304 GHz. RIS, which, however, may oversimplify channel characterization. On the other hand, full-wave simulations provide a reliable means for the RIS characterization, allowing us to extract equivalent rays from the RCS patterns; see the inset of [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗

**Figure 3.** Overview of the unit cell design corresponding to the 4 different states. -20 -15 -10 -5 0 5 10 15 20 -90 -60 -30 0 30 60 90 RCS (dBm2) 𝜃 (degree) 300GHz 304 GHz 310GHz [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗

**Figure 4.** RCS patterns of the designed 100×100 static RIS with 2-bit phase resolution unit cells. manufacturing on Printed Circuit Board (PCB). The designed 2-bit phase resolution unit cell is illustrated in [PITH_FULL_IMAGE:figures/full_fig_p003_4.png] view at source ↗

Figures from the paper (3 more)

**Figure 6.** Figure 6: E-field versus the RIS-RX distance d2 for a fixed observation angle at 30◦ and different operating frequencies. C. Far-Field Approximation The far-field distance of the designed 5×5 cm2 RIS aperture is 10 m at the studied 304 GHz band. This implies that, in almost the …

**Figure 8.** Figure 8: RCS amplitude of the designed static 2-bit RIS as a function of θ, including the detail of the phase at the main reflection directions. expected that ray interference will play a significant role on the channel behavior [24]. In the case of RIS-assisted wireless commun…

**Figure 7.** Figure 7: Full-wave simulations for the RCS amplitude (a) and phase (b) of the designed static 2-bit RIS. and d1 and d2 denote the TX-RIS and the RX-RIS distances, respectively, and σRIS is the RCS in dB which has been obtained for the designed static 100×100 RIS in [PITH_FULL_…

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Reviewed August 10, 2026 · model on record in the stance chip above.

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